A 3 g/L solution of ZSR-P was added to the Cd2+ solution with different pH values under the conditions of solution Cd2+ concentration of 100 mg/L (actual measured value was 108.39 mg/L by ICP-AES) and temperature of 25◦C, and then it was oscillated for 10 min to perform adsorption. **Figure 7** shows the effect of solution pH on adsorption. From **Figure 7**, it can be seen that the solution pH had a great influence on the adsorption of Cd2+ by ZSR-P. When pH value was less than 6, the adsorption

amount of Cd2+ was less than 10 mg/g, indicating that when pH was low, the adsorption of Cd2+ by ZSR-P was poor. When the pH was higher than 7, the adsorption amount increased greatly, and the maximum value was reached at pH 9, which was 43.1 mg/g. It showed that increasing the pH of the Cd2+ solution can greatly enhance the adsorption of Cd2+ by ZSR-P. The main reason was that Cd2+ was prone to hydrolysis at a higher pH, while ZSR-P has a stronger affinity to Cd2+ hydrolysate than free Cd2+ (Bayat, 2002). Obviously, the pH value of the solution was crucial for the adsorption of Cd2+ by ZSR-P.

## ADSORPTION MECHANISM

#### Analysis of Adsorption Position

**Figure 8** shows the SEM images and the surface distribution of the elements, such as O, Si, Zr, and Cd, of ZSR-P raw material and adsorbed Cd2+ product (ZSR-P-Cd). **Table 4** shows the energy spectrum analysis (EDX) values of each element on the surface. It can be seen that there was no Cd in the ZSR-P raw material while there was a large amount of Cd in ZSR-P-Cd, which was consistent with the test results of the adsorption amount of Cd2+. The distribution area of Cd was consistent with the position of ZSR-P particles in the SEM images, indicating that Cd2+ was uniformly adsorbed on the surface of SiO2 particles in ZSR-P.

**Figure 9** shows the TEM images of ZSR-P and ZSR-P-Cd. It can be clearly seen from **Figure 9A** that ZSR-P is mainly composed of aggregates formed by combining primary particles with sizes of about 20 nm. The small particles adhere to each other, and large aggregates of particles were formed. Due to the small particle size of the primary particles, ZSR-P had a large specific surface area, which was consistent with the test results. The large amount of hydroxyl groups present on the surface may provide a large number of adsorption and reaction binding sites for Cd2+. **Figure 9B** shows the TEM images of ZSR-P-Cd. It can be seen that the aggregated shape of ZSR-P-Cd particles was consistent with that of ZSR-P, indicating that no new substances were produced in the adsorption reaction. It was inferred that Cd2+ was adsorbed on the surface of ZSR-P and binding occurs between their interfaces.

#### Adsorbent Properties Changes of Zeta Potential

The zeta potential is the difference between the shear plane of the solid-liquid relative movement and the potential inside the solution. The magnitude and the positive and negative values are determined by the solid surface properties, the Stern layer of the solid-liquid interface, the properties of the medium, and the concentration and nature of the ions in the solvation layer, which can reflect the difference in surface properties of ZSR-P before and after the adsorption of Cd2+ (Moreno-Castilla

#### TABLE 4 | Elemental analysis of ZSR-P and adsorbed Cd2+ product.

et al., 2000, 2001). **Figure 10** shows the surface zeta potential values of ZSR-P raw materials and adsorbed Cd2+ products (ZSR-P-Cd) at different pH conditions. It can be seen that the zeta potential of the surface before and after the adsorption of Cd2+ by ZSR-P in the water system decreased with the increase in the pH value. When pH was less than 5.92, the zeta potential of the ZSR-P surface was positive, which indicated that the surface charge of ZSR-P was positive, and the counter ion of the Stem layer in the solution was negative. As the pH increased, the H+ ions in the solution gradually decreased, and the OH− ions in the Stern layer and the diffusion layer of the ZSR-P surface gradually increased. The zeta potential of the ZSR-P surface was negative, and the absolute value gradually increased. When ZSR-P adsorbed Cd2+, the point of zero charge of ZSR-P-Cd shifted to the right to pH 6.7, indicating that more cations were adsorbed on the surface of the particles. When the pH was less than 6.7, the surface charge value of ZSR-P-Cd was larger than that of ZSR-P under the same pH condition. When the pH was greater than 6.7, the absolute surface charge value of ZSR-P-Cd was smaller than that of ZSR-P at the same pH.

Using the surface hydroxyl test method in formula (2), the surface hydroxyl groups before and after adsorption of Cd2+ by ZSR-P were 1.39/nm2 and 1.06/nm2 , respectively. Therefore, it can be seen from the changes in surface zeta potential and hydroxyl density that under the same pH condition, the number of hydroxyl groups on the surface of the product after adsorption of Cd2+ were much smaller than that on the surface of ZSR-P. This indicates that when Cd2+ was added to the solution, ZSR-P was surrounded by a large number of cations in the solution, and the large amount of hydroxyl groups on the surface became the main binding site of the adsorption reaction; therefore, Cd2+ ions occupy the surface of the ZSR-P particles and the pores formed by the accumulation of particles.

**Figure 11** is the infrared spectra of ZSR-P and its adsorbed Cd2+ product (ZSR-P-Cd). In the infrared spectrum of ZSR-P, the peak at 3,438 cm−1 was OH stretching of water; 1,637 cm−1 was OH deformation of water; the strong absorption band caused by Si-O stretching appeared at 1,090 cm−1 and 791 cm−1 ; the peak at 970 cm−1 belonged to Si-OH stretching; and the peak at 463 cm−1 was Si-O-Si deformation, reflecting the characteristics of amorphous SiO2 in ZSR-P. The peaks located at 1,637 cm and 3,438 cm were attributed by the abundant hydroxyl groups, which existing on the large specific surface area, absorb the surrounding water molecules. The comparison showed that the above absorption peaks appeared in the spectrum of ZSR-P adsorbed Cd2+ product except for the absorption peak at 970 cm−1 , and the peaks at 1,090 cm−1 were slightly moved to 1,096 cm−1 . It was speculated that the Si-O stretching (Da-Qing et al., 2000) formed on the surface of SiO2 hydrolyzed to form –SiOH (Bogdan et al., 1996) and combined with Cd2+ (d'espinose de la Caillerie et al., 1997). The disappearance of the Si-OH stretching at 970 cm−1 was presumed to be due to the dehydroxylation reaction on the surface of ZSR-P after the adsorption of Cd2+.

#### XPS Analysis

X-ray photoelectron spectroscopy analysis of ZSR-P and ZSR-P-Cd was performed, and the results are shown in **Figure 12**. The results show that the peaks of O, Si, and Zr appeared in the ZSR-P spectrum, reflecting the characteristics of ZSR-P being a component of SiO2 and a small quantity of Zr-containing impurities. However, the peak of Cd3d5/2 appeared in the XPS of ZSR-P-Cd, and its binding energy was 405.72eV, indicating that Cd2+ was adsorbed on the surface of SiO2, which was consistent with the results of SEM. In addition, the binding energies of Si2s and Si2p in the XPS of ZSR-P were 153.64 and 102.83 eV, respectively. While their binding energies in the XPS of ZSR-P-Cd were 154.07 and 103.04 eV, respectively. The larger binding energy value indicated that the chemical environment of Si had changed after the adsorption of Cd2+, which was consistent with

the results of Si-OH on the SiO2 surface and Cd2+ hydrate reactions.

# Adsorption Model

Based on the above analysis of the properties and mechanisms of ZSR-P adsorbing Cd2+, an interaction model reflecting this property, mechanism, and process was established and shown in **Figure 13**.

# CONCLUSIONS

(1) Zirconium-containing silica residue purification is mainly composed of aggregates of amorphous SiO2 nanoparticles. It has an excellent adsorption and removal effect on Cd2+ in water. The pH value of the solution, Cd2+ concentration, and adsorption time have a significant effect on the removal effect. In addition, ZSR-P, at a dosage of 3 mg/L, was added to

# spectra of ZSR-P and ZSR-P-Cd.

an aqueous solution, which had a Cd2+ concentration of 100 mg/L and pH 9, and then the mixture was shaken for 10 min; subsequently, the adsorption reached an equilibrium. The adsorption amount of Cd2+ by ZSR-P is 43.1 mg/g.

# REFERENCES

reaction between Si-OH on the surface of SiO2 and Cd2+ hydroxyl compounds.

## AUTHOR CONTRIBUTIONS

WC, HZ, HD, and SS conceived and designed the experiments. WC, HZ, and YL performed the experiments. WC and SS analyzed the data. HZ, YL, and SS contributed reagents. WC wrote the manuscript. YL retouched the document.

#### ACKNOWLEDGMENTS

The authors are thankful to the School of Materials Science and Technology, China University of Geoscience (Beijing), China for providing laboratory facilities.

ion simultaneously with mixed surfactants. Sep. Purif. Technol. 125, 83–89. doi: 10.1016/j.seppur.2014.01.020

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Chen, Zhang, Liang, Ding and Sun. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Carbon Cloth Supported Nano-Mg(OH)2 for the Enrichment and Recovery of Rare Earth Element Eu(III) From Aqueous Solution

Yinong Li, Chen Tian, Weizhen Liu\*, Si Xu, Yunyun Xu, Rongxin Cui and Zhang Lin

*The Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters (Ministry of Education), School of Environment and Energy, South China University of Technology, Guangzhou, China*

Nano-Mg(OH)2 is attracting great attention as adsorbent for pre-concentration and recovery of rare earth elements (REEs) from low-concentration solution, due to its superior removal efficiency for REEs and environmental friendliness. However, the nanoparticles also cause some severe problems during application, including aggregation, blockage in fixed-bed column, as well as the difficulties in separation and reuse. Herein, in order to avoid the mentioned problems, a carbon cloth (CC) supported nano-Mg(OH)2 (nano-Mg(OH)2@CC) was synthesized by electrodeposition. The X-ray diffraction and scanning electron microscopy analysis demonstrated that the interlaced nano-sheet of Mg(OH)2 grew firmly and uniformly on the surface of carbon cloth fibers. Batch adsorption experiments of Eu(III) indicated that the nano-Mg(OH)2@CC composite maintained the excellent adsorption performance of nano-Mg(OH)2 toward Eu(III). After adsorption, the Eu containing composite was calcined under nitrogen atmosphere. The content of Eu2O3 in the calcined material was as high as 99.66%. Fixed-bed column experiments indicated that no blockage for Mg(OH)2@CC composite was observed during the treatment, while the complete blockage of occurred to nano-Mg(OH)2 at an effluent volume of 240 mL. Moreover, the removal efficiency of Mg(OH)2@CC was still higher than 90% until 4,200 mL of effluent volume. This work provides a promising method for feasible application of nanoadsorbents in fixed-bed process to recycle low-concentration REEs from wastewater.

Keywords: Nano-Mg(OH)2 , carbon cloth, composite, rare earth, recovery

# INTRODUCTION

Recent years, Rare earth elements (REEs) are considered to be irreplaceable critical dopants for advanced materials in high-tech applications (Alonso et al., 2012), such as luminescent (Zhang et al., 2018), catalysts (Li et al., 2017; Lin et al., 2017), permanent magnets (Mudryk et al., 2017), and sensor material (Willa et al., 2017), due to their special metallurgical, optical, and electronic properties (Dutta et al., 2016). With the development of the society and industry, the demand for the REEs is increasing all over the world (Dutta et al., 2016; Tansel, 2017). However, the REEs resource is in a serious shortage due to their low reserves and outputs from the natural minerals.

## Edited by:

*Hongfei Cheng, Chang'an University, China*

#### Reviewed by:

*Xuan Dou, Northwestern University, United States Ping Zhang, Nanchang University, China*

> \*Correspondence: *Weizhen Liu<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Green and Sustainable Chemistry, a section of the journal Frontiers in Chemistry*

> Received: *09 February 2018* Accepted: *30 March 2018* Published: *18 April 2018*

#### Citation:

*Li Y, Tian C, Liu W, Xu S, Xu Y, Cui R and Lin Z (2018) Carbon Cloth Supported Nano-Mg(OH)2 for the Enrichment and Recovery of Rare Earth Element Eu(III) From Aqueous Solution. Front. Chem. 6:118. doi: 10.3389/fchem.2018.00118* Therefore, in order to relieve the environmental burden and ease the potential supply crisis, many secondary resources for recycling REEs are developed, including discarded REEscontaining solid waste (Maroufi et al., 2017; Tansel, 2017) and industrial wastewater (Binnemans et al., 2013; Wilfong et al., 2017), etc. Nevertheless, the REE concentrations in most of the secondary resources are as low as hundreds of ppm (Binnemans et al., 2013, 2015), which makes their enrichment and separation extremely difficult.

In view of the above issues, adsorption is regarded as an effective technology for the pre-concentration and recovery of REEs, owing to its simplicity, easy handling, sludge-free operation, and cost effectiveness (Liu et al., 2017; Qi et al., 2017; Wilfong et al., 2017). Compared with other adsorbents, nano-Mg(OH)2, an environmental-friendly material, exhibits rapid kinetics, and high efficiency (with a maximum adsorption capacity of 1,827 mg/g) toward a typical REE of Tb(III) at extremely low concentrations (Li et al., 2013). However, it should be pointed out that, like other nanoadsorbents, the small size of nano-Mg(OH)2 also caused some issues and difficulties in separation and reuse, including mass transfer and excessive pressure drops when applied in fixed bed or any other flowthrough systems (Zhao et al., 2011). Moreover, the release of nanoparticles into the environment is also a possible risk to ecosystems and human health. These drawbacks strongly hinder the application of nano-Mg(OH)2 in the wastewater treatment (Zhao et al., 2011; Tesh and Scott, 2014).

