Abstract

An isotope dilution inductively coupled plasma-mass spectrometry (ICP-MS) technique, using an enriched stable Cd isotope, was applied to determine the labile fraction of Cd in 20 biosolids-amended soils. The measured labile Cd fraction was compared with the amounts of Cd solubilised or extracted by HNO₃, EDTA, CaCl₂, and Ca(NO₃)₂. Labile Cd, as a proportion of total soil Cd (% labile), ranged between 33 and 84% with a mean of 50%, indicating that a significant proportion of the Cd added to these soils from biosolids is in non-labile forms. EDTA was found to release Cd from non-labile sites, solubilising on average 30% more Cd than labile forms measured by isotope dilution. In contrast, the proportion of Ca(NO₃)₂-extractable Cd was significantly less than the measured labile fraction and probably represents Cd in soil solution. Extraction of soil with 1 M CaCl₂, however, was found to provide a very good estimate of labile Cd in these biosolids-amended soils. Isotope dilution ICP-MS using an enriched stable Cd isotope and ICP-MS appears to be an analytical technique which has the potential to measure labile Cd in soils without the problems associated with the use of radioisotopes.