Experimental Study On The Settling Of Microplastics-marine Snow In The Quiescent Water Of Estuarine And Coastal Areas

Marine microplastics is a global environmental problem and challenge,however,the transport and deposition processes and impact mechanisms of microplastics from source to sink are still unclear.The settling velocities of microplastics are important for estimating their possible migration pathways and deposition areas in estuarine and coastal waters,i.e.,predicting the fate of microplastic particles in the water column.In this research,experiments on the settling of microplastics(granular and fibrous)in estuarine coastal waters were conducted,and the effect of marine snow on the settling process of microplastics was considered.The main conclusions are as follows.(1)The occurrence status of microplastic and marine snow in global estuarine coastal areas were investigated,and a conversion method from quantitative concentration to mass concentration based on the average abundance,particle size range,shape of microplastics and mesh pore size was proposed.We also calculated the distribution of microplastics mass concentration in major estuarine and coastal areas worldwide.(2)Using polystyrene particles and polyvinyl chloride particles as research objects,we recorded the settling process of various microplastics in different water bodies by means of the high-precision particle settling observation system,and analyzed and summarized the influence law of marine snow on the settling velocity of various microplastics.The analysis results shows that the settling process of microplastic particles smaller than 50 microns is more influenced by marine snow,and the average settling speed is reduced by more than 35%.The denser or larger the size of the microplastics,the less they are affected by marine snow;The settling process of microplastics with different shapes is affected by marine snow to a different extent.And the effect of marine snow concentration on the settling process of microplastic particles was studied.The experimental results showed that when the mass concentration of marine snow was less than 2 mg/L,the influence of marine snow on the settling process of microplastic particles was very small,and when the concentration of marine snow was greater than 2 mg/L,the hindering effect of marine snow on the settling process of microplastic began to be significant.The equation of the relationship between the influence factor of marine snow and the concentration of marine snow was obtained by a good fit.(3)The settling process of polyamide fibers in different water bodies was observed,and the results show that:marine snow reduces the settling velocity of microplastic fibers,and the average settling velocity of microplastic fibers in water containing marine snow slows down by 12.1%compared with their free settling;with the increase of microfiber length,the hindering effect of marine snow decreases;microplastics with different shapes are affected by marine snow to a different extent.