Adsorption of Cr(III) from an Aqueous Solution by Chitosan Beads Modified with Sodium Dodecyl Sulfate (SDS)

This study is to prepare chitosan beads modified with sodium dodecyl sulfate (SDS) to effectively remove Cr(III) from an aqueous solution. The characte-rizations of SDS-chitosan by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), Fourier transform-infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) proved the successful synthesis of the adsorbent. The adsorption of Cr(III) on the SDS material was investigated by varying experimental conditions such as pH, contact time and adsorbent dosage. The maximum adsorption capacity of SDS-chitosan for Cr(III) was estimated to be 3.42 mg·g −1 . The results of adsorption kinetics and isothermal models show that the adsorption process conforms to the pseu-do-second-order and Langmuir isotherm models, indicating that the adsorption is single- layer chemical adsorption. Thermodynamic analyses indicate that the adsorption of Cr(III) is an endothermic reaction. These results show that the new adsorbent has obvious application prospect to eliminate Cr(III). an Solution by Chitosan Beads Modified


Introduction
Due to the emergence of industrialization and urbanization worldwide, heavy metals are excessively produced and discharged into the environment. The pres-( ) 2 Cr OH + , with the first or second hydrolysis products dominating at pH values from 4 to 8. The low solubility of Cr(OH) 3 considerably limits the concentration of Cr(III) for pH values above about 5 [10] [11]. Cr(III) is considered to be an essential microelement for the maintenance of effective glucose, lipid, and protein metabolism in mammals [12]. Given its high danger to biological systems, many studies have focused on the removal of Cr(VI), while very few articles deal with the adsorption of Cr(III).
Chromium elements, therefore, it is vital to eliminate heavy metals from wastewaters before discharging them into the environment. Various treatment methods have been developed to remove heavy metal ions from different aqueous solutions. These include chemical precipitation [13], adsorption, evaporation, reverse osmosis, ion exchange, membrane filtration, and biosorption [14] [15] [16] [17]. Among these, adsorption is considered to be an alternative process to remove heavy metal ions from wastewater [18] [19] [20]; especially, biological adsorption (biosorption) has emerged as one of the most promising method. It is generally preferred for the removal of lead due to its high efficiency, cost-effectiveness, easy handling, and the availability of different adsorbents [21] [22].
Among the many biosorbents, chitosan is a type of natural polyaminosaccharide, synthesized from the deacetylation of chitin and mainly composed of poly-β-(1,4)-2-deoxy-2-amino-d-glucose. Chitin is the second most abundant polymer in nature after cellulose, and can be extracted from crustacean shells such as prawns, insects, fungi, crabs and other crustaceans [23] [24] [25]. Owing to its many attractive properties such as its hydrophilicity, biocompatibility, biodegradability, and non-toxicity, as well as, the presence of reactive amino (-NH 2 ) and hydroxyl (-OH) groups in its backbone, chitosan has been used as an effective material for the removal of heavy metals from wastewaters [26] [27] [28]. Chitosan has a few defects such as notable swelling in aqueous media and a nonporous structure resulting in a low surface area [29]. Therefore, many types of physical or chemical modifications can be made to chitosan to improve its chemical stability in an acidic medium, its mechanical strength, and its adsorption capacity. The most common chemical modifications are cross-linking the grafting of a new functional group and acetylation. Cross-linking agents (such as TPP) are commonly used to prepare chitosan beads because they form a non-toxic agent [30] [31]. Recently, the synthesis of biocomposites has presented a novel approach for the modification of several properties of polysaccharides and various types of materials have been used to form composites with chitosan such as a zeolitic imidazolate framework (ZIF) [32], polyvinyl alcohol [33], polyvinyl chloride [34], and silicon dioxide [35] [36].
In this work, chitosan with sodium dodecyl sulfate (SDS) was synthesized to enhance the adsorption potential of heavy metal ions. In our previous paper [37], the adsorption of Cr(VI) by SDS-modified chitosan was investigated. The objective of this work was to explore the efficiency of SDS-modified chitosan bead composites and to use them for removing Cr(III) from an aqueous solution.

