Removal of Cr(VI) and Toxic Ions from Aqueous Solutions and Tannery Wastewater Using Polymer-Clay Composites

: Polymer-clay composites were prepared by natural zeolite (clinoptilolite) or naturally local clay deposits in an N , N -methylene-bis-acrylamide as cross-linked. The resultant composites were used for the removal of Cr(VI) from an aqueous solution. Additionally, their effects on soluble ions of tannery wastewater were investigated. The produced composites were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM). The results showed that Cr(VI) removal is dependent upon initial concentrations and pH. The adsorption quantity of Cr(VI) onto the polymerized clay deposit followed by polymerized zeolite exhibited higher values than their original samples. The polymer-clay composite of clay deposit showed the highest removal of 76.3–100% overall initial concentrations of 10–50 mg L − 1 and at initial pH of 2. Kinetics of Cr(VI) removal by various sorbents was predicted using a pseudo–second order model. Our ﬁndings showed that the levels of salinity and various soluble ions (Cr 2+ , Na + , Cl − and SO 42 − ) in tannery wastewater are very high, and their levels were reduced after treatment, especially by polymerized sorbents. It could be concluded that the polymer-clay composites may be employed as a highly efﬁcient sorbent for the removal of Cr(VI) and toxic ions from the wastewater.


Introduction
The leather industry is considered one of the industries that contributes most to environmental pollution [1,2]. Tannery wastewater contains large amounts of pollutants including sodium chloride, sodium sulfate, toxic metallic compounds of chromium, and various other organic and inorganic compounds [1][2][3][4]. The tanning industries discharge their effluents into nearby lands or rivers, damaging the environmental compartments and thus causing serious harm to human health [2,4,5].
The removal of salts and toxic metallic compounds such as chromium ions from tannery liquid waste is a necessary action to protect public health and the environment. In this context, several traditional techniques for the removal of contaminants from wastewater and effluents were conducted using different means, including chemical precipitation, oxidation and reduction, ion exchange, electrochemical treatments, and reverse osmosis [6][7][8][9][10][11]. However, these techniques are mostly characterized by their high cost and difficulty. Recently, the use of clay minerals and their modified forms, such as a high specific surface area and high cation exchange capacity (CEC) with chemical and mechanical stability, has received much attention as an effective sorbent [12,13]. Among aluminosilicate

Sample Collection, Preparation, and Synthesis
Naturally occurring deposits of aluminum silicate rich in zeolite were collected from Shama Mountain (20 • 44 29.1 N; 39 • 42 04.8 E), Saudi Arabia, while the natural zeolite used in this study was clinoptilolite (Morgan Co., Morgan, Mountain Green, UT, USA). The samples of natural zeolite and naturally occurring local clay deposits were washed with deionized water, allowed to air-dry, and then sieved using 50 µm.
The samples of natural zeolite and naturally occurring local clay deposits were grafted using polyacrylamide according to Shigetomi et al. [24]. Specifically, 25.0 g of each clay sample was put in 250 mL aqueous solution (2.5 g N,N-methylenebisacrylamide and 1.5 g potassium peroxydisulfate). Then, 7.5 g of acrylamide was added, the mixture was stirred at 70 • C. After that, the resultant polymerized materials were rinsed with deionized water and dried at 80 • C.

SEM, XRD, and FTIR Analysis
Scanning electron microscope (SEM; FEI, Inspect S50, The Netherlands) was employed to identify the morphological changes in the surface structure of raw and modified samples. X-ray diffraction (XRD) analysis (XRD-7000 Shimadzu, Japan) was used to identify any crystallographic structure in the adsorbents. Fourier transformation infrared analysis (Nicolet 6700 FT-IR, Thermo Fisher Scientific, Madison, WI, USA) of the adsorbent samples was performed in the wave number range of 400-4000 cm −1 in order to characterize their surface functional groups.

The Influence of Initial pH
The influence of initial feed pH of the aqueous solution on Cr(VI) adsorption onto various adsorbents was carried out using an aqueous solution (50 mL) containing K 2 Cr 2 O 7 (at concentration of 20 mg Cr(VI) L −1 adjusted at pH of 2, 4, 6, 8, and 10 using 0.1 M HCl and 0.1 M NaOH. The solutions were shaken for 120 min in plastic containers containing 0.5 g of each adsorbent. After that, suspensions were centrifuged, and the concentrations of Cr(VI) in the supernatant solutions were analyzed colorimetrically using a spectrophotometer following United State Environmental Protection Agency (USEPA) Method-7196A [25].

