“MICRO X-RAY LAB”: A NEW APPROACH FOR HEAVY METAL SPECIATION IN POLLUTED SOILS AND ENVIRONMENTAL MATERIALS

Heavy metals (HM) in soils can be toxic for cultivated plants and living organisms and can pose a serious risk for human health, especially if they can reach underground waters or enter the food chain. Determining HM total concentration in soil is only the starting point of a risk assessment, but nowadays information about HM speciation is crucial to understand the real impact of HM pollutants on the environment. In addition, soil is an extremely heterogeneous matrix where different organic and inorganic components are mixed down to the micrometer or nanometer scale. Therefore analytical instruments capable of resolving such a complexity at the nanometer/micrometer scale are also needed. To this purpose, a proper combination of X-ray analyses can provide both information about HM total concentration and speciation also at the nanometer/micrometer level. At the University of Bari a new laboratory called "Micro X-ray Lab" has been created. In this lab, the potentialities of different X-ray based analytical techniques are combined to solve problems related to soil HM pollution and remediation. To this purpose the following instrumentation is available: a portable X-ray fluorescence spectrometer (pXRF) (for quick semiquantitative in situ analyses), a wavelength dispersive X-ray fluorescence spectrometer (WD-XRF) (for more precise determinations of HM in solid and liquid samples), a total reflection X-ray fluorescence spectrometer (TXRF) (for the determination of HM at ppb levels), a micro X-ray fluorescence spectrometer (µ-XRF) and a Variable Pressure Field Emission Scanning Electron Microscope (for the identification of HM pollutants at the micro/nano-scale). In addition, HM-containing mineral phases of soil samples are investigated by X-ray powder diffraction (XRPD) and thermogravimetrical and differential thermal analyses (TGA/DTA). Finally, X-ray microtomography (µ-CT) is used to characterize the soil structure, its porosity as well as the internal structure of microscopic soil minerals. The same methodological approach can be applied to the study of HM in other environmental materials such as plant and biological matrices and to assess the efficacy of technologies developed for the remediation of HM polluted soils. In this work, preliminary results on HM speciation and biaccessibility studies in polluted soils are presented and discussed with a focus on As-polluted soils. The aim of these studies are to identify the source and the fate of pollution as well as to develop suitable physical-chemical procedures to remediate the contaminated sites.