Suitability of Various Ureolytic Microbes for Potential Soil Strengthening via Biocementation

Abstract

Biocementation or microbially induced calcium carbonate precipitation (MICP) is a feasible biochemical process in enhancing the behavior of geomaterial, that is, soil strengthening and/or remediation. This biochemical process encounters several biotic and abiotic challenges while implementing it in real field conditions. With this, the present study aims to investigate the efficiency and suitability of different ex situ and in situ ureolytic microbes in enhancing the geotechnical properties of the sand in different environmental conditions. The studies of MICP revealed a hindrance of microbial growth and ureolytic enzyme activity of one ex situ strain of S. pasteurii in the prevailing soil anoxic (air restrict) or anaerobic condition. The hindrance was the major reason for the minimal amount of precipitation and no strength gain in the biomodified sample. In contrast, high compressive strength was achieved with an abundant amount of precipitation for the sample catalyzed by another strain of S. pasteurii and isolated Proteus species. The results showed that the specific urease activity varies substantially pertaining to the type of microbes in similar chemical and environmental conditions which directly impact the biomineral precipitation and the rate of strength enhancement. The whole study recommends two major tools, that is, the value of specific urease activity and the ureolysis rate in the prevailing soil condition to compute the suitability of the ureolytic microbe for a successful MICP implementation. Microbially induced calcium carbonate precipitation (MICP) via ureolysis is a cost-effective, ecofriendly biochemical process that helps strengthen or remediate the geomaterial. In this process, the ureolytic microbes precipitate the carbonate biomineral in the soil pore and bridge the soil particles which helps in soil strengthening. Isolating and stimulating the native ureolytic microbes is preferable to successfully implement the MICP process in real field conditions. In addition, it is essential to analyze the suitability or efficiency of ureolytic microbes in the MICP process since the type of ureolytic microbes affects the biomineral precipitation activity and also the soil enhancement process. This study isolated four unique native ureolytic microbes from the soil and investigated their efficiency for soil strengthening. The investigation suggests using the isolated facultative ureolytic microbes during MICP field implementation since most of the soil strengthening or remediation is in the subsurface with low or no oxygen availability. For this, feasible stepwise isolation and selection of the ureolytic microbes process are provided to further use it either in soil strengthening or remediation. The study also provides two major parameters to choose the suitability of microbes, that is, the specific enzyme/ureolytic activity unit and the enzyme activity in the prevailing soil environmental conditions for MICP implementation.