Elemental mercury production from contaminated riparian soil suspensions under air and nitrogen bubbling conditions

The dynamic change of redox conditions is a key factor in emission of elemental mercury (Hg⁰) from riparian soils. The objective of this study was to elucidate the influences of redox conditions on Hg⁰ emission from riparian soils. Soil suspension experiments were conducted to measure Hg⁰ emission from five Hg-contaminated soil samples in two redox conditions (i.e., treated with air or with N₂). In four of the five samples, Hg⁰ emission was higher in air treatment than on N₂ treatment. Remaining one soil, which has higher organic matter than other soils, showed no distinct difference in Hg⁰ production between air and N₂ treatment. In soil suspensions subject to N₂ treatment, the dissolved organic carbon (DOC) and Fe²⁺ concentrations were 3.38- to 1.34-fold and 1.44- to 2.28-fold higher than those in air treatment, respectively. Positive correlations were also found between the DOC and Fe²⁺ (r = 0.911, p < 0.01) and Hg²⁺ (r = 0.815, p < 0.01) concentrations in soil solutions, suggesting Fe²⁺ formation led to the release of DOC, which bound to Hg²⁺ in the soil and, in turn, limited the availability of Hg²⁺ for reduction to Hg⁰ in N₂ treatment. On the other hand, for remaining one soil, more Hg²⁺ might be adsorbed onto the DOM in the air treatment, resulted in the inhibition of Hg⁰ production in air treatment. These results imply that the organic matter is important to prevent Hg⁰ production by changing redox condition. Further study is needed to prove the role of organic matter in the production of Hg⁰.