The mechanism for promoted oxygenation of V(IV) by goethite: Positive effect of surface hydroxyl groups

The most toxic and mobile form of vanadium (V) is V(V). Thus, the oxidation of V is a subject of considerable importance since the process may increase environmental risks of V. V(IV) is known to be stable when pH < 4.5. However, in this study, stable V(IV) was oxidized almost completely in goethite suspension within 60 min at pH 2.0 and pH 4.0, correspondingly, only 25%-30% and 15%-20% of V(IV) were oxidized in equimolar Fe³⁺ solution and the blank solution, respectively. The promoted oxidation of V(IV) by FeOOH is attributed to the formation of the complex, VO(OFe<)⁺ or VO(OFe<)₂. On the one hand, the formation of the complex caused higher pH in the interface of solid FeOOH and corresponding lower conditional potential of V(V)/V(IV) than those in bulk solutions, which facilitates the oxidation of V(IV). On the other hand, the surface group OFe < may serve as a medium that facilitating the electron transfer from V(IV) ions to dissolved oxygen molecules, also favoring the oxidation of V(IV). This study not only deepens our understanding of the transfer, transformation, and risk of V in environments, but also provides a reference to speculate the fate of similar elements in environments.