Lead Leachability in Firing Range Soils

Abstract

This study focuses on investigation of the effects of Pb transformation products (Pb forms in the soil) on Pb leachability by evaluating Toxicity Characteristic Leaching Procedure (TCLP) data from samples taken out of six different firing ranges. In all the soils tested, TCLP Pb leachability levels and total Pb concentrations were found to be above the regulatory limits set by the U.S. EPA and state agencies. However, TCLP Pb leachability levels did not always correlate well with total Pb concentrations. Formation of Pb carbonates was found to play an important role in controlling Pb leachability and precipitation was found to be an important Pb immobilization mechanism in all soils tested. The effect of posttumbling TCLP (leaching) pH, soil buffering capacity, grain size, and mineralogy on Pb leachability was also considered. The leaching pH effect was most pronounced in the range that showed the lowest Pb leachability, and was attributed to the presence of buffering compounds in this soil. Pb leachability was found to decrease with decreasing soil grain size, where lowered Pb leachability levels were attributed to clay, Fe–Mn oxide and aluminum hydroxide sorption mechanisms. The effectiveness of soil treatment with phosphates and pozzolans (cement or lime) in achieving Pb immobilization was also evaluated. Formations of calcite and ettringite were identified in both cement and lime treated soils. PbHPO₄ formations were identified in soils treated by phosphate addition. Effective Pb immobilization in treated soils was attributed to the formation of these minerals and to an increase in the soil’s buffering capacity in soils treated with pozzolans.