Electrochemically assisted photocatalytic degradation of Orange II: Influence of initial pH values

Abstract

The effect of initial pH values in solution was studied in the process of electrochemically assisted photocatalytic degradation (EAPD) of an azo-dye Orange II using a β-PbO 2 electrode modified by TiO 2. The removal of color and total organic carbon (TOC) from the simulated dye wastewater was experimentally investigated at pH 2.29 and 11.52. The degradation products were examined by means of HPLC, UV–vis and FTIR spectra analyses. Similar color removal rate was achieved at pH 2.29 and 11.52, but the removal of TOC at pH 2.29 was 20.0% higher than that at pH 11.52. The dye was oxidized the most completely at pH 2.29 than at 11.52 and 6.88. There was a new absorbance peak at ca. 255 nm generated at ca. 40 min only at pH 11.52, which was ascribed to the quinonic compounds resulted from 1-amino-2-naphtol. A reductive degradation pathway by reductive species such as active electrons was proposed, which only led to the removal of color in parallel with the oxidative degradation pathway. Hole process was proved to be the dominating process at pH 2.29 and the effect of the reductive species was also significant at pH 11.52. Initial pH values were proved to have only influenced the degradation pathways to some extent, and therefore resulted in the disparity of the removal of color and TOC. High mineralization rate was still achieved in spite of the initial pH values in solution due to the prevailing photocatalytic oxidation mechanism.