Simultaneous degradation of phenol and reduction of chromium (VI) using UV/TiO2 photocatalysis

Abstract

Organic compounds can serve as electron donors when reducing metallic pollutants such as hexavalent chromium [Cr(VI)] during a photocatalytic reaction. However, simultaneous Cr(VI) reduction and degradation of phenol has not yet been studied under photolytic and photocatalytic processes. Both processes, reduction of Cr(VI) and phenol degradation, are desirable since both phenol and the hexavalent form of chromium are acutely toxic at high concentrations. Cr(VI) is also carcinogenic under low and subchronic conditions. An investigation was launched to devise an effective method of simultaneously degrading phenol and reducing Cr(VI) in a waste water sample using photocatalysis. TiO2 was added to a solution of Cr(VI) and phenol in a laboratory cooling jacket reactor. This was studied under UV light for an hour and readings recorded. Controls were run under the same set up to determine the impact of the catalyst and UV light. In this study, it was noted that at higher concentrations of phenol while keeping Cr(VI) constant at 50 mg/L, a higher percentage reduction of Cr(VI) to Cr(III) was achieved. This was as a result of a high number of reaction sites available. However, at lower concentrations of phenol and Cr(VI) set at 50 mg/L, higher phenol degradation rates were observed. Photolysis degrades phenol to a much higher percentage (47.27 %) compared to photocatalysis (23.19 %) under the same conditions. A 30.03 % Cr(VI) reduction was achieved during photolysis, while for photocatalysis, only 13.69 % was obtained. It was therefore discovered that the presence of a catalyst inhibits the reaction, hence the effect of UV light is key in the photocatalytic experiment. A better understanding of the effectiveness of the photocatalytic process could be obtained by running the experiment using a different reactor set up.