Mobilization and detoxification of carcinogenic oxyanions from a contaminated soil mediated by micro and nano-sized metallic particles

Soil washing is a treatment process that has been used extensively for toxicant extraction, yet the process does not detoxify the mobilized pollutants per se. The more common detoxification processes include redox reactions that are mediated by zero-valent metals (ZVMs). Reduction of Cr(VI) to Cr(III) is desirable, as the latter specie is toxicologically innocuous to most living organisms and also has a low mobility and bioavailability. The main goals of this project included the development of an efficient treatment for the extraction and inactivation of toxic inorganic anions using ZVMs from contaminated soils. A variety of micron-sized ZVMs (Al, Cu, Fe, Mg, Ni, Si, and Zn); metallic mixtures (Pd/Fe, Ag/Fe, Cu/Fe, Zn/Fe, Co/Fe, Mg/Fe, Ni/Fe, Al/Fe, Si/Fe, Pd/Cu, Pd/Zn, Pd/Cu/Fe, Pd/Zn/Fe, and Zn/Cu/Fe); Cu and Fe nano-sized particles (NPs), and selected nano-sized bimetallic analogs (Pd/Fe, Cu/Fe and Pd/Cu) were evaluated for reactivity towards the reduction of Cr(VI) in a surfactant preparation (TweenRTM 20) under a selection of reaction conditions. At circum neutral pHs, a rapid inactivation of the surface was observed for almost all the tested metals whereas complete reduction of Cr(VI) was achieved at acidic pH only by using Cu, Fe, Mg, or Zn. 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 results suggest that the cementation of a noble metal serves not only as a reaction accelerator but also provides protection of the ZVM surface impeding its rapid inactivation. A considerable increase in reactivity (up to 100-fold) was observed with NPs. The formation of clusters of polymeric structures provided an extra protection of the NPs surface with a striking increase of their reactivity mainly at more alkaline conditions. The influence of the presence of various naturally-occurring inorganic and organic molecules were evaluated for the detoxification of Cr(VI). An appreciable enhancement (up to 50-fold) of the constant rate was achieved for certain organic compounds capable of reducing Cr(VI) (alpha-hydroxyl carboxylate, ascorbic acid, cysteine) and for certain chelating agents (EDTA derivates, siderophores). Moderate improvements in the Cr(VI) reduction were observed for certain inorganic oxyanions Cl-4,NO- 3,SeO2-4 , likely caused by their redox reaction with zero-valent iron.