Facile Carboxylation of Sugarcane Bagasse and the Adsorption Mechanism for Cadmium Ions

The presence of cadmium ions (Cd2+) has been an urgent issue in water environmental protection. In this paper, sugarcane bagasse was treated with nitric acid for the preparation of an adsorbent, on which the adsorption process for Cd2+ was consistent with quasi-second-order kinetics. The adsorption isotherm agreed with the Langmuir expression, and the maximum Cd2+ adsorption capacity reached 119.3 mg/g. After elution with ethylene diamine tetraacetic acid (EDTA), the high adsorption capacity could be completely restored. As the common ions in natural hard water, Ca2+ exhibited a weak inhibition effect on the adsorption for Cd2+ while Mg2+ completely poisoned the adsorbent. Fourier transform infrared spectrometer (FTIR) results showed that carboxylate groups were generated after treatment with nitric acid and neutralization with sodium carbonate, and a wavenumber shift of 9 cm(-1) at the band corresponding to carboxylate groups was observed after Cd2+ doping. X-ray photoelectron spectrometer (XPS) results showed that compared to that of Cd(NO3)(2) the binding energy of Cd 3d(3) and Cd 3d(5) in adsorbed Cd2+ was decreased by 0.9 eV, indicating that the electrons in carboxylate groups were captured by Cd2+ during the adsorption process. Density-functional theory (DFT) calculations clarified that total energy and Gibbs free energy declined by 23.1 and 66.7 kJ/mol due to the chelation behavior between Cd2+ and carboxylate, indicating that carboxylate groups were active sites for Cd2+ removal. According to these results, carboxylate groups are highly recommended in the fabrication of Cd2+ adsorbents.