The Influence of Contact Time
To investigate the equilibrium dynamics, duplicate samples (0.5 g) of each sorbent were shaken for 5, 15, 30, 60, 80, and 120 min in polypropylene centrifuge tubes at room temperature with 50 mL of aqueous solution containing K 2 Cr 2 O 7 (at concentration of 20 mg Cr(VI) L −1 and initial pH of 2). Then, suspensions were centrifuged, and the concentrations of Cr(VI) in the supernatant solutions were analyzed colorimetrically by using a spectrophotometer. The following different kinetic models (Equations (1)-(4)) were applied to describe adsorption of Cr(VI) by using each sorbent [26].

Adsorption Isotherm
An amount (0.5 g) of raw and polymerized adsorbents of natural zeolite and naturally occurring local clay deposits was weighed and put into 100 mL glass bottles. Duplicate samples (0.5 g) of each sorbent were shaken for 120 min in polypropylene centrifuge tubes at room temperature with 50 mL of the solution containing K 2 Cr 2 O 7 (at a concentration of 10-50 mg Cr(VI) L −1 and initial pH of 2). The quantities of Cr(VI) adsorbed by the various adsorbents were determined using the following equation (Equations (5)): where q e is Cr(VI) concentration on the adsorbents (mg g −1 ) at equilibrium, C e is Cr(VI) concentration in solution (mg L −1 ) at equilibrium, C o is initial metal concentration in solution (mg L −1 ), V is the used volume of initial aqueous solution (L), and m is mass of adsorbent (g). Sorption isotherm parameters were obtained by applying the following models (Equations (6)-(8)) [27]: Langmuir model: C e /q e = 1/(K a q m ) + C e /q m (6) Freundlich model Log q e = log K f + (1/n) log C e Temkin model: q e = a + b lnC e (8) where C e = equilibrium concentration (mg L −1 ) in solution phase, q e = equilibrium concentration (mg g −1 ) in solid phase, q m = Maximum sorption capacity (mg g −1 ), K a = sorption constant (L mg −1 ) in relation to enthalpy, n = sorption constant in relation to intensity, K f = sorption capacity constant (L mg −1 ), and a and b = constants in relation to adsorption energy and capacity.

Characterization of Tannery Wastewater and Its Treatment
The wastewater samples were collected from a number of tannery factories from Riyadh, Saudi Arabia. The collected samples were filtered and stored for the chemical analysis. The wastewater samples were analyzed for various chemical properties including pH, electrical conductivity (EC), soluble anions and cations and toxic heavy metals including Cr 2+ , Cd 2+ , and Pb 2+ . The wastewater pH and EC were measured using a digital pH and EC meter, respectively. The concentrations of soluble Na + , SO 4 −2 , and Cl − were also measured using Ion Chromatography (IC; Dionex 5000, Sunnyvale, CA, USA). The toxic heavy metals (Cr 2+ , Cd 2+ , and Pb 2+ ) concentrations were analyzed using Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES; Perkin Elmer Optima 4300 DV, Norwalk, CT, USA). In this study, the influence of raw and modified sorbents through polymerization on the removal of soluble ions (Cr 2+ , Na + , SO 4 2− , and Cl − ) from tannery wastewater was also investigated. The wastewater samples were shaken in plastic containers containing 1, 2, 4, 6, 8, and 10 g sorbent L −1 for 2 h. Then, for equilibrium, the mixtures were allowed to stand for 24 h. Concentrations of soluble Cr 2+ in the supernatant solutions were analyzed after acidification using HNO 3 by ICP-OES. Additionally, soluble ions of Na + , SO 4 2− , and Cl − at a sorbent dose of 10 g L −1 were analyzed using IC.

Characterization of the Raw and Polymer-Clay Composites
Powder X-ray diffraction patterns of natural zeolite or naturally occurring local clay deposits and their obtained composites (polymer-zeolite and polymer-clay deposit) are shown in Figure 1. The XRD spectra of the raw natural zeolite and clay deposit have different peaks. The XRD results indicate that raw natural zeolite is characterized by clinoptilolite as dominated mineral. The XRD spectra of raw clay deposit show the presence of the following minerals: chabazite (at 9.41, 3.93, and 2.12 Å), analcime (3.4, 5.6, and 2.69 Å), feldspar (at 4.02 and 3.19 Å) and quartz (at 3.34 Å). By comparing the XRD data of the polymer-samples with those of raw samples, some deviations in 2 Sustainability 2017, 9,1993

