Heap Leaching of Heavy Metal Contaminated Soil Using Advanced Oxidation Processes for Treatment of Extractants in a Closed Loop.
Domen Lestan and Neza Finzgar. Biotechnical Faculty, Univ of Ljubljana, Jamnikarjeva 101, Ljubljana, Slovenia
Soil contamination is seldom mono-metallic. In soils contaminated primarily with Pb, Zn and other heavy metals are usually also present in elevated concentrations. Current remediation activities have involved excavation and removal of the contaminated soil, immobilization and containment of heavy metals in the soil by mixing or injecting agents such as cement, lime and different phosphates, electrokinetic mobilization and removal of heavy metals from the soil by precipitation on the electrodes, phytoextraction of metals using metal-accumulating plants, and soil washing and flushing. Soil washing involves the separation of contaminants from soil solids by solubilizing them in a washing solution. This can be done in a reactor (extraction of soil slurry) or preferably as soil heap leaching. The heap leaching process is operationally simple. Heavy metal contaminated soil is excavated, screened and mounded on a pad. Heavy metals are removed by passing washing solution through the soil using some type of liquid distribution system. The extractant is collected in a pregnant solution pit and processed to remove heavy metals. In practice, acid washing and chelator soil washing are the two most prevalent heavy metal removal methods. A number of chelators have been tested. For soils contaminated primarily with Pb, EDTA was in most cases the most effective. One of the main problems of current EDTA based soil washing technologies is the separation of chelator-heavy metals complexes from the waste extractant. EDTA is toxic and its complexes are poorly chemo- and biodegradable and must therefore be removed before the extractant can be safely discharged. To treat decontamination wastewaters from the nuclear industry and other aqueous effluents contaminated with EDTA, the chemical destruction of EDTA and its complexes using Advanced Oxidation Processes (AOP) has been proposed. AOP involves the use of ozone, H2O2, ultrasonic waves, UV irradiation, Fenton's reagent (Fe2+ and H2O2), alone or in combination, to generate free hydroxyl radicals; powerful, effective, non-specific oxidizing agents. Ozone is most often used in AOPs. It oxidizes organic compounds in two ways: by direct oxidation with ozone molecules and by the generation of free hydroxyl radicals. The concentration of hydroxyl radicals under normal ozonation conditions is, however, small. Under conditions favoring hydroxyl radical production, such as exposure to UV light, hydroxyl oxidation starts to predominate over oxidation with molecular ozone. Removal of EDTA-heavy metal complexes from soil extractants using AOP has not yet been tested. The aim of this study was to assess the feasibility of a novel soil heap leaching method for remediation of Pb and other heavy metal contaminated soil, using ozone and UV for advanced oxidation of EDTA complexes in soil extractants, removal of released metals from extractant by absorption, and reuse of extractant in a closed process loop. Soil containing 1243 mg/kg Pb and 1190 mg/kg Zn was collected from the 0-30 cm surface layer at an industrial site of a former Pb smelter in the Mežica Valley in Slovenia. An ozone/UV treatment unit for EDTA soil extractants consisted of an ozone generator, ozonation flask, UV-light, and absorption column. A peristaltic pump was used to force the extractant through the unit. Metals, released after advanced oxidation of EDTA complexes, were removed from soil extractants by passing them through the absorption column with commercial sorbent Slovakite. Soil heap leaching was simulated in triplicate in 15 cm diameter soil columns filled with 4.6 kg of air-dried soil. The soil was treated with six consecutive EDTA additions with 2.5 mmol/kg EDTA in 3100 mL unbuffered tap water. Extractant was first circulated through the soil column for 48 h using a peristaltic pump and then through an ozone/UV unit and soil column until essentially all Pb and Zn were absorbed on Slovacite. Six consecutive additions of EDTA (total 15 mmol/kg of soil) removed 49.6±0.6 and 19.7±1.7% of initial total Pb and Zn from soil. The new heap leaching method produced discharge extractant with fairly low final concentrations of Pb, Zn and EDTA (1.98±2.17 mg/L, 4.55±2.36 mg/L, 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.