Escherichia coli transport in two acidic soils: Effect of microbially induced calcite precipitation technology

Microbially induced calcite precipitation (MICP) while neutralizing soil pH, can lead to pore clogging which in turn may reduce bacteria transport. This study aimed to evaluate the effectiveness of the MICP process for E. coli filtration in two acidic soils. Two soil samples from Amlash (Am) and Lahijan (La) areas with pH values of 5.88 and 3.93, repectively, were collected and poured into plastic columns (14.92 × 2.4 cm). For the MICP reaction, the soil columns were saturated with a solidification solution (1:1 urea: CaCl₂, 1.5 M) and Sporosarcina pasteurii (∼10⁸ cell mL^-1), and incubated at 30 °C for 72 h. Leaching experiments were conducted on both MICP-treated and control soil columns at steady-state, saturated flow condition. A pulse of influent (0.1 PV) containing Escherichia coli (ciprofloxacin-resistant) (∼10⁸ cell mL^-1) and bromide tracer (1000 mg L^-1 KBr) was added at the top of the soil columns, followed by sterile water to collect the effluent. Recovered E. coli, and Br^-, HCO₃^-, NH₄⁺, Ca²⁺ ions were measured in the leachate. The profile of residual E. coli count, urease activity, and bioprecipitated CaCO₃ content were also assessed in the soil. Correlated with bioprecipitated CaCO₃, the hydraulic conductivity coefficients (K_s) was reduced by 4.4 and 5.8 times after MICP treatment in Am and La soils, respectively, thus bacteria leaching was significantly reduced. A higher filtration coefficient (λ_f) and recovery rate of E. coli were calculated in the La soil column, likely due to the lower pH and higher anion exchange capacity, which induced greater bacterial mortality and electrostatic attraction, respectively. MICP treatment reduced the average and cumulative count of E. coli by ∼3.4 times compared to the control column. In conclusion, the application of MICP in acidic soil increased soil pH and reduced the risk of E. coli transport to deeper layers by reducing soil hydraulic conductivity.