Laboratory-based X-ray multianalytical approach to assess the bioavailability of arsenic in contaminated soils

Arsenic (As) is a metalloid element, naturally associated with gold, sulphur, iron and heavy metals and is often found in soils around former mines and industrial sites treating As-bearing minerals. The assessment of the bioavailability of As in these soils is very important in order to protect human and ecosystem health. Earthworms are the most used organisms to assess the bioavailability of As in contaminated soils. The present work aims at evaluating the bioavailability of As in six polluted soils sampled in the industrial area of Scarlino (Tuscany, Italy) and the gold mining site of the Anzasca Valley (Piedmont, Italy), using Eisenia andrei as a bioindicator and a multianalytical X-ray based approach. The soils were preliminary analysed in situ by portable energy dispersive X-ray fluorescence spectrometry (pEDXRF) which detected As concentrations ranging from 20 to 800 mg/kg in Scarlino, and from 20 to 20000 mg/kg in Anzasca Valley. X-ray powder diffraction (XRPD) detected no As-bearing mineral. Only in the most contaminated soil from Anzasca Valley, jarosite and goethite were detected as minerals formed as a consequence of the alteration of pyrite and arsenopyrite. In order to have more information about As speciation, micro X-ray fluorescence (μXRF) analysis were performed on soil thin sections, which showed the co-presence of Fe and As around quartz or feldspar grains. This is in accordance with the results of sequential extractions which indicated that As was mainly associated with amorphous and/or well-crystallized Fe oxides and hydroxides. To study As bioavailability, ten sexually mature earthworms were exposed to contaminated and control soils. After 14 days, mortality (which gives information about acute toxicity) and oxidative stress were measured. The chronic toxicity was estimated with reproduction tests after 28 days. μXRF analyses on earthworm thin sections revealed that As accumulates mainly in the coelomic cavity while no As was detected in other parts of the body. Given that As detoxification in earthworms seemed to involve mainly the coelom, coelomic fluids were electrically extruded and the As concentration was estimated via total-reflection X-ray fluorescence (TXRF) spectroscopy. In all cases, no earthworm died after 14 days of exposure to contaminated soils even if an oxidative stress was measured. A chronic toxicity was observed after 28 days with a reduction in new born organisms with the increase of As concentration. TXRF analyses on coelomic fluids suggested that the As concentration in the fluids was closely related with the As available fraction (adsorbed or loosely bound to Fe oxides and hydroxides) and not with the total As concentration in soils. In fact, when a consistent part of the As was associated with well crystallized Fe-hydrous oxides (the low available fraction) the concentration of As in the coelomic fluids was comparable to that found in earthworms exposed to control soils. Finally, this study showed the usefulness of laboratory x-ray based techniques for bioavailability studies and could be used as a base for the development of new procedures for the assessment of As bioavailability in contaminated soils.