Polymerization Of Phenolic Compounds On The Interface Of Structural Fe(Ⅲ)-Bearing Clay Minerals

Iron in the lattice of clay minerals is termed as structural iron.Almost all clay minerals have a certain amount of structural iron and it is one of the most important iron forms in nature.As the basic raw materials for organic chemical industry,phenolic compounds are widely used and have been detected in various environmental media.Polymerization of phenolic compounds at the interface of clay minerals has been focused by researches for many years.On the one hand,the polymerization of chlorophenols could produce dioxins or dioxin-like compounds,which are potential pathways for the natural formation of dioxins.On the other hand,the polymerization of phenolic compounds,such as pyrogallol and guaiacol,might be an important pathway to form humic substances in nature.However,most of the clay minerals examind by previous studies were Fe(Ⅲ)-saturated clays,i.e.replacing the interlayer cations of the clay minerals by Fe(Ⅲ)through ion exchange.However,iron asscociated with clay minerals is rarely present in the interlayers and interlayer Fe(Ⅲ)and structural Fe(Ⅲ)differ greatly in properties.Therefore,it is necessary to explore the role of structural Fe(Ⅲ)in the clay minerals in the above reactions.Meanwhile,mineral dust is a common particulate matter in the atmosphere with clay minerals as the important component.If the above reaction occurs on the clay mineral particles in the atmosphere,it might greatly increase the toxicity of the particles,which risks to human health.However,current researches on the interaction between mineral dust and organic rarely concern about the toxicity change of the particles,especially for the process that would potentially increase the particles toxicity.Moreover,since mineral dust is an important source of iron to the ocean,the dissolution of iron is a controlling factor for phytoplankton in the ocean to take in sulphate,nitrate and phosphate,which could directly affect the primary productivity of the ocean and finially affects the global carbon cycle and climate.But at present,the effects of the interfacial reactions mediated by mineral dust under ambient conditions on the iron dissolution behavior of the particles have not reported.Since these surface reactions usually involve the reduction of iron,and the formed polymers often photisentive,it is worthwhile to further investigate the effect of the above similar interfacial reactions on the iron dissolution from the particles.In order to investigate the oxidative polymerization of chlorophenols mediated by structural Fe(Ⅲ)bearing clay minerals,three kinds of clay minerals including montmorillonite SWy-2,nontronite NAu-1 and nontronite NAu-2 were selected,considering their difference in iron content and site occupancy,synthetic montmorillnite SYn-1 without structural iron was used as the control.The reactions between these clay minerals and 2,4,6-trichlorophenol under simulated environmental unsaturated soil conditions were investigated.The experimental results turned out that structural iron is the indispensable factor for the clay mineral to mediate 2,4,6-trichlorophenol dimerization to form hydroxylated polychlorinated diphenyl ethers.The reaction is controlled by the Fe(Ⅲ)site occupancy and environmental humidity.The active structural Fe(Ⅲ)in NAu-2 is close to the interlayer and has a strong interaction with 2,4,6-trichlorophenol molecules at low humidity,whereas the interaction would be weakened at high humidity,resulting in a decrease in reaction of NAu-2 with 2,4,6-trichlorophenol.The structural Fe(Ⅲ)in SWy-2 and NAu-1 is located far from the interlayer and water molecules can form hydrogen bonds with the organics to reduce the activation energy of the products formation,hence the reaction of 2,4,6-trichlorophenol on these two clay minerals is increase with the increase of humidity.To evaluate the reactivity of this kind of reaction in atmospheric environment and its potential impact on human health,we designed a reactor that can simulate the reaction between atmospheric particulate matter with volatile organic compounds.The most common clay minerals in mineral dust,namely,montmorillonite SWy-2,illite IMt-2 and kaolinite KGa-2,were selected to react with gaseous 2-bromophenol.The experimental results showed that SWy-2 and IMt-2 could mediate the dimerization of 2-bromophenol to form hydroxylated polybrominated biphenyl and hydroxylated polybrominated diphenyl ether.KGa-2 showed no reactivity due to its low iron content.Further analysis indicated that besides structural Fe(Ⅲ),iron oxides associated with IMt-2 also contributed to the reaction.A humid environment was favored for the dimerization reaction.Subsequent toxicity examination results certified that the dioxin-like products formed on the clay mineral surface increased the toxicity of the particles to A549 by destroying the cell membrane and protein.To investigate the effect of similar interfacial reactions on iron dissolution behavior of the clay minerals in atmospheric environment,we conduced the iron dissolution test of the clay minerals having been reacted with gaseous guaiacol.The clay minerals here included montmorillonite SWY-2,illite IMT-2 and Arizona test dust AZTD.The results showed that the structural Fe(Ⅲ)in clay minerals and the associated iron oxides could mediate guaiacol to form dimers,trimers and tetramers.The effect of humidity on the reaction was firstly positive and then negative.The iron dissolution of the clay minerals all increased as their Fe(Ⅲ)had been reduced by guaiacol.For SWy-2,the adsorption of guaiacol and the formed polymers could further affect its iron dissolution behavivor.Under ligh condition,the surface of SWy-2 could polarize oxygen to generate hydroxyl radicals to oxidize of the formed Fe(II),which inhibite the iron dissolution.However,with the gas-solid phase reaction with guaiacol proceeding,SWy-2 would adsorb large amounts of guaiacol to consume the hroxyl radicals,meanwhile the formed high polymers could complex with the structural Fe(Ⅲ)and photoreduced to further promore the iron dissolution.This research indicates that phenolic compounds could polymerize to form dioxin-like compounds on the interface of structural iron bearing clay minerals,providing important theoretical support for the natural formation of dioxins.Meanwhile,it is confirmed that the reaction could occur in the atmospheric environment,and will increase the adverse effects of the particles on human health.Therefore,not only the particulate matter itself,but also the relevant surface reactions should be considered to fully assess the toxity of the particles.Further,similar reactions could increase the dissolution of iron.Therefore,the interfacial reactions involved in iron in mineral dust should be taken into consideration when evaluating the iron dissolution of iron in mineral dust.These results provide new perspective and data support for further exploration of the abiotic transformation of organics on iron-bearing clay minerals and the potential effects of this process on human health and biochemical cycle.