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

Highlights: • 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. - Abstract: 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.