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.