The chemistry of aqueous S(IV)-Fe-O2 system: state of the art

The aqueous Fe-S(IV)-O2 system is kinetically superior to pure O2 in oxidizing inorganic (e.g., As(III) to As(V), S(IV) to S(VI)) or organic (e.g., phenol) compounds and can achieve EH levels (>1.3 V) that exceeds the O2-H2O couple. Fe-S(IV)-O2 is important in various geochemical, biochemical, atmospheric, and industrial processes, and thus much effort has been devoted to understanding the mechanistic aspect of its redox chemistry. Despite the many advances made recently, Fe-S(IV)-O2 redox chemistry has not been fully understood and elucidated. Clarification is needed on the redox chemistry of Fe-S(IV)-O2: from a mechanistic level (e.g. mode of catalysis, effects of ligand) to its relative importance in various natural processes (e.g., acid rain formation). We attempt to initiate some of these clarifications by: 1) critically examining experimental results, conclusions, and disagreements found in literature, 2) considering the Fe-S(IV)-O2 system in the light of other relevant chemistries, 3) highlighting difficulties in experimental practice that can interfere with the chemistry of Fe-S(IV)-O2, and 4) discussing future research needs. This review ends with a large compilation of available thermodynamic properties (complex stability constants, ) and kinetic data (rate expression, rate constants) relevant to Fe-S(IV)-O2 system.