Effect of the Initial SDS Concentration
To estimate the optimal initial concentration of SDS loading on chitosan beads for Cr(III) removal, the chitosan beads were modified with SDS solutions by varying the initial SDS concentrations from 10 to 9000 mg/L. The adsorption experiments were performed under the following conditions: an initial concentration of Cr(III) of 1 mg/L, a contact time of 2 days, an adsorbent dosage of 0.05 g, a pH of 4, and a temperature of 25˚C. The results are shown in Figure 1 the increase in SDS concentration, the adsorption capacity continuously increased within 6000 mg/L but the adsorption amount was almost constant at further higher concentrations. Therefore, 6000 mg/L was considered to be the optimum initial SDS concentration for Cr(III).

Effect of pH
Solution pH is most important parameter affecting adsorption characteristics. In this experiment, the effect of pH on Cr(III) adsorption by SDS-chitosan was studied in the pH range of 4 -7; the contact time was 24 h, the temperature was 25˚C, the adsorbent dosage was 0.4 mg/L, and the initial Cr(III) concentration was 1 mg/L. The results are shown in Figure 2.
The distribution of Cr(III) in environmental waters is predominantly as Cr 3+ at a pH of 1 -3 whereas it exists predominantly as CrOH 2+ at a pH of 4-6. Cr(III) precipitates as Cr(OH) 3 at a pH of 5.5 or above [38]. For the confirmation of precipitation, the change of the initial Cr(III) concentration on the effect of the pH was measured in the pH range of 4 -7, as seen in Figure 3.  As can be seen in Figure 2, the adsorption capacity of Cr(III) increased with the increase in the pH value from 4 to 7. The initial Cr(III) concentration in the aqueous solution decreased with the increase in the pH value from 4 to 7 ( Figure   3). It was observed that Cr(III) precipitated as Cr(OH) 3 at a pH of 6 and 7 due to the decrease in the initial Cr(III) concentration. Therefore, a pH of 6 and 7 were not suitable for the adsorption experiments and a pH of 4 was chosen.

Effect of Contact Time
The

Effect of the Adsorbent Dosage
The adsorbent dosage is an important factor that affects the adsorption capacity.
Adsorption experiments were performed to determine the adsorbent dosage under the conditions of a pH of 4, a temperature of 25˚C, a contact time of 24 h, and an initial concentration of Cr(III) of 1 mg/L. Figure 5 shows the effect of the adsorbent dosage on Cr(III) adsorption by SDS-chitosan. The adsorption rate increased with an increase in the adsorbent dosage and reached approximately 70% at 0.8 mg/L; after that, there was no appreciable increase. Therefore, 1 mg/L was selected as the optimized adsorbent dosage.  Figure 6. It was confirmed that the adsorption capacity of Cr(III) did not decrease at all even in the presence of other metal ions.

SEM-EDS Micrographs
The SEM-EDS images of the chitosan beads and SDS-chitosan are shown in Figure 7. These images showed an unevenness on the surface. This was probably due to the release of water contained in the adsorbent during the drying process of chitosan. In addition to the aforementioned irregularities, SDS-chitosan showed a mesh-like pattern. This was thought to be due to the effect of SDS   modified on the chitosan surface. In particular, the modified chitosan beads with a high SDS concentration showed a difference compared with the other beads.
As the irregularities on the surface of the adsorbent were considered, the adsorption proceeded in two ways: physical and chemical adsorption. From the mapping images, it was confirmed that Cr ions were actually adsorbed onto the adsorbent surface.

FT-IR and XPS Spectra
The surface functional groups and the chemical compositions on the modified SDS-chitosan beads were identified by FTIR and XPS analysis, respectively. The results are fundamentally shown in our previous work [37], and more detailed features are denoted as follows. The FT-IR results are shown in Figure 8, where the peaks of SDS-chitosan and the chitosan beads can clearly be seen. It is apparent from this figure that the main peaks common to each adsorbent were due to the -OH group at 3400 -3500 cm -1 and the aliphatic methylene group at 2871 cm −1 . The wide peaks at 1560 -1640 cm −1 and 1110 cm −1 show the amine group and ether group, respectively. For SDS-chitosan, the peak at 1248 cm −1 was characteristic of the asymmetrical vibration of the C-O-S group, which confirmed that the prepared adsorbent was a composite of SDS and chitosan.
The XPS results are shown in Figure 9, where the chitosan beads with initial

Adsorption Isotherms
Adsorption isotherms explain the interactive process between the adsorbents and adsorbates in aqueous medium at the attained saturation point. Adsorption     (1 1 n > ) [32]. The correlation coefficient (R 2 ) of these isotherms for Cr(III) on SDS-chitosan is shown in Table 1 along with other relevant parameters.
As shown in Figure 11 and Figure 12, and Table 1, it was found that the R 2 values for Cr(III) were relatively large and a favorable adsorption of Cr(III) on SDS-chitosan was presented. The R 2 value in the Langmuir isotherm was larger than that in the Freundlich isotherm. The maximum adsorption capacity ( max q ) calculated from the Langmuir model was 3.42 mg/g, which suggested that the adsorption of Cr(III) on SDS-chitosan mainly occurred by a monolayer reaction.

