Adsorption performance and optimization by response surface methodology on tetracycline using Fe-doped ZIF-8-loaded multi-walled carbon nanotubes

Herein, an iron-doped ZIF-8-loaded multi-walled carbon nanotube (FZM) was synthesized and its adsorption performance on tetracycline (TC) was investigated. The experimental conditions (solution pH, temperature, adsorbent dose) were optimized by Box–Behnken design (BBD) in response surface methodology (RSM). The results show that the adsorption effect of TC by FZM is optimal under the conditions of temperature = 298 K, pH = 6, and contact time = 360 min. The adsorption processes of TC by FZM follow the pseudo-second-order (PSO) kinetic and Freundlich isotherm models, indicating that chemisorption is the dominant factor and the adsorption reaction is multi-layer, with a theoretical maximum saturation capacity of 1111.11 mg/g at 298 K. The adsorption thermodynamic results indicate that the adsorption of TC by FZM is a spontaneous and endothermic process. The mechanism of TC adsorption by FZM possibly occurs through hydrogen bonding, surface complexation, π–π interaction, and electrostatic interaction. From the statistical results, the optimal adsorption capacity of TC by FZM is 599.78 mg/g at a pH of 7.1, a temperature of 312.5 K, and an adsorbent dose of 64.43 mg/L, with a deviation of 1.73% from the actual value. Furthermore, regeneration experiments demonstrate that FZM has excellent reusability with a 15% loss of adsorption capacity after four cycles. This study provides some insights to study the adsorption behavior of TC by MOFs and the optimization of the adsorption experimental conditions, and also shows the potential of FZM for TC removal.


Introduction
Nowadays, antibiotics have been identified in significant amounts in water and soil ecosystems . For example, TC has become one of the most widely prescribed and is extensively utilized in disease treatment and livestock breeding (Akbari et al. 2021). A total of 70-90% of TC will be excreted into the sewage system through urine and feces due to the inability of TC to be fully absorbed in the human body (Minale et al. 2020). Due to the low toxicity of tetracycline, if it is not completely eliminated, it will cause adverse reactions such as nausea and vomiting to the human body, and inhibit the growth of microorganisms (Chen et al. 2020;Wang et al. 2021a, b;Zhou et al. 2021). TC is widely produced and used in China, yet there are no corresponding discharge standards, which results in serious water contamination. Hence, the elimination of TC is an urgent environmental concern that must be solved immediately.
Recently, metal-organic frameworks (MOFs) have been investigated for applications in gas separation (Wang et al. 2015), photocatalysis , desalination (Wei et al. 2020), and adsorption ) by their properties including porosity, high surface area, and structural flexibility (Feng et al. 2019). For example,  to remove tetracycline and achieved a removal rate of 48.8% (Tian et al. 2016). Ahmadi et al. successfully synthesized core-shell activated carbon-ZIF-8 nanomaterials, which had an adsorption capacity of 35.64 mg/g for tetracycline (Ahmadi et al. 2021).  for the removal of tetracycline and obtained a maximum adsorption capacity of 145 mg/g (Xia et al. 2021). Zhao et al. investigated the adsorption performance of Fe 3 O 4 @MOF-525 on TC, and the results showed that the maximum adsorption capacity of TC was 277 mg/g (Zhao et al. 2022). In summary, the adsorption of tetracycline by MOFs also has some shortcomings. However, ZIF-8 showed excellent performance in tetracycline removal (Sun et al. 2020;Nguyen et al. 2022). Based on previous researches, iron-doped ZIF-8/ MWCNTs have not been investigated for TC adsorption from wastewater.
In this paper, iron-doped ZIF-8/MWCNTs were prepared using a post-synthesis approach and applied to remove TC. The effects of solution pH, reaction time, and initial concentration were all systematically discussed to evaluate the adsorption performance of TC by iron-doped ZIF-8-loaded multi-walled carbon nanotube (FZM). The adsorption kinetics, isotherms, and thermodynamics were also explored in depth to further explain the mechanisms. The optimal conditions (pH, adsorbent dosage, and temperature) for the adsorption of tetracycline by FZM were determined using Box-Behnken design (BBD). Finally, the production cost of FZM was estimated, and regeneration trials were conducted to investigate the feasibility of its practical application.

