In-depth analysis of the roles and mechanisms of sulfate radical and hydroxyl radical in the degradation of metal-cyanide complexes

Abstract

Persulfate-based advanced oxidation processes (PS-based AOPs), characterized by the coexistence of SO₄•⁻ and HO•, have been proven effective in treating a series of cyanide-bearing pollutants. However, the mechanisms of these reactive species in the degradation of cyanides, especially metal-cyanide complexes, remain unclear or contradictory. The degradation behavior of representative cyanides (including potassium cyanide and potassium ferricyanide) at different pH conditions (2, 7 and 12) in thermally activated persulfate system (T = 60 °C) was explored, and the roles of SO₄•⁻ and HO• in cyanide degradation were explored by leveraging the distinct characteristics of reactive species under different pH conditions. The study found that both HO• and SO₄•⁻ can react with free cyanide (CN⁻ and HCN). However, the reaction barrier between CN⁻ and HO• is lower than that between HCN and SO₄•⁻, resulting in a higher removal rate of free cyanide under alkaline conditions compared to acidic and neutral conditions. For complexed cyanide, the complex bonds in ferricyanide were completely broken within 24 h by thermally activated persulfate at pH 2, releasing free cyanide, indicating the effectiveness of SO₄•⁻ in breaking the Fe-C bonds in ferricyanide. In contrast, ferricyanide was barely decomposed at pH 12, implying the inefficacy of HO• in breaking the Fe-C bonds. This study also innovatively found that SO₄•⁻ breaks the Fe-C bonds by oxidizing Fe(Ⅲ) in ferricyanide to Fe(Ⅳ) or Fe(Ⅴ), releasing CN⁻, which is then converted to CNO⁻ by SO₄•⁻ and HO•. CNO⁻ is further mineralized to NO₃⁻, NH₄⁺, and N₂ through hydrolysis or oxidation reactions. This research clarifies, for the first time, the activity of SO₄•⁻ and HO• toward cyanide degradation in PS-based AOPs.