Study On The Microplastics Pollution In Environment By Using The Coherent Anti-stokes Raman Scattering Spectroscopy

With the development of the plastic industry,plastic pollution in environment has becoming prominent and already caused worldwide attention.Microplastics is defined as plastic particles with a diameter less than 5 mm.Because microplastics is with a tiny size and in its high ratio of surface area with volume,they can bear the ability to absorb the other pollutants.Flowing with rivers,air to spreading on the earth,it is even eaten by creatures and pose a major threat to life.The survey of microplastics in the environment has become highly concerned in recent years.The methods employed to extract microplastics are based on flotation,density or centrifugation separation system to make a pretreatment.And then,a physical or chemical methods was designed to identify.Nonetheless,these methods required a long waiting time for pretreatment and could easily be affected by the subjective factors.Therefore,this article proposed to use the Raman spectroscopy to detect plastic microspheres in sea sand in a qualitative way.And the Coherent Anti-Stokes Raman Scattering（CARS）method was used as quantitatively method by imaging the distribution of plastic microspheres in sea sand.The experiments were designed by mixed two microplastic microspheres,PS and PMMA,with sea sand and simulated polluted sea sand by microplastics.The Raman spectroscopy demonstrated:the Raman frequency shifts in the range between 0 and 500cm^-1of seawater and sand with no overlap with that of the two microplastics,PS and PMMA.Even if sand also has a strong fluorescence signal,and it becomes stronger for the sand in darker color.But this signal can serve as the base line and without interfere the reading of the Raman shift.The results proved that the Raman spectroscopy could serve as a method to detect the plastic microspheres in sea sand.And the Raman signal becomes stronger for the microspheres in lager size.In order to study the distribution of this pollution in a quantitative way,we mixed PS microspheres with a diameter of 10μm into sea sand,and the CARS spectral imaging was carried out at 1605cm^-1.The results were showed as:（1）The CARS signal of seawater is weak and separated from that of PS microspheres.An image processing algorithm based on morphology can extract the information of the microspheres to make the distribution clearly.（2）Sand can generate a strong CARS signal,which can interfere with the detection of PS microspheres.So CARS spectral imaging can detect PS microspheres between the grains of sand.Although there are some interference signals,the signal can be treated by median filtering morphological analysis signal,removing a portion of the interference signal in order to achieve detection purposes.However,this method is not effective in detecting PS microspheres when it being adsorbed on the surface of sand particles.This thesis combined Raman spectroscopy and CARS spectral imaging method to study the pollution of plastic microspheres in sea sand.Both methods are non-destructive and real-time detection methods.The results showed that both methods were feasible for detecting plastic microspheres in sea sand.Raman spectroscopy can more accurately determine the types of plastic particles,and CARS spectral imaging method can quickly detect the distribution of plastic particles in the sample.In practice a suitable detection method can be selected according to the requirements.