Influences Of Ph And Humic Acid On Kinetics Of Phenols Oxidation By Permanganate

There has long been increasing occurrence of phenolic compounds consideredas the ubiquitous organic micro-pollutants and the major substituent of severaltopical emerging pollutants in various studies of aquatic environment.Permanganate as a green oxidant can be widely applied to control contamination indrinking water treatment processes and environmental remediation, however, themechanisms of phenols oxidation by permanganate is not clear. In this work, theinfluence of pH and humic acid (HA) on the kinetics of phenols oxidation bypermanganate were investigated, furthermore, linear free energy relationships(LFERs) were also established to understand the mechanisms of permanganateoxidation.The oxidation kinetics of phenol, chlorophenols, nitrophenols andmethylphenols by permanganate was determined at the pH range of5.0-9.0. Theexperiments implied that the rate constants of phenol oxidation by permanganateshowed a monotonic increase with increasing pH while that of pentachlorophenol(PCP) exhibited an opposite trend. The increasing pH-rate profiles of phenol couldnot be modeled by the acid-base speciation model, at the same time, the decreasingpH-rate profiles of PCP also could not be explained by the electron-withdrawingmechanism. These experiments indicated that the reaction rates of chlorophenolswith permanganate increased with pH to a maximum, then decreased with furtherpH increase, in a word, the pH-rate profiles of chlorophenols exhibitedparabola-like shapes with the maximum reaction rate obtained at pH close to theirrespective pKa. Furthermore, the pH-rate profiles of phenol and PCP showed themaximum of rates at pH=9.0and5.0, which also were the special examples ofparabola-like shapes (both pKaof phenol and PCP were beyond the pH range of5.0-9.0). A proton transfer mechanism is proposed in which the protonated phenol isdirectly oxidized by permanganate to generate products while aphenolate-permanganate adduct, intermediate, is formed between deprotonatedphenol and permanganate ion. Rate equations have been derived based on thecombined mechanisms include the direct oxidation of the protonated phenols bypermanganate and the reactions of phenolates with permanganate involving pronton transfer, moreover, the rate equations could modeled the parabola-like shapedpH-rate profiles well.LFERs were established based on the parameters obtained from the modeling,Hammett constants and oxygen Natural charges in phenols and phenolates. Theresults revealed that chlorine substituents could weak the susceptibility ofphenolates to permanganate oxidation because of the electrophilic reactions; on thecontrary, phenols were more easily oxidized than their dissociated counterpartssingularly which were very different from the other oxidation system. The oppositeresults were independent of permanganate, but were determined by thecharacteristics of chlorine substituents which could form intramolecular hydrogenbonding with the phenolic group, and shifted the oxidation potential of the phenols.The oxidation kinetics of nitrophenols and methylphenols by permanganate showedthe electro-withdrawing substituents could restrain the reaction rates ofpermanganate with phenols and phenolates.Phenol and chlorophenols oxidation by permanganate was enhanced by thepresence of HA increasing with pH at pH <7.0, and the influence of HA could beignored at neutral solution; while HA inhibited permanganate oxidation and showedthe parabola-like shaped pH-rate profiles at pH>7.0. Phenol and chlorophenolsoxidation by permanganate was enhanced by the presence of HA at pH <7.0, and theinfluence of HA could be ignored at neutral solution; while HA inhibitedpermanganate oxidation and showed the parabola-like shaped pH-rate profiles atpH>7.0. There were no differences of the oxidation kinetics of nitrophenols bypermanganate with and without HA, but the influence of pH on the oxidationkinetics of methylphenols was same to the chlorophenols. The presence of pH didnot change the electrophilic mechanism of permanganate oxidation according to theeffect of substituents. The UV-Vis spectra analysis indicated that the acceleration ofrate constants at pH <7.0with HA because HA could enhance the production ofMn(V). The increasing of Mn(V) could correspond to the degradation of phenolswithout HA. The mechanisms of HA were totally different from the enhancingeffect of some ligands such as phosphate which could combine intermediate stateMn(III) or Mn(II) in phenols oxidation by permanganate.