Measuring reactive metal in soil: a comparison of multi‐element isotopic dilution and chemical extraction

Abstract

Summary The isotopically exchangeable metal pool ( E ‐value) of zinc ( Zn ), cadmium ( Cd ) and lead ( Pb ) were simultaneously measured, using stable isotope dilution, in soils contaminated by Pb/Zn mining activities and varying in properties likely to affect metal reactivity, including pH , organic matter content, metal concentration and land use. E ‐values were compared with single and sequential extraction schemes. Results showed a wide range of metal reactivity (approximately 1–100% of total) depending on the extent of contamination and on the prevailing soil conditions. Across the range of soils, the E ‐values showed no consistent correspondence to any single chemical extraction procedure ( EDTA , DTPA and HNO 3 ) although there was reasonable agreement with the extractants 0.05 m EDTA and 0.43 m HNO 3 in acidic organic soils. Extraction with 0.005 m DTPA substantially under‐estimated the isotopically exchangeable metal content. E ‐values corresponded reasonably well with the exchangeable metal (fraction 1 ( F1 ) of the sequential extraction procedure) in calcareous soils but relatively poorly and inconsistently with F1–F2 , F1–F3 or F1–F4 in acidic‐neutral soils. Operational aspects associated with determination of multi‐element E ‐values are discussed.