In all cases, the squareness ratio (S, 0.075 for ZnFe2O4 at 700 ◦C—0.727 for NiFe2O4 at 1000 ◦C), the anisotropy constant (K, 0.002 × 103 erg/cm3 for ZnFe2O4 at 700 ◦C—2.44 × 103 erg/cm3 for CoFe2O4 at 1000 ◦C) also increase with the increase of heating temperature. Compared to the same ferrites embedded in SiO2 matrix, the K value is much lower [33].

### **3. Discussion**

The thermal behavior of FeIII(NO3)3–MII(NO3)2–1,3-propanediol solutions was studied by DTA. The endothermic effect at 70–100 ◦C is attributed to the loss of moisture and of crystallization water from the metallic nitrates used in the synthesis. The endothermic effect at 132–213 ◦C on the DTA diagram is attributed to formation of metal-malonate precursor. The formation of metal-malonate precursor for CoFe2O4 and ZnFe2O4 takes place in two stages as indicated by the two endothermic effects. The first endothermic effect was assigned to the divalent metal-malonate formation (143 ◦C for Co-malonate and 132 ◦C for Mn-malonate), while the second endothermic effect (around 201 ◦C) was assigned to the formation of Fe-malonate. In case of the other synthesis, the divalent metal (Ni, Zn, Cu) malonates and the trivalent Fe-malonate formation takes place in a single stage (204–213 ◦C). The formation of ferrites by decomposition of malonate precursors is indicated on the DTA diagram by a single exothermic effect at 250–350 ◦C, except for CoFe2O4, where two exothermic effects appear at 253 and 303 ◦C. The two-stage formation of CoFe2O4 in the metal nitrates–diol mixture could be explained by the fact that the aqua cation [Fe(H2O)6] 3+ is a stronger acid than the aqua cation [Co(H2O)6] 2+ [17,36]. The highest mass loss shown on the TG diagram is attributed to CoFe2O4 (65%), while the lowest mass loss is attributed to ZnFe2O4 (57%), probably due to the fact that ZnFe2O4 is quantitatively obtained at lower temperatures compared to other ferrites accompanied by other crystalline or amorphous secondary phases.

The increase of heating temperature from 700 to 1000 ◦C did not affect the crystal structure of the studied ferrites but improved the phase purity [10]. Moreover, by increasing the heating temperature, the diffraction peaks become sharper and narrower, indicating the formation of larger particles due to grain growth [25,38]. After heat treatment at 1000 ◦C, an important agglomeration of the particles takes place without consequent recrystallization, supporting the formation of single crystals rather than polycrystals [1,2,6]. Oppositely, at 700 ◦C, the surface dipole–dipole interactions, high surface energy and tension, as well as the change of cation distribution within the nanocrystallite, induces lattice shrinking, which further inhibits grain growth [1–6]. Generally, the size of the crystallite is higher than the size of the corresponding ferrites embedded in SiO2 matrix, produced by sol–gel method [15–19]. These findings indicate that the heating temperature and the synthesis route plays a key role in determining the crystallite size [8]. Crystallite size has a significant effect on the magnetic and optical properties of the material, especially when the grain size is approaching the crystallite size [4–6,38].

The different particle size of the produced ferrites may be attributed to different kinetics of metal oxides formation, different particle growth rate or presence of structural disorder and strain in the lattice caused by different ionic radii [39] The different particle arrangement is attributed to the formation of well-delimited particles that generate solid boundaries. Moreover, interfacial surface tensions appear most likely due to the agglomeration tendency of small particles, weak surface interaction due to Van der Waals forces and magnetic interactions [39].

The magnetic properties of nanoferrites are strongly influenced by the cation distribution between the tetrahedral (A) and octahedral (B) sites, as well as by the interactions between the magnetic ions [37,40]. Different size and morphology of the nanoparticles at different heating temperatures results in different surface spin disorder, pinned magnetic moment, different surface spin canting and consequently different cation inversion in the spinel structure and magnetic features [8]. The lower MS values of ferrites heat treated at 700 ◦C compared to those at 1000 ◦C result from the lower crystallinity at 700 ◦C, presence of vacancies, interatomic spacing, low coordination number and surface spin disorder [37].

At 1000 ◦C, the CoFe2O4 has superparamagnetic behavior, while the other ferrites display paramagnetic behavior. The superparamagnetic behavior is attributed to the high disorder of the magnetic moment orientation with the increase in the surface-area-tovolume ratio [41]. For MnFe2O4, the sharp increase in MS with the increase of heating temperature could be explained by the formation of trace paramagnetic α-Fe2O3 [27,30]. The ZnFe2O4 heat treated at 700 ◦C is paramagnetic, the low MS values being attributed to the lattice defects, core–shell interactions, spin canting, disordered cation distribution, A–B super exchange interaction and random spin orientation on the surface of nanoparticles [30]. The MS values reported for ZnFe2O4 differ from study to study, indicating that MS strongly depends on the synthesis route and heating temperature [27,41]. The changes of CuFe2O4 magnetic properties following the reduction in bulk grain size of CuFe2O4 nanoparticles by milling was reported by Soufi et al. [12]. The MS value of NiFe2O4 is lower than that of MnFe2O4 and CoFe2O4, probably due to the increase in surface effects with the decrease in particle size [20]. The influences of surface effect on the magnetic properties may be explained by the different exchange interactions and presence of magnetic defects on the nanoparticles surface [20]. Priyadharsini et al. also reported the MS, MR and HC values increasing with the heating temperature [25].

Generally, the HC of the spinel ferrite nanoparticles is governed by the magnetocrystalline anisotropy, strain, interparticle interaction, grain size and morphology [42]. The HC also increase with the increase in the surface potential barrier caused by crystalline lattice defects such as the deviation of atoms from the normal positions in the surface layers [43]. The influence of the particle sizes, internal strain, magnetic domain structure, shape and magnetocrystalline anisotropy of the nanoparticles on the HC value is not fully explained [43]. The low HC of all ferrites for both heating temperatures indicate an enhanced coalescence of the crystallites that further results in strong magnetic coupling and high magnetization [43]. At both temperatures, the HC of CoFe2O4 nanoparticles prepared by thermal decomposition increases with the particle-size increase, suggesting the presence of a single magnetic domain [8]. The transition from superparamagnetic to ferromagnetic behavior of CoFe2O4 was noticed after heat treatment at 1000 ◦C [25]. The different HC of NiFe2O4 is attributed both to the crystallite size and the presence of shape anisotropy [30].

The increasing squareness ratio (S) at high heating temperatures could be the consequence of the reorientation of grains along the easy axis of magnetization when the field is switched off [25]. The main factors that influence the magnetic anisotropy are the crystallographic directions, surface defects and irregularities [44,45]. The high HC and K of CoFe2O4 are the consequence of Co2+ ions in octahedral (B) sites, which induce frozen orbital angular momentum and strong spin-orbital coupling [8,46].

### **4. Materials and Methods**

Fe(NO3)3·9H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O, Zn(NO3)2·6H2O, Cu(NO3)2·3H2O, Mn(NO3)2·3H2O and 1,3-propanediol of purity higher than 98% were purchased from Merck (Darmstadt, Germany) and used as received.

MFe2O4 (M = Co, Zn, Ni, Cu, Mn) were synthesized by mixing the metal nitrates in 1M/2Fe molar ratio with 1,3-propanediol in equimolecular ratio of NO3 −/1,3-propanediol. The resulted solutions were heat treated at 700, and 1000 ◦C (5 h) in air using a LT9 muffle furnace (Nabertherm, Lilienthal, Germany).

The ferrite formation was investigated by thermogravimetry (TG) and differential thermal analysis (DTA) using a Q600 SDT (TA Instruments, New Castle, DE, USA) analyzer, in air, up to 1000 ◦C, at 10 ◦C·min−1 using alumina standards. The crystalline phases were investigated by X-ray diffraction using a D8 Advance (Bruker, Karlsruhe, Germany), at ambient temperature, with CuKα radiation (λ = 1.54060 Å) and LynxEye detector, operating at 40 kV and 40 mA. The Co/Fe (CoFe2O4), Ni/Fe (NiFe2O4), Zn/Fe (ZnFe2O4), Cu/Fe (CuFe2O4) and Mn/Fe (MnFe2O4) molar ratios were confirmed using an Optima 5300 DV (Perkin Elmer, Norwalk, CT, USA) inductively coupled plasma optical emission spectrometer (ICP-OES), spectrometer, after microwave digestion (Xpert microwave system, Berghof, Eningen, Germany) with aqua regia. Specific surface area (SSA) was calculated using the BET model from N2 adsorption–desorption isotherms recorded at 196 ◦C on samples degassed for 4 h at 150 ◦C and 2 Pa pressure using a Sorptomatic 1990 (Thermo Fisher Scientific, Waltham, MA, USA) instrument. The shape and clustering of nanoparticles were studied on samples deposited and dried on carbon-coated copper grids

using a transmission electron microscope (TEM, HD-2700, Hitachi, Tokyo, Japan). The magnetic measurements were performed using a 7400 vibrating-sample magnetometer (VSM, LakeShore Cryotronics, Westerville, OH, USA). The hysteresis loops were recorded at room temperature in magnetic fields between −2 to 2 Tesla.

### **5. Conclusions**

The structural, morphological and magnetic characteristics of nanosized CoFe2O4, NiFe2O4, ZnFe2O4, CuFe2O4 and MnFe2O4 obtained by thermal decomposition were investigated. The formation of ferrites appeared as a single exothermic effect at 250–350 ◦C, excepting CoFe2O4 with two exothermic effects. The highest mass loss was attributed to CoFe2O4 (65%), while the lowest mass loss was assigned to ZnFe2O4 (55%). Unlike similar ferrites embedded in SiO2 matrix, at both temperatures, single crystalline phases were remarked, excepting the presence of Co3O4 (in case of CoFe2O4) and α-Fe2O3 (in case of MnFe2O4) at 700 ◦C. The SSA values lower than 0.5 m2/g indicated a non–porous structure due to the particle agglomeration. CuFe2O4 showed the largest particle size (85 nm), while MnFe2O4 had the smallest particle size (32 nm). The crystalline CoFe2O4 heat treated at 1000 ◦C displayed the highest Ms, HC and K values, presenting superparamagnetic behavior, while the other ferrites exhibited paramagnetic behavior.

**Author Contributions:** Conceptualization, methodology, investigation, writing—original draft, visualization, supervision, T.D.; methodology, investigation, writing—review, E.A.L. and O.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** The APC was funded by Technical University of Cluj-Napoca Grant Support GC1/2021.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This study was supported by the Ministry of Research, Innovation and Digitization through Program 1—Development of the national research & development system, Subprogram 1.2—Institutional performance—Projects that finance the RDI excellence, Contract no. 18PFE/30 December 2021.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

### **References**

## *Article* **Impact of Ni Content on the Structure and Sonophotocatalytic Activity of Ni-Zn-Co Ferrite Nanoparticles**

**Thomas Dippong 1 , Oana Cadar 2 , Firuta Goga 3 , Dana Toloman 4 and Erika Andrea Levei 2,\***

**Abstract:** The structure, morphology, and sonophotocatalytic activity of Ni-Zn-Co ferrite nanoparticles, embedded in a SiO2 matrix and produced by a modified sol-gel method, followed by thermal treatment, were investigated. The thermal analysis confirmed the formation of metal succinate precursors up to 200 ◦C, their decomposition to metal oxides and the formation of Ni-Zn-Co ferrites up to 500 ◦C. The crystalline phases, crystallite size and lattice parameter were determined based on X-ray diffraction patterns. Transmission electron microscopy revealed the shape, size, and distribution pattern of the ferrite nanoparticles. The particle sizes ranged between 34 and 40 nm. All the samples showed optical responses in the visible range. The best sonophotocatalytic activity against the rhodamine B solution under visible irradiation was obtained for Ni0.3Zn0.3Co0.4Fe2O4@SiO2 .

**Keywords:** nickel-zinc-cobalt ferrite; thermal behavior; crystalline phase; sonophotocatalysis

**Citation:** Dippong, T.; Cadar, O.; Goga, F.; Toloman, D.; Levei, E.A. Impact of Ni Content on the Structure and Sonophotocatalytic Activity of Ni-Zn-Co Ferrite Nanoparticles. *Int. J. Mol. Sci.* **2022**, *23*, 14167. https:// doi.org/10.3390/ijms232214167

Academic Editor: Shaojun Yuan

Received: 18 October 2022 Accepted: 14 November 2022 Published: 16 November 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

### **1. Introduction**

Despite the measures taken to reduce pollution, industrial effluents containing dyes and pigments used in the textile industry often resurface in the surrounding environment. Dyes are complex organic structures with a high resistance to chemical and biological degradation, high water solubility, and have a negative impact on the environment, particularly aquatic ecosystems [1,2]. Therefore, the efficient treatment of industrial effluents and wastewaters containing dyes is crucial for environmental protection.

The photocatalytic degradation of dyes is a simple, cost-effective, and environmentally friendly approach for wastewater treatment as it allows the decomposition of complex organic structures into CO2 and water [3,4]. In the last few years, the use of sonophotocatalysis for the degradation of a wide range of organic pollutants in aqueous systems has been the topic of several studies [5,6]. Sonophotocatalysis use the synergistic effects of ultrasonic waves, UV-Vis irradiation and photocatalyst to form highly reactive free radicals in an aqueous medium that further react with dyes and lead to their degradation [7]. By providing additional nuclei, the heterogeneous catalyst enhances the formation of cavitation bubbles, which in turn increases the formation of reactive radicals through water pyrolysis [5]. The mechanism of sonophotocatalytic degradation, as well as the main advantages of combined ultrasound and photocatalytic processes, are presented by Abdurahman et al. [5]. The high energy consumption of sonophotocatalysis makes its large-scale application difficult, however, the high costs could be compensated by the low time required for the degradation of organic compounds [7]. Due to their magnetic properties, their recovery using an external magnetic field and their reuse is possible [8,9].

Oxides containing at least two types of metals are potential candidates for photoelectrochemistry and photocatalysis due to their band structure and energy position [10]. The bandgap of several MFe2O4-type ferrites are presented by Dillert et al. [11]. Nanosized spineltype ferrites, containing first-row transition metals such are Ni, Co, Zn, Mn are attractive materials in electronics, magnetic storage, ferrofluid technology, gas sensors, catalysis, photocatalysis and biomedicine, including magnet-guided drug carriers, contrast agents and tracers for positive magnetic resonance imaging [12–17]. They are also promising candidates for wastewater treatment as they could act both as adsorbents, due to their high specific surface area, and as photocatalysis, due to their low energy band gap, that allow the conversion of UV or visible light into chemical energy that favors the degradation of dyes [8,17–21]. The strong photodegradation capacity of 3d transition metal ferrites such as CoFe2O4, CuFe2O4, NiFe2O4, and ZnFe2O4, with magnetic properties, was previously demonstrated for different organic compounds [12,13,21–27]. The surface coating of the ferrite nanoparticles with different materials, especially semiconductor materials such as TiO2 and SiO2, was proven to enhance the photocatalytic activity [28]. CoFe2O4 nanoparticles coated with TiO2–SiO2 efficiently degraded (up to 98%) the methylene blue dye [2], while Rhodamine B (RhB) degradation by CoFe2O4, was only 80% [29,30]. The enhancement of the photocatalytic performance of NixCo1-xFe2O4, prepared by the coprecipitation method against methylene blue at a high Ni content, was reported by Lassoued and Li [31]. A good photodegradation efficiency (about 80%) of methylene blue under visible light irradiation was also delineated for Co-Zn ferrite with various Co and Zn contents, obtained using the citrate precursor method [32]. The increase of the rate constant with the increasing Co content was also observed [32]. The photocatalytic activity of Co0.6Zn0.4NixFe2-xO4 powders, obtained by the sol-gel method under visible light against methyl orange dye in an aqueous solution, was also reported [33].

