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 R eference) sequential extraction procedure. This type of environmental analy sis is now increasingly used, but its reliability is questioned due to conc erns regarding redistribution of extracted metals occurring during the proc edure. Using radiotracer techniques, the redistribution of Cd and Zn during the sequential extraction was directly quantified. For a fill material tak en from downtown Toronto, metal partitioning behavior conformed with the th eory 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 aban doned 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 th e soil was readsorbed to the reducible mineral oxide fraction. The extent o f the phenomenon is less than previously suspected and does not invalidate sequential extraction results for these metals.