Chromate reduction by zero-valent Al metal as catalyzed by polyoxometalate

In spite of a high reduction potential of zero-valent Al (ZVAl), its ability to reduce Cr(VI), a widespread pollutant, to less toxic Cr(III) remains to be uncovered. In the present study, Cr(VI) reduction by ZVAl was conducted to evaluate the potential application of Al as a reductant for Cr(VI). Polyoxometalate (POM, HNa₂PW₁₂O₄₀), a catalyst, was used to accelerate Cr(VI) reduction by Al. The reaction of 0.192 mM Cr(VI) on ZVAl was investigated in the presence of N₂ or O₂ at pH 1. A slight decrease in Cr(VI) concentration was observed on as-received (uncleaned) ZVAl due to the presence of oxide layer with a low surface area (ca. 3.4 × 10^-3 m²/g) of ZVAl. On addition of 0.1 mM POM, Cr(VI) reduction on uncleaned ZVAl increased significantly. This is attributed to the unique properties of POM, which has a Brφnsted acidity higher than usual inorganic acids such as H₂SO₄ and HCl. Thus, POM could remove rapidly the oxidize layer on ZVAl, followed by acting as a shuttle for electron transfer from ZVAl to Cr(VI). Under a N₂ atmosphere, one- or two-electron reduction of POM by ZVAl was responsible for Cr(VI) reduction in the early stage of the reaction. However, during reaction with ZVAl over 120 min, three-electron reduction of POM predominated over Cr(VI) reduction. On interaction of O₂ with reduced POM, the formation of H₂O₂ was responsible for subsequent Cr(VI) reduction. The results suggest that POM is an efficient catalyst for Cr(VI) reduction by Al due to the extremely rapid consumption of reduced POM or H₂O₂ by Cr(VI).