Synthesis and Characterization of Thiolated Chitosan Beads for Removal of Cu(II) and Cd(II) from Wastewater

Abstract

Removal of Cu(II) and Cd(II) from wastewater using porous chitosan beads is likely to be enhanced by the introduction of thiol groups (−SH). This is because, in accordance with the Hard Soft Acid Base concept, the soft Lewis base of –SH forms a strong bond with soft Lewis acid of Cd(II) or with borderline Lewis acids such as Cu(II). Possible formation of thiourea and disulfide crosslinks (−S–S–) may also confer increased bead stability in acidic solution. Thiolated chitosan beads (ETB) prepared and investigated in this study had a total sulfur content of 7.9 %. The thiolation process slightly increased the Brunauer-Emmett-Teller surface area of the chitosan beads from 39.5 to 46.3 m2/g. This ETB was categorised as a microporous material (pore aperture: 1.8 nm) with multiple and uniform porous layers. Analysis by X-ray photoelectron spectroscopy indicated the presence of three sulfur species, S(−I), S(−II) and S(V) attributed to –S–S–, –SH and sulfonate (−SO3 −) groups. The Langmuir sorption capacity, q max, for Cd(II) was improved by 18 times by thiolation of chitosan. However, the q max for Cu(II) by ETB was seven times lower than that of pristine chitosan beads, possibly due to exhaustion of amine groups (−NH2). The batch sorption data was generally fitted well by a linearised Freundlich isotherm model and a Ho’s pseudo-second-order kinetic model, indicating metal interaction with the heterogeneous surface of ETB and chemical adsorption as the possible rate-limiting step, respectively. The metal uptake has resulted in the oxidation of –SH to –SO3 − group in ETB, thereby decreasing the stability of metal-sulfide bonds as well as their metal uptake.