Study On Generation Mechanism And Prediction Of Photochemically Reactive Species From Dissolved Organic Matter In Songliao Watershed Water

Under light irradiation,DOM can absorb photons to generate its excited state,and excited triplet DOM(3DOM*)can sensitize oxygen in water to generate photochemically produced reactive intermediates(PPRIs),such as hydroxyl radicals(·OH)and singlet oxygen(1O2).These PPRIs can play an important role in chemical transformation of pollutants and bacterial inactivation.The structural composition of DOM in different watersheds varies somewhat,resulting in differences in their ability to generate PPRIs.However,there are still insufficient studies on the photogeneration of PPRIs from DOM in watersheds in the cold area of Northeast China.The quantitative prediction of the generation quantum yield and other parameters of these PPRIs from DOM isolated from these watersheds is important for the comprehensive interpretation of their environmental geochemical processes.In this study,the generation quantum yields of 3DOM*,1O2 and ·OH were tetermined by sampling and extracting DOM from Liaohe River,Songhua River and its secondary tributary Yitong River in Jilin Province.The quantitative prediction models of photophysical and photochemical parameters of water samples and DOM were subsequently constructed.Furthermore,the generation mechanism of 3DOM*was further investigated in depth.The main contents and conclusions are as follows:(1)Extraction and optical property study of DOM in water in Songliao basin.Water samples were collected from 17 sampling points in the Songhua River,Liaohe River and Yitong River within Jilin Province,the basic indexes and photophysical property parameters of the water samples were measured,it was found that the light absorption parameters and fluorescence intensity of water samples in different basins and sampling points were significantly different.The DOM was extracted from the water samples by solid phase extraction(SPE)and the photophysical and fluorescence intensity parameters were measured.Parallel factor analysis(PARAFAC)indicated that DOM in Jilin watershed mainly contained four components:terrestrial humic substances,tryptophan-like,microbial humic substances and humic-like substances.Determined the apparent quantum yield of 3DOM*,1O2 and ·OH(Φ3DOM*,Φ1O2 and Φ·OH),the range of values are(0.69 ± 0.08)× 10-12-(1.19 ± 0.14)× 10-2,(1.30 ± 0.10)× 10-2-(2.19 ± 0.18)× 10-2 and(1.05 ± 0.09)× 10-5-(2.87 ± 0.19)× 10-5,respectively.(2)Construction of prediction model of DOM photochemical property parameters in watershed water.By partial least-squares regression(PLS),based on the determination of water samples of TOC concentration([TOC]),optical physics properties,and nonmetal ion concentration,build the Φ3DOM*,Φ1O2 and Φ·OH models.The cumulative fitting coefficient(R2Y(cum))of the model is 0.896,indicating the good forecasting ability.Based on the constructed model,[TOC]was found to be the most effective predictor among the response variables,indicating it has an important contribution to the DOM photochemical property parameters in watershed water.In addition,based on analysis of multiple linear regression(MLR)to build models for the quantum yield,formation rate and steady-state concentration of PPRIs,found that the constructed 3DOM*parameter(Φ3DOM*、R3DOM*、[3DOM*]ss),Φ1O2 and R1O2(the 1O2 rate of formation)models have good fitting and predictive.(3)A comparative and mechanistic study of different chemical probes for the determination of 3DOM*.The chemical probes 2,4-hexadien-1-alcohol(HDO)and 2,4,6trimethylphenol(TMP)were selected to determine the Φ3DOM*of the basin DOM and commercial DOM by energy transfer and electron transfer pathways,respectively,and it was found that the Φ3DOM*(HDO)and Φ3DOM*(TMP)determined using the two probes for the same DOM were significantly different,and the Φ3DOM*values of DOM extracted from Songhua and Liaohe River were significantly higher than those of commercial DOM.17 DOM-analogs with different structures were selected to determine ΦHDO and Φ3DOM*quantum yield coefficients(fTMP)using HDO and TMP,respectively.The results showed that the ΦHDO and fTMP exhibited significant substituent effects,and the differences in substituents significantly affected the values of ΦHDO and fTMP determined using TMP and HDO.(4)Quantitative structure-activity relationship(QSAR)models for Φ3DOM*and fTMP were constructed.Multiple linear regression(MLR)was used to construct the QSAR models of ΦHDO and fTMP based on the measured OHDO and fTMP values of 17 DOM-analogs.The results showed that the R2adj and Q2Loo of QSARHDO and QSARTMP were 0.811,0.778 and 0.815,0.726,respectively,indicating that they have good good fit and robustness,and have good prediction ability for Swanee River fulvic acid(SRFA),riboflavin,fenofibrate and naproxen.Based on the constructed QSAR models,the differences and substituent effects of ΦHDO and fTMP determined by different probes were further revealed.The results of the paper provide a method for the quantitative prediction of DOM photochemical properties in watershed water,and provide insight into the microscopic mechanism of 3DOM*generation at the molecular level by different chromophores,which is important for the comprehensive interpretation of the photochemical behavior of DOM in watershed water.