Mercury (Hg) contamination in floodplains poses long-term threats to the surrounding environment due to its persistence in nature and complex biogeochemical cycling. Frequent drying and rewetting in floodplains alter redox oscillations affecting biogeochemical conditions. Sulfurized materials have become popular in Hg remediation as sulfur (S) has a great affinity to bond with Hg. Previous studies demonstrated sulfurized biochar has great potential for use as a soil amendment in Hg contaminated soils to stabilize Hg release and supress net methylmercury (MeHg) production under long-term saturated conditions. This study evaluated the impacts of drying and rewetting in sulfurized biochar amended systems. Laboratory experiments were conducted representing flooding, drying and rewetting conditions in floodplains. Microcosm experiments were first conducted with Hg contaminated soil, biochar, and river water under initial flooding, drying, and rewetting conditions. The sulfurized biochar was further evaluated in a cyclic column experiment representing repeated drying and wetting conditions. This study incorporated aqueous concentrations, solid-phase concentrations, synchrotron-based measurements, and microbial taxonomy. In the microcosm experiment, increased aqueous and solid MeHg concentrations occurred after drying and rewetting. The increased MeHg concentrations were associated with increased concentrations of Fe, Mn, SO₄^2-, and HS^- as well as the changes in S chemistry and the microbial community in the solid phase. In the cyclic column experiment, enhanced release of MeHg associated with elevated concentrations of Mn and DOC were observed during the initial cycles. Release of SO₄^2-associated with shifts in microbial community towards sulfur oxidizing bacteria and changes in S chemistry in solid phases occurred as wetting and drying cycles continued, likely related to microbially-driven oxidation. Results of this study suggest that sulfurized biochar is sensitive to redox oscillations from drying and rewetting. Impacts of drying and rewetting on biogeochemical conditions need to be considered when planning for future remediation projects.

Dr. Alana Ou Wang

University of Waterloo

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