DEVELOPMENT AND EVALUATION OF AN IN-SITU REMEDIATION TECHNOLOGY FOR MERCURY CONTAMINATED SEDIMENTS

Sediments are a primary sink for anthropogenic and natural mercury (Hg), which can mobilize into aquatic ecosystems at sediment-water interfaces. Methylmercury (MeHg) is a bioaccumulating neurotoxin formed largely by bacterial conversion of inorganic Hg under anaerobic conditions. Legacy Hg-contaminated sites adversely impact freshwater ecosystems with limited remediation options available. For example, historical Hg use at Y-12 National Security Complex (Oak Ridge, TN) resulted in dispersal of Hg throughout the East Fork Poplar Creek (EFPC) watershed, found primarily in streambank deposits. We synthesized manganese(III)/(IV) oxide-modified activated carbon (MOMAC) as a cost-effective sediment amendment intended to lower net Hg methylation through redox control and limit Hg and MeHg release from contaminated sediments to freshwater through sorption. Amendments were characterized and performance was evaluated in bench-top batch and flow-through column experiments with artificial creek water (ACW), comparing Hg and MeHg released from MOMAC-treated, activated carbon-treated, and untreated sediments. Microbial activity was stimulated in select columns by addition of organic carbon (OC) as acetate + pyruvate into ACW or as spirulina powder into sediment to create conditions favorable for Hg methylation. For each column, ~90 pore volumes of ACW flowed through, the flow was stopped for 3 days, and flushed for ~110 pore volumes. Batch experiments showed similar redox control for Mn oxide and MOMAC, but Hg release to porewater was 6.2 µg/L from Mn oxide-treated sediment compared to 2.0 µg/L in MOMAC-treated sediment. Before the stopped-flow period in column experiments, MeHg concentrations ranged from 0.6 to 12 ng/L in effluent from untreated sediment stimulated with spirulina, while effluent from MOMAC-treated sediment had low or non-detectable MeHg concentrations. However, after the stopped-flow, effluxes of Mn, OC, and MeHg were observed in both columns suggesting reduction of solid Mn(III)/(IV) from native sediment or MOMAC. Addition of MOMAC to sediment lowered Hg and MeHg concentrations compared to untreated sediment but can result in reductive dissolution of Mn in systems with high OC, sulfides, or other electron donors. However, Mn-reduction pathways may be disfavored in systems with electron acceptors (i.e. nitrate, native Mn oxides) such as EFPC.