The spinel ferrite properties are determined by their composition, structure, particle size, and morphology [34–37]. These characteristics strongly depend on the synthesis route, chemical composition, doping cations, and sintering conditions [36,38]. The change in preparation method and temperature affects the microstructure, cation distribution among tetrahedral (A) and octahedral (B) sites and magnetic properties. Thus, to produce spinel ferrite nanoparticles with a desired stoichiometry, high compositional control, excellent chemical stability, high purity, crystallinity, saturation magnetization, and low coercivity are of interest and the selection of the synthesis route is critical [15,38].

Soft chemical routes such as sol-gel, solid-phase, hydrothermal, coprecipitation, sonochemical, spray pyrolysis, citrate gel, microwave refluxing, flash auto combustion, etc., are currently preferred for the synthesis of nano ferrites [17,37,39,40]. The solid-state reaction is a simple and attractive preparation method that allows large productivity and well-controllable grain sizes [41]. The conventional ceramic method is based on the solid-state reaction of metal oxides/carbonates at high temperatures (>1000 ◦C) and produces particles in the micrometer regime; however, agglomeration due to slow reaction kinetics is unavoidable [39]. Hence, wet chemical methods have been used intensively to avoid the limitations of conventional ceramic methods and to produce nanoscale materials with improved magnetic properties [35]. Wet chemical routes, such as hydrothermal, sol-gel method, and auto combustion, have been employed to obtain ferrite nanoparticles at low temperatures [34,39,41]. The main drawback of the wet methods is the formation of different oxide impurities, particularly Fe2O3 [17]. Highly homogenous Ni-Zn nanoferrite powders can be easily produced by a wet chemical route, using low-cost raw materials, and in air atmospheres. Its properties can be adjusted to fit the requirements of different applications by appropriately adjusting the Ni-Zn ratio and the sintering process [37,42]. The properties of Ni-Zn nanoferrites can be further improved by adding low amounts of other divalent ions such as Co2+ [37].

This paper investigates the formation, structure, morphology, and sonophotocatalytic activity of mixed Ni-Zn-Co ferrites embedded in SiO2, obtained by a modified sol-gel method and followed by thermal treatment. The reaction progress was investigated through thermal analysis (TG-DTA) and Fourier transform infrared spectroscopy (FT-IR), while the Ni-Zn-Co ferrites composition was investigated by inductively coupled plasma optical emission spectrometry (ICP-OES). The formation of the crystalline phase, crystallite size, and lattice constant were monitored by X-ray diffraction (XRD). The surface (specific surface area and porosity) was investigated using the Brunauer-Emmett-Teller (BET) method. The sonophotocatalytic properties of the samples were evaluated under visible light irradiation, assisted by sonication against RhB.

### **2. Results and Discussion**

### *2.1. Thermal Analysis*

The TG-DTA curves of all of the samples show three endothermic and two exothermic processes characterized by very close, overlapping peaks (Figure 1).

The endothermic effect at 61–70 ◦C, accompanied by 3–5% mass loss, is attributed to the loss of crystallization and constitution water. The endothermic effect at 136–144 ◦C, accompanied by 17–26% mass loss, is assigned to the formation of divalent metal precursors (Co, Ni, and Zn succinates), while the endothermic effect at 187–201 ◦C accompanied by 9–14% mass loss, is ascribed to the formation of a trivalent metal precursor (Fe succinate). The distinct behavior of Fe succinate, compared to the divalent metal (Co, Ni, Zn) succinates, can be attributed to the redox reaction between Fe(NO3)3 and 1,4BD, as well as to the stronger acidity of the aqua-cation [Fe(H2O)6]3 [43]. The overlapping exothermic effects, at 270–292 ◦C and 310–325 ◦C, accompanied by 19–25% mass loss, are attributed to the decomposition of metal succinates to metal oxides, which leads to the formation of ferrites.

The exothermic peak, characteristic of the decomposition of divalent metal succinates, decreases with the increasing Zn content and shifts toward higher temperatures, leading to the increase of the exothermic peak, attributed to the decomposition of the Fe succinates. The SiO2 matrix suffers various transformations during the thermal process, making the demarcation of the processes attributed to the formation and decomposition of succinate precursors difficult [44]. The total mass loss increases in the following order: Ni0.5Zn0.1Co0.4Fe2O4@SiO2 < Zn0.6Co0.4Fe2O4@SiO2 < Ni0.3Zn0.3Co0.4Fe2O4@SiO2 < Ni0.6 Co0.4Fe2O4@SiO2 < Ni0.2Zn0.4Co0.4Fe2O4@SiO2 < Ni0.4Zn0.2Co0.4Fe2O4@SiO2 < Ni0.1Zn0.5 Co0.4Fe2O4@SiO2.

In the case of the samples dried at 40 ◦C, the intense band at 1379–1389 cm−1 is associated with the N-O bonds stretching vibration in metal nitrates. This band disappears in the case of samples dried at 200 ◦C, indicating the decomposition of nitrates [44]. The bands at 2958–2963 cm−1 and 2872–2888 cm−1 are specific to the symmetric and asymmetric vibration of C-H bonds in 1,4-BD or succinate precursors. These bands also disappear in samples heattreated at 200 ◦C. The bands at 1578–1605 and 3200–3210 cm−1 are attributed to the stretching and bending vibrations of O-H in 1,4-BD and adsorbed molecular water [44,45]. In the samples dried at 200 ◦C, the band at 3200–3210 cm−1 is shifted towards higher wavenumbers (3421–3437 cm−1 ), indicating that the metal succinates are hygroscopic [44,45]. The presence of this absorption band could also be due to the O-H stretching vibration and Si-OH deformation vibration caused by the hydrolysis of –Si(OC2H5)4 [44,45]. For samples dried at 40 and 200 ◦C, the bands at 557–568 cm−1 are attributed to Ni-O and Zn-O vibrations, while the band at 433–452 cm−1 is attributed to the Fe-O vibration in the nitrates [44,45]. In samples at 40 ◦C, the band at 683–393 cm−1 is assigned to the Co-O bond vibration in the cobalt nitrate [44,45]. The formation of the SiO2 matrix in the samples dried at 40 and 200 ◦C is confirmed by the presence of specific bands of Si-O bond vibration (433–452 cm−1 ), cyclic Si-O-Si bonds vibration (557–568 cm−1 , more noticeable in samples dried at 40 ◦C), Si-O symmetric stretching and bending vibration (792–815 cm−1 ), Si-OH bonds (938–943 cm−1 , well delimited only in case of samples dried at 40 ◦C) vibration and Si-O-Si bonds stretching vibration (1045 cm−1 at 40 ◦C and 1058–1068 cm−1 at 200 ◦C). The band at 938–943 cm−1 is distinguishable for the samples dried at 40 ◦C, indicating the presence of unreacted TEOS, while the band at 1045 cm−1 suggests the formation of amorphous SiO2 [44,45]. Figure 3a shows the FT-IR spectra of NCs thermally treated at 1000 ◦C. The band at 618–626 cm−1 is attributed to the vibration of the M(II)-O (Co-O, Ni-O, Zn-O) bonds, while that at 485–490 cm−1 is attributed to the Fe-O bond [44–46].

*Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 4 of 16

**Figure 1.** TG-DTA diagrams of Zn0.6Co0.4Fe2O4@SiO2, Ni0.1Zn0.5Co0.4Fe2O4@SiO2, Ni0.2Zn0.4Co0.4Fe2O4@SiO2, Ni0.3Zn0.3Co0.4Fe2O4@SiO2, Ni0.4Zn0.2Co0.4Fe2O4@SiO2, Ni0.5Zn0.1Co0.4Fe2O4@SiO2 and Ni0.6Co0.4Fe2O4@SiO2 samples. **Figure 1.** TG-DTA diagrams of Zn0.6Co0.4Fe2O4@SiO2 , Ni0.1Zn0.5Co0.4Fe2O4@SiO2 , Ni0.2Zn0.4Co0.4 Fe2O4@SiO2 , Ni0.3Zn0.3Co0.4Fe2O4@SiO2 , Ni0.4Zn0.2Co0.4Fe2O4@SiO2 , Ni0.5Zn0.1Co0.4Fe2O4@SiO2 and Ni0.6Co0.4Fe2O4@SiO2 samples.

*2.2. FT-IR Analysis*

### *2.2. FT-IR Analysis*

As vibrational modes in FT-IR spectroscopy are determined by the bond type, the symmetry of the lattice sites and the elements in the crystal lattice, the monitoring of the ferrite formation process is possible [44]. The FT-IR spectra of the gels dried at 40 and 200 ◦C are presented in Figure 2. As vibrational modes in FT-IR spectroscopy are determined by the bond type, the symmetry of the lattice sites and the elements in the crystal lattice, the monitoring of the ferrite formation process is possible [44]. The FT-IR spectra of the gels dried at 40 and 200 °C are presented in Figure 2.

*Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 5 of 16

**Figure 2.** FT-IR spectra of Zn0.6Co0.4Fe2O4@SiO2 (1) Ni0.1Zn0.5Co0.4Fe2O4@SiO2 (2), Ni0.2Zn0.4Co0.4Fe2O4@SiO2 (3), Ni0.3Zn0.3Co0.4Fe2O4@SiO2 (4) Ni0.4Zn0.2Co0.4Fe2O4@SiO2 (5) Ni0.5Zn0.1Co0.4Fe2O4@SiO2 (6) and Ni0.6Co0.4Fe2O4@SiO2 (7) samples at 40 and 200 °C. **Figure 2.** FT-IR spectra of Zn0.6Co0.4Fe2O4@SiO2 (1) Ni0.1Zn0.5Co0.4Fe2O4@SiO2 (2), Ni0.2Zn0.4Co0.4 Fe2O4@SiO2 (3), Ni0.3Zn0.3Co0.4Fe2O4@SiO2 (4) Ni0.4Zn0.2Co0.4Fe2O4@SiO2 (5) Ni0.5Zn0.1Co0.4Fe2O4 @SiO2 (6) and Ni0.6Co0.4Fe2O4@SiO2 (7) samples at 40 and 200 ◦C. [47]. The Fe2+ ions may result from the hopping process, namely M2+ + Fe3+ ↔ M3+ + Fe2+ (M = Co, Ni, Zn). The Co3+ ion may migrate to tetrahedral (A) sites, while the Fe2+ ions remain in their sites [31].

ence of Fe2+ ions in the sublattices, can lead to band splitting, small bands and/or shoulders

samples dried at 40 °C), Si-O symmetric stretching and bending vibration (792–815 cm−1), Si-OH bonds (938–943 cm−1, well delimited only in case of samples dried at 40 °C) vibration and Si-O-Si bonds stretching vibration (1045 cm−1 at 40 °C and 1058–1068 cm−1 at 200 °C). The band at 938–943 cm−1 is distinguishable for the samples dried at 40 °C, indicating **Figure 3.** FT-IR spectra (**a**) and XRD patterns (**b**) of Zn0.6Co0.4Fe2O4@SiO2 (1), Ni0.1Zn0.5Co0.4Fe2O4@SiO2 (2), Ni0.2Zn0.4Co0.4Fe2O4@SiO2 (3), Ni0.3Zn0.3Co0.4Fe2O4@SiO2 (4), Ni0.4Zn0.2Co0.4Fe2O4@SiO2 (5), Ni0.5Zn0.1Co0.4Fe2O4@SiO2 (6) and Ni0.6Co0.4Fe2O4@SiO2 (7) samples at 1000 °C. **Figure 3.** FT-IR spectra (**a**) and XRD patterns (**b**) of Zn0.6Co0.4Fe2O4@SiO2 (1), Ni0.1Zn0.5Co0.4Fe2O4 @SiO2 (2), Ni0.2Zn0.4Co0.4Fe2O4@SiO2 (3), Ni0.3Zn0.3Co0.4Fe2O4@SiO2 (4), Ni0.4Zn0.2Co0.4Fe2O4@SiO2 (5), Ni0.5Zn0.1Co0.4Fe2O4@SiO2 (6) and Ni0.6Co0.4Fe2O4@SiO2 (7) samples at 1000 ◦C.

the presence of unreacted TEOS, while the band at 1045 cm−1 suggests the formation of amorphous SiO2 [44,45]. Figure 3a shows the FT-IR spectra of NCs thermally treated at 1000 °C. The band at 618–626 cm−1 is attributed to the vibration of the M(II)-O (Co-O, Ni-O, Zn-O) bonds, while that at 485–490 cm−1 is attributed to the Fe-O bond [44–46]. In comparison to the samples dried at 40 °C and 200 °C, in the samples thermally treated at 1000 °C, the wavenumbers specific to Co-O bond vibration decrease, and the The bands at 790–795 cm−1 are characteristic for the vibration of the Si-O bond in SiO2 matrix, while those at 1090–1095 cm−1 and 485–490 cm−1 are characteristic to the stretching and bending vibration of Si-O-Si chains and show a low degree of polycondensation of the SiO2 network [44,45]. The difference in band position could be attributed to the difference in M-O distance in the tetrahedral and octahedral sites [39]. In comparison to the samples dried at 40 ◦C and 200 ◦C, in the samples thermally treated at 1000 ◦C, the wavenumbers specific to Co-O bond vibration decrease, and the wavenumbers specific to M(II)-O increase. The Jan-Teller effect, determined by the presence of Fe2+ ions in the sublattices, can lead to band splitting, small bands and/or shoulders [47]. The Fe2+ ions may result from the hopping process, namely M2+ + Fe3+ ↔ M3+ + Fe2+ (M = Co,

of the samples, while the positions and intensities of the diffraction lines support the spinel structure [47]. The peaks with 2θ values of 30.07, 35.42, 37.07, 43.05, 53.41, 56.94, and 62.52 correspond to the (220), (311), (222), (400), (422), (511), and (440) planes [41,48]. In all of the samples, the local crystal structure is cubic spinel-type, belonging to the *Fd-3m* space group [21,40,41]. Additionally, two crystalline phases of the SiO2 matrix (SiO2-cristobalite, JCPDS card 39-1425 [48] and SiO2-tridymite, JCPDS card 042-1401 [48]) are identified. In the case of Zn0.6Co0.4Fe2O4@SiO2, the well-crystallized spinel is composed by CoFe2O4 (JCPDS card 22-1086 [48]) and ZnFe2O4 (JCPDS card 70-6491 [48]. In the case of Ni0.6Co0.4Fe2O4@SiO2, the well-crystallized spinel is composed of CoFe2O4 and NiFe2O4 (JCPDS card 74-2081 [48]), while in the other samples (Ni0.1Zn0.5Co0.4Fe2O4@SiO2, Ni0.2Zn0.4Co0.4Fe2O4@SiO2, Ni0.3Zn0.3Co0.4Fe2O4@SiO2, Ni0.4Zn0.2Co0.4Fe2O4@SiO2, and Ni0.5Zn0.1Co0.4Fe2O4@SiO2), the crystalline phase contains NiFe2O4, ZnFe2O4 and CoFe2O4. In ferrites with a high Zn content, the presence of hematite (Fe2O3, JCPDS card 89-0599 [48]) is also remarked. The presence of Fe2O3 indicates the decomposition of Fe(NO3)3 into Fe2O3, leading to the formation of spinel ferrite [39]. The excess of metal oxides in insoluble secondary phases (Fe2O3) can contribute to the densification by generating high pore volumes and demagnetizing fields. During synthesis, the homogeneity of the metal oxide particles may result in higher defects and pore volumes in the final products [36,42].

