The effect of Ca-bearing contents in chitosan on Pb 2+, Cd 2+ and Cu 2+ adsorption and its adsorption mechanism

The preparation of chitosan has been investigated for more than half century; however, the application of chitosan for heavy metal (HM) adsorption is still under research. This study investigated the effects of chitosan with chemically controlled Ca-bearing contents (CBC) on Pb2+, Cd2+ and Cu2+ adsorption in the solution with the initial pH values of 2.10, 4.14 and 6.13. Highly purified chitosan showed the optimum HM adsorption at the initial pH values of 4.14 and 6.13, and the adsorption mechanism was chemisorption involving valence forces through sharing or exchange of electrons between the chitosan and HM ions. Highly purified chitosan prepared from HCl treated chitin only showed effective for Pb2+, however, those prepared from CH3COOH treated chitin showed effective for Pb2+, Cd2+ and Cu2+ adsorption due to a little amount of CBC. The HM adsorption mechanisms of partly purified chitosan were precipitation due to CBC and biosorption. Chitosan with 73% CBC showed the optimum adsorption of Pb2+ (755 mg/g) at an initial pH value of 2.10 while Cd2+ (979 mg/g) and Cu2+ (877 mg/g) at the initial pH values of 4.14 and 6.13. High Ca(OH)2-bearing chitosan prepared from HCl and H2SO4 treated chtin showed the optimum Cd2+ (978 mg/g) and Cu2+ (852 mg/g) adsorption at an initial pH value of 2.10. Biosorption isotherm and kinetics models showed that the adsorption data of Pb2+, Cd2+ and Cu2+ onto the surface of chitosan was well-fitted by Langmuir model and Pseudo-second-order model with correlation coefficient (R2 > 0.95 and R2 > 0.91, respectively). Pseudo-second-order model showed that the adsorption capacity strongly depended on CBC in chitosan and initial pH value of HM solution. It is concluded that the HM adsorption by the prepared chitosan is a chemical process that was supported by CBC of chitosan through elevating solution pH value.