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

Mercury sulfide nanoparticles (HgS_NPs) 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 HgS_NPs 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-HgS_NPs as the predominant Hg species in newly contaminated areas, whereas α-HgS_NPs dominated in paddies near mining areas. Subsequent incubation assays indicated minimal phase transformation between α-HgS_NPs and β-HgS_NPs in simulated paddy systems, suggesting their high stability under natural conditions. Compared to α-HgS_NPs, β-HgS_NPs 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-HgS_NPs is closely linked to their crystalline phases and higher atomic binding energy for Hg²⁺ adsorption, as compared to α-HgS_NPs. This study, for the first time, unravels the significance of the crystalline phase in governing the bioavailability of HgS_NPs in paddy fields and provides novel insights into the ecological risk of HgS in wetland-like ecosystems.