**8. Conclusion and Recommendations**

We conclude that fringe communities enjoy some benefits and limited access to the forest reserves, but they do not know their tenure rights, user-rights and responsibilities to the reserves. Most front-line staff of FSD are unaware of the userrights of fringe communities which is the reason for denying access of the reserves to community members. Improving collaborative management means changing the perceptions and attitudes of communities and frontline staff of FSD, respectively, and securing communities rights to the reserves.

## **8.1 Recommendations**

To serve the interest of fringe communities and secure their commitment to responsible collaborative management of forest reserves, we recommend the following: The forestry department should educate community members on their tenure, rights and responsibilities to the reserves and involve them in the processes of decision-making. FSD in collaboration with collaborative forest management Unit (CFMU) of the Ghana forestry commission, should improve the capacity of their frontline staff on the rights and responsibilities of communities in CFM so as to avoid the unlawful denial of fringe communities of what rightfully belong to them.

Forest Fringe communities in the Northern region are not enjoying social responsibility benefits and royalties because the forest reserves were gazetted mainly to protect major rivers within the region. Meanwhile the beneficiaries of these rivers are the Ghana Water Company and the Volta River Authority who are making huge financial gains against the restrictions of right to communities. It is thus recommended that Government ensures that the two beneficiary companies give at least 0.5% of their revenue to FC, fringe communities and land owners as their social responsibility contributions. The part given to the FC could be used to develop the forest reserves through plantation development and to facilitate their activities with communities. That of the communities could be used to provide social amenities for them while the part for the land owners will boost their interest and motivate them to support their communities in sustainable management of forest reserves. This will, in the long term, benefit the two companies since the continuous protection of the rivers depends on the sustainable management of these forest reserves; the success of which in turn depends on the continuous support and cooperation of the fringe communities.

**Author details**

Rikiatu Husseini1

**93**

\*, Stephen B. Kendie2 and Patrick Agbesinyale2

*Rights and Responsibilities: The Reality of Forest Fringe Communities in the Northern Region…*

*DOI: http://dx.doi.org/10.5772/intechopen.93550*

1 Department of Forestry and Forest Resources Management, Faculty of Natural Resources and Environment, University for Development Studies, Tamale, Ghana

2 Department of Integrated Development Studies, School for Development Studies,

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to<EMAIL_ADDRESS><EMAIL_ADDRESS>
University of Cape Coast, Cape Coast-Ghana, Ghana

provided the original work is properly cited.

*Rights and Responsibilities: The Reality of Forest Fringe Communities in the Northern Region… DOI: http://dx.doi.org/10.5772/intechopen.93550*


**1. Introduction**

Polyurethanes (PUs) are characterized by excellent properties such as good resistance to abrasion and good oil and atmospheric resistance. Their main applications are very wide, as flexible foam in upholstered furniture and rigid foam in wall insulations, roofs, and appliances; thermoplastic PU resins in medical devices, automotive parts, and footwear industries; and last but not least their uses as coatings, adhesives, sealants, and elastomers (CASE) which are very important, for example, on floors and pipe protection and again in automotive parts.

It is not unusual that PU have to sustain very high temperatures in several uses, specially in applications such as defense [1]. For example, high-temperature resistant adhesives are required in advanced aircraft, space vehicles, missiles, and ground vehicles [2].

Thermal stability describes thermal durability as well as heat resistance. Polymers with higher thermal stability are characterized by higher melting points, softening and thermal decomposition, smaller mass loss during heating at high temperatures, and higher heat deflection temperature under load, without losing their basic properties which determine its functionality. In respect to analytical techniques, differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were traditionally used to evaluate the thermal properties of several types of polyurethane and are still standard analytical techniques that are utilized. Thermal stability requirements can be summarized in the following statements: retention of mechanical properties (melting/softening point), high resistance to chemical attack, and high resistance to breakdown, specially under oxidative conditions.

The following figure introduces the general reactions involved in PU thermal decomposition:

The first reaction is fast. Flammable gases then react much faster with oxygen, producing more heat and small molecule gaseous degradation products (**Figure 1**). Finally, char reacts with oxygen but in a much slower rate, releasing heat but with a lesser rate. The first step of the degradation includes the scission of the urethane bonds to obtain the polyol and the isocyanate groups apart. In the second set reactions, dimerization gives off gaseous carbon dioxide and carbodiimide, and trimerization gives isocyanurates, while reactions with water render aromatic amines and carbon dioxide again. Heat is released in every reaction step, sustaining degradation until eventually a compact char is left.

PUs have unique properties derived from their two-phase microstructure composed of hard and soft segments. Soft segments (SS) are formed by polyols and have low glass transition temperatures, while hard segments (HS) are derived from diisocyanates and chain extenders and possess high glass transition temperature. PU can be considered as a block copolymer with alternating soft and hard segments along the macromolecule chain. The SS originates from the polyol and imparts extensibility to PU. The HS which is composed of urethane and aromatic rings aggregates into microdomains resulting from the hydrogen bonding, and the domains provide physical cross-linking points for materials [3].

Ingredients for manufacturing PU are polyisocyanate, polyester or polyether polyol, and a chain extender like a diol or diamine. The most reactive component is isocyanate due to its -NCO groups. The quality of PU obtained depends on the ratio of -NCO to -OH groups to obtain a good end product with the required properties. Insufficiency as well as an excess of -NCO groups will result in the formation of allophanate or biuret compounds, with different properties. On the other side, urea and isocyanurate linkages displayed higher thermal stability than polyurethanes [4].

Thermal stability of PU has been extensively studied for many decades. As introduced above, three general reactions can occur during the thermal degradation of polyurethane: (i) dissociation to the original polyol and isocyanate; (ii) formation of a primary mine, alkene, and carbon dioxide; and (iii) formation of a secondary amine and carbon dioxide [5]. The tendency for a particular mechanism depends on the chemical nature of the groups, adjacent to the urethane linkage, and the environmental conditions. Polyurethane degradation usually starts with dissociation of the urethane bonds and carbon dioxide and isocyanate evaporation [6]. The general consensus, however, is that decomposition occurs in three steps at the level

**75**

*Thermal Resistance Properties of Polyurethanes and Its Composites*

of the urethane group between 200 and 300°C [7]. The most important factors that determine thermal stability of PU are the nature of starting materials and the

When polyurethanes undergo thermal degradation, some potentially hazardous chemicals are released. These chemicals could not lead to visible warning. When PU is submitted to high temperatures, special health and safety precautions should be put in practice. It was early noticed that at temperatures above 600°C, cyanide is produced from PU decomposition and polyureas, giving off the so-called yellow smokes [8, 9] and emission of other toxic products [10]. The conditions of synthesis (polycondensation) and the nature of the reagents (initial prepolymers and monomers) influence the composition of the volatile compounds and residues arising from decomposition [7]. Health and safety, apart from material performance, is one of the reasons why it is important to establish heat stability ranges for materials

This review is intended to convey a brief compilation of research in the field of thermal resistance of non-foamed PUs and to identify strategies to augment stability to high temperatures of PUs and its composites. It is not focused in other aspects of PU which has been thoroughly covered by many other experts in reviews and books [6, 10–16]. We will concentrate on the effect of structural changes and on the effect of additives on PU thermal resistance. This contribution has in mind that the vast information about thermal properties of PU cannot be summarized in one single review but tries to present main factors that determine thermal resistance of

The first structural factor that greatly influences thermal resistance is phase microstructure; this is the nature, proportion, and segregation of soft and hard segments (SS and HS, respectively). At the same time, microphase is determined by the chemical structure of PU (polyol, isocyanate, and chain extender type), so the effect of phase microstructure on thermal resistance often overlaps with chemical structure. Therefore, individual effects are rather complex to analyze. Thermal degradation is mainly initiated within the HS, which has normally the faster degradation stage. When it comes to SS, as this is composed by macrodiol, which is typically the weakest link in the oxidation of PU elastomers, using macrodiols that have high oxidative stability could give PUs with better thermo-oxidative stability. A lower flexibility in chains of SS domains produced a lower thermal resistance threshold (temperature where 5% sample weight is lost) as a result of lower crystallinity [17]. The structure of the HS has more influence on thermal stability rather than SS structure. Interurethane hydrogen bonding plays a significant role in the thermal stability of segmented PUs, which can be enhanced by a higher degree of phase separation between SS and HS [18]. The higher the concentration of the urethane group, the lower are both the activation energy for thermal decomposition and the

The polyester polyol-based PUs are more stable than the materials obtained with polyether macrodiols [4, 18]. For example, onset decomposition of polyetherpolyurethane in air is about 245°C. This is anticipated to be about 13°C as compared with that in nitrogen atmosphere. Such anticipation suggested that polyesterpolyurethane is more stable thermally than polyether-polyurethane. It also suggested that the different soft segments will influence the thermal stability of PU [20]. Polycarbonate diols, cured with MDI and chain extended with 1,4-butanediol, showed a drop-off in the weight of samples at around 290°C [21]. Krol and Pilch

*DOI: http://dx.doi.org/10.5772/intechopen.87039*

condition of polymer preparation [4].

with such a wide spectrum of utilization as PUs.

