Methods for determining labile cadmium and zinc in soil

Isotopically exchangeable cadmium and zinc ('E values') were measured on soils historically contaminated by sewage sludge and ones on zinc-rich mine spoil. The E-value assay involves determining the distribution of an added metal isotope, e.g. Cd-109, between the solid and solution phases of a soil suspension. The E values for both metals were found to be robust to changes in the position of the metal solid double left right arrow solution equilibrium, even though the concentration of dissolved metal varied substantially with electrolyte composition and soil:solution ratio. Concentration of labile metal was also invariant over isotope equilibration times of 2-6 days. The use of a submicron filtration procedure, in addition to centrifuging at 2200 g, proved unnecessary if 0.1 M Ca electrolyte was used to suspend the soils. The proportion of 'fixed' metal, in non-labile forms, apparently increased with increasing pH, although there was considerable variation in both sets of contaminated soil. Zinc and cadmium in the sludged soils were similarly labile. Several possible methods for the measurement of chemically reactive metal were explored for comparison with E values, including single extraction with 1 M CaCl2 and a 'pool depletion' (PD) method. The latter involves comparing solid double left right arrow solution metal equilibria in two electrolytes with differing degrees of (solution) complex formation, 0.1 M Ca(NO3)(2) and CaCl2. Both the single extraction and the PD method gave good estimates of E value for Cd, although the single extraction was more consistent. Neither technique was a useful substitute for determining labile Zn, because of weak chloro-complexation of Zn2+. We therefore suggest that 1 M CaCl2 extraction of Cd alone be used as an alternative to E values to avoid the inconvenience of isotopic dilution procedures.