| Iodinated aromatic organics are important precursors of small molecular iodinated organics(e.g.,iodoform and mono-iodoacetic acid)with their toxicity generally higher than aliphatic organics as well as their chlorinated and brominated analogues.Formation of iodinated aromatic organics in water treatment processes shows great impact on water quality and safety.Hence,it has received significant scientific attention.Manganese oxidants(i.e.,permanganate(KMnO4))and manganese dioxide(MnO2))are relatively stable and commercially cheap,and they have been widely used to remove organic and inorganic pollutants in aqueous environment.However,studies show that small molecular iodinated organics are generated during KMnO4 and MnO2 treatment of iodide(I-)-containing waters,which significantly hinders their further application in water treatment.Yet,information on formation mechanism and regulation strategy of iodinated aromatic organics during KMnO4 and MnO2 oxidation remains unclear at present.In this work,phenolic compounds with different structures were selected.Formation mechaniusm of iodinated aromatic organic products during treatment of I--containing waters by KMnO4 and MnO2 was studied.Moreover,regulation strategy of using chemical oxidant peroxymonosulfate to control formation of iodinated aromatic organic products was also proposed.Impact of I-on transformation efficiency of various phenolic compounds by KMnO4 were first explored,along with discussions on related mechanisms.The experimental results showed that:Kinetic modeling using Me P as a model phenolic compound could well fit its transformation by KMnO4 under various conditions,suggesting that hypoiodous acid(HOI)played a key role in phenolic compounds transformation by KMnO4in the presence of I-.The higher the HOI exposures,the stronger the accelerating impact of I-on transformation of phenolic compounds.HOI exposure is related to the structure of phenolic compounds:Phenolic compounds with electron-donating moieties(e.g.,bisphenol A and triclosan)have high reactivity with KMnO4,thus a low HOI exposure was maintained in the system.In contrast,phenolic compounds with electron-withdrawing moieties(e.g.,bisphenol S and methylparaben)have low reactivity with KMnO4,therefore a high HOI exposure was maintained in the system.Iodinated aromatic organic products were generated along with the accelerated transformation of phenolic compounds by KMnO4 in the presence of I-.Electrophilic substitution of HOI on phenolic compounds by one-or multi-steps resulted in their formation.Generally,higher I-Ar Ogs concentrations were obtained in higher HOI exposure.Oxidation efficiency of the three selected bisphenols by MnO2 follows the order of bisphenol A>bisphenol AF>>bisphenol S under similar conditions,mainly resulting from the differences in their structures.The electron-donating alkyl moiety in bisphenol A enhanced its reaction with MnO2,while the electron-withdrawing fluoro-alkyl group in bisphenol AF and thio-oxygen group in bisphenol S reduced their reactivities with MnO2.Phenoxyl radicals were initially formed from MnO2 oxidation of bisphenols via one-electron transfer,and they could be further oxidized to hydroxylated,carbon-carbon or carbon-oxygen polymeric,and bond-cleavage products.Impact of I-on bisphenols oxidation by MnO2 were closely related to HOI exposures.The higher HOI exposure,the stronger the accelerating effect of I-.HOI exposure is determined by structure of phenolic compounds and solution p H.bisphenol A has a high reactivity towards MnO2 than I-,therefore lower HOI exposure was obtained in the system.I-has slight impact on bisphenol A oxidation by MnO2 at p H 5.0-9.0.Bisphenol AF and bisphenol S have lower reactivity towards than I-,resulting in higher HOI exposure.Therefore,I-obviously enhanced bisphenol AF and bisphenol S oxidation by MnO2 at p H 5.0-7.0.At p H 7.0-9.0,MnO2 cannot oxidize I-due to its decreased oxidative ability,wherein I-showed slight impact on bisphenol AF and bisphenol S oxidation by MnO 2.Iodinated aromatic organic products were additionally generated from MnO 2 oxidation of bisphenol AF and bisphenol S in the presence of I-,mainly resulting from the one-or multi-steps electrophilic substitution reactions of HOI with bisphenols and/or their oxidation products.Peroxymonosulfate(PMS)showed high reactivities towards both I-and HOI.The apparent second-order rate constants of PMS reaction with I-was up to 1.1×103M-1s-1 at neutral p H,which was much higher that of KMnO4 with I-(6.9 M-1s-1).The apparent second-order rate constant for PMS with HOI was only second to ozone but higher than KMnO4 in the p H range of 5.0-10.0 with its value reaching around 7.9×104 M-1s-1 at p H 10.0.The iodine mass balance analysis showed that HOI was oxidized to iodate(IO3-),a stable and non-toxic sink of iodine,by PMS.Total organic iodine and iodinated organics with small molecular weight(e.g.,iodoform and monoiodoacetic acid)were formed during PMS oxidation in the co-presence of natural organic matter and I-with their concentration decreasing with increasing p H.Formation of iodoform,and monoiodoacetic acid during PMS oxidation of model natural organic matters in the presence of I-was lower than those formed during chlorination and chloramination processes under similar conditions,mainly due to lower HOI exposures in PMS system.Addition of PMS remarkably reduced the accelerating effect of I-on methylparaben transformation by KMnO4 as well as formation of iodinated aromatic organic products in both synthetic and authentic waters.Stronger inhibitive effect of PMS on iodinated aromatic organic products formation during KMnO4 oxidation of methylparaben in the presence of I-were obtained at higher PMS dosage or p H,meanwhile higher levels of IO3-was formed.Similar inhibitive effect of PMS on formation of iodinated aromatic organic products during MnO2 oxidation of methylparaben were also observed. |
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