**2. Effect of structure on PU thermal resistance**

these important polymeric materials.

thermal stability of the PU [19].

**Figure 1.** *General mechanism of thermal decomposition of PUs.*

#### *Thermal Resistance Properties of Polyurethanes and Its Composites DOI: http://dx.doi.org/10.5772/intechopen.87039*

*Thermosoftening Plastics*

until eventually a compact char is left.

decomposition:

The following figure introduces the general reactions involved in PU thermal

PUs have unique properties derived from their two-phase microstructure composed of hard and soft segments. Soft segments (SS) are formed by polyols and have low glass transition temperatures, while hard segments (HS) are derived from diisocyanates and chain extenders and possess high glass transition temperature. PU can be considered as a block copolymer with alternating soft and hard segments along the macromolecule chain. The SS originates from the polyol and imparts extensibility to PU. The HS which is composed of urethane and aromatic rings aggregates into microdomains resulting from the hydrogen bonding, and the

Ingredients for manufacturing PU are polyisocyanate, polyester or polyether polyol, and a chain extender like a diol or diamine. The most reactive component is isocyanate due to its -NCO groups. The quality of PU obtained depends on the ratio of -NCO to -OH groups to obtain a good end product with the required properties. Insufficiency as well as an excess of -NCO groups will result in the formation of allophanate or biuret compounds, with different properties. On the other side, urea and isocyanurate linkages displayed higher thermal stability than polyurethanes [4]. Thermal stability of PU has been extensively studied for many decades. As introduced above, three general reactions can occur during the thermal degradation of polyurethane: (i) dissociation to the original polyol and isocyanate; (ii) formation of a primary mine, alkene, and carbon dioxide; and (iii) formation of a secondary amine and carbon dioxide [5]. The tendency for a particular mechanism depends on the chemical nature of the groups, adjacent to the urethane linkage, and the environmental conditions. Polyurethane degradation usually starts with dissociation of the urethane bonds and carbon dioxide and isocyanate evaporation [6]. The general consensus, however, is that decomposition occurs in three steps at the level

domains provide physical cross-linking points for materials [3].

The first reaction is fast. Flammable gases then react much faster with oxygen, producing more heat and small molecule gaseous degradation products (**Figure 1**). Finally, char reacts with oxygen but in a much slower rate, releasing heat but with a lesser rate. The first step of the degradation includes the scission of the urethane bonds to obtain the polyol and the isocyanate groups apart. In the second set reactions, dimerization gives off gaseous carbon dioxide and carbodiimide, and trimerization gives isocyanurates, while reactions with water render aromatic amines and carbon dioxide again. Heat is released in every reaction step, sustaining degradation

**74**

**Figure 1.**

*General mechanism of thermal decomposition of PUs.*

of the urethane group between 200 and 300°C [7]. The most important factors that determine thermal stability of PU are the nature of starting materials and the condition of polymer preparation [4].

When polyurethanes undergo thermal degradation, some potentially hazardous chemicals are released. These chemicals could not lead to visible warning. When PU is submitted to high temperatures, special health and safety precautions should be put in practice. It was early noticed that at temperatures above 600°C, cyanide is produced from PU decomposition and polyureas, giving off the so-called yellow smokes [8, 9] and emission of other toxic products [10]. The conditions of synthesis (polycondensation) and the nature of the reagents (initial prepolymers and monomers) influence the composition of the volatile compounds and residues arising from decomposition [7]. Health and safety, apart from material performance, is one of the reasons why it is important to establish heat stability ranges for materials with such a wide spectrum of utilization as PUs.

This review is intended to convey a brief compilation of research in the field of thermal resistance of non-foamed PUs and to identify strategies to augment stability to high temperatures of PUs and its composites. It is not focused in other aspects of PU which has been thoroughly covered by many other experts in reviews and books [6, 10–16]. We will concentrate on the effect of structural changes and on the effect of additives on PU thermal resistance. This contribution has in mind that the vast information about thermal properties of PU cannot be summarized in one single review but tries to present main factors that determine thermal resistance of these important polymeric materials.


*3.1. Surface Morphology and Structural Studies*

The surface morphology of the GAG samples was investigated by SEM. Figure 1 shows the typical SEM images of top view surfaces observed in this work. A single layer GAG-0 sample with a total thickness of oxide of 80 nm and a zero thickness Ag interlayer (Figure 1a) consists of well-defined continuous particles of nearly equal lateral size (~ 40 nm) uniformly covering the smooth substrate. After deposition of 10-nm thick Ag layer on the GZO surface, a well-marked change in morphology appears (Figure 1b). Forming a sufficiently continuous layer of silver makes the surface of the structure even smoother, although some nanovoids on the surface of the GA bilayered structure are still present due to the surface performance of the bottom GZO layer. The observed smoothing of the surface indicates that under the above growth conditions, the process of Ag growth should be described in the following scenario:

**Figure 1.** SEM images GAG-0 (**a**), GA (**b**), and GAG-3 samples (**c**).

In earlier reports [33,34], it was found that the spreading of Ag on the surface of ZnO was better than on SnO2 and In2O3 due to an ameliorated affinity between Ag and ZnO. Nevertheless, it was confirmed that the surface of ZnO was rougher than that of the latter.

As is shown in Figure 1c, the surface morphology of GAG-3 trilayered structure became less smooth but quite compact. The difference between the surfaces of GAG-1 and GAG-3 samples is due to both differences in the nucleation conditions of the top and bottom oxide layers, and the fact that the grain size usually increases with the thickness of the ZnO thin film [35,36].

It should be noted that the SEM studies of other samples did not reveal any noticeable differences in the surface morphology of the samples considering the thickness of the Ag interlayer at a fixed thickness of the GZO, as well as when changing the thickness of the GZO layers in the range of 30–50 nm. Thus, we can reach a conclusion that in our SEM experiment, the top GZO, which was sputtered at room temperature, showed surface features typical of nanocrystalline Ga-doped ZnO thin films, regardless of the Ag interlayer thickness and its morphology. At the same time, additional atomic-force microscopy (AFM) studies (Figure S2 in SM) showed that the surface roughness increases noticeably by introducing an Ag interlayer into GZO. The root mean square (RMS) roughness values of GZO-0 and GZO-1 samples are 0.897 and 1.226 nm, respectively, calculated from the AFM data. A slight decrease in roughness is observed with further increases of both Ag and GZO thicknesses in the trilayer structures.

Figure 2a shows the XRD plots of the GAG multilayered structures with Ag interlayers of different thicknesses. Only four broad peaks were present in the XRD spectra, two of which belong to the (002) ZnO and (004) ZnO reflections, and the other two to the (111) Ag and (222) Ag reflections. The presence of two (002) ZnO and (004) ZnO peaks corresponding to the nanocrystalline hexagonal ZnO wurtzite phase indicates that the GZO has a preferential orientation featuring the c-axis perpendicular to the substrate surface, regardless of the Ag interlayer thickness. The insertion of the Ag interlayer in the middle of the GAG structure does not affect the strongly preferred orientation of GZO toward (001). Additionally, for the GAG structures, the Ag interlayer has highly preferred orientation toward (111). It is often reported that the crystallized ZnO lattice promotes the silver growth along the (111) direction. This might be due to the fact that the (111) plane of a cubic structure has a similar symmetry to that of the (001) plane of ZnO [34]. The inset of Figure 2a shows the XRD spectral region in which the most intense (002) ZnO and (111) Ag peaks are located. The main features of both peaks are given in Table 2.

**Figure 2.** X-ray diffraction (XRD) plots of the prepared multilayer structures: (**a**) with different Ag interlayer thicknesses; (**b**) with different thicknesses for bottom and top GZO layers. The inset of Figure 2a shows the XRD spectral region with the most intense (002) ZnO and (111) Ag peaks.

