Unveiling interactions of norfloxacin with microplastic in surface water by 2D FTIR correlation spectroscopy and X-ray photoelectron spectroscopy analyses

“Microplastics (MPs) has shown adsorption of hydrophilic organic matters (HOMs) in aqueous environments. However, it is still difficult to predict the adsorption behaviors of HOMs by different MPs, especially in authentic water systems. In this study, the adsorption behaviors and mechanisms of norfloxacin (NOR) onto polyamide (PA) MPs were investigated in both simulated and real surface water. The results showed that the adsorption equilibrium of NOR by PA in simulated surface water could be achieved within 15 h, while the adsorption rate of NOR in real surface was slowed down, with the equilibrium time of 25 h. Pseudo-second-order model could well describe the adsorption kinetics data. The experimental maximum adsorption capacity of NOR on PA in real surface water (e. g. 132.54 ug/g) was dramatically reduced by 37.5 % compared with that in simulated surface water (e. g. 212.25 ug/g), and the adsorption isotherm would obey Freundlich model. Besides, the leaching of NOR from the surface of PA could occur obviously at acidic environment. Furthermore, the salinity and natural organic matter exhibited significantly adverse effects on the NOR adsorption. Finally, the results of 2D Fourier transform infrared correlation spectroscopy and X-ray photoelectron spectroscopy indicated that the electrostatic, H-bond and van der Waals interactions were involved in the adsorption. More importantly, the sequential functional groups in the adsorption process followed the orders: 1638 (Cdouble bondO) > 1542 amide II (-NH-Cdouble bondO) > 717 (CH2) > 1445 (Cdouble bondO) > 973 amide IV (CONH). This study could provide an insight into the interactions between PA and NOR in different water environments.”

For XPS analysis, as shown in Fig. 5a, the wide scan spectrum of PA indicated that pristine PA mainly contained elements of C, N and O. While after the adsorption of NOR, the intensity of N and O slightly increased, suggesting the load of NOR onto the PA surface.

Fig. 5

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Fig. 5. XPS wide scan spectra (a) and high-resolution scan spectra of C1s (b) and N1s (c) of PA before and after adsorption of NOR in different water systems.

The high-resolution scan of C1s spectra of PA and NOR-loaded PA in different water systems are shown in Fig. 5b. The spectrum of pristine PA could be decomposed into three peaks, at the binding energy (BE) of 284.8 eV, 285.89 eV, and 287.60 eV, which can be assigned to the C-C, C-N, and -N-Cdouble bondO groups, respectively (Tang et al., 2021). After adsorption, different changes were observed for these peaks. For C-C bond, he BE values after adsorption of NOR in simulated and real surface water did not change, because the nonspecific C-C group can only interact with NOR through van der Waals interaction. Whereas, the BE of C-N in simulated surface water occurred a shift to 286.10 eV, suggesting the involvement of C-N groups in the adsorption process. Meanwhile, the BE of -N-Cdouble bondO shifted to 287.78 eV, indicating that the -N-Cdouble bondO groups acted as an electron donor to interact with NOR (Dong et al., 2020). Similar phenomenon was also observed in the NOR adsorption in real surface water.

The high-resolution scan of N1s spectra are shown in Fig. 5c. The pristine PA can be resolved into a single peak assigned to C-N group. For N1s spectrum of NOR as shown in Fig. S3, two appeared peaks at BE of 398.88 eV and 400.26 eV could be assigned to -N- of trialkylamine and NH- of piperazinyl groups, respectively (Polishchuk et al., 2009). Accordingly, the NH- of the piperazinyl group can be easily protonated to form NOR+ species. After adsorption of NOR in both aqueous systems, the BE of NH- was significantly reduced to a lower BE, e. g. 400.11 eV and 399.80 eV compared to that of NOR (Fig. S3), which indicates an increase in the density of the outer electron cloud of NH- groups. Thus, the NH- groups participated in the adsorption as electron-accepting groups.

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