An effective approach for overcoming the above bottlenecks is to fabricate nano-Mg(OH)2 onto supporting materials of larger size (Zhao et al., 2011; Tesh and Scott, 2014; Chen et al., 2016). The resultant nanocomposite is expected to retain the inherent properties of nano-Mg(OH)2, while the supporting material would provide higher mechanical strength and improve the dispersity of nano-Mg(OH)2 (Chen et al., 2016).  Despite plenty of supporting materials can be alternates for nanomaterials, very few of them were used to support nano-Mg(OH)2. Jia et al. (2014) reported that the nanofibrous membrane of PA6@Mg(OH)2, which was fabricated by electrospinning technique combined with hydrothermal strategy, exhibited preferable removal ability to Cr(VI). Xie et al. (2014) developed a novel modified diatomite adsorbent modified by dispersed magnesium oxide nanoflake for the remediation of eutrophic lakes by removing excess PO4− 4 . Li et al. (2011) synthesized a Mg(OH)2@reduced graphene oxide composite using in-situ chemical deposition method. The composite exhibited excellent adsorption effect to methylene blue. In the above studies, nano-Mg(OH)2 was loaded onto the surface of carriers by coprecipitation or hydrothermal method, which still caused some drawbacks such as complicated operation, poor controllability, and low synthesis efficiency of nano-Mg(OH)2.

Compared with the above methods, electrodeposition is a rapid, simple, and low-cost method without high-temperature treatment to fabricate the nano-composites (Lv et al., 2011; Liu T. et al., 2015). Currently, carbon cloth (CC) has been considered to be an ideal candidate for supporting materials by electrodeposition, owing to the following three advantages: (1) CC has abundant functional group, which can fasten the nanoparticles on its surface; (2) CC has excellent mechanical property, admirable resistance to the acid/alkali and environmental friendliness; (3) CC has superior electrical conductivity, which can control the morphology and the loading amount of nanoparticles during the synthesis. An et al. (2016) fabricated the honeycomb-shaped porous NiCo2O4 on electroetched CC with strong adhesion by electrodeposition. The composites as a bind-free electrode display fast kinetics and superior electrochemical behavior controlled by the surface reaction. Fan et al. (2016) reported a highly flexible electrode synthesized by a facile in-situ electrodeposition of MnO2 and polypyrrole on CC. The composite showed superior electrochemical performance.

In this work, a nanocomposite was synthesized by electrodepositing Mg(OH)2 nanoparticles onto carbon cloth. Adsorption performance of the nano-Mg(OH)2@CC composite to Eu(III), a typical species of REE, was investigated via batch adsorption experiments. The properties of nano-Mg(OH)2@CC nanocomposites before and after adsorption were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and inductively coupled plasma(ICP). Furthermore, fixed-bed adsorption experiments were conducted to assess the enrichment of Eu(III) and the recovery of nano-Mg(OH)2@CC for the treatment of effluent containing the Eu(III).

## EXPERIMENTAL SECTION

#### Reagents and Materials

All solutions were prepared using deionized water. Europium chloride hexahydrate (EuCl3·6H2O) was obtained from Aladdin (China). Magnesium sulfate (MgSO4·7H2O), magnesium nitrate [Mg(NO3)2·6H2O] and sodium hydroxide (NaOH) were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All of the chemicals were analytical grade and used without further purification. Carbon cloth was obtained from PHYCHEMi Co. Ltd. (Taiwan, China).

# Preparation of Adsorbents

Flowerlike nano-Mg(OH)2 was prepared at room temperature (25◦C) by precipitation from NaOH and MgSO4·7H2O solutions according to the method described in our previous study (Liu M. et al., 2015). The fabrication of Mg(OH)2@CC nanocomposite is illustrated in **Scheme 1**. In a typical process, a piece of commercial carbon cloth was washed by acetone, methanol, and isopropanol under sonication for 30 min to remove any contaminants on the surface. After drying in the oven, the piece of carbon cloth was cut into small rectangle piece of

2 ∗1 cm and used as the working electrodes. However, only half of the CC was soaked into the electrolyte to make Mg(OH)2 only grow in an area of 1∗1 cm. The electrodeposition of Mg(OH)2 on CC was conducted in a three-electrode system with a static potential of −1.5 V, soaking in Mg(NO3)2·6H2O aqueous solution of 1 mol/L. The counter and reference electrode utilized in the electrodeposition is platinum (Pt) net electrode and SCE electrode, respectively. After electrodeposition, the CC was rinsed by DI water and dried in a vacuum oven. Generally, the loading content of Mg(OH)2 on CC can be controlled by the electrodeposition time. Therefore, different electrodeposition times were conducted to optimize the preparation condition. Finally, the Mg(OH)2@CC nanocomposite prepared in optimal condition was used in the following study.

#### Batch Adsorption Experiments

A stock solution of Eu(III) at a concentration of 1,000 mg/L was prepared using sodium Eu(III) (EuCl3·6H2O; AR). A series of 500 mL glasses of the solution were added to 200 mL of 10, 40, 70, 100, 130, 160 mg/L Eu(III) solutions and 14 mg Mg(OH)2/CC or 14 mg flowerlike nano-Mg(OH)2. The adsorbent quality used for analyzing the adsorption data of the two adsorbents are both in terms of the quality of magnesium hydroxide. The samples were shaken at 200 rpm at room temperature for 24 h to ensure the adsorption reaching equilibrium. The adsorption data were fitted using Langmuir (Equation 1) and Freundlich model (Equation 2). The Langmuir model expressed by the following equation:

$$\frac{C\_{\varepsilon}}{q\_{\varepsilon}} = \frac{C\_{\varepsilon}}{q\_{m}} + \frac{1}{k\_{L}q\_{m}}\tag{1}$$

Where Ce (mg/L) and qe (mg/g) are the solute concentration and adsorption capacity at equilibrium, respectively, and qm (mg/g) and kL (L/mg) are the maximum monolayer adsorption capacity and the binding energy of adsorption, respectively. The Freundlich adsorption model expressed by the following equation:

$$q\_e = k\_f \, \mathrm{C}\_e^{1/\mathrm{n}} \tag{2}$$

Where kf and n are the Freundlich constants measuring the adsorption capacity and the adsorption intensity, respectively.

Kinetic experiments were conducted by mixing certain amount of Mg(OH)2@CC or flowerlike nano-Mg(OH)2 into a 2,000 mL flask containing 100 mg/L Eu(III) solutions. For milli liter solution was sampled at various time intervals to determine the adsorption kinetics. The kinetic data were fitted by the pseudo-first-order (Equation 3) and pseudo-second-order equation (Equation 4):

$$q\_t = q\_e \left(1 - e^{-k\_l t}\right) \tag{3}$$

$$q\_t = \frac{q\_\text{\\_}^2 K\_2 \text{ t}}{1 + |q\_\text{\\_} K\_2 \text{ t}} \tag{4}$$

Where qe (mg/g) is the adsorption capacity at equilibrium, qt (mg/g) is the adsorption capacity at time t, and k1 (min−1 ) and k2 (g/mg·min) are the rate constants of pseudo-first-order and pseudo-second-order kinetics, respectively. The rate constants k1 and k2 were determined by plotting log(qe-qt) vs. t and t/qt vs. t, respectively.

#### Continuous Effluent System Experiments

Three grams nano-Mg(OH)2 was put into the sealed polytetrafluoroethylene groove with two cores to prevent the loss of nano-Mg(OH)2 (the inner groove size is 200 mm in length, 12 mm in width, 18 mm in height). The initial concentration of Eu(III) is 100 mg/L. The peristaltic pump was used to control the water inlet velocity, while the automatic collector was used for collecting samples at the same interval time. The flow rate is calculated by the volume of per unit time collected by the automatic collector. Figure S1a illustrates the above process in a simplified sequence flow diagram.

The composite loaded 3 g Mg(OH)2 was put into the sealed polytetrafluoroethylene groove with two slots to fix the Mg(OH)2@CC well into the groove (the inner groove size is 200 mm in length, 12 mm in width, 18 mm in height). The initial concentration of Eu(III) is 100 mg/L. The peristaltic pump was used to cycle 200 mL solution containing Eu(III) through the groove for 10 h with constant flow rate. The solution was replaced by 200 mL of fresh Eu(III) solution every 10 h, while each of the 200 mL solution was considered as one cycle to test the treatment effect of the nanocomposite to Eu(III). During processing for 25 circulations, the residual solution of every cycle was collected to measure the Eu(III) concentration. Figure S1b illustrates the above process in a simplified sequence flow diagram.

# Characterization

XRD patterns were collected on a Bruker X-ray powder diffractometer (advance D8) with Cu–Kα radiation. The tube

FIGURE 1 | (a) XRD patterns of carbon cloth and Mg(OH)2@CC. (b) SEM image of the pure carbon cloth (\*2000). (c) SEM image of Mg(OH)2@CC (\*2000), (d) SEM image of Mg(OH)2@CC (\*5000).

FIGURE 2 | (A) Residual Eu(III) concentration in aqueous solution after adsorbed by Mg(OH)2@CC with different dosages. (B,C) Langmuir and Freundlich isotherms for Eu(III) adsorption by Mg(OH)2@CC and nano-Mg(OH)2. (D) Adsorption kinetics of Mg(OH)2@CC and nano-Mg(OH)2 toward Eu(III). (E) Pseudo-first-order kinetic model fitting for Eu(III) adsorption. (F) Pseudo-second-order kinetic model fitting for Eu(III) adsorption.

voltage was 40 kV and the tube current was 40 mA. Diffraction patterns were collected over 2θ = 10◦−80◦ at 1◦ /min. The step size of the scan was 0.02◦ . The morphology of the samples was observed using a JSM-7100F SEM with an Oxford INCA EDS. Eu(III) content in solution was determined by inductively coupled plasma optical emission spectrometry (ICP-OES).

#### RESULTS AND DISCUSSION

#### Mechanism Study

Mechanism of electrodeposition process is shown in **Scheme 1** and Equations (5–7):

$$\text{NO}\_3^- + \text{H}\_2\text{O} + 2e^- = \text{NO}\_2^- + 2\text{OH}^- \qquad E^\theta = 0.01 V \quad \text{(5)}$$

$$2H\_2O + 2e^- = 2OH^- + H\_2 \qquad E^\theta = -0.83V \quad \text{(6)}$$

$$\text{Mg}^{2+} + 2\text{OH}^- = \text{Mg} \,(\text{OH})\_2 \tag{7}$$

Equations (5, 6) are the main electrode reaction equations of synthesizing nano-Mg(OH)2 by electrodeposition. Briefly, when the electrochemical workstation is in running state, the cathode will continuously produce the OH− which combines with Mg2+ in solution, by electrolyzing NO− 3 and H2O. When the concentration product of OH− and Mg2+ reach the solubility product of Mg(OH)2, Equation (7) occurs and the Mg(OH)2 will adhere onto the surface of carbon cloth firmly.

#### Characterization of the Adsorbents

The XRD patterns of the carbon cloth and Mg(OH)2@CC composite are shown in **Figure 1a**, both of which exhibit two strong peaks at 2θ = 26.2 and 44.4◦ . The two peaks are attributed to (002) and (101) planes of the carbon cloth, respectively (Zhang G. et al., 2015). For the Mg(OH)2@CC composite, the characteristic diffraction peaks at 2θ = 18.4, 32.9, 38.0, 50.9, 58.7, 62.2, 68.4, and 72.1◦ (except peaks derived from the carbon cloth) are well indexed to (001), (100), (011), (102), (110), (111), (103), and (201) phases of Mg(OH)2, respectively (JCPDS NO.44-1482). Figure S2 shows the SEM image of Mg(OH)2@CC at different electrodeposition time. It can be found that with too short electrodeposition time, magnesium hydroxide on the surface of carbon cloth is very sparse. While with too long electrodeposition time, the magnesium hydroxide on the surface of carbon cloth will crack. Only with the appropriate electrodeposition time (around 10min), one layer of magnesium hydroxide is interlaced on the surface of the carbon cloth and attached strongly. **Figures 1b–d** show the typical morphologies of pure carbon cloth (∗2000), Mg(OH)2@CC (∗2000), and Mg(OH)2@CC (∗5000), respectively. A low-magnification SEM image (**Figure 1b**) reveals that the fibers of the carbon cloth possess ∼10µm in diameter with a smooth surface. After the synthesis of Mg(OH)2@CC composites via electrodeposition, it is noticed that the interlaced nano-sheets of Mg(OH)2 are firmly and uniformly grown on the fibers with an average diameter of 15µm **(Figures 1c,d)**. The above results collectively demonstrated the successful integration of the Mg(OH)2@CC. In addition, the XRD patterns and SEM image of nano-Mg(OH)2

shown in Figure S3 indicated that the flower-like nano-Mg(OH)2 was successfully synthesized similar to the reported literature.

#### Eu(III) Adsorption Isotherm and Kinetics

To confirm that the carbon cloth carrier in Mg(OH)2@CC has no negative effect on the adsorption performance of nano-Mg(OH)2, the adsorption effect of Mg(OH)2@CC toward Eu(III) was compared with nano-Mg(OH)2. **Figure 2A** shows that with an initial Eu(III) concentration of 100 mg/L (the concentration of REEs in the actual environment), Mg(OH)2@CC could decrease the residual Eu(III) concentration in the solution with increasing adsorbent dosage. When the dosage of adsorbent increased to 0.07 g/L, the removal efficiency can reach 99%. Once exceed 0.07 g/L, the adsorbent will be excessive. Therefore, in the experiments of adsorption kinetics and adsorption thermodynamics, the dosage of adsorbent was 0.07 g/L.

The adsorption isotherm data of nano-Mg(OH)2 and Mg(OH)2@CC to Eu(III) were fitted by the Langmuir and Feriundlich models. The results and related parameters are shown in **Figures 2B,C** and **Table 1**. Obviously, no matter using nano-Mg(OH)2 or Mg(OH)2@CC, the adsorption capacities increase with the concentration of Eu(III), until it reaches equilibrium. Both of Langmuir and Feriundlich models can be used to describe the adsorption of the two adsorbents (R 2 > 0.95), while the R 2 value of Langmuir model is higher than of Feriundlich model. Therefore, Langmuir model is more suitable to describe the adsorption of two adsorbents to Eu(III) and the adsorption type of two materials to Eu(III) is chemical adsorption. The maximum adsorption capacities of nano-Mg(OH)2 and Mg(OH)2@CC calculated by Langmuir model were 1428.571 and 1436.781 mg/g, respectively (**Table 1**). Therefore, the loading Mg(OH)2 on the carbon cloth will not affect the adsorption effect of Mg(OH)2.

