An exploratory study on low-concentration hexavalent chromium adsorption by Fe(III)-cross-linked chitosan beads

In this study, Fe(III)-cross-linked chitosan beads (Fe(III)-CBs) were synthesized and employed to explore the characteristics and primary mechanism of their hexavalent chromium (Cr(VI)) adsorption under low concentration Cr(VI) (less than 20.0 mg l−1) and a pH range from 2.0 to 8.0. Batch tests were conducted to determine the Cr(VI) adsorption capacity and kinetics, and the effects of pH and temperature on the adsorption under low concentration Cr(VI) and a pH range from 2.0 to 8.0. Scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy were employed to explore the characteristics of Fe(III)-CBs and their Cr(VI) adsorption mechanisms. The results show that, unlike the adsorption of other absorbents, the Cr(VI) adsorption was efficient in a wide pH range from 2.0 to 6.0, and well described by the pseudo-first-order model and the Langmuir–Freundlich isotherm model. The capacity of Cr(VI) adsorption by Fe(III)-CBs was as high as 166.3 mg g−1 under temperature 25°C and pH 6.0. The desorption test was also carried out by 0.1 mol l−1 NaOH solution for Fe(III)-CBs regeneration. It was found that Fe(III)-CBs could be re-used for five adsorption–desorption cycles without significant decrease in Cr(VI) adsorption capacity. Ion exchange was confirmed between functional groups (i.e. amino group) and Cr(VI) anions (i.e. CrO42−). The amino-like functional groups played a key role in Cr(VI) distribution on the Fe(III)-CBs surface; Cr(VI) adsorbed on Fe(III)-CBs was partially reduced to Cr(III) with alcoholic group served as electron donor, and then formed another rate-limiting factor. So, Fe(III)-CBs has a good prospect in purifying low concentration Cr(VI) water with a pH range from 2.0 to 6.0.

Na 2 SO 4 , Na 3 PO 4 , Na 2 CO 3 , NaCl and NaNO 3 were all purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). De-ionized water was used in the preparation of all solutions. The stock solution with 100.0 mg l −1 Cr(VI) was prepared by dissolving 0.2848 g K 2 Cr 2 O 7 in 1 l de-ionized water. The studied water with different Cr(VI) levels was obtained by diluting the stock K 2 Cr 2 O 7 standard solution.

Preparation of Fe(III)-CBs
The Fe(III)-CBs were prepared following the method described by Dos Santos et al. [26] and Qi et al. [27]. The main steps were as follows: Chitosan powder (2.0 g) was dissolved in 0.1 mol l −1 FeCl 3 aqueous solution (100 ml), followed by stirring the solution at room temperature for 4 h. Subsequently, the Chitosan-FeCl 3 solution was added into NaOH solution (200 ml, 10%, w/v) dropwise using a syringe to make the beads. Then the beads were separated from NaOH solution and washed with de-ionized water. Afterwards, the beads were added into de-ionized water (100 ml) before pure epichlorohydrin solution (4 ml) was added. The pH of the reaction system was adjusted to 6.0 using 0.1 M HCl or 0.1 M NaOH and kept at 70°C for a further 3 h. The cross-linked beads were dried in an oven at 60°C after the filtration. Finally, the products were treated with HCl solution (0.01 mol l −1 ) for another 3 h to remove the iron compounds on the surface and protonate the cross-linked beads.

Characterization of Fe(III)-CBs
Surface morphology of synthesized Fe(III)-CBs was observed by SEM with an FEI Quanta 200 FEG field emission scanning electron microscope at an accelerating voltage of 20 kV. The surfaces of samples were gold coated before analysis. FT-IR spectra of Fe(III)-CBs before and after sorption were recorded at room temperature using a Bruker TENSOR 27 FT-IR spectrometer in the range of 4000-400 cm −1 . The samples were pre-treated according to the KBr pellet method. XPS analysis was conducted before and after Cr(VI) adsorption by a Kratos Axis Ultra DLD spectrometer. All the binding energies were referenced to the C 1s peak at 284.8 eV from surface adventitious carbon.

