Surface-modified chitin by TEMPO-mediated oxidation and adsorption of Cd(II)

To introduce carboxyl groups on the surface of chitin and improve the adsorption performance of Cd(II) from aqueous solution, chitin was modified by 2, 2, 6, 6-tetramethylpiperidine-1-oxyl radical (TEMPO) with hypochlorite (NaClO) and sodium bromide (NaBr). Structure characterization demonstrated the carboxyl groups were successfully introduced on the surface of chitin. Batch experiments with various parameters of the content of carboxyl group, pH, contact time, initial concentration of Cd(II) and ionic strength on the adsorption performance were investigated. The surface-modified chitin exhibited enhancement for the Cd(II) adsorption capacity than original chitin. The Cd(II) adsorbed on carboxylated chitin increased rapidly in the first 30 min and reached equilibrium within 120 min. The dynamic data followed the pseudo-second-order kinetic model. Both Frendlich and Langmuir isotherm models fitted well to the equilibrium data and the maximum adsorption capacity calculated from Langmuir model was about 207.9 mg/g at 298 K. The adsorption of Cd(II) tended to increase with an increase of pH varying from 2.0 to 5.0, but decrease with an increase of ionic strength. The Cd(II) removal mechanism on carboxylated chitin was mainly through electrostatic interaction to form outer-sphere complexes and ion exchange.