Competitive exchange reactions between divalent metal ions (Cu2+, Zn2+, Ca2+) and Hg(II) bound to organic ligands and natural organic matter

Mercuric ions (Hg2+) are known to form exceptionally strong complexes with various organic ligands, particularly thiols and dissolved organic matter (DOM) in natural water. However, few studies have experimentally determined whether or not the presence of divalent metal ions, such as Ca2+, Cu2+, and Zn2+, would compete with Hg(II) bound to these ligands, as concentrations of these metal ions are usually orders of magnitude higher than Hg(II) in aquatic systems. Using a novel selective reduction approach, we show that a significant fraction of Hg(II) bound to cysteine (CYS), glutathione (GSH), or DOM could be competitively exchanged by Cu2+, but not by Zn2+ or Ca2+. About 2075% of CYS-bound-Hg(II) (at 2:1 CYS:Hg(II) molar ratio) was exchanged rapidly by Cu2+ at concentrations only 13 orders of magnitude greater than Hg(II). Cu2+ could also compete with Hg(II) bound to GSH and DOM, albeit to a lower extent, owing to their compositional and structural differences from CYS. When complexed with ethylenediaminetetraacetate (EDTA), most Hg(II) could be exchanged by Cu2+ and Zn2+, as well as Ca2+ at increasing concentrations. Our results indicate that, despite its orders of magnitude higher stability constants, Hg(II) bound to thiols and DOM could potentially be exchanged by coexisting metal ions, such as Cu2+, and thus have important implications in Hg(II) speciation, biogeochemical transformation, and transport in natural aquatic environments.