Enriched stable isotopes for determining the isotopically exchangeable element content in soils

The proportion of metals in soils in equilibrium with soil solution can be determined using isotopic dilution. For this purpose, an isotope dilution mass spectrometric (IDMS) technique has been applied for the elements Cd, Cr, Cu, Mo, Ni, Pb, Tl and Zn. Conventionally, sorbed amounts of heavy metals in soils are analysed by ethylenediamine tetra-acetic acid (EDTA) extraction. The IDMS technique and EDTA extraction were both applied to 115 soil samples and compared. For Cd, Cu, Ni, Pb and Zn, the results of the IDMS technique correlated well with the results of EDTA extractions (r(s)(Cd) = 0.965, r(s)(Cu) = 0.916, r(s)(Ni) = 0.878, r(s)(Pb) = 0.922, r(s)(Zn) = 0.962; all at P < 0.001). For Cd and Zn, no significant differences between the results of both methods could be observed, which suggests that EDTA and IDMS determined the same pool. EDTA extracted more Cu, Ni and Pb than was determined by IDMS (7, 26 and 13%, respectively). The correlation between EDTA extraction and IDMS for Cr was significant but weak (r(s)(Cr) = 0.361). For Tl and Mo, EDTA extraction and IDMS did not correlate, and IDMS yielded larger concentrations than EDTA. This can be explained by the fact that Tl and Mo do not form stable EDTA complexes, which are essential for the EDTA technique. Recovery experiments demonstrated that added Cd, Cu, Mo, Ni, Pb, Tl and Zn could be recovered successfully by IDMS analysis (mean recovery = 103 +/- 9%). Adsorption isotherms for soil samples were determined for Tl, thereby demonstrating that IDMS gave a better estimation of the native content of sorbed Tl in soils than EDTA extraction.