The adsorption kinetics data of nano-Mg(OH)2 and Mg(OH)2@CC composites to Eu(III) were fitted by the pseudo-first-order kinetic and the pseudo-second-order

TABLE 1 | Parameters of Langmuir and Freundlich isotherm for Eu(III) on Mg(OH)2 and Mg(OH)2@CC.

TABLE 2 | Kinetic parameters calculated from Pseudo-first order and Pseudo-second order kinetic models.

kinetic model. The results and related parameters are shown in **Figures 2D,E** and **Table 2**. **Figure 2D**. showed that the equilibrium adsorption capacity of the two materials varies with time. When the dosage of adsorbent was 0.07 g/L and the initial concentration Eu(III) was 100 mg/L, both of the two adsorbents could reach equilibrium in 400 min and the equilibrium adsorption capacities were around 1,400 mg/g. Therefore, loading Mg(OH)2 on the carbon cloth will not affect its adsorption rate. The kinetic data of two adsorbents were fitted using the pseudo-first-order and pseudosecond-order models. The fitting results (**Table 2**) show that the kinetics of the two adsorbents were better described with pseudo-first-order model kinetics, which suggested that physical and chemical interactions may simultaneously contribute and control the uptake of Eu(III) onto the surface of Mg(OH)2.

As a consequence, loading Mg(OH)2 on the surface of carbon cloth has no negative effect on its adsorption performance. The main reason is that the electrodeposited Mg(OH)2 is evenly distributed on the surface each carbon fiber. Since the carbon fibers are relatively independent, they have no coverage and interference to the active site of Mg(OH)2, leading to enough active sites exposing for Eu(III) adsorption.

# Material Characterization After Adsorption

To explore the adsorption mechanism, Mg(OH)2@CC was exposed to Eu(III) solution with a high concentration. The morphology, distribution, and crystal phase of Mg(OH)2@CC-Eu(III) were determined by SEM-EDS and XRD. The SEM image showed that after adsorption, the original laminar structure of Mg(OH)2@CC slightly bended, but still retained the interlacing laminar structure (**Figure 3a**). This is due to the ions interaction between Eu(III) and Mg(II) that changes the morphology (Li et al., 2013). The distribution map of corresponding elements (Eu, O, Mg) was recorded by SEM-EDS (**Figures 3b–d**). The thick purple areas in **Figure 3b** show that the Eu is evenly distributed on the carbon fiber surface, while the sparse blue areas shown in **Figure 3d** indicate the decrease of Mg(II) content after adsorption. To confirm the enrichment of Mg(OH)2@CC to Eu(III), the adsorbed powder of Mg(OH)2-Eu was collected from the surface of carbon cloth through mechanical methods and calcinated at 800◦C without oxygen. The XRD patterns of the obtained powder shown in **Figure 4** can be well indexed as Europium Oxide phase (ICCD card no.00-034-0392). The corresponding ICP data are shown in **Table 3**. The content of europium in solid phase is 860,727 mg/kg, while the content of magnesium is only 21.3 mg/kg. According to ICP, SEM-EDS, and XRD data, it can be calculated that 99.66% of the obtained powder after calcination is europium oxide, while there is almost no magnesium oxide. Therefore, the Mg(OH)2@CC can extract Eu(III) from solution to the surface of the nanocomposite, which can realize the enrichment and recovery of REEs.

# Continuous System (Fixed Bed-Column)

To further investigate the practical application of Mg(OH)2@CC, a fixed bed-column experiment was conducted to verify the

TABLE 3 | ICP data of the reacted powder.

removal performance of Eu(III) by Mg(OH)2@CC, taking nano-Mg(OH)2 as comparison. As shown in **Figure 5**, with the same quality of Mg(OH)2 and the initial Eu(III) concentration of 100 mg/L, the adsorption column performance of Mg(OH)2@CC is much better than that of pure nano-Mg(OH)2. It can be explained that the main adsorption mechanism of Mg(OH)2@CC to Eu(III) includes two parts: (1) ion exchange of Eu(III) with magnesium hydroxide; (2) the enhanced dispersion of nano-Mg(OH)2 by the surface function groups of carbon cloth. Under the two mechanisms, the treatment capacity of Mg(OH)2@CC is as high as 4,200 mL (C/C0 is <0.10), while the concentration of effluent Eu(III) decreases to the µg/L level after treatment. Meanwhile, the effluent of Mg(OH)2@CC can be processed at a steady flow rate. The curve of nano-Mg(OH)2 toward Eu(III) uptaken in fixed-bed columns is shown in the bottom left corner of **Figure 5A**, where there is a short black curve. The magnification of the short black curve is shown in **Figure 5B**. The effluent effect of nano-Mg(OH)2 is superior (C/C0 approaches 0.00). However, with the increase of effluent volume, the flow rate also decreases rapidly. When the effluent volume reaches 240 mL, the flow rate is close to zero. A possible explanation is that the agglomeration of the nanoparticles reduces the specific surface area of nano-Mg(OH)2, and thus decreases the active sites for Eu(III) adsorption. Moreover, the agglomeration of nano-Mg(OH)2 leads to the increase of particle size, which further causes blockage in fix-bed column. This problem will severely limit the development of nano-Mg(OH)2 in practical application. Therefore, magnesium hydroxide plays an important role in the

absorption of Eu(III) by Mg(OH)2@CC, while the main effect of carbon cloth is to strengthen the dispersion of nano-Mg(OH)2. The combination of nano-Mg(OH)2 with carbon cloth would further improve the efficiency of nano-Mg(OH)2 and prevent its loss.

## CONCLUSIONS

In summary, a new nanocomposite Mg(OH)2@ CC was prepared by electrodeposition, and used to adsorb Eu(III) in fixed-bed. The morphology and phase analyses revealed that the interlaced Mg(OH)2 nano-sheets are uniformly loaded on the carbon cloth fibers. The maximum adsorption capacity (1436.8 mg/g) and removal rate (95% within 300 min) of Mg(OH)2/CC toward Eu(III) is close to those of nano-Mg(OH)2, indicating that loading Mg(OH)2 on the surface of carbon cloth has no negative effect on its adsorption performance. Moreover, up to 99.66% of Eu2O3 is found in the adsorbed powder, indicating Mg(OH)2@CC could be an ideal candidate for the enrichment and recovery of Eu(III) from aqueous solution. In addition, Mg(OH)2@CC exhibited great potential for Eu(III) removal in the fixed-bed compared with nano-Mg(OH)2. Therefore, the excellent performance of Mg(OH)2@CC suggests that loading nanoparticles on CC via electrodeposition is a promising method to improve the application of nano-adsorption materials.

#### AUTHOR CONTRIBUTIONS

YL: Organized the research problem on carbon cloth supported nano-Mg(OH)2 for the enrichment and recovery of Rare Earth Element Eu(III) from aqueous solution, and participated in writing the manuscript; CT: Synthesized Mg(OH)2@CC and nano-Mg(OH)2; SX: Conducted the batch adsorption experiments and the data fitting; YX: Conducted the characterization of the adsorbents before and after adsorption; RC: conducted the fixed bed experiment of two adsorbents; WL and ZL: Analyzed the mechanism of the carbon cloth on enhancing the adsorption ability of Mg(OH)2@CC toward Eu(III) in fixed bed and participated in writing the manuscript.

### REFERENCES

#### ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (Grant No. 21607045 and 21477129), the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2016ZT06N569), Guangzhou Science and Technology Project (No.<PHONE_NUMBER>89), and the Fundamental Research Funds for the Central Universities (No. 2015ZM157 and 2017PY009).

#### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2018.00118/full#supplementary-material

immobilized alginate beads in aqueous phase. Chemosphere 160, 126–133. doi: 10.1016/j.chemosphere.2016.06.074

synergistic adsorption and mechanism. Environ. Sci. Nano 4, 2134–2143. doi: 10.1039/C7EN00758B

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Li, Tian, Liu, Xu, Xu, Cui and Lin. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Separation of CH4/N2 of Low Concentrations From Coal Bed Gas by Sodium-Modified Clinoptilolite

Xiaofei Hao1,2, Zhen Li 1 \*, Hongjie Hu2 , Xueqin Liu1 and Yanqiu Huang1

1 Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, China, 2 Zhengzhou Fulong Science and Technology of New Materials Co., Ltd., Zhengzhou Institute of Multipurpose Utilization of Mineral Resources, Chinese Academy of Geological Sciences, Zhengzhou, China

Clinoptilolite is a widely distributed tectosilicate, mainly composed of Al2O3, SiO2 with exchangeable cations such as Ca, K, Mg, and Na. In this research, raw clinoptilolite was ground, gravimetrically concentrated and ion-exchanged using different concentrations of NaCl solution. Then the modified clinoptilolite powder was formulated into particles as adsorbents. The adsorbents were applied to CH4 separation in coal bed gas. The raw and modified clinoptilolites were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), atomic emission spectrometer (ICP-AES), Fourier transform infrared spectrometer (FTIR), and Brunauer Emmett Teller (BET) specific surface area. The CH4 absorptivity by raw and modified clinoptilolites was evaluated using pressure swing adsorption (PSA) to assess the CH4 separation ability. The results indicated that the ion-exchanged clinoptilolite using 0.2 mol/L NaCl solution was found to be promising for the kinetic PSA separation of CH4/N2, giving a better absorptivity for CH4 separation under different influence factors. Based on the simulated static experiments, it was indicated that both CH4 and N2 were capable of diffusing into clinoptilolite while N2 adsorption by clinoptilolite was excellent. The experiment results also indicated that ion-exchanged clinoptilolite using a 0.2 mol/L NaCl solution was the optimal adsorbent for separating CH4/N2 at the low pressure condition. From the simulated dynamic experiments, the ion-exchanged clinoptilolite using a 0.2 mol/L NaCl solution as a potential sorbent in kinetic PSA processes for N2/CH4 separation, exhibited the best performance at 648 K under 0.2 MPa within 28 min, in comparison to the raw clinoptilolite and clinoptilolite under other modification conditions. In the next phase of research, the modified clinoptilolite will be tested for CH4 separation in real coal bed gas.

Keywords: clinoptilolite, ion-exchange, nitrogen/methane separation, selectivity, low concentrations methane, pressure swing adsorption

# INTRODUCTION

The coalbed gas is found in coal bed with a main composition of methane (CH4), which was absorbed on the surface of the coal particles. Part of coal bed gas was dissociated or dissolved in the hydrocarbon gas in the coal pore and the water of coal bed, which is automatically stored up in the coal bed as the powerful complement to raw gas. CH4 in the coal bed is a high quality gas fuel. Meanwhile, it is also one of detrimental gases influencing mining underground coal and an

#### Edited by:

Wenbo Wang, Lanzhou Institute of Chemical Physics (CAS), China

#### Reviewed by:

Libing Liao, China University of Geosciences, China Federica Valentini, Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma Tor Vergata, Italy Xiazhang Li, Changzhou University, China

> \*Correspondence: Zhen Li<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Green and Sustainable Chemistry, a section of the journal Frontiers in Chemistry

Received: 29 June 2018 Accepted: 06 December 2018 Published: 18 December 2018

#### Citation:

Hao X, Li Z, Hu H, Liu X and Huang Y (2018) Separation of CH4/N2 of Low Concentrations From Coal Bed Gas by Sodium-Modified Clinoptilolite. Front. Chem. 6:633. doi: 10.3389/fchem.2018.00633

**58**

important harmful source leading to atmospheric greenhouse effect. In China, there is up to 13 billion m3 CH4 under the process of coal mine a year, which accounts for around one third of its emissions globally. On the other hand, the utilization ratio of CH4 in the coal bed gas was only 35%, resulting in a huge CH4 resource loss. We know that the greenhouse effect of CH4 is 21 times to the CO2 and power of CH4 for damaging ozone (O3) is 7 times to the CO2. Thus, recycling coal bed gas is of great significance on both energy development and environmental protection. With improving consciousness of human on the coal mine safety and environmental protection, the exploitation of CH4 in the coal bed has been attached great importance to the world in recent years.

The separation technology of CH4 in the coal bed is not effective, which is one of the main reasons for low the utilization ratio. In the separation process of low concentration of coal bed gas, the physicochemical property of N2 and CH4 was similar (Perry et al., 1999; Johnson III, 2015). It led the recycle and separation technology to be a key common technology challenge. It was also one of the most important technological obstacles on gas development, energy saving and emission reduction (Bomberger et al., 1999; Cavenati et al., 2006; Tagliabue et al., 2009). At present, the common technologies were cryogenic distillation, pressure swing adsorption (PSA), membrane separation, hydration technology and dissolutionabsorption technology. The PSA separation method has become the mainstream technology for the purification of coal bed gas at small and medium scales, due to its advantages of low energy consumption, less investment equipment and high degree of automation (Arya et al., 2014; Yin et al., 2015). Its key challenge is the selection of adsorbents.  The equilibrium adsorption capacity of CH4 is higher than that of N2 for AC. Although the separation coefficient is higher and the effect is better based on results from laboratory studies, it is still far away from industrial application. The main reason is that the preparation process of AC is complex and the cost is relatively high. And it obtains CH4 product in vacuum desorption stage, the subsequent operation needs to be compressed, so the power cost is increased, and the economic effect is not obvious. The separation of CH4 and N2 by CMS is based on the kinetic effect. The diffusion rate of N2 in the micropore is higher than CH4. A large amount of N2 is adsorbed into the pore and CH4 remains outside the pore in a relatively short time. Therefore, the product CH4 is obtained by the adsorption or sequestration of the PSA process, instead of the vacuum step. With the increase of adsorption time, the kinetically the effect becomes weaker, and the equilibrium effect will dominate, making CH4 and N2 separation difficult. Although CMS has achieved good results in the laboratory, it is mainly aimed at the coal bed gas with high concentration of CH4 (CH4 content >70%). However, there are few reports on the study of coal bed gas with low concentrations. The natural clinoptilolite as a kind of PSA adsorbents is of great potential for application with the advantage of acid resistance, heat resistance, alkali resistance, stable structure, rich resources, and low price. They can show both equilibrium and kinetic effects. However, the adsorbent prepared from natural clinoptilolite are of different sodium contents and its application in low concentration coal bed gas (CH4 < 30%) has not been reported.