*2.3. XRD Analysis*

Ni, Zn). The Co3+ ion may migrate to tetrahedral (A) sites, while the Fe2+ ions remain in their sites [31].

The bands at 790–795 cm−1 are characteristic for the vibration of the Si-O bond in SiO2 matrix, while those at 1090–1095 cm−1 and 485–490 cm−1 are characteristic to the stretching and bending vibration of Si-O-Si chains and show a low degree of polycondensation of the SiO2 network [44,45]. The difference in band position could be attributed to the difference in M-O distance in the tetrahedral and octahedral sites [39].

### *2.3. XRD Analysis*

The XRD patterns (Figure 3b) confirm the presence of well-crystallized ferrites in all of the samples, while the positions and intensities of the diffraction lines support the spinel structure [47]. The peaks with 2θ values of 30.07, 35.42, 37.07, 43.05, 53.41, 56.94, and 62.52 correspond to the (220), (311), (222), (400), (422), (511), and (440) planes [41,48]. In all of the samples, the local crystal structure is cubic spinel-type, belonging to the *Fd-3m* space group [21,40,41]. Additionally, two crystalline phases of the SiO2 matrix (SiO2 cristobalite, JCPDS card 39-1425 [48] and SiO2-tridymite, JCPDS card 042-1401 [48]) are identified. In the case of Zn0.6Co0.4Fe2O4@SiO2, the well-crystallized spinel is composed by CoFe2O4 (JCPDS card 22-1086 [48]) and ZnFe2O4 (JCPDS card 70-6491 [48]. In the case of Ni0.6Co0.4Fe2O4@SiO2, the well-crystallized spinel is composed of CoFe2O4 and NiFe2O4 (JCPDS card 74-2081 [48]), while in the other samples (Ni0.1Zn0.5Co0.4Fe2O4@SiO2, Ni0.2Zn0.4Co0.4Fe2O4@SiO2, Ni0.3Zn0.3Co0.4Fe2O4@SiO2, Ni0.4Zn0.2Co0.4Fe2O4@SiO2, and Ni0.5Zn0.1Co0.4Fe2O4@SiO2), the crystalline phase contains NiFe2O4, ZnFe2O4 and CoFe2O4. In ferrites with a high Zn content, the presence of hematite (Fe2O3, JCPDS card 89-0599 [48]) is also remarked. The presence of Fe2O3 indicates the decomposition of Fe(NO3)3 into Fe2O3, leading to the formation of spinel ferrite [39]. The excess of metal oxides in insoluble secondary phases (Fe2O3) can contribute to the densification by generating high pore volumes and demagnetizing fields. During synthesis, the homogeneity of the metal oxide particles may result in higher defects and pore volumes in the final products [36,42].

The average crystallites size (DXRD) was calculated using the Scherrer equation (Equation (1)).

$$\text{D}\_{\text{XRD}} = \frac{0.9 \cdot \lambda}{\text{ $\beta$ } \cdot \cos \text{ }\text{\textdegree}} \tag{1}$$

where λ is the wavelength of the CuKα radiation (1.5406 Å), β is the full width at halfmaximum intensity (FWHM), hkl are the Miller indices and θ is the Bragg angle (◦ ) [43,44,49,50].

The lattice constant (a), was calculated from Bragg's law with Nelson-Riley Equation (2) [44,50].

$$\mathbf{a} = \frac{\lambda \sqrt{\mathbf{h}^2 + \mathbf{k}^2 + \mathbf{l}^2}}{2 \cdot \sin \theta} \tag{2}$$

where λ is the wavelength of CuKα radiation (1.5406 Å), hkl are the Miller indices, θ is the Bragg angle (◦ ) [44,50].

The unit cell volume (V) and the hopping length (L) of magnetic ions for tetrahedral (A) and octahedral (B) sites were calculated using Equations (3)–(5) [44,50].

$$\mathbf{V} = \mathbf{a}^3\tag{3}$$

$$\mathbf{L\_A} = 0.25 \mathbf{\bar{a}} \sqrt{3} \tag{4}$$

$$\mathbf{L\_B = 0.25 \cdot a\sqrt{2}}\tag{5}$$

where a is the lattice constant (Å).

The average crystallite size lies in the nanocrystalline range and increases with the increasing Ni content, while the lattice parameter (a) decreases with the increasing Ni content (Table 1).

**Table 1.** Parameters obtained from TEM, XRD and ICP-OES analysis for samples thermally treated at 1000 ◦C.

The change in the lattice constant (a) generates internal stress and suppresses additional grain growth during thermal treatment [44,50,51]. The tetrahedral (A) sites have smaller radii (0.52 Å) than the octahedral (B) site (0.81 Å) [9]. The ionic radii of Ni2+ (0.69 Å), Zn2+ (0.74 Å) and Co2+ (0.75 Å) ions are larger than the ionic radius of Fe3+ (0.64 Å) [3,38,52]. The amorphous to crystalline phase transformation and the relative content of crystalline phases, after thermal treatment at 1000 ◦C, were assessed using the relative degree of crystallinity (DC), calculated as the ratio between the area of diffraction peaks and the total area of diffraction peaks and halos. The DC increases with the increase of the crystallite size and Ni content. The Reference Intensity Ratio (RIR) method was used for the quantitative phase analysis of NCs thermally treated at 1000 ◦C.

### *2.4. Elemental Analysis*

The Ni/Zn/Co molar ratios, determined by microwave digestion and combined with inductively coupled plasma optical emission spectrometry, are in good agreement with the theoretical values (Table 1).

### *2.5. BET Analysis*

Due to the low amount of adsorbed/desorbed nitrogen, the determination of porosity and specific surface area (SSA) for the samples thermally treated at 1000 ◦C was not possible. The SSA below the method detection limit (0.5 m2/g) suggests that all ferrites have a nonporous structure, probably due to particle agglomeration that limits nitrogen absorption.

### *2.6. TEM Analysis*

The TEM images of the mixed Ni-Zn-Co ferrites following thermal treatment at 1000 ◦C (Figure 4) reveal spherical, small (high Zn content), or large (high Ni content) nanoparticles that form large spongy aggregates.

The formation of agglomerates with irregular morphology composed of high Zn content ferrite particles and the homogenous dispersion of high Ni content ferrite particles is also remarked. The small grains have a high surface area to volume ratio and allow faster oxygen diffusion than the larger grains, leading to an increase in the stoichiometry of the sample [35]. Although the small particles are closely arranged together, a clear boundary between adjacent particles is still observed. The average particle size is 34–36 nm, the difference being attributed to the grain boundary motion that exerts a dragging force, while the pores delay the force over the grain [41]. Moreover, the driving force increases the grain boundaries over the pores, resulting in lower pore volume and higher density [41]. The average crystallite size estimated by XRD is close to the particle size determined by

TEM, the slight differences being attributed to the amorphous SiO2 matrix and large-size nanoparticles [43,44,50]. *Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 9 of 16

**Figure 4.** TEM images of Zn0.6Co0.4Fe2O4@SiO2 (1), Ni0.1Zn0.5Co0.4Fe2O4@SiO2 (2), Ni0.2Zn0.4Co0.4Fe2O4@SiO2 (3), Ni0.3Zn0.3Co0.4Fe2O4@SiO2 (4), Ni0.4Zn0.2Co0.4Fe2O4@SiO2 (5), Ni0.5Zn0.1Co0.4Fe2O4@SiO2 (6) and Ni0.6Co0.4Fe2O4@SiO2 (7) samples at 1000 °C. **Figure 4.** TEM images of Zn0.6Co0.4Fe2O4@SiO2 (1), Ni0.1Zn0.5Co0.4Fe2O4@SiO2 (2), Ni0.2Zn0.4Co0.4 Fe2O4@SiO2 (3), Ni0.3Zn0.3Co0.4Fe2O4@SiO2 (4), Ni0.4Zn0.2Co0.4Fe2O4@SiO2 (5), Ni0.5Zn0.1Co0.4Fe2O4 @SiO2 (6) and Ni0.6Co0.4Fe2O4@SiO2 (7) samples at 1000 ◦C.

#### *2.7. UV-VIS Analysis 2.7. UV-VIS Analysis*

The optical response of the samples was evaluated by UV-Vis spectroscopy. The UV-Vis absorption (Figure 5a) shows that all the samples have a broad response in the visible range. Based on the absorption spectra and using the Tauc's relation [50], the band gap energy of the samples was evaluated (Figure 5b). The optical response of the samples was evaluated by UV-Vis spectroscopy. The UV-Vis absorption (Figure 5a) shows that all the samples have a broad response in the visible range. Based on the absorption spectra and using the Tauc's relation [50], the band gap energy of the samples was evaluated (Figure 5b).

**Figure 5.** (**a**) UV-Vis absorption spectra and (**b**) Tauc's plot of the of Zn0.6Co0.4Fe2O4@SiO2 (1), Ni0.1Zn0.5Co0.4Fe2O4@SiO2 (2), Ni0.2Zn0.4Co0.4Fe2O4@SiO2 (3), Ni0.3Zn0.3Co0.4Fe2O4@SiO2 (4), Ni0.4Zn0.2Co0.4Fe2O4@SiO2 (5), Ni0.5Zn0.1Co0.4Fe2O4@SiO2 (6) and Ni0.6Co0.4Fe2O4@SiO2 (7) samples at 1000 °C. **Figure 5.** (**a**) UV-Vis absorption spectra and (**b**) Tauc's plot of the of Zn0.6Co0.4Fe2O4@SiO2 (1), Ni0.1 Zn0.5Co0.4Fe2O4@SiO2 (2), Ni0.2Zn0.4Co0.4Fe2O4@SiO2 (3), Ni0.3Zn0.3Co0.4Fe2O4@SiO2 (4), Ni0.4Zn0.2 Co0.4Fe2O4@SiO2 (5), Ni0.5Zn0.1Co0.4Fe2O4@SiO2 (6) and Ni0.6Co0.4Fe2O4@SiO2 (7) samples at 1000 ◦C.

The band gap energy values are in the range 1.21–1.49 eV, are lower than that of NiFe2O4 (2.2 eV), ZnFe2O4 (1.91 eV) and CoFe2O4 (2.31 eV), and are comparable to those reported for CoFe2O4 xerogel calcined at 500 °C (1.5 eV) [33,50]. The band gap of our samples was also lower than those of NixCo1-xFe2O4 (1.37–1.78 eV) obtained by coprecipitation [52]. The optical band gap of the samples is due to the d-d transition. The crystal field splits the d level in the eg and t2g levels and the band gap energy depends on octahedral (B) and tetrahedral (A) sites. The band gap energy, in the case of the octahedral site, is higher than that of the tetrahedral (A) site [53]. The variation of the band gap energy, by replacing Zn2+ ions with Ni2+, can be explained by the redistribution of Ni2+ ions between the octahedral (B) and tetrahedral (A) sites. In the XRD data, the peaks corresponding to the plane (220) and (422) are sensitive to the tetrahedral (A) site, whereas the peak corresponding to the (222) plane is sensitive to the octahedral (B) site [54,55]. The values of the I(220)/I(222) ratio, for the samples annealed at 1000 °C, are 4.29 for Zn0.6Co0.4Fe2O4@SiO2, 3.79 for Ni0.1Zn0.5Co0.4Fe2O4@SiO2, 4.26 for Ni0.2Zn0.4Co0.4Fe2O4@SiO2, 4.10 for Ni0.3Zn0.3Co0.4Fe2O4@SiO2, 3.52 for Ni0.4Zn0.2Co0.4Fe2O4@SiO2, 3.22 for Ni0.5Zn0.1Co0.4Fe2O4@SiO2 and 3.77 for Ni0.6Co0.4Fe2O4@SiO2, which indicates that the population at the tetrahedral (A) site tends to decrease with the increase of the Ni2+ ions. These findings are correlated with the optical band gap values, which increases with the increase of Ni2+ content. The low band gap energy makes our samples suitable for the absorption of visible light. The activation energy of the Co0.6Zn0.4NixFe2-xO4 ferrite nanoparticles obtained by sol-gel route decreased with the increasing Ni content, from 2.71 eV (x = 0.2) to 1.46 eV (x = 1) [33]. The band gap energy values are in the range 1.21–1.49 eV, are lower than that of NiFe2O4 (2.2 eV), ZnFe2O4 (1.91 eV) and CoFe2O4 (2.31 eV), and are comparable to those reported for CoFe2O4 xerogel calcined at 500 ◦C (1.5 eV) [33,50]. The band gap of our samples was also lower than those of NixCo1-xFe2O4 (1.37–1.78 eV) obtained by coprecipitation [52]. The optical band gap of the samples is due to the d-d transition. The crystal field splits the d level in the eg and t2g levels and the band gap energy depends on octahedral (B) and tetrahedral (A) sites. The band gap energy, in the case of the octahedral site, is higher than that of the tetrahedral (A) site [53]. The variation of the band gap energy, by replacing Zn2+ ions with Ni2+, can be explained by the redistribution of Ni2+ ions between the octahedral (B) and tetrahedral (A) sites. In the XRD data, the peaks corresponding to the plane (220) and (422) are sensitive to the tetrahedral (A) site, whereas the peak corresponding to the (222) plane is sensitive to the octahedral (B) site [54,55]. The values of the I(220)/I(222) ratio, for the samples annealed at 1000 ◦C, are 4.29 for Zn0.6Co0.4Fe2O4@SiO2, 3.79 for Ni0.1Zn0.5Co0.4Fe2O4@SiO2, 4.26 for Ni0.2Zn0.4Co0.4Fe2O4@SiO2, 4.10 for Ni0.3Zn0.3Co0.4Fe2O4@SiO2, 3.52 for Ni0.4Zn0.2Co0.4 Fe2O4@SiO2, 3.22 for Ni0.5Zn0.1Co0.4Fe2O4@SiO2 and 3.77 for Ni0.6Co0.4Fe2O4@SiO2, which indicates that the population at the tetrahedral (A) site tends to decrease with the increase of the Ni2+ ions. These findings are correlated with the optical band gap values, which increases with the increase of Ni2+ content. The low band gap energy makes our samples suitable for the absorption of visible light. The activation energy of the Co0.6Zn0.4NixFe2-xO4 ferrite nanoparticles obtained by sol-gel route decreased with the increasing Ni content, from 2.71 eV (x = 0.2) to 1.46 eV (x = 1) [33].

### *2.8. Sonophotocatalytic Activity* The sonophotocatalytic activity of the samples was evaluated using an RhB synthetic

*2.8. Sonophotocatalytic Activity*

The sonophotocatalytic activity of the samples was evaluated using an RhB synthetic solution under visible irradiation. Before visible irradiation, the samples were kept in the dark for 1 h to reach the adsorption-desorption equilibrium. The adsorption capacity of the sample varied between 7–28%. The adsorption properties depend on the surface sites and specific surface area. In our case, the samples had almost identical particle sizes; thus, those surface sites were responsible for the adsorption properties. The removal rate (Figure 6) was evaluated after 7 h of visible irradiation and varied between 16 and 75%. Similar removal efficiencies (83.9%) for methylene blue were obtained using Ni-Cu-Zn ferrite@SiO2@TiO2 by Chen et al. [52,55]. solution under visible irradiation. Before visible irradiation, the samples were kept in the dark for 1 h to reach the adsorption-desorption equilibrium. The adsorption capacity of the sample varied between 7–28%. The adsorption properties depend on the surface sites and specific surface area. In our case, the samples had almost identical particle sizes; thus, those surface sites were responsible for the adsorption properties. The removal rate (Figure 6) was evaluated after 7 h of visible irradiation and varied between 16 and 75%. Similar removal efficiencies (83.9%) for methylene blue were obtained using Ni-Cu-Zn ferrite@SiO2@TiO2 by Chen et al. [52,55].