**Table 2.** XRD data for GAG multilayer structures.

It can be seen that when the Ag interlayer is introduced into the GAG structure, thereby breaking the GZO layer into two equal parts by thickness, a decrease in intensity and some broadening of the (002) ZnO peak takes place. A further increase of the thickness of the Ag interlayer until 12 nm does not affect the crystallinity of the GZO phase, which is correlated with the SEM data results.

At the same time, the intensity of the (111) Ag peak increases and the integral breadth β decreases with a thickening of the Ag interlayer. The peak shifts from 38.21 to 38.25◦, with an increase in the Ag thickness from 6 to 8 nm, after which its position no longer changes.

Estimation of the averaged crystallite size (CS) from (002) ZnO and (111) Ag peak characteristics using the Scherrer equation (CS = 0.9λ/(βcosθ), where λ is the wavelength of CuKα x-rays, β is the peak integral breadth with no instrumental contribution, and θ is the peak Bragg angle) showed that the crystallite size of GZO decreases from 16 to 14 nm when the Ar interlayer is introduced, and the Ag crystallite size increases continuously with the increase of the Ag thickness from 7 to 10 nm.

The change in the ratio of the intensities I of the (111) Ag and (002) ZnO peaks (IAg/IZnO) is in agreement with the deposition regimes for these GAG structures.

Despite the low substrate temperature during the GZO and Ag sputtering process, the GAG multilayer structures consist of both nanocrystalline GZO and Ag layers (Figure S3 of SM). In addition, we can reach the conclusion that the crystallinity of the top GZO layer is independent of the Ag interlayer.

GZO thickness variation in the range of 30–50 nm at a fixed thickness of the Ag interlayer of 10 nm does not affect the preferential orientation of both GZO and Ag layers. Figure 2b shows the XRD plots of the GAG structures as a function of the top and bottom GZO thickness in the 2θ range of 31–41◦. XRD data for (002) ZnO and (111) Ag peaks of GAG-5 and GAG-6 are shown in Table 2 (in order to compare these with GAG-3).

Comparing these samples, the (002) ZnO peak shifts from 34.01 to 34.10◦ with an increase in the GZO thickness from 30 to 40 nm. A further increase of the GZO thickness does not change the peak position at 34.10. It can also be observed that as the thickness of GZO film increases, the intensity *I* is enhanced and the integral breadth β decreases for the (002) ZnO diffraction peak, indicating that the increased thickness of top and bottom layers improves the crystallinity of the GZO phase.

As for the (111) Ag peak, its features for this sample group were practically independent of the thickness of the oxide layers, which implies the invariability of the crystallinity of the Ag interlayer with the increase of the GZO thickness. By increasing the GZO thickness, the averaged crystallite size for GZO increases continuously from 12 to 15 nm, while the Ag crystallite size remains unchanged and remains in good agreement with the thickness of the Ag interlayer.

Thus, we can conclude that the crystallinity of the Ag interlayer is insensitive to changes in the thickness of the bottom GZO layer for the low substrate temperature sputtering process. In this case, the presence of this GZO layer itself is important as a seed layer for Ag. Changes in the nature of coalescence of Ag nuclei were observed in the presence of seed layers with a thickness of only a few nanometers [37,38].


**4. Long afterglow phosphor-assisted photocatalysts**

Long afterglow phosphor-assisted novel photocatalysts become promising functional mate‐ rials due to their effective utilization of solar light in practical applications of environmental purification. In this section, the recent development on TiO2-based and Ag3PO4-based fluo‐ rescence photocatalyst composites with full-time active photocatalytic properties is reviewed.

The long afterglow phosphor (CaAl2O4:(Eu, Nd)) has a high luminescent brightness around 440 nm of wavelength, long afterglow time, good chemical stability, and low toxicity. The luminescent brightness around 440 nm can excite the visible-light-responsive nitrogen-doped titania (TiO2–xNy). Therefore, TiO2–xNy photocatalyst was expected to possess a novel photo‐ catalytic property after coupling with CaAl2O4:(Eu, Nd).

In our research [147–149], the degradation of continuous NO gas was achieved by TiO2–xNy surface-immobilized CaAl2O4:(Eu, Nd) microparticles (Fig. 9). The results show a persistent deNO*x* ability of the CaAl2O4:(Eu, Nd)/TiO2−xNy composite. In another work [149], the persistent fluorescence-assisted photocatalysts of Ag3PO4 composites have been synthesized to expand the application of Ag3PO4 on dye decomposition day and night. Sr4Al14O25:(Eu,Dy)/ Ag3PO4 composites exhibited excellent photocatalytic activities for the Rhodamine B (RhB) decomposition reaction in the dark without additional light sources (Fig. 9c).

**Figure 9.** (a) Composition and (b) the mechanism of the persistent fluorescence-assisted photocatalysts; (c) variation of RhB solution concentration against illumination time using Sr4Al14O25:(Eu,Dy)/Ag3PO4 composites. Reproduced with permission from ref. 148 © 2013 Elsevier B.V. & ref. 149 © 2013 RSC.


*Article* **Ordered Mesoporous Silica Prepared in Different Solvent Conditions: Application for Cu(II) and Pb(II) Adsorption**

**Ana-Maria Putz 1 , Oleksandr I. Ivankov 2 , Alexander I. Kuklin 2 , Vasyl Ryukhtin 3 , Cătălin Iană¸si 1 , Mihaela Ciopec 4 , Adina Negrea 4 , László Trif 5 , Zsolt Endre Horváth 6 and László Almásy 7,\***

**Abstract:** In this work, the synthesis of ordered mesoporous silica of MCM-41 type was investigated aimed at improving its morphology by varying the synthesis conditions in a one-pot process, employing different temperatures and solvent conditions. 2-methoxyethanol was used as co-solvent to ethanol. The co-solvent ratio and the synthesis temperature were varied. The pore morphology of the materials was characterized by nitrogen porosimetry and small angle neutron scattering (SANS), and the particle morphology by transmission electron microscopy (TEM) and ultra-small angle neutron scattering (USANS). The thermal behavior was investigated by simultaneous thermogravimetrydifferential scanning calorimetry (TG-DSC) measurements. The SANS and N2 sorption results demonstrated that a well-ordered mesoporous structure was obtained at all conditions in the synthesis at room temperature. Addition of methoxyethanol led to an increase of the pore wall thickness. Simultaneously, an increase of methoxyethanol content led to lowering of the mean particle size from 300 to 230 nm, according to the ultra-small angle scattering data. The ordered porosity and high specific surfaces make these materials suitable for applications such as adsorbents in environmental remediation. Batch adsorption measurements of metal ion removal from aqueous solutions of Cu(II) and Pb(II) showed that the materials exhibit dominantly monolayer surface adsorption characteristics. The adsorption capacities were 9.7 mg/g for Cu(II) and 18.8 mg/g for Pb(II) at pH 5, making these materials competitive in performance to various composite materials.

**Keywords:** MCM-41; SANS; USANS; SAXS; Stöber method; 2-methoxyethanol; Langmuir isotherm

### **1. Introduction**

Numerous research has dealt with the problem of obtaining suitable adsorbents in order to remove the hazardous pollutants. Adsorption technology is the most promising and frequently used approach due to its simplicity, high efficiency and low cost [1]. Various kinds of sorbent materials, both natural and synthetic, have been used to remove heavy metal ions from aqueous solution. Low cost industrial and agricultural by-products and waste materials usually have low adsorption efficiency because of their low pore volumes and poor pore structure [2,3]. Novel nanomaterials, including carbon-based nanomaterials,

**Citation:** Putz, A.-M.; Ivankov, O.I.; Kuklin, A.I.; Ryukhtin, V.; Ian˘a¸si, C.; Ciopec, M.; Negrea, A.; Trif, L.; Horváth, Z.E.; Almásy, L. Ordered Mesoporous Silica Prepared in Different Solvent Conditions: Application for Cu(II) and Pb(II) Adsorption. *Gels* **2022**, *8*, 443. https://doi.org/10.3390/ gels8070443

Academic Editors: Francesco Caridi, Giuseppe Paladini and Andrea Fiorati

Received: 23 June 2022 Accepted: 13 July 2022 Published: 15 July 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/).

metal-oxide based nanomaterials, nanoparticles and various nanocomposites have been systematically applied for the removal of heavy metal ions from wastewater [4–11].

Due to their simplicity and abundance, silica and silica-based sorbents obtained from natural bio or waste sources represent an emerging approach in applications for removal of industrial pollution [12–17].

