Mechanisms of inorganic carbon-14 attenuation in contaminated groundwater: Effect of solution pH on isotopic exchange and carbonate precipitation reactions

Abstract

Radioactive 14C is a significant contaminant associated with nuclear fuels and wastes that is potentially highly mobile in the environment as dissolved inorganic carbonate species. This study investigated the mechanisms by which dissolved inorganic 14C is retained in surface and groundwater environments via precipitation and isotopic exchange reactions. Precipitation of calcite in the presence and absence of nucleation sites is considered along with isotopic exchange with both atmospheric CO2 and solid carbonates. Precipitation occurs at calcite supersaturation values of SICAL > 1.5 in the absence of nucleation sites and SICAL > 0–0.5 in the presence of nucleation sites, suggesting that precipitation of 14C-bearing carbonates is much more likely in subsurface environments where nucleation sites are abundant. The maximum 14C removal in solid isotopic exchange experiments occurred after approximately 2 weeks equilibration. In these experiments the amount of 14C removed from solution was proportional to the amount of calcite surface area present, and removal from solution was equivalent to rapid equalisation of the isotope ratio in an 8–10 Å active surface layer. Although the reactivity of natural carbonates may be lower than the calcite samples used in this study, these results suggest isotopic exchange with solids will be an important 14C retardation mechanism in subsurface environments containing only modest TIC concentrations. These results suggest that if inorganic 14C is released into sub-surface environments, both precipitation and solid phase isotopic exchange can result in non-conservative 14C-DIC transport and 14C contamination may persist in groundwater for decades following accidental releases. In contrast, in experiments open to atmosphere with pH values below 9.3, complete loss of dissolved inorganic 14C was very rapid and occurred with timescales of 10's of hours. 14C loss was due to a rapid exchange of dissolved 14C species with 12CO2 (g) and the kinetics of 14C removal increased as pH values were lowered (i.e. atmospheric isotopic exchange was first order with respect to the concentration of carbonic acid present). Thus these results suggest that release of inorganic 14C to surface waters with pH values <9.3 would result in rapid exchange with 12CO2 (g) and 14C would not persist in the aqueous environment, whereas 14C-DIC released to saturated subsurface environments may persist close to the release site for decades due to precipitation and solid phase exchange reactions preventing/retarding transport with the groundwater.