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, 1.5 g Zn(NO 3 ) 2 ·6H 2 O and 75 mg FeSO 4 ·7H 2 O were dissolved in 70 mL methanol (solution A), and 3.3 g of 2-methylimidazole was dissolved in 70 mL methanol (solution B). After stirring for 30 min 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 h at room temperature. After that, the combination was allowed to rest for 1 day in an indoor environment. Finally, the solid was rinsed 3 times using anhydrous methanol after centrifuging the emulsion at 8000 rpm. The solid was dried in a vacuum at 60 °C for 12 h 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- , with the addition of FeSO 4 ·7H 2 O as 0 mg and 150 mg, respectively.

Preparation of FZM
FZM was synthesized by a post-synthesis method. After that, 10 mg of multi-walled carbon nanotubes was added to the previously prepared Fe-ZIF-8 solution and then sonicated for 30 min and magnetically stirred for 3 h to form a uniformly dispersed mixture. The mixture was then deposited in a polytetrafluoroethylene-lined autoclave and kept at 90 °C for 12 h. Samples were collected after the reaction, rinsed 3 times with methanol and ultrapure water, and then placed in vacuum at 60 °C for 12 h to obtain the final product.

Sample analysis
The crystal structures of the samples were determined by X-ray diffraction (XRD; Ultima IV, Rigaku, Japan). The Cu-k X-ray generator had a power of 3 kW, a scanning speed of 0.02°/s, and a test range of 5-50°.
High-resolution images of gold-plated samples were acquired using scanning electron microscopy (SEM; JSM-7600f, Jeol, Japan; accelerating voltage 20 kV) to investigate the materials' morphologies.
Fourier transform infrared spectroscopy (FTIR, Nicolet iS 5, Thermo Scientific, USA) in the wave number range 400-4000 cm −1 was used to examine the chemical compositions of the samples.
An automatic specific surface area and pore size analyzer was used to determine the Brunauer-Emmett-Teller (BET; ASAP 2020 and HD88, Micromeritics, USA) specific surface areas, pore size distributions, and pore volumes of the materials.
After degassing at 150 °C for 24 h, N 2 adsorption-desorption isotherms were obtained at 196 °C (heating rate 10 °C/ min, N 2 flow rate 30 mL/min, test range ambient temperature to 800 °C).
For the sample surface elements, X-ray photoelectron spectroscopy (XPS; ESCALAB 250Xi, Thermo Scientific, USA) was used to determine species, valence, and composition.

Batch adsorption
To investigate the effects of adsorbent dosage, solution pH, and co-existing ionic strength, a certain amount of FZM was added into a centrifuge tube containing 50 mL of 100 mg/L TC solution, and the tube was placed at 160 rpm and 298 K in a constant-temperature oscillator. The solution pH was adjusted by 0.1 mol/L HCl and 0.1 mol/L NaOH. The effects of co-existing ions (NaCl, Na 2 SO 4 , Na 2 CO 3 , MgCl 2 , and CaCl 2 ) on TC adsorption by FZM at various concentrations (50 and 100 mg/L) were studied. After the adsorption reaction, the supernatant was extracted through a 0.22-μm filter and then detected by using an UV spectrophotometer at 357 nm. The removal rate (R) and adsorption capacity (Q e , mg/g) were calculated by Eqs. 2.1 and 2.2, respectively.
where C i and C e are the concentration of TC before and after reaction, respectively, mg/L; V is the volume of the TC solution, L; and m is the amount of FZM, mg.

Optimization by RSM
Response surface methodology (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 the 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 where Y is the predicted response; X i and X j are the independent variables (i and j = 1, 2, 3, 4, or k); and β 0 , β i , β ii , and β ij are the migration, linear, second-order, and interaction coefficients, respectively.

