Kinetics of Inhibition of Dichlorophenol Oxidation by NaCl over Fe-Cu-Pillared Clay

Abstract

Catalytic wet peroxide oxidation (CWPO) of dichlorophenols (DCPs) over Fe-Cu-pillared clay (PILC) without and with NaCl was investigated. Concentration change dynamics in H2O2, Cl−, and hydroxyl radicals (·OH) during DCP oxidation were also studied. Additionally, the intermediates formed were detected by liquid chromatography-mass spectroscopy (LC-MS). The results showed that the presence of NaCl inhibited the DCP oxidation rate to some extent, where induction time (TI) was lengthened from 75.9 to 93.6 min, and half-decomposition time (T1/2) was extended to 20.6 min (from 86.6 to 107.2 min) as compared with control (without NaCl addition). However, NaCl only strongly inhibited the oxidation rate of ortho-substituted DCPs such as 2,6-DCP, where TI was lengthened to 80 min (from 290.7 to 370.4 min), but not meta-DCPs or para-DCPs. Further investigation showed that NaCl did not affect the conversion (dechlorination and hydroxylation) of DCPs and also did not change the DCP oxidation path. It was hypothesized that the inhibitory mechanism of NaCl on ortho-DCP oxidation was mainly due to inhibiting the rate-limiting step of H2O2 decomposition (lengthening the induction period from 350 to 510 min) by competing with H2O2 for the complexation of Fe (III), secondary due to scavenging of ·OH (about 21%) to form chlorine radicals such as ·Cl2−.