Characterization of Tannery Was
The wastewater samples were Arabia. The collected samples we samples were analyzed for variou soluble anions and cations and tox and EC were measured using a dig Na + , SO4 −2 , and Cl − were also measu USA). The toxic heavy metals (Cr 2 Coupled Plasma Optical Emission CT, USA).
In this study, the influence removal of soluble ions (Cr 2+ , Na + The wastewater samples were sha L −1 for 2 h. Then, for equilibrium, soluble Cr 2+ in the supernatant sol Additionally, soluble ions of Na + , S

Characterization of the Raw and
Powder X-ray diffraction pat and their obtained composites (po The XRD spectra of the raw natur indicate that raw natural zeolite i spectra of raw clay deposit show t 2.12 Å), analcime (3.4, 5.6, and 2 comparing the XRD data of the p Ɵ and d-spacing values w Figure 1. The X-ray diffraction The functional groups onto th naturally occurring local clay de and d-spacing values were detected. Sustainability 2017, 9,1993 4 of 14

Characterization of Tannery Wastewater and Its Treatment
The wastewater samples were collected from a number of tannery factories from Riyadh, Saudi Arabia. The collected samples were filtered and stored for the chemical analysis. The wastewater samples were analyzed for various chemical properties including pH, electrical conductivity (EC), soluble anions and cations and toxic heavy metals including Cr 2+ , Cd 2+ , and Pb 2+ . The wastewater pH and EC were measured using a digital pH and EC meter, respectively. The concentrations of soluble Na + , SO4 −2 , and Cl − were also measured using Ion Chromatography (IC; Dionex 5000, Sunnyvale, CA, USA). The toxic heavy metals (Cr 2+ , Cd 2+ , and Pb 2+ ) concentrations were analyzed using Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES; Perkin Elmer Optima 4300 DV, Norwalk, CT, USA).
In this study, the influence of raw and modified sorbents through polymerization on the removal of soluble ions (Cr 2+ , Na + , SO4 2-, and Cl − ) from tannery wastewater was also investigated. The wastewater samples were shaken in plastic containers containing 1, 2, 4, 6, 8, and 10 g sorbent L −1 for 2 h. Then, for equilibrium, the mixtures were allowed to stand for 24 h. Concentrations of soluble Cr 2+ in the supernatant solutions were analyzed after acidification using HNO3 by ICP-OES. Additionally, soluble ions of Na + , SO4 2-, and Cl − at a sorbent dose of 10 g L −1 were analyzed using IC.

Characterization of the Raw and Polymer-Clay Composites
Powder X-ray diffraction patterns of natural zeolite or naturally occurring local clay deposits and their obtained composites (polymer-zeolite and polymer-clay deposit) are shown in Figure 1. The XRD spectra of the raw natural zeolite and clay deposit have different peaks. The XRD results indicate that raw natural zeolite is characterized by clinoptilolite as dominated mineral. The XRD spectra of raw clay deposit show the presence of the following minerals: chabazite (at 9.41, 3.93, and 2.12 Å), analcime (3.4, 5.6, and 2.69 Å), feldspar (at 4.02 and 3.19 Å) and quartz (at 3.34 Å). By comparing the XRD data of the polymer-samples with those of raw samples, some deviations in 2Ɵ and d-spacing values were detected.  The functional groups onto the surface of raw and polymerized materials of natural zeolite and naturally occurring local clay deposits were analyzed by means of FT-IR. FT-IR spectra of all investigated materials are presented in Figure 2. For raw natural zeolite, there are bands at 3628.87, 3449.80 cm −1 , and 1636.0 cm −1 , which are corresponding to a typical stretching and bending of OH groups in water molecules associated with Na and Ca in the zeolite structure. The band corresponding to asymmetry stretch (Si-O or Al-O stretching) in TO 4  For the composite of polymer-clay deposit, its spectrum shows two bands at 3187.62 and 2944.21 cm −1 , which are due to -NH 2 and C-H stretching, respectively. Additionally, the bands corresponding to bond vibrations at 1651.60 cm −1 (C=O stretching), 1525.99 cm −1 (C=C stretching), 1449.57 cm −1 (C-H deformation), and 1311.37 cm −1 (C-N stretching) were detected on the surface of the polymerized clay deposit. Compared to un-polymerized samples of zeolite and clay, the characteristic absorption peaks (around 3440 cm −1 ) of polymerized samples have become strong, mainly due to the amine group (-NH). The FTIR spectra of synthesized composites of polymer-clay through polymerization process have been investigated by several authors [28,29]. Similar to our finding, they found that the stretching vibrations of C-H, C=C, -NH 2 , C-N and C=O bonds were detected on the surface-polymerized samples. This indicates that the functional groups interact with clays, and thus, composites of polymer-clay were synthesized. For the composite of polymer-clay deposit, its spectrum shows two bands at 3187.62 and 2944.21 cm −1 , which are due to -NH2 and C-H stretching, respectively. Additionally, the bands corresponding to bond vibrations at 1651.60 cm −1 (C=O stretching), 1525.99 cm −1 (C=C stretching), 1449.57 cm −1 (C-H deformation), and 1311.37 cm −1 (C-N stretching) were detected on the surface of the polymerized clay deposit. Compared to un-polymerized samples of zeolite and clay, the characteristic absorption peaks (around 3440 cm −1 ) of polymerized samples have become strong, mainly due to the amine group (-NH). The FTIR spectra of synthesized composites of polymer-clay through polymerization process have been investigated by several authors [28,29]. Similar to our finding, they found that the stretching vibrations of C-H, C=C, -NH2, C-N and C=O bonds were detected on the surface-polymerized samples. This indicates that the functional groups interact with clays, and thus, composites of polymer-clay were synthesized.  Figure 3 shows the SEM images for the raw and polymerized zeolite and clay deposit. The SEM images of natural zeolite and local clay deposit after polymerization showed the presence of the large sticking particles with a smooth surface, mainly due to the polymerization process.

