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

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 CrxFe1–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 CrxFe1–x(OH)3 hydration. CrxFe1–x(OH)3 is oxidized through a well-defined pathway involving CrO3 as a key metastable intermediate, which sequentially decomposes into Cr5O12 and CrnFe2–nO3 at higher temperatures. The substitution of Fe(III) enhanced CrxFe1–x(OH)3 oxidation and lowered initial oxidation temperature, because Fe3d orbital hybridization with exogenous O2p 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, PO43–, SO42–, and Cl– ions were introduced to preferentially combine Cr(III) before the CrxFe1–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.