The role of thiocyanate in enhancing the process of sulfite reducing Cr(VI) by inhibiting the formation of reactive oxygen species

•Thiocyanate(SCN) catalytically enhanced the reduction of Cr(VI) by sulfite.•SCN inhibited the formations of SO4− and OHin Cr(VI)/sulfite system.•The consumption of sulfite for Cr(VI) reduction was reduced by SCN.•An optimal SCN concentration for enhancing Cr(VI) reduction by sulfite was assessed. The reductive detoxification of Cr(VI) by sulfite is known as the prevailing strategy and can be successfully implemented for the treatment of Cr(VI)-contaminated waters. However, this method inevitably faces the challenges of excessive consumption of sulfite due to the generations of highly oxidative OH and SO4− during the process of sulfite reducing Cr(VI). In this study, we find that a small quantity of thiocyanate (SCN) can catalytically enhance the process efficiency of Cr(VI) reduction by sulfite and effectively prevent the excessive consumption of sulfite. Specifically, when adding 5μM SCN into 100μM Cr(VI) + 600μM sulfite reaction system at pH 3.5, Cr(VI) reduction amount and [sulfite]oxidation/[Cr(VI)]reduction ratio value were approximately 2 and 0.45, respectively, times those in the SCN-free case. The maximum Cr(VI) reduction amount can be achieved at an initial [SCN]/[Cr(VI)] molar ratio of 2.0. Electron spin resonance measurement, combined with the fluorescence spectrum detection, verified that the process of sulfite reducing Cr(VI) mediated by SCN probably proceeds via the non-radical pathway, avoiding the formation of OH and SO4− under aerobic condition.