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Microbially induced calcite precipitation (MICP) or biocementation is a bio-mediated process that can be used to improve the engineering properties of granular soils through calcium carbonate precipitation. Although most commonly this process is accomplished using microbial urea hydrolysis, other microbial metabolic pathways can be used to enable biocementation with the potential to eliminate ammonium byproducts. Microbial organic acid oxidation presents one alternative pathway by which increases in solution carbonate species can be generated to enable calcium carbonate mineral formation. While past studies have considered the potential of this microbial pathway to enable biocementation for surficial applications, to date few studies have examined the feasibility of this pathway for subsurface applications wherein dissolved oxygen is more limited. In this study, 18 small-scale batch experiments were performed to investigate the ability of microbial organic acid oxidation to enable biocementation soil improvement. Experiments investigated the feasibility of using both acetate and citrate oxidation to mediate biocementation as well as the effect of differences in techniques used to supply dissolved oxygen, the effect of supplied growth factors, bicarbonate salt additions, and solution sampling frequency. Results suggest that aerobic oxidation of acetate and citrate can be used to enable calcium carbonate biocementation, though ensuring dissolved oxygen availability appears to be critical towards enabling this process. 

Bio‐mediated soil improvement technologies leverage microbial enzymatic and metabolic processes to generate minerals, gases and biopolymers that can improve soil engineering behaviours with the potential to reduce detrimental environmental impacts when compared with conventional methods. Ureolytic biocementation is perhaps the most widely researched of these processes and relies on urea hydrolysis in the presence of calcium to initiate the precipitation of calcium carbonate minerals on soil particle surfaces and contacts, thereby improving soil behaviours. Although effective, urea hydrolysis generates aqueous ammonium by‐products that may result in undesirable environmental and human health impacts, if left unaddressed. Recent studies have shown the potential of rinse solution injections to effectively remove generated ammonium following biocementation through both advective flow and the removal of sorbed ammonium from soil surfaces; however, critical gaps remain in our understanding of the effect of rinse solution composition and injection strategies on ammonium removal efficacy. In this study, 16 soil column experiments were performed to investigate the effect of rinse solution cation types, cation concentrations, applied injection sequences and biocementation treatment variations on the removal of ammonium from a biocemented poorly graded sand. All columns receiving cation‐enriched rinse solutions achieved greater than 98% aqueous, 65% sorbed and 95% total ammonium removal after injecting 12 pore volumes, with no detectable impacts on cementation integrity. Cation‐enriched solutions specifically enhanced sorbed ammonium removal and achieved sorbed ammonium concentrations up to 2 orders of magnitude less than those observed in columns rinsed with deionized water alone. Columns receiving K⁺‐based rinse solutions and 12 daily 1‐PV rinse injections also achieved greater total ammonium removal when compared with comparable columns receiving rinse solutions containing other cations and continuous 12‐PV rinse injections.

 

Microbially-induced calcium carbonate precipitation (MICP) is a bio-cementation process that can improve the engineering properties of granular soils through the precipitation of calcium carbonate (CaCO₃) minerals on soil particle surfaces and contacts. The technology has advanced rapidly as an environmentally conscious soil improvement method, however, our understanding of the effect of changes in field-representative environmental conditions on the physical and chemical properties of resulting precipitates has remained limited. An improved understanding of the effect of subsurface geochemical and soil conditions on process reaction kinetics and the morphology and mineralogy of bio-cementation may be critical towards enabling successful field-scale deployment of the technology and improving our understanding of the long-term chemical permanence of bio-cemented soils in different environments. In this study, thirty-five batch experiments were performed to specifically investigate the influence of seawater ions and varying soil materials on the mineralogy, morphology, and reaction kinetics of ureolytic bio-cementation. During experiments, differences in reaction kinetics were quantified to identify conditions inhibiting CaCO₃precipitation and ureolysis. Following experiments, scanning electron microscopy, x-ray diffraction, and chemical composition analyses were employed to quantify differences in mineralogical compositions and material morphology. Ions present in seawater and variations in soil materials were shown to significantly influence ureolytic activity and precipitate mineralogy and morphology, however, calcite remained the predominant CaCO₃polymorph in all experiments with relative percentages exceeding 80% by mass in all precipitates.

