Cadmium reactivity in metal-contaminated soils using a coupled stable isotope dilution-sequential extraction procedure

To better understand the intrinsic reactivity of Cd, four soils having dive rse sources of Cd contamination (total Cd: 22-34 mg kg(-1)) were investigat ed using a stable isotope dilution-sequential extraction procedure (SID-SEP ) during a 59-week incubation. Samples were spiked with carrier-free Cd-111 and periodically extracted into five operationally defined fractions; the Cd-111:Cd-110 ratios were measured by inductively coupled plasma-mass spect rometry. The total labile pool of Cd (E value) was calculated as well as th e labile Cd within each extracted fraction. Results for three of the soils (Cd sources: natural, sewage sludge, smelter emissions) were quite similar. The overall %E after 2-week equilibration was 35-49% of total soil Cd. Wit hin fraction 2 (sorbed/carbonate), 70-75% of the Cd was isotopically labile , while within fraction 3 (oxidizable) only 35-41% of the Cd was labile wit hin 2 weeks;The fourth (reducible) and fifth (residual) fractions were domi nated by nonlabile Cd. Although all E values increased somewhat from 2 to 5 9 weeks, none of the extracted fractions reached isotopic equilibrium with the soluble/exchangeable Cd extracted during step 1. Because fractions 2 an d 3 dominated the native Cd in all three soils, the total labile pool was c ontributed primarily (85-98%) by these two fractions. A fourth soil (mine s poil-contaminated) was demonstrably different after 2 weeks, the overall %E was just 13 and, although 82% of the total Cd was present in the oxidizabl e fraction, just 2% of that was isotopically labile. The nonlability of Cd in this soil could be ascribed to the predominance of inorganic forms, most likely occluded Cd in sphalerite. No single Cd fraction from the SEP nor a ny combination of fractions showed a good correspondence with the size of t he isotopically labile pool. Our results suggest that conventional SEPs may be of limited utility for predicting bioavailability, for example, during ecological risk assessment.