Efficient activation of sulfite autoxidation process with copper oxides for iohexol degradation under mild conditions

Highlights: • Copper oxides exhibit excellent catalytic activity for sulfite autoxidation. • Sulfite activation with copper oxides achieves rapid iohexol abatement for mild condition. • Sulfate radical is identified as the main active species for iohexol degradation. • Inner-sphere complexation occurs between sulfite and Cu(II)/Cu(I) sites on oxide surface. • Copper oxides have great reuse ability in the activation of sulfite autoxidation process. -- Abstract: Sulfite has been recently emerging as an appealing sulfate radical (SO₄^{• −}) precursor for efficient treatment of organic contaminants. Due to the negligible autoxidation of sulfite, activators are often introduced to accelerate sulfite autoxidation and the concomitant generation of SO₄^{• −}. Present heterogeneous activators are mostly not very effective under mild conditions (pH 7.0–8.0). In this work, efficient activation of sulfite with copper oxides including Cu₂O and CuO for iohexol degradation under mild pH conditions is proposed. In a comparison of iohexol degradation efficiency by sulfite autoxidation activated with different metal oxides (Co₃O₄, CoO, α-Fe₂O₃, γ-Fe₂O₃, CuO and Cu₂O), CuO and Cu₂O with lower toxicity are efficient activators and removal efficiencies of ~95% can be obtained at pH 8.0. SO₄^{• −} is identified to be the major species contributing to the removal of iohexol by electron paramagnetic resonance (EPR) spectroscopy and quenching experiment. Based on the effect of ionic strength and copper leaching, sulfite is proposed to interact with copper oxides via inner-sphere coordination. Effect of critical influencing parameters and efficacy of copper oxides in real water matrixes are investigated. The results suggest that using copper oxides as activators is a new alternative to promote sulfite autoxidation process for rapid contaminants degradation.