Magnetic NH2-MIL-101(Al)/Chitosan nanocomposite as a novel adsorbent for the removal of azithromycin: modeling and process optimization

In the present study, the magnetic NH2-MIL-101(Al)/chitosan nanocomposite (MIL/Cs@Fe3O4 NCs) was synthesized and used in the removal of azithromycin (AZT) from an aqueous solution for the first time. The as-synthesized MIL/Cs@Fe3O4 NCs was characterized by SEM, TEM, XRD, FTIR, BET, and VSM techniques. The effect of various key factors in the AZT adsorption process was modeled and optimized using response surface methodology based on central composite design (RSM-CCD). The low value of p-value (1.3101e−06) and RSD (1.873) parameters, along with the coefficient of determination > 0.997 implied that the developed model was well fitted with experimental data. Under the optimized conditions, including pH: 7.992, adsorbent dose: 0.279 g/L, time: 64.256 min and AZT concentration: 10.107 mg/L, removal efficiency and AZT adsorption capacity were obtained as 98.362 ± 3.24% and 238.553 mg/g, respectively. The fitting of data with the Langmuir isotherm (R2: 0.998, X2: 0.011) and Pseudo-second-order kinetics (R2: 0.999, X2: 0.013) showed that the adsorption process is monolayer and chemical in nature. ΔH° > 0, ΔS° > 0, and ∆G° < 0 indicated that AZT removal was spontaneous and endothermic in nature. The effect of Magnesium on AZT adsorption was more complicated than other background ions. Reuse of the adsorbent in 10 consecutive experiments showed that removal efficiency was reduced by about 30.24%. The performance of MIL/Cs@Fe3O4 NCs under real conditions was also tested and promising results were achieved, except in the treatment of AZT from raw wastewater.


Chemicals and reagents
The model antibiotic, Azithromycin (AZT) was supplied by Tehran Pharmaceutical Co, Ltd.

Instrumentation and analytical methods
The stock solution of AZT was produced by dissolving foreordained amounts of AZTs in 1000 mL deionized water, and other required concentrations were prepared through the appropriate dilution of the stock solution. Stock solutions were stored in a dark place at 4 °C (refrigerator).
HCl and NaOH 1 N were used to justify pH to desired values. The pH values were measured using a pH meter (Metrohm 744, Switzerland). The separation of synthesized nano-composite from aquatic solution was carried out through a cubic magnet (magnetic field of 3,000 Gauss). The initial and residual concentration of AZT was determined using UV-Visible spectrophotometer at a maximum absorbance wavelength of 482 nm 1 .

Chitosan obtained from shrimp shell (Cs)
A predetermined amount of shrimp exoskeletons was washed with water, and oven-dried at 80 ℃ for 4 h. Then, demineralization was carried out by adding 1000 mL of 1 M HCl to 100 g of shrimp shells. The reaction proceeded at 30 ℃ under agitation at 250 rpm for 2h. Afterwards, the demineralized shells were filtrated and washed with distilled water until neutral pH. Subsequently, the demineralized shrimp shells were ground, and subjected to chemical treatment in 1 M NaOH solution at a solid/liquid ratio of 1:10 (g/mL). Reaction was carried out under agitation with magnetic stirring at 80 ℃ for 3 h for removal of proteins. The solid was filtrated and washed with distilled water until it achieved neutral pH. Then, it was immersed in ethanol for 10 min for further bleaching, and the resulting chitin was dried in an oven at 80 ℃. A discoloration process was made with a solution of 15% ether, 75% acetone, and 10% distilled water at 50 ℃ for 3 h, with magnetic stirring. Finally, the sample was exhaustively washed with distilled water. The resulting Chitosan was filtrated, washed with distilled water until neutral pH, and dried in an oven at 70℃ 2,3 .

Supplementary Information, Text 3 2.3. Characterization
The crystal phases of the samples were determined using an XRD diffractometer (D8 Advance, Bruker, Germany) with Cu Ka radiation (k= 1.540562 A, 40 kV, 40 mA). The relative intensity was measured throughout a scattering range of 5 to 70 degrees (2=5°-70°). The surface morphology and structure of the sorbents were examined using scanning electron microscopy (SEM, Philips XL-30, USA) at 15 kV. The N2 sorption-desorption isotherms were determined at 77 K using a Micromeritics Tristar 3000 analyzer to examine the textural features of samples. The BET and BJH procedures were used to determine the specific surface area and pore size distribution of materials. Transmission electron microscopy (TEM, Hitachi, H-7500, Japan) operating at 80 kV was used to assess the size distribution and polydispersity of MIL/Cs@Fe3O4 NCs. Fourier transform infrared spectra (FTIR, Shimadzu-8400S spectrometer, Japan) were utilized to identify the chemical functional groups contained in the materials at wavenumbers ranging from 500 to 4000 cm -1 . The magnetic characteristics of the adsorbents were measured using a vibrating-sample magnetometer (VSM, Quantum Design MPMS SQUID, USA).