Understanding the Reduction Kinetics of Aqueous Vanadium(V) and Transformation Products Using Rotating Ring-Disk Electrodes

Vanadium(V) is an emerging contaminant in the most recent Environmental Protection Agency's candidate contaminant list (CCL4). The redox chemistry of vanadium controls its occurrence in the aquatic environment, but the impact of vanadium(V) speciation on the redox properties remains largely unknown. This study utilized the rotating ring-disk electrode technique to examine the reduction kinetics of four pH- and concentration-dependent vanadium(V) species in the presence and the absence of phosphate. Results showed that the reduction of VO₂⁺, H_xV₄O_12+x^(4+x)- (V₄), and HVO₄^2- proceeded via a one-electron transfer, while that of Na_xH_yV₁₀O₂₈^(6-x-y)- (V₁₀) underwent a two-electron transfer. Koutecky-Levich and Tafel analyses showed that the intrinsic reduction rate constants followed the order of V₁₀ > VO₂⁺ > V₄ > HVO₄^2-. Ring-electrode collection efficiency indicated that the reduction product of V₁₀ was stable, while those of VO₂⁺, HVO₄^2-, and V₄ had short half-lives that ranged from milliseconds to seconds. With molar ratios of phosphate to vanadium(V) varying from 0 to 1, phosphate accelerated the reduction kinetics of V₁₀ and V₄ and enhanced the stability of the reduction products of VO₂⁺, V₄, and HVO₄^2-. This study suggests that phosphate complexation could enhance the reductive removal of vanadium(V) and inhibit the reoxidation of its reduction product in water treatment.