Green Rust Reduction of Chromium Part 2: Comparison of Heterogeneous and Homogeneous Chromate Reduction

• Published in: Journal of physical chemistry. C Vol. 114; no. 39; pp. 16408 - 16415
• Main Authors: Wander, Matthew C. F, Schoonen, Martin A. A
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 07.10.2010
• Subjects: C: Surfaces, Interfaces, Catalysis; CHROMATES; CHROMIUM; ELECTRON TRANSFER; Environmental Molecular Sciences Laboratory; FUNCTIONALS; IRON; MATERIALS SCIENCE; PASSIVATION; POLLUTANTS; SCRAP
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp1021328

White and green rusts are the active chemical reagents of buried scrap iron pollutant remediation. In this work, a comparison of the initial electron-transfer step for the reduction of CrO4 −2 by Fe2+ (aq) and Fe(OH)2(s) is presented. Using hybrid density functional theory and Hartree−Fock cluster calculations for the aqueous reaction, the rate constant for the homogeneous reduction of chromium by ferrous iron was determined to be 5 × 10−2 M−1 s−1 for the initial electron transfer. Using a combination of Hartree−Fock slab and cluster calculations for the heterogeneous reaction, the initial electron transfer for the heterogeneous reduction of chromium by ferrous iron was determined to be 1 × 102 s−1. The difference in rates is driven by the respective free energies of reaction: 33.4 vs −653.2 kJ/mol. This computational result is apparently the opposite of what has been observed experimentally, but further analysis suggests that these results are fully convergent with experiment. The experimental heterogeneous rate is limited by surface passivation from slow intersheet electron transfer, while the aqueous reaction may be an autocatalytic heterogeneous reaction involving the iron oxyhydroxide product. As a result, it is possible to produce a clear model of the pollutant reduction reaction sequence for these two reactants.