Potentiometric ion-selective membrane sensing of microbially induced calcite precipitation

Abstract

Microbially induced calcite precipitation (MICP) is an emerging biogeotechnical technique that facilitates a bio-cementation process through biomineralisation and which can bond otherwise loose soil particles, thereby increasing the shear strength and stiffness of the soil while decreasing its permeability and compressibility. However, the uniformity of calcite (calcium carbonate) precipitation may have a relatively low ‘certainty of execution’ in situ due to strict environmental conditions required for the reaction, highlighting a strong need for in situ monitoring of the subsurface during and following the treatment. This research explored an electrochemical approach to monitoring MICP treatment through the development and use of potentiometric ion-selective sensors to measure ammonium and calcium ions continuously in both aqueous batch reactors and small laboratory-scale soil test cells. Sensor calibrations indicate a strong and reliable correlation between the electrochemical potential and target ion concentration, which shows promise in sensor technology for monitoring MICP and other similar processes. Improvements to the current design are, however, needed, as fouling due to calcite precipitation, and to some extent biofouling of the sensor, was observed in the highly dynamic, biomineralisation environment, which caused data drift/interference. These effects should be considered in data analysis and eliminated/minimised for eventual adoption towards monitoring the efficacy of MICP treatment in situ.