The magnetic multi-walled carbon nanotubes (MWCNTs) were synthesized by wet chemical treatments. Firstly, an amount of 1.0 g of purified MWCNTs was suspended in 200 mL of mixed solution containing 1.7 g of ammonium ferrous sulfate and 2.5 g of ammonium ferric sulfate, which was followed by a slow addition of 10.0 mL of 8 mol·L^-1 NH₄OH solution at constant temperature of 50 ℃, under nitrogen atmosphere, with the aid of ultrasonic stirring for 10 min. The pH of the final mixture was controlled in the range of 11～12. The reaction was allowed to continue for 30 min, which results in the color of the suspension changing from black to brown. After the completion of the reaction, the suspension was allowed to cool at room temperature. The magnetic MWCNTs was isolated from the mixture by a permanent magnet, afterwards dried under vacuum. The size and morphology of the synthesized magnetic MWCNTs were characterized by transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and vibrating sample magnetometer. The magnetic MWCNTs was used to adsorbing the three nitroimidazoles,—metronidazole, ornidazole and tinidazole—in aqueous solutions. The adsorption kinetic (or isotherms) experiments were carried out by adding 250 mg of magnetic MWCNTs to a 50 mL flask which containing a solution made up of one or three nitroimidazoles with known initial concentration (10 mg·L^-1) at natural pH, then agitating in a temperature-controlled shaker at (25±1) ℃ for different time intervals (1～480 min). The effect of pH was performed by dispersion of 250 mg of magnetic MWCNTs in 50 mL aqueous solution which containing 10 mg·L^-1 of three nitroimidazoles of each. The results showed that the removal of metronidazole, ornidazole and tinidazole reached 92.86%, 94.44%, 94.91% respectively when the dosage of magnetic MWCNTs was 5 g·L^-1, the pH values was 7 and the time of adsorption was 300 min. Adsorption kinetic analysis demonstrated that pseudo-second order model could interpret the kinetics of the adsorption process, and Freundlich model could better explains the isotherm for adsorption process under different temperatures. The overall adsorption rate appeared to be influenced by both the external mass transfer and the intraparticle diffusion, and mainly governed by the intraparticle diffusion. The adsorption mechanism could be explained by electrostatic attraction between MWCNTs and nitroimidazoles, and by π-π stacking interaction. The obtained thermodynamic parameters (ΔH⁰, ΔS⁰, ΔG⁰), calculated from adsorption process revealed that 1) ΔG⁰＜0 indicated that the adsorption of nitroimidazoles on magnetic MWCNTs was spontaneous; 2) ΔG⁰ indicated that the adsorption is physical process; 3) ΔH⁰＜0 indicated that the absorption reaction was exothermic and that low temperatures were favorable for adsorption, and 4) ΔS⁰＞0 indicated that the adsorption process was an entropy enhancing process in the experimental temperature range. Finally, the leaching of Fe from adsorbent into the treated water was tested, which found that the leaching of Fe was as low as 0.96 mg·L^-1. It indicated that the magnetic MWCNT was very stable in neutral aqueous solution.

Key words: magnetic multi-walled carbon nanotubes, nitroimidazoles, adsorption, kinetics, thermodynamics