Adsorption behaviors of the pristine and aged thermoplastic polyurethane microplastics in Cu(II)-OTC coexisting system

In this work, the hypothesis that thermoplastic polyurethane (TPU) microplastics (MPs) could form complex toxic pollution by absorbing both antibiotics and heavy metals simultaneously was proposed. The unique features of the adsorption of Cu(II) and oxytetracycline (OTC) on the pristine TPU and photo-aged (aged) TPU MPs in single and coexisting system were investigated, which included the kinetics, isothermal equilibrium and thermodynamics. The possibly synergistic or competitive effects between Cu(II) and OTC were also evaluated. The results showed that the adsorption process of Cu(II) and OTC could be described well by pseudo-second-order kinetic equation. The entire process could be divided into two stages: internal diffusion and external diffusion. The Sips model could give good fitting for the isothermal adsorption equilibrium. The thermodynamic parameters depicted the endothermic nature of adsorptions and the process was spontaneous. In the coexisting system, synergistic or competitive effects depended critically on the ratio of concentrations (Cu(II) vs OTC). When the ratio was 1:1, Cu(II) significantly enhanced the adsorption of OTC, while OTC showed a weak effect on Cu(II) adsorption. The synergies could be attributed to the formation of Cu(II)-OTC complex and the bridging effect of Cu(II). Overall, the adsorption capacity of aged TPU was higher than that of pristine TPU, which was due to the differences in morphological characteristics and functional groups. FTIR studies revealed that ester carbonyl and acylamino groups in the TPU may be involved in the adsorption of Cu(II) and OTC.