Use of a chelating agent to determine the metal availability for leaching from soils and wastes

Constituent availability, or the potentially mobile fraction of the total content, is an important characterization parameter for waste materials and soils. The availability of inorganic species is often evaluated from the results of a sequential extraction procedure at a high liquid-to-solid (LS) ratio using pH control. Several tests maintain pH at 4 and 7 to maximize the solubility of cations and anions, respectively. An alternative procedure for measuring inorganic availability was developed using ethylene diamine tetraacetic acid (EDTA) as a chelating agent to eliminate the need for rigorous pH control. Analysis of variance was used to optimize the conditions of the EDTA availability procedure with respect to LS ratio, contact time and the concentration of EDTA in the leachant. The optimized procedure is a single batch extraction of particle size reduced material at near-neutral pH that may be used to determine the availability of both cations and anions. Optimized test conditions were determined to consist of a solution of 50 mM EDTA at an LS ratio of 100 ml/g dry for a contact time of 48 h. Availability values of arsenic, cadmium, copper, manganese and lead from four solid materials were determined in parallel using the optimized EDTA method and a two-step, static pH extraction at pH 7.0 and 4.0. The characterized waste materials represent common waste types in both treated and untreated forms. As determined by the EDTA procedure, the entire total content of contamination elements such as As, Cd, and Pb was found available for release whereas only a fraction of typical mineral substitution elements like Cu and Mn was found to be available. Comparison to the two-step static pH approach showed that significantly greater availability was observed using the EDTA method for all test cases. Since the extraction conditions under which availability was measured differed greatly between the protocols and to a large extent “availability” is operationally defined, it was impossible to conclude that one method was more rigorous than the other. Thus, it could be said that both approaches measure a different operationally defined availability of inorganic constituents in the tested materials.