Aggregation Kinetics Of Microplastics And Its Toxicity To Chlorella Pyrenoidosa

Microplastics have become emerging contaminants in recent years.Because of poor waste management,increasing plastic production,extensive usage and minimal biological degradation,high abundance of microplastics have been identified in ecosystems.An increasing number of studies have implied that microplastics in the environment could pose a potential threat to the ecosystem.Therefore,it is imperative to understand the distribution,environmental hazard and transport of microplastics in the ecosystems.However,most studies focused on marine environments,but limited information is available about the ecological risks of microplastics in freshwater environments(paid close attention to the abundance of microplastics).Against the background,the microplastics pollution was studied in the Yulin River,a typical tributary of Three Gorges Reservoir.The role of environmental and spatial factors in the occurrence of microplastics was discussed.Then,polystyrene micropastics,a typical type of microplastics that are widely identified in aquatic environments,was employed to estimate its toxicological effects on Chlorella pyrenoidosa,and the interaction mechanism was explored.In addition,Influence of electrolyte,p H,weathering and extracellular polymeric substances(EPS)on aggregation kinetics of microplastics were studied with the help of the DLVO theory.The main results of this study are as follows.(1)Microplastics were widely distribution in the main stream,tributaries and bays of the Yulin River and were found in all samples.The abundance of microplastics in the main stream,tributaries and bays reached(1.30±0.28)×10-2 items/L,(1.95±2.91)×10-1 items/L and(3.60±1.53)×10-1 items/L,respectively.Microplastics were classified into 4 groups based on their sizes: group 1(64-100 ?m),group 2(100-250 ?m),group 3(250-500 ?m),group 4(500-5000 ?m).The results of size analysis indicated microplastics abundance increased with a decreasing particle size.64-100 ?m were more frequently found than other size groups,accounting for 20.00-78.57% of total microplastics.Meanwhile,microplastics were classified into 3 groups based on their shapes: fiber(line/filament),film(sheet)and granule(pellet/bead/foam).The film observed in this study was fewer than fiber(15.79%-50.00%)and granule(38.46%-73.68%).Polystyrene,propylene and polyethylene were the most common types in the Yulin River.The results of SEM and XPS indicated that oxidative and mechanical weathering of microplastics occurred in the environments.(2)The distribution of microplastics(jointly shaped by environmental and spatial factors)was affected by anthropogenic activities,hydrodynamic condition and the characteristic of river.The abundance of microplastics in upstream of Yulin River was higher than those in downstream of Yulin River,which was realatd to anthropogenic activities.Backwater of Yangze River resulted in high abundance of microplastics observed in estuary.Microplastics abundance significantly increased downstream of wastewater treatment plant effluent.In terms of spatital factors,microplastics abundance followed a distance-decay pattern(R=-0.441,P<0.001).Meanwhile,microplastics was postively correlated with the NH4-N?NO2-N?DOC and Chla,but negatively correlated with the river wdth.In addition,microplastics abundant were associated with the the diversity(R=-0.677,P<0.05)and composition of planktonic bacterial community.(3)The responses of Chlorella pyrenoidosa under polystyrene microplastics exposure were studied across its whole growth period.Three growth phases(the lag phase,the logarithmic phase and the stationary phase)of Chlorella pyrenoidosa over a 30-day incubation periods were distinguished in the control groups and algal density continuously increased until its stationary phases.Exposure of Chlorella pyrenoidosa to polystyrene microplastics induced a two-stage response over the 30 day cultivation period.During the first stage,which occurred in the lag and earlier logarithmic phases,the growth rate of Chlorella pyrenoidosa was reduced with increasing polystyrene microplastics dosages.The efficiency of oxygen-evolving complex,the reaction center of PSII,electron transport rate,celluar membrane,malondialdehyde(MDA)and superoxide dismutase(SOD)activities were impaired.Algal EPS increased under microplastics exposure.Unclear pyrenoids,distorted thylakoids and damaged cell membrane were also observed.The toxicology effects caused by microplastics were attributed to the physical damage and oxidative stress.During the second stage(from the end of the logarithmic to the stationary phase),the inhibitory effects of microplastics on algae growth,photosynthesis and SOD activities weakened over time(jointly through cell wall thickening,algae homo-aggregation and algae-microplastics hetero-aggregation).Cell structures turned to normal.What's more,when the algal cells adapted to the microplastics stress,a “stimulatory effect” can be observed at the end of the study period.(4)Effects of electrolyte,p H,weathering and EPS on aggregation kinetics of microplastics were studied.At relatively low ion concentration,aggregation rates of microplastics increased with the increased ion concentration,which is known as the reaction-limited regime.When the aggregation kinetics of microplastics reached diffusion-limited regime,the aggregation rates reached maximum and were not dependent on the electrolyte concentration.Higher valence electrolytes had stronger influence on the aggregation rates of polystyrene microplastics and were more in accordance with the Schulzee-Hardy rule as compared to lower valence electrolytes.The difference in aggregation kinetics of microplastics caused by equivalent ion was related to hydration ability of the ions.The critical coagulation concentration(CCC)values of polystyrene microplastics were 590.96,579.99,74.10,71.28,63.10,2.12 and 1.36 mmol/L for Na Cl,KCL,Mg Cl2,Ca Cl2,Ba Cl2,Al Cl3 and Fe Cl3,respectively.The aggregation rates of polystyrene microplastics increased with decreased p H.Van der Waals and electrical double-layer interaction energy played major roles in aggregation.Weathering process for polystyrene microplastics resulted in increased carbonyl index,oxygen content and scission of the polymer backbone.The aggregation rates of polystyrene microplastics following weathering were lower than pristine microplastics,which were related to the change in hydrophobicity of microplastics.The steric repulsive forces,originated from EPS adsorbed on microplastics,could be responsible for the decrease in aggregation rates.The classical and extened DLVO theory were applied to calculate the interaction energies between particles and could interpret the aggregation mechanisms of microplastics in some extent.Results from this study could provide the basic data and theories foundation for establishment of ecological risk and pollution control of microplastics.