Adsorption Kinetics and Isotherms
In this study, different kinetics models (pseudo-second order, Elovich, Power function, Parabolic diffusion) were applied on the obtained adsorption data in relation with time to evaluate the kinetics process for Cr(VI) adsorption onto various sorbents. The equilibrium time was pronounced at 5-80 min among various sorbents ( Figure 4). The results indicate that the adsorption data were well described by the pseudo-second order kinetic model with the correlation coefficients (r 2 ) of 0.9996-1.000 for the investigated sorbents (Table 1). Therefore, the pseudo-second order can be applied for predicting the qe values theoretically for all the studied range of initial Cr(VI) concentrations. Our results suggest that Cr(VI) adsorption onto polymerized and unpolymerized sorbents may be described by chemisorption.

Adsorption Kinetics and Isotherms
In this study, different kinetics models (pseudo-second order, Elovich, Power function, Parabolic diffusion) were applied on the obtained adsorption data in relation with time to evaluate the kinetics process for Cr(VI) adsorption onto various sorbents. The equilibrium time was pronounced at 5-80 min among various sorbents ( Figure 4). The results indicate that the adsorption data were well described by the pseudo-second order kinetic model with the correlation coefficients (r 2 ) of 0.9996-1.000 for the investigated sorbents (Table 1). Therefore, the pseudo-second order can be applied for predicting the q e values theoretically for all the studied range of initial Cr(VI) concentrations. Our results suggest that Cr(VI) adsorption onto polymerized and unpolymerized sorbents may be described by chemisorption.

Adsorption Kinetics and Isotherms
In this study, different kinetics models (pseudo-second order, Elovich, Power function, Parabolic diffusion) were applied on the obtained adsorption data in relation with time to evaluate the kinetics process for Cr(VI) adsorption onto various sorbents. The equilibrium time was pronounced at 5-80 min among various sorbents ( Figure 4). The results indicate that the adsorption data were well described by the pseudo-second order kinetic model with the correlation coefficients (r 2 ) of 0.9996-1.000 for the investigated sorbents (Table 1). Therefore, the pseudo-second order can be applied for predicting the qe values theoretically for all the studied range of initial Cr(VI) concentrations. Our results suggest that Cr(VI) adsorption onto polymerized and unpolymerized sorbents may be described by chemisorption.  The adsorption isotherms show that the adsorption quantity of Cr(VI) on various raw and polymerized sorbents increased with increasing equilibrium Cr(VI) concentrations (Figure 4). The adsorption quantity of Cr(VI) on the polymerized clay deposit followed by polymerized zeolite exhibited higher values than their original samples before polymerization. This finding suggests the potential removal of Cr(VI) using polymerized materials and their higher affinity toward Cr(VI) could be explained by the strong interaction of Cr(VI) with the formed surface groups of polymerized sorbents.
The experimental data on Cr(VI) adsorption were fitted to the all three used models (Langmuir, Freundlich, and Temkin) with the correlation coefficients >0.90 (Table 2). Overall, the correlation coefficient values (r 2 ) are higher for the Freundlich isotherm (0.9560-0.9987) than for the Langmuir isotherm (0.9232-0.9884) and Temkin isotherm (0.9340-0.9818), suggesting that the Freundlich isotherm model described well the Cr(VI) adsorption onto original and polymerized materials of zeolite and clay deposit. This indicates that Cr(VI) ions are sorbed onto heterogeneous sites with different binding energies. The 1/n values of Freundlich model for Cr(VI) adsorption are lower than 1, suggesting a favorable adsorption. Thus, chemisorption can control the adsorption of Cr(VI) onto various raw and polymerized sorbents, indicating a high affinity of Cr (VI) toward the sorbents [30]. The Freundlich constant (K f ), which is generally in relation to the adsorption capacity, showed higher values (0.662) for polymerized clay deposit than those (0.141-0.184) of other sorbents.

