ORGANIC LIGAND-ENHANCED PHOTOCHEMICAL REDUCTION AND IMMOBILIZATION OF CHROMIUM(VI) ON TIO2 PARTICLES IN ACIDIC AQUEOUS MEDIA

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Abstract

Chromium(VI) is both toxic and mutagenic; Cr(III), on the other hand, is less toxic and essential for human nutrition. Thus, the reduction of Cr(VI) to less toxic Cr(III) is extremely important to decrease the potential hazard of Cr(VI) to the ecosystem. The reduction of Cr(VI) by organics alone is a slow proces; however, the simultaneous applications of light energy and activated surface sites may lead to the enhancement of Cr(VI) reduction by organic compounds. The objective of the study was to evaluate the effects of different intensities of photon flux on Cr(VI) reduction catalyzed by titanium dioxide (TiO2), with or without the presence of organic compounds. The results show that less than 5% of 40 μM Cr(VI) was removed by 2 g L−1 TiO2 in the dark. The application of fluorescence lights, however, could catalyze Cr(VI) reduction in the TiO2 suspensions, and 40 μM Cr(VI) would be reduced upon exposure to growth chamber light at pH 2.5 within 2 days. In the absence of organics, photolysis of water molecules provided electrons for Cr(VI) reduction with the release of oxygen. Because organic compounds served as better reductants than water, light-induced TiO2-catalyzed Cr(VI) reduction was further enhanced. For instance, 40 μM Cr(VI) was reduced within 100 minutes in the presence of 0.2 m M HEDTA exposed to growth chamber light at pH 2.5. Therefore, to accelerate Cr(VI) reduction, lower pH (i.e., pH 2.5), stronger light intensity (i.e., growth chamber light), and decomposable organic compounds should be involved in the TiO2 system.

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