CHEMICAL AND PHYSICAL CHARACTERIZATION OF NATURALLY PRECIPITATED HYDROUS FERRIC OXIDES AT THE TAR CREEK SUPERFUND SITE, OKLAHOMA

Nearly 80 years of mining operations left the Tar Creek Superfund Site contaminated with Pb, Zn, and Cd. Over time, metal sulfides in mine waste (“chat”) piles become oxidized, creating more labile and bioavailable species that leach into Tar Creek. Tar Creek flows from upstream chat piles, through several downstream communities, and into the Neosho River, a local recreational water body. Elevated metal concentrations in downstream residential soils and floodplain sediments have raised concern over community exposure and have highlighted the importance of Tar Creek in metal transport. Recent work has identified hydrous ferric oxides (HFOs), which naturally precipitate in the water column of Tar Creek following the mixing of surface water with iron-rich groundwater draining from flooded mines, as an important mechanism for heavy metal transport and storage. Natural HFOs are effective metal sorbents, but their unique chemical and physical characteristics cause their behavior to differ from that predicted by their synthetic counterparts. Characterization of HFO morphology and metal adsorption to HFOs is therefore necessary to achieving a better understanding of metal transport within Tar Creek and similarly contaminated sites.

HFOs deposited on creek-side tree bark during flooding events were collected at 4 different sites along Tar Creek. Investigation of HFOs deposited on tree bark enables consideration of how variables such as HFO age and particle size affect bulk chemistry. HFOs are amorphous with a particle size range of 200-300 nm when disaggregated. X-ray fluorescence analysis shows that metal concentrations decrease away from source, with averages of [Pb] by site of 815, 612, 185, and 150 µg g^-1. The ratio of Pb/Zn also decreases away from source as [Zn] increases relative to [Pb], suggesting that metal affinity relates to sorption stability constants which decrease in the order Pb>Zn>Cd. Analysis of bonding environment through Mössbauer spectroscopy and of colloid stability by zeta potential suggests that tree bark HFOs contain two ferric iron phases. Future work will quantify metals stored in HFOs, their potential for remobilization, and their bioavailability in order to assess the risk of community exposure.