In situ stabilization of arsenic and lead in contaminated soil using iron-rich water treatment residuals

Arsenic (As) and lead (Pb) are common soil contaminants, environmentally hazardous, and threats to public health. Addition of soluble phosphate is known to be effective for in situ remediation of Pb-contaminated soils, but phosphate additions displace As from the soil particles and increase As concentration in soil solution. This study examined the dual use of iron (Fe) and phosphorus (P) amendments to soil that was highly contaminated with As and Pb. The test soil originated from a former smelter site in Utah with As and Pb concentrations of 66,400 mg Pb kg(-1) and 7,520 mg As kg(-1). Goethite, ferrihydrite, and Fe-rich water treatment residuals (Fe-WTRs) were added to immobilize As, and soluble P was added to reduce Pb lability. The Fe amendments were added in Fe/As molar ratios ranging from 1:1 to 1:10, and P was added in P/Pb molar ratios ranging from 0.2:1 to 5:1. Iron-rich water treatment residuals were found to be the most effective Fe amendment. When Fe-WTR and P were added simultaneously, the P concentrations required to immobilize Pb resulted in increased mobilization of As, even when Fe-WTR were added at 10:1 Fe/As. However, when P was added first at 5:1 P/Pb, incubated for 1 wk, and then amended with Fe-WTR at 10:1 Fe/As, both Pb and As were significantly immobilized. The proposed process is a practical remediation approach to soils co-contaminated with Pb and As while encouraging re-use of Fe-WTR as a sustainable amendment.