Controls on the rate of ureolysis and the morphology of carbonate precipitated by S. Pasteurii biofilms and limits due to bacterial encapsulation

Despite the potential contribution of microbially induced carbonate precipitation (MICP) to a range of environmental technologies, little is known about the controls on the rate of ureolysis and precipitate size and morphology using attached bacterial communities. This paper presents results of experiments using Sporosarcina pasteurii biofilms, of varying density, grown on perspex and granite surfaces then immersed in fluids comprising calcium chloride and urea of varying concentrations. Denser biofilms resulting from higher nutrient conditions led to faster nucleation of calcite and higher rates of ammonium production found to be related to crystal size via a power law. The observed morphology of the precipitates was variable depending on precipitation rates and nucleation of calcite was independent of the substrate mineralogy. In some cases the calcite layer became non-porous, and the bridging of pores within the granite was also observed. We show how ureolysis is limited eventually by the encapsulation of the biofilm by calcite and present a novel model that enables the reaction to be optimised to yield maximum calcite precipitation over a desired timescale. Slower reaction rates may in some circumstances be desirable for maximum reaction efficiency. The results have important implications for the design of engineering solutions involving MICP.