Batch adsorption test
The batch test was designed and carried out to evaluate the adsorption capacity of Cr(VI) on Fe(III)-CBs (2.0 g l −1 dosage). The temperature of all batch reactors was kept at 25 ± 1°C in an air-conditioned room and the reactors were well mixed by the shaker (Ningbo Jiangnan Instrument Factory, China) at 100 r.p.m. During the adsorption tests, the mixed liquor sample was withdrawn regularly to determine the concentration of Cr(VI).
To study the effect of pH on the Cr(VI) (5.0 mg l −1 initial Cr(VI) concentration) adsorption on Fe(III)-CBs, the initial pH values of the solution were adjusted to 2.0-8.0 by 0.1 mol l −1 HCl or 0.1 M NaOH solution. Only the optimum pH would be used in the further study.
A measure of 0.2 g Fe(III)-CBs was dosed to five batch reactors containing 100 ml of Cr(VI) solution but with different concentrations (2.5, 5.0, 10.0, 15.0, 20.0 mg l −1 ) to research the effect of Cr(VI) initial concentration for kinetic studies and variable for isotherm studies.
For desorption and regeneration, the Fe(III)-CBs were subjected to successive adsorption-desorption cycle tests. After the test equilibration with the initial Cr(VI) concentration of 5.0 mg l −1 as mentioned above, the adsorbent was separated from the solution and washed well. Thereafter, desorption tests were performed by keeping 50 ml 0.1 M NaOH solution in contact with the spent adsorbent. At the end of each cycle, the Fe(III)-CBs were washed several times with de-ionized water to remove excess base from the adsorbent surface. The reusability of Fe(III)-CBs for Cr(VI) adsorption was evaluated by conducting five cycles.
The Cr(VI) analysis was carried out spectrophotometrically at 540 nm in a spectrophotometer (Spectrovision WFJ7200, Shanghai, China), using diphenylcarbazide as the complexing agent. The amount of Cr(VI) adsorption q e (mg g −1 ) was calculated from the equation as follows:  Table 1. Langmuir, Freundlich and Langmuir-Freundlich isotherm models and their parameters for Cr(VI) adsorption on Fe(III)-CBs (the adsorbent dosage 2.0 g l −1 , the temperature 25 ± 1°C, the contact time 200 min). q m is the maximum adsorption capacity (mg g −1 ); K L is the Langmuir constant (l mg −1 ) related to the energy of adsorption. K F is the Freundlich constant (mg l −1 ); 1/n is the heterogeneity parameter. K L−F is the Langmuir-Freundlich constant (l mg −1 ) related to the energy of adsorption; c is the heterogeneity parameter, which indicates relatively heterogeneous distribution of binding sites, while c is close to zero. isotherm equation parameters   where C 0 is the initial Cr(VI) concentration (mg l −1 ), C e is the instantaneous Cr(VI) concentration (mg l −1 ) during the test, V is the volume of the aqueous phase (l) in the batch reactor and W is the weight of the adsorbent used (g). The adsorption removal rate (R) of Cr(VI) was calculated as follows: Langmuir, Freundlich and Langmuir-Freundlich isotherms [28,29] were used for the mathematical description of the adsorption equilibrium of Cr(VI) ions on Fe(III)-CBs (table 1). The operating parameters were T = 25 ± 1°C, pH = 6.0, time = 200 min. The kinetic data were analysed using the pseudo-first-order, pseudo-second-order and intraparticle diffusion models (

Characterization of synthesized Fe(III)-CBs
The SEM images of the Fe(III)-CBs surface are shown in figure 1. The beads appeared in the wellshaped spherical form with relatively rough and non-porous surface. The diameter of the spheres was approximately 1 mm. These findings were in agreement with the fact that beads were prepared with the suspension in aqueous solution and subsequent cross-linking [26]. Figure 1 also illustrates the existence of folds on the surface. These folds provide a large specific area and numerous active sites distributed on the surface, offering favourable conditions for Cr(VI) adsorption.
The FT-IR spectra of chitosan and Fe(III)-CBs are presented in figure 2. For chitosan, the overlapped peak at 3415 cm −1 corresponded to −OH and −NH 2 stretching vibrations. Two adsorption bands at 2924 and 2873 cm −1 were assigned to asymmetric and symmetric vibration of −CH 2 groups, respectively, and the adsorption peak at 1600 cm −1 was attributed to the characteristic peak of −NH 2 . The spectra of Fe(III)-CBs confirm the presence of both −NH 2 and −OH groups. It can be seen that the spectra had  two significant shifts. The first shift of peak was at 1665-1600 cm −1 . This shift in −NH 2 -bending mode reflects the interaction between Fe 3+ ion and −NH 2 group. The second band at 1080 cm −1 corresponding to primary −OH stretching was shifted to 1068 cm −1 , indicating the interaction between Fe 3+ ion and −OH group due to hydrogen bonding [6]. These depress hydrogen bonding to other ions, but promote the chitosan to be more positive. Thus, Fe(III)-CBs should have larger potential for Cr(VI) adsorption.

