Selective biosorption of silver ions from aqueous solution using chitosan-based biosorbents

Abstract

In this study, Ag⁺-imprinted thiourea/glutaraldehyde grafted O-carboxymethyl chitosan (ITG-OCMC) beads and Ag⁺-imprinted glutaraldehyde-crosslinked chitosan (IGCC) gel beads were synthesized to selectively adsorb Ag⁺ from bimetallic aqueous solutions containing the same molar concentration of Ag⁺ and Cu²⁺. Surface imprinting technology has been applied to achieve extremely high selectivity for Ag⁺ on the prepared sorbents by generating recognizable binding sites. For the ITG-OCMC beads, experimental results indicated that high degree of carboxymethylation and low level of crosslinking would help to achieve higher uptake capacity of Ag⁺. The maximum uptake of Ag⁺ was found to be 156.32 mg g⁻¹ at 40.0 °C with an initial Ag⁺ concentration of 160.50 mg L⁻¹ and the biosorbent dosage of 1.0000 g L⁻¹. Langmuir isotherm and Lagergren’s pseudo-second-order kinetics can be used to describe the sorption process of Ag⁺. Analyses from FTIR and XPS confirmed that selective adsorption of Ag⁺ took place on the surfaces of ITG-OCMC beads by chelation through >C=S, amine, carboxyl and hydroxyl groups. For the IGCC beads, the maximum uptake of Ag⁺ by the ion-imprinted chitosan beads was found to be 89.200 mg g⁻¹ at 25.0 °C with an initial Ag⁺ concentration of 352.95 mg L⁻¹ and the biosorbent dosage of 1.0000 g L⁻¹. The adsorption equilibrium and kinetics of Ag⁺ by IGCC beads can be better described by Langmuir isotherm and the intraparticle diffusion model. FTIR and XPS analyses suggested that amine functional groups involve the binding of Ag⁺ via complexation at higher solution pH (3.0 ≤ pH ≤ 5.0) and ion exchange at lower solution pH (1.0 ≤ pH <3.0).