*Int. J. Mol. Sci.* **2022**, *23*, x FOR PEER REVIEW 11 of 16

**Figure 6.** Removal rate of Zn0.6Co0.4Fe2O4@SiO2 (1), Ni0.1Zn0.5Co0.4Fe2O4@SiO2 (2), Ni0.2Zn0.4Co0.4Fe2O4@SiO2 (3), Ni0.3Zn0.3Co0.4Fe2O4@SiO2 (4), Ni0.4Zn0.2Co0.4Fe2O4@SiO2 (5), Ni0.5Zn0.1Co0.4Fe2O4@SiO2 (6) and Ni0.6Co0.4Fe2O4@SiO2 (7) samples. **Figure 6.** Removal rate of Zn0.6Co0.4Fe2O4@SiO2 (1), Ni0.1Zn0.5Co0.4Fe2O4@SiO2 (2), Ni0.2Zn0.4Co0.4 Fe2O4@SiO2 (3), Ni0.3Zn0.3Co0.4Fe2O4@SiO2 (4), Ni0.4Zn0.2Co0.4Fe2O4@SiO2 (5), Ni0.5Zn0.1Co0.4Fe2O4 @SiO2 (6) and Ni0.6Co0.4Fe2O4@SiO2 (7) samples.

The sample with similar Zn2+ and Ni2+ ions content (Ni0.3Zn0.3Co0.4Fe2O4@SiO2) shows the highest removal capacity, indicating that the equilibrium between Ni-ferrite and Znferrite assures the best photocatalytic performance. In addition, based on the quantitative crystalline phase analysis, this sample contains a lower amount of α-Fe2O3 (2%) compared with samples Zn0.6Co0.4Fe2O4@SiO2 (1), Ni0.1Zn0.5Co0.4Fe2O4@SiO2 (2), Ni0.2Zn0.4Co0.4Fe2O4@SiO2 (3), which means that in the case of this sample, α-Fe2O3 does not The sample with similar Zn2+ and Ni2+ ions content (Ni0.3Zn0.3Co0.4Fe2O4@SiO2) shows the highest removal capacity, indicating that the equilibrium between Ni-ferrite and Zn-ferrite assures the best photocatalytic performance. In addition, based on the quantitative crystalline phase analysis, this sample contains a lower amount of α-Fe2O3 (2%) compared with samples Zn0.6Co0.4Fe2O4@SiO2 (1), Ni0.1Zn0.5Co0.4Fe2O4@SiO2 (2), Ni0.2Zn0.4Co0.4Fe2O4@SiO2 (3), which means that in the case of this sample, α-Fe2O3 does not significantly influence photocatalytic activity.

significantly influence photocatalytic activity. For this sample, the photodegradation kinetic was analyzed with respect to the ab-For this sample, the photodegradation kinetic was analyzed with respect to the absorbance of RhB using the pseudo-first order kinetic model (Equation (6)).

$$-\ln\frac{\mathbf{A\_t}}{\mathbf{A\_0^\*}} = \mathbf{k\_i}\cdot\mathbf{t}\tag{6}$$

− ∗ = ∙ (6) where At is the absorbance of RhB at time t, A0 ∗ is the absorbance of RhB at time t0 and ki where At is the absorbance of RhB at time t, A ∗ 0 is the absorbance of RhB at time t0 and ki is the apparent kinetic constant (min−1 ). A linear relationship with the irradiation time (Figure 7), with a rate constant of 2.79 × 10−3 min was obtained.

(Figure 7), with a rate constant of 2.79 × 10−3 min was obtained.

is the apparent kinetic constant (min−1). A linear relationship with the irradiation time

**Figure 7.** Photodegradation kinetics of RhB in the presence of Ni0.3Zn0.3Co0.4Fe2O4@SiO2. **Figure 7.** Photodegradation kinetics of RhB in the presence of Ni0.3Zn0.3Co0.4Fe2O4@SiO2 .

### **3. Materials and Methods 3. Materials and Methods**

### *3.1. Synthesis*

*3.1. Synthesis*  The Ni-Zn-Co ferrite embedded in SiO2 matrix (60% wt. ferrite, 40% wt. SiO2) were synthesized by sol-gel method using Ni(NO3)2∙6H2O, Zn(NO3)2∙6H2O, Co(NO3)2∙6H2O, Fe(NO3)3∙9H2O, 1,4-butanediol (1,4-BD), tetraethyl orthosilicate (TEOS), ethanol and HNO3 65%, using a Ni:Zn:Co:Fe molar ratio of 0:6:4:20 (Zn0.6Co0.4Fe2O4@SiO2), 1:5:4:20 (Ni0.1Zn0.5Co0.4Fe2O4@SiO2), 1:2:2:10 (Ni0.2Zn0.4Co0.4Fe2O4@SiO2), 3:3:4:20 (Ni0.3Zn0.3Co0.4Fe2O4@SiO2), 2:1:2:10 (Ni0.4Zn0.2Co0.4Fe2O4@SiO2), 5:1:4:20 (Ni0.5Zn0.1Co0.4Fe2O4@SiO2), 6:0:4:20 (Ni0.6Co0.4Fe2O4@SiO2) and a nitrate:1,4-BD:TEOS molar ratio of 1:1:0.67. All chemicals were of analytical grade (Merck) and used without fur-The Ni-Zn-Co ferrite embedded in SiO2 matrix (60% wt. ferrite, 40% wt. SiO2) were synthesized by sol-gel method using Ni(NO3)2·6H2O, Zn(NO3)2·6H2O, Co(NO3)2·6H2O, Fe(NO3)3·9H2O, 1,4-butanediol (1,4-BD), tetraethyl orthosilicate (TEOS), ethanol and HNO3 65%, using a Ni:Zn:Co:Fe molar ratio of 0:6:4:20 (Zn0.6Co0.4Fe2O4@SiO2), 1:5:4:20 (Ni0.1Zn0.5Co0.4Fe2O4@SiO2), 1:2:2:10 (Ni0.2Zn0.4Co0.4Fe2O4@SiO2), 3:3:4:20 (Ni0.3Zn0.3Co0.4 Fe2O4@SiO2), 2:1:2:10 (Ni0.4Zn0.2Co0.4Fe2O4@SiO2), 5:1:4:20 (Ni0.5Zn0.1Co0.4Fe2O4@SiO2), 6:0:4:20 (Ni0.6Co0.4Fe2O4@SiO2) and a nitrate:1,4-BD:TEOS molar ratio of 1:1:0.67. All chemicals were of analytical grade (Merck) and used without further purification. The resulting sols were kept at room temperature until gelation (5 weeks), ground, dried at 40 ◦C (5 h), and then subjected to thermal treatment 1000 ◦C.

### ther purification. The resulting sols were kept at room temperature until gelation (5 *3.2. Characterization*

*3.3. Sonophotocatalysis*

weeks), ground, dried at 40 °C (5 h), and then subjected to thermal treatment 1000 °C. *3.2. Characterization*  The formation and decomposition of the carboxylate-type precursor were investigated by thermogravimetry (TG) and differential thermal analysis (DTA) using the SDT Q600 thermogravimeter, in air, up to 1000 °C, at 10 °C·min−1 heating rate, using alumina standards. The FT-IR spectra were recorded on KBr pellets containing 1% samples using a Perkin Elmer Spectrum BX II spectrometer, while the XRD patterns were recorded at room temperature using a Bruker D8 Advance diffractometer with CuKα1 radiation (λ = 1.54060 Å ). The Ni/Zn/Co molar ratios were confirmed using Perkin Elmer ICP-OES Optima 5300 DV (Norwalk, CT, USA) after closed-vessel microwave-assisted aqua regia digestion using a Speedwave Xpert system (Berghof, Germany). The specific surface area The formation and decomposition of the carboxylate-type precursor were investigated by thermogravimetry (TG) and differential thermal analysis (DTA) using the SDT Q600 thermogravimeter, in air, up to 1000 ◦C, at 10 ◦C·min−1 heating rate, using alumina standards. The FT-IR spectra were recorded on KBr pellets containing 1% samples using a Perkin Elmer Spectrum BX II spectrometer, while the XRD patterns were recorded at room temperature using a Bruker D8 Advance diffractometer with CuKα1 radiation (λ = 1.54060 Å). The Ni/Zn/Co molar ratios were confirmed using Perkin Elmer ICP-OES Optima 5300 DV (Norwalk, CT, USA) after closed-vessel microwave-assisted aqua regia digestion using a Speedwave Xpert system (Berghof, Germany). The specific surface area (SSA) was obtained using the BET model from N2 adsorption-desorption isotherms recorded at −196 ◦C by a Sorptomatic 1990 (Thermo Fisher Scientific) instrument. The UV–VIS absorption spectra were recorded using a JASCO V570 UV–VIS-NIR spectrophotometer, equipped with a JASCO ARN-475 absolute reflectivity measurement accessory.

(SSA) was obtained using the BET model from N2 adsorption-desorption isotherms recorded at −196 °C by a Sorptomatic 1990 (Thermo Fisher Scientific) instrument. The UV– VIS absorption spectra were recorded using a JASCO V570 UV–VIS-NIR spectrophotom-

The sonophotocatalytic activity of the samples was evaluated against RhB solution under visible light irradiation in a Laboratory-Visible-Reactor system using a 400 W

### *3.3. Sonophotocatalysis*

The sonophotocatalytic activity of the samples was evaluated against RhB solution under visible light irradiation in a Laboratory-Visible-Reactor system using a 400 W halogen lamp (Osram) and an ultrasonic bath. The catalyst (10 mg) was suspended in an aqueous solution of RhB (1.0 × 10−5 mol L−1 , 20 mL), and the mixture was stirred in the dark to achieve the adsorption equilibrium on the catalyst surface. Each degradation experiment was conducted for 240 min. Samples from a given mixture (3.5 mL) were withdrawn for analysis every 60 min. After separating the catalyst from the suspensions with a permanent magnet, the solution was analyzed using a UV–Vis spectrophotometer by recording the maximum absorbance of RhB at 554 nm. The sonophotocatalytic activity was estimated based on the calculated degradation rate. Before the sonophotodegradation experiments, the RhB adsorption on the surface of the nanoparticles was analyzed. The adsorption was verified in the dark by mixing the photocatalyst into the RhB solution for 60 min until the adsorption-desorption equilibrium was reached.

### **4. Conclusions**

Ni-Zn-Co ferrites, with different Ni:Zn:Co ratios (Zn0.6Co0.4Fe2O4, Ni0.1Zn0.5Co0.4Fe2O4, Ni0.2Zn0.4Co0.4Fe2O, Ni0.3Zn0.3Co0.4Fe2O4, Ni0.4Zn0.2Co0.4Fe2O4, Ni0.5Zn0.1Co0.4Fe2O4, and Zn0.6Co0.4Fe2O4@SiO2), embedded in SiO2 were obtained by sol-gel method, followed by thermal treatment at 1000 ◦C. The thermal analysis revealed the formation and decomposition of metal succinate precursors in two stages, with distinct formation and decomposition of divalent (Ni2+, Zn2+, Co2+) and trivalent (Fe3+) succinates. The shapes of the DTA curves are similar, with the exception of the divalent metal's succinate decomposition stage, where for samples with high Ni content, the intensity of the exothermic peak decreases and shifts to higher temperatures. The total mass losses vary between 54.4–58.5%. The precursor formation and their decomposition into ferrites, as well as the formation of the silica matrix, are also confirmed by the FT-IR spectra. The XRD revealed the presence of well-crystallized ferrites along two crystalline phases of the SiO2 matrix (cristobalite and tridymite). In samples with high Zn content, traces of hematite were also identified. The agglomeration of particles and the particle size of Ni-Zn-Co ferrites increase with the increasing Ni content, from 34 nm to 40 nm. All of the samples show a good optical response in the visible range, the best sonophotocatalytic performance being found for the Ni0.3Zn0.3Co0.4Fe2O4@SiO2 sample, most likely due to the equilibrium between Ni-ferrite and Zn-ferrite.

**Author Contributions:** Conceptualization, T.D.; methodology, T.D., O.C. and E.A.L.; formal analysis, T.D., O.C., F.G., D.T. and E.A.L.; investigation, T.D., O.C., F.G., D.T. and E.A.L.; resources, T.D., O.C., F.G., D.T. and E.A.L.; data curation, T.D.; writing—original draft preparation, T.D., O.C. and E.A.L.; writing—review and editing, T.D., O.C. and E.A.L.; visualization, T.D.; supervision, T.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** The APC was funded by the Technical University of Cluj-Napoca Grant Support GC1/2021.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This study was supported by the Ministry of Research, Innovation and Digitization through Program 1—Development of the national research and development system, Subprogram 1.2—Institutional performance -Projects that finance the RDI excellence, Contracts no. 18PFE/30.12.2021 (O.C. and E.A.L.) and 37PFE/30.12.2021 (D.T.). The funder had no role in the design of the study; in the collection, analysis and interpretation of data; in the writing of the manuscript; and in the decision to submit the article for publication.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

## *Article* **Selective Recovery of Cadmium, Cobalt, and Nickel from Spent Ni–Cd Batteries Using Adogen® 464 and Mesoporous Silica Derivatives**

**Ahmed R. Weshahy 1 , Ahmed K. Sakr 2,3,\* , Ayman A. Gouda 1 , Bahig M. Atia 3 , H. H. Somaily 4,5 , Mohamed Y. Hanfi 3,6 , M. I. Sayyed 7 , Ragaa El Sheikh 1 , Enass M. El-Sheikh 3 , Hend A. Radwan 3 , Mohamed F. Cheira 3 and Mohamed A. Gado 3,\***

**Abstract:** Spent Ni–Cd batteries are now considered an important source for many valuable metals. The recovery of cadmium, cobalt, and nickel from spent Ni–Cd Batteries has been performed in this study. The optimum leaching process was achieved using 20% H2SO4 , solid/liquid (S/L) 1/5 at 80 ◦C for 6 h. The leaching efficiency of Fe, Cd, and Co was nearly 100%, whereas the leaching efficiency of Ni was 95%. The recovery of the concerned elements was attained using successive different separation techniques. Cd(II) ions were extracted by a solvent, namely, Adogen® 464, and precipitated as CdS with 0.5% Na2S solution at pH of 1.25 and room temperature. The extraction process corresponded to pseudo-2nd-order. The prepared PTU-MS silica was applied for adsorption of Co(II) ions from aqueous solution, while the desorption process was performed using 0.3 M H2SO4 . Cobalt was precipitated at pH 9.0 as Co(OH)2 using NH4OH. The kinetic and thermodynamic parameters were also investigated. Nickel was directly precipitated at pH 8.25 using a 10% NaOH solution at ambient temperature. FTIR, SEM, and EDX confirm the structure of the products.

**Keywords:** spent Ni–Cd batteries; separation; cadmium; nickel; cobalt

**1. Introduction**

Nowadays, cost-effective and low-cost techniques for collecting important metals from secondary sources have been developed to help meet economic and environmental constraints. In this regard, cadmium is prominent among non-ferrous metals. The principal application of cadmium has moved during the past 40 years from coatings to portable Ni–Cd batteries [1]. Rechargeable batteries (such as Ni–Cd batteries) have the highest quality, when utilized at low temperatures, which makes them a key user priority [2,3]. Thanks to their multiple attributes, i.e., long life spans, easy-of-rechargeing, no maintenance requirements, good shelf life, and reliability. The used Ni–Cd batteries wind up in large amounts in landfill, and cadmium waste pollutes the environment [4]. Spent Ni–Cd batteries are composed mostly of Ni–Cd electrode components corresponding to around 43–49% of battery weight. The remainder comprises the outer case steel parts/fundamental plates (40–49%) and feed- and Ni grids (9%), electrolyte, and 2% plastic constituents [5,6].

