The research progress in migration, distribution, biological effects and analytical methods of microplastics

Microplastics (MPs) are synthetic organic polymers. The particle sizes of MPs range from approximately 0.01 to 5 nm. The photodegradation of MPs is more difficult than that of bulk plastics. Therefore, MPs are regarded as potential persistent organic pollutants. Researches of MPs have become a hot spot, and lots of MPs studies were reported recently. Although there are a lot of studies on the migration, distribution, biological effects and analytical methods of MPs in natural environments, systematic and comprehensive review articles are emergent, especially for the recent literatures. The present work summarized the researches involving the migration, distribution, biological effects and analytical methods of MPs in recent years. MPs are generally divided into two types, primary MPs and secondary MPs. Primary MPs are intentionally designed and produced for certain purposes, while secondary MPs generate from the fragmentations of bulk plastics and the breakage of clothes fibers. Currently, researches of MPs distribution focus on in marine environments, and found that MPs distributed throughout the ocean, even in the north and south poles, driven by the flow of sea water. However, researches of on MPs distribution in fresh water and terrene are limited. Researches also showed that MPs could migrate among terrestrial, freshwater and marine environments, where freshwater environments affected the migration and interaction of MPs between terrene and marine environments. Studies of biological effects of MPs focus on two parts, ingestion effects and combined effects with organic contaminants. Ingestion of MPs damages living beings and then MPs transfer through food chains. The responses of living beings to MPs are mainly related to sublethal effects at environmental-relevant concentrations. The inhibition of individual growth and reproduction, disturbance of proteins and genes, and reduction of nutrition uptake were reported for specific physiological effects of MPs. Some researches proposed that MPs could be accumulated in living organisms. Therefore, humans, as a part of the food chains, will inevitably be affected by MPs. The organic contaminants combine with the MPs released from the plasticizers in the production of MPs and then are adsorbed in the natural environments. MPs have been found in seafood. As a result, MPs become one medium of human exposure to organic pollutants. The analysis and identification of MPs are critical to other researches, such as environmental behaviors and toxicity. In general, the analytical methods of plastic include physical and chemical characterizations. Physical characterizations involve visual, microscopy and spectroscopy methods. Chemical characterization methods, such as differential scanning calorimetry (DSC), gas chromatography mass spectrometry (GC-MS), scanning electron microscope and energy disperse spectroscopy (SEM-EDS), and thermal desorption gas chromatography mass spectrometry (TDS-GC-MS) are frequently used in chemical characterizations. Although lots of analytical methods were proposed by researchers, there are still some shortcomings and limitations, for example, the influence from environmental or biological matrices. It is necessary to develop effective and accurate methods for the analysis of MPs. Through many researches of MPs were reported recently, the information of source, migration, distribution, biological effects and analytical methods of MPs is not enough to scientifically evaluate their environmental and health risks. Consequently, the present review also proposes some perspectives for MPs researches. It is worth to study the distribution of MPs in terrestrial and freshwater environments, the biological effects at individual level, the prevention and control of MPs pollution and the environmental behavior of nanoplastics. Integrating the data of MPs source, distribution, behavior and toxicity is necessary to the scientific evaluations of MPs risks. This review provides insights in the control techniques and theoretical researches of MPs.