Indirect Electrochemical Oxidation of Chlorophenols in Dilute Aqueous Solutions

The indirect electrochemical oxidation of chlorophenols was studied. The process involved the simultaneous reduction of water to produce protons that were used for the subsequent electrochemical reduction of oxygen to generate hydrogen peroxide. In the presence of ferrous ions, a Fenton’s reagent was produced. In the process, upon reaction with hydrogen peroxide, ferrous ions were converted to the ferric state and were then regenerated at the cathode to recover ferrous ion—an important reactant in Fenton’s reagent. A series of chlorophenols, including 4-mono-, 6-dichloro-, and 4-trichlorophenol, were studied. The degradation of monochlorophenols proceeded in decreasing order: $3\mathrm{\text{-}}\mathrm{\text{chlorophenol}}>2\mathrm{\text{-}}\mathrm{\text{chlorophenol}}\sim 4\mathrm{\text{-}}\mathrm{\text{chlorophenol}}$; for dichloerophenols, the order was: 3, $5\mathrm{\text{-}}\mathrm{\text{dichlorophenol}}>3$, $4\mathrm{\text{-}}\mathrm{\text{dichlorophenol}}>2$, $3\mathrm{\text{-}}\mathrm{\text{dichlorophenol}}>2$, $5\mathrm{\text{-}}\mathrm{\text{dichlorophenol}}>2$, $4\mathrm{\text{-}}\mathrm{\text{dichlorophenol}}>2$, $6\mathrm{\text{-}}\mathrm{\text{dichlorophenol}}$;and for trichloro-phenols, the order followed: 2, 3, $5\mathrm{\text{-}}\mathrm{\text{trichlorophenol}}>2$, 3, $6\mathrm{\text{-}}\mathrm{\text{trichlorophenol}}>2$, 4, $5\mathrm{\text{-}}\mathrm{\text{trichlorophenol}}>2$, 4, $6\mathrm{\text{-}}\mathrm{\text{trichlorophenol}}$. The results indicated that the number and the relative locations (i.e., ortho, meta, or para) of chlorine substitution on the benzene ring played an important role in determining the rate and extent of chlorophenol oxidation. Regardless of the type of chlorine-substituted phenol, two major reaction pathways were responsible for the intermediate degradation of chlorophenols: hydroxyl radical attack (hydroxylation) and dechlorination. The detailed kinetics of the degradation of chlorophenols was studied in detail using 2-chlorophenol. Formic acid and acetic acid were the major end products that, in combination, contributed to 70% of the residual total organic carbon (TOC). Three other minor intermediates, 2-chloro-hydroquinone (2-Cl-HQ), 2-chloro-catechol (2-Cl-CA), and 4-hydroxyl,2,2’-dichlorodiphenyl ether (HDCDH), were observed; each contributed to a small percentage of the residual TOC.