Nasser Hamdan

Soil improvement via MICP may provide a sustainable and cost-effective means of soil improvement that mitigates a host of geotechnical problems associated with other approaches for soil improvement. Using  Pseudomonas  denitrificans, a gram-negative facultative anaerobe, calcite can be precipitated from calcium-rich pore water using denitrification. As denitrifying bacteria are ubiquitous in the subsurface, denitrification offers the potential for use in bio-stimulation of indigenous microorganisms for MICP. 

Plant-derived urease enzyme has been used to induce carbonate cementation (ureolytic carbonate cementation) in sand and silt, which has been demonstrated through column tests at ASU. In ureolytic carbonate cementation, urea hydrolysis is catalyzed by the urease enzyme (urea amidohydrolase) to precipitate calcium carbonate (CaCO3)  in the presence of calcium. The use of plant-derived urease offers many benefits over the use of microbially-generated urease to induce carbonate cementation (a process that has attracted much attention recently). 

Liquefaction mitigation is another potential application for MICP and urease enzyme processes. Carbonate precipitation forms cementation bonds at inter-particle contacts and can also fill the void space in the soil causing an increase in dilatancy of soils. Soil desaturation via microbial biogas may also be an effective method of liquefaction mitigation.

Bio-inspired surficial stabilization of soils is another important area of research. Soil erosion due to wind and surface water runoff is a major sustainability issue as it can pollute streams and groundwater and cause air quality problems. Recent tests at ASU confirm the feasibility of using a topical application of plant-derived urease to induce carbonate cementation for the stabilization of soils against wind and surface water erosion.

Other potential applications for denitrification may include remediation of groundwater containing radionuclide and metal contaminants. For example, microbially induced carbonate precipitation (MICP), an emerging technology for soil improvement, also may be used to sequester (biomineralize) radionuclides and metal contaminants (e.g., 90Sr2 , Cd2 ) in groundwater, a significant problem at some U.S. Department of Energy sites. Previous work by others using the bacterium Sporosarcina   pasteurii suggests that  in-situ sequestration of these contaminants can be achieved through MICP via hydrolytic ureolysis. Biomineralization through bacterial denitrification offers a promising alternative for in-situ remediation in these cases. Highly ubiquitous denitrifying bacteria, including Pseudomonas denitrificans, are capable of MICP without the production of harmful byproducts.  Biomineralization of metal contaminants through the stimulation of native denitrifying bacteria may provide a more sustainable means of remediating groundwater impacted by radionuclides and metal contaminants.