Role of Biochar in Drained Shear Strength Enhancement and Ammonium Removal of Biostimulated MICP-Treated Calcareous Sand

Biostimulation through the enrichment of indigenous ureolytic bacteria has been proven to be a feasible approach for implementing microbially induced carbonate precipitation (MICP). To apply this method to real-world engineering situations, researchers are focusing on (1) increasing the cementation content to improve the mechanical performance of biocemented sand, and (2) minimizing the negative impact of ammonium, a harmful byproduct. In this study, biochars of different shapes (powdered or flaked) were used as additives in calcareous sand. The effects of biochar on shear strength improvement and ammonium removal were investigated through isotropically consolidated drained (CD) triaxial shear tests, as well as ammonium adsorption and leaching tests, respectively. Experimental results indicated that biochar positively impacted cementation content, particularly for powdered biochar with smaller particle sizes, which enhanced shear strength in CD tests. Microscopic analysis of biochar-amended biocemented sand revealed that biochar can serve as additional nucleation sites for precipitation. Moreover, the contributions of friction, dilatancy, and mobilized cohesion of biocemented sand at peak strength and in the maximum dilatancy state were elucidated. Furthermore, biochar demonstrated significant ammonium removal under both aqueous and sand column conditions. These findings hold practical significance for the stabilization of local calcareous sand, as well as for other coastal cities worldwide.