Biofilm alters tetracycline and copper adsorption behaviors onto polyethylene microplastics

• Co-adsorption of Cu(II) and TC on PE microplastics with biofilm was first studied. • Biofilm enhanced the adsorption and stabilization of Cu(II) and TC on microplastics. • Competition and complexation interactions were involved in the adsorption process. • Microplastics with biofilm act as carriers of heavy metals and antibiotics in water. In this study, the adsorption properties of Cu(II) and tetracycline (TC) onto virgin and biofilm-developed polyethylene (PE) microplastics were investigated in batch sorption experiments. PE microplastics were placed at sewage outlets (Shanghai, China) for 20 days to develop biofilm on their surface. The adsorption and desorption isotherms of Cu(II) and TC were well fitted by the Freundlich model, and revealed that biofilm could enhance the adsorption and stabilization of Cu(II) and TC on microplastics. The linearity test of the film diffusion model in kinetic experiments suggested that the adsorption on virgin and biofilm-developed microplastics was dominated by intra-particle diffusion and film diffusion, respectively. Compared with the virgin microplastics, the adsorption of Cu(II) and TC on biofilm-developed microplastics was additionally affected by pH-dependent complexation interactions in the biofilm and competition interactions. Cu(II) pre-adsorbed on the biofilm could be released into solution because of competition effects of TC. Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) further confirmed that the enhanced adsorption of TC on the biofilm could be attributed to the complexation of TC, Cu(II) and components in biofilm. This study illustrated that biofilms could enhance the role of microplastics in the Cu(II) and TC migration by changing their adsorption properties on microplastics.