Geochemical Forms of Heavy Metals in an Industrial Polluted Soil: A Critical Evaluation by Combined Synchrotron Spectromicroscopy Techniques and Bulk Extraction Methods.
Roberto Terzano1, Matteo Spagnuolo1, Koen Janssens2, Bart Vekemans2, Wout De Nolf2, Gerald Falkenberg3, Saverio Fiore4, and Pacifico Ruggiero1. (1) Dipartimento di Biologia e Chimica Agro-forestale ed Ambientale - University of Bari, Via Amendola 165/A, Bari, Italy, (2) Department of Chemistry - University of Antwerp, Universiteitsplein 1, Antwerp, Belgium, (3) HASYLAB at DESY - Beamline L, Notkestrasse 85, Hamburg, Germany, (4) I.M.A.A. - C.N.R., Contrada S. Loja, Tito Scalo (PZ), Italy
It is well known that heavy metals (HM) bioavailability and toxicity in soil are tightly connected to their solubility and therefore to their geochemical forms. The correct identification of HM chemical forms in soil is of extreme relevance for a proper risk assessment and to formulate effective remediation strategies. Sequential extraction procedures (method generally adopted for HM speciation in soil) can only provide information about some operative pools HM can be divided into, thus without providing subtle details on HM speciation. In addition, these methods can sometimes fail in correctly determining HM chemical forms, in particular when mixed and complex HM pollution sources are involved. Such determination problems are typical of industrial polluted soils. In recent years, the development of new analytical methods exploiting high energy and high intensity synchrotron generated X-rays has provided soil scientists with new powerful tools for shedding light on HM speciation in soil. In particular, X-ray microbeam techniques can reduce soil heterogeneity and complexity, thus allowing for a direct HM speciation at the micrometer scale. In this investigation two soil samples from the industrial site of “Val Basento” (Basilicata, Italy) have been studied by using a combination of synchrotron spectromicroscopy techniques. The polluted soils are characterised by concentrations of HM such as Hg, Pb, Cr, Zn, Cu, V, and Ni strongly exceeding the Italian legislation limits. Micro X-ray Fluorescence (µ-XRF) has been used to localize and quantify trace elements in soil thin sections (20-60 µm thick) and to find correlations among different elements. Simultaneously, micro X-ray diffraction (µ-XRD) patterns have been collected to get information about the minerals HM are related to. Finally, micro X-ray Absorption Near Edge Structure (µ-XANES) spectroscopy point analyses have been performed to determine the oxidation state of pollutants such as Cr and V and to define HM associations with soil minerals by comparison with selected mineral standards. The information revealed by these investigations allows for HM speciation at the microscopic level. The main limit of such analytical approach is represented by the difficulty to reliably extrapolate the obtained results to a higher length scale. To analyse larger amounts of soil samples, a sequential extraction procedure has been adopted. Most of the results obtained at the microscopic level have been confirmed by the sequential extraction procedure. However, sequential extractions failed to correctly identify some HM chemical species. Therefore, for the analysed soil samples, an extrapolation of the results obtained by using microspectroscopic techniques to the macroscopic level seems to be applicable and the above-mentioned limitations of the sequential extractions approach are confirmed. All the obtained results show that, despite a very high HM total concentration in the investigated soils, these HM appear to be present as scarcely mobilisable geochemical forms, indicating that the environmental danger connected to their presence in these soils is much lower than expected. Such a conclusion is also supported by different leaching tests (TCLP, EDTA extractions, deionised water extractions) carried out on the same soil samples. Finally, the HM speciation determined has been compared with the geochemical characteristics of the site under investigation and the possible sources of anthropogenic impact have been determined.