Effect mechanism of copper ions on photocatalytic activity of TiO/graphene oxide composites for phenol-4-sulfonic acid photodegradation.

Organic pollutants in electroplating wastewater can be removed by photodegradation, however the effect mechanism of heavy metal ions on photocatalytic activity still remains unknown. Herein, we firstly reported the self-assembly synthesis of titanium dioxide/reduced graphene oxide (TiO/rGO) composites for phenol-4-sulfonic acid (PSA) removal, and investigated the effects of Cu ions on photocatalytic efficiency. During the self-assemble process, rGO nanosheets were connected together to form network macropores, and simultaneously induced the deposition of hierarchically nanostructured TiO microspheres. The synergetic effects of TiO microspheres and rGO nanosheets improved the photocatalytic activity by enhancing light adsorption ability, stabilizing electron-hole separation and decreasing band gap energy. The Cu ions in wastewater showed positive and negative effects on PSA photodegradation. In the photocatalytic reaction, the electron-induced reduction reaction of Cu(II) into Cu(0) or Cu(I) took place, which inhibited electron-hole recombination and thus enhanced photocatalytic activity. However, the high chemical stability of PSA-Cu(II) complex compounds held back PSA photodegradation. The appropriate concentrations of Cu ions at around 25 mg/L accelerated PSA photodegradation over TiO/rGO composites. The PSA degradation into CO and HO was performed by using hydroquinone, benzoquinone and maleic acid as degradation intermediates. Hence, TiO/rGO composites are novel multifunctional photocatalysts to purify electroplating wastewater.