Mineralogical association and geochemistry of potentially toxic elements in urban soils under the influence of mining

Abstract

Polluted soil is an important source of exposure to potentially toxic elements (PTEs) for humans, especially in urban areas. We studied the fate of PTEs in the total (<2 mm) and fine (<50 μm) fractions of urban soils in playgrounds, passing areas, and vacant lots of the historic mining village of Minas de Riotinto in SW Spain. The mineralogical and chemical observations included analysis by scanning electron microscopy, electron backscatter diffraction, X-ray diffraction, chemical analysis of Al, V, Cr, Mn, Co, Ni, Cu, Zn, Rb, Sr, As, Cd, Ba, Tl, and Pb after acid digestion by Inductively Coupled Plasma-Mass Spectrometry and Sb by X-ray fluorescence. The total and fine fractions of natural and mixed (consisting of natural soils and aggregate pavements) urban soils have significantly higher concentrations of sulfide-associated PTEs (Ni, Cu, Zn, As, Sb, and Pb) and Ba in comparison to the aggregate pavements. Most of the natural and mixed urban soils surpass the regulatory levels set by the regional Government for As and Pb to declare a soil as contaminated. This work highlights the mineralogical source of PTEs in the urban soils. Primary geogenic sulfide minerals are prone to oxidation promoting dissolution of PTEs and acid generation in the future. Additionally, for the first time, we have described arsenian plumbojarosite and beudantite in urban soils which are abundant secondary phases under the circumneutral pH conditions, effectively retaining As and Pb. Inhalable small PTE-rich particles (<10 μm) are present in many soils in playgrounds and garden areas potentially posing health risk to residents upon dusting and resuspension in the air.