Multiple heavy metal immobilization and strength improvement of contaminated soil using bio-mediated calcite precipitation technique.

Bio-mediated calcite precipitation potential for multiple heavy metal immobilization in contaminated soils at industrial, waste dump, abandoned mine, and landfill sites is not explored yet. This study includes investigation of bio-mediated calcite precipitation for strength improvement and immobilization of heavy metals, specifically lead (Pb), zinc (Zn), and hexavalent chromium (Cr(VI)), in contaminated soils. Firstly, the toxicity resistance of bacteria against different concentrations (1000, 2000, 3000, 4000, and 5000 mg/l) of each heavy metals was investigated and observed that Pb and Cr were less toxic to Sporosarcina pasteurii than Zn. The poorly graded sand was spiked with 333-2000 mg/kg concentrations of a selected individual or mixed metal solutions, i.e., 1000 mg/kg and 2000 mg/kg individual concentrations of Pb, Zn, and Cr(VI); 500 mg/kg and 1000 mg/kg concentration of each metal in "Pb and Zn," "Pb and Cr(VI)," and "Zn and Cr(VI)" mixture of heavy metals; and 333 mg/kg and 666 mg/kg concentration of each metal in "Pb, Zn, and Cr(VI)" mixed metal concentration. Contaminated soil was biotreated with Sporosarcina pasteurii and cementation (a solution of urea and calcium chloride dihydrate) solutions for 18 days. Biocemented sand specimens were subjected to testing of hydraulic conductivity, ultrasonic pulse velocity (UPV), unconfined compressive strength (UCS), calcite content, pH, toxicity characteristic leaching procedure (TCLP), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The heavy metal contaminated samples showed decrease in hydraulic conductivity and increase in UPV and UCS after biotreatment; however, the changes in engineering properties were found more moderate than clean biocemented sand. The conversion of Cr(VI) to Cr(III) followed by CrO precipitation in calcite lattice was observed. Zn was precipitated as smithsonite (ZnCO), while no Pb precipitate was identified in XRD results. TCLP leaching showed Pb and Cr immobilized proportional to calcite precipitated amount, and higher calcite amounts yielded levels within regulatory limits. Pb and Cr(VI) immobilization up to 92 % and 94 % was achieved, respectively, in contaminated biocemented sand. Zn was found completely leachable as smithsonite is only stable down to pH~5, and strongly acidic TCLP solution reversed all immobilization at natural soil pH~8-9.