Enhanced MICP for Soil Improvement and Heavy Metal Remediation: Insights from Landfill Leachate-Derived Ureolytic Bacterial Consortium

Abstract

This study investigates the potential of microbial-induced calcium carbonate precipitation (MICP) for soil stabilization and heavy metal immobilization, utilizing landfill leachate-derived ureolytic consortium. Experimental conditions identified yeast extract-based media as most effective for bacterial growth, urease activity, and calcite formation compared to nutrient broth and brown sugar media. Optimal MICP conditions, at pH 8-9 and 30 °C, supported the most efficient biomineralization. The process facilitated the removal of Cd²⁺ (99.10%) and Ni²⁺ (78.33%) while producing stable calcite crystals that enhanced soil strength. Thermal analyses (thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)) confirmed the successful production of CaCO₃ and its role in improving soil stability. DSC analysis revealed endothermic and exothermic peaks, including a significant exothermic peak at 444 °C, corresponding to the thermal decomposition of CaCO₃ into CO₂ and CaO, confirming calcite formation. TGA results showed steady weight loss, consistent with the breakdown of CaCO₃, supporting the formation of stable carbonates. The MICP treatment significantly increased soil strength, with the highest surface strength observed at 440 psi, correlating with the highest CaCO₃ content (18.83%). These findings underscore the effectiveness of MICP in soil stabilization, pollutant removal, and improving geotechnical properties.

Keywords: heavy metal removal; landfill leachate; microbial-induced calcium carbonate precipitation; soil stabilization; ureolytic bacteria.

Figure 1

Biostimulation of ureolytic bacterial cultures was used for biomineralization after being grown in different media (YEM, NBM, and BSM) for 72 h of incubation at 32 °C with shaking at 150 rpm. The figure includes (A) biomass measurements (OD₆₀₀), (B) pH levels, (C) urease activity of the bacteria, and (D) mass of CaCO₃ precipitates following biomineralization tests.

Figure 2

Growth and pH profile of ureolytic bacterial culture in YEM medium. (A) OD₆₀₀ of bacterial cultures over 72 h, and (B) pH profile of the enriched bacterial culture over the same period. Error bars represent standard deviations.

Figure 3

Genus abundance observed in the enriched leachate.

Figure 4

Impact of initial pH and temperature on enriched ureolytic cultures. (A–C) Effect of initial pH levels (6–11): (A) biomass measurements (OD₆₀₀) of the cultures; (B) pH measurements of the cultures; (C) urease activity (urea hydrolyzed/min). (D–F) Effect of temperature (10–50 °C): (D) biomass measurements (OD₆₀₀) of the cultures; (E) pH measurements of the cultures; (F) urease activity (urea hydrolyzed/min). Error bars represent standard deviations.

Figure 5

Removal percentage of heavy metal ions using enriched ureolytic bacterial culture from leachate. Error bars represent standard deviations.

Figure 6

Crystal morphology of MICP-treated soil revealing well-defined rhombohedral and polyhedral crystal forms bridging soil particles and filling voids within the soil matrix. (A) SEM image at ×100 magnification; and (B) SEM image at ×250 magnification.

Figure 7

Elemental composition of MICP-treated soil via EDS analysis. The yellow rectangular lines indicate the specific locations where the EDS analysis was performed on the soil samples.

Figure 8

X-ray diffraction (XRD) pattern of the MICP-treated soil sample.

Figure 9

FTIR image of soil sample after treatment with enriched ureolytic bacterial cultures.

Figure 10

Thermal characterizations of biocemented soil samples after treatment with enriched ureolytic cultures from the leachate wastewater sample. (A) DSC analysis; and (B) TGA analysis.

Figure 11

Measurement of MICP treatment showing surface strength and CaCO₃ content. Error bars represent standard deviations.

Figure 12

Evaluation of effluents during soil biocementation with enriched ureolytic cultures. The pH levels and ammonium concentrations in the effluent samples collected throughout the treatment process are shown. Error bars denote the standard deviations.