Availability of soil cadmium using stable and radioactive isotope dilution

The aim of this research was (i) to compare the use of stable (111Cd) and radioactive (109Cd) isotopes for assessing the isotopically exchangeable pool of Cd (ECd), (ii) to evaluate a simpler alternative method, the extraction by 1 M CaCl2, (iii) to determine ECd in a wide range of soil and contamination types and (iv) to assess how ECd can be affected by the soil characteristics. Measurement with 111Cd gave equivalent but more repeatable results than those of 109Cd. Increasing the acidity of the spike solution led to an overestimation of ECd. Extraction by 1 M CaCl2 generally gave similar values to isotope dilution. Measured on 29 soil samples, ECd was on average 44% of the total Cd (CdT). The cultivated soils showed the highest relative cadmium availability (ECd/CdT = 46% on average) and the geochemically enriched soils the lowest (20%). For the whole sample set, ECd variance was mainly explained by the correlation with CdT (r = 0.86), while ECd/CdT was negatively correlated with pH (r = − 0.73). In the cultivated soils, ECd increased with CEC (r = 0.94), while ECd/CdT decreased with increasing iron content (r = − 0.91).