Final report for grant number DE-FG02-06ER64244 to the University of Idaho (RW Smith)-coupling between flow and precipitation in heterogeneous subsurface environments and effects on contaminant fate and transport

Abstract

Engineered remediation strategies for inducing mineral precipitation in the subsurface typically involve the introduction of at least one reactant either by direct injection or by in situ generation. The localization of reactant sources means a wide range of saturation states and ion ratios will be created as reactants are mixed: These conditions together can result in a wide range of precipitation rates, as well as impact which mineral phase precipitates. This is potentially important for the capacity of the precipitates to take up of trace metal contaminants, for their long term stability. Aragonite, for example, is able to sequester a larger amount of Sr than calcite. However, aragonite is less stable under typical groundwater conditions, and so may release sequestered Sr over time as the aragonite transforms to a more stable phase. In addition, previous experimental studies have indicated that other system constituents may influence calcium carbonate precipitation and consequently the Sr uptake potential of a system. For example, dissolved organic carbon (at levels typical of groundwaters) can suppress crystal growth. As a result, the continuous nucleation of small crystals, rather than growth of existing crystals, may be the dominant mode of precipitation. This has the potential for greater uptake more » of Sr because the smaller crystal sizes associated with nucleated calcite may more readily accommodate the distortion resulting from substitution of the larger Sr ion for Ca ions than can larger crystals. However, these smaller crystals may also be less stable and over the long term release Sr as a result of Ostwald ripening. To better understand the formation and composition of relevant calcium carbonate mineral phases two related series of mineral precipitation experiments were conducted. The first series of experiments, conducted using a Continuously Stirred Tank Reactor (CSTR) operated at steady state rates of precipitation was focused on understanding the influence of pH and ammonium carbonate (the hydrolysis product of urea: ureolytically driven calcium carbonate precipitation has been demonstrated to be a promising method of inducing mineral precipitation in the field) on calcium carbonate polymorph and Sr co-precipitation. The second series of experiments, conducted at constant pH and saturation state, was focused on understanding the influence of aqueous carbonate to calcium ratios on the precipitation rate of calcite. In 12 CSTR experiments (three pH levels, two ammonium carbonate levels, and two strontium levels) we found that lower pH values and ammonium carbonate concentration promoted the precipitation of calcite and the higher pH values and ammonium carbonate concentration promoted the precipitation of aragonite (as determined by X-ray diffraction). Overall, the rate of calcium carbonate precipitation increased with increasing pH and ammonium carbonate concentration, consistent with increasing values of Q/K. Intermediate conditions resulted in the precipitation of a mixture of calcite and aragonite. There was no discernible effect of strontium on the rate of precipitation or the phase precipitated. In our experiments we precipitated rhombohedral calcite, lath-shaped aragonite and inter-grown calcite-aragonite mixtures. Using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometer we characterized the composition of solids from an experiment in which both calcite and aragonite precipitates were identified by X-ray diffraction. We found a range in composition from a high Sr and low Mg phase (inferred to be aragonite) to a coexisting lower Sr and higher but variable Mg phase (inferred to be calcite). Values of the distribution coefficient for strontium of 1.1 and 0.2 for aragonite and calcite, respectively were estimated from the data. These values compare to values of 1.1 and 0.1 for aragonite and calcite, respectively, determined from bulk analysis of precipitates from experiments in which only calcite or only aragonite were detected. In our experiments to assess the influence of solution composition on precipitation rate (at a constant value of Q/K), we found that the precipitation rate varied by approximately a factor of 2 over the range of conditions considered, with a maximum rate observed at a carbonate to calcium ion molar ratio of approximately 0.2. Precipitation kinetics at the “extremes” tested in this study exhibited interesting behavior. At the lowest ion ratio (carbonate to calcium ion molar ratio of 0.004), a metastable solution was not achievable. At the highest ion ratio (carbonate to calcium ion molar ratio of 4), the solution was indefinitely metastable: no amount of seed material added initiated a drop in pH that would indicate the onset of precipitation. Under conditions considered to date, we cannot definitively quantify the influence (if any) of Sr at 0.1 mM on the measured precipitation rates (work continues in this area through the INL SFA). « less