Cadmium removal from water using thiolactic acid-modified titanium dioxide nanoparticles

This study investigated the use of titanium dioxide (TiO₂) nanoparticles to remove aqueous cadmium from simulated wastewaters. Nanosized (45 Å) colloids of anatase TiO₂ were synthesized through the controlled hydrolysis of TiCl₄ and their surfaces modified with the bidental chelating agent thiolactic acid (TLA). Colloids were introduced into 65 ppm cadmium-laden waters, and the suspensions were purged aerobically, anoxically with an inert gas, or by a sequential aerobic/anoxic purge. Suspensions were illuminated with 253.7 nm light. In each experiment, samples were taken from the reactor, filtered, and the filtrates analyzed by atomic absorption spectroscopy for residual cadmium. Results from the aerobic experiments exhibited minimal (approximately 10%) removal of the cadmium from solution and no reduction of the metal on either the modified or the unmodified colloid. Anoxic results were more promising, showing no cadmium reduction on the unmodified colloid but a 40% adsorption and reduction (from a +2 valence state to elemental cadmium as determined by methyl viologen tests) of cadmium on TLA-modified colloid in the presence of light. Results from the mixed atmospheric conditions fared the best and demonstrated that in the absence of light, approximately 20% of aqueous cadmium was sorbed to the modified colloid via a Freundlich adsorption isotherm. Upon illumination, greater than 90% of cadmium was removed by both adsorption and reduction processes onto the TLA-modified TiO₂. These removal and reduction processes were catalytic in nature. Results from this study are significant because to date, no other research in the literature has been able to accomplish cadmium removal and reduction using TiO₂.