Here we present a study of the adsorption isotherms of four adsorbents made from natural clinoptilolite with different sodium contents at 298 K. And the corresponding adsorption kinetics were measured at the same and different pressure using the feed gas containing 20% CH4 and 80% N2 at 298 K. This study will provide technical support for the implementation of industrialization.

# MATERIALS AND METHODS

# Materials

#### Clinoptilolite and All Agents

The raw clinoptilolite was collected from the south of the Liaoxi metallogenic belt in China. The adsorbents used were CH4 (99.95%), N2 (99.95%). The purging gas for adsorbent activation/regeneration was He (99.999%, pre-purified). All gases were provided by Praxair. The reagents, including NaCl, used in this research were of analytically pure and bought from Sinopharm Group Chemical Reagent Co., Ltd.

#### Clinoptilolite Pretreatment

The raw clinoptilolite was ground by a ball grinder to a granular size <70µm. Then the milled pulp with clinoptilolite powders was poured into a Falcon centrifuge to remove some heavy impurities. The purified clinoptilolite was dried at 105◦C and stored in a desiccator. It was used as a raw material for the preparation of adsorbents.

#### Preparation of Modified Clinoptilolite

The processed clinoptilolite powders were mixed NaCl solutions at concentrations of 0.1, 0.2, 0.4, and 0.6 mol/L at a solid to liquid ratio of 1:20 for 2.5 h in Erlenmeyer flasks separately and covered with sealing films and maintained in a 90◦C water bath. The mixture was centrifuged to separate the solids, then washed using deionized water until no Cl−. All ion-exchanged clinoptilolite samples were pressed into a round cake and calcined at 200◦C (to dry the samples) for 2 h. And then they were crushed and sieved. Particles of 0.5–1.5 mm sizes were used as adsorbents.

### Separation Experiment With CH4/N2

The gas mixture of CH4 and N2 was prepared by highpure standard gas, and the ratio of CH4/N2 was 20/80%. The experimental device for adsorption was a single absorbing tower filled with raw and modified clinoptilolites (**Figure 1**). At first, the device was pressurized using high-pure standard He until the adsorption pressure was up to the setting pressure. Then the intake valve of He was closed and the intake valve of mixture gas was opened (it was the start time of data recording). In order to keep the pressure of absorbing tower reaching the experimental

(Copyright©1994–1997 SIEMENS, AG).

value, it was adjusted by control valves (the flow value of gas was set to 50 mL/min). The outlet discharge was set using mass flow controller before the test. In the process of absorption, the change of concentration of CH4 was tested and recorded by a gas analyzer. The test was continued until the concentration of CH4 was the same as the initial concentration of CH4 in the mixture. The activation and regeneration of modified clinoptilolite was not begun until inverse vacuum was pumped for 10 min.

#### Pressure/Vacuum Swing Adsorption Experimental Program

A vacuum pressure swing adsorption (VPSA) cycle was devised to be experimented in the pilot-scale unit, which was the purpose of catching CH4 from a dynamical mix of CH4/N2 simulated coal bed gas. The device, single-tower adsorption layer, was shown in **Figure 1**. Before test, the absorbents were modified clinoptilolites which were activated in the vacuum rotation activation furnace for 6 h in 648 K. When the test began, the pressure of He and pressure of adsorption were both set at the certain pressure, and flow value of tower top was 60 mL/min. It needed to be emphasized that vacuum pumping treatment using vacuum pumps was initiated before the experiments.

#### Analytical Methods

The morphologies of purified clinoptilolite and modified clinoptilolite were observed with TEM (Tecnai G2 TF30) and SEM (Hitachi S-4800).

X-ray diffraction (Rigaku MiniFlex600) measurements were proceeded with copper CuKα1 radiation (λ = 1.5406 Å), utilizing a voltage of 40 kV and a current of 15 mA. The divergence slit was 0.3 mm and data was gathered for 2θ scanned from 3◦ to 80◦ at 10◦ /min.

The chemical components of purified clinoptilolite and modified clinoptilolite samples were analyzed using an inductively coupled plasma atomic emission spectroscopy (ICP-AES, ICAP7400 THERMO Fisher). To obtain a more representative chemical composition of a sample, the analysis was done in triplicates per sample, and the element contents were averaged (**Table 1**).

Mid-infrared spectra were recorded using a Fourier transform infrared (FT-IR) spectrometer (Nicolet IS50) with a Smart EnduranceTM single bounce diamond ATR cell. Spectra were acquired of 4,000–400 cm−1 by the average of 64 scans with a resolution ratio of 4 cm−1 . A mirror speed of 0.6 cm/s was used.

The BET surface area was 57.84 ± 0.20 m2 /g measured by an ASAP 2020 instrument (Micromeritics, USA). N2 (at 298 K) and CH4 (at 298 K) adsorption were measured to determine the BET surface area and micropore size distribution.

## RESULTS AND DISCUSSION

#### Characterization of Clinoptilolites

**Figure 2** showed SEM photographs of unmodified and modified clinoptilolites, They didn't change significantly among them. In **Figure 2**, C-0, unmodified clinoptilolite, was purified clinoptilolite with no NaCl treatment. The modified clinoptilolites were depicted by C-1, C-2, C-3, and C-4, which were treated by 0.1, 0.2, 0.4, and 0.6 mol/L NaCl solution, respectively. These crystals are flaggy or schistose, which appears as parallel conjunctive aggregate. Individual crystals span is from hundreds of nanometers to several microns. A number of ordered small particles were also found on the crystals from **Figure 2a**∗ .

In order to observe the internal microstructure of samples, TEM photographs of modified clinoptilolites were shown in **Figure 3**. From **Figure 3**, inside of C-0 was large lamella stacking. However, inside of C-1, C-2, C-3, and C-4 were stacked with small lamella, in which the number of fissures produced by flake particles. This is because they are stirred on thin sheets during ion exchange, so more lamellar clinoptilolite is stripped, mainly concentrated at 100 × 300 nm, which also makes the exchange easier. The 131 faces of clinoptilolite can be seen in C-2 of photograph c∗ , whose crystal plane spacing is 3.98 Å, which have not seen in other published papers.

ICP-AES was adopted to analyze the ion change before and after modification (**table 1**). It is obviously that with the increasing of nacl concentrations, the Na+ component increased from 0.65 to 3.68 wt% gradually. Meanwhile, the Ca2+ component decreased from 3.8 to 1.89 wt% apparently. In addition, other elements showed a random change with increased nacl concentrations (such as Si, Al, K, Mg, Fe, and Ti). The

phenomenon indicated that the ion exchange of Na+ for Ca 2+ were carried out in clinoptilolite samples with NaCl solutions bath.

The XRD patterns were mainly used for confirmation the difference in all clinoptilolites without and with treatment using NaCl solution (**Figure 4**). The major mineral in all samples was clinoptilolite with minor amounts of quartz. All the lattice parameters were not observably influenced by the different contents of NaCl. We couldn't see the characteristic peak of NaCl in modified clinoptilolites, It showed that the Na+ entered the lattice, and we combined the chemical composition analysis in **Table 2**. So we know it was replaced with Ca2+. The obvious difference was the D value of the characteristic peak was smaller than modified clinoptilolites. It was because the exchange of different ions leaded to the change of lattice spacing.

The broad band at 620 cm−1 in the clinoptilolites spectra were attributed to stretching vibrations related to Si-O tetrahedron structure (**Figure 5**). The characteristic peak of clinoptilolite without ion-exchange was weak. Furthermore, the vibration absorption peak of Si-O-Si and Al-O-Si appears at 795 cm−1 . Compared with clinoptilolite without ion-exchange, some evident changes in characteristic absorption peaks were discovered. The characteristic peak at 985.46 cm−1 deriving from Al-O vibrations shifted to 1043.32 cm−1 , caused a small amount of Al-O losing after ion-exchange. A small account of non-framework Al stuck in the unit cell was shifted by ion-exchange. And a small account of Al was shifted from framework because of Al-O-Si hydrolyzing in the clinoptilolites. Then cavities were formed leading to pore volume increasing. For modified clinoptilolites, the Si-O and/or Al-O out-of-plane bend occurred at 485 and 1,622 cm−1 . Thus, the intensity of Al-O out-of-plane bend in modified clinoptilolites was stronger than that of clinoptilolite. It was because ion-exchange caused a small amount of Al-O losing. The broad band at 2,328 cm−1 mainly resulted from stretching vibrations of OH− groups on modified clinoptilolites after ion-exchange. The broad band at 3,743 cm−1 mainly resulted from stretching vibrations of O-H on modified clinoptilolites after ion-exchange.

TABLE 2 | BET of modified clinoptilolites treated by NaCl of different concentrations with N2 at 77 K.

The N2 adsorption-desorption isotherms at 77 K with the four adsorbents were shown in **Figure 6A**. According to the IUPAC classification, all curves were identified as type IV. The surface has mesopore and macropore, The curve of p/p0 region of low relative pressure is convex up, in the higher p/p0 region, the adsorbed material is condensed by capillary, the isotherm obtained by desorption does not coincide with the isotherm obtained by adsorption, and the desorption isotherm lags over the adsorption isotherm. So they present a hysteresis loop. It belongs to the class D loop. It is mainly due to the slit holes formed by sloping sheet stacking. In **Figure 6B**, we know most of the pores are between 5 and 80 nanometers, They have different pore structure by loading different contents of sodium ions. According to **Table 2**, we know that the internal surface area of C-2 adsorbents is the largest. With the increase of the loading of sodium ions, the micropore surface area of adsorbent increased first and then decreased. So the micropores are adjusted. At the beginning, the calcium ion was replaced by sodium ion, and the pore channel of clinoptilolite became smaller. When loading a certain amount, a large number of sodium ions loaded the surface of clinoptilolite and blocked some channels.

#### Simulated Experiments

To explore the separation capability of CH4/N2 on the modified clinoptilolites, the simulated static experiments were conducted using BET equipment, and the simulated dynamic experiments were conducted using the device of adsorption of single tower (**Figure 1**), in which pure N2 and CH4 gas were chosen as gas-supply.

CH4 adsorption and N2 adsorption of the simulated static experiments were shown in **Figure 7**. It should be noted that adsorption of the two molecules is competitive and thus the gas-supply in the simulated static experiments is high purity N2 and high purity CH4 at 298 K, separately. Before the adsorption experiment begun, The absorbents were vacuum activated for 8 h at 370◦ . From **Figure 7**, there was obvious difference between quantity adsorbed of CH4 and N2 using clinoptilolites. At the same relative pressure condition, quantity adsorbed of N2 on clinoptilolites was much more than that of CH4. From **Figure 7A**, the quantity adsorbed of N2 using clinoptilolites followed: C-3 = C-2> C-4> C-1> C-0. From **Figure 7B**, the quantity adsorbed of CH4 using clinoptilolites was as follows: C-3> C-4> C-0> C-1> C-2. Considering the contradiction between adsorption capability of clinoptilolites for CH4 and N2, it is obvious that the C-2 adsorbents static equilibrium separation coefficient is the larger than other three. It is the most potential adsorbent for separating CH4/N2.

Dynamic experiments had been done. The CH4 volume concentration of product is obtained at 298 K on certain pressure when the feed gas is a mixture of CH4 (20%) and N2 gas (80%), as shown in **Figure 8**. Concentrated CH4 could be obtained directly by using these adsorbents of C-1, C-2, C-3, C-4. In this experiment, before testing, the adsorbent was vacuum activated for 8 h at 648 K, and then the package was sealed for use. When the concentration of CH4 of the top of the tower is 20%, it is put back to normal pressure, and a 30 min vacuum is activated and regenerated. The experiment was repeated three times, and the data are recorded in the third experiment. **Figure 8A** showed the

298 K on 0.1, 0.2, 0.3, 0.5 Mpa.

different adsorbents breakthrough curve of nitrogen adsorption at 298K on 0.2 Mpa. In the dynamic adsorption curve, it showed that CH4 concentration can be increased.C-1 is from 20 to 65.2%, C-2 is from 20 to 70.0%, C-3 is from 20 to 66.1%, C-4 is from 20 to 63.1%. Moreover, they can be continuously regenerated. So the adsorbent of C-2 is the best among these adsorbents, which is consistent with the static adsorption results. **Figure 8B** shows the C-2 breakthrough curve of nitrogen adsorption at 298 K on the different pressure. We can control residence time of raw gas in adsorbent by adjusting the pressure of carrier He gas. The residence time corresponding to 0.1, 0.2, 0.3, and 0.5 Mpa is 5, 7.5, 11, and 14 min, respectively. The peak value of CH4 reaches 70.0% when the residence time is 7.5 min. The results show that the N2 adsorption was bigger than CH4 when the mixture of CH4/N2 were in the absorbing tower. The bigger the pressure, the longer the residence time. CH4 adsorption was bigger than N2 adsorption when they were adsorbed. The separation factors would decrease. So, It is very important to choose the suitable residence time.

From the above, it seemed that the C-2 shows the greatest performance at 648 K under 0.2 MPa within 50 min, in comparison to the other modified clinoptilolites, as a underlying sorbent in kinetic PSA processes for the N2/CH4 separation. The different concentrations of Na+ that were existed in its porous network as well as their distribution were the primary influence factor that specifies the adsorption and kinetic properties of the clinoptilolites. Thus, the ionexchange with differences in the concentration of Na+ disturbed the Na+ distribution as well as the electrostatic field inside the clinoptilolite's pores affecting the adsorption property.

#### CONCLUSION

The material structure and CH4/N2 adsorbability of raw and Na+ ion-exchanged clinoptilolites have been examined in detail using PSA. The effect of adsorbent prepared by clinoptilolite with different sodium ion content on methane nitrogen separation is very different. The clinoptilolite adsorbents can be adjusted for

#### REFERENCES

their pore channel by controlling the loaded amount of sodium ions.

The C-2 adsorbent prepared using 0.2 mol/L NaCl solutions was the most promising for the kinetic PSA separation of CH4/N2, giving the better adsorptivity and influence factors concerning the CH4 separation.

From the simulated static experiments, it indicated that N2 and CH4 are both competent in diffusing into the clinoptilolites while N2 adsorptions of clinoptilolites are more excellent. The pertinent results also indicated that adsorption capability of ionexchanged clinoptilolite using 0.2 mol/L NaCl solutions was the optimal adsorbent for separating CH4/N2 at low pressure, considering the contradiction between adsorption capability of clinoptilolites for CH4 and and N2.

