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

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 5 μM SCN into 100 μM Cr(VI) + 600 μM 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.