Speciation, mobility and adsorption effects of various metals in sediments in an agricultural area surrounding a uranium ore deposit (Nisa, Portugal)

Abstract

The availability and mobility of metals in soil strongly depend on its sediments geochemical forms and, therefore, the sediments’ particles composition evaluation is more meaningful than the estimation of total soil concentrations, particularly when risk and toxicity are under evaluation. The objective of the present study is to evaluate speciation, fractionation and mobility of the considered metals in sediment samples collected in the surrounding area of a very important unexploited uranium ore deposit located in the Alto Alentejo region of Portugal (Nisa). Sequential Extraction and analytical determinations using X-ray fluorescence technique of the tested materials allowed the determination of mobile fractions and the estimation of ionic speciation. Mobility factor for different elements was estimated, with some of them presenting high mobility rates for exchangeable ions and oxide fractions. The results evidence a high geochemical availability for U, Mo, Nb, Ca, Mn, and, in some particular cases for As, V and Cr. This availability is considerably lower for other elements such as Cu, Zn, Pb, Fe, Ba and Zr. Geochemically, regarding its form of occurrence, most of these metals are incorporated in the sediment matrixes as (oxide) secondary minerals and/or included in iron-exchangeable-oxyhydroxides precipitates. By contrast, binocular microscopy observations and Scanning Electron Microscopy analysis evidenced that U, As, Nb, Mo, and (but with less evidence) V and Cr, are adsorbed in iron-oxyhydroxides-clay minerals, being present as aggregates and as coatings at the surface of primary grains. It was possible to prospect that the majority of the more mobile metals, including uranium, probably occur as oxide or in secondary forms minerals being its origin related with weathering and dissolution of primary sources and subsequent transportation by local water courses in the dissolved fractions as hydrochemical solutes or included in colloids. Later, these metals precipitated as secondary forms and/or were adsorbed preferentially at the surface of the iron-oxyhydroxides-clay minerals precipitates.