Oxidative degradation of p-chlorophenol by ferrate(VI): Kinetics, intermediates and pathways

The ability of ferrate(VI) (Fe(VI)) to remove p-chlorophenol (4-CP) during water treatment was evaluated by determining the degradation efficiency, kinetics, stoichiometry, and mechanism. pH and reactants molar ratio significantly affect 4-CP degradation efficiency. The optimal degradation was found to occur at pH 9.20, which coincides closely with pK(a) value for 4-CP (pK(a) = 9.38). 4-CP was completely degraded by Fe(VI) at a 5:1 or higher mole ratio. The kinetics of the oxidation of 4-CP by Fe(VI) were studied as a function of pH (7.59-10.96) and temperature (20.0-33.5 degrees C) using the ABTS method. The reaction of Fe(VI) with 4-CP was found to be first-order with respect to each reactant and second-order overall. The apparent rate constants of the reaction decreased non-linearly from 353.98 +/- 10.93 to 5.91 +/- 0.94 M-1 s(-1) with an increase in pH (7.59-10.96). The pH dependence of the rate constants of the reaction between Fe(VI) and 4-CP was explained by using acid-base equilibrium of Fe(VI) and 4-CP. Four parallel reactions could be considered as plausible reactions between Fe (VI) and 4-CP species, but the reaction between deprotonated Fe(VI) and deprotonated 4-CP was the rate-determining step. Organic chlorine intermediates and chloride ion were identified using HPLC-MS and IC, and the pathways for degradation of 4-CP by Fe(VI) were proposed. The results suggest that Fe(VI) is a promising oxidant for mineralizing hazardous 4-CP to form nontoxic products in water and wastewater treatment.