Adsorption of metronidazole antibiotic using a new magnetic nanocomposite from simulated wastewater (isotherm, kinetic and thermodynamic studies)

Abstract This experimental study was conducted on a laboratory scale to synthesize a new magnetic nano-adsorbent and to examine its efficiency in removing metronidazole antibiotic from aqueous solutions. In this research, a new FeNi 3 /SiO 2 /CuS magnetic nanocomposite was first synthesized and its physical and structural characteristics were analyzed using FESEM, TEM, FTIR, XRD, VSM, and TGA techniques. To determine the thermodynamic parameters, the equilibrium isotherms, and adsorption process kinetics, the effect of parameters including pH (3–11), contact time (5–180 min), initial concentration of pollutant (10–30 mg/L), nanocomposite dose (0.005–0.1 g/L), and temperature (50-5 °C) were studied. Finally, the residual metronidazole concentration was determined using a UV–Vis spectrophotometer T80 + at a wavelength of 320 nm. The results related to the physical properties of the synthesized magnetic nanocomposite indicated that the particle size lied within the range of 20–65 nm and the structure of this new nano-absorbent was amorphous. Also, the FeNi 3 /SiO 2 /CuS nanocomposite had a good magnetism and its magnetic saturation was 18.42 emu/gr. Considering the efficiency of the newly synthesized nanocomposites in metronidazole absorption, the highest percentage of the pollutant adsorption was observed at pH = 7, contact time = 180 min, nanocomposite dose = 0.1 g/L, and temperature = 20 °C. With the increase in the synthesized nano-absorbent dose, the absorption percentage increased significantly (from 24.18 to 62.18). Similarly, with in the growth of the initial concentration of metronidazole from 10 to 30 mg/L, the absorption percentage declined from 85.26 to 44.6% due to the restriction of the absorption sites. Notably, as the temperature rose, the absorption percentage increased (from 37.73% to 65.15%), suggesting that the adsorption reaction was endothermic. The data obtained from Langmuir (R 2  = 0.9991) and Freundlich (R 2  = 0.8148) equilibrium isotherms revealed that the metronidazole absorption process through the synthesized magnetic nanocomposite was consistent with the Langmuir model. Also, the data obtained from the reaction kinetic calculations showed that the absorption of metronidazole by the adsorbent was described in accordance with a pseudo-second-order model. According to the results of the thermodynamic studies including the entropy changes (ΔS) (82.12% J/mol k), the enthalpy changes (ΔH) (0.056 kJ/mol) and the Gibbs negative free energy (ΔG), it could be concluded that the adsorption process was endothermic. In this study, the efficiency and the reusability of the synthesized magnetic nanoadsorbent were examined in the removal of metronidazole. The findings indicated that following five adsorption – desorption cycles, the efficiency of the nanoadsorbent in the removal process have a slight decrease. Regarding the results of this study, it is suggested that the magnetic nanocomposite (FeNi 3 /SiO 2 /CuS) can be a suitable new absorbent for absorption of metronidazole antibiotic from aqueous solutions. The reason is that in addition to its excellent efficiency, it can be separated from an in-vitro setting by an external magnetic field due to its great magnetic properties.