Structure-dependent mercury sequestration and microbial methylation mediated by FeS nanoparticles in contaminated groundwater†

Abstract

Iron sulfide nanoparticles (CMC-FeS) have demonstrated great potential for selective and effective in situ mercury (Hg) immobilization in soil and groundwater through sorption, coprecipitation, or precipitation. Yet, the relative contributions of these immobilization mechanisms on Hg removal and their impacts on microbial mercury methylation in groundwater remain unknown. Here, we revealed that the Hg removal efficiency is ranked sorption (82.2%) > coprecipitation (75.2%) > chemical precipitation (22.3%). Conversely, the net MeHg production exhibited an inverse trend: sorption (46.73 nM) < coprecipitation (50.67 nM) < chemical precipitation (59.82 nM). Other than dissolved Hg(II), the particulate Hg species including sorbed (Hg-CMC-FeS_sorp), coprecipitated (Hg-CMC-FeS_cpt), and precipitated (Hg-CMC-FeS_pre) were bioavailable to Geobacter sulfurreducens PCA and contributed to MeHg production following the order dissolved Hg(II) > Hg-CMC-FeS_cpt > Hg-CMC-FeS_sorp > Hg-CMC-FeS_pre. Particulate Hg effectively prevented the microbial reduction of Hg(II) and thus, the production of Hg(0) during Hg methylation. Methylation potential of particulate Hg was probably correlated with the Hg–S coordination configuration. Hg-CMC-FeS_sorp and Hg-CMC-FeS_pre displayed tetrahedral Hg–S₄ coordination whereas Hg-CMC-FeS_cpt exhibited linear Hg–S₂ coordination. MeHg production correlated linearly with Hg removal efficiency, and the produced MeHg can be predicted based on the known Hg removal performance. The findings highlight the paramount role of Hg speciation and coordination chemistry in controlling microbial methylation and provided a mechanistic basis for developing next-generation Hg sorbents through structural modulation to achieve enhanced Hg immobilization and inhibited bioavailability.