Isotope dilution techniques were evaluated in order to determine the labile pool of cadmium (Cd) in uncontaminated soils. The techniques included determining the
E-value (isotopically exchangeable pool) and isotopic exchange kinetics (IEK). An enriched
113Cd solution was added to the suspension, and then the
113Cd/
114Cd ratio was monitored at different exchange times. After correcting the spectral interference from concomitant elements, the
113Cd/
114Cd ratio in a soil solution (Cd: 0.1 ng mL
−1) could be measured with good precision within RSD 5%. The
E(t)-values increased with time, especially between 1 min and 1 day. After 7 days, it reached 0.11 μg g
−1, corresponding to 50% of the total Cd. The decrease rate of the
113Cd/
114Cd ratio in the soil solution could be fitted by the IEK equation. The
E(t)-values estimated by long-term isotopic kinetics (from 1 min to 15 days) were in good agreement with observed values. The
E(t)-values, estimated by short-term isotopic kinetics (<100 min), were also in good agreement up to 15 days. The IEK technique was applied to evaluating the Cd availability of sewage sludge amended soil. These results suggest that the labile pool of Cd in uncontaminated soils can be estimated by short-term isotopic kinetics.
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