Advanced oxidation technologies often use peroxymonosulfate in the presence of Co II aq . It is commonly assumed that the reaction of Co(H 2 O) 6 2 + with HSO 5 À yields Co III aq and SO 4 *À . DFT results point out that first Co II (SO 5 )(H 2 O) 2 is formed. The homolysis of Co II (SO 5 )(H 2 O) 2 to yield (H 2 O)Co II (SO 5 )OH * + SO 4 * À , is exothermic but has a large activation energy. However the cobalt is not oxidized in this reaction. Co II (SO 5 )(H 2 O) 2 reacts with a second HSO 5 À to form Co II (SO 5 ) 2 (H 2 O) 2À 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 HSO 5À to form LCo II -(OOSO 3 2À ) 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 II L aq , LCo II -OOSO 3aq reacts with Co II L aq to form 2Co III L aq . These results point out that the mechanism of advanced oxidation processes initiated by a mixture of Co(H 2 O) 6 2 + and HSO 5 À must be reconsidered.
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