Insight Into the Effects of Environmental and Structural Factors on the Re-Oxidation of Cr(III) Hydroxides

Chromium hydroxide is an important form present in chromium chemicals and a major product in the reduction of hexavalent chromium pollutants, and the study of chromium hydroxide re-oxidation process is crucial in controlling chromium pollution. The aim of this research was to investigate the re-oxidation performance of different forms of chromium hydroxide in air: crystalline chromium hydroxide (C-Cr(OH)₃), amorphous chromium hydroxide (A-Cr(OH)₃), chromium hydroxide obtained by reduction (R-Cr(OH)₃), and aged R-Cr(OH)₃ (Aged-R-Cr(OH)₃). The results showed that A-Cr(OH)₃ had the highest re-oxidation efficiency and the largest re-oxidation rate constant (k), followed by R-Cr(OH)₃, Aged-R-Cr(OH)₃, and C-Cr(OH)₃. The study found that the re-oxidation rate of chromium hydroxide was mainly affected by the surface Cr–O bond energy and physical water. The advantageous re-oxidation of chromium hydroxide could be attributed to its diminutive bond energy of Cr–O and the presence of physical water on its surface. It was observed that increasing the temperature and adding salt (Na₂SO₄ and Na₂CO₄) promoted the re-oxidation of Cr(III) for different chromium hydroxides. This effect was particularly noticeable under alkaline conditions induced by Na₂CO₃ or at a reaction temperature of 200 °C. The re-oxidation rate constant of chromium hydroxides was up to 39.4 times higher at a reaction temperature of 200 °C than at 80 °C. This would be of great significance for chromium contamination removal by controlling the hexavalent chromium reduction products and environmental conditions.