Adsorption of Cd ²⁺ on carboxyl-terminated superparamagnetic iron oxide nanoparticles

The affinity of Cd ²⁺ toward carboxyl-terminated species covalently bound to monodisperse superparamagnetic iron oxide nanoparticles, Fe ₃O ₄(np)-COOH, was investigated in situ in aqueous electrolytes using rotating disk electrode techniques. Strong evidence that the presence of dispersed Fe ₃O ₄(np)-COOH does not affect the diffusion limiting currents was obtained using negatively and positively charged redox active species in buffered aqueous media (pH = 7) devoid of Cd ²⁺. This finding made it possible to determine the concentration of unbound Cd ²⁺ in solutions containing dispersed Fe ₃O ₄(np)-COOH, 8 and 17 nm in diameter, directly from the Levich equation. The results obtained yielded Cd ²⁺ adsorption efficiencies of ∼20 μg of Cd/mg of Fe ₃O ₄(np)-COOH, which are among the highest reported in the literature employing ex situ methods. Desorption of Cd ²⁺ from Fe ₃O ₄(np)-COOH, as monitored by the same forced convection method, could be accomplished by lowering the pH, a process found to be highly reversible.