**Citation:** Weshahy, A.R.; Sakr, A.K.; Gouda, A.A.; Atia, B.M.; Somaily, H.H.; Hanfi, M.Y.; Sayyed, M.I.; El Sheikh, R.; El-Sheikh, E.M.; Radwan, H.A.; et al. Selective Recovery of Cadmium, Cobalt, and Nickel from Spent Ni–Cd Batteries Using Adogen® 464 and Mesoporous Silica Derivatives. *Int. J. Mol. Sci.* **2022**, *23*, 8677. https://doi.org/10.3390/ ijms23158677

Academic Editor: Dippong Thomas

Received: 6 July 2022 Accepted: 1 August 2022 Published: 4 August 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The recycling of spent Ni–Cd batteries has been thoroughly established and run commercially at scale [7–11]. Huge amounts of spent Ni–Cd batteries have been recycled internationally with pyrometallurgical procedures, including destructive cadmium distillation. The recycling process was established using demounting, crushing, metallurgic vacuum separation, and magnetic separation to produce environmentally friendly technology [12,13].

Acid leaching is a significant stage in hydrometallurgical improvements of the shredding of separated Ni–Cd battery powder. Cd and Ni efficiently dissolve during processes based on Ni–Cd wasted battery liquidation with hydrochloric acid [14–16]. The hydrochloric acid solution actually has a substantially higher leaching capacity than other acids; yet sulfuric acid has mostly been proposed as the total leaching and regeneration agent [17].

The separation and elimination of Cd from solutions involving various metal ions could be attained by adsorption, precipitation, ion exchange, electrolysis, solvent extraction, etc. [18,19]. With almost equivalent valence configurations, metal species allow co-transportation and selective extraction challenging. Selective extraction of Cd was possible in the presence of Ni, Mn, Zn, Fe, Mg, and Ca. However, Pb and Cu were co-extracted with Cd from processes established for HCl liquidation of Ni–Cd spent batteries [20]. Cd in the same mixture was observed to impair other metals' physiological balance [21].

Solvent extraction is a very adaptable technique for the sometimes difficult extraction, purification, recovery, and separation of the aqueous medium incorporating metal ions [22]. It is a quick and straightforward, economical method [23]. Liquid-liquid extraction is employed for metal separation using immiscible fluids (usually one organic and another, an aqueous phase containing metal cations) in contact with each other. Most investigations employed the distribution ratios and metal separation factors with specified extractants for a particular metal [24,25]. Ionic liquids are task-specific extractants. They have useful and adaptable physicochemical properties, such as heat stability, high polarity, negligible steam pressure, inactivity, and a wide range of miscellaneous effects on other organic solvents, which have been highlighted in various scientific publications [26,27]. Adogen® 464 has been widely investigated in the extraction of metal chloride complexes. It is suitable for cadmium separation from cobalt, copper, zinc, and manganese [28,29].

The present work aims to recycle and separate different valuable constituents of spent Ni–Cd batteries in a pure state using successive different separation techniques (solvent extraction for cadmium, adsorption for cobalt, and precipitation for nickel). Significant physical chemistry factors were investigated to develop the solvent extraction and adsorption approach, solvent, adsorbent concentration, pH, temperature, and extraction time.

### **2. Results and Discussion**

### *2.1. Precipitation of Iron*

The solution was treated with 1:1 of 32% hydrogen peroxide under agitation for 65 min at 60 ◦C. A green solution was produced at pH 3.0. The solution pH was increased to pH 4.0 using 10 M NaOH, then raised to pH 5.0 using 32% ammonium solution. After warming for 11 min at 73 ◦C, iron(III) hydroxide was the obtained product and was identified using EDX analysis [30].

### *2.2. Factors Controlling Adsorption of Cadmium*

### 2.2.1. Effect of pH

Using 25 mL battery liquor, 0.1 M Na2SO4 was added into deionized water as inert salt to minimize phase separation problems linked to low ionic strength. The samples were equalized at different pH values with 25 mL 0.02 M Adogen 464/kerosene while maintaining the same phase rate because the pH of the solution substantially affects extraction of the metal ions.

The pH was in the range 2.0–5.5. The pH recorded after extraction is 5.0 which was greater than the original pH value. Subsequently, it declined with a rise in the original pH for the ionic fluids. As the pH of the solution increases, Cd2+ extraction increases; however, at pH 5, the percentage of cadmium extraction was insufficient, with just 19% extraction (Figure 1a). Increased pH with higher levels of Adogen 464 resulted in a higher percentage of cadmium extraction [31]. This may be because acid competes with Cd2+ ions or bisulfate ions at low pH values, hindering Cd2+ extraction. As the pH increased, extractable Cd2+ complexes were generated, which improved the extraction ratio.

**Figure 1.** (**a**) Impacts of pH (0.1 M Na2SO4 , 0.02 M Adogen 464, 5 min, O:A 1:1, 25 ◦C); (**b**) equilibration time on Cd2+ extraction (0.1 M Na2SO4 , pH 5.0, 0.02 M Adogen 464, O:A 1:1, 25 ◦C); (**c**) The pseudo-1st-order; (**d**) pseudo-2nd-order modeling of Cd2+ extraction by Adogen 464.

### 2.2.2. Effect of Time

The kinetics of Cd2+ removal was tested with the ionic liquid Adogen 464 in kerosene and with a 25 mL solution of leaching liquor and 0.1 M Na2SO4 in pH 5.0; the influence of balanced cadmium (II) removal time was investigated. As demonstrated in Figure 1b, the extraction efficiency improved gradually with the growth in contacting time until it reached its maximum efficiency at 10 min of contact time, which is sufficient to achieve equilibrium, after which the extraction efficiency became constant with any further increase in the contact time.

The rate of cadmium ion extraction using Adogen 464 is described by its kinetics. The kinetic parameters assist in assessing sorption rates and offer necessary details for extraction approach setup and modelling. The Cd2+ extraction process by Adogen 464 and the rate constants were determined using pseudo-1st and 2nd-order modelling. The following mathematical Equation (1) describes the pseudo-1st-order modelling [32–34]:

$$
\log(q\_\ell - q\_t) = \log q\_\varepsilon - \left(\frac{K\_1 t}{2.303}\right) \tag{1}
$$

The following formula describes the quantity of metal generated per unit weight (*qe*) and the quantity extracted per unit time (*qt*) increase: (*t*, min−1 ). As Figure 1c illustrates, the extraction rate of the 1st order follows a straight line when plotting log (*qe* − *qt*) vs. *t* values. To discover the practical data in Table 1, one can apply the pseudo-1st-order modelling, which matches the results shown in Table 1. A good correlation was established between the calculated value of *qe* at a modest extraction rate of (*K*1 = 0.1029 mg/g), which is around 18.74 mg/g. This is very different from the practical determining capability of 49.712 mg/g.

**Table 1.** The parameters of Cd2+ extraction kinetic by Adogen 464.

It is now apparent that the 1st-order modelling and the empirical data are not alike, and therefore the modelling is not suitable for the system under investigation. Conversely, the pseudo-2nd-order modelling is symbolized by the subsequent Equation (2) [35–38]:

$$\frac{t}{q\_t} = \frac{1}{\mathcal{K}\_2 q\_\varepsilon^2} + \frac{t}{q\_\varepsilon} \tag{2}$$

The rate constant is given as *K*2 (g/mg·min). The slope of the linear plotting is 1/*qe* , and the intercept is 1/*qe* 2 . From the model in Figure 1d, and the data in Table 2, it is obvious that both the theoretical and experimental uptakes are close, and the correlation coefficient *R* 2 is 0.9959 is higher than the 1st order one. The obtained data establish that the 2nd-order modelling is adapted to the extraction system.

**Table 2.** Thermodynamic parameters acquired from Cd2+ extraction temperature investigation.

2.2.3. Effect of Adogen 464 Concentration

A total of 25 mL battery leach liquor, and extractant concentrations between 0.001 and 0.1 M, were used in the experiments to examine ionic liquid concentration impact on Cd2+ extraction. The results showed that ionic liquid concentration had no marked effect on Cd2+ extraction. Figure 2a illustrates that the percent extraction of cadmium rose as the extractant concentration was increased. 0.2 M Adogen 464 was used to extract 99.9% of the cadmium ions from the solution. At pH 5.0, and with other parameters constant, contact time at room temperature was 10 min.

### 2.2.4. Effect of Sulfate Concentration

To explore the influence of Na2SO4 at concentrations of 0.1–1.0 M upon Cd2+ recovery, experiments were conducted at room temperature using 0.2 M Adogen 464. The experiment presented in Figure 2b showed that the percentage of cadmium extraction in the aqueous feed decreased with an enlargement in the concentration of sulfate ions because of the salting-out effect for Adogen 464/kerosene.

**Figure 2.** (**a**) Impact of Adogen 464 concentration upon Cd2+ extraction (0.1 M Na2SO4 , pH 5.0, O:A 1:1, 25 ◦C, 10 min); (**b**) Impact of sulfate concentration upon Cd2+ extraction (0.2 M Adogen 464, pH 5.0, O:A 1:1, 10 min, 25 ◦C); (**c**) Impact of aqueous to organic ratio upon Cd2+ extraction (0.2 M Adogen 464, pH 5.0, 0.1 M Na2SO4 , 10 min, 25 ◦C); (**d**) FTIR spectra of 0.2 M Adogen 464/toluene and cadmium/Adogen 464/toluene); (**e**) Impact of temp. upon Cd2+ extraction (0.2 M Adogen 464, pH 5.0, 0.1 M Na2SO4 , O:A 1:1, 10 min); (**f**) Influence of temp. on Cd2+ partition coefficient (0.2 M Adogen 464, pH 5.0, 0.1 M Na2SO4 , O:A 1:1, 10 min).

Nayl (2010) described the relationship between quaternary ammonium extractants, which were employed to extract metal ions, as only occurring with a solvation reaction mechanism. The extraction of the Cd2+ mechanism from SO4 2− media by R3R'N.SCN

obeyed the addition mechanism, and the removed dominant species were (R3R'N.CdSO4. SCN) [39].

$$\text{\textbullet Cd}^{2+} \text{\textbullet (aq)} + \text{\textbullet O}\_4^{2-} \text{\textbullet (aq)} + 2\text{H}^+ + \text{\textbullet (R}\_3\text{R'N.SCN}) \text{\textbullet (org)} \leftrightarrow \text{\textbullet (R}\_3\text{R'N.CdSO}\_4\text{SCN}) \text{\textbullet (org)} + 2\text{H}^+$$

where R = C10H21, C9H19, C8H17 and R' = CH3 groups. The extraction (*Kex*) constant for Adogen 464 is gained from Equation (3).

$$K\_{\rm ex} = \frac{[\left(\text{R}\_3\text{R}\text{N}\text{N}\text{dSO}\_4\text{SCN}\right)]\_{\text{(org)}}^3}{[\text{Cd}^{2+}]\_{\text{(aq)}}^3[\text{SO}\_4^{2-}]\_{\text{(aq)}}^3[\text{(R}\_3\text{R}\text{N}\text{SCN)}]\_{\text{(org)}}^3} \tag{3}$$

### 2.2.5. Effect of Organic to Aqueous Ratio

The ratio between aqueous/organic phases was studied with various ratios or relations of aqueous leach liquid with the fitting organic solvent, generating the balance curve. The influence of varying the aqueous to organic phase ratio (A:O) upon Cd2+ extraction was examined using different aqueous/organic phase ratios from 4:1 to 1:4 at 25 ◦C, 10 min of shaking time, pH 5.0, and 0.2 M Adogen 464/kerosene. According to Figure 2c, the A:O ratio slightly influences the extraction efficiency in the case of increasing the organic volume. In the other case, however, the extraction efficiency decreases gradually with increasing the aqueous phase. Hence, the O:A ratio of 1:1 was selected as the optimal ratio, with 96.4% extraction.

Fourier infrared spectroscopy technique (FTIR) has been employed to detect many characteristic vibrational bands [40]; it can detect the functional groups of Adogen 464 before and after the extraction of cadmium. A study was conducted using Adogen 464, diluted in kerosene and presented in Figure 2d. The peak at 1030 cm−1 for Adogen 464 was assigned for –C–N stretching vibration that was transformed to 1044 cm−1 for Cd-Adogen 464 complex, after the bonding among Cd2+ and Adogen 464 happened. The wide absorption band at 3604 cm−1 for Cd-Adogen 464 was assigned for the stretching vibration of –OH of soluble moisture [41]. The distinctive peak of quaternary amine at 1464 cm−1 appeared in both spectra and was induced via (CH3)N+ . The asymmetric –CH peaks (2923 cm−1 ) for a combination of Adogen 464 prior to and after extraction investigations were equivalent.

### 2.2.6. Effect of Temperature

The extraction rate may rise or decrease depending on the temperature of the environment. Temperatures ranging from 30 ◦C to 80 ◦C were used to scrutinize the temperature influence upon Cd2+ extraction via ionic liquid Adogen 464. The aqueous solution used in this experiment was the battery leach liquor with 0.1 M Na2SO4, pH 5.0, and 0.2 M Adogen 464 in kerosene, as shown in Figure 2e. Altogether, the outcomes demonstrated that temperature had a favourable influence on Cd2+ extraction. The extraction of Cd2+ ions rose as the temperature increased; it reached its maximum *E*% (99.9%) at 50 ◦C. From the successive van't Hoff Equation (4) [42–45].

$$
\log K\_d = \frac{-\Delta H^\circ}{2.303RT} + \frac{\Delta S^\circ}{2.303R} \tag{4}
$$

The enthalpy (∆*H*◦ ) and entropy (∆*S* ◦ ) were determined via the plot of log *Kd* vs. 1/*T* (Figure 2f). From the relation of ∆*G* ◦ = ∆*H*◦ − *T*∆*S* ◦ , the values of ∆*G* ◦ were computed and are listed in Table 2.

The positive significance value of the ∆*H*◦ in Table 2 shows that the extraction of cadmium ions through Adogen 464 is an endothermic mechanism, which is confirmed by the temperature impact that affects the *E*% of Cd2+ ions. Negative ∆*G* ◦ values show that the suggested extraction contrivance is spontaneous and feasible. The Arrhenius relation is important as the straight-line slope in Figure 2f is applicable for assessing the apparent activation energy (*Ea*) of cadmium ions extraction at different temperatures [46]. Values are calculated from the following Equation (5) [47].

$$
\log K\_d = \frac{-2.303E\_d}{RT} + \log A \tag{5}
$$

The partition coefficient, *Kd* , and *Ea* (kJ·mol−1 ) denote the extraction activation energy, *R* (8.314 J·mol−1 ·K −1 is the molar gas constant), and *T* is the absolute temperature in Kelvin. Cadmium ions extraction required 1.882 kJ·mol−1 of activation energy, and the Arrhenius constant is 436.4. This signifies that Cd2+ ions extraction by Adogen 464 is an endothermic approach, which means that the reaction requires energy to be completed.

### *2.3. Cadmium Stripping from Cadmium/Adogen 464 and Preciptation*

The removal of Cd2+ ions from the loaded Adogen 464 in toluene was accomplished using different concentrations of H2SO4 and EDTA. Figure 3a demonstrates that raising the acid concentration from 0.1–2.0 M causes an increase in Cd2+ stripping from 25% to 83.5%. The use of 2.0 M H2SO4 and/or 0.1 M EDTA in two successive stages at an O:A ratio of 1:2 resulted in a significant increase in Cd2+ stripping from 23.4% to 69.5% with an increase in EDTA concentration from 0.01 to 0.1 M, resulting in the complete removal of the entire cadmium content of the solvent, as shown in Figure 3b. As a result, 2.0 M H2SO4 was selected as the most effective stripping agent for this work.

