Vanadium sorption by mineral soils: Development of a predictive model.

The toxicity of vanadium in soils depends on its sorption to soil components. Here we studied the vanadate(V) sorption properties of 26 mineral soils. The data were used to optimise parameters for a Freundlich equation with a pH term. Vanadium K-edge XANES spectroscopy for three selected soils confirmed that the added vanadate(V) had accumulated mostly as adsorbed vanadate(V) on Fe and Al hydrous oxides, with only minor contributions from organically complexed vanadium(IV). Data on pH-dependent V solubility for seven soils showed that on average 0.36 H+ accompanied each V during adsorption and desorption. The resulting model provided reasonable fits to the V sorption data, with r2 > 0.99 for 20 of 26 soils. The observed KdS value, i.e. the ratio of total to dissolved V, was strongly dependent on V addition and soil; it varied between 3 and 4 orders of magnitude. The model was used to calculate the Freundlich sorption strength (FSS), i.e. the amount of V sorbed at [V] = 2.5 mg L-1, in the concentration range of observed plant toxicities. A close relationship between FSS and oxalate-extractable Fe and Al was found (r2 = 0.85) when one acidic soil was removed from the regression. The FSS varied between 27 and 8718 mg V kg-1, showing that the current environmental guidelines can be both under- and overprotective for vanadium.