Degradation Of Polycyclic Aromatic Hydrocarbons On Montmorillonite And Formation Mechanism Of Environmental Persistent Free Radicals

Environmentally persistent free radicals?EPFRs?are formed during the degradation of organic chemicals.They rapidly attracted research attention due to the harmful effects on the environment and human health.The process and stability mechanism of EPFRs have been initially recognized.But the research about their environmental behavior and risk are still rare.The effects of geologic and environmental conditions on the formation of EPFRs are still unknown.On one hand,the EPFRs as intermediate products are formed during the degradation process of organic chemicals.On the other hand,the PFRs can affect the environmental behavior of precursor compounds through either promoting or inhibiting their degradation.Therefore,a new perspective for investgating the environmental behavior of PAHs is to incorporate the impacts from EPFRs.As one kind of persistent organic pollutants with high health risks,polycyclic aromatic hydrocarbons?PAHs?are ubiquitous in the environment.The environmental behavior of PAHs on particulate matter has an important effect on their environmental risks.The environmental risk assessment is based on the concentration of organic pollutants.Some important informations on the existence of new environmental pollutants,such as environmentally persistent free radicals?EPFRs?,are ignored.This may be the reason why some environmental health risks are not well explained.A comprehensive understanding about the environmental behavior of PAHs and the formation and stabilization mechanisms of EPFRs will provide new ideas for evaluating pollution characteristics and treatment technologies.In this study,the formation mechanism of EPFRs were studied by laboratory simulation of PAHs degradation.The pollutant level of environmental samples was analysised.It was hypothesized that the formed EPFRs was the intermediate products of the PAHs degradation on soil minerals.EPFRs were stablized in the environment after being combined on the surface of solid particles and their high health risk were often neglected.The main results of this study are as following:1)The degradation mechanism of PAHs at different sites of montmorillonite?MT?was studied.Anthracene?ANT?as PAHs model compound was bounded on the surface of montmorillonite through different loading forms.The degradation kinetics of ANT was studied.The overall degradation of ANT on MT is in the range of80-90%after 24 hours.The degradation process of ANT can be divided into surface adsorption system and coating system on surface of MT which showed inconsistent degradation process.The degradation of ANT is significantly inhibited in the adsorption system.It is mainly due to the fact that the hydrophobic surface,nano or micropores of MT are preserved or protected through the strong adsorption.Conversely,the degradation rate of ANT in the coating system exceeded 90%,which was mainly attributed to the loose combination of ANT on the MT edge structure.Superoxide radical(O₂^·-)and silanol on the edge structure of MT are the main active sites.ANT is more easily converted and degraded by oxidation on these sites.The selective screening site further confirmed that the silanol and metal oxide active sites on the MT surface are the main active sites for the degradation of ANT.The contribution of different metal oxides to the degradation of ANT are obviously different.The Fe₂O₃ loading MT can promote the degradation of ANT in coating system at low solid phase concentration,while the appearance of ZnO and CuO inhibite its degradation.At high solid phase concentration,the degradation of ANT on MT are inhibited by metal oxides.2)The adsorption of PAHs with gaseous molecule onto particles is a key process for their environmental fate.In this study,the degradation of ANT in the volatilization adsorption system is significantly higher than that in the liquid phase adsorption system.The degradation and formation EPFRs of ANT on MT,Al₂O₃ and SiO₂ were compared.The high reactive sites are formed on special layered structure and isomorphous substitution of MT.The sites can accelerate the degradation of ANT precursor compound.The formation mechanism of EPFRs during the degradation of PAHs on minerals were studied through different loading forms.EPFRs can be formed in ANT adsorption system through the volatilization on mineral surface.It is emphasized that the combination of ANT with the gaseous molecules on the particulate matter surface can form EPFRs.The health risk of the system is often overlooked in the risk assessment process.In addition,a significant EPR signal was observed in the coating degradation system,but no free radicals were formed in the adsorption degradation system.Some oxygen center radical groups were formed in intermediate products through the g-factor values of the EPR signal.There was a significant negative correlation between free radical strength and the degradation rate?r=-0.793,p<0.01?on the MT surface with metal oxides.These results indicate that the addition of metal oxides can enhance the stability of EPFRs,which may inhibit the degradation of ANT.3)The mining and burning of low-rank coal in Xuanwei,China have attracted a great deal of research attention because of the generated PAHs and the high incidence of lung cancer in this region.Given the abundant transition metals in the allitic soil,we hypothesized that environmentally persistent free radicals?EPFRs?are formed in this region and the potential risk had not been addressed.Strong electron paramagnetic resonance?EPR?signals of 3.20×10¹⁷-3.10×10¹99 spins/g were detected in environmental samples,including chimney soot,coal,soil and total suspended particles?TSP?.These EPR signals did not significantly change after 18-months storage and had g-values in the range of 2.0039-2.0046,suggesting typical organic free radicals.Similar strong EPR signals were observed in PAH?anthracene and pyrene as model compounds?degradation on simulated soil particles and lasted over one month even when the applied PAHs were 100%degraded.Based on g-value and bond width,we propose that EPR signals detected in TSP and soot originated from both coal combustion and PAH photodegradation.Further research is thus urgently required to investigate EPFR generation,exposure and risk in Xuanwei to better understand the cause of high lung cancer incidence.In the previous study for routinely evaluating the environmental pollution level,the target pollutants were separated by chemical methods to quantify their concentrations.However,the environmental risks from the combination of various pollutants were ignored.As a new type of environmental pollutant,EPFRs may be one of the reasons for the abnormal environmental health risks in the low rank coal areas of Xuanwei.The formation of EPFRs may also be used to explain the phenomenon that the characteristics of conventional pollutants do not match the local high health risk.Studies on the degradation process of PAHs and the formation mechanism of EPFRs will help to increase the understanding of their environmental risks.