The investigation of microbial induced calcium carbonate precipitation for soil improvement

Haystead, Jamie Daniel (2023) The investigation of microbial induced calcium carbonate precipitation for soil improvement. Doctoral thesis, Northumbria University.

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Abstract

Microbial induced calcium carbonate precipitation (MICCP) has been studied increasingly over the years as a method for soil improvement. An advantage of MICCP to existing techniques is that it does not yield greenhouse gases or contaminate groundwater with toxic chemicals. The ureolytic approach is used most widely and utilises the biological activities of ureolytic bacteria to hydrolyse urea to form reactants necessary for CaCO3 precipitation, as well as provide favourable conditions for CaCO3 precipitation which include local pH rise and provision of nucleation sites. The bacteria most used in studies is Sporosarcina pasteurii due to its high urease content per cell. Despite the attraction of this technique, there are challenges associated. One challenge is to make accurate predictions of MICCP at a large scale for field application. This is difficult due to the many factors associated with MICCP. Computational models have been formed that address this issue and function to make predictions of MICCP outcome whilst considering a range of factors. There are limitations with these models which include use of assumptions either to reduce computational complexity or from lack of existing data. To improve the accuracy of these models or to verify that the assumptions made are correct, laboratory data needs to be collected to address these data gaps.

The objectives of this research were to investigate MICCP factors in greater detail, as a means of improving understanding of MICCP processes for greater efficient application of this technique and for addressing computational model data gaps. Firstly, several factors were investigated to understand the effects on S. pasteurii growth and urease activity in both whole cell and cell free extract conditions which were the growth stage of bacteria, the addition of NH4Cl to growth media, temperature and pH. Then reaction rates of ureolysis and CaCO3 precipitation were measured at a range of reactant concentrations. This was followed by cell tests in sand columns which focused towards understanding the impact of using bacteria cultures of different growth phases as well as understanding in greater detail the distribution profile of both S. pasteurii and CaCO3 in sand columns. Finally, the impact of charged additives on CaCO3 was explored, to determine if morphological changes of CaCO3 had an effect on binding properties and unconfined compressive strength of sand columns.

This work found significantly different reaction rates between ureolysis and CaCO3 precipitation rates, with ureolysis occurring at a higher rate than CaCO3 precipitation. This evidences that computational models that input these rates to be the same are not correct. An optimised method of bacteria distribution measurement in sand columns was determined which has the potential to address data gaps of bacteria distribution in computational models through collaborative work Other work gained further understanding of factors that affect S. pasteurii urease activity and showed that activity was higher: for mid exponential bacteria compared to late exponential bacteria; when additional nitrogen was added to the growth media; at higher temperatures in the range 20-37 oC; and at pH 7-8 within the range 7-10. Additionally, links between CaCO3 morphology and binding properties were made from the use of additives. The knowledge from these other works is beneficial to the advancement of MICCP technology to be utilised commercially for soil improvement.

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