Synthetic silica based mesoporous materials, in spite of their higher costs, have also received wide attention and have been widely used for the adsorption of heavy metal ions due to their exceptionally large specific surface area, regular pore structure and suitability for surface modifications [18–21].

Silica materials with ordered porosity and open pore network, such as MCM-41 and SBA-15, are especially suitable for capturing species from aqueous solutions. In the classical synthesis of MCM-41-type materials, the silica precursors and the surfactant molecules used for pore templates are dissolved in water-ethanol mixtures. Full or partial replacement of ethanol with another co-solvent can modify the reaction speed of the sol-gel process, and change the morphology of the resulting silica gel [22,23]. The more hydrophobic 2-methoxyethanol has been used previously as a co-solvent in sol-gel synthesis of uniform spherical silica nanoparticles [24]. Hydrophobic organic co-solvents can affect the pore size by mingling with the surfactant molecules and acting as spacers inside the micelles [25,26]. For example, it was proven that ethoxyethanol acts not only as solvent but also as cosurfactant, controlling the morphology and pore structure. In alkali-catalyzed reactions, the co-surfactants promote the water solubility of the silica precursor tetraethoxysilane (TEOS), facilitating the hydrolysis reactions [27,28]. In previous works, 2-methoxyethanol was used as a protic polar solvent playing different specific roles: as solvent and complexing agent in the synthesis of mixed silica–titania [29], or as a solvent as well as a stabilizer of alkoxide in the hydrolysis-precipitation reaction, allowing one to control the reactivity of the precursors by adjustment of the quantity of 2-methoxyethanol [30].

In the present study, we applied a simple sol-gel synthesis procedure to obtain sorbent materials suitable, among other applications, for water pollutant removal. Two series of ordered mesoporous silica were synthetized, one at room temperature and the other one at 50 ◦C, and different amounts of 2-methoxyethanol were used in the range of 0:1 to 3:1 relative to the ethanol content. Cetyltrimethylammonium bromide (CTAB) was used as a pore forming agent. The influence of the mixed solvent composition and synthesis temperature on the particle morphology and the pore network was studied by small angle neutron scattering and nitrogen porosimetry.

The applicability of the obtained materials as adsorbents for heavy metals ions from water was studied with the example of Pb(II) and Cu(II).

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

### *2.1. FT-IR Analysis*

All samples (xerogels and thermally treated materials) show the specific vibration bands for the silica skeleton at 1055–1085 cm−1 , 785–805 cm−1 and 450–455 cm−1 (Figure 1). This is assigned to the asymmetric stretching [31,32], symmetric stretching [33,34] and bending rocking mode vibration of the Si-O-Si network [32], respectively. For all calcined samples, the asymmetric stretching vibration bands are shifted to higher values by approximately 20–30 cm−1 . This effect is related to the densification of the silica network [34]. The 1640 cm−1 vibration band belongs to molecular water [35]. The presence of the silanol groups was confirmed by the presence of the band centered about 960 cm−1 , which is associated with the stretching mode of the Si-OH groups [35]. The broad band centered at around 3440–3405 cm−1 corresponds to the O-H stretching bands of hydrogen-bonded water molecules (H-O-H···H) and the Si-O-H stretching of surface silanols hydrogen-bonded to water (-SiOH···H2O) [33]. They become more intense after calcination due to the increased hygroscopicity of the calcined samples, because the removal of the surfactant molecules allows some water to enter the pores [36].

water molecules (H-O-H … H) and the Si-O-H stretching of surface silanols hydrogenbonded to water (-SiOH … H2O) [33]. They become more intense after calcination due to the increased hygroscopicity of the calcined samples, because the removal of the

surfactant molecules allows some water to enter the pores [36].

**Figure 1.** FT-IR spectra of mesoporous silica synthesized at room temperature (**a**) and at 50 °C (**b**), before and after calcination. Vertical lines in panel (**a**) show the characteristic bands of CTAB that disappear after thermal treatment (red); the vibrations related to water (blue) and Si-OH vibrations (dark grey). The vertical lines in panel (**b**) show the characteristic bands of the silica network. **Figure 1.** FT-IR spectra of mesoporous silica synthesized at room temperature (**a**) and at 50 ◦C (**b**), before and after calcination. Vertical lines in panel (**a**) show the characteristic bands of CTAB that disappear after thermal treatment (red); the vibrations related to water (blue) and Si-OH vibrations (dark grey). The vertical lines in panel (**b**) show the characteristic bands of the silica network.

Another group of characteristic vibrations is due to surfactant molecules. In the noncalcined samples, the bands at 2924 and 2855 cm−l are the asymmetric and symmetric stretching vibrations of the alkyl chain, respectively, and the band around 1475 cm−1, is the bending vibration of the C–H groups [35]. These bands completely disappear after calcination for all samples (Figure 1). Another group of characteristic vibrations is due to surfactant molecules. In the noncalcined samples, the bands at 2924 and 2855 cm−l are the asymmetric and symmetric stretching vibrations of the alkyl chain, respectively, and the band around 1475 cm−1 , is the bending vibration of the C–H groups [35]. These bands completely disappear after calcination for all samples (Figure 1).

### *2.2. Thermal Analysis*

*2.2. Thermal Analysis*  Thermogravimetric and scanning calorimetry analyses in synthetic air were performed on the RT series of xerogels not subjected to calcination. The weight loss curves for the four samples are shown in Figure 2a. On the DTG curves (Figure 2b), five distinct weight loss regions can be identified, from which the second and the third steps are overlapped. In the first temperature range between 25 and 150 °C, physisorbed water evaporates, which is accompanied by a small and broad endotherm (Figure 2c). Between 150 and 350 °C, the surfactant decomposes and burns out. The two overlapped peaks on the DTG curve can be attributed to the separate oxidation and decomposition of the tetramethyl ammonium headgroups and the long alkyl chains of the CTAB. The oxidative burning of the organic matter is accompanied by a sharp and large exothermic signal on the heat flow curves around 340 °C. Between 400 and 700 °C, the small weight loss, accompanied by a small and broad exotherm, can be attributed to the complete oxidation Thermogravimetric and scanning calorimetry analyses in synthetic air were performed on the RT series of xerogels not subjected to calcination. The weight loss curves for the four samples are shown in Figure 2a. On the DTG curves (Figure 2b), five distinct weight loss regions can be identified, from which the second and the third steps are overlapped. In the first temperature range between 25 and 150 ◦C, physisorbed water evaporates, which is accompanied by a small and broad endotherm (Figure 2c). Between 150 and 350 ◦C, the surfactant decomposes and burns out. The two overlapped peaks on the DTG curve can be attributed to the separate oxidation and decomposition of the tetramethyl ammonium headgroups and the long alkyl chains of the CTAB. The oxidative burning of the organic matter is accompanied by a sharp and large exothermic signal on the heat flow curves around 340 ◦C. Between 400 and 700 ◦C, the small weight loss, accompanied by a small and broad exotherm, can be attributed to the complete oxidation and elimination of the decomposed organic compounds from the pores.

and elimination of the decomposed organic compounds from the pores. The spike-like exothermic peaks in the DSC curves at around 720–740 °C indicate The spike-like exothermic peaks in the DSC curves at around 720–740 ◦C indicate phase transformations in the silica matrix.

phase transformations in the silica matrix. In their thermal behavior, no systematic differences between the four samples were observed, which can be explained by the small differences in the morphology, and indicate that not much of methoxyethanol remained entrapped in the pores after the washing procedure during synthesis. The total weight loss of about 50% in the whole temperature range corresponds to the typical content of CTAB surfactant in the MCM-41 type materials [36].

