Mechanistic roles of particulate organic carbon in mercury (Hg) immobilization in Hg-contaminated soil around the Yuping mercury mining area, Guizhou, China

Mercury (Hg) is a highly toxic heavy metal pollutant characterized by bioaccumulation potential and long-range transport capacity. As one of its critical environmental sinks, soil Hg contamination has attracted significant scientific and regulatory attention. Particulate organic carbon (POC) plays a pivotal role in governing the speciation and stabilization of Hg in soils due to its rapid turnover rate and strong metal-binding capacity, yet current understanding of these mechanisms remains notably inadequate. This study systematically investigated the variability in SOC fraction enrichment across different soil types and elucidated the POC-mediated Hg stabilization mechanisms in soil through comprehensive characterization analyses. The experimental results demonstrate that POC can adsorb more Hg²⁺, thereby promoting Hg reduction in the soil environment. At the same time, the content of available Hg in POC is lower than that of mineral-associated organic carbon (MAOC), and the residual Hg is higher than that of MAOC. Therefore, it can be considered that POC has a certain promoting effect on the stability of Hg in soil. Furthermore, SEM-EDS results showed that the Hg concentration in POC was higher than that in MAOC. Fourier transform infrared spectroscopy (FTIR) showed that -OH, -COOH and -SH were involved in the adsorption of Hg. These findings establish that POC content directly regulates both the concentration and chemical forms of Hg in farmland soil, suggesting that POC management could serve as a nature-based strategy for mitigating Hg availability and mobility in Hg-contaminated agricultural soil.