According to the simulated dynamic experiments, the ionexchanged clinoptilolite using 0.2 mol/L NaCl solutions exhibits the best performance at 648 K under 0.2 MPa within 50 min, in comparison to raw and other modified clinoptilolites, as a underlying sorbent in kinetic PSA processes for the N2/CH4 separation.

The ion-exchange with differences in the concentration of Na+ as well as the electrostatic field inside the clinoptilolite's pores affecting the adsorption property. Finally, further manipulation for CH4 separation of the clinoptilolite is underway with coal bed gas as gas-supply.

# AUTHOR CONTRIBUTIONS

XH and ZL conceived and designed the project. XH performed the experiments and wrote the manuscript. HH, XL, and YH analyzed the data.

#### FUNDING

The work was financially supported by the Special Fund for National/International Science and Technology Cooperation in China (Grant no. 2015DFR60640) and the Science and Technology Plan of Henan Province (No.<PHONE_NUMBER>31).

of natural gas. Chem. Eng. Sci. 61, 3893–3906. doi: 10.1016/j.ces.2006. 01.023

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**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Hao, Li, Hu, Liu and Huang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Synthesis and Microwave Absorbing Properties of Porous One-Dimensional Nickel Sulfide Nanostructures

Min Lu1 , Qian Wu1 , Xiao-Hui Guan1 , Wei Xu2 , Hao-Yue Zhang2 , Xin Di 3 , Guang-Sheng Wang2 \* and Shao-Hua Dong3 \*

*1 School of Chemical Engineering, Northeast Electric Power University, Jilin, China, 2 School of Chemistry, Beihang University, Beijing, China, 3 Pipeline Technology Research Center, China University of Petroleum–Beijing, Beijing, China*

#### Edited by:

*Wenbo Wang, Lanzhou Institute of Chemical Physics (CAS), China*

#### Reviewed by:

*Zheng Ren, University of Connecticut, United States Aiwei Tang, Beijing Jiaotong University, China*

#### \*Correspondence:

*Guang-Sheng Wang<EMAIL_ADDRESS>Shao-Hua Dong<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Green and Sustainable Chemistry, a section of the journal Frontiers in Chemistry*

> Received: *30 June 2018* Accepted: *20 August 2018* Published: *11 October 2018*

#### Citation:

*Lu M, Wu Q, Guan X-H, Xu W, Zhang H-Y, Di X, Wang G-S and Dong S-H (2018) Synthesis and Microwave Absorbing Properties of Porous One-Dimensional Nickel Sulfide Nanostructures. Front. Chem. 6:405. doi: 10.3389/fchem.2018.00405* One-dimensional (1D) porous NixSy nanostructures have been successfully fabricated by two-step method consisting of solvothermal and subsequent annealing process. The suitable heat treatment temperature and reaction time play crucial roles in the final structure, morphology, as well as performance. The uniform and perfect porous NixSy nanostructures obtained at 310◦C exhibit outstanding microwave absorption performances. A minimum reflection loss of −35.6 dB is achieved at 8.5 GHz, and the effective absorption bandwidth almost covers 14.5 GHz with the absorber thickness range of 2.0–5.0 mm. It can be supposed that this porous structure with rough surface which is favor for increasing the microwave multiple reflection and scattering, contributes a high-performance electromagnetic absorption.

Keywords: porous, nickel sulfide, one-dimensional, dielectric loss, microwave absorption

# INTRODUCTION

Microwave absorber with strong capacity in absorption, low proportion in filler loading, thin thickness in coating, and wide bandwidth in absorption frequency, has aroused burgeoning research interest because of their great potential applications both in military and civil fields, including stealth technology, information security, electromagnetic interference shielding, and healthcare (Zhu et al., 2010; Zhao H. et al., 2014). As is well known that many factors such as morphology, geometry and structure, have vital impacts on determining the microwave absorption (MA) properties (He et al., 2013). Conventional microwave absorbers with different morphologies that have been divided into three classes are as follows: (1) one-dimensional nanostructures such as ZnO nanowires (Wang et al., 2014), Bi3S2 nanorods (Luo et al., 2014); (2) two-dimensional materials such as MoS2 nanosheets (Ning et al., 2015), α-Fe2O3 flakes (Lv et al., 2015a); and (3) three-dimensional network structures, including Co20Ni80 hierarchical nanospheres (Liu et al., 2015), Ni chains nets (Liu et al., 2016), Fe3O4@carbon ordered arrays (Yuan et al., 2015) and so on. However, the aforementioned materials usually possess high density, thus leading to severe limitations to their practical applications in some specialized fields. In this regard, materials with characteristics of low density and special void spaces, such as yolk–shell structural microspheres (Liu et al., 2013; Yu et al., 2014; Qiang et al., 2016), foam composites (Zhang Y. et al., 2015; Zhao H. B. et al., 2016), as well as porous nanostructures (Yan et al., 2009; Zhou et al., 2010; Zhu et al., 2011), are highly beneficial to obtaining superior microwave absorption performance.

Considerable attention has been concentrated on the porous structure of MA materials owing to its fascinating characteristics. For example, Liu et al. fabricated porous carbon/Co composites, and the results suggested that the composites with large dielectric loss could achieve a minimum RL of −40 dB at 4.2 GHz with a coating thickness of 5 mm (Liu et al., 2008). Lv et al. synthesized Co/CoO porous 3-D flower nanostructure through annealing process at 400◦C and found that the minimal reflection loss was up to −50 dB when the coating thickness was 3.5 mm (Lv et al., 2015b). Similarly, Wang et al. reported the formation of porous flower-like NiO decorated graphene, and the composites with a filler loading of 25 wt% exhibited highly MA performance (−59.6 dB) because of their special porous structures and numerous void spaces (Wang et al., 2017). Benefiting from the porous structure, the above-mentioned materials show superior electromagnetic wave absorption performance, which reveals that these porous materials are effective as MA materials.

Metal sulfides, as semiconductor materials, have been proven to have promising potential as an ideal microwave absorber on account of their typical dielectric loss mechanism (Zhang X. J. et al., 2017). Recent studies have suggested that metal sulfides including MoS2 (Wang et al., 2015), CoS2 (Zhang C. et al., 2017), CdS (Zhang et al., 2014), and CuS (He et al., 2014), as well as various phase of nickel sulfides (Zhou et al., 2010) can effectively absorb electromagnetic waves and attenuate them in the form of thermal energy. However, the microwave-absorbing properties of NixSy with special porous structure have not been reported previously. Based on the above study, we demonstrated the successful design and fabrication of porous one-dimensional NixSy nanomaterial through a facile solvothermal route together with annealing process. The MA properties of resultant composites were investigated in detail for the first time. As expected, the synthesized NixSy nanostructure exhibited excellent microwave absorption property confirming that this material can be used as high-performance microwave absorber.

# MATERIALS AND METHODS

# Preparation of Porous NixSy

Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), elemental sulfur, ethylene glycol (EG), and ethylenediamine (EN) were purchased from Nanjing Chemical Reagent Co. All of the chemical reagents were analytical-grade purity and used without further purification.

Typically, nickel sulfide was synthesized by the reaction of Ni(NO3)2·6H2O, EN and sulfur powder in EG. At first, Ni(NO3)2·6H2O (0.3489 g) was added to EG (135 mL) under strong magnetic stirring to form a light green homogeneous solution. Then the sulfur powder (0.0288 g) was dissolved in EN (15 mL) through ultrasonic treatment. Mix the two solutions together and put it into oil bath, maintained at 120◦C for 6 h. After being cooled to room temperature, the resulting solid precursors were centrifuged, washed with alcohol to remove possible remnant, and finally dried in air at 60◦C for 24 h. The dried precursors were treated at 310◦C for 2 h with a heating rate of 2◦C/min under N2 atmosphere to get the final NixSy products.

# Preparation of Nickel Sulfide/PVDF Nanocomposites

The polyvinylidene fluoride (PVDF) was first dispersed in N-N dimethylformamide (20 mL) under magnetic stirring for 1 h. Then, the desired amount of nickel sulfide was added into the suspension. After ultrasonication for another 1 h, the mixture was poured onto a glass plate and dried at 80◦C for 24 h. The samples for testing were also compacted into a cylindrical compact (Φout = 7.00 mm and Φin = 3.04 mm) by hot pressing at 210◦C under 5 MPa (pressed for 15 min, followed by cooling to room temperature under the same pressure).

#### Instrumental Analyses

The X-ray diffraction (XRD) pattern of the nickel sulfide product was carried out on a Rigaku, Dmax2200 diffractometer equipped with a CuKa radiation source (λ = 1.5416 Å) in the range of 2θ = 10–80◦ . For the phase analysis. Further microstructural analyses were performed by using a FEI Quanta 250 field emission gun environmental scanning electron microscope (JSM-6700F microscope) at 15 kV. In brief, SEM samples were prepared by diluting the final products with alcohol by ultrasonic treatment and dropping it on the silicon slice. The relative permittivity (ε ′ , ε ′′) and permeability (µ ′ , µ ′′) values were measured using two-port vector network analyzer (Agilent E5071C) over the frequency of 2–18 GHz at room temperature, coupled with a coaxial wire setup. Finally, the reflection loss (RL, dB) value which presents the ratio of the total reflected microwave power against the incident microwave power can be calculated by using the following formulas (Abbas et al., 2006; Xu et al., 2018).

$$Z\_{in} = \sqrt{\frac{\mu\_r}{\varepsilon\_r}} \tanh\left[j\left(\frac{2f\pi d}{c}\right)\sqrt{\mu\_r\varepsilon\_r}\right] \tag{1}$$

$$RL(dB) = 20\log\left|\frac{Z\_{in} - 1}{Z\_{in} + 1}\right|\tag{2}$$

where Zin is the normalized input characteristic impedance, f is the frequency of microwave, d is the thickness of the absorber. A lower RL value stands for a better MA performance.

### RESULTS AND DISSCUSSION

An illustration of the synthesis of the NixSy is shown in **Figure S1**. We first prepared the precursors of NixSy nanorods by a simple solvothermal method in a controlled way as described later. Then the collected dried precursors were transferred into a tube furnace and annealed at 310◦C for 2 h under an N2 atmosphere, which eventually led to the generation of the porous NixSy. It is well recognized that reaction parameters such as temperature, pressure, reaction time, type of solvent, and concentration of reagents, have a huge effect on the morphology of the products. In the present reaction system, Ethylene glycol and Ethylenediamine were applied as solvents for the precursors synthesis of NixSy, and the temperature as well as reaction time was tightly regulated so that the precursors at a well-defined state were obtained.

**Figure 1** shows the representative SEM images of the precursors prepared at different temperature of 80, 100, 120, 140,

160, and 180◦C, respectively. When the reaction is performed under the lower temperature (80 and 100◦C), although onedimensional structures of products can be formed, their thicknesses and lengths are varying greatly. On the contrary, when the temperature reaches 180◦C, the 1D nanostructures are nonuniform, and a certain number of spherical impurities emerge. By comparing the morphology of the products under different temperature conditions, the best reaction temperature is determined. The products are well-defined nanorods with diameters of about 50 nm and lengths of several micrometers at the suitable reaction temperature of 120◦C (**Figure 1c**). On the other hand, the stages of the growth process are monitored at 120◦C by varying the reaction time from 2 to 10 h (**Figure S2**). It is interesting to find that reaction time does not change the overall morphology of the precursors. However, the yield of the products is very low within a short period of 2 h, indicating the slow reaction rate in such reaction system. Further increasing the reaction time produce more products, while the diameter and structure of the nanorods remain stable.

To obtain porous 1D nanostructures, the precursors were annealed at different temperatures ranging from 280 to 330◦C under the flowing nitrogen gas, and the final products are exhibited in **Figure 2**. It can be seen that the surfaces are actually becoming porous with annealing temperature, and the as-obtained NixSy retain the rods morphology with appropriate porosity when prepared at 310◦C. Further increasing the annealing temperature will led to 1D structural instability and collapse. The energy-dispersive X-ray spectroscopy (EDS) indicates that the obtained product is composed of Ni, S, C, N, and O elements, also demonstrating a very homogeneous elemental distribution (**Figure 3**). It is worth mentioning that the C, N, and O element signals originate from the incomplete decomposition of organic compositions of precursors during heat treatment. Meanwhile, the XRD results suggest that the

as-synthesized products are poorly crystallized, containing NiS2 phase and Ni7S6 phase (**Figure S3**).

**Figures 4A,B** show the frequency dependence of the real part (ε ′ ) and imaginary part (ε ′′) of the complex permittivity for NixSy/PVDF composites with 10, 20, 30, and 40 wt% filler loadings in the frequency range of 2–18 GHz. Generally, the ε ′ is known to stand the storage capability of electromagnetic energy, and ε ′′ associated with various of polarization present the energy dissipation (Zhang X. et al., 2015; Zhao B. et al., 2016). As can be seen in **Figure 4**, with the increase of NixSy, the ε ′

and ε ′′ values show a similar tendency. For nanohybrids with low concentration of filler (10 and 20 wt%), the values of ε ′ and ε ′′ are approximately equal to some certain constant in the whole frequency range (ε ′ = 4, ε ′′ = 0.5, and ε ′ = 6, ε ′′ = 1). With the NixSy proportion increasing from 20 to 30 wt%, the ε ′ increases from 6 to 13 and the ε ′′ changes from 1 to 4.5 at 2 GHz. However, when the proportion of fillers is increased to 40 wt%, both the ε ′ and ε ′′ dramatically decrease, which is possibly due to the

FIGURE 4 | Frequency dependence of (A) real part and (B) imaginary part of permittivity, (C) dielectric loss tangent, and (D) reflection loss under a 3 mm thickness for NixSy composites with different filler loadings.

fact that the higher concentration of NixSy in this nanohybrid may result in severe agglomeration. Similar phenomena could be observed in dielectric loss tangent (tan δe = ε ′′/ε ′ ) which is universally applied to evaluate the dielectric loss capacity of the microwave absorber (Yang et al., 2017), shown in **Figure 4C**. It can be seen that the dielectric loss tangent increases with the NixSy proportion first, getting a maximum value of 0.35 with 30 wt% NixSy, and then decreases to a value of 0.1.

