Guidance for investigating calcite precipitation by urea hydrolysis for geomaterials

Microbially Induced Calcite Precipitation (MICP) is a sustainable method of stabilizing (i.e., cementing) loose sandy deposits and/or to create an impervious barrier within the soil mass. MICP can occur through various biochemical pathways, among which ‘Urea Hydrolysis (UH)’ is considered to be the most efficient method of biochemically inducing calcite precipitation. To date, the geotechnical engineering community investigating MICP has tended to focus on the hydro-mechanical behaviour of the end product, i.e. MICP cemented sands; however, many biochemical factors that affect reaction-rate kinetics and MICP outcome have been understudied or neglected. This study investigated the kinetics of UH and compared different sources of urease enzyme: those microbially cultivated in the laboratory (i.e., Sporosarcina pasteurii) and those extracted from plants (i.e., Jack bean meal), to investigate the influence of urea concentration, buffer capacity, and cell harvesting method on UH. Through this study, an attempt has been made to arrive at an optimal concentration of urea, under the influence of the above mentioned parameters along with the buffering action of the soil, on urea hydrolysis. These results have implications towards optimising MICP and, in particular, for upscaling these methods to in-situ applications.