Mechanism On The Adsorption And Photodegradation Of PAHs At The Interface Of Natural Minerals In Water Environment

With the rapid development of industrial production and the world population,the water pollution has become increasingly serious.As the largest energy consumption in the world today,the petroleum related products have released a large amount of polycyclic aromatic hydrocarbons（PAHs）into the environment during the combustion,emission or accidental leakage.Some studies have revealed that PAHs can be converted into more toxic secondary pollutants through a series of physical and chemical processes in the water environment,but there are few studies on the migration and transformation behavior of PAHs in the environmental media,especially the transformation mechanism in the presence of natural inorganic mineral.There are a large number of natural minerals in the natural water environment.Some of them are semiconductors,they can drive some chemical reactions in the natural environment,thus affecting the photochemical conversion and toxicity of pollutants in the environment.In this study,acenaphthene（Ace）and anthracene（Ant）were used as model PAHs,and the natural hematite（NH）and pyrite（NP）were used as typical natural minerals.The adsorption and transformation behavior of PAHs at the interface of natural minerals were studied,and their mechanisms were revealed.The chemical composition,crystal structure,morphology and surface area of natural minerals were determined by mineralogical characterization.The adsorption mechanism of acenaphthene on natural hematite was studied by using adsorption kinetics and isothermal adsorption model.In different water environment systems,the formation rate of active species in the photocatalysis/Fenton reaction,the steady-state concentration and the second order reaction rate constant with PAHs were measured by combining molecular probe method and competitive reaction.The effect of natural pyrite on the photodegradation of anthracene was studied,and the degradation intermediates and toxicity in the photodegradation process were analyzed.Finally,a photochemical model was established using simulation software to predict the photochemical degradation and natural fate of PAHs in the actual water environment.The main conclusions are as follows:（1）The maximum adsorption capacity of acenaphthene on natural hematite is 31.6μg g^-1,which is easy to be adsorbed.Weber-Morris model reveals that the adsorption types are surface adsorption and micropore filling,and mainly occured on the initial stage of adsorption.In addition,Langmuir model and isothermal adsorption data can be well fitted.Therefore,the adsorption of acenaphthene on the surface of natural hematite is mainly uniform monolayer adsorption.In the study of adsorption stability,we have studied the photocatalytic activity of natural hematite and the adsorption property of acenaphthene on natural hematite.The results show that the photochemical degradation of anthracene in the presence of small amount of natural hematite can be almost ignored,and the adsorption on its surface is physical adsorption,and there is almost no chemical decomposition.（2）Low dose of natural pyrite can promote the photodegradation rate of anthracene.Compared with fresh water system,the light shielding effect caused by natural pyrite in seawater has a more significant impact on anthracene photodegradation.There is a small amount of H₂O₂ produced in the photocatalytic reaction system.The steady-state concentration of ¹O₂,O₂^·-and ·OH is 1.05×10^-13,4.47×10^-14 and 3×10^-15 M in fresh water system.The steady-state concentration of ¹O₂ and O₂^·-is 1.63×10^-13 and 1.5 × 10^-13 M in seawater system.（3）The photodegradation rate of anthracene in seawater is greater than that in fresh water.The photodegradation of fresh water is dominated by 102 and OH.The contribution of ¹O₂,·OH and O₂^·-to anthracene degradation is 20.8%,10.0%and 3.5%respectively.In the seawater system,O₂^·-and ¹O₂ plays a leading role in the intermediate photodegradation,and their contribution are 28.9%and 23.1%,respectively.（4）The oxidation of NP in air can broaden its band gap.In this work,the band gap of NP is 1.56 eV,the valence band position is 1.35 eV,and the conduction band position is -0.21 eV.NP can produce active free radicals by photocatalysis and Fenton reaction,then attack the organic pollutants（anthracene）and decompose them into smaller intermediates.The GC-MS analysis detected the formation of intermediate products,and we proposed two degradation pathways of anthracene.The toxicity analysis software was used to clarify the impact of intermediate products on organisms in the environment.（5）APEX software tool is used to predict the fate of anthracene in natural water environment.The results show that direct photodegradation is the main phototransformation pathway of anthracene in natural water.Direct photolysis is affected by water depth and NP dose,and the contribution rate of direct photolysis in seawater is higher than that in freshwater.