Effect of initial pH of the solution on Cr(VI) adsorption
The pH could significantly impact the adsorption process, because it affects not only the surface charge of the adsorbent and the degree of ionization, but also the species of adsorbate [6,31,32]. In this study, the effect of initial pH (from 2.0 to 8.0) on the adsorption process was investigated and the results are shown in figure 3.
The Cr(VI) adsorption removal rate strongly depended on the initial pH (figure 3). After 200 min reaction, Cr(VI) adsorption rate increased from 74.9 to 99.8% with pH increasing from 2.0 to 3.0, but kept similar between pH 3.0 and 6.0, finally decreased from 99.36 to 31.61% with pH constantly increasing from 6.0 to 8.0. Therefore, the optimal pH for Cr(VI) adsorption by Fe(III)-CBs ranged from 3.0 to 6.0. Similar pH-dependent trend was also found in Cr(VI) adsorption by magnetic chitosan nanoparticles [33] and protonated cross-linked chitosan [34]. In this work, the initial pH may exert effect on Cr(VI) adsorption via two facets: the surface characteristics of Fe(III)-CBs and the species of Cr(VI). The protonation by hydrogen ions (H + ) presented in the amino groups (−NH 2 ) of Fe(III)-CBs in acid solutions is depressed under basic conditions [6].     CrO 2− 4 with pH higher than 6.0 [3]. Both of them worked and resulted in the characteristics of the initial pH effects, and concluded that Fe(III)-CBs adsorbed Cr(VI) as HCrO − 4 form. The optimal pH value for Fe(III)-CBs is approximately 6.0, which is higher than others (table 3). Especially, pH is a restrictive factor for Cr(VI) absorbent and it could be overcome.
All the parameters as well as the regression coefficients of Langmuir, Freundlich and Langmuir-Freundlich models are obtained and presented in table 1. As the coefficients of correlation (R 2 ) were close to each other (table 1), χ 2 analysis was carried out to identify the suitable isotherm for the adsorption of Cr(VI). The χ 2 was analysed according to the following equation: where q e is the quantity of Cr(VI) adsorbed at the equilibrium (mg g −1 ), q m is the maximum adsorption capacity based on the model prediction (mg g −1 ). It can be found that the Langmuir-Freundlich isotherm fitted best with the experimental data, i.e. the R 2 values were higher, and χ 2 values were lower. And these results (table 1) suggested that the active sites on the surface of Fe(III)-CBs are not homogeneous [38], and the Langmuir-Freundlich isotherm model is the best in the present study.  Based on the Langmuir-Freundlich isotherm model, Cr(VI) adsorption capacity of Fe(III)-CBs was determined to be 166.3 mg g −1 under the present conditions. As mentioned above, chitosan and its modified derivatives were characterized as great potential for Cr(VI) adsorption. The adsorption capacities of Fe(III)-CBs and other adsorbents are listed in table 3. The highest Cr(VI) removal capacity could be achieved by Fe(III)-CBs, probably attributed to its own surface characteristics (see §3.1). In view of its high adsorption capacity, Fe(III)-CBs can be recommended as an efficient alternative for the low concentration Cr(VI) adsorption.

Adsorption kinetics
The effect of initial Cr(VI) concentration on adsorption capacity on Fe(III)-CBs is presented in figure 5. Figure 5 shows that the adsorbed amount of Cr(VI) increased rapidly in the first 100 min, contributing to approximately 80% of the ultimate adsorption capacity. And the adsorption equilibrium reached within 200 min. Comparatively, the required times to achieve equilibrium for the zirconium-chitosan composite and γ-Fe 2 O 3 −chitosan beads were approximately 300 and 360 min [39] respectively; as a result, the Fe(III)-CBs shows better performance in Cr(VI) adsorption.
To determine the adsorption kinetics, the models in table 2 were applied here to show whether they could match the experimental data (table 4). Table 4 shows that the coefficient of correlation (R 2 ) was much higher, and the theoretical q e,cal value was closer to the experimental q e,exp value in the case of the pseudo-first-order kinetic model. Therefore, the pseudo-first-order model fitted the experimental    data better than the pseudo-second-order model. This is in accordance with the Cr(VI) adsorption on Fe(III) hydroxide-loaded sugar beet pulp [11]. Because the pseudo-first-order model is based on the adsorption rate being proportional to the difference between the equilibrium adsorption capacity and the adsorbed amount [20,40], while the adsorbed amount is controlled by Cr(VI) diffusion to the adsorbent, Fe(III)-CBs have the higher kinetic constant k 1 , indicating that this adsorbent is easy to close for Cr(VI).

