Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, ≥99%) was purchased from Chemical Reagent Co., Ltd. (Nanjing, China), 2-methylimidazole (C4H6N2, 99%) was purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China), heptahydrate Ferrous sulfate (FeSO4·7H2O, ≥99%), methanol (CH4O, ≥99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Tetracycline (99%) and multi-walled carbon nanotubes (>95%, outer diameter: 8-15 nm, length: ~50 μm) were provided by McLean Biochemical Technology Co., Ltd (Shanghai, China). Without additional treatment, all reagents were utilized as purchased.
2.2 Synthesis of adsorbents
2.2.1 Preparation of ZIF-8 and Fe-ZIF-8
The preparations for ZIF-8 and Fe-ZIF-8 adopt the simple precipitation method. To be more specific, 1.5g of Zn(NO3)2·6H2O and 75mg of FeSO4·7H2O were dissolved in 70mL of methanol (solution A), and 3.3g of 2-methylimidazole was dissolved in 70mL of methanol (solution B). After stirring for 30 minutes and generating a homogeneous solution, solution B was gently added to solution A to make a mixed solution. The mixture was then stirred vigorously for 3 hours at room temperature. After that, the combination was allowed to rest for 1 day in an indoor environment. Finally, the solid was rinsed three times using anhydrous methanol after centrifuging the emulsion at 8000 rpm. The solid was dried in a vacuum at 60 °C for 12 hours to obtain a white ZIF-8 powder and a light-yellow Fe-ZIF-8 powder. Moreover, Fe-ZIF-8 (0) and Fe-ZIF-8 (150) were prepared similarly to Fe-ZIF-8 (75), with the addition of FeSO4·7H2O was 0 mg and 150 mg, respectively.
2.2.2 Preparation of Fe doped ZIF-8/MWCNTs composite
Fe-ZIF-8/MWCNTs was synthesized by a post-synthesis method. Following that, 10mg of multi-walled carbon nanotubes were added to the previously prepared Fe-ZIF-8 solution, which was then ultrasonically stirred for 30 minutes and magnetically stirred for 3 hours to form a uniformly dispersed mixture. The mixture is then deposited in a polytetrafluoroethylene-lined autoclave and maintained at 90 °C for 12 hours. Collect the sample after the reaction is finished, rinse it 3 times using methanol and ultrapure water, and then place it in a vacuum at 60 °C for 12 h to acquire the final product.
2.3 Sample analysis
The crystal structures of the samples were obtained by X-ray diffraction (XRD; Ultima IV, Rigaku, Japan). The power of the Cu-kα X-ray generator was 3 kW, the scanning speed was 0.02 °/s, and the 2θ test range was 5°–50°.
To observe the morphologies of the materials, high-resolution images of gold-plated samples were taken using scanning electron microscopy (SEM; JSM-7600f, Jeol, Japan; accelerating voltage 20 kV).
The chemical compositions of the samples were analyzed by Fourier-transform infrared (FT-IR) spectroscopy (Nicolet IS5, Thermo Scientific, USA) in the wavenumber range 400–4000 cm−1.
The Brunauer-Emmett-Teller (BET) specific surface areas, pore-size distributions, and pore volumes of samples were determined using an automatic specific surface area and pore size analyzer (ASAP2020 and HD88, Micromeritics, USA).
N2 adsorption-desorption isotherms were obtained at −196 °C after degassing at 150 °C for 24 h (heating rate 10 °C/min, N2 flow rate 30 mL/min, test range room temperature to 800 °C).
The thermal stabilities of the samples were evaluated by thermogravimetric analysis (TGA; Q500, Ta, USA)
X-ray photoelectron spectroscopy (XPS; ESCALAB 250Xi, Thermo Scientific, USA) was used to obtain species, valence, and composition information for the sample surface elements.
2.4 Evaluation of adsorption performance
In batch experiments, the adsorption properties of as-prepared adsorbents for TC were studied. All of the TC solutions were made from the 100-mg/L stock solution, which was developed by dissolving quantitative TC powder in ultrapure water and preserving it at 4°C in the refrigerator.
A centrifuge tube containing 50 mL of TC solution (100 mg/L) was infused with a specified amount of adsorbent and placed in a constant temperature oscillator at 160 rpm and 298 K to evaluate the effect of adsorbent dosage, solution pH, and co-existing ionic strength. The solution pH was adjusted from 2 to 12 using 0.1mol/L HCl and 0.1mol/L NaOH. The effect of ionic strength on TC adsorption by FZM at various concentrations (50, 100mg/L) was researched using NaCl, Na2SO4, Na2CO3, MgCl2, and CaCl2. After adsorption, a 0.22-μm filter was used to obtain a clear solution, and the residual TC concentration was detected using an ultraviolet spectrophotometer at 357nm, and the removal rate (R) and adsorption capacity (Qe, mg/g) were estimated by equations 2.1 and 2.2.
In which, Ci and Ce are the concentration of TC before and after reaction, respectively, mg/L; V is the volume of TC solution, mL; m is the amount of adsorbents, mg.
2.5 Optimization by RSM
RSM is a statistical approach for fitting correlations between factors and response values that employs quadratic polynomial equations. It requires consideration of experimental features such as experiment design, model applicability, and the ideal combination of conditions. The BBD model of RSM was used to explore the effect of three significant individual variables on TC adsorption capacity on FZM, including pH (A, 4-8), temperature (B, 298–318 K), and adsorbent dosage (C, 50-150 mg/L), as well as the projected response (Y) to the adsorption capacity. The TC optimization criteria are described in Table S1.
Use Design-Expert 12 software for statistical investigations such as analysis of variance (ANOVA), 3D surface plots, and fit statistics. A binary regression equation is used to represent the relationship between the response variable (Y) and the individual variable (X), as shown below (Equation 2.3).
Where Y is the predicted response; Xi and Xj are the independent variables (i and j = 1, 2, 3, 4, or k); and β0, βi, βii, and βij are the migration, linear, second-order, and interaction coefficients, respectively.