On the basis of the above analysis, it can be deduced that the nanohybrids with 30 wt% NixSy possess the best microwave absorption properties which is highly consistent with the test results, shown in **Figure 4D**. The minimum RL reaches −34 dB at 7.5 GHz with a thickness of 3 mm, indicating that 99.9% of incident electromagnetic wave is attenuated. Since NixSy is a typical semiconductive material, various polarization, and related relaxation resulting in a strong dielectric loss are the dominant mechanism for microwave attenuation (Zhao et al., 2015). The dielectric loss of NixSy mainly originate from the defect dipole polarization, the interfacial polarization, and the electronic relaxation loss. First, the defect dipoles are generated by the charge unbalance around the sulfur vacancies in the NixSy lattice, while the interfacial polarizations come from the existence of plentiful interfaces between porous fillers and polymer matrix. Moreover, the porous structures of fillers with rough surfaces further induce the multiple reflection and scattering, resulting in more longer propagation path and greater energy loss (**Figure 5**). Second, the carbonation of precursors benefits electrical conduction, while the internal

doping with nitrogen favors electron transport thus further improving electrical conduction. Furthermore, the calcination also induces a structural disorder and defects into NixSy that frequently lead to an enhanced electrical activity. This calcination process results in significant electronic relaxation polarization, which would enhance the dielectric loss obviously.

**Figures 6A–D** shows three-dimensional presentations of calculated reflection loss for the NixSy/PVDF with different filler loadings. Clearly, the composite with 30 wt% NixSy has the best performance, and the RL values under different thickness are shown in **Figure 6E**. The minimal reflection loss of −35.6 dB is obtained at 8.5 GHz with a coating thickness of 2.7 mm and the effective bandwidth is about 3 GHz. Furthermore, the RLs exceeding −10 dB in the frequency range of 3.68–18 GHz are obtained for a variation in absorber thicknesses of 2.0–5.0 mm, demonstrating that this kind of materials has great potential for use as a microwave absorber. Meanwhile, there is an interesting phenomenon that with the increasing thickness of absorber the RL peaks shift to the lower-region frequency. This phenomenon is consistent with a so-called quarter-wavelength (λ/4) matching model (Deng and Han, 2007; Wang et al., 2011, 2013) which plays another significant role in electromagnetic attenuation. The model can be expressed as tm = nλ/4 = nc/4fm √ |µr | |εr |(n = 1, 3, 5), where |µr | and |εr | are the moduli of µr and εr , respectively. Besides, when the thickness of absorber satisfies above equation, the curves corresponding to wavelengths of λ/4 (crescent shape) can be observed (seen in **Figure 6C**).

#### CONCLUSIONS

In summary, we have successfully demonstrated an approach for the large-scale production of 1D porous NixSy nanostructures via solvothermal synthesis together with an annealing process.

#### REFERENCES

Through controlling the reaction temperature and time, products with uniform morphology are obtained. The results reveal that the interesting porous structure of NixSy might benefit the access of incident microwave and offer more active sites for multiple reflections and scattering, and thereby improve microwave absorbing performance. The minimum RL of −35.6 dB is achieved with a thickness of 2.7 mm at −35.6 dB GHz. The absorption bandwidth with RLs below −10 dB is up to 14.5 GHz when considering thicknesses of 2.0–5.0 mm. Combining the synergistic effect with intrinsic chemical properties and special structures, the NixSy nanorods are promising for utilization as MA materials in various fields, such as aeroplanes and spacecraft.

#### AUTHOR CONTRIBUTIONS

ML designed experiments, carried out experiments, analyzed experimental results, and wrote the manuscript. QW, WX, H-YZ, and XD helped experimental result. X-HG gave guidance on the revision of the experimental plan and manuscript. G-SW and S-HD funded experimental topics.

#### ACKNOWLEDGMENTS

This project was supported by the National Nature Science Foundation of China (No. 51472012), and the Fundamental Research Funds for the Central Universities.

#### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2018.00405/full#supplementary-material

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Lu, Wu, Guan, Xu, Zhang, Di, Wang and Dong. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# CoAl2O4/Kaoline Hybrid Pigment Prepared via Solid-Phase Method for Anticorrosion Application

Anjie Zhang1,2, Bin Mu1 \*, Xiaowen Wang1,2 and Aiqin Wang1 \*

*1 Key Laboratory of Clay Mineral Applied Research of Gansu Province, Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China, 2 Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, China*

In this study, kaoline is incorporated to prepare CoAl2O4/kaoline hybrid pigments *via* traditional solid-state reaction, and the introduction of kaoline decreases the preparation temperature for formation of spinel CoAl2O4, and reduces the production cost of cobalt blue as well. More importantly, kaoline may participate in the crystallization of spinel CoAl2O4 during calcining process, and the hybrid pigments prepared using 8.1% Co3O4 and 81.5% kaoline features bright blue and good chemical resistance. Due to the synergistic effect between the sheet-like kaoline and the loaded CoAl2O4, the as-prepared CoAl2O4/kaoline hybrid pigments can be incorporated into epoxy paint system to obtain the high-performance blue anticorrosion coating, especially for acetic acid-salt fog corrosion.

#### Edited by:

*Pu-Xian Gao, University of Connecticut, United States*

#### Reviewed by:

*Na Tian, China University of Geosciences, China Adimali Piyadasa, Intel, United States*

#### \*Correspondence:

*Bin Mu<EMAIL_ADDRESS>Aiqin Wang<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Green and Sustainable Chemistry, a section of the journal Frontiers in Chemistry*

Received: *31 July 2018* Accepted: *09 November 2018* Published: *29 November 2018*

#### Citation:

*Zhang A, Mu B, Wang X and Wang A (2018) CoAl2O4/Kaoline Hybrid Pigment Prepared via Solid-Phase Method for Anticorrosion Application. Front. Chem. 6:586. doi: 10.3389/fchem.2018.00586* Keywords: CoAl2O4 , hybrid pigments, kaoline, anticorrosion coating, acetic acid-salt fog

# INTRODUCTION

It is well-known that corrosion of the metal materials is one of the common and key problems to industry applications, especially the high risk fields including aviation, ocean, and chemical industry, etc. (De Leon et al., 2012; Daham et al., 2014; Hao et al., 2017). The organic anticorrosion coating has been recognized as the most effective and economical method for metal protection (Vesely et al., 2010; Golru et al., 2015; Gu et al., 2015; Madhup et al., 2017). The anticorrosion mechanisms of the coatings usually include protection of barrier type, inhibition type protection and electrochemical protection (Al-Sabagh et al., 2017), and the organic anticorrosion coating is served as a physical barrier between corrosive electrolyte and steel substrate. However, the main disadvantage of the organic anticorrosion coating, especially epoxy system, is the creation of holes and defects over the film due to the high crosslink density.

Several approaches have been proposed to enhance the anticorrosion properties of organic anticorrosion coating. By contrast, incorporation of inorganic pigments is efficient and economical way (Kartsonakis et al., 2012; Montemor et al., 2012; Jeon et al., 2013; Naderi et al., 2014). It is conducive to improving the aesthetics (e.g., gloss, opacity, and color) and the mechanical properties of the film (El Saeed et al., 2012). However, the relevant application of some commercial available inorganic pigments is limited due to their disadvantages, such as poor adhesive force and optical transparency, weak abrasion and scratch resistance performance (Zhou et al., 2002; Cayton and Sawitowski, 2005; El-Wahab et al., 2009). Therefore, many attempts have been carried out to prepare high-performance pigments with good anticorrosion properties by designing lamellarbased nano-materials preventing water, oxygen, or ions from penetrating film (Kalendová et al., 2008; Ahmed et al., 2012; Ammar et al., 2016; Zhang et al., 2016).

Cobalt aluminate (cobalt blue, CoAl2O4) is a spinel type structure blue eco-friendly pigments with excellent thermal and chemical stability, and it has been widely used in the fields of ceramics, plastics, paint, rubber and glass (Jafari and Hassanzadeh-Tabriz, 2014; Álvarez-Docio et al., 2017; He et al., 2017; Yoneda et al., 2018). Therefore, cobalt blue may be expected to develop the high-performance anticorrosion coating, especially for acetic acid-salt fog corrosion. However, the agglomeration of CoAl2O4 nanoparticles is inevitable during preparation process, which goes against the dispersion of cobalt blue in coating substrate. In addition, the high-cost of cobalt blue also limits its wide application in anticorrosion coating (Tirsoaga et al., 2011; Dandapat and De, 2012; Zou and Zheng, 2016). In our previous work (Mu et al., 2015; Zhang A. J. et al., 2017), clay minerals were employed to fabricate the high-performance CoAl2O4 hybrid pigment with low-cost and prefect color properties. Kaoline (Kaol) is a 1:1 type layered silicate mineral with one tetrahedral sheet of silica (SiO4) linked through oxygen atoms to one octahedral sheet of alumina (AlO6) octahedral (Liu et al., 2016; Kloprogge, 2017; Zhang S. L. et al., 2017). Owing to the advantages of excellent mechanical properties, thermal stability, high whiteness and unique flakelike morphology (Vesely et al., 2010; Ahmed et al., 2012; Qu et al., 2017), it is a promising candidate for preparation of cobalt blue hybrid pigment to design the color high-performance anticorrosion coating (Zhang et al., 2018a,b). However, the preparation process of co-precipitation usually involves in the discharge of the wastewater and lengthy steps including washing, solid-liquid separation and drying.

In this study, CoAl2O4/Kaol hybrid pigments were prepared by the traditional solid-state reaction after grinding the mixture of γ-Al2O3, Co3O4, and Kaol. The preparation conditions were systematically investigated including the grinding time, the calcining temperature and the added amount of Kaol. Furthermore, the formation and coloring mechanisms of CoAl2O4/Kaol hybrid pigments were also studied and discussed. In order to evaluate the anticorrosion application of the asprepared hybrid pigment, it was incorporated into the epoxy paint system to obtain the blue anticorrosion coating, and then placed in acetic acid-salt spray testing chamber.

#### MATERIALS AND METHODS

#### Materials

Kaol was obtained from Longyan kaoline Development Co., Ltd., (FuJian, China), Kaol were firstly crushed and purified by 4% HCl (wt%), and then the solid were filtered by passing through a 200-mesh sieve to remove quartz sand, and the XRF (X-ray fluorescence) chemical compositions of Kaol were presented in **Table S1** (see ESI) before and after being treated by HCl. γ-Al2O3 (purity > 99.9%, particle size = 20 ∼ 50 nm) and Co3O4 ( purity > 99.9%, D50: 4 ∼ 6µm) were obtained from Shanghai Reagent Factory (Shanghai, China).

### Preparation of CoAl2O4/Kaol Hybrid Pigments

8.00 g of Kaol, 0.80 g of Co3O4 and 1.02 g of Al2O3 with a Co to Al mole ratio of 2:1 were mixed and grinded in a TABLE 1 | Conditions for preparation of the samples.

mortar mill (CRINOER, MG100, China) for 2 h in anhydrous ethanol medium, and then the mixture was calcined to obtain CoAl2O4/Kaol hybrid pigments. The optimum preparation conditions were systematically investigated including the calcining temperatures (900, 1,000, 1,100, and 1,200◦C), the grinding time (0.5, 1, 2, and 4 h) and the added amount of Kaol (0, 1, 2, 4, 6, and 8 g), as summarized in **Table 1**.

#### Characterization

The Fourier Transform infrared (FTIR) spectra were collected on a Thermo Nicolet NEXUS TM spectro photometer using KBr pellets. The morphology was observed using transmission electron microscopy (TEM, JEM-1200EX/S, JEOL). XRD test was conducted on X'pert PRO diffractometer with a scan step size of 0.02◦ per second. Raman spectra were tested using a Labram HR Evolution Raman spectrometer (Horiba). The chemical compositions were measured on a MiniPal 4 XRF spectrometer (PANalytical Co., Netherland). The anticorrosion performance was evaluated on a salt spray testing chamber (YWX-750, Nanjing Huanke experimental equipment Co. Ltd, China). The colorimetric values and reflectance spectra were measured on a Color-Eye automatic differential colorimeter (X-Rite, Ci 7800).

# Stability Evaluation of CoAl2O4/Kaol Hybrid Pigments

In order to evaluate the environmental stability of CoAl2O4/Kaol hybrid pigment, it was sprayed onto glass substrate and dried in the open atmosphere after being ultrasonically dispersed into ethanol for 30 min. And then the glass plates were placed for 15 days in a UV Accelerated Weathering Tester (ZN-P, Xinlang, Shanghai, China) with eight UV-B (280-315 nm) bulbs (40 W) at 60◦C under UV-B exposure with a radiation intensity of 320 W/m2 . The color properties were measured before and after being exposed in a UV accelerated weathering tester to evaluate UV irradiation stability.

The obtained sample plates were also immersed into 3M HCl, 3M NaOH, and ethanol at room temperature for 72 h to study the chemical stability of CoAl2O4/Kaol hybrid pigment, respectively. Unlike with above glass substrate, the hybrid pigments were sprayed onto ceramic substrate and then placed in a muffle furnace to be calcined at 1,000◦C for 2 h to evaluate the thermal stability.

# Anticorrosion Evaluation of the Blue Epoxy Coating

The anticorrosion coating was prepared by following procedures: Firstly, epoxy resin, 5 wt% CoAl2O4/Kaol hybrid pigment, dispersing agent, flatting agent, auxiliary solvents, talcum powder, ethyl acetate, and some grading beads added in a 500 mL stainless steel under high shear for 30 min (2,000 rpm), and then the antifoaming agent, wetting agent were added into the above mill base and stirred under low shear for 60 min (500 rpm). If the maximum pigment size was lower than 30µm, the grinding was considered for meet the requirements, and then the viscosity of the coating was adjusted by ethyl acetate to 20∼30 s. And then it was sprayed onto steel plate substrate of size 15 cm × 10 cm × 0.1 cm and dried under the standard conditions (temperature: 25◦C and humidity: 40%) for 7 days ( the coating thickness: D = 50 ± 2µm). The obtained sample plates were placed in salt spray testing chamber to evaluate the salt fog corrosion resistance performance (35◦C, 100% humidity and 5.0 wt% NaCl solution). In addition, the obtained sample plates were also placed in acetic acid-salt spray testing chamber to evaluate the acetic acid-salt fog corrosion resistance performance (35◦C, 100% humidity and 5.0 wt% NaCl solution, pH = 2.8 ∼ 3.0).

