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 Ga(2)O(3), In(2)O(3), and TiO(2) photocatalysts was investigated. In the Ga(2)O(3)/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 Ga(2)O(3) system were identified to be h(VB)(+) and O(2)˙(−). 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 e(CB)(−) by Cr(vi), which retarded the recombination of e(CB)(−) and h(VB)(+). The In(2)O(3)/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 TiO(2)/UVC system, because Cr(iii) adsorbed on TiO(2) 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 Ga(2)O(3)/UVC, followed by a neutralization step that converts dissolved Cr(iii) into Cr(OH)(3(S)).