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BackgroundA proposed strategy for immobilizing trace metals in the subsurface is to stimulate calcium carbonate precipitation and incorporate contaminants by co-precipitation. Such an approach will require injecting chemical amendments into the subsurface to generate supersaturated conditions that promote mineral precipitation. However, the formation of reactant mixing zones will create gradients in both the saturation state and ion activity ratios (i.e., aCO32MathClass-bin-MathClass-bin/aCa2MathClass-bin+). To better understand the effect of ion activity ratios on CaCO3 precipitation kinetics and Sr2+ co-precipitation, experiments were conducted under constant composition conditions where the supersaturation state (Ω) for calcite was held constant at 9.4, but the ion activity ratio (rMathClass-rel=aCO32MathClass-bin-MathClass-bin/aCa2MathClass-bin+) was varied between 0.0032 and 4.15.ResultsCalcite was the only phase observed, by XRD, at the end of the experiments. Precipitation rates increased from 41.3 ± 3.4 μmol m-2 min-1 at r = 0.0315 to a maximum rate of 74.5 ± 4.8 μmol m-2 min-1 at r = 0.306 followed by a decrease to 46.3 ± 9.6 μmol m-2 min-1 at r = 1.822. The trend was simulated using a simple mass transfer model for solute uptake at the calcite surface. However, precipitation rates at fixed saturation states also evolved with time. Precipitation rates accelerated for low r values but slowed for high r values. These trends may be related to changes in effective reactive surface area. The aCO32MathClass-bin-MathClass-bin/aCa2MathClass-bin+ ratios did not affect the distribution coefficient for Sr in calcite (DPSr2+), apart from the indirect effect associated with the established positive correlation between DPSr2+ and calcite precipitation rate.ConclusionAt a constant supersaturation state (Ω = 9.4), varying the ion activity ratio affects the calcite precipitation rate. This behavior is not predicted by affinity-based rate models. Furthermore, at the highest ion ratio tested, no precipitation was observed, while at the lowest ion ratio precipitation occurred immediately and valid rate measurements could not be made. The maximum measured precipitation rate was 2-fold greater than the minima, and occurred at a carbonate to calcium ion activity ratio of 0.306. These findings have implications for predicting the progress and cost of remediation operations involving enhanced calcite precipitation where mineral precipitation rates, and the spatial/temporal distribution of those rates, can have significant impacts on the mobility of contaminants.

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Precipitation" Scientific Focus Area (SFA). The ultimate goals of the EMSP project and the continuing INL SFA are to provide a scientific basis for the development of methods of amendment distribution in porous media that leads to sequestration of metal contaminants (e.g. strontium-90), and to determine how macroscopic field-scale modeling can be applied to predict the outcome of remediation activities. Using calcium carbonate as a model system, physical experiments, in addition to modeling at the pore-scale and continuum-scale, were and continue to be used to improve the conceptual approach to predicting the impact of flow-precipitation coupling on solute migration. The University of Idaho has responsibility for physical experiments aimed at characterizing calcium carbonate precipitation kinetics, polymorphs formed, and metal contaminant (i.e., Sr) co-precipitation over a wide range of saturation indices University of Idaho conducted calcium carbonate and strontium co-precipitation experiments that attempted to maintain conditions of constant solution composition and levels of supersaturation that are likely to be relevant in the implementation of subsurface remediation strategies. These experiments stand in contrast to our previous investigations using microbially drive precipitation in batch experiments in which individual experiments exhibit wide ranges in composition, saturation state, and precipitation (and co-precipitation) rates. The results from this study provide the basis for developing generalized precipitation and co-precipitation kinetic models that explicitly account for the extent of supersaturation and variations in solution compositions.The application of the kinetic models will enable the more effective deployment of DOE relevant subsurface remediation strategies.