Study of mineralogical speciation of arsenic in soils using X ray microfluorescence and scanning electronic microscopy

In this paper we studied the As content in natural contaminated soils, classified as Dystric Leptosol, Chromic Luvisol, Eutric Cambisol and Mollic Leptosol. In soil samples, sieved (<2 mm), total As was determined by XRF and chemical speciation by sequential extraction. As-bearing minerals were concentrated from fine sand fraction of soil (200–20 μm) using heavy liquid. In this fraction, mineralogical speciation was studied by X-ray microfluorescence, XRD with Göbbel mirror and SEM–BEI–EDX. Total As contents ranging from 61.00 to 131.00 mg kg−1. The results of the sequential extraction showed that As was, mainly, in the residual fraction (52.51–98.76 mg kg−1) and in the fraction bound to iron oxyhydroxides (0–36.5 mg kg−1). Mapping of As with X-ray microfluorescence show strongly relationship between Fe and As. Iron (III) oxyhydroxides (FeOHs) (lepidocrocite and goethite), scorodite, angelellite, schultenite and dussertite were identified by XRD analysis as most likely mineral phases. The contents of As, Fe, Pb and Ba obtained with EDX-microprobe, confirmed the results of XRD. The results of sequential extraction and X-ray microfluorescence indicate that As is strongly bound to the soils because the identified As-bearing mineral phases are very stable at the pH conditions of studied soils. Consequently, a low mobility of As can be assumed in these soils.