A kinetic study of the redox behaviour of Fe-III(TPPS) [TPPS=5,10,15,20-tetrakis(p-sulfonato)porphyrinate] in the presence of peroxomonosulfate, hydrogen peroxide, and sulfite/oxygen. Direct evidence for multiple redox cycling and suggested mechanisms

The reactions of the water soluble complex Fe-III(TPPS) [TPPS = 5,10,15,20- tetrakis(p-sulfonatophenyl)porphyrinate] with peroxomonosulfate, hydrogen p eroxide and sulfite/oxygen have been investigated kinetically as a function of reactant concentration and pH. The spectral changes recorded for the re actions between the Fe-III(TPPS) dimer and peroxomonosulfate and hydrogen p eroxide can be interpreted in terms of a redox cycle between (TPPS)(FeOFeII I)-O-III(TPPS) and (TPPS)(FeOFeIV)-O-III(O)(TPPS+), and in terms of multipl e redox cycles also involving Fe-II(TPPS) for the Fe-III(TPPS)-sulfite-oxyg en system. In the case of peroxomonosulfate and hydrogen peroxide a slow re dox cycle (1000 s) between iron-(III) and -(IV) complexes is observed at lo w [SO52-] and [H2O2]. In the case of sulfite-oxygen the kinetic traces are quite different; the Fe-III/Fe-IV redox cycle is very fast (a few seconds) and is only observed after what appears to be an induction period. Furtherm ore, it also depends significantly on the selected experimental conditions (pH, sulfite and oxygen concentration). Rapid-scan techniques were used to study these redox cycles. Reaction mechanisms for the redox cycling of the Fe-III(TPPS)-SO52- system, and for the multiple redox cycling of the Fe-III (TPPS)-sulfite-oxygen system, are proposed. They are based on reactions tha t participate in the suggested mechanism for the iron-catalysed autoxidatio n of sulfite. In contrast to the Fe-III(TPPS)-HSO5- system, which is insens itive to oxygen, oxygen plays an essential role in the multiple redox cycle s of the Fe-III(TPPS)-sulfite-oxygen system, which is accounted for in the proposed mechanism. Computer simulations based on the proposed reaction mec hanisms are in good agreement with the observed experimental kinetic traces and indicate that for the Fe-III(TPPS)-sulfite-oxygen system the formation of the SO5.- radical is the main oxygen-consuming step during the overall redox process.