The Influence Of Microplastic On The Migration And Transformation Of Microcystin That Regulated By Turbulence:the Mechanisms Study

Microcystins are one of the most common toxic organic pollutants in freshwaters,and their toxicity are largely linked with their environmental behavior in the aquatic environment.Due to the extensive application of plastic products,microplastics have been frequently detected in environments in recent years.In aqueous environmental systems,microplastics have potential effects on the environmental behavior and effects of microcystins,and the extent and process of the effects are closely related to hydrodynamic conditions.Therefore,the present study was performed in order to elucidate the interfacial processes of microplastics affecting the migration and transformation of microcystins in the aqueous and microplastic phases,to analyze the key mechanisms of hydrodynamically mediated environmental fate of microplastics and microcystins,and to deepen the current understanding of the ecological effects of microcystins-microplastics complex as well as assess the ecological risks of the microcystins-microplastic complex.This study focuses on the effect of microplastics on the environmental behavior of microcystins.Firstly,we investigated the adsorption of microcystins by different types of microplastics,studied the influence of algogenic organic matter on the adsorption processes and the key mechanisms involved.Then,the effect of microplastics on the migration and transformation of microcystins under turbulent conditions was studied in an approximately homogeneous turbulence simulation system(AHTS),which obtained water that collected from the Yulin River.The transport and transformation of microcystins infulencecd by microplastics were conducted,during which the dynamic of microcystins concentration,the formation process of biofilm on the surface of microplastic,and the biodegradation process of microcystins were evaluated.The main findings are listed as follows:(1)The results of experiments on the adsorption of microcystins and algogenic organic matter by different types of microplastics found that the adsorption of microcystins by polystyrene(PS),polyethylene(PE)and polymethyl methacrylate(PMMA)shared the same equilibration time(24 h)at p H=7,25°C.After reaching adsorption equilibrium,the maximum adsorption capacity of PS for microcystins was 843μg/g,while the maximum adsorption capacities of PE and PMMA for microcystins were722 and 521μg/g,respectively,the adsorption processes fit the pseudo-second-order kinetics(R~2>0.99)and the Langmuir isotherm model(R~2>0.92).The maximum adsorption capacities of PE,PS and PMMA were 1400,1470 and 1300μg/g when adsorbing algogenic organic matter,respectively.Which were significantly greater than microcystins,indicating that the three microplastics displayed greater affinity for algogenic organic matter.The adsorption capacity of microplastics to microcystins and algogenic organic matter is closely related to the property of microplastics.The strongest adsorption capacity of PS is due to its maximum specific surface area(5.136 m~2/g)and can formπ-πdouble bonds with microcystins or algogenic organic matter.(2)The adsorption of microcystins by PE,PS and PMMA under the influence of algogenic organic matter revealed that the maximum adsorption capacities were 558,655and 493μg/g,which decreased by 22.3%,22.7%and 5.4%,respectively.This indicates that algogenic organic matter inhibits the adsorption of microplastics to microcystins.The results of Fourier transform infrared spectroscopy(FTIRs)analysis showed that the adsorption mechanism of PE,PS and PMMA on microcystins and algogenic organic matter is physical adsorption,including intermolecular van der Waals forces,electrostatic interactions,and microporous filling.(3)The effect of PS on microcystins transport and transformation processes was investigated in the AHTS for 37 days.It was found that the concentration of microcystins did not change significantly in the system with the addition of Na N?,indicating the biodegradation of microcystins in waters is inhibited.In the PS(20 mg/L)and Na N?added system,the microcystins concentration was reduced by 9%compared to the initial value,suggesting the adsorption of microcystins occurred.When PS was present in the system and without the addition of Na N?,the removal rate of microcystins increased to35.8%,which was contributed to the joint effects of microcystins adsorption and biodegradation.The removal of microcystins in the static state of water(20 mg/L PS,without Na N?)was 24.9%.The results of microcystins concentration dynamics in the static and turbulent water confirmed that turbulent benefited microcystins adsorption and biodegradation.To further investigate the transformation process of microcystins that impacted by microplastic,the microcystins degradation products were measured.The typical degradation products of microcystins,Adda(m/z=331),appeared in the mass spectra of all the Na N?free treatments,and the highest relative intensity of the ion peak was found in the treatment with PS added in the turbulent waters.Additionaly,the biofilms on the microplastics surface was analyszed using Scanning electron microscopy and Confocal laser scanning microscope also supported the judgements.Besdies,the microcystins-degrading bacteria,including Sphingomonadaceae,Rhizobiaceae,Saprospiraceae and Methylophilaceae,was recorded in the waters and microplastics surface.This study proved that microplastics can influence the degradation of microcystins by adsorption,and microbial degradation in water disturbance system.