Revealing the formation of nanoparticulate mercury sulfide in Hg(II)-DOM-Sulfide(-II) systems with nanomolar Hg(II) by using liquid chromatography-ICP-MS analysis.

Sulfidation of mercury (Hg) in Hg(II)-dissolved organic matter (DOM)-sulfide (S(-II)) systems has crucial impacts on its transport, transformation, and bioavailability risks in food chains (e.g., from fish to humans) under anoxic environments. Probing this process remains a significant challenge due to low Hg concentrations and poor sensitivity of conventional characterization methods. Here, we systematically identified nanoparticulate mercury sulfide (nano-HgS) formation in Hg(II)-DOM-S(-II) systems at nanomolar Hg(II) levels and deciphered its environmental factors by integrating liquid chromatography (LC)-ICP-MS combined with ultrafiltration-transmission electron microscopy (TEM). Ultrafiltration-TEM with energy dispersive spectroscopy confirmed the spherical nano-HgS (∼5 nm) formation. Formation kinetics demonstrated a rapid formation of nano-HgS, followed by a slow size increase. Environmental factors, including pH, DOM, S(-II), and Ca2+, modulated nano-HgS size distributions by altering surface charge interactions (e.g., electrostatic repulsion), whereas natural ligands (Cl-, citrate, mercaptopropionic acid) showed negligible effects. Crucially, nano-HgS was the predominant Hg species in the Hg(II)-DOM-S(-II) systems, rather than dissolved HgS species (e.g., HgS0aq and HgS22-). Although nano-HgS, especially aged nano-HgS exhibits lower methylmercury production than Hg(II)-DOM complex at 24 h incubation by Geobacter sulfurreducens PCA, higher methylation for newborn nano-HgS at the initial 8 h further highlights the previously proposed "Trojan horse" mechanism of uptake followed by intracellular dissolution of nano-HgS. This work provides a significant analytical tool for accurately identifying and quantifying nano-HgS in the Hg(II)-DOM-S(-II) systems and highlights the importance of assessing the methylation of newborn nano-HgS because it may amplify Hg toxicity in food chains through providing higher methylation production.