Strontium-90 is a principal radionuclide contaminant in the subsurface at several Department of Energy sites in the Western U.S., causing a threat to groundwater quality in areas such as Hanford, WA. In this work, we used laboratory-scale porous media flow cells to examine a potential remediation strategy employing coprecipitation of strontium in carbonate minerals. CaCO(3) precipitation and strontium coprecipitation were induced via ureolysis by Sporosarcina pasteurii in two-dimensional porous media reactors. An injection strategy using pulsed injection of calcium mineralization medium was tested against a continuous injection strategy. The pulsed injection strategy involved periods of lowered calcite saturation index combined with short high fluid velocity flow periods of calcium mineralization medium followed by stagnation (no-flow) periods to promote homogeneous CaCO(3) precipitation. By alternating the addition of mineralization and growth media the pulsed strategy promoted CaCO(3) precipitation while sustaining the ureolytic culture over time. Both injection strategies achieved ureolysis with subsequent CaCO(3) precipitation and strontium coprecipitation. The pulsed injection strategy precipitated 71-85% of calcium and 59% of strontium, while the continuous injection was less efficient and precipitated 61% of calcium and 56% of strontium. Over the 60 day operation of the pulsed reactors, ureolysis was continually observed, suggesting that the balance between growth and precipitation phases allowed for continued cell viability. Our results support the pulsed injection strategy as a viable option for ureolysis-induced strontium coprecipitation because it may reduce the likelihood of injection well accumulation caused by localized mineral plugging while Sr coprecipitation efficiency is maintained in field-scale applications.

The use of microbiologically induced mineralization to plug pore spaces is a novel biotechnology to mitigate the potential leakage of geologically sequestered carbon dioxide from preferential leakage pathways. The bacterial hydrolysis of urea (ureolysis) which can induce calcium carbonate precipitation, via a pH increase and the production of carbonate ions, was investigated under conditions that approximate subsurface storage environments, using a unique high pressure (∼7.5 MPa) moderate temperature (32 • C) flow reactor housing a synthetic porous media core. The synthetic core was inoculated with the ureolytic organism Sporosarcina pasteurii and pulse-flow of a urea inclusive saline growth medium was established through the core. The system was gradually pressurized to 7.5 MPa over the first 29 days. Concentrations of NH 4 + , a by-product of urea hydrolysis, increased in the flow reactor effluent over the first 20 days, and then stabilized at a maximum concentration consistent with the hydrolysis of all the available urea. pH increased over the first 6 days from 7 to 9.1, consistent with buffering by NH 4 + ⇔ NH 3 + H +. Ureolytic colony forming units were consistently detected in the reactor effluent, indicating a biofilm developed in the high pressure system and maintained viability at pressures up to 7.5 MPa. All available calcium was precipitated as calcite. Calcite precipitates were exposed to dry supercritical CO 2 (scCO 2), water-saturated scCO 2 , scCO 2-saturated brine, and atmospheric pressure brine. Calcite precipitates were resilient to dry scCO 2 , but suffered some mass loss in water-saturated scCO 2 (mass loss 17 ± 3.6% after 48 h, 36 ± 7.5% after 2 h). Observations in the presence of scCO 2 saturated brine were ambiguous due to an artifact associated with the depressurization of the scCO 2 saturated brine before sampling. The degassing of pressurized brine resulted in significant abrasion of calcite crystals and resulted in a mass loss of approximately 92 ± 50% after 48 h. However dissolution of calcite crystals in brine at atmospheric pressure, but at the pH of the scCO 2 saturated brine, accounted for only approximately 7.8 ± 2.2% of the mass loss over the 48 h period. These data suggest that microbially induced mineralization, with the purpose of reducing the permeability of preferential leakage pathways during the operation of GCS, can occur under high pressure scCO 2 injection conditions.

We have measured infrared spectra from several types of calcite: chalk, freshly cultured coccoliths produced by three species of algae, natural calcite (Iceland Spar), and two types of synthetic calcite. The most intense infrared band, the asymmetric carbonate stretch vibration, is clearly asymmetric for the coccoliths and the synthetic calcite prepared using the carbonation method. It can be very well fitted by two peaks: a narrow Lorenzian at lower frequency and a broader Gaussian at higher frequency. These two samples both have a high specific surface area. Density functional theory for bulk calcite and several calcite surface systems allows for assignment of the infrared bands. The two peaks that make up the asymmetric carbonate stretch band come from the bulk (narrow Lorenzian) and from a combination of two effects (broad Gaussian): the surface or near surface of calcite and line broadening from macroscopic dielectric effects. We detect water adsorbed on the high surface area synthetic calcite, which permits observation of the chemistry of thin liquid films on calcite using transmission infrared spectroscopy. The combination of infrared spectroscopy and density functional theory also allowed us to quantify the amount of polysaccharides associated with the coccoliths. The amount of polysaccharides left in chalk, demonstrated to be present in other work, is below the IR detection limit, which is 0.5% by mass.

These results have implications for the detection and treatment of struvite stones. Currently this model is being used to study specific urinary factors that regulate struvite formation to identify treatment options, which combined with antibiotics would improve treatment of these stones and decrease recurrence.

Schultz, L., Pitts, B., Mitchell, A. C., Cunningham, A. B., Gerlach, R. (2011). Imaging Biologically Induced Mineralization in Fully Hydrated Flow Systems. Microscopy Today, 19 (5), 12-15.preprintPeer reviewe