XRD analysis
that the structure of ZIF-8 did not change during the synthesis of FZM (Wang et al. 2020a, b). Furthermore, FZM shows a stronger peak intensity around 2θ = 26°, indicating that ZIF-8 and MWCNTs were successfully recombined. In addition, the high-intensity characteristic peaks indicate that the synthesized sample has high crystallinity. Figure 2b shows the FTIR spectra of MWCNTs, ZIF-8, Fe-ZIF-8, and FZM. ZIF-8 and Fe-ZIF-8 have similar spectra, indicating that the introduction of Fe did not change the functional groups on ZIF-8. The absence of broad peaks between 3300 and 3500 cm −1 indicates the successful binding of N-H to Zn (Tsai et al. 2019). The characteristic peaks induced by the stretching vibration absorption of C-H appear at 2930 and 3135 cm −1 , respectively, which correspond to the saturated hydrocarbon C-H (CH 3 ) and the unsaturated hydrocarbon C-H of 2-methylimidazole. The stretching vibration of Zn-N is responsible for the sharp peak at 423 cm −1 , which also indicates that Zn is linked to N successfully (Nagarjun & Dhakshinamoorthy 2019). Furthermore, the characteristic peak in the range of 700-1600 cm −1 is the peak for the 2-methylimidazole ligands, and the stretching vibration peak of the C = N bond in the imidazole ring appears at 1584 cm −1 , indicating that Fe doping does not cause the disappearance of organic ligands (Sun et al. 2020). It can be inferred that the functional groups on FZM, such as C-H, C = N, and Zn-N, will provide sufficient active sites for TC adsorption.

BET analysis
Figure S1(a), (b), and (c) show the N 2 adsorption-desorption isotherms for ZIF-8 and FZM, demonstrating that ZIF-8 and FZM have type I and IV isotherms, as well as type H1 and H4 hysteresis loops, respectively ). In addition, the hysteresis loops are in the 0.45 < P/P 0 < 1.0 range, indicating that FZM has a hierarchical porous structure. Compared with ZIF-8 (983.44 m 2 /g), FZM (1110.39 m 2 /g) has a higher specific surface area. Moreover, the pore volume of FZM is 0.521 cm 3 /g, which is higher than that of ZIF-8 (0.443 cm 3 /g). And the average pore size decreases from 6.30 nm of ZIF-8 to 5.81 nm of FZM, indicating that the addition of Fe and MWCNTs changes the pore structure of the crystals. Furthermore, the reduced pore size provides more active sites for the attachment of pollutants and can effectively improve the adsorption performance.

TG analysis
Figure S1(d) shows the TG curves of ZIF-8 and FZM. The results show that both adsorbents are thermally stable. At 550 °C, the weights of ZIF-8 and FZM drop by 7.62% and 11.11%, respectively, which is attributed to water and organic solvents as well as unreacted 2-methylimidazole. When the temperature reaches 600 °C, the weight of both adsorbents decreases sharply due to the fracture and skeleton collapse of Fe-N or Zn-N. It is shown that both ZIF-8 and FZM have good thermal stability (Wu et al. 2015). Figure S2 shows the XPS spectrum of FZM. Figure S2(a) shows that the composite is mainly composed of C, N, Fe, and Zn. The presence of Fe implies that the metal-organic framework has been successfully doped with Fe. In Fig. S2(b), C 1 s is divided into two peaks, C = C/C-C and C-N/C-H. N 1 s in Fig. S2(c) is similarly divided into two peaks, pyridine N and Fe-N/Zn-N; the presence of Fe-N confirms the successful synthesis of Fe-ZIF-8 (Yang et al. 2021). Figure S2(d) shows that Zn 2p 3/2 and Zn 2p 1/2 are located at 1021.8 eV and 1044.8 eV, respectively, confirming that the introduction of Fe has no effect on the synthesis of ZIF-8. The Fe peaks in Figure S2(e) include Fe 2+ 2p 3/2 (710.58 eV), Fe 3+ 2p 3/2 (713.13 eV), Fe 2+ 2p 1/2 (723.43 eV), and Fe 3+ 2p 1/2 (727.86 eV). The presence of Fe 3+ may be due to the exposure of Fe 2+ to air and oxidation to Fe 3+ during the synthesis process.

