Study On The Oxidation Of Low-density Polyethylene Microplastics Via Hydrothermal Method Coupled With Persulfate Advanced Oxidation Technology

"Microplastics（MPs）",with the particle size<5 mm.Due to its environmental persistence,bioaccumulation and wide transmission,MPs poses underlying threat to the ecology and human health.Therefore,it is urgent to develop effective methods to remove MPs from water body for the security of environment and ecology.Hydrothermal treatment technology is an emerging and prevalent method for the degradation of macromolecular organic pollutants into small molecules or monomers via taking use of the subcritical water with excellent physicochemical properties and produced heat energy under high temperature and pressure.In addition,persulfate-based advanced oxidation technology has aroused great interests in the degradation of organics field benefited by its strong oxidation ability and simple operation.Inspired by above works,it is highly anticipated that the hydrothermal process coupled with and persulfate-based advanced oxidation technology can achieve the removal of microplastics from water,which exhibited potential application prospects.In this paper,the basic physicochemical properties of low-density polyethylene microplastics（LDPE）were firstly investigated by SEM,FT-IR,Raman,particle size analysis and other characterization methods.The stability of LDPE in water was determined by the TOC values after reaction and the p H changes of aqueous solution.The main factors,including reaction temperature,reaction time and persulfate concentration,were explored to identify their effect on the degradation of low-density polyethylene（LDPE）.Scanning electron microscopy（SEM）,infrared spectroscopy（FTIR）,Raman spectroscopy,X-ray photoelectron spectroscopy（XPS）were employed to analyze the microscopic morphology,surface functional groups,element content of LDPE before and after reaction.In order to get insight into the degradation mechanism of LDPE,the dominant radical in the reaction system was analyzed by electron spin resonance and radical quenching experiment.The specific research results are as follows:（1）The results of microplastic characterization showed that:the surface of LDPE was rough,rugged,and slightly wrinkled.The particle size of microplastics was different,which accounted for 93%in the range of 10-100μm.Characteristic peaks of LDPE such as-CH₂-,-CH₃,C-H and C-C were observed by FTIR and Raman spectra,indicating that LDPE was a hydrocarbon polymer composed of carbon chains.In addition,the stability of LDPE properties was determined by monitoring the change of p H and TOC at different times.Results demonstrated that the p H was stable at 6.4-6.6.TOC was maintained at 6 mg/L,indicating that LDPE was stable at room temperature and pressure.（2）The degradation of LDPE was greatly influenced by hydrothermal temperature,time,and oxidant concentration.With the increase of hydrothermal temperature,reaction time and oxidant concentration,the degradation efficiency of LDPE was enhanced,as reflected by the increased TOC in liquid phase after reaction as well as emerging more microfolds and cracks on the surface of microplastic observed by SEM.Especially,many holes appeared at 220℃under 12 h and the oxidant concentration of100m M.Correspondingly,the persulfate content in the hydrothermal solution decreased significantly after the reaction,with the remaining content about 3 mmol/L.These results indicated that the persulfate was consumed during the reaction.（3）After reaction,a series of new absorption peaks representing C-H and C-O-C appeared at 719 and 1110 cm^-1 as compared with the original LDPE sample.Combined with CI value（reflecting the degradation degree of microplastics）,it implied that the C-H bond of LDPE was oxidized and broken to form oxygen-containing groups under the synergy interaction of hydrothermal and persulfate.In addition,the intensity of methylene peak decreased observed by Raman spectra.Moreover,oxygen-containing groups（C-O-C,C-OH,C=O）emerged in the O 1S and C 1S XPS spectra.The above results indicated that the LDPE became more easily broken and oxidized.（4）The degradation mechanism of LDPE was further investigated by EPR and free radical quenching experiment.Results demonstrated that the main active groups involved in the reaction were·OH and SO₄^·-.