Removal Efficiency and Mechanism
The removal efficiency of sorbents for Cr(VI) is presented in Figure 5. The initial solution pH of the 25 mL of an initial Cr(VI) concentration of 25 mg L −1 was in a range of 2-10. The results showed that the removal efficiency of Cr(VI) from aqueous solutions was dependent upon initial pH. The highest removal efficiency of Cr(VI) for all the investigated sorbents (raw and polymerized sorbents of zeolite and naturally occurring local clay deposit) was pronounced for the lowest initial solution pH of 2. It has been reported that the sorbents' surfaces that are positively charged are predominant under acid conditions, which are easily attracted toward negatively charged chromium species (HCrO 4 and Cr 2 O 4 2− ) [31,32]. Therefore, the maximum removal efficiency was pronounced for a pH value of 2. It was noticed that the removal percentage of Cr(VI) onto various sorbents was 59.3% (raw zeolite), 69.0% (polymerized zeolite), 62.7% (raw clay deposit), and 90.1% (polymerized clay deposit). However, this highest removal efficiency was decreased with increasing initial pH of aqueous solutions and reached 44.6% (raw zeolite), 50.4% (polymerized zeolite), 45.7% (raw clay deposit), and 55.3% (polymerized clay deposit) at the highest initial pH of 10. This behavior suggests that the adsorption process depends on the dominated species of hexavalent Cr. Previous studies suggested that the monovalent form (HCrO 4 − ) dominated at low pH required only a single site on the clay minerals' surface [33]. On the contrary, at high pH values, the divalent form (CrO 4 2− ) dominated at high pH values required two sites with a positive charge on the surface of clay minerals. Additionally, the excess of OH ions at higher pH is possible to compete with (CrO 4 2− ), resulting in surprising Cr(VI) immobilization onto the surface of the sorbents. Our results may also suggest that the formed functional groups on the surfaces of sorbents due to polymerization stay in protonation form at acidic conditions, resulting in an increase of the adsorbed chromate quantity. However, as the solution pH increases, the functional groups de-protonation/ionization of the polymerized sorbents tended to occur, resulting in decreasing the adsorbed chromate quantity. It has been previously reported that the adsorption of hexavalent chromium onto the surface of various materials is mainly governed by ion-exchange or outer-sphere complexation at low pH values and by inner-sphere complexation at high pH [34].
Sustainability 2017, 9, 1993 8 of 14 suggested that the monovalent form (HCrO4 − ) dominated at low pH required only a single site on the clay minerals' surface [33]. On the contrary, at high pH values, the divalent form (CrO4 2− ) dominated at high pH values required two sites with a positive charge on the surface of clay minerals. Additionally, the excess of OH ions at higher pH is possible to compete with (CrO4 2− ), resulting in surprising Cr(VI) immobilization onto the surface of the sorbents. Our results may also suggest that the formed functional groups on the surfaces of sorbents due to polymerization stay in protonation form at acidic conditions, resulting in an increase of the adsorbed chromate quantity. However, as the solution pH increases, the functional groups de-protonation/ionization of the polymerized sorbents tended to occur, resulting in decreasing the adsorbed chromate quantity. It has been previously reported that the adsorption of hexavalent chromium onto the surface of various materials is mainly governed by ion-exchange or outer-sphere complexation at low pH values and by inner-sphere complexation at high pH [34]. The influence of the feed initial Cr(VI) concentrations on removal efficiency by various sorbents of raw and polymerized sorbents of zeolite and clay deposit is shown in Figure 5. It was noted that the removal efficiency of Cr(VI) decreased with an increase in Ci, indicating that the availability of binding sites for Cr(VI) may be occupied, and hence preventing further adsorption of Cr(VI) onto the surface of the investigated adsorbents. It was generally observed that the removal efficiency increased in the following order: polymer clay deposit > polymer-zeolite > raw clay deposit > raw zeolite. The high ability of polymerized sorbents (especially polymerized clay deposit) to remove Cr(VI) from aqueous solutions may be explained by the strong basic nature and high ionization degree of the amine moiety, resulting in sites loaded with positive charges (≡N + ) being able to interact electrostatically with anionic forms of Cr(VI) [35]. Several studies have shown that enhancing adsorption capacity of various materials to remove negatively charged ions can be achieved by polymerization process [23,36]. In order to confirm the presence of Cr(VI) on the sorbents, FTIR analysis was conducted before and after the adsorption process (Figures 6 and 7). Comparing the results of the FTIR spectra before and after adsorption onto raw zeolite and clay deposits, some shifts were detected in the stretching vibrations of OH and Si-O or Al-O groups groups ( Figure 6). The results of the FTIR spectra before and after the adsorption onto polymerized clay deposits showed a shift in the stretching vibration for the hydrogen bonds of OH at 3435.23 cm −1 (before adsorption) to 3431.08 cm −1 (after adsorption), for -NH2 at 3187.62 cm −1 (before adsorption) to 3200.36 cm −1 (after adsorption), for C-H at 2944.21 cm −1 (before adsorption) to 2941.49 cm −1 (after adsorption), for C=O at 1651.60 cm −1 (before adsorption) to 1659.13 cm −1 (after adsorption), and for C-N at 1311.37 cm −1 to 1292.26 cm −1 upon Cr(VI) adsorption (Figure 7). This confirms that the OH, -NH2, C-H, C=O, and C-N groups are in connection with the Cr(VI) adsorption. In addition, there is a shift in the stretching vibrations of OH, C-H, and Si-O or Al-O groups for polymerized zeolite after The influence of the feed initial Cr(VI) concentrations on removal efficiency by various sorbents of raw and polymerized sorbents of zeolite and clay deposit is shown in Figure 5. It was noted that the removal efficiency of Cr(VI) decreased with an increase in C i , indicating that the availability of binding sites for Cr(VI) may be occupied, and hence preventing further adsorption of Cr(VI) onto the surface of the investigated adsorbents. It was generally observed that the removal efficiency increased in the following order: polymer clay deposit > polymer-zeolite > raw clay deposit > raw zeolite. The high ability of polymerized sorbents (especially polymerized clay deposit) to remove Cr(VI) from aqueous solutions may be explained by the strong basic nature and high ionization degree of the amine moiety, resulting in sites loaded with positive charges (≡N + ) being able to interact electrostatically with anionic forms of Cr(VI) [35]. Several studies have shown that enhancing adsorption capacity of various materials to remove negatively charged ions can be achieved by polymerization process [23,36]. In order to confirm the presence of Cr(VI) on the sorbents, FTIR analysis was conducted before and after the adsorption process (Figures 6 and 7). Comparing the results of the FTIR spectra before and after adsorption onto raw zeolite and clay deposits, some shifts were detected in the stretching vibrations of OH and Si-O or Al-O groups groups ( Figure 6). The results of the FTIR spectra before and after the adsorption onto polymerized clay deposits showed a shift in the stretching vibration for the hydrogen bonds of OH at 3435.23 cm −1 (before adsorption) to 3431.08 cm −1 (after adsorption), for -NH 2 at 3187.62 cm −1 (before adsorption) to 3200.36 cm −1 (after adsorption), for C-H at 2944.21 cm −1 (before adsorption) to 2941.49 cm −1 (after adsorption), for C=O at 1651.60 cm −1 (before adsorption) to 1659.13 cm −1 (after adsorption), and for C-N at 1311.37 cm −1 to 1292.26 cm −1 upon Cr(VI) adsorption (Figure 7). This confirms that the OH, -NH 2 , C-H, C=O, and C-N groups are in connection with the Cr(VI) adsorption. In addition, there is a shift in the stretching vibrations of OH, C-H, and Si-O or Al-O groups for polymerized zeolite after Cr(VI) adsorption. After Cr(VI) adsorption onto polymerized zeolite, the stretching vibrations of the OH at 3442.61 cm −1 , C-H at 2944.17 cm −1 and 2774.95 cm −1 , and Si-O or Al-O at 1069.7 cm −1 are respectively shifted to 3439. 15, 2946.33, 2879.05 and 1059.32 cm −1 . This suggests that these functional groups with several interactions after binding may be responsible for contribution in the process of Cr(VI) adsorption onto polymerized zeolite. Pandey and Mishra [36] suggested that the anionic forms of hexavalent chromium may be adsorbed onto polymer-clay nanocomposite via electrostatic attraction with the functional groups (such as amine). This suggests that these functional groups with several interactions after binding may be responsible for contribution in the process of Cr(VI) adsorption onto polymerized zeolite. Pandey and Mishra [36] suggested that the anionic forms of hexavalent chromium may be adsorbed onto polymer-clay nanocomposite via electrostatic attraction with the functional groups (such as amine).    This suggests that these functional groups with several interactions after binding may be responsible for contribution in the process of Cr(VI) adsorption onto polymerized zeolite. Pandey and Mishra [36] suggested that the anionic forms of hexavalent chromium may be adsorbed onto polymer-clay nanocomposite via electrostatic attraction with the functional groups (such as amine).

