Study On The Mechanism Of The Effect Of Biofilm On The Uptake And Fate Of HOCs In Plastics

Marine microplastic（MP）has attracted considerable attention.Due to its wide distribution,small size and high hydrophobic surface,the harm of microplastics is not limited to biological feeding.Its enrichment behavior to microorganisms and pollutants makes it a carrier for long-distance transmission of pollutants.At present,there are many studies about the chemical mechanism of microplastic adsorption under laboratory conditions.However,the effect of the biofilm formed naturally on the surface of microplastic is not clear.The biofilm may cause huge deviation in theoretical adsorption data and affect the accurate evaluation of microplastic carrier mechanism.Based on this,in-situ experiments have become a necessary means to study the behavior of microplastic carriers.In this study,batch experiments were used to study the theoretical adsorption behavior of microplastics in water,and then in-situ adsorption experiments were used to investigate the enrichment and transformation behavior of hydrophobic organic pollutants such as PAHs under the conditions of different biofilm bacterial community composition,bacterial abundance and physical abscission.The main results are as follows. （1）Theoretical adsorption studies show that the adsorption of microplastics to phenanthrene is affected by type and size of plastics and dissolved organic matter,mainly as follows:polyethylene（PE）fiber has stronger adsorption capacity of phenanthrene than nylon（PA）fiber;smaller fiber size shows a clear elevating effect on the sorption of pollutants;humic acid will reduce the amount of adsorption of phenanthrene by competitive adsorption,while the effect of tryptophan on the adsorption capacity of microplastics is not obvious.In addition,four kinds of pollutants on the surface of plastics collected in the actual water body were evaluated.The results showed that the plastics are both source and sink of pollutants,while the strong adsorption capacity of PAHs onto PE was confirmed in the collected samples. （2）By in-situ experiment in the culture area,we found that higher summer temperatures（31.4±1.07°C）promote colonized bacterial assemblages with larger biomasses,higher activity and more degrading bacteria than winter temperatures（13.3±2.49°C）by the methods of OD₅₉₅ and 16S r RNA.Consequently,some sorbed pollutants underwent significant decline in the summer,and this decline was particularly the case for PAHs with low（2-3 rings）and median（4 rings）rings such as phenanthrene（59.4±1.6%）,chrysene（70.6±4.2%）,fluoranthene（77.1±13.3%）and benz[a]anthracene（71.5±11.0%）.In our winter test,however,most pollutants underwent a consistent increase throughout the 8-week exposure period.Two putative PAH-degrading bacteria appeared with high relative abundances during the summer test,i.e.,family Rhodobacteraceae（18.6±0.5%）and genus Sphingomicrobium（22.4±3.6%）,positively associated with drastic decreases in low（45.2±0.4%）and median（66.0±2.5%）rings PAHs,respectively.Bacterial degradation effects of biofilms on PAHs are also supported by the correlative dynamics of salicylic acid,an important degradation intermediate of PAHs. （3）By in-situ experiments in urban river,the degradation of pollutants by biofilm was also found.Under the condition of shading,the larger biomass of plastic,higher relative abundance of heterotrophic bacteria and the stronger synergistic effect between PAHs degradation bacteria lead to the decrease of PAHs enrichment.In addition,it was found that biofilm could be used as an additional adsorption phase to promote the accumulation of PAHs on plastics.With the falling off of the biofilm,most of the PAHs with high rings will enter the sediment together with the biofilm.It can be seen that the biofilm on plastic accelerates the transfer of pollutants from water to sediment,especially for PAHs which are difficult to degrade and highly toxic,and increases the pollution load of sediment. The above results show that biofilm plays an important role in the enrichment of hydrophobic organic pollutants by microplastics.The above results deepen the understanding of the environmental behavior of microplastics and provide some basis for the accurate evaluation of the environmental risk of microplastics.