Dissolved Organic Matter Of Stormwater Runoff In Beijing Urban Area:Characterization And Its Interaction With Copper Ions

The non-point source pollution caused by the stormwater runoff has become one of the important factors of restricting economic and social sustainable development.The pollution load of stormwater runoff significantly changes with space and time.In addition,its research,control and dispose are challenging.Beijing,the capital of China is facing dual pressure of water resource shortage and water environment pollution.Thus,the study on the status of the water quality of stormwater runoff in urban areas is quite meaningful.However,current studies have paid more attention to the monitoring and the interaction of conventional parameters of stormwater runoff（such as BOD,COD,TSS,TN,TP,heavy metals and some specific organic contaminants）,but nearly nothing is known about the nature and role of dissolved organic matter（DOM）in urban stormwater runoff.DOM can interact with numerous heavy metals and organic contaminants by complexation,acidity cushion,light attenuation and so on,influencing the toxicity,biodegradability and transport of contaminants in aquatic ecosystem.Therefore,to study the existential state of DOM in stormwater runoff and the relationship between DOM and the coexistences will be conducive to grasp its subsequent impact to the stormwater treatment facilities,possessing the important scientific meaning and the values of practice and application.In this study,researchers firstly collected the stormwater runoff samples at three typical regions（business,residential and campus regions）in Beijing.We not only used the routine technologies to characterize the DOM,including UV-visible spectrometry（UV-vis）,excitation–emission matrix（EEM）fluorescence and proton nuclear magnetic resonance spectroscopy（1H NMR）,but also adopt ultra-performance liquid chromatography quadrupole time-of-flight mass spectrometry（UHPLC-Q-TOF-MS）to characterize the DOM at molecular level.Secondly,the higher content of contaminant in stormwater runoff,Cu²⁺ was selected as the representative of heavy metals.And both UV absorbance and fluorescence titration methods were introduced to determine the complex capacities（CL）and conditional stability constants（lgKM）of DOM’s bonding with Cu²⁺.Furthermore,scanning electron microscopy-energy dispersive X-ray spectroscopy（SEM-EDX）and X-ray photoelectron spectroscopy（XPS）were applied to clarify the reaction mechanism.Finally,the molecular weight distribution of DOM was obtained by ultrafiltration membrane separation to quantify the binding strength between Cu²⁺ and DOM of different molecular weight.Main results of this thesis are listed as follows:（1）Spectroscopic techniques demonstrated that the DOM of stormwater runoff was the UV humic-like substances.Compared with the wet precipitation,its molecular weight was biger and the aromaticity was stronger.It mainly contained carbonyl,carboxyl,aliphatic/phenolic hydroxy and alkyl functional groups,and the substituent degree of oxygen-containing functional groups in aromatic structures was higher.With respect to the mass spectrometric analysis,a total of 12247 and 4776 m/z values were found in ESI-positive and ESI-negative ionization mode,respectively,suggesting the types of DOM were countless and its microstructures were complex.Combining with the van Krevelen plots,we further discovered that lipid,protein,cellulose,lignin and condensed hydrocarbon substances were the components of DOM in stormwater runoff,and lignin and condensed hydrocarbon substances were the dominated ones;（2）The UV absorbance and fluorescence titration methods were used to quantify the binding strength between Cu²⁺ ions and the DOM.The significant negative correlations can be found between DOM’s UV absorbance and fluorescence intensity during the titration（r2 = 0.98）.The fitting results indicated that the values of lgKM and CL were 5.37 and 5.48,85.62 and 87.23 μmol/mg,respectively.Compared to the organics in other surface water,higher values of lgKM and CL can be attributed to the more aromatic structures and effective complexing sites,such as carboxyl,carbonyl,hydroxy,etc.Meanwhile,the XPS spectrum of copper-loaded sample showed that two distinct peaks of Cu 2p3/2 and Cu 2p1/2,which located at 933.7 eV and 954.3 eV,respectively.And the pronounced shake-up peaks of Cu-O further confirmed that Cu²⁺ mainly coordinated with oxygen-containing functional groups of DOM in stormwater runoff.（3）The results revealed that the fraction of molecular weight less than 1 kDa possessed the largest ratio in total organics,and the DOC of this fraction accounted for about 41% of the total DOC.UV humic-like compounds are the major fluorophores components of all fractions with no significant difference.However,we found that the fraction of 1-3 k Da contained more aromatic carbon and conjugated unsaturated double bonds,and the molecular substituent degree of some oxygenated functional groups（e.g.,carbonyl,hydroxyl,carboxyl and ester）of 3-5 kDa’s fraction was higher than the others.In addition,compared with the values of KM and CL of all fractions,the results suggested KM of 1-3 kDa’s fraction and CL of 3-5 kDa’s fraction were the largest than the others.The above-stated results implied that low molecular weight organics（< 5 kDa）were the major constituents of DOM in stormwater runoff,which had the better combinative potential with Cu²⁺.