Characterization of Tannery Wastewater and Its Treatments Using Polymer Clay
The values of tannery wastewater pH were in the alkaline range (8.32-9.05) mainly due to the alkaline chemicals used in the tanning processes (Table 3). Higher EC values and concentrations of the investigated soluble Na + , Cl − , and SO 4 2− in wastewater were observed comparing to standards limit.
The high concentrations of these ions may result from the use of significant quantities of their salts in the industry via auxiliary chemicals such as sodium sulphide salts or the salts used in hiding and skin preservation such as chloride salts [2]. Among the investigated heavy toxic metals, the soluble total Cr 2+ (195-878.4 µg L −1 ) of wastewater was very high compared to standards limit [37][38][39] ( Table 3). The presence of high concentration of Cr 2+ in the wastewater samples of the tannery effluents can be attributed mainly to the addition of significant quantities of chromium salts in the chrome tanning process [2].  Figure 8 presents treatment effects on total soluble Cr 2+ concentration and percentage of Cr 2+ removal from tannery wastewater. The removal efficiency of soluble Cr 2+ increased as the sorbent dose was increased. The highest removal capacities of Cr 2+ were recorded for polymer-zeolite followed by unmodified one, giving a maximum total chromium removal of 84.4-86.2% at a dosage of 6-10 g L −1 for polymer-zeolite and 83.5% at a dose of 10 g L −1 for raw zeolite. Caputo and Pepe [40] found a reduction up to 99.99% of pollutants by using zeolite, depending upon the type of zeolite, charged cations, and the type and concentration of salts in the wastewater. Sustainability 2017, 9,1993 10 of 14