**Figure 3.** Influence of (**a**) H2SO4 ; (**b**) EDTA concentration upon Cd2+ stripping from the loaded Adogen 464.

The sulfate solution containing Cd2+ produced from the stripping process was subjected to precipitation using 0.5% Na2S solution. Adjusting the pH to 1.25 at room temperature, a yellow precipitate of CdS was formed; it was separated via filtration and dried in the oven at 70 ◦C. The obtained product was about 2.0 g. The product has also been subjected to SEM-EDX analysis, as shown in Figure 4, which reveals a maximum purity.

**Figure 4.** SEM-EDX chart for CdS product.

### *2.4. Adsorption of Cobalt Using Prepared Silica Adsorbant*

The leachate is now free of cadmium and iron and contains nickel and cobalt. The remaining solution was subjected to the selective separation of Co2+ from Ni2+ utilizing a new adsorbent, PTU-MSA.

### 2.4.1. Characterization of PTU-MSA

The morphology of PTU-MS was gained via an electron scanning microscope (SEM-EDX). The matrix mainly consisted of a hexagonal formation and showed a rather homogeneous particle size of about 0.85 mm PTU-MSA adsorbent (mean size) [48]. It was different from PTU-MS (Figure 5a,b). Moreover, the PTU-MS was analyzed using FTIR. The data showed major typical peaks in the range of 2900–3000 cm−1 due to aliphatic stretching of C–H. The feature at 1655 cm−1 is C=O for urea, that at 3410 cm−1 is due to phenolic –OH, and peaks at 1454 cm−1 and 1568 cm−1 correspond to N–H and N–C, respectively. Three bands are attributed to silica at 1075, 794, and 456 cm−1 of Si–O–Si. Moreover, a peak at 949 cm−1 is assigned to Si–OH [49–54].

The spectrum of sulphonic acid-modified PTU-MSA shows new bands at 1151 cm−1 and a specific peak is recorded at around 584 cm−1 , resulting from the SO3 group. The peak at 650 cm−1 is dispersed due to the stretching vibration of the S–N. The S–N band was predicted to overlap with the Si–O stretching band of the identical area of energy (Figure 5c). The spectrum of FT-IR confirmed that the surface of prepared PTU-MS and PTU-MSA material was successfully functionalized and synthesized [55,56].

The nitrogen adsorption/desorption isotherm curves (BET) of the two samples (PTU-MS and PTU-MSA) were obtained through steep condensing/evaporation capillary stages [57–59]. Recognizable H1 hysteresis loops were shown for the adsorption/desorption of nitrogen in the two samples (PTU-MS and PTU-MSA), which are typical for mesoporous materials with cylindrical mesoporous following the IUPAC classification. The PTU-MSA's specific surface area is decreased due to the sulphonic acid group's incorporation into the PTU-MS framework. The results in Table 3 along with Figure 5d, showed a reduction in the pore diameter and pore volume indicating successful incorporation of the group.

**Table 3.** Physico-chemical properties of PTU-MS material sorption, for the examination of N2 prior and after incorporation of a sulphonic group.

2.4.2. Factors Controlling the Adsorption Process of Cobalt

The adsorption of cobalt was initially tested using PTU-MSA from a synthetic solution. It was then applied on the leach liquor free of cadmium and iron, containing only cobalt and nickel.

1. Impact of pH

The pH has a vital role in reducing or increasing the adsorption selectivity of efficiently hydrolysable metal ions. As already established, most metal ions are hydrated in water [60]. Figure 6a indicates the pH dependency of Co2+ ions on PTU-MSA. The Co2+ ions form

insoluble aqueous complexes with increased pH, when Co2+ ions undergo hydrolysis reactions in water.

**Figure 6.** (**a**) impact of pH upon Co2+ adsorption (20 mL of 250 mg/L Co2+, 10 min, 0.1 g of PTU-MSA, 25 ◦C); (**b**) Impact of contacting time upon Co2+ adsorption (20 mL of 250 mg/L Co2+, pH 8.0, 0.08 g of PTU-MSA, 25 ◦C); (**c**) 1st-order kinetic plot; (**d**) 2nd-order kinetic plot of PTU-MSA's Co2+ adsorption (20 mL of 250 mg/L Co2+, pH 8.0, 0.08 g of PTU-MSA, 25 ◦C).

The solution pH influences the speed of surface reactions. The adsorption capacity changes with pH scope mostly due to the impact of pH upon the adsorption aspects of the PTU-MSA surface. Over the pH range 4.0–10.0, Co2+ adsorption efficiency increased with higher pH, as demonstrated in Figure 6a. The adsorption conditions were kept constant at Co2+ concentration of 250 mg/L, 10 min, and 0.1 g of PTU-MSA at room temperature, while the pH was varied.

The results shown in Figure 6a indicate that the Co2+ adsorption on the surface of PTU-MSA steadily rose with the solution pH rising to the highest or maximal at pH 8. The Co2+ species exclusively exist in a divalent ionic state at this pH, enhancing the removal of the adsorbent from the solution. The pH effect can be explained by the presence in working solutions with pH values just below 8.0 of various ionic shapes such as Co2+ , Co(OH)+ , Co(OH)2, and Co(OH)3 −, which reduce the overall effectiveness of the Co2+ removal [61,62]. PTU-MSA has the maximum adsorption efficiency at pH 8.0. At rising pH levels (pH > 8.0), the Co2+ was precipitated as cobalt hydroxide. This outcome displays the pH effect on the adsorbent.

### 2. Impact of time and kinetics

The foremost important feature of cobalt adsorption was the adsorption time. Sequences of experiments with 0.08 g PTU-MSA adsorbent were examined at differing contact times (5–70 min) while the rest of the experimental conditions were maintained constant. As shown in Figure 6b, Co2+ sorption capacity rose by 5 min to 61.69 mg·g −1 upon PTU-MSA. However, the Co2+ ions uptake was not affected when the contact period extended after 20 min. As a result, the balance was extended to 20 min, and the adsorption conditions of cobalt were then improved.

The kinetics of the adsorption process of Co2+ regulates the Co2+ uptake rate and, in turn, the adsorbate residence time was controlled by this rate at the solid-solution interface. Lagergren's 1st- and 2nd-order kinetic modellings derived PTU-MSA rate constants were used to quantify Co2+ adsorption [63,64]. The correlation coefficient was utilized to measure the consistency of the investigational outcomes by the values estimated through the two models (*R* 2 ). A higher *R* 2 assessment suggests that Co2+ adsorption kinetics were accurately characterized by a particular model.

The pseudo-1st- and pseudo-2nd-order kinetics (see Equations (1) and (2)) were shown in Table 4 and Figure 6c,d. The 2nd-order correlation coefficient is 0.9758 and the theoretical uptake is 61.69 mg·g −1 , which resembles the investigational finding. The result suggests that Co2+ adsorption upon PTU-MSA hinges on the initial Co2+ concentration. Thus, pseudo-2nd-order kinetics anticipates the adsorption performance throughout the extent of the whole concentration examined [61].

**Table 4.** Kinetic factors of Co2+ adsorption upon PTU-MSA.

### 3. Impact of PTU-MSA dose

The influence of PTU-MSA dosage upon Co2+ adsorption was investigated over the range 0.02–0.2 g. As demonstrated in Figure 7a, with augmented PTU-MSA dosage from 0.02 to 0.08 g, the Co2+ uptake improved [58,59]. The Co2+ adsorption upon PTU-MSA does not differ substantially when the PTU-MSA dosage is more than 0.08 g with the same constant Co2+ concentration in the medium. For the subsequent experiments, 0.08 g of PTU-MSA was therefore chosen.

## 4. Impact of initial Co2+ concentration and adsorption isotherms features

For three constant temperatures (298, 323, and 343 K), the effect of the initial Co2+ concentration was explored. The Co2+ adsorption ability improved as the initial Co2+ concentration was increased, according to the results reported in Figure 7b. Increased Co2+ ion concentrations enhanced the forces in the aqueous and solid phase that suppress the mass transfer. In addition, a beneficial effect was seen on the uptake of Co2+ ions in PTU-MSA which shows the process could have an endothermal origin.

Two isothermal models have been utilized to explore the interaction and distribution mechanism between Co2+ and solid interface, the isotherms of Freundlich and Langmuir. The Langmuir isothermal pattern implies that monolayers with homogenous binding sites form on the surface of the PTU-MSA (Figure 7c) [65–67]. The model is usually described by Equation (6):

$$\frac{\mathcal{C}\_{\mathfrak{e}}}{q\_{\mathfrak{e}}} = \frac{1}{bq\_{\max}} + \frac{\mathcal{C}\_{\mathfrak{e}}}{q\_{\max}} \tag{6}$$

where *Ce* (mg·L −1 ) is the equilibrium concentration, *qe* (mg·g −1 ) is the equilibrating uptake of adsorbed Co2+ , *qmax* (mg·g −1 ) is maximal sorption capability, and *b* (L·mg−1 ) is

equilibrating adsorbance constant. Freundlich modeling is presumed such that adsorption performance is carried out with variable binding energies in scenarios with a heterogeneous surface (Figure 7d) [68–73]. The Freundlich modeling is expressed as Equation (7):

$$
\ln q\_{\varepsilon} = \ln k\_f + \frac{1}{n} \ln \mathbb{C}\_{\varepsilon} \tag{7}
$$

where *kf* (mg·g −1 ) is the uptake of the Freundlich continuum and *n* is the sorption intensity parameter. Table 5 shows that the result agrees more with the Langmuir than Freundlich isothermal for Co2+ adsorption. This may be due to the greater correlation coefficient (*R* 2 = 0.9992) and the maximal sorption capacity at all three temperatures, which are closer to the experimental. The outcomes showed that there was sorption upon a homogenous monolayer surface and that the energy of every metal-binding place is the same [74–76]. In the Freundlich isotherm, the computed value is 1/*n* higher than 1.0 (Table 5), which showed normal adsorption. It may be inferred that the higher the *kf* , the higher is the adsorption intensity [77,78].

**Figure 7.** (**a**) Impact of PTU-MSA dose upon Co2+ adsorption (20 mL of 250 mg/L Co2+, pH 8.0, 20 min, 25 ◦C); (**b**) Impact of initial Co2+ concentration upon PTU-MSA uptake (20 mL of 250 mg/L Co2+, pH 8.0, 20 min, 0.08 g PTU-MAS); (**c**) Langmuir isotherm model; (**d**) Freundlich isotherm mode for Co2+ adsorption upon PTU-MSA (20 mL of 250 mg/L Co2+, pH 8.0, 20 min, 0.08 g PTU-MAS).

**Table 5.** Langmuir and Freundlich data for Co2+ adsorption by PTU-MSA.

When the values of the separation factor constant (*RL*) are calculated, the degree of adsorptive capacity of PTU-MSA towards Co2+ can be predicted, and this gives an indicator of whether the adsorption development can occur. A suitable environment for Co/PTU-MSA adsorption exists when the *RL* values fall within the range of 0 to 1.0. It is found that the relative lightness (*RL*) ranges between 0.003614 and 0.10626, indicating that the PTU-MSA is capable of adsorbing Co2+ from the aqueous phase. The following thermodynamic evaluation factors can prove this Equation (8):

$$R\_L = \frac{1}{1 + K\_L \mathcal{C}\_o} \tag{8}$$

### 5. Thermodynamic parameters for systems of cobalt adsorption

The intercept and slope of the plot of log *Kd* vs. 1/*T* for the cobalt adsorption system were used to compute the values of ∆*H*◦ and ∆*S* ◦ , besides the values of ∆*G* ◦ from Equation (4) [79]. The thermodynamic parameters are shown in Table 6. Both ∆*H*◦ and ∆*S* ◦ are positive, and *T*∆*S* ◦ is larger than ∆*H*◦ .

**Table 6.** Factors controlling the thermodynamics of Co2+ adsorption upon PTU-MSA at different temperatures.

These findings support the idea that this adsorption mechanism is endothermic. Positive values of ∆*S* ◦ indicate that there is a strong affinity between PTU-MSA and Co2+ ions. Furthermore, a decline in negative ∆*G* ◦ quantities with increasing temperature demonstrates that sorption processes are more effective at more elevated temperatures, most likely because ions are more mobile in the solution [46].

6. Impact of Ni2+ ion concentration on PTU-MSA selectivity for Co2+ adsorption

In this experiment, we use a mixture of Co2+ and Ni2+ in the same solution, while changing the concentration of Ni2+ to study the influence of Ni2+ on the selectivity of PTU-MSA for Co2+ adsorption. The data in Figure 8 demonstrate that increasing the concentration of nickel to more than 15-fold the concentration of Co2+ ions has no influence on the adsorption of cobalt. There is no affinity of PTU-MSA for adsorption of Ni2+ ions. This study covered the application of PTU-MSA to leach liquor that contained only nickel and cobalt ions.

**Figure 8.** Influence of nickel concentration upon the selectivity of PTU-MSA for Co2+ ions.

### *2.5. Desorption and Precipitation of Cobalt*

The cobalt was eluted from Co/PTU-MSA to a varied phase ratio in the range of 0.01–0.5 M for a 5 min duration using different concentrations of sulfuric acid and ethylene diamine tetraacetic acid (EDTA). Metal values were examined in the aqueous phase. The elution percentage of Co2+ vs. H2SO4 concentration (M) is shown in Figure 9a. The cobalt desorption grew from 10.8 to 99.9% by enhancing the concentration of H2SO4 from 0.01 to 0.2 M. In case of EDTA, the maximal desorption of Co2+ was realized at 0.5 M; the highest elution percentage in H2SO4 concentration, 0.3 M H2SO4 was unique as the greatest eluting agent.

**Figure 9.** (**a**) Effect of eluents concentration on elution of cobalt loaded on PTU-MSA; (**b**) Reusability of PTU-MSA.

At this juncture, to identify how often the adsorbent can be used for the adsorption process, and whether its effectiveness changes or not, the regeneration process has been carried out seven times after every adsorption cycle. Figure 9b shows that the adsorption capacity was reduced with higher numbers of cycles. In the first 4 cycles, the positive results were evident. However, the adsorption percentage fell in cycle number 5 compared to the first cycle from around 99 to 84%. Cycles 6 and 7 were extremely close in their adsorption percentages. A decrease in adsorption percentage may be due to poisoning of the active sites or partial leaching.

After desorption of cobalt ions, the precipitation of Co2+ was achieved by increasing the pH of the solution pH to 9.0 using NH4OH; a pale pink precipitate of Co(OH)2 was formed. It was filtrated and cleaned with distilled H2O numerous times, after which the precipitate was dried at 60 ◦C for 2 h. Later, it was calcined at 900 ◦C to obtain a cobalt (II) oxide (CoO) product. The final product was analyzed by SEM-EDX (Figure 10), which showed that the product is entirely cobalt.

### *2.6. Nickel Recovery*

The final solution contains only nickel ions after separation of cadmium and cobalt from the Cd–Ni batteries leach liquor. 200 mL of the sulfate leach liquor contains about 0.823 g of Ni. This has been precipitated at pH 8.25 using 10% NaOH, added to the Ni solution at room temperature. After filtration, the precipitate Ni(OH)2 was washed several times with distilled H2O to eliminate any impurities and ignited at 650 ◦C for 1.5 h. The NiO product was recovered and identified using chemical and SEM-EDX techniques, as shown in Figure 11. The recovery of Ni was attained 98% with a purity ≥ 97%. In fact, the interference problem from Co2+ ions has been overcome by the prior removal of Co2+ ions.

