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A newly emerging microbiological soil stabilization method, known as microbially induced calcite precipitation (MICP), has been tested for geotechnical engineering applications. MICP is a promising technique that utilizes the metabolic pathways of bacteria to form calcite precipitation throughout the soil matrix, leading to an increase in soil strength and stiffness. This paper investigates the geotechnical properties of sand bio-cemented under different degrees of saturation. A series of laboratory experiments was conducted, including sieve analysis, permeability, unconfined compressive strength, consolidated undrained triaxial, and durability tests. The results indicate that higher soil strength can be obtained at similar CaCO3 content when the treatment is performed under a low degree of saturation. The experimental results are further explained with a mathematical model, which shows that the crystallization efficiency, i.e., actual volume of crystals forming at the contact point where they contribute the most to strength, can be calculated from the degree of saturation and grain size. Fine sand samples exhibited higher cohesion, but lower friction angle than coarse sand samples with similar CaCO3 content. The results also confirm the potential of MICP as a viable alternative technique for soil improvement in many geotechnical engineering applications, including liquefiable sand deposits, slope stabilization, and subgrade reinforcement. The freeze–thaw and acid rain resistance of MICP-treated sand has also been tested.

The possibility of using microbiological processes to improve the mechanical properties of soil by undisturbed in-situ application has gained attention over recent years. This paper describes a new variation of in-situ soil reinforcement technology based on Microbially Induced Carbonate Precipitation (MICP), which involves both the hydrolysis of urea by soil bacteria enzyme and calcium carbonate precipitation in the presence of dissolved calcium ions. In contrast to other previously published approaches, the current work uses surface percolation for in-situ placement of bacteria and cementation solution. Bacteria could be immobilised over the full length of a 1 m column by surface percolation. To accomplish this it was necessary to percolate alternate solutions containing either bacteria or fixation solution containing calcium ions.The biologically triggered cementation resulted in homogeneous cementation over the entire length of the 1-meter sand column. The efficiency of calcite crystals to form strength was found to be related to the pore water content of the continuously drained column with less water content enabling more efficient strength formation. Scanning electron microscopy supported the idea that lower water contents lead to selective positioning of crystals at the bridging points between sand grains. These findings imply that the cost of MICP technology can be reduced by optimizing the conditions for effective crystals precipitation. This is expected to make this technology more readily acceptable for large scale applications

An extracellular electron carrier excreted into the growth medium by cells of Geobacter sulfurreducens was identified as ac-type cytochrome. The cytochrome was found to be distributed in about equal amounts in the membrane fraction, the periplasmic space, and the surrounding medium during all phases of growth with acetate plus fumarate. It was isolated from periplasmic preparations and purified to homogeneity by cation-exchange chromatography, gel filtration, and hydrophobic interaction chromatography. The electrophoretically homogeneous cytochrome had a molecular mass of 9.57 ± 0.02 kDa and exhibited in its reduced state absorption maxima at wavelengths of 552, 522, and 419 nm. The midpoint redox potential determined by redox titration was −0.167 V. With respect to molecular mass, redox properties, and molecular features, this cytochrome exhibited its highest similarity to the cytochromes c of Desulfovibrio salexigensand Desulfuromonas acetoxidans. TheG. sulfurreducens cytochrome c reduced ferrihydrite (Fe(OH)3), Fe(III) nitrilotriacetic acid, Fe(III) citrate, and manganese dioxide at high rates. Elemental sulfur, anthraquinone disulfonate, and humic acids were reduced more slowly. G. sulfurreducens reduced the cytochrome with acetate as an electron donor and oxidized it with fumarate. Wolinella succinogenes was able to reduce externally provided cytochrome c of G. sulfurreducens with molecular hydrogen or formate as an electron donor and oxidized it with fumarate or nitrate as an electron acceptor. A coculture could be established in which G. sulfurreducens reduced the cytochrome with acetate, and the reduced cytochrome was reoxidized by W. succinogenes in the presence of nitrate. We conclude that this cytochrome can act as iron(III) reductase for electron transfer to insoluble iron hydroxides or to sulfur, manganese dioxide, or other oxidized compounds, and it can transfer electrons to partner bacteria.