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), Modified Fenton (Fe/HO), Alkali-activated persulfate (SO/OH), and Fe-activated persulfate (SO/Fe). Our results indicated that the KMnO, Fe/HO, and SO/Fe processes progressively oxidized Cr(III) to Cr(Ⅵ) during the 2-CP degradation. The KMnO process likely involved direct electron transfer, while the Fe/HO and SO/Fe processes primarily relied on HO• and/or SO• for the Cr(III) oxidation. Notably, after 4 h of 2-CP degradation, the Cr(VI) content in the KMnO process surpassed China's 3.0 mg kg risk screening threshold for Class I construction sites, and further exceeded the 5.7 mg kg limit for Class II construction sites after 8 h. Conversely, the SO/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/HO 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.