Sequential extraction of metal contaminated soils with radiochemical assessment of readsorption effects

The operational speciation of selected trace metals in several contaminated Toronto area soils was determined by a modified BCR (Community Bureau of Reference) sequential extraction procedure. This type of environmental analysis is now increasingly used, but its reliability is questioned due to concerns regarding redistribution of extracted metals occurring during the procedure. Using radiotracer techniques, the redistribution of Cd and Zn during the sequential extraction was directly quantified. For a fill material taken from downtown Toronto, metal partitioning behavior conformed with the theory and anecdotal evidence in the soil chemistry literature: Cd was highly mobile, while Cu and Pb were primarily associated with oxidizable organic matter, and Zn was distributed across all soil fractions. Soil from an abandoned metals recycling operation had unusual physical characteristics, and the trace metals within were largely nonextractable. In the soils examined, some 20-30% of the Cd and Zn released from the acid-soluble fraction of the soil was readsorbed to the reducible mineral oxide fraction. The extent of the phenomenon is less than previously suspected and does not invalidate sequential extraction results for these metals.