An integrated geochemical and mineralogical approach for the evaluation of arsenic mobility in mining soils

Purpose: The assessment of risk related to the presence of potentially toxic elements in soils is strictly related to the knowledge of their form and mobility. These relevant properties depend on the complex interactions of the elements of concern with the soil particles that generally cannot be addressed by a single technique. This study presents an integrated approach implementing geochemical and mineralogical investigation techniques on samples from a former mining area (Tolfa Mountains district, northern Latium, Italy), where exploiting activities occurred until the recent past. In particular, the As total concentration and the As distribution in solid phases is studied with the aim to evaluate the possibility of environmental pollution and consequent risks for the health of people living in the area and possibly affected in case of significant mobilization of this toxic element. Materials and methods: Chemical (ICP-MS, ICP-OES) and mineralogical (Xray diffraction (XRD), scanning electron microscopy–energy dispersive X-ray (SEM-EDX), X-ray absorption near-edge structure (XANES)) analyses and the evaluation of the heavy metal mobility (by means of a sequential extraction procedure) were performed. Results and discussion: Chemical analyses show a high As content in the soils collected immediately downstream the marcasite mine waste deposit, that is the starting point of As pollution over the whole area. XANES analyses show that As occurs in two oxidation states (AsIII and AsV) simultaneously in the tailing samples and nearby, while it has been mainly or totally transformed into AsV at increasing distance from the mine. XRD data show that sheet silicates don’t affect As behavior, whereas sequential extractions and SEM-EDX analyses reveal the evident association of As and Fe oxyhydroxides content. Conclusions: The use of an integrated geochemical-mineralogical approach allows to single out the contaminant forms and the relative potential threat. Arsenic in the surface level of such soils could be mobilized in the case of extreme meteorological or of a not well planned human event, even though there is no evidence yet of any serious contamination of either underground or flowing waters.