**Figure 2.** TG (**a**), DTG (**b**) and DSC (**c**) curves for samples prepared at room temperature.

#### **Figure 2.** TG (**a**), DTG (**b**) and DSC (**c**) curves for samples prepared at room temperature. *2.3. Nitrogen Porosimetry*

In their thermal behavior, no systematic differences between the four samples were observed, which can be explained by the small differences in the morphology, and indicate that not much of methoxyethanol remained entrapped in the pores after the washing procedure during synthesis. The total weight loss of about 50% in the whole temperature range corresponds to the typical content of CTAB surfactant in the MCM-41 The nitrogen physisorption isotherms for all materials are shown in Figure 3. The materials present type IVb isotherm according to IUPAC classification. Type IVb is specific for mesoporous materials having conical or cylindrical pores, closed at one end [37,38]. The narrow hysteresis in the interval 0.2–0.4 P/P0 suggests the presence of some wide pores that could have more access to the external surface [38]. *Gels* **2022**, *8*, x FOR PEER REVIEW 5 of 18

**Figure 3.** N2 adsorption-desorption isotherms of calcined samples prepared at room temperature (**a**) **Figure 3.** N2 adsorption-desorption isotherms of calcined samples prepared at room temperature (**a**) and at 50 ◦C (**b**).

and at 50 °C (**b**). The pore size distribution (Figure 4) is narrow and centered around 3.5 nm for all samples. The BJH mean pore size shows a small increase from 3.0 to 3.3 nm, with the increase of methoxyethanol concentration in the solvent. The total pore volume does not change appreciably, except for the samples prepared with the highest methoxyethanol content at 50 °C when it drops by about 40%. This is related to the partial collapse of the The pore size distribution (Figure 4) is narrow and centered around 3.5 nm for all samples. The BJH mean pore size shows a small increase from 3.0 to 3.3 nm, with the increase of methoxyethanol concentration in the solvent. The total pore volume does not change appreciably, except for the samples prepared with the highest methoxyethanol content at 50 ◦C when it drops by about 40%. This is related to the partial collapse of the pores, as shown later by the SANS results.

**Figure 4.** Pore size distribution of samples synthetized at room temperature (**a**) and at 50 °C (**b**).

0.0

0.4

0.8

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b

The different texture of the materials was also observed in the surface fractal dimensions evaluated by Frenkel-Halsey-Hill (FHH) method [39]. The FHH model is used

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 MeOE-0%-50C MeOE-25%-50C MeOE-50%-50C MeOE-75%-50C

pore width [nm]

 MeOE-25%-RT MeOE-50%-RT MeOE-75%-RT

pores, as shown later by the SANS results.

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pore width [nm]

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 MeOE-0%-RT MeOE-25%-RT MeOE-50%-RT MeOE-75%-RT

100

**Figure 3.** N2 adsorption-desorption isotherms of calcined samples prepared at room temperature (**a**)

The pore size distribution (Figure 4) is narrow and centered around 3.5 nm for all samples. The BJH mean pore size shows a small increase from 3.0 to 3.3 nm, with the increase of methoxyethanol concentration in the solvent. The total pore volume does not change appreciably, except for the samples prepared with the highest methoxyethanol content at 50 °C when it drops by about 40%. This is related to the partial collapse of the

200

300

400

500

600

700

b

P/P0

pores, as shown later by the SANS results.

100

and at 50 °C (**b**).

200

300

400

volume adsorbed [cm3/g]

500

600

700

a

**Figure 4.** Pore size distribution of samples synthetized at room temperature (**a**) and at 50 ◦C (**b**).

0.0 0.2 0.4 0.6 0.8 1.0

 MeOE-0%-50C MeOE-25%-50C MeOE-50%-50C MeOE-75%-50C

P/P0

**Figure 4.** Pore size distribution of samples synthetized at room temperature (**a**) and at 50 °C (**b**). The different texture of the materials was also observed in the surface fractal dimensions evaluated by Frenkel-Halsey-Hill (FHH) method [39]. The FHH model is used The different texture of the materials was also observed in the surface fractal dimensions evaluated by Frenkel-Halsey-Hill (FHH) method [39]. The FHH model is used to determine the fractal geometry and calculate surface structure and irregularities. Value of Df 2 indicates smooth surface and 3 is characteristic for a porous surface [39]. The FHH data (Table 1) indicate that using methoxyethanol as co-solvent a smoother surface is obtained compared to the materials prepared only with ethanol.

**Table 1.** Textural parameters of the calcined MCM-41 samples.

Materials synthetized at 50 ◦C present almost the same characteristics. The isotherms are of type IVb with a narrow hysteresis (Figure 3b). The surface area decreases with increasing methoxyethanol content. For this series, the pore size distribution is somewhat broader, especially for the highest methoxyethanol content (Figure 4b).

All textural parameters for surface area, total pore volume and mean pore diameter are shown in Table 1. The highest BET surface area of 1793 m2/g was obtained for sample synthesized at room temperature, and all samples had surface areas above 1000 m2/g. With increasing 2-methoxyethanol content in the solvent mixture, the surface area decreased in both series. The changes in the surface area and roughness can be related to the lowering of the speed of hydrolysis and condensation with addition of methoxyethanol having hydrophobic methyl and methylene groups.

### *2.4. Electron Microscopy*

TEM micrographs of the two investigated samples showed spherical and slightly elongated particles with sizes of 200–400 nm, typical for silica obtained in the Stöber synthesis process (Figure 5). Well-developed parallel channels with hexagonal ordering

confirmed the structure of the MCM-41 materials. There were no noticeable differences between the samples MeOE-0%-RT and MeOE-75%-RT, in the selected areas of the TEM images. *Gels* **2022**, *8*, x FOR PEER REVIEW 7 of 18

**Figure 5.** Characteristic TEM images of mesoporous silica prepared at room temperature without 2 methoxyethanol (**a**) and in solvent with 3:1 methoxyethanol/ethanol ratio (**b**). **Figure 5.** Characteristic TEM images of mesoporous silica prepared at room temperature without 2-methoxyethanol (**a**) and in solvent with 3:1 methoxyethanol/ethanol ratio (**b**).

#### *2.5. Small Angle Neutron Scattering 2.5. Small Angle Neutron Scattering*

SANS curves for the two series taken on spectrometer YuMO are shown in Figure 6 a,b. All samples show the typical organized structure of MCM-41 type, exhibiting the (100), (110), (200) reflections of the hexagonal pore structure [40–42]. The (110) and (200) peaks are smeared because of the moderate instrumental resolution related to the relaxed collimation and finite detector ring size, characteristic for small-angle neutron instruments. The sample prepared at 50 °C using 75% methoxyethanol is disordered (Figure 6b) and the diffraction peaks are nearly invisible. No well-ordered structure formed in the synthesis at these conditions, and the xerogel filled with CTAB was weaker and the pore structure was destroyed during calcination. Another characteristic difference between the samples prepared at the two temperatures is the width of the first diffraction peak, which is broader for the 50 °C synthesis, indicating a weaker long-range ordering. For samples prepared at room temperature, a significant difference was seen in the (100) peak position, between the material prepared in pure ethanol solvent, and the three samples prepared in mixed solvents. The peak position for MeOE-0%-RT sample was smaller by 3%, showing larger interpore distance, which indicates a larger wall thickness (Figure 7). This effect can be related to the slower condensation rate of the silica the presence of 2-methoxyethanol as cosolvent. SANS curves for the two series taken on spectrometer YuMO are shown in Figure 6a,b. All samples show the typical organized structure of MCM-41 type, exhibiting the (100), (110), (200) reflections of the hexagonal pore structure [40–42]. The (110) and (200) peaks are smeared because of the moderate instrumental resolution related to the relaxed collimation and finite detector ring size, characteristic for small-angle neutron instruments. The sample prepared at 50 ◦C using 75% methoxyethanol is disordered (Figure 6b) and the diffraction peaks are nearly invisible. No well-ordered structure formed in the synthesis at these conditions, and the xerogel filled with CTAB was weaker and the pore structure was destroyed during calcination. Another characteristic difference between the samples prepared at the two temperatures is the width of the first diffraction peak, which is broader for the 50 ◦C synthesis, indicating a weaker long-range ordering. For samples prepared at room temperature, a significant difference was seen in the (100) peak position, between the material prepared in pure ethanol solvent, and the three samples prepared in mixed solvents. The peak position for MeOE-0%-RT sample was smaller by 3%, showing larger interpore distance, which indicates a larger wall thickness (Figure 7). This effect can be related to the slower condensation rate of the silica the presence of 2-methoxyethanol as cosolvent.

Measurements of the RT sample series in the very low *q* range, performed on the MAUD instrument, allowed us to assess the overall dimensions of the nanoparticles. The scattering curves measured with the medium and low-resolution instrument settings (solid and open symbols) are shown in Figure 8. The scattering data were fitted to a model of polydisperse spherical particles with log-normal size distribution. The resulting mean diameters were in the range of 200–300 nm, which is typical for Stöber synthesis. Interestingly, a well distinguished monotonous change of the particle size in the function of methoxyethanol content was observed: the larger (300 nm) particles were obtained using pure ethanol-water solvent mixture, and the gradual replacement of ethanol by 2-methoxyethanol resulted in smaller (230 nm) particles.

1

]

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**Figure 6.** SANS scattering curves of samples prepared at room temperature (**a**) and at 50 ◦C (**b**).

