Mechanisms of Reaction Between Co(II) Complexes and Peroxymonosulfate

Advanced oxidation technologies often use peroxymonosulfate in the presence of Co^II_aq. It is commonly assumed that the reaction of Co(H₂O)₆²⁺ with HSO₅⁻ yields Co^III_aq and SO₄^.−. DFT results point out that first Co^II(SO₅)(H₂O)₂ is formed. The homolysis of Co^II(SO₅)(H₂O)₂ to yield (H₂O)Co^II(SO₅)OH^.+SO_4.⁻, is exothermic but has a large activation energy. However the cobalt is not oxidized in this reaction. Co^II(SO₅)(H₂O)₂ reacts with a second HSO₅⁻ to form Co^II(SO₅)₂(H₂O)²⁻ that decomposes via disproportionation of the monoperoxysulfate ions without oxidation of the central cobalt ion. Surprisingly even in the presence of ligands, L, that stabilize Co^III, i. e., pyrophosphate; tri-polyphosphate and ATP, the experimentally observed reaction mechanism involves the formation of LCo^II-OOSO_3aq which then reacts with another HSO₅⁻ to form LCo^II-(OOSO₃²⁻)₂. The latter complex decomposes via disproportionation of the monoperoxysulfate ligands followed by oxidation of the central cobalt cation. Alternatively, in the presence of excess Co^IIL_aq, LCo^II-OOSO_3aq reacts with Co^IIL_aq to form 2Co^IIIL_aq. These results point out that the mechanism of advanced oxidation processes initiated by a mixture of Co(H₂O)₆²⁺ and HSO₅⁻ must be re-considered.