Adsorption of Silver (I) From Aqueous Solution Using Chitosan/Montmorillonite Composite Beads

Chitosan/montmorillonite (CTS-MMT) composite beads were used as adsorbents for the removal of silver ion (Ag+) from aqueous solution. Equilibrium adsorption was achieved within 150 min at 3% MMT of chitosan solution. The optimum pH value for Ag+ removal was found to be 6-7. The maximum adsorbent dosage for Ag+ removal was 5 g/L. Under above maximum conditions the Ag+ removal was 95.7%. The maximum adsorption capacities of CTS and CTS-MMT composite beads as obtained from Langmuir isotherm were found to be 38.46 and 43.48 mg/g, respectively. The adsorption kinetic agrees well with the pseudo second order model. The Ag+ desorption of CTS-MMT composite bead was 17.39% at pH=4. SEM/EDX images confirm that after adsorption the Ag+ were dispersed onto the composite bead surface. The adsorption and desorption experiment demonstrated that the CTS-MMT composite beads can be used as an effective adsorbents for removal of Ag+ from aqueous solution.


Introduction
The pollution of water resources due to the indiscriminate disposal of heavy metal ions has been causing worldwide concern. Wastewater from many industries, such as metallurgical, mining, tannery, chemical manufacturing, and battery manufacturing industries, contains one or more toxic metal ions 1 . Therefore, the remediation of heavy metal ions from the industrial wastewaters is great interested.
Silver is the one of the precious metals that has significant roles in many parts of human life 2 . Due to its excellent properties, for instance, malleability, ductility, electrical and thermal conductivity, corrosion and oxidation resistance, antimicrobial and luster, Silver has been extensively investigated for various uses in industrial activities including the production of mirrors, photographic films, electroplating, catalyst, antimicrobial material, batteries and electronic devices, and jewelry 3,4 . Therefore, all these industrial activities have been given the silver ion pollution in wastewaters. The contact with soluble silver compounds may create some toxic effects, for example, argyria, a disease related to skin pigmentation, liver and kidney degeneration and respiratory disorders 5,6 .
The toxicity and high economic value of silver result in the requirement of a treatment that both remove the silver from wastewater and recover a valuable material. A variety techniques have been developed to remove the silver from wastewater such as ion-exchange 4,7 , precipitation 8 , solid phase extraction 9 , coagulation 10 , biosorption 11 and adsorption 12 . Adsorption is an attracted attention because of new material types available for the recovery process. Several research studies have been focused on the utilization of the natural adsorbents which are extracted from agriculture wastes and seafood by-products [13][14][15][16][17] due to their biodegradability and eco-friendly properties.
Among these natural materials, chitosan has been proved to be a suitable biopolymer for the removal of metal ions from aqueous solution [18][19][20][21] . Chitosan is chemically identical to cellulose except that the hydroxyl groups in the cellulose molecule are substituted with amino groups in chitosan. The amino and hydroxyl groups in chitosan can be coordinated with metal ions to from stable complex. However, chitosan bead, as an adsorbent, has some disadvantages such as dissolution in highly acidic solution, low surface area, high cost, poor thermal and mechanical properties [22][23][24] . Physical or chemical modification of chitosan using different materials has been studied to improve its properties and adsorption capacity. Chitosan composites such as chitosan/zeolite 25 , chitosan/ lignin 26 , chitosan/MgO 27 , nano-hydroxyapatite/chitosan 28 , and chitosan/polyacrylonitrile 29 have been developed to exhibit better adsorption together with enhancement in other physical and chemical properties.
Clay is a cheap natural raw material that has been widely used for many years as inorganic filler for plastics and rubbers to reduce the polymer consumption and cost. Montmorillonite (MMT), the main component of bentonite, is a layered aluminosilicate mineral that belongs to the montmorillonite/smectite group of clay minerals. Chitosan/ montmorillonite composite prepared by different approaches has successfully employed to adsorb dye 30 , Cobalt (II) 31 , Copper (II) 32 , and tannic acid 33 . However, there have been few reports for using chitosan/montmorillonite composite as an adsorbent to remove silver ion from aqueous solution. In this study, chitosan (CTS) and chitosan-montmorillonite (CTS-MMT) composite beads were used to remove silver ions from aqueous solutions by batch adsorption process. Characterization of the composite beads, effect of initial pH, initial metal concentration, contact time and adsorbent dose on the adsorption efficiency of Ag + were studied systemically. The silver ions desorption of adsorbents was also investigated. In addition, the experimental data was examined by evaluating the adsorption isotherms and adsorption kinetic parameters.

Materials
Chitosan (90% deacetylation degree) was purchased from Seafresh Chitosan (Lab) Co., Ltd. (Thailand). The montmorillonite powder was supplied by Thai Nippon Chemical Industry Co., Ltd. Silver nitrate (AgNO 3 ) was used as the source Ag + . Glacial acetic acid was purchased from J.T. Baker (Thailand).