Characterization of Tannery Wastewater and Its Treatments Using Polymer Clay
The values of tannery wastewater pH were in the alkaline range (8.32-9.05) mainly due to the alkaline chemicals used in the tanning processes (Table 3). Higher EC values and concentrations of the investigated soluble Na + , Cl -, and SO4 2-in wastewater were observed comparing to standards limit. The high concentrations of these ions may result from the use of significant quantities of their salts in the industry via auxiliary chemicals such as sodium sulphide salts or the salts used in hiding and skin preservation such as chloride salts [2]. Among the investigated heavy toxic metals, the soluble total Cr 2+ (195-878.4 μg L −1 ) of wastewater was very high compared to standards limit [37][38][39] (Table 3). The presence of high concentration of Cr 2+ in the wastewater samples of the tannery effluents can be attributed mainly to the addition of significant quantities of chromium salts in the chrome tanning process [2].  Figure 8 presents treatment effects on total soluble Cr 2+ concentration and percentage of Cr 2+ removal from tannery wastewater. The removal efficiency of soluble Cr 2+ increased as the sorbent dose was increased. The highest removal capacities of Cr 2+ were recorded for polymer-zeolite followed by unmodified one, giving a maximum total chromium removal of 84.4-86.2% at a dosage of 6-10 g L −1 for polymer-zeolite and 83.5% at a dose of 10 g L −1 for raw zeolite. Caputo and Pepe [40] found a reduction up to 99.99% of pollutants by using zeolite, depending upon the type of zeolite, charged cations, and the type and concentration of salts in the wastewater.  A significant low quality of the tannery wastewater is indicated by the high EC values and high concentrations of cations (Na + ) and anions (Cl − and SO4 2− ). Figure 9 shows the effects of raw and polymerized materials on salinity (EC values), and soluble Cl − , SO4 2− , and Na + . Results showed that the salinity was removed from the tannery wastewater to varying degrees. The EC value of in wastewater samples before treatment amounted to 33.3 (dS m −1 ), which was decreased after treatment by 48.9% (raw zeolite) and 55.6% (polymer-zeolite) and 56.7% (raw clay deposit) and 57.1% (polymer clay deposit).
The results showed that the ions of Cl − , SO4 2− , and Na + on the studied sorbents are retained ( Figure 9). The removal percentage of Na + ions onto various sorbents was 32.7% (raw zeolite), 65.2% (polymer-zeolite), 55.7% (raw clay deposit), and 67.4% (polymer-clay deposit). The removal percentage of Cl − ions onto various sorbents was 33.7% (raw zeolite), 63.1% (polymer-zeolite), 56% (raw clay deposit), and 65.8% (polymer-clay deposit). Meanwhile, the removal percentage of SO4 2− ions onto various sorbents was 60.8% (raw zeolite), 88.6% (polymer-zeolite), 87.9% (raw clay deposit), and 92.6% (polymer-clay deposit). The retention of different ions on zeolite and clay minerals or mineral-containing materials may be a reasonable reason for reducing the electrical conductivity. In addition, the precipitation process may lead to a reduction of the concentration of dissolved salts. According to numerous studies conducted by a number of researchers [40,41], the use of zeolite may be highly efficient in wastewater purification and desalination. Zhao et al. [41] reported that ammonium reduction in the municipal water sanitation reached 89.8% using zeolite. In a study by Ghaly and Verma [42] on the water from the sludge saltwater desalination by ion exchange column of zeolite, the efficiency of removal of sodium salts amounted to 75.34%. In the current study, the highest removal capacities of Na + , Cl − , and SO4 2− were recorded for polymer-clay deposit, followed by polymer-zeolite. The findings of the current study suggest that the polymerized materials of clay deposit or zeolite may be employed as highly efficient sorbents for the removal of toxic ions from the wastewater. A significant low quality of the tannery wastewater is indicated by the high EC values and high concentrations of cations (Na + ) and anions (Cl − and SO 4 2− ). Figure 9 shows the effects of raw and polymerized materials on salinity (EC values), and soluble Cl − , SO 4 2− , and Na + . Results showed that the salinity was removed from the tannery wastewater to varying degrees. The EC value of in wastewater samples before treatment amounted to 33.3 (dS m −1 ), which was decreased after treatment by 48.9% (raw zeolite) and 55.6% (polymer-zeolite) and 56.7% (raw clay deposit) and 57.1% (polymer clay deposit). The results showed that the ions of Cl − , SO 4 2− , and Na + on the studied sorbents are retained ( Figure 9). The removal percentage of Na + ions onto various sorbents was 32.7% (raw zeolite), 65.2% (polymer-zeolite), 55.7% (raw clay deposit), and 67.4% (polymer-clay deposit). The removal percentage of Cl − ions onto various sorbents was 33.7% (raw zeolite), 63.1% (polymer-zeolite), 56% (raw clay deposit), and 65.8% (polymer-clay deposit). Meanwhile, the removal percentage of SO 4 2− ions onto various sorbents was 60.8% (raw zeolite), 88.6% (polymer-zeolite), 87.9% (raw clay deposit), and 92.6% (polymer-clay deposit). The retention of different ions on zeolite and clay minerals or mineral-containing materials may be a reasonable reason for reducing the electrical conductivity. In addition, the precipitation process may lead to a reduction of the concentration of dissolved salts. According to numerous studies conducted by a number of researchers [40,41], the use of zeolite may be highly efficient in wastewater purification and desalination. Zhao et al. [41] reported that ammonium reduction in the municipal water sanitation reached 89.8% using zeolite. In a study by Ghaly and Verma [42] on the water from the sludge saltwater desalination by ion exchange column of zeolite, the efficiency of removal of sodium salts amounted to 75.34%. In the current study, the highest removal capacities of Na + , Cl − , and SO 4 2− were recorded for polymer-clay deposit, followed by polymer-zeolite. The findings of the current study suggest that the polymerized materials of clay deposit or zeolite may be employed as highly efficient sorbents for the removal of toxic ions from the wastewater.

