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