Effects of Fe doping and adsorbent dosing
Figure S3(a) shows the effect of Fe doping on TC adsorption by FZM at a TC concentration of 100 mg/L, 298 K, and pH = 6 for 6 h. When the Fe doping is raised from 0 to 150 mg, the adsorption capacity of TC increases from 321.7 to 416.1 mg/g, and then reduces to 364.8 mg/g, suggesting that Fe doping improves the adsorption performance of ZIF-8. Furthermore, the addition of MWCNTs significantly increases the adsorption capacity of TC, demonstrating that MWCNTs also promote the adsorption performance of ZIF-8. Figure S3(b) shows the effect of FZM dosing on TC adsorption at 100 mg/L TC, 298 K, and pH = 6. The removal rate of TC increases from 31.19 to 55.11% as the FZM dosing increases from 50 to 250 mg/L, whereas the adsorption capacity decreases from 623.8 to 220.44 mg/g. The increase in TC removal could be attributed to a higher adsorbent dosage, which offered underutilized adsorption sites, resulting in a considerable reduction in adsorption capacity (Yu and Wu 2020). The adsorbent dosage of 100 mg/L was set as optimal.

Effect of solution pH
The solution pH is also crucial in the adsorption process. Figure 3a shows the effect of pH on TC adsorption by FZM under the conditions of 100 mg/L TC, 100 mg/L FZM, and 298 K. The results show that the adsorption capacity of TC increases when the pH increases from 2 to 6. The adsorption capacity decreases significantly when the pH increases from 6 to 12. This suggests that TC adsorption by FZM is pH-dependent.
According to previous research, TC has cationic species TC + (pH < 3.3), oligomeric ionic species TC 0 (3.3 < pH < 7.8), and anionic species TC − /TC 2− (pH > 7.8) ionic states in aqueous solution (Wang et al. 2020a, b). When pH < 3.3, TC exists in the form of TC + , and a huge number of positive charges accumulate on the surface of FZM, resulting in the obvious electrostatic repulsion between FZM and TC + . When pH is between 3.3 and 7.8, the proton concentration decreases, lowering the positive charge on FZM's surface, and TC exists in the form of TC 0 , weakening the electrostatic repulsion. However, FZM maintains a better adsorption effect on TC at this time. It is suggested that the hydrogen bonds formed between FZM and TC molecules and π-π interactions play a dominant role in the adsorption process. When pH > 7.8, TC molecules exist in the form of anionic species TC − /TC 2− , and the OH − content in the solution gradually increases, resulting in the accumulation of negative charges on the FZM's surface, and the adsorbent and anions generate electrostatic repulsion, obstructing the contact between the TC and FZM, lowering the adsorption capacity.

Effect of co-existing ions
TC usually co-exists with other ions in natural water, and these ions can affect the adsorption of TC. Therefore, in this work, the effects of NaCl, Na 2 SO 4 , Na 2 CO 3 , MgCl 2 , and CaCl 2 at concentrations of 50 and 100 mg/L on TC adsorption were investigated at 100 mg/L TC, 100 mg/L FZM, pH = 6, and 298 K.
In Fig. S4, the adsorption of TC by FZM is significantly inhibited in the presence of NaCl, Na 2 SO 4 , Na 2 CO 3 , and MgCl 2 , and the inhibition increases as the ion concentration increases. This indicates that these ions have a competitive effect on TC adsorption; probably Cl − , SO 4 2− , and Mg 2+ occupy the adsorption sites on the adsorbent surface, resulting in the hindrance of TC contact with FZM. The hydrolysis of CO 3 2− to create OH − makes the solution alkaline and affects the adsorption performance of FZM. The adsorption capacity of TC by FZM is promoted in the presence of Ca 2+ , but decreases when the concentration of Ca 2+ increases from 50 to 100 mg/L. Figure 4a shows the effect of reaction time on TC adsorption by FZM under the conditions of 100 mg/L TC, 298 K, and pH = 6. The results show that as time passes, the adsorption capacity increases rapidly, and then slows down before 360 min. The adsorption capacity rises rapidly because there are adequate adsorption sites on the surface of FZM. The number of adsorption sites decreases as reaction time increases, rendering TC adsorption harder and driving the TC adsorption capacity to plateau.

Adsorption kinetics
Pseudo-first-order (PFO), pseudo-second-order (PSO), and intra-particle diffusion (ID) kinetic models were used to investigate the adsorption data of TC by FZM, to understand the adsorption mechanism deeply. Figure 4 and Table S2 demonstrate the linear fitting curves of the experimental data of TC adsorption through three adsorption kinetic models, as well as the corresponding model parameters. Table S2 shows that the PSO model's correlation coefficient (R 2 ) value (R 2 = 0.9856) is substantially closer to 1, indicating that chemisorption may be the rate-determining step and controlling factor in the TC adsorption process (Suganya and Kumar 2018). Although Q t and t 0.5 in the ID model have a good linear correlation, the fitted straight line in the figure does not pass through the origin, indicating that the ID model is not the only rate-limiting step.

