Photochemical redox reactions of mercury

Mercury is a unique heavy metal element with several oxidation states, whose changes are sensitive to photo energy, directly or indirectly. Because of the existence of stable oxidation states of mercury, redox reactions are important in the element's chemistry. Photochemical redox reactions of mercury involve electron transfer induced, directly, by absorption of light and consequent electronic excitation of a mercury species in a specific oxidation state, or indirectly, by another non-mercury species, i.e., the reactive intermediates (reductants or oxidants) photo-induced. In this chapter, some recent development in the photochemical redox chemistry of mercury in the last five to ten years is reviewed. First, aqueous phase and heterogeneous photochemical redox reactions of mercury in artificial media are discussed. The review is then centered on photochemical redox reactions of mercury in aquatic media relevant to natural surface waters. Notable progress has been made in the aqueous photochemistry of Hg(II) complexes. Recent studies on heterogeneous reduction of Hg(II) photocatalyzed by semiconductor TiO2 indicate that a group of parameters control the process, including characteristics of TiO2 (surface properties, particle size, surface coating, concentration, etc.), PH, irradiation, hole scavengers, and interfering ions (e.g., Cl-). Aquatic Hg(H) species could be reduced through secondary photochemical processes mediated by intermediate reactive reductants (e.g., O-2(-)/HO2) photochemically produced involving aquatic organic matter and through heterogeneous reduction photocatalyzed by particles (e.g., TiO2), and probably also through primary photochemical processes (e.g., direct photolysis of Hg(OH)(2) and Hg(H)-oxalate). Aquatic organic substances are the major electron donors for Hg(H) reduction, but the mechanisms remain to be fully uncovered. The role of metal ions (e.g., Fe, Cu) in photoredox chemistry of aquatic mercury warrants more attention. While the role of the strong oxidants, i.e., OH, in photo-induced secondary oxidation of aquatic Hg(0) is known, other possible oxidants are to be revealed. Much still remains unknown about the photochemical behavior of the unstable Hg(I) species in aquatic photochemical redox chemistry of mercury, which may hold a special key to understanding the photochemoredox cycle of aquatic mercury.