Heap leaching of Pb and Zn contaminated soil using ozone/UV treatment of EDTA extractants

The feasibility of a novel EDTA-based soil heap leaching method with treatment and reuse of extractants in a closed process loop was evaluated on a laboratory scale. Ozone and UV irradiation were used for oxidative decomposition of EDTA-metal complexes in extractants from Pb (1243 mg kg−1) and Zn (1190 mg kg−1) contaminated soil. Released metals were absorbed in a commercial metal absorbent Slovakite. Six-consecutive additions of 2.5 mmol kg−1 EDTA (total 15 mmol kg−1 EDTA) removed 49.6 ± 0.6% and 19.7 ± 1.7% of initial total Pb and Zn from soil (4.6 kg) packed in 22 cm high columns. The efficiency of extraction was similar to small-scale simulations of heap leaching (150 g of soil), where EDTA used in the same manner removed 49.7 ± 1.0% and 13.7 ± 0.4% of Pb and Zn. The new heap leaching method produced discharge extractant with fairly low final concentrations of Pb, Zn and EDTA (1.98 ± 2.17 mg l−1, 4.55 ± 2.36 mg l−1, and 0.05 ± 0.04 mM, respectively), which could presumably be reduced even further with continuation of treatment. The results of our study indicate that for soils contaminated primarily with Pb, treating the EDTA extractants with ozone/UV and reuse of extractants enables efficient soil heap leaching with very little or no wastewater generation, easy control over emissions, and lowers the requirements for process water.