Extraction mechanism of lead from shooting range soil by ferric salts

The conventional soil washing process uses inorganic acids to extract metal from contaminated soils; however, soil structure disturbance is a negative by-product of the process using the acids. To minimize the adverse effects of soil washing, the use of alternative extractants is needed. Ferric chloride has a low pK_a value, and anionic chloride ligand can enhance the extraction of toxic cationic metals from soils. In this study, a soil washing process using ferric chloride and ferric nitrate was applied to a Pb-contaminated model and field shooting range soils, and the outcome was compared to the use of conventional inorganic acids. The extraction of Pb from both soils was mostly dependent on the equilibrium solution pH regardless of the extractants used, and chloride could not significantly enhance that. Ferric iron enhanced the extraction of Pb compared to the inorganic acid treatment because ferric could generate continuously hydrogen ions by the equilibrium reaction between ferric ions and ferric hydroxide. Ferric chloride may be an attractive alternative extractant for the soil washing of Pb-contaminated soils. However, although Pb concentration in soil was significantly decreased by washing process, soil enzyme activity and plant growth was inhibited by severe soil acidification. Additionally, ferric salt-based washing showed similar seed germination test using tomato (Solanum lycopersicum) with HCl-based washing. Therefore, ferric salt based soil washing could remove more Pb from the shooting range soil with similar quality of soil compared to inorganic-based washing.