Preparation of chitosan-montmorillonite composite beads
Chitosan-montmorillonite composite beads were prepared by adding montmorillonite powder (1 to 5 wt% of chitosan solution) to 1 g of chitosan in 50 mL of 2% (v/v) acetic acid solution. The solution was stirred with sonication for 6 h. The chitosan-montmorillonite solution was dropped through a syringe into a precipitation bath containing 1 L of an alkaline coagulating mixture gave rise to the composite beads. The beads were washed with deionized water and preserved in an aqueous environment for future use.

Adsorption studies
Batch adsorption experiments were carried out using a water bath shaker. For each adsorption experiment, 1 g adsorbent was dispersed in 100 mL of 10 mg/L AgNO 3 solution without adjusting the pH value. The dispersion was stirred at a speed of 100 rpm at 30 °C. The Ag + concentrations were determined by Atomic Absorption Spectroscopy (AAS, Analyst 300). Each experiment was repeated five time under the same controlled conditions. The effect of pH on adsorption capacities was determined in the pH range from 4 to 10. The pH was adjusted with 0.1 M NaOH or 0.1 M HCl. Different amounts of the adsorbent in the range of 1 to 5 g were used to examine the effect of an adsorbent dosage on adsorption of Ag + .

Adsorption isotherms
Adsorption isotherms were obtained by using 1 g of adsorbent beads and 100 mL of AgNO 3 solution with different concentrations (1-10 mg/L). These solutions were buffered at an optimum pH for adsorption, i.e., pH 6. The dispersions were stirred in a water bath shaker until they reached adsorption equilibrium, i.e., 150 min. The quantity of adsorbed Ag + was derived from the concentration change.

Adsorption kinetics
Adsorption kinetics experiments were carried out by varying concentration of AgNO 3 solution from 2-10 mL and stirring in water bath shaker at 30 o C for time varying from 0-150 min. At the end of each adsorption period and the Ag + concentrations were analyzed similarly.

Desorption studies
Desorption experiment were performed in order to estimate to the metal releasing capacity of chitosan-montmorillonite with Ag + . The adsorbent was used 1 g for adsorbing Ag + ions in 100 mL of AgNO 3 solution (10 mg/L) at pH 4, 7 and 10. The adsorbent was washed with deionized water to remove any unabsorbed Ag + . For desorption experiment was carried out using 100 mL of 10 mM acetic acid solution with a constant stirring (100 rpm) at 30 °C for 150 min. The desorption ratio was calculated from the number of metal ions adsorbed on the adsorbent and the final metal concentration in desorption medium, according to the following equation:

Characterization
The X-ray diffraction (XRD) patterns were obtained using a Bruker D8 advance with a monochromatic Cu Kα source (λ=0.15418 nm) operating at 40 kV and 40 mA. The diffraction patterns were recorded from 3 o and 40 o with a scan rate of 0.4 sec/ step. The morphology of CTS and CTS-MMT composite beads were characterized by scanning electron microscopy (SEM, JEOL JSM-6335F) and energy dispersive X-ray spectroscopy (EDX, Oxford). The Zeta potential was measured using a Zetasizer (ZS90, Malvern). The pH value of the CTS and CTS-MMT composite beads were adjusted between 4-10 by adding HCl and NaOH.

Structure analysis
The XRD patterns of CTS, MMT and CTS-MMT are shown in Figure 1. The (001) diffraction peak of MMT occurred at 2θ of 6.05 o , corresponding to a d001 spacing of 14.58 Å. However, the d001 spacing of CTS-1%MMT was slightly increased to 15.29 Å (2θ=5.78 o ), and CTS-3%MMT was shifted to 15.39 Å (2θ=5.74 o ). It indicated that the MMT interlayer was expanded, which can be reasonably ascribed to the intercalation of CTS into the MMT interlayer region, leading to the formation of CTS-MMT composite bead.

