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 c ontent, is an important characterization parameter for waste materials and soils. The availability of inorganic species is often evaluated from the re sults of a sequential extraction procedure at a high liquid-to-solid (LS) r atio using pH control. Several tests maintain pH at 4 and 7 to maximize the solubility of cations and anions, respectively. An alternative procedure f or measuring inorganic availability was developed using ethylene diamine te traacetic acid (EDTA) as a chelating agent to eliminate the need for rigoro us 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 sin gle batch extraction of particle size reduced material at near-neutral pH t hat may be used to determine the availability of both cations and anions. O ptimized 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. Availabilit y values of arsenic, cadmium, copper, manganese and lead from four solid ma terials were determined in parallel using the optimized EDTA method and a t wo-step, static pH extraction at pH 7.0 and 4.0. The characterized waste ma terials represent common waste types in both treated and untreated forms. A s determined by the EDTA procedure, the entire total content of contaminati on 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 showe d that significantly greater availability was observed using the EDTA metho d for all test cases. Since the extraction conditions under which availabil ity was measured differed greatly between the protocols and to a large exte nt "availability" is operationally defined, it was impossible to conclude t hat one method was more rigorous than the other. Thus, it could be said tha t both approaches measure a different operationally defined availability of inorganic constituents in the tested materials. (C) 2000 Elsevier Science Ltd. All rights reserved.