**Figure 7.** Variation of the first diffraction peak position with the content of 2-methoxyethanol in the solvent mixture.

**Figure 7.** Variation of the first diffraction peak position with the content of 2-methoxyethanol in the

Interestingly, a well distinguished monotonous change of the particle size in the function of methoxyethanol content was observed: the larger (300 nm) particles were obtained

Measurements of the RT sample series in the very low *q* range, performed on the MAUD instrument, allowed us to assess the overall dimensions of the nanoparticles. The scattering curves measured with the medium and low-resolution instrument settings (solid and open symbols) are shown in Figure 8. The scattering data were fitted to a model of polydisperse spherical particles with log-normal size distribution. The resulting mean

**Figure 7.** Variation of the first diffraction peak position with the content of 2-methoxyethanol in the

MAUD instrument, allowed us to assess the overall dimensions of the nanoparticles. The scattering curves measured with the medium and low-resolution instrument settings (solid and open symbols) are shown in Figure 8. The scattering data were fitted to a model

diameters were in the range of 200–300 nm, which is typical for Stöber synthesis. Interestingly, a well distinguished monotonous change of the particle size in the function of methoxyethanol content was observed: the larger (300 nm) particles were obtained

Measurements of the RT sample series in the very low *q* range, performed on the

solvent mixture.

solvent mixture.

2-methoxyethanol resulted in smaller (230 nm) particles.

**Figure 8.** USANS scattering curves of samples prepared at room temperature. Symbols are the measured data, and red solid lines are the fitted model of polydisperse spheres. Data are shifted vertically. In the inset, the mean radius of the particles is shown in the function of 2-methoxyethanol **Figure 8.** USANS scattering curves of samples prepared at room temperature. Symbols are the measured data, and red solid lines are the fitted model of polydisperse spheres. Data are shifted vertically. In the inset, the mean radius of the particles is shown in the function of 2-methoxyethanol content in the sol.

using pure ethanol-water solvent mixture, and the gradual replacement of ethanol by

#### content in the sol. *2.6. Metal Ion Adsorption*

*2.6. Metal Ion Adsorption*  Adsorption isotherms are powerful tools for the analysis of adsorption processes. They establish the relationship between the equilibrium concentration and the amount adsorbed by unit mass of the adsorbent at a constant temperature. Adsorption studies were performed at pH 5 because it was shown previously for these kind of materials that the maximum adsorption capacities for Cu2+ and Pb2+ were observed at pH 5.0 [19]. First, all eight samples were tested for adsorption of Cu(II) and Pb(II), and then the material with the best adsorption capacity for Cu(II), (sample MeOE-75%-RT), and the material with the best adsorption capacity for Pb(II), (sample MeOE-0%-RT) were selected for Adsorption isotherms are powerful tools for the analysis of adsorption processes. They establish the relationship between the equilibrium concentration and the amount adsorbed by unit mass of the adsorbent at a constant temperature. Adsorption studies were performed at pH 5 because it was shown previously for these kind of materials that the maximum adsorption capacities for Cu2+ and Pb2+ were observed at pH 5.0 [19]. First, all eight samples were tested for adsorption of Cu(II) and Pb(II), and then the material with the best adsorption capacity for Cu(II), (sample MeOE-75%-RT), and the material with the best adsorption capacity for Pb(II), (sample MeOE-0%-RT) were selected for construction of the adsorption isotherms. The equilibrium time for adsorption was determined varying the time in the range 15–180 min for a Me(II) ions concentration of 10 mg/L. After 2 h, the adsorption capacity has a constant value. Two hours of contact time was used in constructing the adsorption isotherms.

construction of the adsorption isotherms. The equilibrium time for adsorption was The adsorption capacity *q* (mg/g), was determined using the following equation:

$$q = (\mathbb{C}\_0 - \mathbb{C}\_f) \frac{V}{m} \tag{1}$$

was used in constructing the adsorption isotherms. The adsorption capacity *q* (mg/g), was determined using the following equation: where *C*0 and *Cf* are the initial and final concentrations of metallic ions in solution in the beginning and the end of the adsorption test, respectively (mg/L), *V* is the volume of the solution (L) and *m* is the mass of the adsorbent (g).

*m V q C Cf* ( ) = 0 − (1) The adsorption isotherms of Me(II) ions are presented in Figure 9. The Langmuir, Freundlich and Sips isotherms were used to model experimental data in order to establish the adsorption mechanism and the maximum adsorption capacity [43,44].

the adsorption mechanism and the maximum adsorption capacity [43,44].

where *C*0 and *Cf* are the initial and final concentrations of metallic ions in solution in the beginning and the end of the adsorption test, respectively (mg/L), *V* is the volume of the

The adsorption isotherms of Me(II) ions are presented in Figure 9. The Langmuir, Freundlich and Sips isotherms were used to model experimental data in order to establish

solution (L) and *m* is the mass of the adsorbent (g).

MeOE-75%-RT

follows:

capacity [47].

and *KL* is the Langmuir constant.

**Figure 9.** Adsorption isotherms of (**a**) Cu(II) on MeOE-75%-RT and (**b**) Pb(II) on MeOE-0%-RT. **Figure 9.** Adsorption isotherms of (**a**) Cu(II) on MeOE-75%-RT and (**b**) Pb(II) on MeOE-0%-RT.

The non-linear form of the Langmuir isotherm equation [45] can be expressed as The non-linear form of the Langmuir isotherm equation [45] can be expressed as follows:

$$q\_{\text{e}} = \frac{q\_L \,\text{K}\_L \,\text{C}\_f}{1 + \text{K}\_L \,\text{C}\_f} \tag{2}$$

*L f L L f K C q K C q* + = 1 e (2) where *qe* is the equilibrium absorption capacity (mg/g); *Cf* is the equilibrium ion concentration in solution (mg/L); *qL* is the Langmuir maximum adsorption capacity (mg/g) and *KL* is the Langmuir constant.

where *qe* is the equilibrium absorption capacity (mg/g); *Cf* is the equilibrium ion concentration in solution (mg/L); *qL* is the Langmuir maximum adsorption capacity (mg/g) The Freundlich isotherm is applicable to heterogeneous adsorption surfaces. The non-linear form of the Freundlich isotherm equation [46] is:

$$q\_{\rm e} = \mathcal{K}\_{\rm F} \mathcal{C}\_{f}^{1/n\_{\rm f}} \tag{3}$$

1 f e *n q* = *K F C f* (3) where *KF* and *nf* are the characteristic constants related to the adsorption capacity of the adsorbent and the intensity of adsorption.

where *KF* and *nf* are the characteristic constants related to the adsorption capacity of the adsorbent and the intensity of adsorption. The Sips or Langmuir-Freundlich isotherm (Equation (4)) derives from the Freundlich and Langmuir models. It reduces to a Freundlich isotherm at low adsorbate The Sips or Langmuir-Freundlich isotherm (Equation (4)) derives from the Freundlich and Langmuir models. It reduces to a Freundlich isotherm at low adsorbate concentrations. At high adsorbate concentrations, the Sips isotherm has the characteristics of the Langmuir isotherm and therefore can be used to calculate the monolayer adsorption capacity [47].

$$q\_{\ell} = \frac{q\_{\rm S} \,\mathrm{K}\_{\rm S} \,\mathrm{C}\_{\rm \varepsilon}^{1/n\_{\rm S}}}{1 + \mathrm{K}\_{\rm S} \,\mathrm{C}\_{\rm \varepsilon}^{1/n\_{\rm S}}} \tag{4}$$

S S 1 e 1 e e 1 *n S n S S K C q K C q* + = (4) where *KS* (mg/g) is a constant related to the adsorption capacity of the adsorbent and *ns* is where *KS* (mg/g) is a constant related to the adsorption capacity of the adsorbent and *ns* is the heterogeneity factor. As the surface heterogeneity is higher, the deviation of 1/*ns* value from 1 will be higher. The parameters of Langmuir, Freundlich and Sips isotherms determined using non-linear regression are listed in Table 2.

the heterogeneity factor. As the surface heterogeneity is higher, the deviation of 1/*ns* value from 1 will be higher. The parameters of Langmuir, Freundlich and Sips isotherms **Table 2.** Langmuir, Freundlich and Sips isotherm parameters for Cu(II) ions adsorption on sample MeOE-75%-RT and Pb(II) on MeOE-0%-RT.

non-linear form of the Freundlich isotherm equation [46] is:

/Cu2+ 9.7 7.3 0.022 0.988 1.14 0.41 0.978 0.0026 10.2 0.3 0.990 The adsorption capacity increased with increasing equilibrium concentration of Me(II) ions. The experimental isotherms show the approach of the adsorption capacity to constant value and, correspondingly, the large deviation of the fitted Freundlich model curves

from the data. The maximum adsorption capacity of Cu(II) on sample MeOE-75%-RT for an initial Cu(II) concentration of 250 mg/L is *q*m,exp = 9.7 mg/g, and the maximum adsorption capacity of Pb(II) on sample MeOE-0%-RT is *qm*,exp = 18.8 mg/g for an initial Pb(II) concentration of 500 mg/L.

