Long-term stability evaluation of reduced Cr-containing waste residue under aging test and the underlying Cr reoxidation mechanism

Abstract

The reduction of highly toxic Cr(VI) to immobilized Cr(III) using ferrite has been widely used to diminish the toxicity of Cr(VI)-containing waste residues (CWR). However, the Cr(III) in the reduced Cr-containing waste residues (RCWR) may be reoxidized into Cr(VI), posing potential environmental risks. In this work, the long-term stability of the real RCWR samples were evaluated through aging test, showing the reoxidation of Cr(III) to Cr(VI). In order to reveal the underlying mechanism of the Cr(III) reoxidation, FexCr1-x(OH)3, a common Cr(III)-containing mineral in the RCWR, was synthesized and used as simulated RCWR, in which the release and oxidation mechanism of Cr(III) were investigated under the influence of ambient conditions (such as sunshine irradiation and different pH). The results showed that FexCr1-x(OH)3 was stable at pH of 5 and 7, but became unstable at pH of 3 and pH of 9 ∼ 11. Specifically, the dissolution of Cr(III) and Fe(III) from FexCr1-x(OH)3 was greatly promoted by simulated sunshine irradiation at pH 3, while the oxidation rate of Cr(III) were increased by sunshine irradiation with the increasing pH value from 9 to 11. The mechanism study showed that Fe0.5Cr0.5(OH)3 is a n-type semiconductor with a band gap of 2.06 eV. When irradiated by light with wavelength below 602 nm, photo-generated electrons and holes can form and immigrate to the surface of particles. The promoted dissolution of Cr(III) at pH of 3 might be ascribed to ·OH deriving from photoexcitation of Fe0.5Cr0.5(OH)3, while the oxidation of Cr(III) to Cr(VI) at pH of 9 ∼ 11 may be related to photo-generated holes. This work contributes to clarifying the migration and transformation of Cr in FexCr1-x(OH)3 and provides a theoretical supporting for the management and disposal of CWR.