Study On The Mechanism Of Photodegradation Of Naphthalene With The Presence Of Dissolved Organic Matter In Seawater

Dissolved organic matter(DOM)is an important organic compound with photochemical activity in natural water environment.Toxicity and transformation process of organic pollutants is strongly dependent on the interaction between hydrophobic organic matter and DOM.Recently,due to the frequent occurrence of oil spills in the ocean,the research on oil-polluted areas has attracted widespread attention worldwide.Polycyclic aromatic hydrocarbons(PAHs)are typical petroleum pollutants with high toxicity in the marine environment.Previous studies have investigated the photolysis process of PAHs in freshwater based on the photochemical properties of DOM,but the internal relationship between PAHs and DOM in marine systems and the effects of DOM on photo transformation of PAHs need to be further elucidated.In this work,humic acid(HA)and fulvic acid(FA)were selected as typical DOM,and naphthalene(Nap)was chosen as the target PAHs to investigate the role of DOM and its separated components in marine systems so as to reveal the environmental behavior and photodegradation mechanism of PAHs.Spectral analysis was used to indicate the photochemical characteristics and component structures of DOM.Combined with molecular probes and competitive reactions,the steady-state concentration,quantum yield and second-order reaction rate constant of reactive species were determined,and their effects on Nap photodegradation were thoroughly investigated.The degradation mechanism was proposed based on the identified intermediates and the toxicity of produced compounds was further revealed.The photochemical model was established to predict the photochemical degradation and fate of PAHs in real environments.In addition,separation of humic substances was carried out based on the solubility difference and molecular weight size,and the effect of different fractionated components on the photodegradation of Nap was investigated.The main obtained results are as follows:(1)Direct degradation mainly accounts for the photodegradation of Nap in simulated seawater.Photogenerated 3DOM*and 1O2 dominant the indirect photolysis,and ·OH contributes limited role.HA and FA at low concentration can promote the photodegradation rate of Nap,but DOM at high concentration will inhibit the photochemical process of Nap by light screening and quenching binding effects.Compared with HA,FA is less aromatic and more susceptible to photobleaching combined with weaker quenching effect of Nap.Although the steady-state concentration and quantum yield of 3DOM*/1O2 of FA are lower,its reactivity with Nap is higher,thus it is a more effective natural photosensitizer for PAHs.(2)The photolysis rate of Nap in DOM solution shows a trend of increasing initially and then decreasing with the increase of salinity,but the promotion effect of FA is always higher than that of HA.Carbonate shows negligible effect on the conversion of Nap.The degradation rate of Nap in simulated seawater is lower than that in simulated seawater without bromide ions,but higher than that in real seawater.(3)The photochemical model APEX shows that the degradation rate of Nap in freshwater is slightly higher than that in seawater,and decreases with the increase of water depth and DOM concentration.3DOM*-induced reaction and direct photolysis are important conversion pathways of Nap in natural waters.In addition,three degradation pathways of Nap are proposed based on GC-MS analysis,and most of identified intermediates are found to be more toxic than Nap using Toxicity Estimation Software evaluation.(4)The acid-base separation product of humic acid,HA1,has larger molecular weight and aromaticity,and shows a stronger binding effect with Nap compared with FA1.At the same low concentration of DOM,the promotion effect of FA1 on photodegradation of Nap is stronger than that of HA1,but the inhibitory effect is weaker than that of HA1 at high concentration.With the increase of DOM,the generation rate of reactive species is affected by the light absorption of DOM,thus limiting the photoreactivity of Nap.In addition to the major role of 3DOM*and 1O2,·OH also play little but not negligible role in the photodegradation of Nap.(5)HA1 can be separated into five components,such as HA<1k、HA1k-3k、HA3k-5k、HA5k-10k and HA>10k,based on the molecular weight size.With the increase of molecular weight,the fluorescence quenching effect between HA and Nap becomes more visible.The photosensitization promotion effect of HA3k-5k is the most significant among all separated components.3DOM*makes the biggest contribution to the photodegradation process,and the yield of 1O2 and the steady-state concentration of·OH are negatively correlated with molecular weight.(6)FA<1k and FA>1k,which are the isolated products of FA1,are highly similar in composition,but FA>1k contains more chromophore structures.Neither of these two components has significant fluorescence quenching effect on Nap.Similarly,it shows a promoting effect at low concentration and an inhibitory effect at high concentration,but the photosensitization effect of FA<1k is stronger and the generation efficiency of reactive species is higher than that of FA>1k.