Conclusions
The results of this study indicated that the tannery wastewater contained very high values of EC, soluble Na + , Cl − , SO4 2− , and Cr 2+ . Our findings showed that the highest removal of Na + , Cl − , and SO4 2− from tannery wastewater was recorded for polymer-clay deposit, followed by polymer-zeolite. However, the highest removal of soluble Cr 2+ from tannery wastewater was recorded for polymer-zeolite. For removal of Cr(VI) from aqueous solutions, the highest removal efficiency was produced by the polymer-clay deposit composite. Our findings also suggest that the environmental assessment of tannery effluents should be taken into account, and their treatment using modified sorbents is necessary for preventing pollutants from entering the environment.

Conclusions
The results of this study indicated that the tannery wastewater contained very high values of EC, soluble Na + , Cl − , SO 4 2− , and Cr 2+ . Our findings showed that the highest removal of Na + , Cl − , and SO 4 2− from tannery wastewater was recorded for polymer-clay deposit, followed by polymer-zeolite. However, the highest removal of soluble Cr 2+ from tannery wastewater was recorded for polymer-zeolite. For removal of Cr(VI) from aqueous solutions, the highest removal efficiency was produced by the polymer-clay deposit composite. Our findings also suggest that the environmental assessment of tannery effluents should be taken into account, and their treatment using modified sorbents is necessary for preventing pollutants from entering the environment.