FINAL REPORT: Coupling Sorption to Soil Weathering During Reactive Transport: Impacts of Mineral Transformation and Sorbent Aging on Contaminant Speciation and Mobility

Abstract

The goal of our project is a predictive-mechanistic understanding of the coupling between mineral weathering and contaminant (Cs, Sr, I) fate in caustic waste-impacted sediments at the Hanford Site. Through bench-scale experiments, we have identified geochemical transformations that alter the mobility of priority pollutants (Cs, Sr, I) in subsurface environments characteristic of high-level radioactive waste (HLRW)-impacted DoE sites. Our studies are designed to model the unique chemistry of this subsurface contamination, to quantify rates of contaminant uptake and release, and to identify molecular mechanisms of time-dependent, irreversible sequestration of contaminants into the solid phase. Our approach is to link quantitative macroscopic measures of contaminant mobility and partitioning to the molecular-scale mechanisms that mediate them. We have found that the molecular mechanisms themselves change with time and system composition in response to the evolving chemistry of contaminant-solution-mineral interactions. Specifically, our results show that contaminant fate is closely coupled to the major silicate incongruent weathering reactions that occur when soil solids are contacted with aqueous solutions under conditions that are far from equilibrium. Neoformed precipitates - including carbonate, feldspathoid and zeolite phases, have been observed to sequester Cs and Sr under caustic waste conditions. In contrast, iodide is less effectively sequestered into the neoformed precipitates. In any case, the long-term stability of these co-precipitates must be assessed, particularly in respect to the site closure scenario wherein sediment pore water is expected to return to a “natural” pH and ionic strength fed by rainwater recharge after removal of the caustic source. Our research centers on a series of saturated and unsaturated column studies conducted on Hanford Formation sediments that had been previously reacted in batch or column systems with synthetic tank waste leachate (STWL) for up to 1 year in the presence and absence of CO2. Batch or column STWLreacted sediments are then subjected to column leaching experiments using simulated background pore water (BPW) solution to assess subsequent contaminant release. We used a multi-faceted approach (XAS, XRD, DRIFT, NMR, TEM/EDS, wet chemistry) to investigate molecular-scale mechanisms that give rise to macroscale response. In order to develop efficient predictive tools applicable to the Hanford site our work involves (1) production of laboratory-weathered sediments under conditions representative of caustic waste release; (2) mineralogical characterization and contaminant molecular speciation of the STWL-weathered (time = 0) sediments; (3) intensive monitoring of element release trajectories during controlled BPW infiltration; (4) mineralogical characterization and contaminant molecular speciation of BPW-leached (time = final) sediments; use of (2) through (4) to constrain mechanistic reactive transport modeling of contaminant release from weathered sediments.