Production of chitosan coated, citric acid modified almond, and hazelnut shell adsorbents for Cr(VI) removal and investigation of equilibrium, kinetics, and thermodynamics of adsorption

Removal of potentially toxic elements such as Cr(VI) from water bodies is crucial for health of aquatic organisms and human health. In order to produce an adsorbent with high Cr(VI) adsorption capacity, two different composite adsorbents were synthesized by coating citric acid modified almond and hazelnut shells with chitosan. Then, chitosan in the structure was cross-linked with glutaraldehyde. Cr(VI) adsorption performances of chitosan-citric acid modified almond shell composite (C-CA-AS) and chitosan-citric acid modified hazelnut shell composite (C-CA-HS) were investigated. The physical and chemical structures of the adsorbents before and after adsorption and chitosan coating were determined by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), and energy dispersive X-ray (EDX) analyses. Optimum adsorbent dosage, Cr(VI) concentration, pH, contact time, and temperature parameters were determined by batch adsorption experiments. Accordingly, the optimum adsorbent dosages for the adsorption of Cr(VI) from the 55 mg/L Cr(VI) solution using C-CA-AS and C-CA-HS adsorbents were 1.8 g/L and 1.0 g/L, respectively. For both adsorbents, the optimum contact time is 120 min; the optimum pH is 2.0. The increase in temperature enhanced the adsorption. Langmuir, Freundlich, Scatchard, Dubinin-Radushkevich, and Temkin isotherm graphs were drawn by using equilibrium adsorption data and the adsorption parameters were calculated from these isotherms. The adsorption equilibrium was found more compatible with Langmuir isotherm. The maximum adsorption capacities calculated by Langmuir isotherm were 100.3 mg/g for C-CA-AS and 89.5 mg/g for C-CA-HS. In addition, as a result of kinetic calculations, adsorption was found to be compatible with pseudo-second-order kinetic model, and as a result of calculation of thermodynamic parameters, it was determined that adsorption was spontaneous and endothermic. The data obtained in this study showed that the recommended adsorbents have higher adsorption capacities than many adsorbents described in the literature and commercial resins. In addition, since agricultural wastes are used in the adsorbents obtained, the adsorbents are low cost.