Feasibility of extracting toxic metals from soil using anhydrous ammonia

Abstract

Pure anhydrous liquid ammonia is a logical choice for extraction of metals from contaminated soil because it forms strong, soluble ammonia complexes with many toxic metals. Ammonia's low cost, availability, and routine use in farming make it even more attractive. Thus, it was the purpose of this research to establish the feasibility of using anhydrous ammonia alone and with enhancing ligands to extract common metal contaminants—lead (Pb), cadmium (Cd), mercury (Hg) copper (Cu), and zinc (Zn)—from soil. To evaluate its effectiveness for soil extraction, three devices were used; a pressurized Soxhlet extractor, a plug flow extractor, and a pressurized stirred batch reactor. They all gave essentially the same performance, which suggests that the metal removed was readily extractable. Sixty to seventy percent removal of cadmium, copper, and zinc could be obtained from soils spiked and aged for four months to two years with 50,000 mg/kg of the metal nitrate. Lead and mercury removals were low (<15%) with pure ammonia. Lead removal increased dramatically, however, when ethylenediaminetetraacetate (EDTA) was added to the ammonia and when aqueous EDTA alone was used as an extractant. When the metal-nitrate spiking concentration was lowered to 5000 mg/kg, the percent removals for all the metals were generally less than 30%. The presence of water in the soil at the start of extraction significantly reduced the removals of all the metals except cadmium. Overall, ammonia extraction of toxic metals does not appear promising for complete site cleanup. However, to determine the practical effectiveness of ammonia extraction as a pretreatment for soil remediation, additional tests need to be performed, and the criterion for success should be a Toxicity Characteristic Leachate Procedure (TCLP) test on the extracted soil.