Adsorption isotherms
The effect of initial TC concentrations in the range of 10-100 mg/L was studied under the conditions of 298 K, pH = 6, and dosage of FZM = 100 mg/L in this research. The results are shown in Fig. 5a. The adsorption capacity increases significantly with the TC concentration increasing from 10 to 100 mg/L. This demonstrates that the surface of FZM has enough adsorption sites for TC adsorption. Therefore, 100 mg/L TC concentration was selected.    Table 1 lists the parameters of the models. It can be determined that the Freundlich model's R 2 is 0.9978 and closest to 1, revealing that the Freundlich isotherm can describe the TC adsorption by FZM the best. TC adsorption by FZM is multi-layer adsorption, according to the assumptions of the Freundlich model (Saravanan et al. 2018). Furthermore, the maximum saturation adsorption capacity fitted to the Langmuir model is 1111.11 mg/g, which is superior to the previously reported material (as listed in Table 2), showing that FZM has a greater potential for TC adsorption.

Adsorption thermodynamics
Temperature is also a consideration that should not be overlooked in adsorption processes. The effects of temperature (298 K, 308 K, and 318 K) on TC adsorption by FZM were investigated at 100 mg/L TC, pH = 6, and 100 mg/L FZM. Table 3, the values of ΔG 0 obtained at all three temperatures are negative, showing that the TC adsorption by FZM is spontaneously. Furthermore, the ΔH 0 value of the adsorption process becomes positive, indicating that the reaction is heat-absorbing, and high temperatures are advantageous for boosting TC adsorption. Additionally, ΔS 0 is greater than 0, implying that the adsorbent has a stronger affinity for TC molecules.

Adsorption mechanism
The mechanism of TC adsorption by FZM was investigated in depth to appropriately improve the current adsorbent and manufacture a higher performance material. The findings of the FTIR characterization examination of the FZM materials before and after TC adsorption are presented in Fig. 6.
The FZM diffraction peaks after TC adsorption are nearly identical to those before adsorption, implying that the structure of FZM remains stable during the adsorption process. In comparison with before adsorption, the FZM after adsorption of TC exhibits a new characteristic peak at 1258 cm −1 , which is caused by the stretching vibration of the -CH 3 group in the TC molecule. Moreover, the intensity of the characteristic peak at 1591 cm −1 is significantly enhanced and redshifted, which may be due to the absorption of the deformation vibration of N-H in the TC molecule (Gao et al. 2012). Furthermore, a broad peak is observed at 3200-3500 cm −1 and the intensity of the characteristic peak is significantly enhanced. This may be attributed to the stretching vibration of intermolecular hydrogen bonding, indicating that -OH on TC is adsorbed on the surface of FZM through hydrogen bonding ). Meanwhile, a new characteristic peak appears at 592 cm −1 , which is thought to be caused by Fe-O stretching, indicating the binding of Fe to the oxygen-containing functional group in the TC molecule. This also demonstrates that the incorporation of Fe introduces active sites for TC adsorption and enhances the adsorption capacity of FZM (Gu et al. 2021).
No new characteristic peak has been found at 423 cm −1 , indicating that Zn is not involved in TC adsorption. In pHinfluenced experiments, the results indicate the existence of electrostatic interactions between FZM and TC molecules. In addition, FZM still has a high adsorption capacity for TC at high pH, indicating that π-π interaction also exists between TC and FZM (Xiong et al. 2018). In summary, the adsorption mechanism of TC by FZM mainly includes H-bonding, surface complexation, π-π interaction, and electrostatic interaction. The adsorption mechanism diagram is shown in Fig. 7.

