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 h_VB⁺ 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 e_CB⁻ by Cr(vi), which retarded the recombination of e_CB⁻ and h_VB⁺. 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).

6:2 fluorotelomer sulfonate (6:2 FtS) and chromate (Cr(vi)) in chromate plating wastewaters can be simultaneously removed by photocatalysis.