Enzyme-induced carbonate precipitation process for soil improvement using expired soybean seeds as urease source

Enzyme-induced carbonate precipitation (EICP) has emerged as a sustainable alternative to cement-based soil stabilization. However, its large-scale application is often limited by enzyme sourcing, mechanical performance uncertainty, and ammonium generation. This study presents an engineering-oriented evaluation of EICP using urease extracted from soybean seeds rejected from the food supply chain. Process parameters, including enzyme concentration, calcium source concentration, temperature, and injection mode, were optimized for the treatment of a sandy soil. The results demonstrated that 25 g·L–1 of soybean seeds, combined with 0.5 M calcium and a 1:1 urea-to-calcium molar ratio, provided optimal conditions for biocementation, carbonate precipitation, and pH maintenance (8−9). Mechanical testing revealed that EICP significantly enhances both strength and stiffness of the treated soil. Stiffness was evaluated using the secant Young’s modulus at 50% of peak stress (E50), and the biocemented specimens exhibited approximately fourfold higher stiffness compared to OPC-treated specimens, while achieving comparable unconfined compressive strength. The enhanced stiffness is attributed to contact-level carbonate cementation, which differs fundamentally from the pore-filling hydration products formed during cement stabilization. Ammonium generation was also quantified, showing that controlled hydraulic flushing allows recovery of a significant fraction of nitrogen in the exhausted solution, while a residual fraction remains within the treated soil. These results indicate that ammonium accumulation can be mitigated, though not eliminated, through engineering controls, and that a downstream effluent treatment is necessary.