Degradation of 2,6-dichlorophenol by ferrate (VI) oxidation: Kinetics, performance, and mechanism

Abstract

• Complete removal of 2,6-DCP by Fe(VI) was achieved at a 6:1 or higher molar ratio. • The reaction activation energy was obtained using temperature dependence of rate constants. • The reaction rate constants were found to decrease non-linearly as pH increases. • Species-specific rate constants were calculated using pH dependence of speciation. • A detailed dechlorination and ring-opening mechanism of 2,6-DCP degradation by Fe(VI) was proposed. The kinetics, performance and mechanism of the oxidative degradation of 2,6-dichlorophenol (2,6-DCP) by ferrate(VI) (Fe(VI)) were investigated in this study. The kinetics of oxidation of 2,6-DCP with Fe(VI) were studied as a function of pH (8.23–10.45) and temperature (20.0–33.5 ℃). The reaction follows a second-order rate law with first order in each reactant. The reaction rate constants were found to decrease non-linearly from 265.14 ± 14.10 M −1 ·s −1 at pH 8.23 to 9.80 ± 0.72 M −1 ·s −1 at pH 10.45. The individual species-specific second-order rate constants were calculated using pH dependence of species distributions of Fe(VI) (HFeO 4 - and FeO 4 2- ) and 2,6-DCP (2,6-DCP and 2,6-DCP - ). The reaction of deprotonated 2,6-DCP with protonated Fe(VI) was found to occur fastest among four parallel reactions between Fe(VI) and 2,6-DCP species. The correlation between temperature and rate constant showed that the activation energy of the reaction was 19.00 ± 1.82 kJ·mol −1 . Removal performance depends on pH and molar ratio of Fe(VI) to 2,6-DCP. 2,6-DCP degradation efficiency decreased with increasing pH, which was in accord with pH dependence of the reaction constants. Complete removal of 2,6-DCP by Fe(VI) was achieved at a 6:1 or higher molar ratio. The oxidized products (OPs) of 2,6-DCP were identified using high performance liquid chromatography - mass spectrometry (HPLC-MS) and ion-selective electrode (ISE), and a dechlorination and ring-opening mechanism of 2,6-DCP degradation by Fe(VI) was proposed in detail. The results indicate that it is feasible and highly efficient for 2,6-DCP degraded by Fe(VI) to form nontoxic products.