Selection of wet digestion methods for metal quantification in hazardous solid wastes

Abstract

Quantifying metal concentration in a hazardous solid waste is critical in evaluating techniques for metal removal or recovery from the waste. Numerous protocols have been published on the digestion of hazardous waste, with diverse methods yielding dissimilar extraction efficiencies for different metals, due to differences in composition of the wastes and chemical forms of the metals. The lack of standard methods (or failure to state the method used) has resulted in inconsistencies in data reported by different laboratories, leading to variations in metal recoveries. Using fly ash as an example, this study compares 11 metal extraction methods including ASTM, USEPA, ultrasonic-assisted and microwave digestion methods as well as a modified Tessier’s sequential extraction procedure for the extraction of metals (Al, Cd, Cr, Cu, Fe, Ni, Pb and Zn). The efficacy of acid digestion was metal dependent. ASTM method D 6357-11 was found to be an effective digestion method for most metals primarily due to the dissolution of metals bound in silicate matrices, while a microwave digestion technique (which hastened the digestion process significantly) was effective for Cd and Pb. This study highlights the importance in the selection of the most appropriate protocol for the digestion of hazardous solid waste.