Effects Of Microplastics On The Bioaccumulation Of Coexisting Organic Pollutants In Aquatic Environments

As a new type of environmental pollutant,microplastics（MPs）are ubiquitous in global environmental media,and their existence may affect the distribution,transport,and environmental fate of organic pollutants.MPs are highly hydrophobic and have large specific surface areas,which enable them to effectively sorb organic pollutants and transport them into living organisms,thereby changing MPs’ potential environmental risks.However,there is no unified conclusion on whether MPs can enhance the bioaccumulation of organic pollutants or not according to current research literature.Therefore,this paper carried out a study about the bioaccumulation effects of MPs on coexisting organic pollutants under different scenarios and compared it with that of natural food-borne particles.We aimed to provide an objective theoretical basis for evaluating the environmental risks of MPs in the real aquatic environment.The main research contents and results are summarized as follows:（1）We designed two exposure scenarios: in the first scenario,the distribution equilibrium of organic pollutants among environmental matrices in the aquatic environment had been reached,which was called as “equilibrium state”.To explore the effect of microplastics on the bioaccumulation of loaded organic pollutants in the“equilibrium state”,we selected three pairs of polychlorinated biphenyls（PCBs）congeners with similar physical and chemical properties（CB-44 and-52,CB-101 and-118,CB-138 and-153）,and the effects of MPs and FBPs alone and co-exposure on the bioaccumulation of PCBs in zebrafish were carried out.MPs and FBPs were preloaded with the equilibrated state（1 μg/L,CB-44,-101,-138）PCBs.The results showed that the bioaccumulated concentration of PCBs in zebrafish was 142.2-444.6ng/g when MPs were exposed alone,and no significant difference was found compared to the control group（176.8-478.6 ng/g）.This suggests that the effect of MPs on bioaccumulation is extremely limited when pollutants are in equilibrium with the surrounding environment.However,FBPs（464.4-1379.3 ng/g）significantly increased the bioaccumulation level of PCBs.This is because the fugacity value of PCBs loaded on FBPs（0.59-11.5E-05 Pa）was higher than that of zebrafish muscle（0.22-9.17E-05Pa）,so PCBs could continuously transport from FBPs to zebrafish in vivo,but the presence of MPs alleviated this enhanced PCBs bioaccumulation by 14.1% upon coexposure.As the fugacity value（0.16-6.36E-06 Pa）of PCBs loaded with MPs was lower than that of FBPs and zebrafish muscles,the fugacity gradient of PCBs from FBPs to MPs was higher than that of zebrafish muscle,so the presence of MPs reduced the bioaccumulation of PCBs in zebrafish muscle upon co-exposure.（2）In the second scenario,organic pollutants were present on plastics in the form of additives,and they were far from equilibrium with the surrounding environmental matrices,which was called as “nonequilibrium state”.To explore the effect of MPs on the bioaccumulation of coexisting organic pollutants when the pollutants were at the nonequilibrium state,we retained the system of 1 μg/L PCBs in the equilibrium state,and also selected the corresponding 3 congeners（CB-52,-118,-153,at 20 μg/L）with similar physicochemical properties to those in the equilibrium state to be loaded on MPs or FBPs to represent the additive state.The results showed that MPs exposure alone could significantly increase the bioaccumulation of PCBs at a nonequilibrium state,up to 754.9 ng/g（1.8-fold increase）.The enhancing effect of FBPs on the bioaccumulation of PCBs was even more significant（4.2-fold increase）,which was because PCBs carried by FBPs（0.10-1.23E-03 Pa）had a higher fugacity gradient relative to zebrafish（0.30-16.3E-05 Pa）.In the co-exposed group,the presence of FBPs（equilibrium,0.59-11.5E-05 Pa）did not affect MPs’ effects on PCBs bioaccumulation（nonequilibrium,0.39-5.96E-05 Pa）,because the PCBs carried by the two particles did not show obvious fugacity gradients between each other.（3）Based on the above results,we recognize that MPs can enhance the bioaccumulation of co-existing additives.Therefore,we further investigated the effects of MPs on bioaccumulation and the toxic effects of a novel brominated flame retardant tris（2,3-dibromopropyl）isocyanurate（TBC）,which is a plastic additive commonly found in the aquatic environment.The results showed that the bioaccumulation concentration in zebrafish（40.2-190.0 ng/g）in the TBC alone exposure group（environmentally relevant concentration: 5 μg/L）was slightly lower than that in the TBC and MPs co-exposure group（54.1-260.1 ng/g）.Moreover,the presence of MPs changed the swimming behavior of zebrafish,making its swimming velocity much quicker and more manic.Meanwhile,the activity of neurotransmitter acetylcholine in the brain tissue of zebrafish in the co-exposure group was higher than that in the TBC alone exposure group.Both results indicated that the presence of MPs could trigger neurotoxic effects in zebrafish.Thus,we further verified that MPs can slightly enhance the bioaccumulation of coexisting additive-type organic pollutants at environmentally relevant concentrations,and they can even enhance the toxic effects of coexisting pollutants.Collectively,we confirmed that MPs have little effect on the bioaccumulation of co-existing pollutants at an equilibrium state,but they can enhance the bioaccumulation of “nonequilibrium state” additive pollutants and change their toxic effects.Therefore,this study provides a solid scientific basis for an objective evaluation of the effects of MP carriers for organic pollutants,and also gives theoretical support for the environmental risk assessment of MPs in the direction of combined pollution.