For both materials, the adsorption is better described by the Sips isotherm. The maximum adsorption capacity obtained by modelling the experimental data with Sips isotherm is *q*s= 10.2 mg Cu(II)/g sorbent, a value very close to the experimental value *q*m,exp = 9.7 mg Cu(II)/g sorbent for sample MeOE-75%-RT and *q*s = 22.2 mg/g, a value very close to the experimental value *q*m,exp = 18.8 mg Pb(II)/g for sample MeOE-0%-RT at the highest salt concentration. Parameter 1/*n* in the Sips model is a measure of the adsorption intensity or surface heterogeneity. For 1/*n* = 1, the partition between the two phases is independent of the concentration. The situation 1/*n* < 1 is the most commonly encountered and corresponds to Langmuir isotherm, while 1/*n* > 1 indicates cooperative adsorption involving strong interactions between the molecules of the adsorbate [48,49]. The value of the heterogeneity factor 1/*ns* was obtained as 0.3 for sample MeOE-75%-RT and 0.6 for sample MeOE-0%-RT, indicating that the heterogeneity of the surface was very low. This suggests that the adsorption mechanism approaches the monolayer adsorption and the adsorption of Me(II) ions onto adsorbent materials was a favourable process.

The maximum adsorption capacities of these materials were compared to some other adsorbents from recent literature (Table 3). Concerning the Pb(II) adsorption, the obtained value of 18.8 mg/g proved that our mesoporous silica has better adsorption properties compared with kaolin, activated carbon and zeolite, but worse than the functionalized mesoporous silica and the composite materials of iron oxide nanoparticles with functionalized porous silica or cellulose (Table 3). Concerning the Cu(II) adsorption, the obtained value of 9.7 mg/g for the adsorption capacity is higher than that of zeolites, similar to magnetite nanoparticles, but worse than the adsorption capacities of other metal oxide nanoparticles or mesoporous silica having different functional groups. Therefore, the functionalization of mesoporous silica materials is necessary to improve their adsorption capacity for metal ions removal. On the other hand, the ease of preparation and low cost of the nonfunctionalized mesoporous silica may compensate for their lower performance in certain applications.

**Table 3.** Comparison of the Me(II) adsorption capacity of the prepared mesoporous silica samples to other adsorbent materials.

### **3. Conclusions**

In this paper we studied morphology evolution of ordered mesoporous silica nanoparticles of MCM-41 type, prepared at different solvent conditions using 2-methoxyethanol as a co-solvent and CTAB as a pore forming molecule, by conventional pin-hole and slitsmeared ultra-small-angle neutron scattering. The use of mixed solvent resulted in larger wall thickness and interpore distance, and the hexagonal mesoporous structure was maintained up to 75/25 methoxyethanol/ethanol ratio. This effect can be related to the slower condensation rate of the silica network in the presence of 2-methoxyethanol as cosolvent. These structural changes were accompanied by a 10% decrease of the surface area, as measured by nitrogen sorption. A weaker long-range ordering was obtained for the samples synthesized at elevated temperature of 50 ◦C. These textural and structural observations show that varying the solvent conditions and temperature allows one to tune and optimize the porosity and intrapore surface of the materials, which is of primary importance for their applications as sorbent materials. In future work the effect of other synthesis conditions should be explored aiming to achieve the highest porosity, high specific surface and surface quality modification by functional groups, for achieving efficient binding of metal and organic pollutants.

Selected samples were tested for adsorption of metal pollutants from aqueous phase. The adsorption equilibrium data of Pb(II) and Cu(II) followed the Sips and the Langmuir isotherms, while the Freundlich model did not fit the experimental data. The modeling showed that the heterogeneity of the adsorbent materials surface was very low, suggesting that the adsorption mechanism approached monolayer adsorption. Comparison of the adsorption performance of the prepared materials to various adsorbents from the literature

revealed that the adsorption capacity of the present mesoporous silica is comparable to some simple materials, but lower than many of the much more expensive composite materials.

This study showed that while the modification of the structure, overall dimension and pore size can be tuned easily within narrow limits by changing the solvent composition, the resulting effect on the adsorption performance is not particularly strong within the investigated synthesis conditions. This is attributed to the relatively small change of the pore size and internal surface by the simple solvent variation method. Replacement of the traditional CTAB with longer or shorter alkyl chain surfactants can efficiently vary the porosity within a certain range. Furthermore, using more eco-friendly biosurfactants may be beneficial in long term, after resolving the most prohibitive factors such as their high costs and low productivity [76].

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

### *4.1. Sample Preparation*

The following chemicals were used: tetraethoxysilane (TEOS), (99%, for analysis, Fluka Chemie, Buchs, Switzerland); hexadecyltrimethyl ammonium bromide (CTAB, Sigma-Aldrich, Darmstadt, Germany); ethanol (Chimopar, Bucharest, Romania); 2-methoxyethanol (99%, for analysis, Sigma-Aldrich); ammonia solution 25% (Adra Chim SRL, Bucharest, Romania).

Ordered mesoporous silica materials were prepared by sol-gel synthesis in alkaline conditions, using TEOS as silica precursor and CTAB as pore forming agent, adopting traditional methods [22,77] for using solvent mixtures. As solvent, ethanol and 2-methoxyethanol were used in different proportions. A quantity of 1 g CTAB was added to 192.49 mL of distilled H2O under stirring. After the solution turned clear, 68 mL of alcohol was added and then 23.2 mL of aqueous ammonia solution (25%) was added, and the samples were stirred for 30 min. Ethanol was used for the first sample from each series and it was progressively substituted with 2-methoxyethanol for the next three samples in each series. After that, 4 mL of TEOS was poured into the solution slowly under stirring. Stirring continued for 3 h. After about 20 h, the solid product was recovered by filtration and washed several times with distilled water with repeated filtrations until the pH of the washing water approached the pH value of the distilled water. The samples were dried at room temperature until the next day, then further dried for several hours at 90 ◦C. CTAB was removed by calcination at 550 ◦C during 6 h, after a heating ramp with 1 ◦C/min. The sample names and synthesis parameters are shown in Table 4.

**Table 4.** Synopsis of the synthesized samples.

### *4.2. Characterization Methods*

FTIR spectra were taken on KBr pellets with a JASCO–FT/IR-4200 apparatus. Samples after drying (the xerogel samples) as well as after calcination were studied. N2 physisorption measurements were done at 77 K using a Quantachrome Nova 1200e apparatus. Prior to the analysis the samples were dried and degassed in vacuum at 80 ◦C for 4 h.

The surface area was determined by the BET (Brunauer–Emmet–Teller) equation in the relative pressure range (P/P0) 0.01–0.25. Pore size distribution was evaluated with BJH (Barrett-Joyner-Halenda) and DFT (Density Functional Theory) methods. Total pore volumes were determined using the relative pressure point closest to 1.

Thermal measurements were performed on a Setaram LabsysEvo (Lyon, France) TG-DSC system, in flowing (90 mL/min) high purity (99.999%) synthetic air (20% O2 in N2) atmosphere. Samples were weighed into 100 µL Al2O3 crucibles (the reference cell was empty) and were heated from 25 ◦C to 800 ◦C with a heating rate of 10 ◦C/min. The obtained data was blank corrected and further processed with the software Calisto Processing, ver. 2.08. The temperature scale and calorimetric sensitivity were calibrated by a multipoint calibration method in which seven different certified reference materials were used to cover the thermal analyzer's entire operating temperature range.

Transmission electron microscopy (TEM) investigation was carried out on a Philips CM20 transmission electron microscope equipped with LaB6 electron gun operating at 200 kV. The samples for TEM were prepared by drop-drying suspensions on holey carbon foil coated copper grids.

Small-angle neutron scattering measurements were performed on the YuMO time-offlight instrument operating at the IBR-2 pulsed reactor source in Dubna, Russia [78,79]. The scattered neutrons were detected using the time-of-flight method by a two-detector set-up with ring wire detectors [80]. Measurements were performed on dry powders at room temperature. Corrections for transmission, background and empty aluminium container scattering were performed using SAS software [81].

