Photocatalytic reduction of Cr(VI) from agricultural soil column leachates using zinc oxide under UV light irradiation

Abstract

The photocatalytic reduction of Cr(VI) from agricultural soil leachates irrigated with Cr(VI)-containing waste hydroponic solution was evaluated in this work. For this purpose, zinc oxide was used as a catalyst under UV irradiation (λ=365 nm ). The reduction of Cr(VI) was preliminarily evaluated on synthetic solutions with a concentration of 15 mg L ⁻¹ to optimize the catalyst loading and the solution pH and to determine the effect of organic matter. Greater removal of Cr(VI) was observed at pH 7, and the optimum catalyst loading was found to be 2 g L ⁻¹, which achieved an 84% Cr(VI) reduction in 6 h. The influence of dissolved organic matter on the reduction of Cr(VI) was evaluated through the addition of different concentrations of humic acid (HA) to the chromium solution. The removal of Cr(VI) was continuously enhanced as the HA concentration gradually increased from 0 to 14 mg L ⁻¹. The percentage of hexavalent chromium reduction from soil leachates was in the range of 13–99%, and the rate constant was significantly enhanced by the presence of organic compounds in the soil pore water. Thus, a marked synergistic effect between the photocatalytic reduction of Cr(VI) and the organic matter in soil (e.g. humic substances) was observed in real samples and was similar to that observed in the Cr(VI) synthetic solution that contained HA.

Keywords:

• hexavalent chromium
• ZnO
• photocatalysis
• pore soil water
• humic substances

Acknowledgements

Financial support from the Department of Chemistry Sciences of the Universidad Autónoma de Nuevo León, PAICyT-UANL 2010 and SEP-PROMEP/103.5/11/4330 is gratefully acknowledged.