Study On The Degradation Characteristics Of Typical Secondary Microplastics And Their Adsorption Properties For Phenanthrene

Microplastics are considered as potential adsorption carriers of organics in the environment,but the adsorption mechanism between primary microplastics?PMP?and secondary micro-plastics?SMP?is not clear,and whether the dominant adsorption factors are identical in different environmental systems also needs to be studied.Therefore,the current thesis explored the degradation rules and mechanisms of plastic under ultraviolet light?UPVC?UPP?UPET?and the plastic under thermal oxygen?TPVC?TPP?TPET?through degradation experiment,with plastic?PVC,PP,PET?being degraded.With the prinmary microplastics and the secondary microplastics,which are made of three typical degraded plastic films,as research objects,this research aims to investigate the adsorption of phenanthrene and its mechanisms under the system of phenanthrene-methanol,phenanthrene-pure water and phenanthrene-seawater and the major findings are as follows.Under 6 months of ultraviolet light and thermal oxygen degradation,the yellowness index of the plastic grows with the degradation time passing,in which the change under ultraviolet light conditions is more significant than that of the thermal oxygen degradation conditions,and the PVC is more sensitive to light and heat and has the highest yellowness index in all samples.The surface morphology changes during the degradation process are characterized by dents,particles,caking,flakes and cracks.With time passing,the changes are carried out from“point to surface”,“line to surface”,“block to surface”and“outside to inside”,and the main functional groups formed during the degradation are hydroxyl(-OH,3500?3000 cm^-1)and carbonyl(C=O,?1700 cm^-1).The infrared spectrum under ultraviolet light degradation and thermal oxygen degradation shows different positions of characteristic peaks while the absorption intensity under both conditions increase with the degradation time,with no significant new functional groups being formed in PET.Finally,different materials have undergone degradation under different conditions,and their thermal stability has changed to different degrees.It is concluded that the thermal stability of TPVC,UPP and TPP is decreased while the thermal stability of UPET and UPVC is increased,and the thermal stability of TPET remains unchanged.Microplastics exhibit different adsorption capacities under different systems.The saturation adsorption capacity of PMP and SMP in the phenanthrene-methanol system,phenanthrene-pure water system and phenanthrene-seawater system are 0.59-8.56?g/g,18.39-25.53?g/g and 28.46-30.71?g/g respectively.The kinetic adsorption of phenanthrene by PMP and SMP in the three systems conforms to the quasi-second-order kinetic equation,with the R² value above 0.93,and the deviation between the predicted value and the experimental value is small.Among them,the R² value in the seawater system is above 0.99,and the difference between the adsorption amounts of SMP and PMP is small.Therefore,the actual adsorption capacity of microplastics in the environment can be predicted through the simulation of phenanthrene by the primary microplastic.Through the simulation of isothermal curve,it is found that Freundlich equation has the highest fitting degree,followed by Linear equation with the Langmuir equation having the lowest fitting degree.According to the adsorption analysis of three systems,the adsorption of microplastics to phenanthrene is a state of coexistence of chemical adsorption and physical adsorption,and it is a multi-molecular layer adsorption.The factors leading to various adsorption capacity and adsorption property are different under different types of microplastics and background solutions.In the Phe-methanol system,because of the hydrophobic distribution,the microplastics adsorption capacity is far less than that of the other two systems,with the PMP being of distribution while the SMP of chemical adsorption.And the reason for the SMP adsorption capacity being larger than that of PMP is that SMP has the function of hydrogen bond.In the Phe-pure water system,the main adsorption is physical adsorption,and the reason for the smaller adsorption of SMP than PMP is the decrease of specific surface.In the Phe-sea water system,the main adsorption is physical adsorption,and the reason why SMP adsorbs more than PMP is due to the interaction between the ions in the background solution and the exposed hydroxyl and carbonyl groups on the plastic surface.Salinity is the main factor for why the adsorption capacity of phe-seawater systemis larger than that of phe-pure water system.The adsorption capacity of microplastics in three systems is as follows,Phe-sea water system>Phe-pure water system>Phe-methanol system,and the type rule is:PP>PVC>PET.