Effect of coexisting anions
During the adsorption process, the coexisting anions may compete with Cr(VI) for active sites on the adsorbent, and thus negatively affect Cr(VI) adsorption performance. This competition is the result of electrostatic interaction and ligand exchange reaction [24,31]. So the effects of several common anions in natural water [17], such as HCO − 3 , H 2 PO − 4 , SO 2− 4 , Cl − and NO − 3 on Cr(VI) adsorption, were evaluated.
The results show that all the anions selected here could depress the Cr(VI) adsorption capacity ( figure 6). Nevertheless, the extent of depression by each anion is different and ranked as When HCO − 3 , H 2 PO − 4 , SO 2− 4 , Cl − or NO − 3 presented with 10-fold of Cr(VI) concentration, the equilibrium Cr(VI) adsorption capacities were lowered by 16.1%, 18.1%, 45.8%, 79.5% and 83.5% compared with that without any coexisting anion. And increasing the above coexisting anion concentration to 50-fold of Cr(VI) concentration, the adsorption capacities were further decreased by 6.4%, 4.5%, 29.7%, 46.2% and 50.0%, respectively. Compared with the influence of Cl − and NO − 3 , HCO − 3 and H 2 PO − 4 may lead to stronger competition with Cr(VI) in the activated sites of Fe(III)-CBs and depress the Cr(VI) removal to a larger extent.

Regeneration and reuse of Fe(III)-CBs
Reusability is one of the most important parameters to assess the applicability of an adsorbent in practice. Based on the results above, 0.1 mol l −1 Na 2 SO 4 , Na 2 CO 3 , Na 3 PO 4 , NaOH and EDTA were  used as desorption solution. The desorption removal rates were 2.54%, 18.40%, 48.00%, 65.80% and 0.58% for the five desorption solutions, respectively. Among them, NaOH solution desorbing Cr(VI) was most effective [25]. This result was consistent with the results of the pH effect above, indicating that electrostatic interaction between the adsorbent and Cr(VI) plays a significant role in the adsorption. So, in this view, adjusting pH is a practical way to regenerate Fe(III)-CBs. So NaOH solution was used as desorption solution for the regeneration and reuse of Fe(III)-CBs. The tests were conducted with 0.01, 0.05, 0.1, 0.5 or 1.0 mol l −1 NaOH concentration to determine its optimal concentration. Their results, lower than 0.1 mol l −1 of NaOH solution, more or less the desorption rate; and higher than 0.1 mol l −1 , the desorption rate, hardly varied and kept close to 70%. So, the optimal concentration is 0.1 mol l −1 for NaOH solution as desorption solution.
The tests were still conducted to evaluate the possibility of Fe(III)-CBs reuse for Cr(VI) adsorption undergoing five cycles ( figure 7). Their data proved that the adsorption capacity for Cr(VI) was not significantly changed with the cycle number and more than 93% of initial adsorption capacity was maintained after five adsorption/desorption/regeneration cycles.

