Effects Of Pre-Oxidation By Mn(Ⅶ) Or Fe(Ⅵ) Coupling S(Ⅳ) Systems On Disinfection By-Products Generation During Subsequent Chlorination

Drinking water security heavily relates health and life of human being,as well as industrial production.Therefore,disinfection is a crucial point during the process of drinking water treatment.Since the early 1990s,chlorine has become the most common chemical disinfectant used in water treatment.But chlorine can react with organic matter to produce toxic and harmful disinfection by-products(DBPs).Thus,how to control DBPs generation during chlorination has become a very important issue in drinking water treatment.In the process of water treatment,pre-oxidation is a common method to remove pollutants and control DBPs generation during chlorination,because it can change the composition and structure of precursors,mineralize some organic matters,and enhance coagulation.Recently,permanganate(Mn(Ⅶ))or ferrate(Fe(Ⅵ))coupling sulfite(S(Ⅳ))system,as a new kind of advanced oxidation technology,has excellent performance in degrading organic pollutants,and has received wide attention at home and abroad.There is still much work to be done to evaluate the application prospect of this kind of oxidation technology in water treatment process,of which the effects of these oxidation technologies as pre-oxidation methods on DBPs generation during chlorination is indispensable to be studied.Therefore,Mn(Ⅶ)or Fe(Ⅵ)coupling S(Ⅳ)systems were selected as the pre-oxidation technology in this paper to evaluate the influence of pre-oxidizing different precursors on DBPs generation during subsequent chlorination and the mechanisms were explored therein.Four DBPs precursors were selected,including humic acid,halide ions,and two artificial halogenated organic pollutants:iopamidol(IPM)and tetrabromobisphenol A(TBBPA).Mn(Ⅶ)/S(Ⅳ)or Fe(Ⅵ)/S(Ⅳ)systems pre-oxidation could reduce the molecular weight of humic acid and the intensity of three-dimensional fluorescence absorption peaks,indicating that the structure of humic acid was changed and bond fracture occurred inside the molecule.Nevertheless,pre-oxidation hardly affected the UV254 of humic acid solutions,suggesting that C=C groups which had UV absorption at 254 nm were not influenced.Since C=C groups are important precursors of trichloromethane,dichloroacetic acid,and trichloroacetic acid,the pre-oxidation of humic acid by Mn(Ⅶ)/S(Ⅳ)or Fe(Ⅵ)/S(Ⅳ)systems had little effect on generation of the three DBPs during subsequent chlorination.During chlorination,trichloromethane,dichloroacetic acid,and trichloroacetic acid were major DBPs,thus,pre-oxidation would not increase the toxicity risk arisen from DBPs.Sulfate radical(SO4·-)is one of the most important actⅣe oxidants in Mn(Ⅶ)/S(Ⅳ)system and can react with I-to form IO3-.Nevertheless,S(Ⅳ)in Mn(Ⅶ)/S(Ⅳ)system can inhibit the formation of reactive iodine species during the reaction between SO4·-and I-,thus inhibiting the generation of IO3-.By constructing a competitive reaction system and combining with the type distribution model,the second order reaction rate constant between S(Ⅳ)and HOI/OI-was obtained.The reduction ability of S(Ⅳ)to HOI/OI-enhanced with the increase of pH from 7.0 to 9.0.In Fe(Ⅵ)/S(Ⅳ)system,excessⅣe S(Ⅳ)could inhibit the oxidation of I-by Fe(Ⅵ)obviously.The generation of bromate was not observed during the oxidation of Br-by Mn(Ⅶ)/S(Ⅳ)or Fe(VI)/S(Ⅳ)technologies.During the chlorination of Br-,six brominated DBPs(Br-DBPs)were detected.The pre-oxidation of Br-by Mn(Ⅶ)or Fe(Ⅵ)coupling S(Ⅳ)systems had slight effects on the formation of DBPs,but the formation of several Br-DBPs were enhanced.Only iodoacetic acid(IAA)was detected during the chlorination of I-and pre-oxidation by Mn(Ⅶ)/S(Ⅳ)or Fe(VI)/S(Ⅳ)systems could lead to the increased of IAA.Especially at pH 7.0,the concentration of IAA in pre-oxidation groups were 4.9 times as much as that in non-pre-oxidation group.In this study,the removal of IPM in Mn(Ⅶ)/S(Ⅳ)system was improved from 56%to 93%by dosing S(Ⅳ)in batches at pH 7.0.The results of competitive dynamics experiment and radical-quenching experiment showed that the major active oxidizing agent for IPM degradation in Mn(Ⅶ)/S(Ⅳ)system was SO4·-.This study compared the experimental results between Mn(Ⅶ)/S(Ⅳ),UV,UV/H2O2,and UV/Cl2 systems.Only I-release was observed during the degradation of IPM in UV and UV/H2O2 systems,while the transformation of I-to IO3-could also be observed in UV/Cl2 and Mn(Ⅶ)/S(Ⅳ)systems.Furthermore,compared with the three UV-based oxidation systems,pre-oxidation of IPM by Mn(Ⅶ)/S(Ⅳ)system could be more beneficial to control DBPs generation during subsequent chlorination.After the removal of IPM increased to 90%,the chlorination of work solutions began and four I-DBPs were detected.Pre-oxidation of IPM by four oxidation systems could enhance the generation of I-DBPs during chlorination.Among the four oxidation systems,Mn(Ⅶ)/S(Ⅳ)system was most beneficial to controlling the formation of DBPs during disinfection.In Fe(Ⅵ)/S(Ⅳ)system,CaSO3 was selected as a slow-releasing source of S(Ⅳ).With the increase of pH from 7.0 to 9.0,the apparent rate constant of TBBPA degradation in Fe(Ⅵ)/S(Ⅳ)system decreased gradually,which could be attributed to the weaker oxidation capacity of Fe(Ⅵ)and slower release rate of S(Ⅳ)from CaSO3 at higher pH value.At pH 7.0-8.0,the addition of 5.0 mg C/L humic acid significantly inhibited the degradation of TBBPA in Fe(Ⅵ)/S(Ⅳ)system,while dosing lower concentration of humic acid had little effect.TentatⅣe pathways for TBBPA degradation in Fe(Ⅵ)/S(Ⅳ)system were proposed,including electron abstraction,debromination,and ring-opening pathways,accompanied by the release of Br-.In the presence of HA,pre-oxidation of TBBPA by Fe(Ⅵ)/S(Ⅳ)system could decrease the generation of DBPs during post chlorination at pH 8.0 and the toxicity risk was reduced by～20%.Pre-oxidation by Fe(Ⅵ)/S(Ⅳ)system could degrade 98%of TBBPA in real water at pH 8.0 and the toxicity risk of DBPs during chlorination could be reduced by～10%.In conclusion,the results indicated that pre-oxidation of humic acid or Br-by Mn(Ⅶ)/S(Ⅳ)or Fe(Ⅵ)/S(Ⅳ)systems would hardly increased the toxicity risk of DBPs during subsequent chlorination.Nevertheless,pre-oxidation of I-by these two systems would lead to the increase of I-DBPs during disinfection.For halogenated organic pollutants,pre-oxidation by the two systems could effectively control the generation of DBPs during chlorination under certain reaction conditions.Therefore,Mn(Ⅶ)or Fe(Ⅵ)coupling S(Ⅳ)systems have the potential to be applied as a preoxidation technology in water treatment.