Kinetics For Oxidation Of Bromophenols/Bromoanilines/Inorganic Iodine By Permanganate And Iodoform Formation Mechanism

Halogenated phenols and anilines have been widely used as chemical intermediates and then frequently detected in aquatic systems.Biological and photochemical reactions induced by iron and manganese oxides in soil and sediments can lead to the transformation and degradation of halogenated phenols in the environmental systems with the concomitant release of halides.As an oxidant,permanganate with high selectivity towards organics and stability has been widely applied in water treatment.Bromophenols and bromoanilines were chosen as the model halogenated organics and iodide as the model halide in this study.Then the oxidation characteristics of these target compounds by permanganate were investigated.In order to study the reactivity for the reaction of permanganate with the bromophenols and bromoanilines,2-bromophenol(2-BP),3-bromophenol(3-BP),4-bromophenol(4-BP),2-bromoaniline(2-BA),3-bromoaniline(3-BA)and 4-bromoaniline(4-BA)were chosen as the model phenol and aniline compounds.Regarding the lower reactivity between permanganate and these phenols and anilines,carbon nanotubes were introduced to study the synergistic enhancement of phenols and anilines removal.Permanganate decay and total manganese concentration decrease in the presence of CNTs indicate considerable formation of manganese dioxide(MnO2)on carbon surface.The CNTs-enhanced phenol degradation is more favorable under lower pH,confirming that the generation of MnO2 induced by the reaction between permanganate and CNTs enhances the phenol oxidation by permanganate.The first order rate constants of respective bromophenols and bromoanilines suggest the synergistic enhancement of CNTs and permanganate in removing these micropollutants.Bromide release experiments clearly demonstrate that the subsequent dehalogenation accompanying the oxidation of bromophenols and bromoanilines is significant for permanganate oxidation in the absence/presence of CNTs.Dehalogenation by permanganate follows the order of 4-BP > 2-BP > 3-BP and 4-BA > 2-BA > 3-BA,respectively.With respect to the significant dehalogenation in the oxidation of halogenated organic compounds by permanganate,iodide was chosen to study the reaction kinetics for permanganate with halo ions due to that no reactions occur between permanganate and chloride or bromide.Iodide oxidation and iodate formation were faster at lower pH.The apparent second order rate constants(kapp)for iodide oxidation by permanganate decrease with increasing pH from 29 M-1s-1 at pH 5.0,6.9 M-1s-1 at pH 7.0,to 2.7 M-1s-1 at pH 10.0.Iodate yields over HOI abatement decrease from 98% at pH 6.0 to 33% for pH(29)9.5,demonstrating that HOI disproportionation dominates HOI transformation by permanganate at pH(29)8.0.A kinetic model was proposed to simulate the reaction for permanganate with iodide or HOI.Mn(VII)-I-or Mn(VII)-HOI are formed in a fast pre-equilibrium process and the rate determining step is the subsequent electron transfer.For pH(27)6.0,HOI/I2 is quickly oxidized by permanganate to iodate whereas for pH(29)8.0,HOI/OI-undergoes a fast permanganate-mediated disproportionation.During the treatment of iodide-containing waters,the potential for the iodinated disinfection by-products(I-DBPs)formation is highest at p H 7.0~8.0 due to the long lifetime of HOI.MnO2 forms as a product from permanganate reduction and can exert an influence on the further transformation of HOI.Colloidal MnO2 was prepared by mixing stoichiometric amounts of permanganate and Na2S2O3 and the role of MnO2 on the HOI abatement and comparison of permanganate and MnO2 were explored.The maximum rate for HOI abatement is observed close to the pKa of HOI(10.4).Thus,it seems that both HOI and OI-are involved in the MnO2-catalyzed reaction.Two main mechanisms can be postulated.For pH 5.0~7.0,HOI oxidation is the major pathway with an iodate yield of ca.90%.For the pH range 8.0~11.0,the MnO2-promoted HOI disproportionation is dominant with an iodate yield of ~33%.Based on the different iodate yields and HOI abatement in presence of MnO2 in the pH range of 5.0~11.0,a kinetic model can be proposed.MnO2-enhanced HOI abatement is initiated by the formation of a MnO2-HOI complex in a fast pre-equilibrium step.At p H(27)7.0,the rate-determining step is a pH-dependent reaction,leading to the formation of IO2-and subsequently to iodate.For pH(29)8.0,the reaction between MnO2-HOI and OI-is the dominant reaction pathway in the MnO2-enhanced HOI disproportionation process.This kinetic model can well describe the oxidation or disproportionation process.The HOI abatement was measured in presence of MnO2 at pH 10.0 in analogy to permanganate.The HOI abatement promoted by permanganate is more effective than for MnO2.Reactivity and pathways of HOI with organic compounds exert significant impact on the formation of iodinated byproducts.The kinetics for the reactions of hypoiodous acid(HOI)with various phenols(phenol,4-nitrophenol,4-hydroxybenzoic acid),3-oxopentanedioic acid(3-OPA),and flavone were investigated in the pH range of 6.0~11.0.The apparent second order rate constants for the reactions of HOI with phenolic compounds,3-OPA,flavone and citric acid at pH 8.0 are 10~107 M-1s-1,(4.0?0.3)?103 M-1s-1,(2.5?0.2)?103 M-1s-1 and(27)1 M-1s-1,respectively.The effect of buffer type and concentration was investigated with acetate,phosphate and borate.All tested buffers promote the HOI reactions with phenols.The percentage of iodine incorparation for various(hydroxyl)phenolic compounds and two NOM extracts ranges from 5% to 98%,indicating that electrophilic aromatic substitution and/or eletron transfer can occur.The extent of these reactions depends on the number and relative position of the hydroxyl moieties on the phenolic compounds.Iodoform formation rates increase with increasing pH and iodoform yields increase from 9% to 67% for pH 6.0~10.0 for the HOI/3-OPA reactions.In the permanganate/HOI/3-OPA and permanganate/iodide/3-OPA system at pH(27)8.0,iodoform formation is elevated compared to the HOI/3-OPA system in absence of permanganate.For pH(29)8.0,in presence of permanganate,iodoform formation is significantly inhibited and iodate formation enhanced,which is due to a faster permanganate-mediated HOI disproportionation to iodate compared to the iodination process.The production of reactive iodine in real waters containing iodide in contact with permanganate may lead to the formation of iodinated organic compounds.