Chemical Pathway and Kinetics of Phenol Oxidation by Fenton's Reagent

摘要

Phenol oxidation by Fenton's reagent(H_2O_2 + Fe~(2+))in aqueous solution has been studied in depth for the purpose of learning more about the reactions involved and the extent of the oxidation process,under various operating conditions.An initial phenol concentration of 100 mg/L was used as representative of a phenolic industrial wastewater.Working temperatures of 25 and 50 deg C were tested,and the initial pH was set at 3.The H_2O_2 + Fe~(2+)doses were varied in the range of 500-5000 and 1-100 mg/ L,respectively,corresponding to 1-10 times the stoichio-metric ratio.A series of intermediates were identified,corresponding mainly to ring compounds and short-chain organic acids.Most significant among the former were catechol,hydroquinone,and p-benzoquinone;the main organic acids were maleic,acetic,oxalic,and formic,with substantially lower amounts of muconic,fumaric,and malonic acids.Under milder operating conditions(H_2O_2 and Fe~(2+)at lower concentrations),a great difference was found between the measured total organic carbon(TOC)and the amount of carbon in all analyzed species in the reaction medium.This difference decreased as the doses of H_2O_2 and Fe~(2+)increased,indicating that the unidentified compounds must correspond to oxidation intermediates between phenol and the organic acids.To establish a complete oxidation pathway,experiments were carried out using each of the identified intermediates as starting compounds.Dihydroxybenzenes were identified in the earlier oxidation stages.Muconic acid was detected in catechol but not in the hydroquinone and p-benzoquinone oxidation runs;the last two compounds were oxidized to maleic acid.Oxalic and acetic acid appeared to be fairly refractory to this oxidation treatment.A detailed knowledge of the time evolution of the oxidation intermediates is of environmental interest particularly in the case of hydroquinone and p-benzoquinone because their toxicities are several orders of magnitudes higher than that of phenol itself.The time evolution of the intermediates and TOC was fitted to a simple second-order kinetic equation,and the values of the kinetic constants were determined.This provides a simplified approach useful for design purposes.