Cadmium ion adsorption on carboxyl-terminated iron oxide beads

The affinity of Cd ²⁺ toward carboxyl-terminated iron oxide beads (C-TIOB, Sigma) suspended in aqueous solutions was investigated using anodic stripping voltammetry (ASV) as a function of both pH and [Cd ²⁺] in the μM range. Advantage was taken of the superparamagnetic character of C-TIOB to confine and/or remove the beads within the enclosed vessel, using an external magnet, and thus assess the influence of dispersed C-TIOB on the amount of Cd ²⁺ detected by ASV. Within experimental error, the difference between ASV data recorded before and after adding and then confining the particles by magnetic means, which provides a measure of the amount of Cd ²⁺ adsorbed on C-TIOB, was found to yield results comparable to those of identical measurements with fully dispersed C-TIOB. Adsorption data collected as a function of [Cd ²⁺] and pH revealed a marked increase in the binding constant of Cd ²⁺ for carboxylate ions upon attachment to the solid support, over values predicted strictly from standard solution phase equilibria.