The redox chemistry of humic substances (HSs) is important in many processes relevant to the fate and transport of contaminants in terrestrial and aquatic environments. In particular, the role of HSs in the abiotic and microbially mediated transformation of organic and inorganic contaminants is of great current interest. Humic substances can serve as electron donors and terminal electron acceptors for microbial anaerobic respiration [1-5], as reductants for organic and inorganic contaminants [6-8], and as electron transfer mediators [9-11].