Oxidation of chromium(Ⅲ): A potential risk of using chemical oxidation processes for the remediation of 2-chlorophenol contaminated soils

Chemical oxidation processes are widely used for the remediation of organically contaminated soils, but their potential impact on variable-valence and toxic metals such as chromium (Cr) is often overlooked. In this study, we investigated the risk of Cr(Ⅲ) oxidation in soils during the remediation of 2-chlorophenol (2-CP) contaminated soils using four different processes: Potassium permanganate (KMnO 4 ), Modified Fenton (Fe 2+ /H 2 O 2 ), Alkali-activated persulfate (S 2 O 8 2− /OH − ), and Fe 2+ -activated persulfate (S 2 O 8 2− /Fe 2+ ). Our results indicated that the KMnO 4 , Fe 2+ /H 2 O 2 , and S 2 O 8 2− /Fe 2+ processes progressively oxidized Cr(III) to Cr(Ⅵ) during the 2-CP degradation. The KMnO 4 process likely involved direct electron transfer , while the Fe 2+ /H 2 O 2 and S 2 O 8 2− /Fe 2+ processes primarily relied on HO• and/or SO 4 • - for the Cr(III) oxidation. Notably, after 4 h of 2-CP degradation, the Cr(VI) content in the KMnO 4 process surpassed China's 3.0 mg kg −1 risk screening threshold for Class I construction sites, and further exceeded the 5.7 mg kg −1 limit for Class II construction sites after 8 h. Conversely, the S 2 O 8 2− /OH − process exhibited negligible oxidation of Cr(III), maintaining a low oxidation ratio of 0.13%, as highly alkaline conditions induced Cr(III) precipitation, reducing its exposure to free radicals . Cr(III) oxidation ratio was directly proportional to oxidant dosage, whereas the Fe 2+ /H 2 O 2 process showed a different trend, influenced by the concentration of reductants. This study provides insights into the selection and optimization of chemical oxidation processes for soil remediation , emphasizing the imperative for thorough risk evaluation of Cr(III) oxidation before their application.