Study On The Effect Of Microplastics On The Adsorption And Transport Of Cd In Riverine Sediment

The invention of plastics has been hailed as one of the masterpieces of the 20th century and is rapidly becoming a vital raw material for modern civilization.However,plastics entering the environment have brought all sorts of related problems over the decades.Today,plastic pollution is widely recognized as a major environmental burden,especially in the water environment.The pollution of marine ecosystems by microplastics(MPs)has been reported worldwide.Land is one of the sources of plastic in the ocean,and the river provides an important channel for the transport of plastic debris from land to the sea.Moreover,river sediments are considered as sinks of MPs.Although numerous studies have been conducted on MPs pollution in river sediments,the impact of MPs on the environmental behaviors of Cd in river sediments is still unknown.In this work,the effects of six virgin MPs(polyethylene,polypropylene,polystyrene,polyvinyl chloride,polymethyl methacrylate and polylactic acid)on the adsorption of Cd by Xiangjiang River sediment and the transport of Cd in sediment were studied.The findings revealed that the adsorption ability of sediment to Cd decreased with the increase of the content of MPs in sediment.The adsorption process can be commendably depicted by Freundlich isotherm model(R~2≥0.970)and quasi-second-order kinetic model(R~2≥0.931).When the content of MPs in sediment increased to10%,polypropylene had the greatest effect on the adsorption affinity of sediments to Cd while polymethyl methacrylate had the least effect on it.The adsorption capacity of six microplastics for Cd was also investigated.It was found that although PP had the highest specific surface area(61.81 m~2/g)and PMMA had the highest surface charge(48.0±0.7 m V,Zeta potential),PS showed the best adsorption capacity for Cd.Moreover,the addition of MPs accelerated the transport of Cd in sediment,and the transport of Cd in sediment increased with the increase of the content of MPs.The reason may be that after adding MPs,the adsorption capacity of sediment to Cd decreases,and the mass transfer resistance of Cd to sediment reduces,which leads to a faster transport of Cd in sediment.When the contents of MPs in sediment were 0.05%,0.5%,5%and 10%,the saturation points of the breakthrough curves were reduced by about 10,20,40,and 70 pore volumes,respectively.In addition,MPs could reduce the safe value and retention of Cd in sediment,and the vertical spatial distribution of Cd in sediment tended to be uniformed in the presence of MPs.The findings have important implications for understanding the transport of Cd in the presence of MPs,especially in“hot-spot”areas of MPs-contaminated sediments.When MPs are released into river sediment,on the one hand,they can reduce Cd exposure to the upper sediment microbial community;on the other hand,can increase Cd exposure to the deeper sediment microbial community and zooplankton,phytoplankton and bottom dweller.Moreover,the loss of Cd retention in sediment facilitates the transport of Cd into underground water,which presents additional environmental and health risks.This work hopes to provide helpful views on the environmental behavior and risk assessment of Cd in the presence of MPs.