Measuring reactive metal in soil: A comparison of multi-element isotopic dilution and chemical extraction

Abstract

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 HNO3) although there was reasonable agreement with the extractants 0.05m EDTA and 0.43m HNO3 in acidic organic soils. Extraction with 0.005m 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. © 2013 The Authors. Journal compilation © 2013 British Society of Soil Science.