Hexavalent Chromium Removal in Industrial Relevant Water Matrices 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 the P90 form of TiO₂ . 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.

A few common industrial additives were examined and the improvement on Cr(VI) removal using NaTaO₃ from high to low was: sodium sulfite > ammonium chloride > sodium formate. At neutral pH, citric acid was found to inhibit Cr(VI) reduction with NaTaO₃. Sulfite alone could remove Cr(VI) by chemical reduction, but required large quantities in excess. The combination of sulfite and photocatalyst greatly improved the Cr(VI) removal. Cr(VI) removal using photocatalysts was only slightly affected by other constituents in cooling tower blowdown and had similar removal rates as those observed in pH 7 DI water. 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.

Different regeneration protocols were applied to the used photocatalysts. The alkaline treatment was more effective for removing adsorbed Cr(III) compared to nitric acid treatment, but acid treated TiO₂ had better subsequent Cr(VI) reduction capabilities due to surface modifications that enhanced Cr(VI) adsorption. Both acid regenerated TiO₂ and NaTaO₃ demonstrated comparable Cr(VI) reduction rates with the pristine forms, indicating that these photocatalysts could be stable during regeneration and recycled for multiple uses. These results show that large scale wastewater treatment using metal oxide photocatalysts to remove Cr(VI) may be possible.