Photochemical reduction of divalent mercury (Hg(II)) in UVB light by humic acids and low molecular mass organic acids (oxalate, cysteine): A probe into mechanism and kinetics

Aquatic photochemical redox of mercury (Hg) is important in its cycle and fate in the environment. A laboratory study was conducted to probe the kinetics and mechanism of photochemical reduction of Hg(II) by humic acids (HAs) and organic acids (OAs) by following the change of Hg(II) concentrations in photochemical experiments using spectrophotometric analysis of Hg(II) with dithizone. We first studied the effects of HAs (Aldrich and Acros humic acids, ADHA, ACHA) and pH on this Hg(II) analysis method. We found that the method was valid and applicable at acidic pH (2-3) with HA up to100 ppm and basic pH (6-9) until pH ≤ 10 with good linearity. The HAs lowered the methods sensitivity and this effect was similar at both acidic and basic pHs. Excessive dithizone should be used to ensure complete complexation of Hg(II) with dithizone. The dithizone solution should be used, instead of chloroform, to zero the spectrophotometer.;The Hg(II) photo-reduction by the ADHA was found to depend on the ADHA concentrations with minimum levels required for detectable Hg(II) reduction and higher levels that inhibited Hg(II) reduction. The Hg(II) reduction at the levels of Hg(II) and ADHA used, can yield the kinetics similar to that obtained at field Hg(II) levels with the apparent rate constants on the scale of h-1. Mechanistically, Hg(II) photo-reduction occurs probably through secondary photochemical processes in which Hg(II) is reduced by organic reducing agents generated through primary and secondary photochemical processes. Free Hg(II) species, rather than Hg(II) strongly bound to HA or OA ligands, are highly reducible. Deprotonated HA may be most favorable for the Hg(II) reduction. Dissolved Oxygen (DO) is probably not involved in the Hg(II) photo-reduction at high pH either in Hg(0) oxidation or generation of organic reducing agents but it is involved at low pH. The Hg(II) photo-reduction differs mechanistically at low and high pH, mainly in the role of DO in the Hg(II) reduction. Hg(II) was also reduced in the dark in the presence of the ADHA or OAs in a quick initial pulse immediately after Hg(II) was mixed with the HA or OA at both low and high pH (but quite less at pH 2-3 than 6-9). The findings of this study implicate that the "fresh" (free) Hg(II) species deposited from the atmosphere are highly photochemically reducible and can be removed rapidly from surface waters. Further research can be done on Hg(II) photo-reduction by cysteine. Radical scavengers can be used to further study on the Hg(II) photo-reduction by humic acids.