SUITMA 2005 Cairo - A comparison of extracting techniques for measurement of metals in urban soils
SUITMA 2005 Cairo
| This abstract is about one of the papers of the Methodology and classification theme of the SUITMA 2005 symposium. |
| E. Ruiz-Cortés,i | Fernando Madrid,i |
| Rocio Reinoso,i | Encarnacion Diaz-Barrientos,i |
| Luis Madrid.i |
The environmental hazard that large concentrations of potentially toxic metals in soils represent for living organisms, e.g. through the plants growing on such soils, is strongly dependent upon the most available metal forms present in the soil. Therefore a good estimate of the more easily soluble fractions is probably more important from an environmental viewpoint than the determination of the total contents. This is well known in the case of agricultural soils. In urban areas, soils are predominantly used for recreational and residential purposes and their use for growing plants for food is often of marginal significance. However, the risk for humans becomes significant through direct contact, ingestion or inhalation. In such cases, it is again the more soluble metal fractions those more likely to be harmful.
Several solutions have been used to estimate the more easily removable metals in soils and sediments, e.g. complexing agents as EDTA or DTPA, dilute acids as HCl or CH3COOH, neutral salts as CaCl2 or NaNO3. Also, sequential extraction with various solutions of progressively stronger ability to dissolve metals has been considered a way to estimate metal fractions of different availability to organisms. The 3-step sequential extraction protocol proposed by the European SMTP (formerly BCR) is considered to dissolve most of the metals of environmental interest. This technique is laborious and time consuming, so that extraction with 0.5 M HCl has been proposed as a good alternative for soils of urban environments, as the amounts dissolved are very highly related with the sum of the 3 steps of the BCR protocol.
In this presentation, evidence showing that such alternative is not applicable to any metal in urban soils of Seville. Some of the metals considered as of anthropogenic origin (‘urban’ metals, Cu, Pb, Zn) give results with the dilute HCl method that are very similar to the BCR protocol, but many other metals, of natural or mixed origin, do not show any correspondence between the results obtained by both methods. EDTA extraction also gives good correlation with the sequential or HCl methods for those urban metals, but the slopes of the plots between both sets of data are clearly below unity. Therefore EDTA extraction clearly overestimates the amounts of metals dissolved by both sequential or HCl extractions.
New evidence is given here of the statistical association between urban metals, either in their pseudo-total (aqua regia) or available amounts, and some characteristics of the soils.
