Photocatalytic reduction, chromium

Gkika E, Troupis A, Hiskia A, Papaconstantinou E. Photocatalytic reduction of chromium and oxidation of organics by polyoxometalates. Appl Catalysis B Environ 2006 62 28-34. 

This activity is particularly useful for degradation of strongly hazardous substances or recalcitrant pollutants that are difficult to remove in chemical or biochemical processes. In this respect any pathway leading to abatement of chromate(VI) pollution arouses interest. One such pathway seems to be created by cooperation between iron and chromium photocatalytic cycles, which were reported as effective in conversion of chromate(VI) into Crm species [20-23,97]. A synergistic photoreduction of CrVI and Cu11 mediated by Ti02 [98], or photocatalytic reduction of Crvl and oxidation of organic matter by environmental polyoxometallates as photocatalysts [99], may constitute alternative possibilities. 

In aspect of its toxicity, any pathway leading to abatement of chromate(VI) pollution arouse a vivid interest. One of such pathways seems to be created by cooperations between the iron and chromium photocatalytic cycles, which were reported as effectively converting chromate(Vl) into Cr(III) species. Photochemical coupling reactions between polycarboxylate Fe(III) complexes and chromate(Vl) were studied and strong collaboration between both photocatalysts was demonstrated, which was significantly affected by the oxygen concentration (16,17,95,261). On the other hand, chromium(Vl) reduction pho-toinduced by iron(lll) nitrilotriacetate accompanied by nta degradation was found to be independent of the O2 concentration, whereas the oxidation state of the chromium product depended on the pH (257). 

The effect of the addition of citric acid on the photocatalytic reduction of hazardous Cr(VI) to less hazardous Cr(III)) with titania catalysts has been studied by means of in situ EPR of chromium species by Meichtry et al. [52] using titania P25 as photocatalyst. Reduction experiments of Cr(VI) solution were performed under near-UV (366 nm) irradiation under acidic conditions (pH 2) with bubbling air. It is found that the addition of citric acid fadhtates Cr(VI) reduction with a stepwise reduction of the chromate CrO/ (V)) via formation of Cr(V) and Cr( IV) and finally Cr(III) spedes observed. In the absence of dtric add, a cycHng between the different valence states of chromium occurs because of reduction and reoxidation processes by OH radicals. The maximum rate (fivefold increase) of Cr(VI) reduction is achieved at an initial citric add/Cr(VI) molar ratio of 1.25. Citric acid is oxidized to its anionic radical by electron abstraction of surface-trapped holes Cit + h+ j, -> Cit . 

Figure 10. Photocatalytic activity of Au/titania nanocomposites containing 0 to 5 % Au in phenol-oxidation and chromium-reduction reactions [57].

It is now realized that copper as metal next to iron and chromium participates in photoredox cycles and its role cannot be ignored. The most important part of the cycle is photoreduction of Cu(II) to Cu(I) induced by solar light and oxidation of ligands to their environmentally benign forms. Then Cu(I) is easily re-oxidized to Cu(II), which can coordinate the next ligand molecule, and thereby the Cu photocatalytic cycles contribute to continuous environmental cleaning. Besides oxida-tion/reduction, other critical processes relevant to the copper cycles are adsorption/desorption and precipitation/dissolution... 

This type of photocatalytic cycle can efiicientiy be used for the degradation of organic substrates with the total recovery of the photocatalyst. They have also been used for the reduction of metal ions [25] and in simultaneous conversion of dye and hexavalent chromium using visible light illumination [26]. Some specific examples are described below. 

---