# RESULTS AND DISCUSSION

## Preparation of CoAl2O4/Kaol Hybrid Pigments

The calcining temperature is an important factor to form spinel CoAl2O4 with prefect blue color (Yang et al., 2013), and thus the effect of calcining temperatures on the color

parameters of hybrid pigment was studied. **Figure 1a** gives the color parameters of CoAl2O4/Kaol hybrid pigments after being calcined at 900∼1,200◦C. The b ∗ value of hybrid pigments firstly decreases with the increase in the temperatures, and then it almost has no obvious change as the temperature is above 1,100◦C. The values of L ∗ and a ∗ continuously increase with the increase in the calcining temperatures. With the increase in the calcining temperature from 900 to 1,100◦C, the color of the hybrid pigments transforms from green to blue to bright blue, which is also consistent with the change of their color parameters. The blue color is derived from Co2+ in tetrahedral sites, while the green color is attributed to Co3+ in octahedral coordination (Álvarez-Docio et al., 2017). By contrast, the b ∗ value of hybrid pigments reaches the minimum value after being calcined at 1,100◦C, suggesting that the optimum calcining temperature is 1,100◦C. As shown in **Figure 1b**, the reflection bands and absorption bands are observed at 440∼500 nm and at 550∼650 nm, respectively, except for hybrid pigment prepared at 900◦C. These bands are associated with the blue color of CoAl2O4 due to 4A2 (F) → 4T1 (P) of the Co2+ d-d transition in tetrahedral coordination (Tielens et al., 2006; Kurajica et al., 2012; Gomesan et al., 2015; Tahereh et al., 2016). And for the hybrid pigment prepared at 900◦C, the bands at 380 and 670 nm are indicative of the occurrence of octahedrally coordinated Co3+, and are ascribed to the 1A1g→ 1T2g and 1A1g→ 1T1g transitions of low spin Co3+ in octahedral symmetry (Brik et al., 2001; Herrero et al., 2007), the sample exhibits greenish hue annealed at low temperature, which is also in accordance with the digital photos of the hybrid pigment calcined at 900◦C (Kurajica et al., 2012). In addition, the intensity of the reflection peaks also increases with the increase in the preparation temperatures until 1,100◦C, and then it decreases as the calcining temperature reaches 1,200◦C, which may be due to crystal phase and structural transformation of Kaol (Juneja et al., 2010; Yeo, 2011).

**Figure S1a** depicts the effect of grinding time on color parameters of hybrid pigment, the b ∗ value of CoAl2O4/Kaol hybrid pigments firstly decreases with the increase in the grinding time from 0.5 to 1 h, and no obvious change is observed with the continuous increasing grinding time. In order to ensure the reaction uniformity, the grinding time is selected to 2 h. **Figure S1b** exhibits the effect of the added amount of Kaol on color parameters. At the same calcining temperature (1,100◦C), the L ∗ value of hybrid pigments gradually increases with increase

in the added amount of Kaol, while the a ∗ and b ∗ values of the sample containing Kaol of 35.5 wt% (1 g Kaol) reach maximum. Interestingly, CoAl2O4/Kaol hybrid pigments still exhibits good color properties (L ∗ = 29.9, a ∗ = 2.01, b ∗ = −54.93) as the added amount of Kaol is up to 81.5 wt% (8 g Kaol). However, the sample is atrovirens prepared without Kaol at 1,100◦C, suggesting that the introduction the low-cost Kaol not only reduces the production cost of CoAl2O4, but also decreases the high temperature crystallization temperature for formation of spinel structure. During calcining process, Kaol (2SiO2 · Al2O3 · 2H2O) transforms into metakaolinite (2SiO2 · Al2O3) at 400∼600◦C, and then it turns into SiAl2O5 and amorphous SiO2 (>900◦C), even amorphous SiO2 and loworder crystalline α-Al2O3 at above 1,100◦C (Ribeiro et al., 2005; Gilkes and Prakongkep, 2016). Because the mass transfer process is the rate controlling step during CoAl2O4 preparation (He and Becker, 1997), the uniform distribution Co3O4 and Al2O3 on the surface of Kaol after being grinded is in favor of reducing the mass transfer resistance, thus the incorporation of Kaol decreases the calcining temperature for formation of spinel CoAl2O4 (Wang et al., 2006; Gabrovska et al., 2014). Based on above results, the optimum conditions for preparing hybrid pigment with the prefect color are 1,100◦C and 81.5 wt% of Kaol.

## Characterization of CoAl2O4/Kaol Hybrid Pigment

**Figure 1c** presents FTIR spectra of Kaol, the calcined Kaol at 1,100◦C and CoAl2O4/Kaol hybrid pigment, respectively. FTIR spectrum of Kaol exhibits the characteristic bands of Si-O-Si (at about 471, 522, 699, 1,032, and 1,114 cm−1 ), Si-O-Al (at around 525, 750, and 795 cm−1 ), Al-Al-OH (at around 522 and 911 cm−1 ), -OH (at about 3,684, 3,684, 3,651, and 3,440 cm−1 ), while the bending vibration of the inter-layer water is observed at 1,636 cm−1 (Rekik et al., 2017; Sreelekshmi et al., 2017). After being calcined, these typical absorption bands disappear compared with that of the raw Kaol, which is ascribed to the dehydroxylation and the crystal phase transition of Kaol during calcining process. In the case of FTIR spectrum of hybrid pigment, several new absorption bands appear at 668, 557, and 509 cm−1 , which can be attributed to the stretching vibration of Al-O of AlO6 and Co-O of CoO4, respectively (Zayat and Levy, 2000; Zhang A. J. et al., 2017), indicating the formation of CoAl2O4.

**Figure 1d** shows the XRD patterns of Kaol, the calcined Kaol at 1,100◦C and CoAl2O4/Kaol hybrid pigment, respectively. It is clear that the associated minerals are quartz (2θ = 20.8◦ and 26.7◦ ) and mica (2θ = 8.9◦ ) (Chhikara et al., 2015; Mymrin et al., 2017), and the typical characteristic peaks of kaolinite presents the well-defined reflections at 2θ = 12◦ and 25◦ . When Kaol was calcined at 1,100◦C, only diffraction peaks of quartz are remained. For hybrid pigment, new diffraction peaks are observed at 2θ = 31.1◦ , 36.8◦ , 44.8◦ , 49.0◦ , 55.5◦ , 59.2◦ and 65.2◦ , which correspond to (220), (311), (400), (331), (422), (511), and (440) planes of CoAl2O4 according to JCPD card No. 10-458, respectively (Xi et al., 2012; Zhu et al., 2015). In fact, CoO also reacts with silica derived from Kaol to form cobaltous silicate, but this reaction is considered to be a minor reaction since most of CoO react with Al2O3 because CoO has high affinity to Al2O3 (Ahmed et al., 2012).

**Figure S2** shows Raman spectra of CoAl2O4 pigment without Kaol and CoAl2O4/Kaol hybrid pigment. CoAl2O4 pigment with a spinel structure usually exhibits five Raman active modes: A1g (764 cm−1 ), F2g (644, 511, and 203 cm−1 ) and Eg (413 cm−1 ). Hybrid pigment also clearly presents the five expected Raman active modes at 760, 641, 510, 410, and 202 cm−1 , indicating that hybrid pigments is assigned to the spinel structure (Zha et al., 2016; Liu et al., 2017). Compared with the Raman spectrum of CoAl2O4, all modes of hybrid pigment shift to lower frequencies, and this red shift suggests that primary coordinative environments of tetrahedron and octahedron positions may change after incorporation of Kaol.

The typical structures and the morphology evolution of the samples can be observed by TEM. As illustrated in **Figure 2a**, Kaol presents a typical lamellar structure with a smooth surface. After being calcined at 1,100◦C (**Figure S3a**), the lamellar morphology remains well, which reveals that Kaol possesses the good thermal stability. With the introduction of CoAl2O4 nanoparticles, the surface of lamellar morphology becomes coarse (**Figures 2b,c**), and CoAl2O4 nanoparticles are uniformly distributed on the substrate surface with a diameter of 10∼20 nm. Furthermore, no obvious aggregation is observed, thus it indicates that incorporation of Kaol effectively prevents from the aggregation and controls the size of CoAl2O4 nanoparticles. The selected area electron diffraction pattern of hybrid pigment also indicates the successful formation of spinel CoAl2O4 (**Figure S3b**) (Naderi et al., 2014; Zou and Zheng, 2016). **Figure 2d** gives an enlarged electron micrograph, and it provides a well-resolved lattice plane with an interplanar spacing of 0.244 nm, corresponding to (311) plane of the cubic Fd3m space group, which is identified on the basis of data of the standard CoAl2O4 database JCPD card No. 10-458 (Kim et al., 2012). Meanwhile, the micrograph also displays the coexistence of amorphous and crystalline phases, which is attributed to the amorphous SiO2 derived from Kaol and spinel CoAl2O4, respectively (Cho and Kakihana, 1999). In addition, **Figure 3** illustrates the elemental mapping of hybrid pigment, it is clear that hybrid pigment is mainly composed of Co, Al, O, and Si elements. Co element is uniformly distributed on the surface of lamellar substrates, suggesting that the generated CoAl2O4 is uniformly anchored on the surface of substrate.

Based on our previous studies, Kaol is not merely a carrier for loading of CoAl2O4 nanoparticles, Al2O3 as one of the main compositions of Kaol might participate in the high-temperature crystallization to form clay mineral doped spinel CoAl2O4. In order to confirm above inference, the sample was prepared using Co3O4 and Kaol without Al2O3 by the same procedure with CoAl2O4/Kaol hybrid pigments. As shown in **Figure 4** and **Table S2**, the color of the as-prepared pigment is blue, and the values of L ∗ and a ∗ decrease with the increase of the addition amount of Co3O4, but b ∗ firstly decrease with the increase of the added amount of Co3O4 to 13%, and then it begins to increase as Co3O4 reaches 16%. At this moment, the obtained pigment is dark-green color, which is mainly ascribed to the fact that the excess Co3O4 is not fully involved in the reaction and anchors on the pigment surface (Aguilar-Elguézabal et al., 2017). At the low addition amount of Co3O4 (4.8%), CoO derived from the thermal reduction of Co3O4 can react with Al2O3 derived from Kaol to form spinel CoAl2O4 at 1,100◦C, which can be confirmed by their XRD patterns and FTIR spectra (**Figure S4**). Therefore, it can be safely concluded that Kaol may be served as

an aluminum source to take part in the crystallization process of CoAl2O4.

# Possible Formation Mechanism of CoAl2O4/Kaol Hybrid Pigment

According to the above results, the as-prepared hybrid pigment exhibit good color performances at a low Co content compared with the commercial CoAl2O4 pigments even 7 wt%. The possible formation mechanism of CoAl2O4/Kaol hybrid pigments can be described as follows: Co3O4 and Al2O3 can be uniformly distributed on the surface of Kaol during grinding. With the increase of the calcining temperature (400∼600◦C), Kaol (2SiO2·Al2O3·2H2O) is turned into metakaolinite (2SiO2·Al2O3). As the calcining temperature increases to 900◦C, Co3O4 begins to thermally reduce into CoO, subsequently the increase in the calcining temperature results in the formation of spinel CoAl2O4 based on the reaction between CoO and Al2O3 on Kaol substrate accompanied with the crystal phase transition of Kaol to amorphous SiO2 and low-order crystalline α-Al2O3. In this process, the uniform distribution of Co3O4 and Al2O3 may be expected to decrease the mass transfer resistance for formation of spinel CoAl2O4, which reduces the processing time and calcining temperature, while the traditional solid-phase method for preparation of CoAl2O4 spends the long time ranging from several hours to days at high temperature (>1,200◦C) (Wang et al., 2006). Furthermore, α-Al2O3 from Kaol also simultaneously reacts with CoO during high-temperature crystallization, thus the generated CoAl2O4 and the calcined production of Kaol finally form a solid solution. The possible formation mechanism of CoAl2O4/Kaol hybrid pigment can be illustrated in **Scheme 1**.

# Environmental Stability of CoAl2O4/Kaol Hybrid Pigments

As shown in **Figure S5a**, the reflectance spectra of hybrid pigments are nearly same before and after being exposed under UV light for 15 days, indicating the excellent stability of hybrid pigment to UV light. In addition, the color of the sample plates has no obvious difference before and after being immersed into HCl, NaOH and ethanol, suggesting the excellent resistance of hybrid pigments to the corresponding reagents, which also can be confirmed from their color parameters (**Table 2**). Meanwhile, CoAl2O4/Kaol hybrid pigment presents the excellent thermal stability (**Figures S5b,c**).

In this study, the potential applications of the as-prepared hybrid pigment were also explored in this study. The thermal resistance coating was prepared by following procedures: Firstly, 5 wt% CoAl2O4/Kaol hybrid pigment was dispersed into ethyl acetate and stirred under high shear for 30 min (2,000 rpm), and the above obtained mill base were added into commercial thermal resistance coating and stirred under low shear for 60 min (500 rpm). The viscosity of the coating was adjusted by ethyl acetate to 20∼30 s. And then it was sprayed onto steel plate substrate of size 15 cm × 10 cm × 0.1 cm and dried under the standard conditions (temperature: 25◦C and humidity: 40%) for 7 days ( the coating thickness: D = 50 ± 2µm). The obtained sample plates were shown in **Figure S6c**, it is clear that the surface of the sample plate is smooth after hybrid pigment (5 wt%) is added into the commercial thermal resistance coating, indicating hybrid pigment disperses well in the painting. Furthermore, the film containing hybrid pigments exhibits the similar color parameters (L ∗ = 24.88, a ∗ = 13.54, b ∗ = −57.80) (**Figure S6d**) after being calcined at 800◦C for 2 h. However, the steel plate coated the

TABLE 2 | Color parameters of the sample plates before and after being immersed into 3M HCl, 3M NaOH, and ethanol for 72 h, respectively.

