Enhanced oxidative activation of chlorine dioxide by divalent manganese ion for efficient removal of PAHs in industrial soil

Chlorine dioxide (ClO2) takes effect as a promising chemical oxidant to remove polycyclic aromatic hydrocarbons (PAHs) from contaminated soil in situ. However, the conventional ClO2 degradation procedure is limited by its reaction selectivity to organic pollutants, as well as its low economic efficiency with huge dosage. Systematical study of activated ClO2 oxidation which is more efficient to remove PAHs in soil is still scarce. Herein, the impact of divalent manganese ion [Mn(Ⅱ)] on activating ClO2 to oxidize PAHs in real contaminated soil was currently studied, including the kinetics, degradation products, mechanisms, and biological toxicity. The results revealed that the PAHs removal rate constants (k1) had a quadratic relationship with Mn(II) concentrations (k1 = -0.005 × [Mn(Ⅱ)]2 + 0.0033 × [Mn(Ⅱ)] + 0.0222, R2 = 0.9084, [Mn(Ⅱ)] = 0 ∼ 5 mM), the best degradation performance was obtained when the molar ratio of Mn(II)/ClO2 was 1/20. Furthermore, the optimal initial ClO2 concentration of 2500 mg/L could satisfy the comprehensive considerations of efficiency and economy. At the pH range of 3 ∼ 11, the removal efficiency decreased with increasing pH. Four representative PAH congeners (anthracene, phenanthrene, pyrene, and fluoranthene) had been chlorine substituted and oxygenated, but benzene ring rupture was observed only in fluoranthene simultaneously. XPS spectra, EPR analysis, and quenching experiments indicated the mechanisms that HOCl and •OH were the principal active species in the oxidation reaction. The toxicity evaluation by earthworms Eisenia fetida resulted out that high toxic effect to organism still remained in remediated soil which should be paid attention to.