Mechanisms of Reaction Between Co(II) Complexes and Peroxymonosulfate

Advanced oxidation technologies often use peroxymonosulfate in the presence of CoIIaq. It is commonly assumed that the reaction of Co(H2O)62+ with HSO5− yields CoIIIaq and SO4.−. DFT results point out that first CoII(SO5)(H2O)2 is formed. The homolysis of CoII(SO5)(H2O)2 to yield (H2O)CoII(SO5)OH.+SO4.−, is exothermic but has a large activation energy. However the cobalt is not oxidized in this reaction. CoII(SO5)(H2O)2 reacts with a second HSO5− to form CoII(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 CoIII, i. e., pyrophosphate; tri‐polyphosphate and ATP, the experimentally observed reaction mechanism involves the formation of LCoII‐OOSO3aq which then reacts with another HSO5− to form LCoII‐(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 CoIILaq, LCoII‐OOSO3aq reacts with CoIILaq to form 2CoIIILaq. 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. The mechanisms of Advanced Oxidation Processes initiated by the reaction of Co(H2O)62+ with peroxymonosulfate have to be re‐considered. Heterogenous cleavage of the O−O bond and formation of Co(IV), are also excluded, as these structures failed to converge. the exothermic cleavage reactions the O−O bond in CoII(SO5)2(H2O) and of CoII(SO5) (H2O)2 are preferred.