Zeta potential analysis
Solution pH was an important parameter that affected the adsorption of Ag + . The Zeta potential of CTS and CTS-MMT composite beads as a function of the solution pH values were shown in Figure 2. Solution pH determined the level of electrostatic or molecular interaction between the adsorption surface and adsorbate owing to charge distribution on the materials. The zero point charge or isoelectric point (pH zpc ) of the CTS was about 7.1 and then shifted to 7.5 after being modified by MMT. The difference in pH zpc between them confirmed that CTS was successfully coated on MMT composite bead. The pH zpc of CTS was obtained at pH=7.1 for amino group in chitosan. This result was similar to some finding in previous studies 24 . At below pH 7.5, more amino groups in the CTS-MMT composite beads were protonated (i.e., from -NH 2 to -NH 3 + ). From pH 7.5 to 10, the negative zeta potential of CTS-MMT composite bead, which amino group in CTS-MMT composite bead was not deprotonated under this pH condition (i.e., from -NH 2 to -NH -). Moreover, Figure 2 revealed that the adsorption capacity of CTS-MMT composite bead was highly depended on the positive charge at pH<7.5. Figure 3 showed the SEM image of the CTS and CTS-MMT composite bead. It can be seen that the morphology of CTS and CTS-MMT composite beads exhibited notable smooth surface and spherical structure (Figure 3 (a) and (d)). The EDX mapping also indicated that Ag was found to be uniformly distributed along surface of the beads (Figure 1 (b) and (e)). In comparison, the bright contrast speckles of Ag  were more densely distributed at CTS-MMT than CTS beads. From Figure 3 (c) and (f), the SEM image of CTS-MMT composite bead showed that high porous and agglomerated particles due to the irregular stacking of MMT particles. This result suggested that the adsorption of composite bead was governed by electrostatic forces and natural entrapment in to the porous CTS and MMT materials.

Effect of contact time
Contact time was another important parameter for adsorption efficiency of metal ions. The effect of contact time on the absorption of Ag + ion on CTS and CTS-MMT composite bead was shown in Figure 4. The result showed that the Ag + was rapidly adsorbed in the first 30 min with the increase of contact time from 0-30 min and the Ag + removal increased to about 84% within 150 min. After 150 min, the adsorption rate becomes constant and the adsorption reach equilibrium. At this (equilibrium) time, the adsorption rate of CTS-MMT composite beads was higher than pure CTS bead. In addition, the Ag + removal of CTS-MMT composite bead increased with increasing of MMT particles concentration in the composite beads. The maximum of Ag + removal was 84.11% for CTS-3%MMT and 84.44% for CTS-5%MMT composite bead at contact time 150 min.

Effect of pH
The pH of the aqueous solution, the most important parameter on adsorption studies, strongly affected the adsorption property of beads for heavy metal ions. From Figure 5, the pH value of the solution increased from 5 to 6, the Ag + removal of adsorbent increased sharply from 57.57% to 77.28% for CTS-3%MMT composite bead, and slightly increased to 78.31% with the increasing of the pH from 6 to 7. However, the mechanisms of adsorption of Ag + may be considered the electrostatic interaction between the protonated group of CTS and Ag + and chemical reaction between adsorbate and the adsorbent. The primary amine group of the CTS structure was about pH 6 to 7. It was noted that an acidic pH value was necessary to provide -NH 3 + groups in the CTS structure facilitating an increase of Ag + adsorption, which corresponding to the point of zero charge result ( Figure 2). The relative lower adsorption below solution pH of 6 may be due to the competitive adsorption between proton and Ag + ions on CTS. Above pH 7, as insoluble silver hydroxide started to precipitate from solution, both adsorption and precipitate were effective mechanism in the removal of Ag + ions from aqueous solution. The precipitation of silver hydroxide increased dramatically when the solution pH of 10. The higher precipitation can be attributed to more hydroxyl group in the solution. Hence adsorption of Ag + onto CTS-MMT was at optimum pH range of 6-7.

Effect of adsorption dosage
The adsorbent dosage was an important parameter for the adjusted during wastewater treatment ( Figure 6). It was clearly seen that the removal efficiency of Ag + increased from 56% to 95.7% which increased in the dosage of from 1 g to 5 g at 30 min. Therefore, the removal of Ag + increased with increasing of the adsorbent dosage was attributed to the availability of a larger surface area and more adsorption sites. The adsorption time required to reach equilibrium was decreased with increase dose of adsorbent. Thus, the increase of adsorption rate may be depended on the ratio of adsorbent weight to solution volume, which is corresponded to the increased dose of adsorbent.

