Identification And Removal Mechanism Of Chlorinated Paraffins And Their Transformation Products In Typical Urban Sewage Treatment Process

Chlorinated paraffins（CPs）are a class of n-alkane chlorinated derivatives,which are classified into short-chain paraffins(C_10-13,SCCPs),medium-chain paraffins(C_14-17,MCCPs)and long-chain paraffins(C_18-30,LCCPs)according to their carbon chain length.Because of their persistence,accumulation,long-distance transport and biotoxicity,SCCPs were listed in the Stockholm Convention on Persistent Organic Pollutants in 2017 and became a chemical substance for global cooperative control.As SCCPs are phased out,a shift to increased production of MCCPs is expected in the future as the preferred alternative.Therefore,MCCPs and LCCPs should also receive the same attention in the environment.Moreover,the application of chlorinated paraffins is actually an industrial mixture with a very complex composition.It contains not only saturated chlorinated alkanes,but also potentially unsaturated components（un-CPs）,and possibly more unknown components.Due to the lack of effective detection technology,the research on its environmental pollution status,distribution characteristics and its removal behavior in the sewage treatment process is very limited.Without the identification and assessment of these potentially hazardous substances,it is likely to underestimate the environmental hazards and risks caused by this type of pollutants,hindering effective risk prevention and control.Therefore,this study addresses the frontier issue of environmental analysis and behavior of CPs in the environmental field,and establishes a highly sensitive and high-throughput simultaneous analysis method for SCCPs,MCCPs and LCCPs using high resolution mass spectrometry.The non-targeted identification is used to further identify and characterize un-CPs and degradation transformation products.On this basis,the multi-media content distribution,removal and mechanism of saturated CPs will be systematically investigated for two important pollution pathways of wastewater treatment plants and constructed wetlands.Meanwhile,the identification and characterization of un-CPs and degradation transformation products will provide a basis for scientific assessment of environmental risks of CPs-like pollutants.Using liquid chromatography coupled with chlorine-enhanced high-resolution mass spectrometry,targeted and non-targeted screening systems were established based on precise molecular weight analysis and isotopic distribution characteristics of CPs-like substances.Combined with liquid-liquid extraction,microwave extraction and column chromatography cleanup techniques,we can achieve comprehensive analysis of CPs-like substances in complex environments.In this study,we collected sewage,sludge and plants samples to verifiy our method.One hundred and eighty-two kinds of saturated CPs were targeted and identified,including 31 SCCPs,45 MCCPs and 106 LCCPs.At the same time,95 unreported or rarely reported transformation products were identified by non-target screening.It included that 24 un-CPs（carbon numbers:10-20;chlorine numbers:4-7;unsaturated degrees:1-2）,65 oxygenated CPs（CP（O_x）s;carbon numbers:10-19;chlorine numbers:3-7;oxygen numbers:1-5）and 6 ultra-short-chain CPs(v-SCCPs（carbon number:8-9;chlorine number:4-7）.Based on the eatablished targeted and non-targeted screening systems,the occurrence,distribution and mass loading of CPs in typical treatment processes of sewage treatment plants were systematically investigated.By quantitative and semi-quantitative analyses（un-CPs and CP（O_x）s）,saturated CPs（529-1732 ng/L）,un-CPs（29-90 ng/L）,and CP（O_x）s（93-343 ng/L）were detected in the wastewater.CPs may be converted into un-CPs and CP（O_x）s during biological treatment.Based on mass balance calculations,the removal efficiency of CPs,un-CPs and CP（O_x）s were 75-97%,63-80%and 83-93%by the inversion A²/O process,respectively.The mass distribution indicated that sludge adsorption was the main removal route,especially for longer chain and highly chlorinated CPs.More attention should be paid to sludge treatment.The distribution,annual mass load and removal efficiencies of CPs,un-CPs and CP（O_x）s in the water,sediment and plant media were further studied of constructed wetland system in Beijing Olympic Park.In the constructed wetlands,CPs（90-429 ng/L）,un-CPs（6.0-16 ng/L）and CP（O_x）s（11-78 ng/L）were also identified at slightly lower concentration levels compared to the wastewater treatment plants.The removal efficiencies of CPs,un-CPs and CP（O_x）s in the constructed wetlands were 51-91%,32-89%and47-71%,respectively,based on mass flux estimates by one year.Combined with the mass distribution in the aqueous phase,mud phase and plant media,bioaccumulation of CPs-like pollutants in plants was found to be 1.9-12%,which should not be neglected.The enrichment of CPs in plants was positively correlated with its log K_OW value,and the enrichment coefficient increased with carbon chain increased.It is noteworthy that this is the first study of CPs-like pollutants in a wetland environment.