Kinetic Studies
The rate-controlling steps of the adsorption system are essential to examine the mechanism of Cr(III). Kinetic studies were performed to explain the adsorption mechanism of Cr(III) ions onto the SDS-chitosan beads. The effects of contact time on the kinetics of Cr(III) adsorption by SDS-chitosan adsorbent are displayed in Figure 13. As can be seen in Figure 13, the removal of Cr(III) from To investigate the mechanism of adsorption of Cr(III) on SDS-chitosan, fitting was determined according to the pseudo-first-order (Figure 14), pseudo-second-order ( Figure 15) and intraparticle diffusion kinetic models ( Figure   16).
The equations of these models are given by: Pseudo-first-order model: Pseudo-second-order model: where e q and t q are the adsorption capacities of Cr using the SDS-chitosan beads at the equilibrium and time, t, respectively (mg•g −1 ), 1 k is the rate constant of the pseudo-first-order adsorption (h −1 ), 2 k is the rate constant of the pseudo-second-order adsorption (g•mg −1 •h −1 ) and id K is the rate constant of The sorption kinetic parameters including 1 2 , , , id e k k K q and the correlation coefficients R 2 are listed in Table 2. As can be seen, the correlation coefficient R 2 for the pseudo-second-order model was higher than the pseudo-first-order and intraparticle diffusion models. These results suggest that the sorption kinetics of the Cr(III) ion on SDS-chitosan could be described by the pseudo-second-order model, that the chemical adsorption could be the rate-limiting step of the process of the adsorption system. Based on above, assuming adsorbent particle to be sphere of radius "a" and the diffusion follows Fick's law that was applied to confirm it. In the law, the relationship between adsorption capacity and time was given by the following Equations [45].
After D is replaced with D 1 , at a short time, Equation (6) becomes as: The film diffusion coefficient D 1 is calculated from the slope of the plot of q t /q e versus t 1/2 . For moderate and large times, the diffusion equation given by The pore diffusion coefficient D 2 is obtained from the slope of the plot of ln(1 − q t /q e ) versus t. The values of D 1 and D 2 are presented in Table 3.
It is also known that if the adsorption is controlled by the film diffusion, the values of the film diffusion coefficient may be in the range of 10 −6 -10 −8 cm 2 /s. While if the adsorption mechanism is controlled by the pore diffusion mechanism, the value of the pore diffusion coefficient belongs to the range of 10 −11 -10 −13 cm 2 /s [46]. As can be seen in Table 3, The D 1 and D 2 values in the model based on the Fick's law was larger than that above mentioned ranges. It is confirmed that the sorption kinetics of the Cr(III) ion on SDS-chitosan could be described by the pseudo-second-order model.

Thermodynamic Study
To investigate the effect of temperature on Cr(III) adsorption by SDS-chitosan, adsorption experiments were conducted from 298 to 318 K. The results are shown in Figure 17. In these temperature ranges, the adsorption amount of Cr(III) on SDS-chitosan increased with the increase in temperature.
In addition, based on the experimental results, the thermodynamic parameters of the adsorption, i.e. standard Gibb's free energy change ( 0 G ∆ ), a change in standard enthalpy ( 0 H ∆ ) and standard entropy ( 0 S ∆ ), were calculated from the Van 't Hoff equation [47] [48] [49].
where R is the universal gas constant (8.314 J/mol −1 •K −1 ) and T is the temperature (K). The slope and intercept of the plot of ln c K versus 1/T were used to deter- The thermodynamic parameters are shown in Table 4. Values of 0 G ∆ >   Table 4. Thermodynamic parameters for the adsorption of Cr(III) on SDS-chitosan.

