| Summary: | 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. © 2013 Springer Science+Business Media Dordrecht.
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