Adsorption of Aquatic Cadmium (II) by Chestnut Inner Shell

Removal of aquatic heavy metals by biosorption was of practical importance. In this paper, the effect of pH, reaction time and adsorbent dosage on the adsorption of cadmium by chestnut inner shell as adsorbent were studied, and the adsorption mechanism was probed by means of model simulation, ion exchange experiment, scanning electron microscope and infrared spectroscopy. It was revealed that chestnut inner shell was an ideal cadmium absorbent characterized by wide adaption range of pH (from 3 to 6), short sorption equilibrium time (15 min), high adsorption capacity (maximum of 14.706 mg·g^-1) and high removal efficiency (under the condition with an initial C_d²⁺ concentration of 250 mg·L^-1 and chestnut inner shell dosage of 10 g·L^-1, the removal rate of aquatic C_d²⁺ reached 95%). The adsorption process could be well fitted not only with pseudo-second order kinetics model (R²=1.000), but also with Langmuir models (R²=0.998), and SEM photos showed that a lot of particle or ash substance was deposited on the surface of chestnut inner shell after adsorption, implying that this was a surface adsorption with adsorption rate was mainly controlled by chemical adsorption. Dubinin-Radushkevich model simulation and ion exchange experiments implied that ion exchange was the main adsorption mechanism for Cd adsorption, with K⁺ and Ca²⁺ as the main exchange ions, while FTIR spectra revealed that -OH, -NH, -COO-, -P=O acted as functional groups to chelated with C_d during adsorption either.