Remediation of Heavy Metals-Contaminated Soils through Phosphate-Induced Immobilization Technology: From Lab to Field.

The amendment-induced immobilization of heavy metals has been considered as a cost-effective and easily-operational method for remediation of contaminated soils. This method relies on the addition of soil amendments to convert heavy metals into in the stable forms, by sorption, precipitation, complexation, ion exchange or redox process, thereby decreasing mobility and bioavailability of contaminants. Among the amendments, phosphate-bearing materials was most-commonly used. In this paper, a summary was made on the development of our research on such an immobilization treatment with more attention paid to the laboratory test and field demonstration.

The laboratory test showed that phosphate rock (PR) has the highest affinity for Pb, followed by Cu and Zn, with the sorption capacities of 138, 114, and 83.2 mmol kg^-1, respectively. The greatest stability of Pb retention by PR can be attributed to the formation of insoluble fluoropyromorphite (Pb₁₀(PO₄)₆F₂), which was primarily responsible for Pb immobilization (up to 78.3%), with less contribution from the surface adsorption or complexation (21.7%), compared to 74.5% for Cu and 95.7% for Zn. Flow calorimetry indicated that Pb and Cu adsorption onto PR was exothermic, while Zn sorption was endothermic. The results indicated that PR can effectively immobilize Pb, Cu, and Zn with the most effective for Pb. Combination of PR with FeSO₄ was effective in simultaneously immobilizing Zn and Cr(VI) within the tested solution pH range of 5.5–8.5. Over 95% Zn and Cr(VI) was removed by PR with FeSO₄. Zn was stabilized via formation of insoluble minerals such as Zn₃(PO₄)₂ or CaZn(CO₃)₂ while Fe²⁺ induced reduction of Cr(VI) into stable Cr(OH)₃ or Cr_xFe_(1-x)(OH)₃ which was responsible for Cr(VI) immobilization.

To assess the efficiency of P-induced metal immobilization in soils, a pilot-scale field experiment was conducted in a heavy metal contaminated soil at an abandoned battery recycling site. All P amendments including H₃PO₄ (PA), Ca(H₂PO₄)₂ (PC), and PA+PR transformed soil Pb from the non-residual (sum of exchangeable, carbonate, Fe/Mn, and organic) to the residual fraction, with residual Pb increase by11-55%. TCLP-Pb in the P-treated soils was reduced from 82 mg L^-1 to below EPA's regulatory level of 5 mg L^-1 in the surface soil. Lead immobilization was attributed to the P-induced formation of chloropyromorphite [Pb₁₀(PO₄)₆Cl₂]. Occurrence of chloropyromorphite was evident 220 days after P addition for PA and PC treatments and 330 days for PA+PR treatment. Lead immobilization shows a long-term stability. Another field work was carried out at an abandoned electroplating site. Application of the PR integrated with FeSO₄ in the electroplating contaminated soil rapidly reduced the TCLP extractable Zn and Cr(VI) to below the standard limits, with decrease by 50% and 94%, respectively. This field demonstration revealed that combination of PR with FeSO₄ could be an effective amendment for remediation of Zn and Cr(VI) contaminated soil.