First-principles investigation of vanadium isotope fractionation in solution and during adsorption

Equilibrium fractionation factors of vanadium (V) isotopes among tri- (V(III)), tetra- (V(IV)) and penta-valent (V(V)) inorganic V species in aqueous system and during adsorption of V(V) to goethite are estimated using first-principles calculation. Our results highlight the dependence of V isotope fractionation on valence states and the chemical binding environment. The heavy V isotope (V-51) is enriched in the main V species following a sequence of V(III) < V(IV) < V(V). According to our calculations, at 25 degrees C, the equilibrium isotope fractionation factor between [V5+O2(OH)(2)](-) and [V4+O(H2O)(5)](2+) (In alpha(V(V)-V(IV))) is 3.9 parts per thousand, and the equilibrium isotope fractionation factor between [V5+O2(OH)(2)](-) and [V3+(OH)(3)(H2O)(3)] (In alpha(V(V)-V(III))) is 6.4 parts per thousand. In addition, isotope fractionation between +5 valence species [V5+O2(OH)(2)](-) and [V5+O2(H2O)(4)](+) is 1.5 parts per thousand at 25 degrees C, which is caused by their different bond lengths and coordination numbers (CN). Theoretical calculations also show that light V isotope (V-50) is preferentially adsorbed on the surface of goethite. Our work reveals that V isotopes can be significantly fractionated in the Earth's surface environments due to redox reaction and mineral adsorption, indicating that V isotope data can be used to monitor toxic V(V) attenuation processes through reduction or adsorption in natural water systems. In addition, a simple mass balance model suggests that V isotope composition of seawater might vary with change of ambient oxygen levels. Thus our theoretical investigations imply a promising future for V isotopes as a potential new paleo-redox tracer. (C) 2015 Elsevier B.V. All rights reserved.