Study On The Oxidative Degradation Process Of Plastics And The Mechanism Of Biodegradation Based On Density Functional Theory

Plastics are synthetic polymers,which are widely used in modern society because of their portability.However,a large amount of plastic wastes in the environment has posed a serious threat to ecosystems and human health.Polystyrene(PS),polyethylene(PE)and polyvinyl chloride(PVC)are three kinds of petroleum-based plastics with high yield and difficult degradation.The research on them is helpful to solve the practical problem of plastic waste degradation.Laccases are regarded as versatile green biocatalysts and potential plastic degradation enzymes,recent scientific research has focused on improving their redox potential for broader industrial and environmental applications.The density functional theory(DFT)quantum mechanics approach,sufficiently rigorous and efficient for the calculation of electronic structures,is conducted to better comprehend the connection between the redox potential and the atomic structural feature of laccases.According to the crystal structure of wild type laccase Cue O and its variant,a truncated miniature cluster model method was established in this research.On the basic of thermodynamic cycle,the overall Gibbs free energy variations before and after the one-electron reduction were calculated.It turned out that the trends of redox potentials to increase after variant predicted by the theoretical calculations correlated well with those obtained by experiments,thereby validating the feasibility of this cluster model method for simulating the redox potentials of laccases.Biodegradation of plastics is a kind of green and pollution-free degradation method,but the microbial strains are generally difficult to grow on the plastics.In order to achieve higher efficiency of microbial degradation,ultraviolet irradiation and dielectric barrier discharge(DBD)plasma treatment were carried out on the selected plastics.The PS,PE and PVC powder were subjected to UV irradiation for 24 h and DBD plasma treatment for 30 min respectively.The results of FTIR detection showed that new carbonyl and hydroxyl peaks appeared in PS,PE and PVC after the above two treatments,indicating that the surface of the plastic powder were oxidized.The results of XPS test showed that the surface O/C of PS after UV irradiation and DBD plasma treatment was increased by about 6.5 and 13 fold respectively than those without treatment,indicating that the oxidation effect of PS surface after DBD plasma treatment was more obvious.The results of GCMS test showed that a variety of oxygencontaining intermediates or products such as aldehydes,ketones,acids,phenols and esters with low molecular weight appeared in PS treated by the two methods,among which more oxygen-containing compounds were detected after DBD plasma treatment,indicating that DBD plasma treatment had better oxidation effect.Therefore,using DBD plasma treating the PS power up to 2 h,the O/C molar mass ratio of the treated PS was 0.0118 detected by the elemental analyzer.