Formation Potential And Reaction Mechanism Of Nitrogen-containing Disinfection By-products By Pre-oxidation Process

With the development of drinking water treatment technology and environmental testing technology,disinfection by-products(DBPs)after chlorine disinfection will pose a potential threat to the human.The conventional drinking water treatment processes were not efficient in removing precursors of DBPs.Easily and cost-effective processing techniques such as pre-oxidation,deep oxidation or membrane treatment can be developed to reduce the DBPs.Studies have shown that pre-oxidation is an effective method to enhance the removal of DBPs precursors.In recent years,nitrogen-containing disinfection by-products(N-DBPs)have attracted attention due to their low content but high toxicity.Carbon-containing disinfection by-products(C-DBPs),which are included in the water quality test indicators have a large concentration.Therefore,this paper compares C-DBPs with N-DBPs.The dissolved organic matter is a precursor of DBPs.Dissolved organic nitrogen compounds are part of dissolved organic matter and are important precursors of N-DBPs.The amino acid in the natural and the synthetic nitrogen-containing pesticide oxazinone(MET)were selected as model compounds to study the influence of pre-oxidation process on the formation potential and reaction mechanism of N-DBPs.Finally,the natural water samples was used to verify and select the best pre-oxidation.Ozone(O3),potassium ferrate(K2Fe O4),potassium permanganate(PM),and potassium permanganate/sodium bisulfite(PM/BS)were selected as pre-oxidants for this study.The pre-oxidation treatment is used to eliminate the precursors of DBPs to control the formation of DBPs after chlorination.First,10 amino acids were selected for chlorination experiments to investigate the detection concentrations and type of DBPs.The concentration of DBPs after chlorination of each amino acid was different.The concentration of C-DBPs was significantly greater than that of N-DBPs.The C-DBPs species include chloroform(TCM),trichloroacetone(TCP)and dichloroacetone(DCP),and the concentration of TCM is much higher than DCP and TCP.The N-DBPs include dichloroacetonitrile(DCAN),trichloroacetonitrile(TCAN),and trichloronitromethane(TCNM).Then pre-oxidation experiments were carried out using O3,PM,K2 Fe O4,and suitable pre-oxidants were screened by the total concentration of DBPs.The effect of three pre-oxidation processes on reducing the DBPs is that PM is better than K2 Fe O4than O3.Comprehensive,PM is the optimal pre-oxidant,which greatly reduces the formation potential of N-DBPs and C-DBPs.The increase in PM dose gradually reduces the DBPs,which is related to the oxidation mechanism of PM and the formation pathway of DBPs.Pre-oxidation of K2 Fe O4 also reduced the DBPs.At low K2 Fe O4 doses,the formation potential of DBPs was small.The effect of O3 pre-oxidation on the destruction of DBPs precursors was not significant,which led to a significant increase in TCNM,resulting in an increase in the total DBPs.Finally,tyrosine was used to infer the transformation pathways of various DBPs to study the reaction mechanism.TCM has a pathway of hydrolysis after ring opening and formation of CH intermediate then hydrolysis.The first step in the formation ofN-DBPs is the chlorination of the amino group.DCAN can be formed by substitution,elimination and "decarboxylation pathway" or "aldehyde pathway".A typical pathway for TCNM formation involves amine nitration followed by halogenation.Select synthetic pesticides for research.In the pre-oxidation process of PM,a certain concentration of BS is added to cause the MET concentration to decrease instantaneously.For this phenomenon,pre-oxidation comparison experiments are carried out using PM and PM/BS.The degradation efficiency of MET,the formation potential,the toxicity,the reaction mechanism of DBPs and the transformation pathway of the precursors were studied.After PM/BS oxidation,the degradation efficiency of MET reached 92% within 10 s,which is much higher than the PM oxidation.MET achieved the highest degradation efficiency when the p H was 6.5 and the molar concentration of PM:BS was 1:5.Mn(III)plays a major role in the oxidative degradation of MET by PM/BS.SO5·-and SO4·-also contribute to the degradation,but HO· plays a minor role.The concentration of DBPs after PM/BS pre-oxidation is lower than the concentration of DBPs after PM pre-oxidation.This indicates that the addition of BS during PM oxidation accelerates the degradation of MET and reduces the formation potential of DBPs.This may be a degradation product that is not DBPs precursor.The formation potential of DBPs,cytotoxicity and genotoxicity increased with the increase in the amount of pre-oxidant,but these values decreased when the dose of PM/BS exceeded 50/250 ?mol L-1.It is worth noting that when the p H is increased from 6.5 to 8.5,the DCAN,TCNM and TCP is increasing,especially DCAN.Which leads to an increase in toxicity.This indicates that PM/BS degradation of pollutants needs to be controlled under p H <7.0 conditions,which can increase the degradation efficiency and reduce the toxicity of DBPs.PM/BS is highly oxidized and completely degrades MET.Eight oxidative intermediates of MET are identified after PM/BS oxidation,while four intermediates are identified after PM oxidation.The oxidation of the triazine ring,the cleavage of the amino group and the cleavage of the double bond in the ring were observed in both PM/BS and PM oxidized MET,and hydroxylation was observed only in the oxidation of PM/BS.The natural water sample was tested using filtered water from river water and water plants.The DBPs,TOC and UV254 of natural water have a gradual increase from upstream to downstream,and these values are higher in summer and autumn than in spring and winter.The formation potential of C-DBPs is much higher than N-DBPs.The increase in chlorine dose leads to a gradual increase in the formation potential of DBPs.The comparison of PM and PM/BS(molar ratio 1/5)found that PM/BS(1/5 ?mol L-1)pre-oxidation can reduce the DBPs in A water plant.The increase of PM/BS dose leads to the increase of DBPs first and then decrease.PM pre-oxidation increases the DBPs,and the increase in PM dose leads to a gradual increase in the DBPs.When the concentration of PM/BS is 1/5 ?mol L-1,the DBPs was reduced in B water plant.The increase of PM/BS concentration leads to the continuous increase of DBPs;PM preoxidation will increase the DBPs.This shows that the treatment effect of PM/BS pre-oxidation of natural water is better than PM pre-oxidation.Low-dose PM/BS pre-oxidation can degrade pollutants and reduce the formation potential of DBPs.It isa pre-oxidation process worthy of promotion.Three-dimensional fluorescence analysis showed that the two water plants mainly contain aromatic protein organic substances,microbial metabolites and a small amount of fulvic acid organic matter and humic acid organic matter.There is a linear relationship between the total intensity of three-dimensional fluorescence and N-DBPs(R2=0.60~0.81).This paper mainly provides a suitable pre-oxidation process for water mainly composed of amino acid and dissolved organic nitrogen-containing pesticides to reduce the formation of DBPs after chlorination,especially N-DBPs.For amino acid micro-contamination waters,rapid oxidation mechanisms and barrier pathways for DBPs make PM the optimal pre-oxidant.For pesticide micro-contamination waters,PM/BS pre-oxidation can accelerate the degradation of pesticides and reduce the concentration and toxicity of DBPs compared to PM pre-oxidation,but it must be controlled at p H <7.0.The natural water sample research proves that the treatment effect of PM/BS pre-oxidation is better than PM pre-oxidation,which is a pre-oxidation process worthy of promotion.