**Figure 11.** EDX and SEM analysis of NiO.

### **3. Materials and Methods**

### *3.1. Chemicals and Instruments*

Cadmium sulfate, xylenol orange, Adogen 464, KSCN, and sulfuric acid were supplied from Riedel-Dehaen AG and Merck, Merck, Darmstadt, Germany. 3-(Triethoxysilyl) propylisocyanate (95%), poly 2,4-diaminophenol (98%), and ethylene glycol were bought from Sigma-Aldrich chemicals, St. Louis, MO, USA. Tetraethyl orthosilicate, nickel nitrate, block polypropylene glycol-block polyethylene glycol (P123, EO20PO70EO20, MW = 5800 g/mol), chromium nitrate, cobalt nitrate anhydrous, chlorosulphonic acid, triethylamine trihydrochloride and copper nitrate anhydrous were obtained from Merck, Darmstadt, Germany. Distilled water was utilized in all processes. The structure and molecular weight of Adogen 464 are shown in Figure 12. A pH meter (systronics µ pH-System 362) was used to monitor pH for the aqueous phase before and after extraction. An ICP-OES spectrometer (OPTIMA 5300 DV, PerkinElmer, Waltham, MA, USA) was utilized for elemental analysis. A mass balance provided the metal quantities extracted by the solvent extractor. FTIR Affinity-1S (Shimadzu, Kyoto, Japan) was applied to detect the FTIR spectra of samples.

**Figure 12.** Structure of Adogen® 464.

### *3.2. Preparation of Cd–Ni Batteries Leachate*

The spent batteries were manually broken into various fractions. Plastic and paper wastes were removed while the electrodes (anode and cathode-active materials) were separated and mixed, ground then washed with distilled water to remove electrolyte (KOH). Figure 13 shows the SEM diagram powder of a deconstructed Ni–Cd battery. After that, the mixture was roasted at 550 ◦C to oxidize metallic cadmium and decompose cadmium and nickel salts. The net weight of the mixture was about 50 g; the leaching process occurred using 20% H2SO4, solid/liquid (S/L) of 1/5 at 80 ◦C for 6 h. Before leaching, a combination of conventional and tooling methods was utilized to scrutinize the chemical formatting of the examined powder (originated from Ni–Cd battery) [80]. Table 7 shows the concentrations of the components that agree with published data before and after the leaching procedure [81–83]. Therefore, the solution contained some iron, which had to be precipitated. The iron-free leaching solution was also employed for selective extraction of Cd2+ ions using Adogen 464.

**Figure 13.** SEM image of Ni–Cd battery powder and a digital photograph of a dismantled spent Ni–Cd battery.

**Table 7.** Chemical composition of consumed Ni-Cd battery powder.

## *3.3. Preparation of Adogen® 464 for Extraction Process*

Adogen® 464 (30%) was converted from chloride to thiocyanate form; this was exposed to 1.5 M KSCN solution, at a 1:1 ratio of water to organic material. It was agitated for 20 min. After two injections of organic solution with a new SCN− solution, there were still traces of thiocyanate in the organic solution. After being in contact with the organic solution, thiocyanate titration was indicated by AgNO3 concentration [29]. An appropriate volume of Adogen® 464 with commercial-grade kerosene (diluent) was diluted and/or dissolved to generate organic solutions of varied levels unless otherwise indicated. Concentration levels of the ionic fluid were determined.

### *3.4. Extraction Processes and Measurements*

A 100 mL separation funnel with the aqueous phase of Cd2+ was balanced with organic Adogen 464 solvent at room temperature (except for temperature variation). After the water layer separation, the pH was determined with a pH meter. Each trial was explored three times, with error bars added on each occasion. Following extraction, the amount of a metal in water increases when it moves from the aqueous stage to the organic stage, according to the equation (Equation (8)). [*M*]*o* equals the metal amount in the organic layer, and [*M*]*A* equals the metal amount in the aqueous layer following extraction:

$$D = \frac{[M]\_o}{[M]\_A} = \frac{[M]\_{i,A} - [M]\_A}{[M]\_A} \tag{9}$$

$$D = \frac{[M]\_{i,A} - [M]\_A}{[M]\_A} \times \frac{V\_A}{V\_o} \tag{10}$$

*VA* and *VO* are volumes of aqueous and organic layers. Hence, the extraction efficiency was applied as observed [84]:

$$E\% = \frac{100D}{D + \left(\frac{V\_A}{V\_o}\right)}\tag{11}$$

#### *3.5. Preparation of Precursor, 1,1*0 *-(4-hydroxy-1,3-phenylene)bis(3-(3-(triethoxysilyl)propyl)urea (PTU)*

A 2,4-Isocyanate of propyl-di-aminophenol (1.2 g) and 3-(triethoxysilly)propyl isocyanate (5.0 g) was liquefied at the concentration of 1:2 in 85 mL dry acetonitrile (Figure 14). The mix was warmed to 85 ◦C for 24 h via static reflux. Thin-layer chromatography (TLC) was utilized to assess reaction advancement. The solvent was then evaporated to produce a white precipitate during over 5 h in dry hexane. The puffed white outcome was purified, cleaned with hexane, and dried with a vacuum.

**Figure 14.** Preparation of PTU-MS and PTU-MSA.

### *3.6. Synthesis of PTU Bridged Mesoporous Organosilica (PTU-MS)*

N1,10 -(4-hydroxy-1,3-phenylene) bis(3-(3-(triethoxysilyl)propyl)urea, a bridging mesoporous material, was prepared [85]. TEOS as the parent source of silica and the structuredirecting agent, PTU as the organic bridging groups, and pluronic P-123 as a sample were used and mixed in a flask. The synthesized materials were designated PTU-MS. Solutions of P123, HCl, and water were mixed with a strong stirring at 35 ◦C in a typical process. The combination of 2 mixtures was allowed to mix for 24 h at the above temperature until white precipitation was established, and the heterogeneous blend aged with mixing for a further 24 h and 80 ◦C. The white precipitate was filtered, cleaned with H2O, and dried in air for various times.

Before using the produced adsorbent PTU-MS, it was subjected to washing with HCl (3 mL)–ethanol (150 mL) solution for 12 h at 25 ◦C to get rid of the surfactant template. This was performed three times until the surfactants had been eliminated. To obtain the final product, PTU-MS was filtered, cleaned with ethanol, and desiccated overnight at 60 ◦C.

Finally, 2.0 g of PTU-MS was dipped in chloroform, stirred at 45 ◦C for 1 h and mixed with 0.08 mL of chlorosulphonic acid (ClSO3H), to generate enough triethylamine to eliminate the released hydrochloric acid with 18 h of stirring (Figure 14). The yield was filtered, cleaned with excess chloroform, and vacuum-dried to gain organic-inorganic mesoporous silica improved by a group of sulphonic acids. The end product received the symbol PTU-MSA. The preparation of PTU-MS and PTU-MSA are illustrated in Figure 14.

### **4. Conclusions**

The Scarp Ni–Cd batteries (50 g) were treated with 20% H2SO4, with the 1:5 solid/liquid (S:L) ratio at 80 ◦C for 6 h. The leaching efficiency of Cd, Fe, and Co was nearly 100%, whereas the leaching efficiency of Ni was 95%. Both newly prepared materials were confirmed via FTIR and SEM techniques. Adogen 464 was used for Cd2+ extraction, followed by precipitation as a yellow CdS product with 0.5% Na2S solution by setting the pH at 1.25, at room temperature. PTU-MS silica adsorbent was used as an ion exchanger for Co2+ ions. 0.3 M H2SO4 was employed as an eluant and the precipitation of cobalt was achieved at up to pH 9.0 as Co(OH)2. The kinetic and thermodynamic parameters were also investigated. Lastly, nickel was directly precipitated at pH 8.25, using a 10% NaOH solution at ambient temperature.

**Author Contributions:** Conceptualization, A.A.G. and A.K.S.; methodology, A.R.W. and M.A.G.; software, A.K.S. and M.A.G.; validation, M.Y.H. and M.I.S.; formal analysis, B.M.A.; investigation, A.R.W.; resources, M.A.G.; data curation, A.A.G.; writing—original draft preparation, E.M.E.-S.; writing—review and editing, A.K.S.; visualization, A.R.W.; supervision, R.E.S. and H.A.R.; project administration, M.F.C.; funding acquisition, H.H.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by King Khalid University through a grant (KKU/RCAMS/22) under the Research Center for Advanced Materials Science (RCAMS) at King Khalid University, Saudi Arabia.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This work was supported by King Khalid University through a grant (KKU/ RCAMS/22) under the Research Center for Advanced Materials Science (RCAMS) at King Khalid University, Saudi Arabia.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

## *Article* **Adjusting Some Properties of Poly(methacrylic acid) (Nano)Composite Hydrogels by Means of Silicon-Containing Inorganic Fillers**

**Claudia Mihaela Ninciuleanu 1,2, Raluca Ianchis, 1 , Elvira Alexandrescu 1 , Cătălin Ionut, Mihăescu 1 , Sabina Burlacu 1 , Bogdan Trică 1 , Cristina Lavinia Nistor 1 , Silviu Preda 3 , Cristina Scomoroscenco 1 , Cătălina Gîfu 1 , Cristian Petcu 1 and Mircea Teodorescu 2,\***

**Abstract:** The present work aims to show how the main properties of poly(methacrylic acid) (PMAA) hydrogels can be engineered by means of several silicon-based fillers (Laponite XLS/XLG, montmorillonite (Mt), pyrogenic silica (PS)) employed at 10 wt% concentration based on MAA. Various techniques (FT-IR, XRD, TGA, SEM, TEM, DLS, rheological measurements, UV-VIS) were used to comparatively study the effect of these fillers, in correlation with their characteristics, upon the structure and swelling, viscoelastic, and water decontamination properties of (nano)composite hydrogels. The experiments demonstrated that the nanocomposite hydrogel morphology was dictated by the way the filler particles dispersed in water. The equilibrium swelling degree (SDe) depended on both the pH of the environment and the filler nature. At pH 1.2, a slight crosslinking effect of the fillers was evidenced, increasing in the order Mt < Laponite < PS. At pH > pKaMAA (pH 5.4; 7.4; 9.5), the Laponite/Mt-containing hydrogels displayed a higher SDe as compared to the neat one, while at pH 7.4/9.5 the PS-filled hydrogels surprisingly displayed the highest SDe. Rheological measurements on as-prepared hydrogels showed that the filler addition improved the mechanical properties. After equilibrium swelling at pH 5.4, G' and G" depended on the filler, the Laponite-reinforced hydrogels proving to be the strongest. The (nano)composite hydrogels synthesized displayed filler-dependent absorption properties of two cationic dyes used as model water pollutants, Laponite XLS-reinforced hydrogel demonstrating both the highest absorption rate and absorption capacity. Besides wastewater purification, the (nano)composite hydrogels described here may also find applications in the pharmaceutical field as devices for the controlled release of drugs.

**Keywords:** nanocomposites; poly(methacrylic acid); hydrogel; montmorillonite; Laponite; pyrogenic silica; water decontamination

### **1. Introduction**

Composite materials have increasingly attracted the attention of researchers within the last few decades due to their properties being far superior to simple materials [1–5]. A special class of such materials is represented by composite hydrogels, which may generally be defined as hydrophilic polymer networks capable of retaining certain quantities of aqueous solutions in the presence of small particles acting as reinforcing agents [6–8]. Composite hydrogels have been and are being studied a great deal due to their improved mechanical, electrical, thermal, and optical properties, absorption capacity, and sensitivity to different stimuli as compared to their unreinforced counterparts [9,10]. The improved

**Citation:** Ninciuleanu, C.M.; Ianchis, , R.; Alexandrescu, E.; Mih˘aescu, C.I.; Burlacu, S.; Tric˘a, B.; Nistor, C.L.; Preda, S.; Scomoroscenco, C.; Gîfu, C.; et al. Adjusting Some Properties of Poly(methacrylic acid) (Nano)Composite Hydrogels by Means of Silicon-Containing Inorganic Fillers. *Int. J. Mol. Sci.* **2022**, *23*, 10320. https://doi.org/10.3390/ ijms231810320

Academic Editor: Dippong Thomas

Received: 8 August 2022 Accepted: 4 September 2022 Published: 7 September 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

properties of the composite hydrogels are a consequence of the synergistic effect of the individual phases, i.e., the polymer matrix and the inorganic filler [11]. Various nanoand microparticles have been used as reinforcing agents for hydrogels over the years, such as graphene and its derivatives (graphene oxide, carbon nanotubes) [12,13], bioactive glass [14], metal nanoparticles (Ag, Au), and metal oxides (Fe3O4, Fe2O3, alumina, etc.) [15], the distinct properties of the reinforcing agents allowing the design of the final hydrogel in agreement with the required characteristics.

A special class of fillers for hydrogels is represented by materials that contain Si-O groups within their structure. This category includes various types of silica-based nanoparticles [16], such as pyrogenic silica [17–20] and layered clays such as montmorillonite (Mt) [21] and Laponite (Lap) [22], which have the advantage of being cheaper than the agents listed above. Among them, clays have attracted more and more attention as fillers due to their remarkable properties, such as high specific surface area and adsorption capacity, optimal rheological properties, chemical inertia, and low toxicity, Lap and Mt being the most used for hydrogel reinforcement [6,21–24]. From a structural point of view, they have similarities, as both belong to the smectite class. Clays are usually micro- or nanometric particles made up of layered sheets of 2D silicate (Figure S1). The empirical formula of Mt is (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O [25]. Its layers, approximately 1 nm thick and 100 nm × 100 nm width × length, are composed of two tetrahedral sheets formed by O-Si-O bonds and an octahedral sheet formed by O-Al(Mg)–O bonds [26]. The Lap layers are instead in the form of disks of approximately 25 nm diameter and 0.92 nm thickness [26]. The empirical formula of Lap reported in the literature is Na0.7 + [(Si8Mg5.5Li0.3)O2(OH)4] −0.7 [27]. Commercial Lap is available in different varieties: XLG, XLS, RD, RDS, XLG, and XLS possessing a higher purity and a lower content of heavy metals than Mt. In aqueous dispersions with concentrations higher than 2%, Laponite XLG can form the so-called "house of cards" structure, leading to gelation due to the ionic interactions among the positively-charged edges of some sheets and the negatively-charged faces of others [27]. Laponite XLS does not form the "house of cards" structure because it is modified with pyrophosphate ions [28,29].

Pyrogenic (fumed) silica nanoparticles (Figure S1) are obtained by SiCl4 pyrolysis at temperatures above 1000 ◦C and are in the form of silica spheres with 5–30 nm diameter, forming particle chains of 100–1000 nm length. These particle chains lead eventually to porous networks that extend up to 250 µm. The primary particles are composed of SiO4 tetrahedra, while on the surface they display both oxygen atoms belonging to siloxane groups (Si-O-Si) and silicon atoms from silanol groups (Si-OH) [30]. Pure pyrogenic silica is hydrophilic and has a high surface energy due to the presence of these groups [31]. Pyrogenic silica nanoparticles are stable in aqueous solutions [32], but unlike layered silicates, they do not dissociate with the formation of free ions [17].

