Simultaneous extraction and recovery of lead using citrate and micro-scale zero-valent iron for decontamination of polluted shooting range soils

Abstract

Lead (Pb) contaminated soils pose serious risks to human health and the environment. One remediation technique that permanently removes Pb is chemical soil washing. This study investigated a modified chemical soil washing technique whereby Pb extraction in citrate solution and recovery of dissolved Pb by reductive precipitation (cementation) were simultaneously carried out in a single reactor by the addition of micro-sized zero-valent iron (mZVI) to decontaminate Pb-contaminated soil from a shooting range. Pb-loaded mZVI were then recovered by magnetic separation. The effects of mZVI dosage, citrate concentration, pH, and hydrodynamics on Pb removal were evaluated. Under acidic pH, Pb removal increased with increasing citrate concentration. The addition of mZVI suppressed Pb removal from 59.6 to 30% in 0.1 M citrate solution, which was attributed to dissolved Fe ions competing for free citrate ions to form soluble Fe-citrate complexes. Increasing the citrate concentration and shaking speed enhanced Pb extraction from the soil and subsequent recovery via cementation onto mZVI. Pb removal and recovery onto mZVI of up to 99.0% were achieved at citrate concentration of 0.7 M, pH of 5, and shaking speed of 165 strokes/min. The addition of mZVI during chemical soil washing in citrate solution not only extracted and recovered Pb from the contaminated soil but also eliminated the need to wash soil residues obtained after treatment because the bulk of dissolved Pb was sequestered from the solution.