Investigate On The Interaction Differences Of Algogenic Dissolved Organic Matter With Phenanthrene And 3-Methylphenanthrene Using Spectroscopy

Algogenic Dissolved Organic Matter(A-DOM),as the main component of the autochthonous DOM that existed widely in the aquatic environment,participated in the carbon cycle,and also influenced the environmental behavior and fate of pollutants.As a group of important derivatives of Polycyclic Aromatic Hydrocarbons(PAHs),Alkylated PAHs(A-PAHs)existed extensively in the environment,especially in oil pollution areas.A-PAHs generally showed a higher content and biotoxicity than that corresponding Parent PAHs(P-PAHs).Among them,Methylated PAHs(M-PAHs)with 3 to 5 benzene ring numbers were the main contributors to the biological toxicity of APAHs in oil spill pollution areas.PAHs could interact with DOM to change their environmental behavior and bioavailability in aquatic environment.However,due to the lability and the complex environmental behavior of A-DOM,there were relatively few studies on the impact of A-DOM on the environmental behavior of PAHs,especially on A-PAHs.A-DOM had strong hydrophilicity.Alkylation increased the molecular weight and hydrophobicity of A-PAHs,which further increased the difficulty of determining the interaction between A-DOM and A-PAHs.Meanwhile,the extreme heterogeneity of the composition and structure of A-DOM made it rare to distinguish the contribution of its components in affecting the environmental behavior of PAHs.Therefore,Phenanthrene(Phe)and 3-Methylphenanthrene(3-MPhe)were selected as representative model of P-PAHs and A-PAHs,respectively.Excitation-Emission Matrix(EEM)fluorescence spectra combined Parallel Factor(PARAFAC)(EEM-PARAFAC)analysis method were used to investigate on the changes and differences of interactions between A-DOM fluorescence components and P-PAHs/A-PAHs.The characteristics of A-DOM depended on its growth stages,and also affected by its own physiological state.Microplastics(MPs),which were widespread in the marine environment,have a certain toxic effect on the growth and metabolism of algae.It would inevitably affect the release and characteristics of A-DOM,and then change the environmental behavior of PAHs.However,whether the impact of MPs on the physiological state of algae cells will further change the release and characteristics of A-DOM,it has rarely been reported.Therefore,it is necessary to explore the effect of Polystyrene(PS)-MPs on the A-DOM characteristics.The main results are as follows:(1)Ultraviolet-Visible(UV-Vis)absorption spectra and EEM-PARAFAC combined with fluorescence quenching method were used to investigate the interaction behaviors between A-DOM released by Prorocentrum donghaiense(P.donghaiense)at different growth phases and Phe/3-MPhe,and the relevant mechanism was also preliminarily explored.The contents of the three main fluorescent components in A-DOM showed an increasing trend with the growth of algae.However,the proportion of protein-like decreased from 73.70%to 59.58%,the total proportion of humic-like increased from 26.30%at the initial phase to 40.42%at the decline stage.Pearson correlation analysis showed that the humification index(HIX),aromaticity(SUVA254)and hydrophobicity(A260)of A-DOM were significantly positively correlated with the proportion of humiclike components(p<0.01).The interaction capability between the three fluorescence components and Phe/3-MPhe showed that humic-like components had a significantly stronger interaction with Phe/3-MPhe than protein-like component.These interaction characteristics and increased content of humic-like components indicated their prominent role in A-DOM and Phe/3-MPhe interaction.The different interaction capabilities between Phe and 3-MPhe with A-DOM fluorescent component at different growth phases were also observed.Methyl substitution increased π-π interaction and hydrophobic interactions of 3-MPhe with humic-like components on day 28.However,methyl group also increased the molecular weight and steric hindrance of 3-MPhe,resulting in a lower interaction capability with A-DOM than that of Phe on day 16.(2)UV-Vis absorption spectra and EEM-PARAFAC were used to explore the effects of changes in the growth and photosynthetic activity of P.donghaiense exposed to different concentrations(0，2,5,10,20 mg/L)of PS on the A-DOM content and fluorescence characteristics released during the entire growth process.The results showed that short-term PS exposure changed the physiological state of algal cells,which affected cell metabolism and secretion,and further reduced the release of ADOM.On the day 10 with high concentration(20 mg/L)PS exposure,the humification degree and hydrophobicity ratio of A-DOM were reduced to 57.27%and 27.04%in the control group.With the increase of culture time,the cells entered into normal growth and metabolism,and the content of each fluorescence component showed no significant difference compared with the control group.Meanwhile,the humification degree and hydrophobicity ratio of A-DOM decreased by 17.05%and 12.19%compared with the control group on day 24,but without significant differences compared with the control group,Pearson correlation analysis showed that the content of protein-like component in the control group was significantly correlated with Chlorophyll a(Chl-a)(p<0.01).The protein-like component,marine humic-like and long-wavelength humic-like component contents in the 2 mg/L and 10 mg/L PS exposure showed a correlation with the Chl-a content(p<0.05),meanwhile,protein-like component and marine humiclike component contents were more significantly affected by photosynthetic activity(Fv/Fm)in the 10 mg/L PS exposure group(p<0.05).The results showed.that the growth and photosynthetic efficiency of P.donghaiense under different growth phases and different concentrations of PS exposure affected the A-DOM content and spectral characteristics.In this work,EEM-PARAFAC analysis method combined with the fluorescence quenching method had realized the interaction behaviors and differences between ADOM fluorescent components and Phe/3-MPhe at the molecular level.It was proved that the interaction between different fluorescent components in A-DOM and PAHs were obviously different,and methyl substitution affected the interaction behavior of P-PAHs/A-PAHs with A-DOM at different growth phases.It revealed that the important influence of dynamic A-DOM on the environmental behavior of PAHs.At the same time,more attention should be paid to the different environment behavior of P-PAHs and A-PAHs in the actual environment.The dynamic characteristics of A-DOM not only depended on the algal growth phases,but the existence of MPs would also change the contents and characteristics of A-DOM.This result also indicated that in addition to its own transformation and degradation,the influence of pollutants also should pay more attention to the influence of the A-DOM characteristics,which could further lead to A-DOM interaction with PAHs behavior change.