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

Abstract

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 (Cd962; 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) ¼ 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 ¼ 103 AE 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.

Lability, solubility and speciation of Cd, Pb and Zn in alluvial soils of the River Trent catchment UK

“…This high reactivity is in agreement with literature values for a range of unpolluted to heavily polluted 60 soils i.e. 3-100% and typically 30-60% 25,[27][28][29][30] . Cadmium is often the most labile metal in soils 29,31 .…”

“…These values are in agreement with typical mean values in a range of polluted and unpolluted soils i.e. 10-33% 25,27,30 . Whilst the linear correlations between the total pools of Cd and Zn and their distribution patterns across the sampling area 70 support a similar origin, their lability is decidely different.…”

“…The effectiveness of EDTA as an analogue for 15 the E-value appears to be metal specific and dependent on the concentration of EDTA used. For example, Gäbler et al 27 reported a good agreement between the labile pool and a 0.025M EDTA extractable pool for a set (n=115) unpolluted soils (mean Cd 33% lability) and for polluted soil (>70% lability). However, 20 Degryse et al 25, Nakhone and Young 28 and Young et al 30 all reported mobilisation of non-labile Cd when concentrations >0.025M EDTA were used.…”

“…The measurement of the labile pool using isotopic dilution (ID) assays is generally accepted as the most robust and mechanistically sound estimate 40 of reactive metals 23 . Methods for Cd, Zn and Pb have used both stable and radio isotopes [24][25][26][27][28][29][30][31] . However, as a result of the ID assay being analytically challenging through the use of either stable isotopes or potentially hazardous using radio-isotopes, analogues of the labile pool have been suggested and these 45 include 0.43M HNO 3 and the EU standard 0.05M EDTA extractions [32][33][34] .…”

Measuring reactive pools of Cd, Pb and Zn in coal fly ash from the UK using isotopic dilution assays

“…Three electrolytes commonly used in determining the isotopically exchangeable pool of metals in soils were evaluated: 0.1M and 0.01M Ca(NO 3 ) 2 , and 0.0005M EDTA (Atkinson et al, 2011;Gäbler et al, 2007;Young et al, 2000). The use of Ca(NO 3 ) 2 (0.1M and 0.01M) does not risk mobilisation of non-labile metal, but it was found to mobilise insignificant amounts of Cd, Zn and Pb from ash, which resulted in analytically unreliable isotope measurements due to a poor contrast with the background.…”