Ultrasmall-angle neutron scattering measurements were performed on a double-bent crystal instrument MAUD operating at the thermal channel of the LVR15 10 MW research reactor in Rež, Czech Republic [ ˇ 82].

### *4.3. Metal Ion Adsorption Measurements*

For the metallic ion adsorption experiments, 1000 mg/L metallic ion solution in HNO3 0.5 mol/L (Merck standard solution) was used. All other metallic ion solutions were prepared from this solution with appropriate dilution. The pH of the sample solutions was pH 5 and was adjusted by using NaOH buffer solutions with the concentration in the 0.05–0.2 M range, measured using a Mettler Toledo Seven Compact S210 pH meter. Distilled water was used in all experiments.

Equilibrium isotherms were constructed using 0.1 g adsorbent materials placed in 25 mL Cu(II) solution at different initial concentrations (5–250 mg/L), and 25 mL Pb(II) solution at different initial concentrations (5–500 mg/L), at room temperature (25 ± 1 ◦C) and with a stirring time 2 h, using a thermostated shaker bath Julabo SW23, and shaking speed 200 rpm. The equilibrium time for isotherms adsorption was determined varying the time in the range 15–180 min for a Me(II) ions concentration of 10mg/L. After determination of the residual concentration of the metallic ions and the adsorption capacity, it was found that after 2 h the adsorption capacity has a constant value. After stirring, the samples were separated trough filtration. The residual concentration of the metallic ions in filtrate was analysed by atomic absorption spectrometry using a Varian SpectrAA 280 Fast Sequential Atomic Absorption Spectrometer with an air-acetylene flame.

**Author Contributions:** Conceptualization, A.-M.P. and L.A.; methodology, A.-M.P., L.T., A.N., M.C. and L.A.; investigation, A.-M.P., C.I., L.T., A.N., M.C., O.I.I., A.I.K., V.R. and Z.E.H.; resources, A.-M.P. and O.I.I.; data curation, A.-M.P., A.N., M.C., L.A. and V.R.; writing—original draft preparation, A.-M.P., A.N., M.C. and L.A.; writing—review and editing, A.-M.P. and L.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Experimental data are available from the authors.

**Acknowledgments:** A.-M.P. and L.A. thank the Inter-Academic Exchange Program between Romanian Academy and the Hungarian Academy of Sciences. A.-M.P. and O.I.I. thank the scientific cooperation program between Romania and Joint Institute for Nuclear Research, Dubna, Russia. USANS measurements were carried out at the CANAM infrastructure of NPI CAS Rež with the use ˇ of reactor LVR-15 supported by the project LM2018120. V.R. acknowledges support from the Czech

Academy of Sciences in the frame of the program "Strategie AV21, No. 23". L.A. is grateful to A. Elbakyan for facilitating access to scientific literature.

**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**


*4.4.1 Anti IL-1*

The IL-1 cytokine family in general and IL-1α and IL-1β, specifically have pro-inflammatory effects, which are neutralized by using the IL-1R antagonist. [59]. Several IL-1 mutations (NLRP3 genes) are collectively defined as auto-inflammatory syndromes, which cause the increased secretion of IL-1β. This is associated with a heterogeneous syndrome (NLRP3-AID (consisting of familial cold autoinflammatory syndrome, Muckle–Wells syndrome, and chronic infantile neurological, cutaneous and articular syndrome. The urticarial-like rash is one of most common hallmarks of these syndromes [60, 61]. IL-1 inhibitors, such as canakinumab (monoclonal antibody against of IL-1β), anakinra (recombinant IL-1R antagonist), and rilonacept (IL-1α/β blocker) are very effective in reducing inflammation and the clinical spectrum of these syndromes [62]. It is worth mentioning, that the emerging knowledge regarding the use of IL-1-blocking agents in the treatment of Schnitzler's syndrome, is characterized by the presence of urticarial rash and systemic inflammation [63, 64]. In on-going clinical trials, the effectiveness and safety of RPH-104 (a novel molecule against IL-1β), rilonacept, and canakinumab has been confirmed in Schnitzler syndrome (NCT04213274), acquired cold-contact urticaria (NCT02171416), and CSU (NCT01635127). The results of these trials have not yet been published. In few sporadic reports, anti-IL-1drugs were shown to

be beneficial in CSU patients, who remained resistant to all classical therapies for CSU [65]. A new somatic mutation in NLRP3 was recently reported in two elderly patients with long-standing, refractory CSU associated with fever and increased CRP. Both of these patients improved dramatically following the usage of anakinra. As a result, it is assumed assumed that in patients with refractory urticaria and markers of systemic inflammation (a possible underlying NLRP3-related disorder), anti-IL-1 treatment requires further evaluation [66].

#### *4.4.2 Anti-IL-4/13*

In the process of Th2 differentiation several cytokines are produced. The most important cytokines in this process are interleukin-4 and IL-13 [59]. Dupilumab, a new monoclonal antibody directed against the alpha subunit of IL-4 and IL-13 receptors, was recently approved for the treatment of asthma, nasal polyposis, and atopic dermatitis [67]. Increased levels of IL-4 were recently demonstrated in patients with CSU, thereby suggesting a pathogenic role of both Th1/Th2 responses and raising the option of treating CSU with Dupilumab [68]. A recent case report involving six patients with concomitant atopic dermatitis and CSU who were refractory to high dose of omalizumab (600 mg\4 weeks) documented their successful treated with Dupilumab. In this report, it was postulated that the beneficial therapeutic effect of Dupilumab could be the result of its blocking Th2 inflammatory pathways by inhibiting IL-4 and IL-13, respective [69]. Currently, there are three ongoing, phase II/III clinical studies investigating the efficacy and safety of Dupilumab in CSU (NCT03749135, NCT04180488 (EFC16461-CUPID)) and cholinergic urticaria (NCT03749148) unresponsive to a high dosage of antihistamines and omalizumab.

#### *4.4.3 Anti IL-5*

Eosinophils, are considered to have a pivotal role in the pathogenesis of CSU. Many reports have demonstrated elevated numbers of eosinophils in urticarial lesions when compared with normal skin. Their contribution to CSU pathogenesis is probably achieved through interactions with mast cells, the secretion of histamine and other inflammatory mediators and the activation of the coagulation cascade [70]. The important role of interleukin-5 (IL-5) in eosinophil development and maturation, as well as in increased chemotaxis towards skin urticarial lesions has been well documented [59]. Several monoclonal antibodies were recently approved for the treatment of eosinophil related airway diseases (e.g. asthma, Churg-Strauss syndrome, nasal polyposis etc.) by targeting IL-5 (reslizumab, mepolizumab) or its receptor, IL-5R (benralizumab). These drugs were recently used in three CSU patients who were refractory to classical therapies; two patients responded well and showed a significant improvement with Reslizumab and mepolizumab [71, 72], while the other patient who suffered from symptomatic dermographism (SD) benefited from their treatment with benralizumab [73]. In a recent single-blind, repeated measures study, 12 CSU patients were treated with benralizumab (30 mg subcutaneously) every 4 weeks for 12 weeks following a single dose of a placebo. Among the nine patients who completed the study, five had complete response. Their UAS7 and CU-Q2oL scores improved significantly with benralizumab when compared with the placebo [74]. Gene-expression analysis in patients with CSU following benralizumab treatment demonstrated the normalization of SIGLEC-8 expression and IL-4/5 induced inflammation [75]. Although the results imply that eosinophils play a role in CSU, the exact mechanism of action has not yet been understood. Two clinical trials investigating the efficacy of benralizumab

(NCT03183024) and mepolizumab (NCT03494881) in CSU are still in progress, and their results are not yet available. Regarding benralizumab, a phase IIb study (ARROYO Trial- D3259C00001) is set to start soon.


**Abstract**

Nucleation is one of the processes that involves at the beginning of the certain process like freezing, melting, boiling, condensation and crystallization. This process normally occurs in the industry, where it is involving the thermodynamic phase that involves work, energy and temperature. The nuclei growth happens when the initial phase changes to the other phase. Unfortunately, the detail for the theory of nucleation is not well-known around the people who are working in the industry, even though there are many reports or writings available. Thus, few types of nucleation like homogeneous and heterogeneous nucleation and the other theory of nucleation have been summarized in this chapter.

**Keywords:** homogeneous, heterogeneous, ice crystal