Adsorption Effect of Oxalic Acid-Chitosan-Bentonite Composite on Cr6+ in Aqueous Solution

In this paper, a gel adsorption material (CS@OB) was prepared by compounded oxalic acid modified bentonite with chitosan, which was used to remove Cr6+ from aqueous solution. Batch adsorption was carried out under different experimental conditions, such as pH value, adsorption dose, Cr6+ concentration, temperature, time and coexisting ions, to evaluate the adsorption effect of CS@OB. The experimental data were fitted and analyzed by adsorption kinetic model, adsorption isotherm model, and adsorption thermodynamic, and the structure and adsorption mechanism of CS@OB were characterized by SEM–EDS, FTIR and XPS. The results showed that pH had a great influence on the removal of Cr6+ by CS@OB, and the removal efficiency could reach above 99% when pH was 2–3. According to the fitting results of adsorption isotherms (Langmuir and Freundlich), adsorption kinetics (pseudo-first-order and pseudo-second-order) and adsorption thermodynamics, Langmuir isotherm model and pseudo-second-order kinetic model were more suitable for experimental data. The adsorption mechanism was spontaneous and endothermic monolayer chemical adsorption, and the maximum adsorption capacity could reach 111.47 mg·g−1. The adsorption of Cr6+ by CS@OB mainly included electrostatic attraction and redox. Nitrogen-containing groups were protonated with positive charge, which effectively combined with Cr6+ (HCrO4−/Cr2O72−) in solution through electrostatic attraction. Therefore, CS@OB can be used as an effective adsorbent to remove Cr6+ ions from water samples.