Mechanisms of Reaction Between Co(II) Complexes and Peroxymonosulfate

Advanced oxidation technologies often use peroxymonosulfate in the presence of Co-aq(II). It is commonly assumed that the reaction of Co(H2O)(6)(2+) with HSO5- yields Co-aq(III) 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.+SO4.-, 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 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 (CoLaq)-L-II, LCoII-OOSO3aq reacts with (CoLaq)-L-II to form 2Co(III)L(aq). These results point out that the mechanism of advanced oxidation processes initiated by a mixture of Co(H2O)(6)(2+) and HSO5- must be re-considered.