Desorption Behavior And Mechanism Of Microplastic-loaded Heavy Metals In Simulated Gastrointestinal Fluids

Microplastics(MPs)are widely present in the natural environment and are prone to interact with various organic pollutants and heavy metals in the environment.When MPs carrying pollutants are ingested by organisms and enter the gastrointestinal system,they will cause complex toxic effects and increase biological health risks.Therefore,exploring the release of MPs-loaded pollutants in the gastrointestinal tract is of great significance for evaluating the health risks of MPs.In this paper,polyamide(PA)and polyvinyl chloride(PVC)are selected as the representatives of microplastics to explore the effect of microplastics on three typical heavy metal ions lead(Pb(II)),chromium(Cr(III)),cadmium(Cd(II)),and to explore the desorption behavior of microplastics loaded with heavy metals in the simulated gastrointestinal tract,the factors affecting desorption and the internal desorption mechanism,to provide basic data for revealing the composite environmental effects and toxicological effects of microplastics and heavy metals.The adsorption results of microplastics on heavy metals in the gastrointestinal environment show that the adsorption capacity of PA and PVC for Pb(II)is significantly stronger than that of Cd(II)and Cr(III),and the adsorption process conforms to the standard First-order adsorption kinetic model and Langmuir adsorption isotherm model,the adsorption of heavy metals by microplastics is electrostatic adsorption.In the simulated gastrointestinal tract environment,the desorption of the three heavy metals loaded on the microplastic surface in the enzymatic gastrointestinal fluid was significantly higher than that in the non-enzymatic gastrointestinal fluid and water,and Pb(II)was more easily desorbed in the simulated gastrointestinal fluid.In the continuous desorption experiment of the gastrointestinal tract,only a small amount of Cr-PA and Cr-PVC were found to be desorbed in saliva.Except that the desorption of Cr-PA and Cr-PVC gradually increased during the continuous desorption process,the desorption of Pb and Cd The microplastics re-adsorbed after entering the intestinal phase with the mixed solution.By exploring the effects of p H,ionic strength,and three digestive enzymes(pepsin,trypsin,and pancreatic lipase concentrations)on the desorption behavior,it was found that strong acid conditions can promote the desorption of heavy metals,and the effect of digestive enzymes on desorption is more significant.In order to further explore the desorption mechanism of MPs-loaded heavy metals in gastrointestinal fluid,molecular dynamics were used to simulate the adsorption and desorption process in this paper.The study found that the digestive enzymes interacted with MPs mainly through van der Waals forces and hydrogen bonds during the desorption process,competing for adsorption sites on the surface of MPs or changing the surface electrical properties of MPs,thereby promoting the desorption of heavy metals.During the interaction between the digestive enzymes and MPs,the secondary structure of the digestive enzymes changes,the amino acid residues of the digestive enzymes undergo electronic transitions,and the residue microenvironment changes.