RSM optimization
Three models in BBD were employed to explore the relationships between the adsorption capacity of TC and pH, temperature, and adsorbent dosage. The quadratic polynomial model fits the data better.
As stated in Eq. 3.1, a quadratic polynomial between the response variable Y (adsorption capacity) and the independent variables A (pH), B (temperature), and C (dosage) is constructed based on the results of the quadratic polynomial model fitting.  where Y represents the adsorption capability of TC and A, B, and C represent solution pH, temperature, and adsorbent dosage, respectively.
The equations reveal that increasing pH and adsorbent dosage inhibit the improvement of TC adsorption capacity, but increasing temperature promotes the improvement of adsorption capacity. The coefficient of adsorbent dosage is found to be the largest in comparison with pH and temperature, indicating that the amount of dosage has the greatest effect on TC adsorption. The order of effect of the independent variables is adsorbent dosage > pH > temperature.
The ANOVA results of the quadratic model obtained from the BBD are listed in Table 4. From the interpretation of the model fitting parameters, the larger the F value and the smaller the P-value, the more significant the model is. It can be seen that an F-value of 229.68 and a P-value less than 0.05 indicate that the model is significant, implying that the model can be used to explain the TC adsorption by FZM and to predict the optimal conditions. The P-values for the individual factors pH (A), temperature (B), and dosage (C) and the interaction term AC are less than 0.05, indicating that all are significant model terms.
Adequate precision (AP), coefficient of variance (CV %), standard deviation (Std. dev.), and regression coefficient (R 2 ) were employed to evaluate the reliability and accuracy of the experimental response (R 2 ). The AP value is larger than 4.0, and the CV % value is 1.84, indicating that the experimental data is reliable. The "adjusted R 2 " and "predicted R 2 ," respectively, are 0.9923 and 0.9547, with a difference of less than 0.2, showing that the quadratic model is suitable for the experimental data. Meanwhile, the R 2 value of the TC adsorption model is 0.9966, which implies a linear correlation between the actual response and the predicted response, indicating that this model is reliable and applicable. Thus, the results reconfirm that the proposed adsorption optimization model for TC by FZM is statistically significant and can be used to predict the optimal conditions. The effects of pH-temperature, temperature-dosage, and pH-dosage on TC adsorption by FZM were determined, and the results are depicted as three-dimensional surface plots and contours. The results are shown in Fig. 8a and b, c and d, and e and f, respectively. The optimal conditions for TC adsorption by FZM are a pH of 7.1, a temperature of 312.5 K, and a dose of 64.43 mg/L using this model. The TC adsorption capacity anticipated is 599.78 mg/g. Experiments were conducted to verify the accuracy and reliability of this model. The results show that the adsorption capacity of TC by FZM under these conditions is 589.42 mg/g, with a 1.73% deviation from the predicted value. This reveals that the optimization of TC adsorption by FZM using RSM is effective, and RSM can be used to determine the optimal adsorption conditions.

Reusability of FZM
Reusability is a determinant of the practical applicability of an adsorbent. FZM was added to a centrifuge tube containing 100 mg/L TC and mixed for 360 min at 298 K and pH = 6. The reacted solid was washed three times with 1 M NaOH and ultrapure water alternately, and then dried in vacuum at 60 °C for 12 h. The above steps were repeated four times, and the results are shown in Fig. 9. The results reveal that the adsorption capacity of FZM for TC decreases significantly after four cycles, but still exceeds 370 mg/g. This confirms that the adsorption performance of FZM remains excellent after four regenerations. This also indicates that FZM has promising reusability and potential for practical application in TC removal.

Production cost estimation
Low yield is a non-negligible shortcoming of MOFs, which largely limits the mass production and practical application of the material. Therefore, we estimated the cost of manufacturing and using FZM, to analyze and explore alternatives for cost reduction. The preparation cost of FZM mainly consists of raw materials (cost A), cleaning solvents (cost B), electricity, tap water, and ultrapure water (cost C). On the basis of the current laboratory scale and the bulk purchase price of the materials, it is estimated that cost A is US Based on the results of RSM, the adsorption capacity under the predicted optimal conditions is 589.42 mg/g, and the cost of adsorption TC is US $0.36/g. Additionally, the cleaning solvent accounted for 71% of the total cost. In the laboratory, the cleaning solvent is disposable. However, the recovery rate of the cleaning solvent can be up to 90% in large-scale production . Therefore, the total cost would be reduced to US $75.10/kg. Currently, the cost of treating 100 mg/L TC wastewater is about US $4.84/m 3 at a 64.43 mg/L dose of FZM. In summary, the practical application of FZM is promising.  Fig. 9 Effect of the number of FZM regeneration cycles of on its TC adsorption capacity