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.
Investigating the Dissolution Behavior of Calcium Carbonate Bio-Cemented Sands
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.
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- Award ID(s):
- 1824647
- NSF-PAR ID:
- 10374118
- Date Published:
- Journal Name:
- Geo-Congress 2022 GSP 331
- Page Range / eLocation ID:
- 385 to 395
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 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 Vsmeasurements 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.
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Viana da Fonseca, António ; Ferreira, Cristiana (Ed.)Microbially induced carbonate precipitation (MICP) is a bio-mediated ground improvement technique that can increase soil stiffness and produce cohesion within granular material. Most experimental investigations on MICP-treated soils are performed on idealized granular materials. Evaluating a narrow range of particle sizes dismisses the potential influence of soil fabric on MICP treatment efficiency. Therefore, little is known regarding the influence of soil fabric on the level of improvement achievable post-MICP treatment. We investigate the influence of the coefficient of uniformity (Cu) on the level of improvement that can be obtained from MICP treatment. This study couples unconfined compression testing with microscale observations obtained from x-ray computed tomography (CT) of two sand mixtures with different Cu values. A soil column and CT specimen of each sand mixture were prepared and received the same number of MICP- injections. The shear wave velocity (Vs) of the soil columns was monitored to evaluate the increase in soil stiffness over time. After MICP treatment, the bio-cemented columns were subjected to unconfined compressive strength testing. Results indicate that for a similar mass of carbonate, the soil with a larger Cu experienced a greater increase in Vs but a lower maximum unconfined compressive strength. Through CT imaging, the soil with a smaller Cu was observed to have a more uniform distribution of carbonate within the sand matrix whereas the soil with a larger Cu has more sporadic MICP trends. This study elucidates the influence of soil fabric on the level of improvement that can be achieved through MICP treatment and assesses the reliability of x-ray CT scanning of MICP-treated sands with moderate carbonate content.more » « less
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null (Ed.)The end goal of this research is assessing the feasibility of using enzyme induced carbonate precipitation (EICP) to create a cemented top layer to control runoff erosion in sloping sandy soil. The paper presents the results of an experimental study of bench-scale tests on EICP-treated sands to determine a treatment method feasible for field placement for this application. The soils tested were two natural sands and Ottawa 20-30 sand used as control. The EICP application methods were percolation by gravity, one-step mix-compact, and two-step mix-compact. Other conditions considered were pre-rinsing the sand prior to treatment, adjusting soil pH prior to treatment, and changing the EICP solution concentration. Promising results for this field application were obtained using the two-step mix-compact when the soil was first mixed with the urease enzyme solution before compaction. Considering that the EICP reaction starts once all components are added, this method would ensure that the reaction does not take place before the protective layer of treated soil has been installed. The effect of pre-rinsing the natural sand was not consistent throughout the testing conditions and its role in improving soil cementation in natural sand needs further study.more » « less
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Coastal bluff erosion and recession are among the common mechanisms altering the geomorphology of the coastline in California. The accelerated erosion rate increasingly threatens the stability of structures located on these bluffs. Previous researchers have investigated the effect of material properties and strength on the generation of the shear plane and failure modes of coastal bluffs and cliffs. Monitoring the morphology of the moderately cemented coastal bluffs with time has indicated that a comparison of material strength with the expected insitu minor principal stress distribution can be used as a criterion to assess bluff stability. However, the effect of varying factors such as cementation levels and bluff geometry and dimensions on stress distribution patterns and material properties that determine bluff failure susceptibility requires further investigation. While bond breakage and disturbance during sampling and transportation undermine the quality of recovered soil samples, artificial cementation methods (e.g., Portland cement) may not properly replicate the natural formation processes. Instead, microbially induced carbonate precipitation (MICP) is a ground improvement method that simulates the cementation processes that occur in natural geological settings. This method harnesses the activities of bacteria to generate cementitious precipitation among soil particles. The formation of the cementing agent improves the mechanical properties of the soil. In the past two decades, extensive studies have been devoted to understanding the cementation formation mechanism and the improvement of mechanical properties that can be used as a proxy for natural cemented soil for stability analysis. In the study presented herein, a series of FEM models were developed in SIGMA/W software. The effect of the different cementation levels and variation of bluff geometry on minor principal stress distribution was investigated. Results of the study demonstrated that although the cementation level of the materials determines the failure mode, the stress distribution mainly depends on the bluff geometry. The obtained results offer further insights into the failure mechanism of coastal bluffs as well as MICP-treated slopes for future field implementation of this soil improvement method.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1061/9780784484012.040
