| As typical persistent organic pollutants,polycyclic aromatic hydrocarbons(PAHs)exist widely in environment.Many of them have carcinogenic,teratogenic and mutagenic effects,and their ecological effects have attracted widespread attention.With the continuous increase of human activities,the pollution of PAHs in natural aqueous environment is becoming more serious.Photodegradation is one of the main natural weathering processes of PAHs.Natural aqueous environment is rich in photosensitizers,such as dissolved organic matters(DOM)and nitrate,which greatly affect the degradation and transformation of PAHs.However,the researches in this area are still limited currently,especially lacking of researches about the effects of photosensitizers in seawater.In this study,typical PAHs(phenanthrene,PHE)were used as a target pollutant to explore the effects of nitrogen compounds,halogen ions and DOM on the photodegradation of PHE in fresh water and seawater,and determine the degradation process and mechanism of PHE under various environmental factors by free radical determination experiment.The main research contents and conclusions of this study are as follows:(1)The effect of nitrogen compounds on the photodegradation of PHE has been evaluated.The results show that nitrate can promote the photodegradation of PHE as an effective photosensitizer;nitrite shows a slight inhibition effect on the photodegradation of PHE;the effect of ammonia on PHE photodegradation can be negligible.Dissolved oxygen(DO)can inhibit photodegradation of PHE by quenching the prominent triplet excited state of PHE,and photochemical ionization is the key process of photodegradation of PHE with the absence of DO.Nitrate or nitrite changes the photodegradation mechanism of PHE by changing the role of free radicals in the system,and DO plays an important role in the free radical generation.The quenching experiments and the free radical concentration analysis show that ·OH is produced by the photosensitization of nitrate,which is an important free radical that causes the degradation of PHE,while the photosensitive process of nitrite has negligible effect on the generation of ·OH.It is also found that 1O2 plays an important role in the photodegradation of PHE.It is determined that the presence of nitrate increases[·OH]ss from 2.08×10-15 M to 1.04×10-14 M in fresh water and from 1.50×10-16 M to 2.08×10-15 M in simulated seawater.The difference between the nitrite and nitrate on quenching experiments indicates that reactive nitrogen radicals may be involved in the reaction in the nitrite system.(2)The effect of halogen ions,as well as the effect of halogen ions and nitrate coexisting condition,on the photodegradation of PHE have been explored.The results show that both Cland Br-can promote the photodegradation of PHE,and the promotion effect firstly increased and then weakened with the increase of Br-concentration.PHE is also a photosensitizer,which can be excited by light to produce a triplet excited state and then react with halogen ions to generate halogen radicals.On the one hand,halogen ions generate halogen radicals to promote the degradation of PHE,and on the other hand,they quench the excited state of PHE and inhibit its degradation.These two effects will cancel each other out,so the effect of halogen ions on the photodegradation of PHE is not significant.When nitrate was added to the Br’ system,the degradation process of PHE was slowed down,and the increase of Br-more significantly decreased the degradation.After adding nitrate to the Cl-system,the photodegradation of PHE under acidic conditions is significantly accelerated,which is twice as high as that under neutral and alkaline conditions.When nitrate was added,more halogen radicals participate in the reaction.The concentration of halogen radicals increases with the increasing of nitrate concentration,and its contribution to the reaction rate is enhanced.Cl-and Br-have different roles in the reaction,which is related to their reactivity with free radicals and pollutants.Specially,Cl-has a different reaction mechanism than Br-under acidic conditions,which can react with H3O+ and ·OH to generate chlorine radicals to accelerate the photodegradation of PHE.In the presence of photosensitizers,halogen ions will generate halogen radicals that react with PHE to form intermediate products,which can compete with PHE for photons and free radicals,then show an inhibitory effect on the degradation of PHE.(3)Humic acid and fulvic acid were used as two representative DOM to explore their effects on the photodegradation of PHE,Results show that fulvic acid promotes the photodegradation of PHE at low concentration through photosensitization to generate ROS and 3DOM*.At high concentration,they all inhibit the degradation of the PHE due to the light shielding effect of DOM and quenching of free radicals in the system.The promoting and inhibiting effects of DOM on the photolysis of PHE exist simultaneously,but which effect is dominant depends on the concentration and species of DOM.In this experiment,fulvic acid has a significantly promoting effect on the photodegradation of PHE than humic acid.The photosensitization of nitrate can weaken the inhibitory effect of DOM on the photodegradation of PHE,while the effect of nitrite on DOM can be negligible.Br-can enhance the photodegradation rate of PHE at low concentration of DOM,while Cl" has no significant effect.The photodegradation of PHE with different concentrations of DOM was inhibited when both nitrate and halogen ions were present in the system.When the composition of the system is complex,the components will interact with each other,and the photodegradation rate of PHE can be is different from that with single component.(4)Two ·OH-induced photodegradation pathways of PHE in simulated seawater containing nitrate are proposed based on GC-MS analysis and DFT calculation.During the photodegradation process,the partially degraded intermediates of PHE perform higher toxicity than PHE,but the amount of these intermediates is much less than that of PHE,so it is believed that the environmental risk of PHE would be reduced during photolysis. |
