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 (Fe2+/H 2 O 2), Alkali-activated persulfate (S 2 O 8 2−/OH−), and Fe2+-activated persulfate (S 2 O 8 2−/Fe2+). Our results indicated that the KMnO 4 , Fe2+/H 2 O 2 , and S 2 O 8 2−/Fe2+ processes progressively oxidized Cr(III) to Cr(Ⅵ) during the 2-CP degradation. The KMnO 4 process likely involved direct electron transfer, while the Fe2+/H 2 O 2 and S 2 O 8 2−/Fe2+ 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 Fe2+/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. [Display omitted] • Environmental risk associated with Cr(III) oxidation was investigated in the remediation of 2-CP contaminated soil. • Cr(III) in soil oxidized gradually during 2-CP degradation in the KMnO 4, Fe2+/H 2 O 2 , and S 2 O 8 2−/Fe2+ processes. • The KMnO 4 process generated the highest content of Cr(VI) at 5.75 mg kg−1 among all tested systems. • The S 2 O 8 2−/OH− process demonstrated the lowest Cr(III) oxidation risk. • Direct electron transfer mediated the oxidation of Cr(III) in the KMnO 4 process. [ABSTRACT FROM AUTHOR]