Comparison of solvents for ex situ removal of chromium and lead from contaminated soil

Soil washing is one of few practical methods for the remediation of metal-contaminated soils. Ethylenediaminetetraacetic acid (EDTA, 0.1, 0.01, 0.001, 0.0001 M); nitrilotriacetic acid (NTA, 0.1 and 0.001 M), sodium dodecyl sulfate (SDS, 0.1 and 0.001 M); and HCl (0.25 to 8% v/v) were compared, in batch studies, for their ability to solubilize chromium (Cr) and lead (Pb) from a contaminated soil (Cr-tot = 4940 mg/kg; Pb-tot = 1300 mg/kg; pH = 10.3) collected from an abandoned industrial facility. The EDTA, NTA, and SDS were reacted with the soil over a wide pH range. The extent of Cr and Pb solubilization was strongly influenced by both solution pH and chelant-metal chemistry; the hexadentate EDTA solubilized both Cr and Pb more effectively than did the tetradentate NTA. Chromium and lead recovery increased with higher EDTA concentrations, with maximum recovery occurring at greater than 1:1 ratios of chelant:metal. The 0.1 M EDTA solution was most effective in soil decontamination: 100% Pb was recovered up to pH 4.3, and 54% Cr and 96.2% Pb were recovered at approximately pH 12. The NTA was less effective: 33%-48% Cr was removed (pH 8.9-11.0), and a maximum of 38% Ph was removed (pH 4.5). The SDS recovered 30%-40.5% Pb from pH 4.4-10.9, and 29%-35% Cr from pH 2.2-3.2. A single soil washing with chelant or SDS was not sufficient to bring soil Cr and Pb concentrations below United Kingdom "trigger concentrations." The acid wash using 2% through 8% HCl removed 100% of the Cr and Pb; however, 49%-51% of the matrix solids were dissolved, which created an impractical wastewater treatment situation. A level of 1% acid was much less effective in metal removal (15.7% and 3.8% Cr and Pb recovery, respectively). The recommended mashing treatment for the alkaline soil/waste mixture studied, based on pH adjustment and other practical considerations, includes 0.1 M EDTA without acidification.