Adsorption isotherms
The adsorption process can be generally expressed by two isotherm equations, the Langmuir 34  Where q e is the amount of Ag + adsorbed per unit mass of adsorbent (mg/g) and C e is the equilibrium concentration of Ag + in solution (mg/L). The constant X m is the monolayer adsorption capacity (mg/g), and b is the Langmuir constant related to the adsorption energy. K (mg/g) is the Freundlich constant related to the adsorption capacity of the adsorption, and n measures the surface heterogeneity. The linear regression analysis was performed using OriginPro 8 software to determine K and X m and standard errors regarding with the model parameter. The isotherm constants of Langmuir and Freundlich were calculated from the plots of C e /q e vs. C e and log q e vs. log C e as depicted from Figure 7, respectively, and the results were listed in Table 1. Therefore, the sorption capacity X m , which was a measure of the maximum sorption capacity corresponding to complete monolayer coverage, showed that the CTS-MMT composite bead had a higher adsorption capacity (X m =43.48 mg/g) than that of CTS bead (X m =38.46 mg/g). The b value was 1.00 for CTS and 2.87 for CTS-MMT, indicating that adsorption strength was not very strong. Moreover, the Freundlich isotherm parameter 1/n is a measure of the deviation from linearity of the adsorption. The low value of CTS (0.84) and CTS-MMT composite bead (0.74) less than 1 showed the favorable sorption and confirmed the heterogeneity of the adsorbent. Also, the results indicated that the bond between Ag + and composite bead were strong. The adsorption capacity K of the adsorbent was calculated from the isothermal linear regression equation. The K value of CTS-MMT composite bead (1.30 mg/g) was higher than that of CTS bead (1.01 mg/g), confirming that CTS-MMT composite bead had higher adsorption. For fitting to Langmuir and Freundlich model, the linear regression coefficients (R 2 ) were 0.95 and 0.92 for CTS-MMT composite bead, respectively (Table 1). It was clear that the correlation coefficients for the Langmuir isotherm were somewhat higher than for the Freundlich isotherm. This indicated that the uptake occurred on a homogenous surface by monolayer adsorption, and can be described in terms of chemisorption as the formation of ionic bonds between adsorbent and adsorbate.

Adsorption kinetics
The kinetic parameter, which was helpful for the prediction of adsorption rate, gives important information for designing and modeling the processes. The kinetic of the adsorption data was analyzed using different kinetic models such as pseudo-first-order and pseudo-second-order models, which is represented by the following equation: pseudo-first-order model: where q e is the amount of Ag + ions adsorption equilibrium (mg/g), q t (mg/g) is the amount of Ag + ions adsorbed at any time t (min), and K 1 (min -1 ) and K 2 (g/mg min) are the rate constants of the pseudo first order and pseudo second order adsorption models. The values of K 1 and q e were calculated from slope and intercept of the plot of log (q e -q t ) vs. t ( Figure  8), while the values of K 2 and q e were evaluated from the intercept and slope of a plot t/q t vs. t. Table 2 summarized the value of the corresponding model parameters.
From Table 2, it can be seen that the linear correlation coefficients (R 2 ) for the pseudo-first order kinetic model were very low. The large difference between the experiment q e value (q e(exp) ) and the calculated q e value (q e(cal) ), which indicated the pseudo first order kinetic model was poor fit for the adsorption processes of CTS-MMT composite bead for Ag + . It can also be found from Table 2 that R 2 for pseudo second order kinetic model were all over 0.9. Moreover, the q e(cal) value for the pseudo second order kinetic model were consistent with the q e(exp) value, Therefore, the adsorption process of CTS-MMT composite beads for Ag + can be well described by the pseudo second order kinetic model. This result indicated that the sorption process was complex and involved more than one mechanism. Table 3 showed that the maximum adsorption capacities reported of different adsorbents for Ag + removal from aqueous solutions 3,17,21,24,36 . It was found that the CTS-MMT composite bead has a higher adsorption capacity than some other adsorbents. This result could be demonstrated that the CTS-MMT composite beads can be used as effective adsorbents for Ag + removal from aqueous solution.

Desorption studies
Recovery of adsorbent was very important feature for the industrial applications. Table 4 showed the relationship between the pH and the desorption efficiency of the CTS and CTS-MMT composite bead. It can be seen that the maximum desorption of saturated adsorbent was 30.39% and 17.39% (pH=4) for CTS and CTS-MMT composite bead, respectively. This result indicated that adsorption Ag + on adsorbents can be desorption by acetic acid solution. The desorption efficiency of CTS and CTS-MMT composite bead was increased with decease of pH. The decrease in desorption efficiency of the metal ions may be explained by the fact that at the MMT surface was higher negative charged, hence large attraction between the positive Ag + occurred. This implied that the CTS and CTS-MMT composite bead were potential of regeneration and reuse.

Conclusions
This study proved that Ag + could be adsorbed and thus higher significant amount was removed by CTS-MMT composite bead from aqueous solution. The maximum adsorption capacity of CTS-MMT composite bead was higher than those CTS bead. The Ag + removal was increased with the increase of contact time and amount of adsorbent. The optimum pH value for Ag + removal was found to be 6-7. The equilibrium data could be described by the Langmuir and Freundlich isotherm equation. However, the Langmuir model better represented the sorption process than the Freundlich model. The maximum adsorption capacity of CTS-MMT composite bead was 43.48 mg/g at initial pH 6 for 10 mg/L (Ag + ) solution. Kinetic modeling results showed that the pseudo second order equation was appropriated for the description of this type of adsorption and removal. The desorption studies revealed that the CTS and CTS-MMT composite bead provide the potential for reused after Ag + adsorption.