Study of the mechanisms of cadmium biosorption by dealginated seaweed waste

The ability of dealginated seaweed waste, a waste material derived from the commercial processing of seaweed for alginate production, to remove cadmiu m from solution was determined. Cadmium sorption was found to be rapid (91% removal within 5 min), achieving a residual concentration of 0.8 mg L-1 af ter 1-h contact time from an initial solution concentration of 10 mg L-1 Th e binding of cadmium by dealginate was found to be pH dependent, optimal so rption occurring at around pH 6-8. The mechanism of cadmium ion binding by dealginate was investigated by a number of techniques. Potentiometric titra tion of the dealginate revealed two distinct pK(a) values, the first having a value similar to carboxyl groups and the second comparable with that of saturated thiols and amines. Esterification of the dealginate resulted in t he subsequent reduction in cadmium sorption (95% to 17%), indicating that c arboxyl groups are largely responsible for sorption. Evidence from FT-IR sp ectra confirmed the presence of carboxyl groups in untreated dealginate, wh ile the number of carboxyl groups was markedly reduced in the esterified sa mple. Furthermore, the FT-IR spectrum for dealginate was found to be simila r to that previously reported for mannuronic acid-rich calcium alginate. De termination of a molar ratio in the displacement of calcium by cadmium on d ealginate further supported the presence of an ion-exchange relationship. T he ion-exchange constant was calculated to be 0.329 x 10(-6). The speciatio n of cadmium in solution both before and after sorption was determined by a n ion-selective electrode (ISE)technique. The findings of this study sugges t that the sorption of cadmium by dealginate is mainly due to an ion-exchan ge mechanism.