Effect Of Cyanobacterial EPS On Microplastic Aggregation Behavior Under Different Environmental Factor Conditions

The widespread presence of microplastics in the natural water environment is of increasing concern because of the potential harm they can cause to aquatic organisms and the natural environment and even human health.In recent years,water blooms caused by cyanobacteria have occurred due to eutrophication problems in water bodies.Extracellular polymeric substances（EPS）are secreted by cyanobacteria in large quantities during their growth.These macromolecular organic substances are complex in composition and have physicochemical properties such as hydrophobicity,which can have a significant impact on the transport and transformation of microplastics in the water column.The transport of microplastics not only has an impact on aquatic life and the environment,but also poses a potential hazard to human health.Therefore,in this study,Polystyrene（PS）,a widespread microplastic in the aqueous environment,and Dissolved Organic Matter（DOM）,a representative of EPS secreted by Microcystis aeruginosa in shallow lakes,were selected as representative microplastics.The aggregation behaviour of polystyrene microplastics（PS-MPs）was analysed by measuring the changes in particle size of microplastics,including p H,ionic strength and light.At the same time,the chemical structure changes of organic matter during the microplastic aggregation process,as well as the surface charge and structural functional groups of microplastics,were characterized and analyzed by combining the techniques of zeta potential,UV-Vis absorption spectroscopy,3D fluorescence spectroscopy,Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy to further explore the potential influence of changes in the physicochemical properties of water bodies on the aggregation of microplastics in the presence of organic matter.This study will further explore the potential mechanism of microplastic aggregation in the presence of organic matter,and provide a theoretical basis for the migration and transformation of microplastics in the water environment.The main findings are as follows:（1）The presence of algal EPS can reduce the particle size and increase the stability of PS-MPs aggregated in the aquatic environment,enhancing the adverse effects of PS-MPs on the aquatic environment.The relatively small molecular weight of cyanobacterial EPS compared to humic acid（HA）(E₂₅₄/E₃₆₅=3.0～3.5)is influenced by the transfer of electrons from the surface of the microplastic to its surface as a spatial site barrier,making the PS-MPs more stable in aqueous solution.HA organic matter has a large molecular weight(E₂₅₄/E₃₆₅=2.0～2.4),in which large aromatic proteins and humic substances are adsorbed by PS-MPs during the aggregation process,forming bridging effects on the surface of the microplastics,leading to their enhanced ability to aggregate in the aqueous environment.（2）The presence of cyanobacterial EPS,influenced by p H conditions,causes a differential reduction in the ability of PS-MPs to aggregate.In the absence of EPS,as the p H of the solution increases（p H=7～10）,the OH^-in the aqueous solution increases,the surface electronegativity of the PS-MPs increases and electrostatic repulsion makes the PS-MPs progressively more stable in aqueous solution.However,cyanobacterial EPS,being inherently more electronegative,is less affected by p H changes in the structure of the aromatic material and therefore,the presence of EPS does not bring about significant differences in PS-MPs aggregation under different p H conditions.（3）The ability of PS-MPs to aggregate increases with increasing ion concentration in the presence of cyanobacterial EPS.However,the ability of PS-MPs to aggregate decreases significantly with increasing ionic valence,especially in the presence of Fe³⁺,and stability increases.This is mainly because Fe³⁺enhances the adsorption of EPS on the surface of PS-MPs,which ionises the hydroxyl and carboxyl groups of the aromatics,greatly increasing the negative charge on the surface of the microplastics and increasing the electrostatic repulsion between the particles,thus improving the stability of the system in aqueous media.Under Ca²⁺conditions the polysaccharides in EPS adsorb to the surface of the microplastics forming hydrogen bonds and bridging between the particles resulting in increased aggregation of PS-MPs and reduced stability.（4）The addition of cyanobacterial EPS increases the stability of PS-MPs under photoresponsive conditions,and the stability of PS-MPs in the aqueous environment is further enhanced by the combined effect of EPS and Fe³⁺.This is mainly due to the fact that the adsorption of EPS on the surface of PS-MPs is further promoted by the complexation of Fe³⁺under the influence of light,and the complexation of Fe³⁺with EPS generates more·OH under light to accelerate the change in the surface properties of PS-MPs,resulting in an increase in the index of C=O groups,thus creating a stronger spatial site resistance on the surface of the microplastic to make it more stable in the aqueous environment.