The precipitation, growth and stability of mercury sulfide nanoparticles formed in the presence of marine dissolved organic matter

The methylation of mercury is known to depend on the chemical forms of mercury (Hg) present in the environment and the methylating bacterial activity. In sullidic sediments, under conditions of supersaturation with respect to metacinnabar, recent research has shown that mercury precipitates as beta-HgS(s) nanoparticles (beta-HgS(s)(nano)). Few studies have examined the precipitation of beta-HgS(s)(nano) in the presence of marine dissolved organic matter (DOM). In this work, we used dynamic Eight scattering (DLS) coupled with UV-Vis spectroscopy and transmission electron microscopy (TEM) to investigate the formation and fate of beta-HgS(s)(nano) formed in association with marine DOM extracted from the east and west of Long Island Sound, and at the shelf break of the North Atlantic Ocean, as well as with low molecular weight thiols. We found that while the beta-HgS(s)(nano) formed in the presence of oceanic DOM doubled in size after 5 weeks, those forming in solutions with coastal DOM did not grow over time. In addition, when the Hg-II : DOM ratio was varied, beta-HgS(s)(nano) only rapidly aggregated at high ratios (>41 mu mol Hg-II per mg C) where the concentration of thiol groups was determined to be substantially low relative to Hg-II. This suggests that functional groups other than thiols could be involved in the stabilization of beta-HgS(s)(nano) Furthermore, we showed that beta-HgS(s)(nano)forming under anoxic conditions remained stable and could therefore persist in the environment sufficiently to impact the methylation potential. Exposure of beta-HgS(s)(nano) to sunlit and oxic environments, however, caused rapid aggregation and sedimentation of the nanoparticles, suggesting that photo-induced changes or oxidation of organic matter adsorbed on the surface of beta-HgS(s)(nano) affected their stability in surface waters.