Molybdenum powder induced Fe2+/Fe3+ cycling enhanced activation peroxydisulfate for degradation of AO7: cost analysis, degradation mechanism, and toxicity evaluation

Abstract

To address the disadvantage of poor degradation effect of Fe²⁺/peroxydisulfate (PDS) system, a co-catalyst was introduced in this study to construct a molybdenum (Mo)/Fe²⁺/PDS system for the degradation of Acid Orange 7 (AO7), and the degradation rate could reach 95.67%. The effects of different parameters (initial pH, Fe²⁺ concentration, etc.) on the degradation effect of AO7 in this system were investigated, and the optimal degradation conditions were obtained by combining cost analysis. Cycling experiments showed that Mo had good stability. The co-catalytic mechanism of Mo powder was determined using X-Ray Diffraction, Scanning Electron Microscope, and X-ray Photoelectron Spectroscopy. The effects of different inorganic anions on AO7 were discussed: Cl⁻ and NO₃⁻ had little effect on degradation, while SO₄²⁻ and HCO₃⁻ both inhibited the removal of AO7 from the system to different degrees, with HCO₃⁻ showing a greater inhibition than SO₄²⁻. The main active radicals were identified as sulfate radical (SO₄^·−) and hydroxyl radical (·OH). In addition, intermediates were identified, and the attack sites were determined by combining the highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels and the Fukui index to hypothesize the degradation pathways. Toxicity indicators such as the mutagenicity of AO7 and intermediates were evaluated, and the overall trend of toxicity was decreasing. This study is expected to provide an efficient treatment process for dye wastewater.