Transformation Of Aromatic Iodinated Disinfection Byproducts (DBPs) To Aliphatic Iodinated DBPs During Chloramine Disinfection

Aromatic iodinated disinfection byproducts(DBPs)are an emerging category of DBPs which are widely present in drinking water.They are mainly classified into four groups based on their structures:iodinated phenols,iodinated nitrophenols,iodinated hydroxybenzaldehydes,and iodinated hydroxybenzoic acids.Previous toxicological studies have demonstrated that these aromatic iodinated DBPs were generally more toxic than their chlorinated and brominated analogues as well as the commonly known aliphatic halogenated DBPs,such as trihalomethanes(THMs)and haloacetic acids(HAAs),and thus they have drawn much attention.Previously,it was reported that some aromatic iodinated DBPs underwent transformation to form other aromatic iodinated DBPs during chloramination,and they were very likely to undergo further transformation to form aliphatic iodinated DBPs.Such transformation may occur during disinfection in drinking water treatment plants as well as in drinking water distribution systems,which may affect the DBP species and overall toxicity of tap water,thus deserving further investigation.Among all aliphatic iodinated DBPs,iodinated THMs and HAAs are two categories with the highest occurrence frequencies and levels in drinking water and they showed diverse health risks,thus deserving particular concern.Hence,in this thesis,the transformation of aromatic iodinated DBPs to aliphatic iodinated DBPs(especially iodinated THMs and HAAs)during chloramine disinfection was systematically explored.Firstly,it was found that the traditional liquid-liquid extraction method with methyl tert-butyl ether was actually not suitable for the determination of iodinated HAAs(iodoacetic acid(MIAA)and diiodoacetic acid(DIAA))due to the low recoveries,and thus a new solid phase extraction-high performance liquid chromatograph/electrospray ionization triple quadrupole mass spectrometer(SPE-HPLC-MS/MS)method was developed for the determination of iodinated HAAs.Oasis MAX cartridges were adopted,and SPE parameters were optimized.In addition,The applicability of this method for the determination of four aromatic iodinated DBPs(3,5-diiodo-4-hydroxybenzaldehyde,3,5-diiodosalicylic acid,2,6-diiodo-4-nitrophenol,and 2,4,6-triiodophenol)was also tested.The results suggested that the recoveries of the two iodinated HAAs and four aromatic iodinated DBPs were in the range of 70-110%,the method detection limits were in the range of0.02-0.15 ng/L,the method quantitation limits were in the range of 0.06-0.48 ng/L,and the precisions were in the range of 6.0%-15.6%,indicating that the method was applicable for the simultaneous determination of iodinated HAAs and aromatic iodinated DBPs.Further,the new method was applied to the determination of the above six polar iodinated DBPs in tap water samples,and their concentrations ranged from 0.03 to 3.97 ng/L.On the basis of this newly developed method,the transformation of four aromatic iodinated DBPs to aliphatic iodinated DBPs during chloramination was investigated.The results revealed that the decomposition of the four aromatic iodinated DBPs all followed a pseudo-first-order decay during chloramination,and the rank order of their decomposition rate constants was as follows:2,4,6-triiodophenol>3,5-diiodo-4-hydroxybenzaldehyde?3,5-diiodosalicylic acid>2,6-diiodo-4-nitrophenol,which may be related with the electron-withdrawing property of their substituting groups.They all underwent transformation to form iodinated THMs(triiodomethane(TIM))and iodinated HAAs(MIAA and DIAA).Among the four aromatic iodinated DBPs,2,4,6-triiodophenol had the highest decomposition rate and generated the highest levels of iodinated THMs and HAAs during chloramination.Hence,2,4,6-triiodophenol was selected as a representative of the aromatic iodinated DBPs for further investigation,including the transformation products and pathways,the factors affecting the transformation,and the toxicity change.Based on the iodinated intermediates(e.g.,2,6-diiodo-1,4-benzoquinone(2,6-DIBQ),2,6-diiodo-3-hydroxy-1,4-benzoquinone,and iodobutenedioic