Both clays [24,33] and pyrogenic silica nanoparticles [17–19] have proven their effect on hydrogels, especially by increasing their mechanical and/or thermal properties or changing the swelling degree, but silica nanoparticles are less studied as reinforcing agents for hydrogels as compared to clays. Incorporating clays into the hydrogel matrix is also a good way to increase the final absorption properties of the hydrogel because they can absorb positively-charged pollutants, such as heavy metals and cationic dyes [34]. A study in this regard was conducted by Peng et al., who synthesized a hydrogel from cellulose and Mt that was tested for the adsorption of methylene blue, obtaining an absorption capacity of over 90% [35]. For methylene blue absorption as well, Yi et al. synthesized hybrid hydrogels based on polyacrylamide, sodium humate, and Lap [36].

Acrylic hydrogels are used in many biomedical fields such as drug delivery, intraocular and contact lenses, bone cement for orthopedics, dressings, and implants for regenerative medicine [37]. Another possible application is for removing pollutants from wastewater through possible interactions between the polymer functional groups and the pollutant. It is well known that one of the big challenges of the environment is the pollution of water by dyes, and various studies have shown that hydrogels represent a promising way to solve this problem [38]. Among the acrylic hydrogels, those based on methacrylic acid (MAA) have an important share due to their remarkable properties, such as pH-sensitive character, mucoadhesive characteristics [33], good absorption properties [39], lack of toxicity, etc. [40]. Composite hydrogels based on poly(methacrylic acid) (PMAA) were obtained by reinforcing hydrogels with different types of clays: bentonite [41], Mt [42–44], modified Mt [11], Lap [45,46], kaolin [47], gold nanoparticles [48], pyrogenic silica [17], and carbon nanotubes [49]. By introducing different nanoparticles into the PMAA matrix, properties such as the mechanical and thermal ones, and water absorption have been improved. Potential applications for PMAA-based composite hydrogels reported in the literature include controlled drug release [48,50,51] or treatment of contaminated water [39,44,49]. Regarding the employment of PMAA hydrogels as dye absorbents, there are only two studies on this subject, as far as we know, describing the use of both unreinforced and zeolite-reinforced hydrogels in connection with Yellow 28 dye [39,52].

The present work aims to show how the main properties of the PMAA hydrogels can be engineered by means of several silicon-based fillers, namely Laponite XLS (XLS), Laponite XLG (XLG), montmorillonite (Mt), and two commercial brands of pyrogenic silica differing mainly by the particle size and zeta potential of their 1.5% aqueous dispersion. For this purpose, the effect of these fillers on the structure and swelling, viscoelastic properties, and water decontamination ability of the (nano)composite hydrogels was comparatively studied in correlation with the filler characteristics. The hydrogels obtained were structurally characterized by FT-IR spectroscopy, X-ray diffractometry, thermogravimetric analysis, and electron microscopy (SEM, TEM). The viscoelastic properties were studied for both as-prepared and after equilibrium swelling hydrogels, while their swelling capacity was analyzed in aqueous media with various pHs, as a function of the nature of the reinforcing agent. Finally, the absorption properties of two cationic dyes, namely methylene blue and crystal violet, which are often found in the wastewater from several industries, were studied. To the best of our knowledge, this is the first report of such a comparative study of the influence of various reinforcing nano-agents, in particular clays and pyrogenic silica, in correlation with their characteristics, on the properties of some pH-sensitive PMAA-crosslinked hydrogels. This study also comparatively shows, for the first time, how the cationic dye absorption ability of a composite hydrogel depends on the reinforcing agent used. It should also be mentioned that the nanocomposite PMAA hydrogels reinforced with pyrogenic silica have already been reported in a paper that analyzed only the physical interactions occurring in the nanocomposite hydrogel [17]. Unlike that paper, the present work provides, for the first time, the extended characterization of these hydrogels, both structurally and in terms of rheological and swelling properties. The results presented in this paper may be useful in selecting an appropriate filler in order to adjust the properties of a PMAA hydrogel in agreement with the intended application. Previously, Zhang and Wang [Zhang, J; Wang, A. Study on superabsorbent composites. IX: Synthesis, characterization and swelling behaviours of polyacrylamide/clay composites based on various clays. React. Func. Polym. 2007, 67, 737–745] compared the effect of five clays (attapulgite, kaolinite, mica, vermiculite and Na+ -montmorillonite) upon the thermal stability and swelling properties of some superabsorbent polyacrylamide hydrogels.

### **2. Results and Discussion**

The studied hydrogels were obtained by the radical copolymerization of MAA with N,N'-methylenebisacrylamide (BIS) in aqueous solution, in the presence of ammonium persulfate (APS) as the initiator, by using different reinforcing agents (XLG, XLS, and Mt clays and two types of pyrogenic silica nanoparticles—HDK and FS—differing mainly by the particle size and zeta potential of their 1.5% aqueous dispersion). The characteristics of both reinforcing agents in water and their aqueous dispersions, determined by us, are displayed in Table 1. According to the measured zeta potential (Table 1), the aqueous dispersions of these fillers are stable over time and allow the acquisition of hydrogels with a uniform distribution of the reinforcing agent. Measurement of their particle size in 1.5 wt% aqueous

dispersion showed that only XLG and XLS were exfoliated and dispersed as individual sheets. Mt was in the form of particles composed of non-exfoliated stacked clay layers, possibly mixed with individual sheets, while FS and HDK appeared as agglomerations of silica nanospheres (Table 1). In addition, the measurement of the pH of the filler aqueous dispersion indicated the presence of ionized basic functional groups in the case of clays, while the Si-OH functional groups on the surface of the pyrogenic silica particles were shown to be practically non-ionized in DI (pH ≈ 5.4).

.

**Table 1.** Characteristics of the investigated fillers and their aqueous dispersions 1

1 Concentration of the aqueous dispersion = 1.5 wt%; room temperature; dispersing time in deionized water = 24 h.

The control hydrogel was prepared from an aqueous solution containing 15 wt% MAA and 2 mol% BIS based on MAA, plus the initiator (APS, 1 mol% to MAA), whereas in the case of (nano)composite hydrogels, reinforcing agents in a proportion of 1.5 wt% to the whole reaction mass were added. Hydrogels are indicated by an "H" followed by the abbreviation for the reinforcing agent. For example, "H" indicates the control hydrogel without reinforcing agent, while "HXLG" and "HHDK" stand for the hydrogels having Laponite XLG and HDK N20 pyrogenic silica as reinforcing agents, respectively.

The hydrogels were structurally characterized, and their viscoelastic and swelling properties were also investigated. The viscoelastic properties were investigated both in as-prepared state, when the composition of the hydrogels was the same in all cases, as well as after swelling at equilibrium. In addition, for a correct interpretation of the results, the water absorption/swelling degree was determined after the purification of the hydrogels and calculated only in relation to the amount of polymer in the hydrogel, excluding the mass of reinforcing agent incorporated in the case of composite hydrogels (Equation (6)). Because both the viscoelastic properties of hydrogels and their swelling degree depend on monomer conversion, this was determined in each case (Equation (1)). The results showed very high conversions (92–95%), which allowed comparison of the rheological and swelling measurements in all cases.

### *2.1. Hydrogel Structure*

The structure of the synthesized hydrogels was studied by FT-IR spectroscopy, X-ray diffraction (XRD), electron microscopy (SEM, TEM), and thermogravimetric analysis (TGA) measurements. FT-IR spectroscopy was used to identify the interactions between the polymer matrix and each reinforcing agent, and also to observe the influence of the pH of the swelling aqueous medium upon the hydrogel structure. By comparing the FT-IR spectra of the (nano)composite hydrogels swelled in deionized water (pH 5.4) with both the control hydrogel and corresponding reinforcing agent (Figure S2), a strong solid-state interaction between the polymeric matrix and the inorganic agent was revealed by the shift of the Si-O band, characteristic to both clays and pyrogenic silica, to higher wavenumber values. The Si-O band was present in the spectrum of xerogels as a shoulder more (HFS, HDK, Figure S2d,e) or less (HXLG, HXLS, Figure S2a,b) pronounced or as a well-defined peak (HMt, Figure S2c).

Due to the pH-sensitive nature of PMAA hydrogels, their contact with aqueous solutions with different pHs causes changes of the contained functional groups, leading to different swelling behavior. To observe these changes, the hydrogels were analyzed by FT-IR after swelling at pH 1.2, 5.4, 7.4, and 9.5; drying; and grinding (Figure 1). The

FT-IR spectra of the hydrogels swelled at pH 7.4 and 9.5 displayed a band characteristic of the carboxylate group at 1537 cm−1 , simultaneously with the decrease of the COOH band from 1650 cm−1 . This may be explained by the basic character of the swelling medium [53], whose pH was appreciably higher than the pKa value of the MAA units in the hydrogel. No notable differences were seen between the spectra at pH 7.4 and 9.5, although the hydrogels behaved differently at these pH values from a swelling point of view (see below). The decrease of pH to 1.2 led to the conversion of all hydrogel groups to COOH, and therefore, to the disappearance of the COO− band in all spectra. At pH 5.4, which is the pH of the deionized water (DI) in which the hydrogels were synthesized and purified, different situations were encountered in the case of various nanocomposite hydrogels. carboxylate group at 1537 cm−1, simultaneously with the decrease of the COOH band from 1650 cm−1. This may be explained by the basic character of the swelling medium [53], whose pH was appreciably higher than the pKa value of the MAA units in the hydrogel. No notable differences were seen between the spectra at pH 7.4 and 9.5, although the hydrogels behaved differently at these pH values from a swelling point of view (see below). The decrease of pH to 1.2 led to the conversion of all hydrogel groups to COOH, and therefore, to the disappearance of the COO− band in all spectra. At pH 5.4, which is the pH of the deionized water (DI) in which the hydrogels were synthesized and purified, different situations were encountered in the case of various nanocomposite hydrogels.

 Due to the pH-sensitive nature of PMAA hydrogels, their contact with aqueous solutions with different pHs causes changes of the contained functional groups, leading to different swelling behavior. To observe these changes, the hydrogels were analyzed by FT-IR after swelling at pH 1.2, 5.4, 7.4, and 9.5; drying; and grinding (Figure 1). The FT-IR spectra of the hydrogels swelled at pH 7.4 and 9.5 displayed a band characteristic of the

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**Figure 1.** FT-IR spectra of (nano)composite hydrogels swelled at different pH values. (**a**) H; (**b**) HXLG; (**c**) HXLS; (**d**) HMt; (**e**) HHDK; (**f**) HFS. **Figure 1.** FT-IR spectra of (nano)composite hydrogels swelled at different pH values. (**a**) H; (**b**) HXLG; (**c**) HXLS; (**d**) HMt; (**e**) HHDK; (**f**) HFS.

Thus, the FT-IR spectra of HXLG and HXLS (Figure 1b,c) displayed the COO− group characteristic band, but of a much lower intensity than at pH 7.4/9.5, which can be explained by the basic groups contained by Lap clays, as proven by the basic pH of their aqueous dispersions (Table 1). They reacted with MAA in the hydrogel synthesis step to form COO− groups, which were then preserved in the hydrogel structure after the purification step [45]. Very interesting is the fact that, although the aqueous dispersion of Mt also had a basic pH, practically identical to that of Lap, the FT-IR spectrum of HMt swelled to pH 5.4 did not show the COO− characteristic band (Figure 1d). This may be explained by the smaller number of COO− groups in the hydrogel formed in the synthesis stage as compared to the Lap-reinforced hydrogels. The smaller number of carboxylate groups was probably due to the much weaker exfoliated structure of Mt in water, leading to a lower contact with the monomer. The COO− group band was not observed at pH 5.4 in the case of HHDK and HFS, due to the lack of basic character of the pyrogenic silica aqueous dispersions (Table 1). As it will be shown below, some of the COOH groups were ionized at pH 5.4 because of the lower pKa of the MAA units, but their concentration was probably too small to become visible in the FT-IR spectrum of H hydrogel (Figure 1a). The Thus, the FT-IR spectra of HXLG and HXLS (Figure 1b,c) displayed the COO− group characteristic band, but of a much lower intensity than at pH 7.4/9.5, which can be explained by the basic groups contained by Lap clays, as proven by the basic pH of their aqueous dispersions (Table 1). They reacted with MAA in the hydrogel synthesis step to form COO− groups, which were then preserved in the hydrogel structure after the purification step [45]. Very interesting is the fact that, although the aqueous dispersion of Mt also had a basic pH, practically identical to that of Lap, the FT-IR spectrum of HMt swelled to pH 5.4 did not show the COO− characteristic band (Figure 1d). This may be explained by the smaller number of COO− groups in the hydrogel formed in the synthesis stage as compared to the Lap-reinforced hydrogels. The smaller number of carboxylate groups was probably due to the much weaker exfoliated structure of Mt in water, leading to a lower contact with the monomer. The COO− group band was not observed at pH 5.4 in the case of HHDK and HFS, due to the lack of basic character of the pyrogenic silica aqueous dispersions (Table 1). As it will be shown below, some of the COOH groups were ionized at pH 5.4 because of the lower pKa of the MAA units, but their concentration was probably too small to become visible in the FT-IR spectrum of H hydrogel (Figure 1a). The presence of COO− groups in the hydrogels, as indicated by the FT-IR spectra, correlated very well with the swelling experiments that will be presented later in Section 2.2.

The XRD analysis revealed the complete disappearance of the clay characteristic peaks in the case of Lap-reinforced hydrogels (Figure 2a,b), proving that exfoliated nanocomposite hydrogels were obtained [54]. In the case of Mt-reinforced xerogels (Figure 2c), the Mt characteristic reflection at 2θ = 6.6◦ was present, indicating the formation of intercalated composite hydrogels [42]. Pyrogenic silica-reinforced hydrogels were also characterized by XRD analysis (Figure 2d,e). The X-ray diffraction spectra of the silica nanoparticles showed only a wide band, indicative of the amorphous nature of commercial silica [55]. This band was no longer present in the corresponding hydrogel spectra, suggesting a strong polymer matrix-silica interaction. peaks in the case of Lap-reinforced hydrogels (Figure 2a,b), proving that exfoliated nanocomposite hydrogels were obtained [54]. In the case of Mt-reinforced xerogels (Figure 2c), the Mt characteristic reflection at 2θ = 6.6 ° was present, indicating the formation of intercalated composite hydrogels [42]. Pyrogenic silica-reinforced hydrogels were also characterized by XRD analysis (Figure 2d,e). The X-ray diffraction spectra of the silica nanoparticles showed only a wide band, indicative of the amorphous nature of commercial silica [55]. This band was no longer present in the corresponding hydrogel spectra, suggesting a strong polymer matrix-silica interaction.

presence of COO− groups in the hydrogels, as indicated by the FT-IR spectra, correlated

The XRD analysis revealed the complete disappearance of the clay characteristic

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**Figure 2.** XRD spectra of both hydrogels investigated and reinforcing agents used. (**a**) HXLG; (**b**) HXLS; (**c**) HMt; (**d**) HFS; (**e**) HHDK. **Figure 2.** XRD spectra of both hydrogels investigated and reinforcing agents used. (**a**) HXLG; (**b**) HXLS; (**c**) HMt; (**d**) HFS; (**e**) HHDK.

The TEM analysis of HHDK and HFS xerogels showed that the fillers were dispersed within the xerogel in the form of approximately spherical particles of several tens of nanometers diameter, associated in large groups (Figure 3). In the case of the Mt-reinforced xerogel, the multi-layer clay particles were visible in the polymer matrix, suggesting the The TEM analysis of HHDK and HFS xerogels showed that the fillers were dispersed within the xerogel in the form of approximately spherical particles of several tens of nanometers diameter, associated in large groups (Figure 3). In the case of the Mt-reinforced xerogel, the multi-layer clay particles were visible in the polymer matrix, suggesting the formation of intercalated nanocomposites, while for the XLS- and XLG-filled xerogels

formation of intercalated nanocomposites, while for the XLS- and XLG-filled xerogels the