Revealing the Oxidation Mechanism of Cr(III)–Fe(III) Hydroxides during Thermal Drying of Cr-Containing Sludge: Molecular Pathway and Inhibition Strategy

Abstract

Thermal drying (100–300 °C) is usually required to reduce moisture from chromium (Cr)-containing sludge before incineration, storage, or other resource recovery processes. Yet, part of trivalent chromium (Cr­(III)) is oxidized to the toxic hexavalent chromium (Cr­(VI)) during thermal drying. Currently, the molecular-level Cr­(III) oxidization pathways during thermal drying remain poorly understood. In this paper, the molecular reaction pathway and the influence of Fe­(III) substitution on Cr_xFe_1–x(OH)₃ oxidation in thermal drying are clearly elucidated. CrO₃ is identified as a formed Cr­(VI) product, and the oxidation pathway is heavily dependent on temperature and Cr_xFe_1–x(OH)₃ hydration. Cr_xFe_1–x(OH)₃ is oxidized through a well-defined pathway involving CrO₃ as a key metastable intermediate, which sequentially decomposes into Cr₅O₁₂ and Cr_nFe_2–nO₃ at higher temperatures. The substitution of Fe­(III) enhanced Cr_xFe_1–x(OH)₃ oxidation and lowered initial oxidation temperature, because Fe_3d orbital hybridization with exogenous O_2p orbitals facilitated electron transfer between Cr–O systems, and expanded electron transition regions. Since Cr­(OH)O is an essential intermediate product during the Cr­(VI) formation process, PO₄^3–, SO₄^2–, and Cl^– ions were introduced to preferentially combine Cr­(III) before the Cr_xFe_1–x(OH)₃ dehydration process, thereby preventing the generation of Cr­(OH)O and subsequent Cr­(VI) products. This study provides fundamental insights into the molecular-level mechanism of Cr­(III) oxidation during thermal drying of Cr-containing sludge.