Exploration And Application Of Confocal Micro-raman Spectroscopy In Detection Of Marine Sediment Microplastics

Microplastics has attracted increasing attention in recent years as a new type of environmental pollutant.Since the report of microplastics in sediment in 2004,microplastics have been found in offshore coastal zones and in the deep sea,in the Antarctic,the Mariana Trench,within marine organisms,and even in human bodies.As for the marine environment,marine sediments are hypothesized to be major sinks for the accumulation of microplastics.However,the detection methods for microplastics in marine sediments are lack of unified standards and vary according to the study area.For microplastics separated from sediments,the selection of appropriate detection and identification methods is of vital importance to the research,which could ensure the efficiency of detection and identification without damaging the samples.Currently,in addition to optical visual identification could provide physical information of particles,Fourier transform infrared transform and Raman spectroscopy are regarded as nondestructive spectroscopic identification methods which can realize the characterization of the internal structure of molecules.FTIR characterize the structure of molecules according to infrared absorption spectrum,and Raman spectroscopy reflects the information of molecular structure by Raman scattering.However,at present,for the detection of microplastics in sediments,the pretreatment steps of samples are quite complicated,such as acid-base digestion of organic matter,staining of sample particles and other steps to assist qualitative identification and sample counting statistics.Additionally,FTIR spectroscopic method could hardly detect particles less than 20 ?m,which would become the main constraint for the detection of microparticles in the lower size range.Based on confocal micro-Raman spectroscopy,this study aims to simplify the pretreatment procedure of microplastic samples and explore fast and efficient detection methods of smaller microplastics from marine sediment by applying simulation experiment and identification of actual marine sediments samples.In this study,a Raman reference library of microplastics was established,including18 kinds of plastics for commercial and industry use.Raman peaks were assigned in detail,which could be compared with the characteristic peaks of samples to acquire the qualitative characteristics of microplastics samples.For the pretreatment steps of microplastic samples in sediments,the filtration device was improved and the digestion of organic matter was omitted to achieve rapid detection.The recovery experiment of microplastics was carried out by simulating the sediment environment,in addition,the characteristics of microplastics were systematically summarized,and the detection parameters suitable for Raman technology were obtained.Furthermore,this simplified detection process was conducted to marine sediment samples to verify the effectiveness of the method.In this regard,the conclusions are as follows:(1)Raman spectra reflect the structural information of microplastics,and different kinds of microplastics from marine sediment could be distinguished.Since the Raman spectra represents the vibration characteristics of molecules,the non-polar molecular vibrations which are not obvious for FTIR could be reflected clearly,especially for microplastics which have carbon-carbon double bonds.The internal structure of the molecule could be obviously characterized.Other Raman characteristic peaks,such as carbon-hydrogen bond vibration,benzene ring respiratory vibration and carbonyl vibration,could be characterized by different types of microplastics.(2)Raman spectra of microplastics without digestion of organic matter with high signal-to-noise ratio could be obtained by confocal micro-Raman detection.In the environmental recovery of simulated sediments in the laboratory,the microplastic samples were extracted without digestion of organic matter,besides,the Raman signals were still clearly displayed,and the classification of microplastics could be realized.The Raman spectra of microplastic from the marine sediments without digestion of organic matter could still present high quality.Moreover,the characteristic information of microplastic type,particle size,shape could be reached.(3)The laser power,the particle size of the microplastic and the pigment coating on the surface of the microplastic all have different effects on the detection,and could be characterized in the Raman signal.Due to the change of the plastic surface temperature caused by the change of power,the microplastics with the same particle size appear spectral deformation in the performance from low power to high power,on the contrary,the background level would change with different particle size at the same power.When the microplastic surface is covered with pigment,the original Raman effective signal would be masked.Therefore,the spectra of specific pigment particles should also be included in the reference library to ensure the completeness.(4)Based on the established detection process and Raman analysis parameters,the detection process of microplastics with small particle size was applied to the marine sediments of Huiquan Bay.The characteristics of microplastics with a particle size of less than 500 um in the marine sediment samples of Huiquan Bay were obtained,and the analysis of PP particles less than 10 microns was realized.The results indicate that microplastics with particle size less than 50 microns account for 56% of the samples with particle size less than 500 microns,which provides data support for exploring the pollution of microplastics in the study area and the existing pattern of microplastics in the marine environment.