Assessment of vanadium sorption by different soils.

Vanadium is a white bright metal that belongs to group 5 in the periodic table of elements. It can exist in different oxidation states from -2 to +5 although the forms can be found naturally in the environment are (III), (IV) and (V). As vanadium is toxic at high concentrations, and as vanadium is a common contaminant from e.g., steel slags, more detailed knowledge on the environmental behavior of this metal is required. One important property is its sorption to soils, as this will determine the bioavailability and the risk of leaching from soils. In surface soils vanadium(V) is commonly the predominating redox species. Therefore the purpose of this study was to determine vanadium(V) sorption in 7 different soils in order to investigate the factors determining vanadium(V) sorption and to estimate the capacity of the soils to bind vanadium. From laboratory adsorption experiments, vanadium sorption has been studied as a function of pH, vanadium(V) concentration, and phosphorus status. The adsorbed vanadium(V) of investigated soils was compared on the basis of the Freundlich parameters m and log Kf. The clay content of the soil and the content of oxalate soluble iron and aluminum were two important factors for the vanadium(V) sorption behavior. The higher the values of these soil properties, the stronger was vanadium(V) sorption. Among the soils investigated here the sorption strength was highest for the Kungängen A3 soil and then decreased in the following order Säby, Kungsängen D3, Pustnäs, Termunck, Guadalajara and Zwijnaarde. It is notable that the three soils with the strongest vanadium(V) sorption were clay soils, whereas the other four were sandy or silty soils. The pH dependence of vanadium sorption was also determined. The results show that the percentage sorbed vanadium(V) increases with decreasing pH. This is due probably to the anion properties of vanadium(V) (i.e. vanadate) in combination with increased positive surface charge on the soil colloids at lower pH. Moreover there is a competition between phosphate and vanadium(V) for sorption sites, which will cause less vanadium(V) sorption in soils. Therefore both the pH value and the phosphorus status are two additional factors that influence the vanadium sorption properties of soils