*The Commission Internationale de l'Eclairage (CIE) 1976 L*\**,a*\**,b*\* *colorimetric method was revised based on the Hunter L*\**a*\**b (1958). And the parameters of L*\* *is the lightness axis (0 for black and 100 for white). The parameters of a*\* *(negative values for green and positive values for red) and b\*(negative values for blue and positive values for yellow) denote the hue or color dimensions.*

commercial thermal resistance coating (**Figure S6a**) is damaged heavily (**Figure S6b**). It indicates that the film containing hybrid pigment has a good thermal stability, and it can effectively protect steel plate substrate to avoid excessive damage. Meanwhile, this film also exhibited excellent fireproof properties, which can be illustrated by its fire test using a flame gun (800∼1,000◦C) (**Figure S6e**). It is found that this film has no obvious change after being placed on the fire for 30 s (**Movie S1** and **Figure S6f**), revealing that the hybrid pigment may be used as fireproof paint. In addition, the as-prepared hybrid pigment also can be used as a colored painting and artistic pigment (**Figures S7a,b**).

# Anticorrosion Coating Application of Hybrid Pigments

Corrosion is a natural and gradual destruction process of metal materials resulting from chemical and/or electrochemical reaction with their environment, which leads to enormous losses to our lives, finances, etc. Therefore, corrosion engineering is the field to control and stop corrosion using various methods, especially surface coating protection technology, in which inorganic pigments (e.g., ZnO and TiO2) are commonly incorporated to improve the anticorrosion properties of coating, and perhaps provide the color for coating (e.g., α-Fe2O3). Therefore, the as-prepared hybrid pigment is incorporated into the common epoxy anticorrosion coating to investigate its anticorrosion properties. As shown in **Figure 5**, the steel plate substrate suffers from an aggravating corrosion referred from the large-scale red rust without the epoxy anticorrosion coating (**Figure 5e**). After coating of epoxy anticorrosion coating, the corrosion of steel plate is slightly improved (**Figure 5f**). In particular, this anticorrosion effect is obvious after introducing the commercial CoAl2O4 pigments (**Figure 5g**). With the incorporation of the as-prepared hybrid pigment, no obvious rust was found on the surface of coating (**Figure 5h**). It suggests that the introduction of hybrid pigment obviously improves the corrosion resistance of coating. This is mainly ascribed to the synergistic effect between the lamellar layered structure of Kaol and the excellent weatherability of CoAl2O4, which can prevents small molecules (H2O and O2) or ions from penetrating into the coating film. Thus, the relevant mechanisms of anticorrosion can be illustrated in **Scheme 2**.

Furthermore, the chemical stability of the anticorrosion coating film was also evaluated by immersing into 3M HCl, 3M NaOH, and ethanol at room temperature for 72 h, respectively.

coated the paint containing hybrid pigment. (e–h) Their corresponding images after being exposed to salt spray test for 60 days, respectively.

The film has no obvious difference before and after being immersed, exhibiting the excellent resistance to HCl, NaOH, and ethanol (**Figure S8**). This is also confirmed by their color

TABLE 3 | Color parameters of the film before and after being immersed into 3M HCl, 3M NaOH and ethanol for 72 h, respectively.

*The Commission Internationale de l'Eclairage (CIE) 1976 L*\**,a*\**,b*\* *colorimetric method was revised based on the Hunter L*\**a*\**b (1958). And the parameters of L*\* *is the lightness axis (0 for black and 100 for white). The parameters of a*\* *(negative values for green and positive values for red) and b*\**(negative values for blue and positive values for yellow) denote the hue or color dimensions.*

parameters before and after being immersed in the corresponding reagents (**Table 3**). More importantly, the obtained sample plates were also immersed into 3.5 wt% saline water to study the salted water resistance test for 60 days. As shown in **Figure 6**, the surface of coating film without pigment is smooth and dense before immersing (**Figure 6a**). After adding of CoAl2O4 without Kaol, the film color seems darker compared with that of containing CoAl2O4/Kaol hybrid pigment (**Figures 6b,c**). As expected, the film without pigment is swelling, blistering, and rusty after immersed into saline water for 60 days (**Figure 6d**). Furthermore, the same phenomenon also is observed from the one containing CoAl2O4 (**Figure 6e**), but the situation is better than the former due to the good weatherability of CoAl2O4 to acid, salt, etc. Surprisingly, the film incorporating of CoAl2O4/Kaol hybrid pigment was perfect without any defects (**Figure 6f**). It suggests that the coating film exhibited the excellent salted water resistance, which may be ascribed to the synergistic effect of the lamellar layered structure of Kaol and the excellent weatherability of CoAl2O4.

Furthermore, the obtained sample plate containing hybrid pigment presents the better acid resistance than that of incorporating of ultramarine after contacting with 36.5 wt% HCl, which changes from blue to white due to its instability to acid (**Figure S9**). Therefore, the obtained sample plate was also placed in acetic acid-salt spray testing chamber to evaluate the acetic acid-salt fog corrosion resistance of the anticorrosion coating. As a control, the coatings containing Kaol, ZnO, Fe2O3 and CoAl2O4 calcined at 1,100◦C are also investigated, and the components of paints were provided in **Table S3**. Firstly, the different amounts of the CoAl2O4/Kaol hybrid pigment were added into epoxy resin to determine the amounts of the hybrid pigment, and the results are shown in **Figure S10**. It is clear that the sample that containing 5 wt% CoAl2O4/Kaol hybrid pigment is bright blue, so the added amounts of the hybrid pigment is 5 wt%. And then the sample containing Kaol, ZnO, Fe2O3, and CoAl2O4 calcined at 1,100◦C are shown in **Figure 7**, the corrosion of the films containing Fe2O3 (**Figure 7f**), CoAl2O4 calcined at 1,100◦C (**Figure 7g**), and ZnO (**Figure 7j**) are very serious after be exposed in salt spray testing chamber 60 days. Interestingly, the corrosion of the sample containing

CoAl2O4/Kaol hybrid pigment (**Figure 7h**) and Kaol (**Figure 7j**) were negligible, especially the sample of hybrid pigment. It may be ascribed to the excellent acid resistance and lamellar layered structure of hybrid pigment and Kaol, respectively. In addition, **Figure 8** gives the anticorrosion ability of the coatings containing CoAl2O4/Kaol hybrid pigments, ZnO, Fe2O3, CoAl2O4 calcined at 1,100◦C and the one without pigment. Bode plots indicates that the addition of CoAl2O4/Kaol hybrid pigments in epoxy paint shows a significant improvement in the anticorrosion properties compared with the pure epoxy paint and others pigment. It also suggests the synergistic effect between the lamellar layered structure of Kaol and the excellent weatherability of CoAl2O4 is in favor of improving the resistance to salts, which also is superior to the reported works (**Table 4**).

Anti-corrosion of the anticorrosive film is commonly determined from electrochemical measurements such as corrosion potential (Ecorr) and corrosion current (Icorr), and Electrochemical Impedance Spectroscopy (EIS) measurements were carried out with the use of an electrochemical working station (CHI660E) at open circuit potential. A three-electrode cell including the Ag/AgCl (3M KCl) reference electrode, the studied

FIGURE 8 | Bode plots of the coatings containing CoAl2O4/Kaol hybrid pigment, ZnO, Fe2O3, CoAl2O4, the control without pigment and steel before (A) and after (B) being immersed in the 3.5 wt% NaCl solution for 60 days.

TABLE 4 | Comparison with the anticorrosion performance of the reported additives and hybrid pigment in epoxy anticorrosive coating.

sample as working electrode and the platinum counter electrode was used to run EIS tests. EIS measurements are studied in the frequency range from 100 kHz to 0.005 Hz with a perturbation of 5 mV, and the working electrode area was 1 cm2 . **Figure 9** shows the Tafel plots generated for the anticorrosion coating containing CoAl2O4/hybrid pigment, ZnO, Fe2O3, CoAl2O4 calcined at 1,100◦C, control (without pigment) and blank immersed in 3.5 wt% NaCl solution about 30 min. And the Tafel curve parameters are given in **Table 5**. As shown in **Figure 9** and **Table 5**, the corrosion current of the control (without pigment) decreases from −1.52 × 10−7 A to −3.39 × 10−8 A compared with the blank, and the corrosion current obviously declines when the pigments were added into the anticorrosion coating. After incorporation of CoAl2O4/hybrid pigment, the corrosion current decreases from −3.39 × 10−8 A to −5.75 × 10−12 A, while the corrosion potential increases from −1.439 V to −0.443 V. In fact, the anticorrosion ability of the coating is related to the chemical composition and microstructure. The lower of the corrosion current or the higher corrosion potential suggests the better of the anticorrosion ability (Liang et al., 2007; Chang et al., 2012). Therefore, the anticorrosion coatings containing CoAl2O4/hybrid pigment, ZnO, Fe2O3 present good anticorrosion ability, and CoAl2O4/hybrid pigment exhibits superior anticorrosion ability compared with than that of ZnO and Fe2O3, which is consistent with that of the Bode plots (**Figure 8**), implying that the addition of CoAl2O4/Kaol hybrid pigments in epoxy paint shows a significant improvement in the electrochemical corrosion properties compared with the pure epoxy paint and other pigments.

#### CONCLUSION

In conclusion, the low-cost bright blue CoAl2O4/Kaol hybrid pigments were successfully prepared by the traditional solidstate reaction. The introduction of Kaol not only reduces the Co consumption and the temperature for formation of spinel CoAl2O4, but also enhances the color properties of cobalt blue pigments with bright blue and high NIR reflectance, especially prepared using 8.1% Co3O4 and 81.5% Kaol. During preparation, Co3O4 and Al2O3 can be uniformly distributed on the surface of Kaol, which reduces the mass transfer resistance and prevents from the aggregation CoAl2O4 nanoparticles during high-temperature crystallization process. In addition, the Al2O3 originated from Kaol also may participate in the formation of spinel CoAl2O4 pigments. Due to the synergistic effect between the lamellar layered structure of Kaol and the excellent

#### REFERENCES

weatherability of CoAl2O4, the anticorrosion coating containing hybrid pigments exhibited the excellent corrosion resistance toward the salt and acetic acid-salt system. Furthermore, the CoAl2O4/Kaol hybrid pigments prepared by traditional solidphase method can be applied in thermal resistance coating, wall painting and artistic pigment.

#### AUTHOR CONTRIBUTIONS

AZ and XW contribute to the experiment process, samples characterization, data analysis, and paper preparation. BM and AW are mainly responsible for the design of experiment, data analysis, and paper revision.

#### ACKNOWLEDGMENTS

This work is supported by the Regional Key Project of the Science and Technology Service of the Chinese Academy of Sciences ([2016] No. 70), the Youth Innovation Promotion Association CAS (2017458), the Major Projects of the National Natural Science Foundation of Gansu, China (18JR4RA001), and The Funds for Creative Research Groups of Gansu, China (17JR5RA306).

#### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2018.00586/full#supplementary-material

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Zhang, Mu, Wang and Wang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Formation and Coloring Mechanism of Typical Aluminosilicate Clay Minerals for CoAl2O4 Hybrid Pigment Preparation

#### Anjie Zhang1,2,3, Bin Mu1 \*, Xiaowen Wang1,2, Lixin Wen3 and Aiqin Wang1 \*

*1 Key Laboratory of Clay Mineral Applied Research of Gansu Province, Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China, 2 University of Chinese Academy of Sciences, Beijing, China, 3 Northwest Yongxin Coatings Limited Company, Lanzhou, China*

#### Edited by:

*Pu-Xian Gao, University of Connecticut, United States*

#### Reviewed by:

*Sibo Wang, University of Connecticut, United States Kaixuan Bu, Rutgers University, The State University of New Jersey, United States Quansheng Liu, Changchun University of Technology, China*

#### \*Correspondence:

*Bin Mu<EMAIL_ADDRESS>Aiqin Wang<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Green and Sustainable Chemistry, a section of the journal Frontiers in Chemistry*

> Received: *05 February 2018* Accepted: *03 April 2018* Published: *19 April 2018*

#### Citation:

*Zhang A, Mu B, Wang X, Wen L and Wang A (2018) Formation and Coloring Mechanism of Typical Aluminosilicate Clay Minerals for CoAl*2*O*4 *Hybrid Pigment Preparation. Front. Chem. 6:125. doi: 10.3389/fchem.2018.00125* Different kinds of aluminosilicate minerals were employed to fabricate CoAl2O4 hybrid pigment for studying its formation and coloring mechanism. It revealed that the color of the obtained hybrid pigments was determined by the content of Al2O3 and lightness of clay minerals. The higher the Al2O3 content and the lightness of clay minerals, the better the color parameters of hybrid pigments. During the preparation of hybrid pigments, CoAl2O4 nanoparticles were confined to be loaded on the surface of the aluminosilicate minerals, which effectively prevented from the aggregation and the size increase of CoAl2O4 nanoparticles. What's more, aluminosilicate mineral might be an ideal natural aluminum source to compensate the aluminum loss due to the dissolution of Al(OH)3 at alkaline medium during precursor preparation, keeping an optimum molar ratio of Co2+/Al3+ for formation of spinel CoAl2O4 pigments in the process of high-temperature crystallization.

 As a high-grade eco-friendly intense blue pigment, it can be applied in the fields of ceramics, plastics, paint, glass, and color TV tubes due to high refractive index, excellent chemical, and thermal stability (Mahé et al., 2008; Ryu et al., 2008; Tirsoaga et al., 2011; Merino et al., 2015; Soleimani-Gorgania et al., 2015). However, the high cost of CoAl2O4 pigments has severely restrained their wide applications because of the expensive cobalt compounds (De Souza et al., 2009; Gholizadeh and Malekzadeh, 2017; Zhang et al., 2017). In addition, the traditional method for preparation of cobalt blue pigment was involved in the calcination of CoO and Al2O3 at above 1,300◦C for a long time (Armijo, 1969; Salavati-Niasari et al., 2009; Sale, 2015; He et al., 2017; Zhang et al., 2017), which was obviously time-consuming. Therefore, it is urgent to develop a strategy to prepare the low-cost cobalt blue pigment with prefect color property in order to realize its wide applications.