Crystalline phase regulates transformation and methylation of mercury sulfide nanoparticles in paddy systems.

Mercury sulfide nanoparticles (HgSNPs) represent an important source of bioavailable mercury (Hg) for microbial methylation in paddy systems, depending on their size and crystalline phases. However, little is known about the phase compositions of HgSNPs in Hg-contaminated paddy fields with dynamically changed redox conditions, their transformation, and methylation potential. Applying transmission electron microscopy (TEM) and synchrotron radiation X-ray absorption spectroscopy (SR-XAS), we found β-HgSNPs as the predominant Hg species in newly contaminated areas, whereas α-HgSNPs dominated in paddies near mining areas. Subsequent incubation assays indicated minimal phase transformation between α-HgSNPs and β-HgSNPs in simulated paddy systems, suggesting their high stability under natural conditions. Compared to α-HgSNPs, β-HgSNPs exhibited a higher methylation potential, as evidenced by greater production of methylmercury (MeHg) and elevated levels of Sn(II)-reducible Hg(II), a proxy for bioavailable Hg. Further experiments and density functional theory (DFT) calculations reveal that the higher bioavailability of β-HgSNPs is closely linked to their crystalline phases and higher atomic binding energy for Hg2+ adsorption, as compared to α-HgSNPs. This study, for the first time, unravels the significance of the crystalline phase in governing the bioavailability of HgSNPs in paddy fields and provides novel insights into the ecological risk of HgS in wetland-like ecosystems.