acid)detected in chloraminated 2,4,6-triiodophenol samples,the formation pathways of TIM,MIAA,and DIAA from 2,4,6-triiodophenol during chloramination were proposed and further validated.The results also revealed that monochloramine dose,p H,temperature,and short free chlorine contact time all affected the formation of TIM,MIAA,and DIAA from 2,4,6-triiodophenol during chloramination.The cytotoxicity rank order of the eight iodinated DBPs was MIAA>2,6-diiodo-4-nitrophenol>2,4,6-triiodophenol>2,6-DIBQ>DIAA?3,5-diiodosalicylic acid>3,5-diiodo-4-hydroxybenzaldehyde>TIM.The toxicity of the chloraminated 2,4,6-triiodophenol sample first decreased and then increased over time due to the transformation.Since it was found that the formation of iodinated THMs from aromatic iodinated DBPs during chloramination was significantly higher than that of iodinated HAAs,the formation of iodinated THMs from these four aromatic iodinated DBPs were further quantified.For better comparison,the formation of chlorinated and brominated THMs from aromatic chlorinated or brominated DBPs during chlorination was quantified,and the formation of trichloromethane from four aromatic iodinated DBPs during chloramination was also quantified.The results revealed that aromatic iodinated DBPs were an important category of THM precursors.Triiodomethane yields from aromatic iodinated DBPs during chloramination were close to trichloromethane yields.Among the aromatic iodinated DBPs,2,4,6-triiodophenol had the highest THM yields while 2,6-diiodo-4-nitrophenol had the lowest one.Alkaline conditions and higher temperatures favored the THM yields from aromatic iodinated DBPs during chloramination.With high disinfectant dose,the THM species might shift from iodinated ones to chlorinated ones.Many similar results were also observed during the transformation of aromatic chlorinated or brominated DBPs to THMs during chlorination.In addition,since it was also found that iodinated halobenzoquinones were an important category of intermediates in the transformation of aromatic iodinated DBPs to iodinated THMs and HAAs during chloramination,while few studies reported iodinated halobenzoquinones in drinking water.Thus,the occurrence of new iodinated halobenzoquinones(2-iodo-1,4-benzoquinone(2-MIBQ),2,3-diiodo-1,4-benzoquinone(2,3-DIBQ),2-chloro-6-iodo-1,4-benzoquinone(2,6-CIBQ),2-bromo-6-iodo-1,4-benzoquinone(2,6-BIBQ),and 2,6-DIBQ)in drinking water was examined and their cytotoxicity was evaluated.Three new iodinated halobenzoquinones,including 2,6-CIBQ(0.7-1.3 ng/L),2,6-BIBQ(1.8-8.0ng/L),and 2,6-DIBQ(0.4-15.9 ng/L),were detected and identified in drinking water for the first time.Toxicological results suggested that compared with a commonly detected halobenzoquinone in drinking water,i.e.2,6-dibromo-1,4-benzoquinone,iodinated halobenzoquinones generally showed higher cytotoxicity.Among the five tested iodinated halobenzoquinones,2,6-DIBQ was ubiquitous in both simulated drinking water and tap water samples,and it showed relatively high toxicity.Hence,it was selected as a representative of the new iodinated halobenzoquinones for further investigation in this study,including its formation and decomposition during chloramination.The results demonstrated that the formation of 2,6-DIBQ during chloramination was significantly affected by the natural organic matter,iodide concentration,and p H.All the five tested iodinated halobenzoquinones underwent decomposition during chloramination,and their decomposition followed pseudo-first-order decay(k,0.17–0.41 h^–1).The rank order of their decomposition rate constants was 2,6-CIBQ>2,6-BIBQ>2,6-DIBQ>2,3-DIBQ>2-MIBQ,which may be related with their electrophilicity.Moreover,2,6-DIBQ underwent transformation to form both carbonous aliphatic iodinated DBPs(e.g.,THMs and HAAs)and nitrogenous aliphatic iodinated DBPs(e.g.,haloacetonitriles and haloacetamides).Based on nine polar halogenated intermediates,including halobenzoquinones,halohydroxybenzoquinones,halohydroxycyclopentenediones,and halobutenedioic acids,the formation pathways of aliphatic iodinated DBPs from2,6-DIBQ during chloramination were tentatively proposed and verified.