Adsorption of divalent metal ions by succinylated and oxidized corn starches

Corn starch was succinylated (degree of substitution, DS 0.03¯0.07) with succinic anhydride in an aqueous alkaline medium (pH 10), or oxidized (DS 0.13¯0.29) with sodium hypochlorite in the presence of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and sodium bromide, and the adsorption properties of the starches for Cu2+, Zn2+, Pb2+, and Cd2+ in their aqueous salt solutions were investigated under various conditions. Regardless of the metal type, the adsorption capacity of both ionic starches reached a state of equilibrium within 5¯10 min for starch dispersion in metal solutions. The succinylated starch was most effective in binding Pb2+, whereas the oxidized starch was effective for Cu2+ among the tested metal ions. Cd2+ was least effective in binding either to the succinylated or to the oxidized starch. However, the oxidized starch was partially soluble in aqueous solutions, and thus the starch dissolution resulted in reduced metal-adsorption efficiency. The metal-adsorption of both ionic starches followed the Langmuir adsorption isotherm. The maximum adsorption capacity (Q) and Langmuir constant (b) for a succinylated starch (DS 0.07) for Pb2+ were 0.534 mmol g-1 and 2.276×10-3 ppm-1, respectively. These values were higher than those of Cu2+, Zn2+, and Cd2+. The Q and b values for an oxidized starch (DS 0.29) for Cu2+ were significantly higher (1.245 mmol g-1 and 14.98×10-3 ppm-1, respectively) than those of Pb2+, Zn2+, and Cd2+. Therefore, among the tested ions, Pb2+ was adsorbed most effectively by the succinylated starch, and Cu2+ by the oxidized starch.