Treatment of electrochemical plating wastewater by heterogeneous photocatalysis: the simultaneous removal of 6:2 fluorotelomer sulfonate and hexavalent chromium.

6:2 fluorotelomer sulfonate (6:2 FtS) is being widely used as a mist suppressant in the chromate (Cr(vi)) plating process. As a result, it is often present alongside Cr(vi) in the chromate plating wastewater (CPW). While the removal of Cr(vi) from CPW has been studied for decades, little attention has been paid to the treatment of 6:2 FtS. In this study, the removal of Cr(vi) and 6:2 FtS by Ga2O3, In2O3, and TiO2 photocatalysts was investigated. In the Ga2O3/UVC system, over 95% of Cr(vi) was reduced into Cr(iii) after only 5 min. Simultaneously, 6:2 FtS was degraded into F- and several perfluorocarboxylates. The predominant reactive species responsible for the degradation of 6:2 FtS in the Ga2O3 system were identified to be hVB + and O2˙-. In addition, it was observed that the presence of Cr(vi) helped accelerate the degradation of 6:2 FtS. This synergy between Cr(vi) and 6:2 FtS was attributable to the scavenging of eCB - by Cr(vi), which retarded the recombination of eCB - and hVB +. The In2O3/UVC system was also capable of removing Cr(vi) and 6:2 FtS, although at significantly slower rates. In contrast, poor removal of 6:2 FtS was achieved with the TiO2/UVC system, because Cr(iii) adsorbed on TiO2 and inhibited its reactivity. Based on the results of this study, it is proposed that CPW can be treated by a treatment train that consists of an oxidation-reduction step driven by Ga2O3/UVC, followed by a neutralization step that converts dissolved Cr(iii) into Cr(OH)3(S).