Mechanism And Kinetics Of The Degradation Of Pentachlorophenol By The Ultrasound/zero-valent Iron System

In this paper, the mechanism and kinetics of the degradation of pentachlorophenol by the ultrasound/zero-valent iron system (US/Fe⁰) were further reported based on the success of the prophase's researches. Its degradation intermediates were analyzed and identified by the Gas Chromatograph-Mass Spectrometer. On the basis of the above studies, the degradation pathway of pentachlorophenol was proposed. The following is main achievements. The achievements would provide a basis for the better understanding removal of other chlorophenols.(1) The degradation of pentachlorophenol in the US/Fe⁰ system was found to obey the pseudo-first-order reaction. The pseudo-first-order rate constant in the US/Fe⁰ system is 4 times as big as that of ultrasound alone and 15 times as big as that of zero-valent iron alone, and it is more than the sum of the latter two. With the initial PCP concentration of 0.5-20 mg?L-1, the initial pH range of 2-8 and the iron loading range of 0.5-6 mg?L-1, the rate constant in the combined system decreases with the increase of the initial concentration and the initial pH value, but it increases with the increase of the dosage of iron.(2) In the US/Fe0 system, the results indicate that the pentachlorophenol degradations are mainly attributed to the oxidation of hydroxyl radicals and the reduction of iron, and the oxidation of hydroxyl radicals plays a dominant role.(3) There are four main pathways in the hydroxyl radical production: molecular pyrolysis in the the cavitation process, Fenton reaction, the synergism of ultrasound and modified Fenton reaction, and the granular effect of Fe0. Its production was greater at pH value of 2 and 3 and the iron dosage of 2.0 g·L^-1.(4) Characterization of iron surface is highly improved by erosion, fragmentation, cleaning, cavitation erosion and melted of the acoustic cavitation; the effect of ultrasonic on the PCP degradation is strengthened by iron through advancing the generation of hydroxyl radical.(5) The ferrous ion and ferric ion were detected in the combined system, and the former concentration is larger than the latter. The ferrous ion can enhance the catalytic decomposition of hydrogen peroxide as to increasing the hydroxyl radical quantity. The infrared spectrophometry analysis of iron polymeric compound had indicated the presence ofδ-FeOOH,γ?FeOOH,α-Fe2O3 and Fe-OH-Fe.(6) 2,3,4,6-TeCP, TeCHQ and OCDD are identified as intermediates in the US/Fe0 system. Three pathways of the oxidative dechlorination of hydroxyl radicals, the coupling of pentachlorophenoxy radicals and the zero-valent iron dechlorination of pentachlorophenol are proposed on the basis of major intermediates and correlation studies.