Hexavalent chromium removal using metal oxide photocatalysts

Photocatalysis is an attractive treatment method for removing hexavalent chromium (Cr(VI)) from water. Thus far, photocatalytic reduction of Cr(VI) has been investigated mostly using TiO₂ photocatalysts in acidic water solutions. Here we investigate Cr(VI) removal using zinc oxide (ZnO), tungsten trioxide (WO₃), and sodium tantalate (NaTaO₃), metal oxides that display good activity for other photocatalytic reactions such as water splitting, as well as titanium oxide (TiO₂, Evonik P90). The efficiency for Cr(VI) removal using these photocatalysts was investigated in synthetic neutral and alkaline water, as well as in cooling tower blowdown water. The effect of several additives used in water treatment processes on the Cr(VI) removal rate was also studied. For NaTaO₃, citric acid was found to have a detrimental effect to Cr(VI) removal, while sodium formate, ammonium chloride, and sodium sulfite were beneficial. While sulfite alone could chemically reduce Cr(VI), sulfite in combination with a photocatalyst resulted in faster and complete removal of Cr(VI) in 10min using a SO₃^2-/Cr(VI) ratio >35 in pH~8 solutions. NaTaO₃ was found to display the highest Cr(VI) removal rates on a photon basis at pH 3 and in the presence of sodium sulfite, while ZnO and TiO₂ showed the best performance in pH 7 and cooling tower blowdown water.