Proposed process and mechanism
Generally, the adsorption process can be divided into three steps: (i) external surface adsorption, (ii) intraparticle diffusion and (iii) the final equilibrium [20,41]. Among them, the intraparticle diffusion is generally regarded as the rate-limiting step [42]. Based on the intraparticle diffusion model, when the plot of q t versus t 1/2 gives a straight line and passes through the origin, the adsorption process is considered to be controlled only by intraparticle diffusion. When the plot of q t versus t 1/2 exhibits the multilinear plot, the adsorption process should be controlled by both intraparticle diffusion step and other steps [20,[42][43][44][45]. In this work, the value of k p became higher as initial Cr(VI) concentration increased (table 5), suggesting the concentration of Cr(VI) has significant influence on the intraparticle diffusion. Meanwhile, the value of R 2 higher than 0.99 revealed that the intraparticle diffusion plays a vital role in the adsorption process. However, the plot of q t versus t 1/2 shown in figure 8 illustrates that the intraparticle diffusion was not the only rate-limiting step.  It has been the Cr(VI) adsorption by Chitosan mainly through the interaction between several function groups (such as amino group) and Cr(VI) [6]. So, the quantities of groups and their exposure to Cr(VI) are essential for the adsorption. As shown in figure 2, the FT-IR spectrum of Cr(VI) adsorbed on Fe(III)-CBs displayed an obvious formation of new bands at 945 and 807 cm −1 . According to the literature [24,44], these two peaks were attributed to Cr(VI)−O and Cr(III)−OH vibration. This confirms the presence of Cr(VI) on the surface of Fe(III)-CBs and demonstrates a part of adsorbed Cr(VI) was reduced to Cr(III) on the sorbent.
XPS spectra of Fe(III)-CBs before and after Cr(VI) adsorption are presented as figure 9. The photoelectron peaks for the Cr 2p 3/2 and Cr 2p 1/2 are located at 576.9 and 586.8 eV (figure 9a), respectively. The broad peak of Cr 2p 3/2 could be fitted into two peaks at binding energies of 579.3 and 576.8 eV, which were the characteristics of Cr(VI) and Cr(III), respectively. On the other hand, the peaks at 588.0 and 586.5 eV could be attributed to Cr(VI) and Cr(III) binding energies for 2p 1/2 orbits [24,45], manifesting both Cr(VI) and Cr(III) coexisted on the surface of the Fe(III)-CBs after adsorption. The photoelectron peaks for O 1s could be fitted by three peaks at binding energies of 530.8, 531.9 and 532.6 eV (figure 9b), respectively. The dominant peak located at 532.6 eV assigned to C−O, O=H or bound H 2 O, and the peak at around 530.8 suggested the existence of Fe(III)=O. As the adsorption proceeded, the peak that appeared at 531.9 eV showed an obvious increasing intensity, indicating that the bond of C=O enhanced significantly after adsorption of Cr(VI). The C=O band was attributed to the oxidation of alcoholic group (−CH 2 OH) on the molecular chain of chitosan [24]. It is therefore suggested that the alcoholic group was the electron donor in the process of the redox reaction, in which Cr(VI) was reduced to Cr(III), while the CH 2 OH was oxidized to C=O. When the pH ranges from 3.0 to 6.0, Cr(III) exists predominantly as Cr(OH) 2+ [46], the bond between Cr(VI) and Fe(III)-CBs was broken due to the change of the electrostatic interaction. Furthermore, the free Cr(OH) 2+ may be chelated with amine groups or precipitated onto the surface of Fe(III)-CBs in the hydroxide nature. The reduction of Cr(VI) to Cr(III) is the reason for not only the desorption rate, but also the adsorption rate-limiting step.
Therefore, the possible mechanism of Cr(VI) adsorbed on Fe(III)-CBs can be proposed: HCrO 4 − was first adsorbed onto Fe(III)-CBs through electrostatic interaction and ligand-exchange interaction. Then, the Cr(VI) adsorbed via electrostatic interaction was partially reduced to Cr(III) with the −CH 2 OH on the adsorbent served as the electron donor. The resulted Cr(III) was either chelated with NH 2 on the Fe(III)-CBs or precipitated onto the surface of the adsorbent.

Conclusion
In this study, Fe(III)-CBs were synthesized and characterized by SEM, FT-IR and XPS, and applied for low concentration Cr(VI) adsorption. The efficient Cr(VI) adsorption was achieved by the composites within low concentration Cr(VI) and pH range from 2.0 to 6.0. And the adsorption process was well agreed with the Langmuir-Freundlich adsorption model and pseudo-first-order kinetic model. The maximum adsorption efficiency and capacity were 99.36% and 166.3 mg g −1 at pH 6.0, respectively. Both electrostatic interaction and ligand exchange reaction play important roles in the adsorption processes, and the anions including OH − , SO 2− 4 , CO 2− 3 and PO 2− 4 could compete with Cr(VI) on the active adsorption sites. The beads were about 1 mm in average diameter and easy to be separated by filtration, and then regenerated and re-used effectively for the adsorption of Cr(VI). A part of Cr(VI) adsorbed on the beads was reduced to Cr(III) with the −CH 2 OH serving as the electron donor, which reduced the toxicity of chromium. Over all, Fe(III)-CBs have the potential of practical application for the adsorption of Cr(VI) from water.
Data accessibility. All of the data in this investigation have been reported in the paper and are freely available. Authors' contributions. Y.W. designed the experiments, coordinated the study and wrote the manuscript. Y.Z., S.H. and