Stepwise Reduction Approach Reveals Mercury Competitive Binding and Exchange Reactions within Natural Organic Matter and Mixed Organic Ligands

Abstract

The kinetics of mercuric ion (Hg 2+ ) binding with heterogeneous naturally dissolved organic matter (DOM) has been hypothesized to result from competitive interactions among different organic ligands and functional groups of DOM for Hg 2+ . However, an experimental protocol is lacking to determine Hg 2+ binding with various competitive ligands and DOM, their binding strengths, and their dynamic exchange reactions. In this study, a stepwise reduction approach using ascorbic acid (AA) and stannous tin [Sn(II)] was devised to differentiate Hg(II) species in the presence of two major functional groups in DOM: the carboxylate-bound Hg(II) is reducible by both AA and Sn(II), whereas the thiolate-bound Hg(II) is reducible only by Sn(II). Using this operational approach, the relative binding strength of Hg 2+ with selected organic ligands was found in the order dimercaptopropanesulfonate (DMPS) > glutathione (GSH) > penicillamine (PEN) > cysteine (CYS) > ethylenediaminetetraacetate > citrate, acetate, and glycine at the ligand-to-Hg molar ratio < 2. Dynamic, competitive ligand exchanges for Hg 2+ from weak carboxylate to strong thiolate functional groups were observed among these ligands and within DOM, and the reaction depended on the relative binding strength and abundance of thiols and carboxylates, as well as reaction time. These results provide additional insights into dynamic exchange reactions of Hg 2+ within multicompositional DOM in controlling the transformation and bioavailability of Hg(II) in natural aquatic environments.

Mercury mobility, colloid formation and methylation in a polluted Fluvisol as affected by manure application and flooding–draining cycle

“…The addition of manure accelerated the release of Hg through reductive dissolution of Mn oxyhydroxides in the cornfield soil (HMLC). Mercury was released 4 d earlier, as a result of additional labile carbon of the liquid manure (1) acting as an electron donor enhancing microbial soil reduction (Liu et al, 2020) and (2) acting directly as a reductant of the Mn oxyhydroxides (Remucal and Ginder-Vogel, 2014). In the manure treatment, we observed a fast decrease in Hg concentration and a constantly high proportion of particulate P-HgT rel.…”

Natural organic matter facilitates formation and microbial methylation of mercury selenide nanoparticles

“…S5c †). These results suggest the formation of inner-sphere coordination bonds between the surface mercury atoms of nano-tiemannite and the carboxyl moieties of NOM, [39][40][41] which occurred to a greater degree for SRFA than SRHA.…”

Synergistic Effects of a Chalkophore, Methanobactin, on Microbial Methylation of Mercury

“…Similar experiments were performed in the presence of various concentrations of MB (2.5 to 50 M), cysteine, glutathione, or DMPS, representing important thiol ligands commonly observed and/or used in biogeochemical investigations of Hg(II) transformations (9,13,14). Additional methylation assays were performed at low Hg(II) (1 nM) and MB (1 and 10 M) concentrations either in the absence or presence of DOM (2 mg liter Ϫ1 ), as previously described (11,48,49). Parallel experiments were also performed without cells as negative controls.…”