Desorption Study
Repeated use of adsorbent and recovery of the adsorbed metal ions are important parameters indicating economic efficiency. In this study, regeneration experiments were carried out using the SDS-chitosan beads after adsorption of Cr(III). In desorption experiment, the spent adsorbent after adsorption was treated with 50 ml of 0.1 mol/L NaOH solution and ultrapure water as desorption agent, and then filtered. Cr(III) content in the filtrate was determined by ICP-MS [37]. The results of this experiment are shown in Figure 18. As can be seen in Figure 18, the desorption efficiency of Cr(III) was found to be 57% using 0.1 mol/L NaOH. The desorption of NaOH was larger than ultrapure water. This suggested that NaOH could be a desorption agent for Cr(III).

Comparison with Other Adsorbents
The comparison of the maximum adsorption capacity of Cr(III) by SDS-chitosan Journal of Environmental Protection in the present study with those of other adsorbents in the literature is presented in Table 5. As can be seen in Table 5, the adsorption capacity of SDS-chitosan for Cr(III) in this work was not necessarily high compared with other adsorbents. However, it could be regarded as a potential adsorbent for treating Cr(III) from wastewater for practical use as, the synthesis method of the adsorbent is relatively simple and that it can be also available for the removal of Cr(VI) [37].

Materials and Methods
Chemical reagents, namely, chitosan and sodium dodecyl sulfate (SDS) were purchased from the Tokyo Chemical Industry Co., Inc. (Tokyo, Japan).   This study-

Characterization of the Adsorbent
The surface morphology of the chitosan beads was observed by scanning electron microscopy (SEM) (JSM-6000, JEOL, Japan) and the elemental analyze of the chitosan samples before and after the adsorption of Cr(III) was performed by EDS (JED-2300, JEOL, Japan). Fourier transform-infrared spectroscopy (FT-IR-4200, Jasco, Japan) was used to identify the functional groups. The KBr was ground and mixed with the dried sample, and the mixture was pressed into thin plates for infrared testing. The surface chemistry properties of the SDS-modified chitosan beads were also identified by X-ray photoelectron spectroscopy (XPS; K-Alpha, Thermo Scientific Center, Waltham, MA, USA).

Adsorption Experiments
The beads were put into contact with 50 mL of an aqueous solution containing Cr(III) ion with a known initial concentration. The pH of each solution was adjusted using 0.1 mol/L -1 NaOH and 0.1 mol/L -1 HNO 3 . The flask was then placed in an automatic shaker. The adsorption experiments were conducted in the pH range of 1 -7, with a contact time of 1 -72 h, an SDS initial concentration of 10 -9000 mg•L -1 , an adsorbent dosage of 0.01 -0.06 g•L -3 , a temperature of 288 -318 K, and an initial Cr(III) concentration of 0.1 -3.0 mg•L -1 . The concentrations of Cr in the filtrate were measured by ICP-MS (Thermo Scientific Center: X-series II). The adsorption capacities of the chitosan beads modified with SDS were calculated by the following equation [9]: where e q represents the adsorption capacity at the equilibrium (mg·g -1 ), i C and e C are the initial and equilibrium concentrations of Cr in the batch system, respectively (mg·L -1 ), V is the volume of the solution (L), and m is the weight of the adsorbents (g).

Conclusions
The efficiency of the SDS-chitosan beads synthesized as an adsorbent for Cr(III) Journal of Environmental Protection was investigated by batch techniques. The following conclusions can be drawn considering the results of this work: 1) The optimal conditions of the adsorption of Cr(III) using the SDS-modified chitosan beads were determined. The optimal initial SDS concentration was 6000 mg/L, the optimal contact time was 48 h, the optimal dosage was 1 mg/L, and 4 was considered to be the optimum pH.
2) The adsorption isotherm of Cr(III) by SDS-chitosan was more suitably described by the Langmuir model and the correlation coefficient was 0.84. It suggests that the monolayer chemical adsorption of Cr(III) on SDS-chitosan was more dominant. The maximum adsorption capacity was estimated to be 3.42 mg•g −1 for Cr(III) under the optimum conditions. The best fit was obtained with a pseudo-second-order model when investigating the adsorption kinetics of Cr(III) adsorption on SDS-chitosan and the correlation coefficient was 0.98. The thermodynamic studies indicated that the adsorption process was favorable under the higher temperature condition.
From this work, the SDS-chitosan beads synthesized in this work can be effectively used for removing Cr(III) ions.