 

Microbially Induced Calcite Precipitation (MICP) is a bio-mediated cementation process that uses microbial enzymatic activity to catalyze the precipitation of CaCO3 minerals on soil particle surfaces and contacts. Extensive research has focused on understanding various aspects of MICP-treated soils including soil behavioral enhancements and process reaction chemistry, however, almost no research has explored the permanence of bio-cemented geomaterials. As the technology matures, an improved understanding of the longevity of bio-cementation improved soils will be critical towards identifying favorable field applications, quantifying environmental impacts, and understanding their long-term performance. In this study, a series of batch experiments were performed to investigate the dissolution kinetics of CaCO3-based bio- cemented sands with the specific aim of incorporating these behaviors into geochemical models. All batch experiments involved previously bio-cemented poorly graded sands that were exposed to different dissolution treatments intended to explore the magnitude and rate of CaCO3 dissolution as a function of acid type, concentration, initial pH, and other factors. During experiments, changes in solution pH and calcium concentrations indicative of CaCO3 dissolution were monitored. After experiments, aqueous measurements were compared to those simulated using two different dissolution kinetic frameworks. While not exhaustive, the results of these experiments suggest that the dissolution behavior of bio-cementation can be well-approximated using existing chemically controlled kinetic models, particularly when surrounding solutions are more strongly buffered. 

Numerous laboratory studies in the past decade have demonstrated the ability of microbially induced calcite precipitation (MICP), a bio-mediated soil improvement method, to favorably transform a soil’s engineering properties including increased shear strength and stiffness with reductions in hydraulic conductivity and porosity. Despite significant advances in treatment application techniques and characterization of post-treatment engineering properties, relationships between biogeochemical conditions during precipitation and post-treatment material properties have remained poorly understood. Bacterial augmentation, stimulation, and cementation treatments can vary dramatically in their chemical constituents, concentrations, and ratios between researchers, with specific formulas oftentimes perpetuating despite limited understanding of their engineering implications. In this study, small-scale batch experiments were used to systematically investigate how biogeochemical conditions during precipitate synthesis may influence resulting bio-cementation and related material engineering behaviors. Aqueous solution chemistry was monitored in time to better understand the relationship between the kinetics of ureolysis and calcium carbonate precipitation, and resulting precipitates. Following all experiments, precipitates were evaluated using x-ray diffraction and scanning electron microscopy to characterize mineralogy and morphology. Results obtained from these investigations are expected to help identify the primary chemical and biological factors during synthesis that may control bio-cementation material properties and 

Microbially Induced Calcite Precipitation (MICP), or bio-cementation, is a promising bio-mediated technology that can improve the engineering properties of soils through the precipitation of calcium carbonate. Despite significant advances in the technology, concerns regarding the fate of produced NH₄⁺by-products have remained largely unaddressed. In this study, five 3.7-meter long soil columns each containing one of three different soils were improved using ureolytic bio-cementation, and post-treatment NH₄⁺by-product removal was investigated during the application of 525 L of a high pH and high ionic strength rinse solution. During rinsing, reductions in aqueous NH₄⁺were observed in all columns from initial concentrations between ≈100 mM to 500 mM to final values between ≈0.3 mM and 20 mM with higher NH₄⁺concentrations observed at distances furthest from the injection well. In addition, soil V_smeasurements completed during rinse injections suggested that no significant changes in cementation integrity occurred during NH₄⁺removal. After rinsing and a 12 hour stop flow period, all column solutions achieved cumulative NH₄⁺removals exceeding 97.9%. Soil samples collected following rinsing, however, contained significant sorbed NH₄⁺masses that appeared to have a near linear relationship with surrounding aqueous NH₄⁺concentrations. While these results suggest that NH₄⁺can be successfully removed from bio-cemented soils, acceptable limits for NH₄⁺aqueous concentrations and sorbed NH₄⁺masses will likely be governed by site-specific requirements and may require further investigation and refinement of the developed techniques.