Formation And Transformation Mechanism Of Carbonyl Compounds During Ozonation Of Source Water In East Taihu Lake

Carbonyl compounds were the main intermediate products of ozonation,and they were lack of relevant testing methods and related standards.Compared with the lower threshold of geosmin and 2-MIB,the smell of carbonyl compounds were not significant,thus carbonyl compounds had not attracted wide attention from people and related departments.Besiades,carbonyl compounds can induce the regeneration of microorganisms in the dispersion system,and they have the properties of carcinogenic and mutagenic.In order to clarify the main source,transformation mechanism and control pathway of carbonyl compounds,the article based on ozonation treatment whose the raw water was East Taihu Lake,the kinds,concentrations and seasonal variations of DBPs,as well as the influence of each process unit on its generation or removal were investigated.Besides,the characterization of soluble organic matter indirectly reflects DBPs precursor.Thus,the investigation not only can reveal the generation and transformation mechanisms of carbonyl compounds,but also can provide insights on the control and removal methods of carbonyl compounds.The detection method of aromatic organic acids?ACAs?were not required,but ACAs were the raw materials of dye factory,pharmaceutical factory,food factory,cosmetics factory and paper mills,so ACAs were widespread in surface water environment.Benzoic acid?BA?,phthalic acid?Ph A?,3-hydroxybenzoic acid?3-OHBA?and nitrobenzoic acid were detected in the river.The method for the determination of ACAs was established.Aromatic organic acids in water,including fumaric acid,protocatechuic acid?PA?,3-OHBA,Ph A,o-nitrobenzoic acid,mnitrobenzoic acid,p-nitrobenzoic acid,BA,m-toluic acid,were determined by solid phase extraction-ultra performance liquid chromatograph?SPE-UPLC?.The recovery rate of PA was 79%,and the recoveries of the other ACAs were 85.7%¹02.8%.Besides,the precision of the detection method?RSD?was 0.26⁵.1%.Based on the SPE-UPLC method,the changes of carbonyl compounds in two parallel water treatment plants?conventional treatment and ozonation treatment?were studied.The results shown that 7¹0 carbonyl compounds were detected in raw water of East Taihu Lake due to seasonal variation.Formaldehyde?2.98⁴3.66 ?g/l?,acetaldehyde?1.33¹6.87 ?g/l?,glyoxal?1.08²17.18 ?g/l?and methylglyoxal?15.6¹68.79 ?g/l?were the dominant aldehydes.Acetic acid,fumaric acid,PA,3-OHBA,BA and Ph A were the dominant carboxylic acids.The concentration of total organic acid was 243.79⁷44.14 ?g/l.As compared with aldehydes,the concentrations of carboxylic acids were higher in raw water.The concentrations of carbonyl compounds were variable according to the season.In general,the lower concentrations of carbonyl compounds were found during warm months.In addition to the water temperature,other environmental and meteorological parameters such as drought,frequency of precipitation,intensity precipitation and solar radiation may also affect the concentrations of carbonyl compounds in surface waters.Compared with carbonyl compounds contained in source water,ozonation process produced more kinds of carbonyl compounds.The concentrations of carbonyl compounds in the source water were improved after ozonation.The ozonation step had a similar effect irrespective of season.Thus,it further increased the total average concentration of the carbonyl compounds in relation to raw water,and caused crotonaldehyde,glyoxal,methyl-glyoxal,formic acid,oxalic acid and 3-OHBA appear.The coagulation/flocculation-sand filtration and activated carbon filter processes had the ability to decrease the concentrations of carbonyl compounds.Finally,after the chlorination process,low-molecular-weight aldehydes showed a slight increase and aromatic acids remained almost constant.By using three dimensional fluorescence to characterize dissolved organic matter in water,the result shown that a certain amount of fulvic acid?FA?in the sand filter effluent water.This phenomenon indicated that it was difficult to completely remove small molecule natural organic matter.In the subsequent treatment process,the reaction between FA with oxidant contributed to the formation of carbonyl compounds,or FA became the precursor of DBPs during disinfection process.It was concluded that FA was the main source of carbonyl compounds in the actual water treatment.PA,3-OHBA,BA and Ph A,fumaric acid,malonic acid,formic acid,acetic acid,formaldehyde,acetaldehyde,glyoxal and methyl-glyoxal were detected in ozonated FA.Their generation were variable according contact time,O₃ dosage and pH.Besides,aromatic acids could be opened ring by O₃ to produce lowermolecular-weight carbonyl compounds.The reaction rates of aromatic acids with O₃ were mainly determined by the chemical structure.If benzene ring with some electron-rich moieties,the reaction rates will be fast.If benzene ring with some electron-withdrawing groups,the reaction rates will be slow.Therefore,the concentrations of PA and 3-OHBA are low,and they are even difficult to be detected.In addition,crotonaldehyde may be the direct product of benzene ring opening.Due to the lively chemical properties of aldehydes,they were easy to be oxidized to the corresponding carboxylic acids.Crotonaldehyde is oxidized to fumaric acid.Thus,it was difficult to detect crotonaldehyde,but a lot of fumaric acid was detected.Moreover,compared with FA ozonation alone,the combination of O₃/UV and O₃/H₂O₂ generated less kinds and concentrations of carbonyl compounds.The system of O₃/UV and O₃/H₂O₂ increased the concentration of ·OH,decreased the ability of selectivity,and improved the ability of mineralization,as well as the reaction rates were accelerated.Therefore,advanced oxidation technology should be used to control and removal of carbonyl compounds in the actual water treatment process.The generation and transformation mechanisms of carbonyl compounds were as the following.Under the common oxidation of O₃ and hydroxyl radical,FA experienced a series of reactions,such as electrophilic reaction,Criegee mechanism,dehydrogenation and decarboxylation reaction,resulting in ACAs formation.Besides,ACAs ozonation may contain three stages.Firstly,ACAs hydroxylation.Secondly,hydroxylation products of ACAs was oxidized to ring-open,and meanwhile unsaturated carbonyl compounds were generated.Thirdly,the shortchain aldehydes and carboxylic acids were formed.As for mineralization,the shortchain aldehydes and carboxylic acids were finally transformed into CO2 and H2 O.The reaction between ozone and ACAs mainly depended on the functional groups?electron-donating group/electron withdrawing group?on the benzene ring.In the presence of hypochlorous acid,polyhydroxylated aromatic acid and small molecule weight aldehydes/ketones acids would take place hydrolysis reaction,structure enolization,substitution reaction,oxidation reaction,decarboxylation reaction and so on.Subsequently,DBPs were generated.