Microbial induced calcite (MICP) is a bio-mediated ground improvement method that can be used to increase the shear strength and stiffness of soils. As the continued rehabilitation and expansion of civil infrastructure becomes more pressing and the requirement to meet growing societal needs increases, the demand for construction on already used ground conditions becomes ever more common. The growing infrastructure development is often limited by poor soil conditions, ground improvement methods can improve poor soil conditions to support the rehabilitated of infrastructure, such as densification of soil with mechanical energy or injecting a binding agent such as cement, epoxy, or silicates (Karol 2003).However, in recent years the emergence of biological alternatives has grown in popularity due to providing a more natural and sustainable solution (Mitchell and Santamarina 2005; Ivanov and Chu 2008; DeJong et al. 2013). The idea behind biological based alternatives is to utilize biological metabolic prosses to mediate the rehabilitation and improvement of ground conditions, (e.g. soil strength and stiffness) (DeJong et al. 2010, 2011).MICP is a naturally biologically mediated method used to create cementation in in-situ soil and improve soil properties (Chou et al. 2011; van Paassen et al. 2010; Whiffin et al. 2007; DeJong et al. 2006).Where other chemical sealants or surface treatment agents can be detrimental to the surrounding area, due to economic environmental concerns, MICP has been proposed as an alternative method to improve ground conditions and aid in the rehabilitation of formerly used ground. However, research into the formation of nucleation points within the MICP has been a topic of interest as for those studying the effects of MICP. Mitchell & Ferris (2007) undertook microcosm experiments to identify the influence of bacterial cell surfaces on the morphology, mineralogy, size and solubility in ground water and the influence of Bacillus Pasteurii on the nucleation growth of calcium carbonate, it was found that nucleation can occur without the presence of solid material for bacteria to grow upon, instead showing evidence that nucleation can occur purely in groundwater alone. This suggests that the presence of granular material may not be necessary for nucleation to occur, leading to the notion that nucleation occurs in the voids in-between soil particles.
Microbial induced calcite
Microorganisms are naturally present in soils and sediments, and can facilitate the precipitation of minerals in two different ways by:
- Acting as nucleation sites on their cell walls
- Through activity that perturbs aqueous geochemistry and shifts mineral equilibrium
The binding cations to microbial cell surfaces act as potential nucleation points for nucleation and the growth of minerals (Beveridge and Fyfe, 1985; Francios et al., 2011). Microorganisms are particularly important in the constitution of soil, due to their size and high surface area to volume ratio microorganisms are adapted to interacting with metal ions in the environment. As a result of this interaction, several microbial metabolic processes result in the bio-precipitation of minerals and amorphous phases (Douglas and Beveridge, 1998). The formation of silica by cyanobacteria (Yee et al., 2003),iron oxides at acid mine drainage sites (Mann and Fyfe, 1989), and calcite presciptatin in soils and groundwater throughout the world are commonly mediated by microbial processes (e.g., Barabesi et al., 2007).
Calcite precipitation is of particular importance and is considered a widespread phenomenon due to large quantities of natural calcium present in groundwater (Baskar et al. 2006). The geochemical processes driving by microbial metabolism and cell surface reactivity result in significant changes in both the microenvironment and the properties of the soil structure.
Sporosarcina pasteurii is a bacterium which inhabits a wide range of habitats, found in soils and aqueous environments throughout the world, and is a common calcite precipitating bacterium. Urease is the enzyme responsible for catalysing the reaction that forms calcite and is abundant in the natural environment (Mobley and Hausinger, 1989). Sporosarcina pasteurii is an alkaliphilic, urease positive, facultative anaerobe, it precipitates large amounts of calcite I the presence of ammonia-present in urea-and dissolved calcium, thus making it a model for use in neutral to alkaline soils. As Sporosarcina pasteurii metabolises, the solution pH increases and creates conditions that are supersaturated with respect to calcite. The bio-precipitation of calcite is fast and eventually is able to encapsulate nearby bacteria and ultimately results in the death of said bacteria (Ramachandran, 2001.)Recent studies suggest that the precipitation of calcite increases with respect to higher bacterial cell counts (Tobler et al., 2011).This suggests that more bacterial rich environments can be used to optimize the nucleation of MICP.
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The precipitation of calcite using As Sporosarcina pasteurii has been analysed for its ability to capture small amounts of C02. It has been shown that through hydrolysis, urea can trap atmospheric C02 in bio-precipitated calcite Dupraz et al., 2009; Mitchell et al., 2010. Furthermore, Sporosarcina pasteurii has been used in many modern applications to facilitate bio-precipitation. Such applications include solid-phase carbon capture of heavy metals and radionuclides (Fujita et al., 2004), wastewater treatment (De Muynck et al., 2010), carbon sequestration (Dupraz et al., 2009), concrete crack repair (De Muynck et al., 2010), and soil stabilization (Whiffin et al., 2007). The method has been used to reduce shear during earthquakes and reduce dangers of landslides and liquefaction.
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