Study On Photocatalytic Degradation Of Microplastics By Layered Double Hydroxide Composite

While modern industrially produced plastic products have greatly facilitated people’s daily lives,they have also brought about serious environmental pollution problems.Plastics degrade slowly in the natural environment and can have a lasting impact on the environment.Plastic waste decomposes into small fragments or particles-microplastics-after long periods of exposure to light,erosion,and weathering in the environment.As an emerging pollutant,microplastics have come into the limelight in recent years.Microplastics in water bodies can easily enter the human body through the food chain,posing a potential threat to human life and health.Therefore,the removal of microplastics from the water environment is urgent.Currently,advanced oxidation processes,represented by photocatalysis,has proven to be effective in removing organic pollutants,making it a viable solution to the problem of microplastic pollution.It can generate strong oxidizing radicals such as hydroxyl radicals（^·OH）and superoxide radicals（^·O₂^-）to completely mineralize microplastics into H₂O and CO₂,achieving complete removal of microplastics.However,the existing photocatalytic technology has limited degradation of microplastics,and the problems of large catalyst band gap,photogenerated electrons and holes are extremely easy to compound,and degradation is mostly carried out under UV light,which hinder further applications.Therefore,it is the future development direction of photocatalytic technology to explore the efficient photocatalysts with better performance and degradation under visible light.In this study,a variety of composite photocatalytic materials were prepared and applied to the degradation of polystyrene（PS）and polyethylene（PE）microplastics for the first time by taking advantage of the high thermal stability and tunable main board cations of layered double hydroxides（LDHs）,also known as hydrotalcite,with the following main research and conclusions:（1）TiO₂@Mg Al-LDH and TiO₂@Zn Al-LDH loaded systems photocatalysts were prepared with Mg Al hydrotalcite and Zn Al hydrotalcite as the carriers and TiO₂ as the loading substance,respectively.The characterization results proved that TiO₂@Mg Al-LDH has higher crystallinity,smaller band gap and possesses better photocatalytic activity.In the photocatalytic degradation of polystyrene（PS）and polyethylene（PE）with different particle sizes in water under UV light,TiO₂@Mg Al-LDH showed better degradation effect than TiO₂@Zn Al-LDH.When p H=7,catalyst dosage=1g/L,and light exposure time of 200 h,the degradation rate of TiO₂@Mg Al-LDH for smaller particle sizes of PS3 and PE3 microplastics could reach 78.6%and 63.8%.The heterojunction generated by Mg Al-LDH and TiO₂ inhibited the compounding of holes and photogenerated electrons,which effectively improved the photocatalytic activity and made the degradation rate much higher than that of the blank control group（39.6%and 33.8%）.And the smaller the particle size of microplastics,the better the degradation effect.（2）The tetrameric Mg Al-based hydrotalcite composite photocatalytic material CuMgAlTi-RLDH was prepared by regulating the metal cation species with a strong response to visible light and a wide range using a co-precipitation method.When the photocatalytic degradation of PS and PE microplastics in water was carried out under visible light,the average particle size of PS and PE decreased by 54.2%and 33.7%,respectively,compared with the initial particle size after 300 h and 200 h of visible light irradiation.The introduced new metal elements copper and titanium increased the compounding sites of excited electrons and inhibited the compounding of photogenerated electrons with holes,thus improving the photocatalytic activity of the catalysts.（3）After the degradation experiments were completed,the degradation mechanism of microplastics in the photocatalytic reaction system was revealed by analyzing the characteristics of the physicochemical changes of microplastics before and after the reaction and the intermediate substances generated during the degradation process,and by free radical quenching experiments:FTIR,SEM,GC-MS and microscopy confirmed that during the degradation process of microplastics,the surface of the polymer was the first to be attacked by the active substances generated by the catalyst（^·OH and^·O₂^-）,and with the extension of the degradation time,the polymer molecular chain of microplastics breaks,converts from large molecular weight polymers to small molecular weight oligomers,and produces ketones,aldehydes,esters and other oxygen-containing intermediates.The loss of volatile intermediates leads to wrinkles,cracks and cavities on the surface of the plastic,and microplastics are degraded from large particles to small particles and even to dissolved organic matter until they are degraded to CO₂and H₂O.