Photocatalytic reduction of chromium and oxidation of organics by polyoxometalates

The photocatalyfic reduction of Cr(VI) to the less toxic Cr(III) is presented in the presence of the polyoxometalates (POM) PW12O403- or SiW12O404- as photocatalyst and an organic substrate (salicylic acid or propan-2-ol) as electron donor. Cr(VI), as dichromate, is reduced to Cr(III), according to the 6:1 stoichiometry of PW12O404- versus Cr2O72- indicated from experiments in the dark. Increase of POM or salicylic acid (SA) concentration accelerates, till a saturation value, both the reduction of metal and the oxidation of the organic, suggesting that these two conjugate reactions act synergistically. The photocatalytic action of POM is not so important in the case of highly concentrated solutions of organics that exhibit direct photochemical reduction of Cr(VI), i.e. propan-2-ol (i-prOH), while it becomes important at low concentrations of i-prOH, especially for organics that do not react directly photochemically with Cr(VI), such as SA. Increase of Cr(VI) concentration enhances consumption of SA and Cr(VI) till an optimum value, due to inner filter effect. The method is suitable for a range of chromium concentration from 5-100 ppm achieving complete reduction of Cr(VI) to Cr(III) up to non-detected traces (> 98%). The presence of oxygen does not influence the efficiency of SA and Cr(VI) consumption. In contrast to the semiconductor-based heterogeneous photocatalysis, the POM-based homogeneous process seems superior in the frame that: (i) it remains catalytic throughout illumination by providing more active sites and (ii) among the two POM used, the one that is more efficient in the degradation of the organic, that is PW12O403- compared to SiW12O404-, is also more efficient in reducing Cr(VI), due to a kinetic effect, and a compromise is not needed. (c) 2005 Elsevier B.V. All rights reserved.