Reduction of hexavalent chromium mediated by micro- and nano-sized mixed metallic particles.

A variety of micron-scale mixed metallic particles (Pd/Fe, Ag/Fe, Cu/Fe, Zn/Fe, Co/Fe, Mg/Fe, Ni/Fe, Al/Fe, Si/Fe, Pd/Cu and Pd/Zn; and Pd/Cu/Fe, Pd/Zn/Fe and Zn/Cu/Fe) and selected nano-sized bimetallic analogs (Pd/Fe, Cu/Fe and Pd/Cu) were evaluated for reducting activity with Cr(VI) in a surfactant preparation (Tween 20) under a variety of reaction conditions. Relative to the reactivity of the zero-valent iron, the tested bimetallic mixtures (Pd/Fe>Pd/Zn>Ag/Fe>Ni/Fe>Zn/Fe>Pd/Cu>Cu/Fe) appreciably increased the pseudo-first-order rate constant. The Zn/Cu/Fe represented a cost-effective preparation providing comparable or improved kinetic parameters relative to the more expensive palladized bimetallic mixtures. The Pd/Fe, Cu/Fe and Pd/Cu nano-sized particles proved to be more reactive (up to 100-fold) for the reductive remediation of Cr(VI). The results were comparable when working with nano-sized Cu and Fe particles, suggesting that the cementation of a noble metal serves not only as a reaction catalyst but also provides protection to the metallic surface, impeding its rapid inactivation.