Study On Nitrosamines Formation During Ozonation

Ozonation has been widely used in the oxidation and disinfection of drinking water with the increasing complexity of water quality.Compared with chlorine and chloramine as oxidant,ozonation can not only remove pathogenic microorganisms in drinking water,but also has a significant effect on the removal of toxic and harmful substances.However,it also produced a highly toxic byproduct of nitrosamines.In view of the strong carcinogenicity of nitrosamines,it is an important problem to study the mechanism and influencing factors of the corresponding nitrosamines formation during ozonation of main precursors.This study relates to formation efficiency of corresponging nitrosamines?N-nitrosodimethyl amine?NDMA??N-nitrosodiethylamine?NDEA??N-nitrosomethylethylamine?NMEA??N-nitrosomorpho line?NMor?and N-nitrosopyrrolidine?Npyr??from their relevant precursors dimethylamine?DMA??die thylamine?DEA??methylethylamine?MEA??morpholine?Mor?and pyrrolidine?Pyr?during ozonation.Furthermore,the coexisted substances?nitrate,nitrite,tert butanol,hydroxylamine,humic acid?on nitrosamines formation during ozonation were also discussed.In addition,NDMA formation from herbicide diuron during ozonation was studied.The main conclusions were as follows:?1?Five nitrosamines were formed during ozonation.With the increase of reaction time?30 s¹5min?,the yields of five nitrosamines increased first and then decreased.When the contact time was 10 min,the yield of each nitrosamine reaches the maximum,the concentrations were 74.1 ng/L?NDMA?,612.5ng/L?NDEA?,41.9 ng/L?NMEA?,43.4 ng/L?NMor?and 191.2 ng/L?Npyr?respectively.This is due to the formation of nitrosamines gradually increased with the increase of the reaction time.However,with the secondary amines were converted into nitrosamines completely,the generated nitrosamines were degraded by ozone.Thus the concentrations were reduced.?2?With the increase of pH value,the yields of five nitrosamines increase gradually.When pH=10.3,the yields of nitrosamines reached the maximum.The highest concentrations were 38.7 ng/L?NDMA?,181.2 ng/L?NDEA?,44.7 ng/L?NMEA?,46.7 ng/L?NMor?and 126.2 ng/L?Npyr?.The reason is the interaction between ozone and amines involves two reaction steps: the first step reaction leads toamination,secondary amines were converted to hydroxylamine firstly;the second step is the electron transfer process,electrons are first transferred to ozone,which gives rise to free radicals.The stability of O₃^-× radical is only good at the high p H.?3?The study of reaction temperature showed that the yields of nitrosamines were the highest at25?.The highest concentrations were 23.9 ng/L?NDMA?,458.3 ng/L?NDEA?,72.0 ng/L?NMEA?,28.3ng/L?NMor?and 213.3 ng/L?Npyr?.The amount of formation decreased at low temperature?5??and high temperature?35??,the reason is that the molecular motion is slow and the collision between molecules decreases at lower temperatures,so the formation of nitrosamines decreased.In theory,a higher temperature may increase the activity of the reaction and change the equilibrium and rate of the reaction,and then have a potential impact on the formation of nitrosamines.However,due to the decrease of the solubility of ozone at higher temperature,the formation rate of nitrosamines can be slowed down.?4?The results showed that the yields of NDMA,NDEA,and NMEA with the linear structure increased firstly and then decreased with the increase of ozone concentration.When the concentration of ozone was 2.59 mg/L,the yields of nitrosamines with the linear structure of were the highest.The highest concentrations were 77.0 ng/L?NDMA?,268.2 ng/L?NDEA?and 52.4 ng/L?NMEA?.The generation of Npyr showed a significant linear trend?r2=0.999?.?5?By studing coexistence,the presence of NO₃^-,NO₂^-,tert butyl alcohol and humic acid inhibited the formation of nitrosamines.When single nitrate existed?2.5 mg/L?,the yields of nitrosamines decreased 9.1%?NDMA?,7.0%?NDEA?,14.7%?NMEA?,42.9%?NMor?and 20%?Npyr?.When single nitrite existed?0.25 mg/L?,the yields of nitrosamines decreased 36.5%?NDMA?,16.2%?NDEA?,28.0%?NMEA?,56.9%?NMor?and 22.9%?Npyr?.Nitrite is a reductive group,the consumption of ozone increased with the increase of the concentration of nitrite,which inhibited the formation of nitrosamines.Tert butyl alcohol was added into the solution of p H=7 or pH=9.5,the yields of NDMA decreased by84.7% and 50.5% respectively.The presence of tert butyl alcohol resulted in a decrease of NDMA production by 84.7%?pH=7?and 50.5%?pH=9.5?,respectively.Tert butyl alcohol as a free radical scavenger can significantly reduce the amount of ×OH,the reduction of hydroxyl radical leads to the fact that the secondary amines can not be protonated to nitrosamines.In the experiments of presence of hydroxylamine,it was found that hydroxylamine can react withsecondary amines in the absence of ozone,The production of nitrosamines were 29.7 ng/L?NDMA?,74.0ng/L?NDEA?,51.0 ng/L?NMEA?,70.8 ng/L?NMor?and 163.1 ng/L?Npyr?.When ozone was added,the formation of nitrosamines can be significantly improved.,the production concentration were 161.4 ng/L?NDMA?,293.7 ng/L?NDEA?,94.8 ng/L?NMEA?,143.6 ng/L?NMor?and 257.4 ng/L?Npyr?.?6?With the addition of humic acid,the formation of nitrosamines decreased by 34.8%?NDMA?,50.8%?NDEA?,45.5%?NMEA?,55.1%?NMor?and 57.7%?Npyr?.As we know,humic acid can be oxidized to nitrosamines.However,because of the competitive relationship with the secondary amines,the total production of nitrosamines is affected.?7?Diuron can be oxidized into nitrosamines during ozonation,diuron concentration was 1 mg/L,the amount of NDMA was 65.9 ng/L.The addition of nitrate or nitrite significantly increased the yield of NDMA,which was increased by 3 times?187.6 ng/L and 189.3 ng/L?.However,no signi?cant difference was observed in NDMA formation between the two nitrogen forms.Further,when both nitrate and nitrite were added,NDMA formation was almost identical to that observed in the experiments with only nitrate or nitrite.The existence of Na2CO3,NaHCO3 and H2O2 promoted the formation of NDMA.The concentrations were 104.3 ng/L,117.4 ng/L and 100.8 ng/L.The yields increased by 21.4%,36.7% and17.3%.The addition of tert butyl alcohol leads to the yield of NDMA decreased by 39.8%.