Preparation of Zr - MOFs for the adsorption of doxycycline hydrochloride from wastewater

: Zr - metal - organic frameworks ( Zr - MOFs ) were prepared by a solvothermal method and characterized by X - ray di ﬀ raction, scanning electron microscopy, and thermogravimetry. Zr - MOFs were used to remove doxycy - cline hydrochloride ( DOC ) from wastewater. According to the experimental results, the maximum adsorption capa - city of DOC by Zr - MOFs within 5 h was 148.7 mg · g − 1 . From the pseudo - second - order kinetics model, all R 2 values were greater than 0.99, which proved that the adsorp - tion of DOC by Zr - MOFs was consistent with practice. According to the Freundlich isotherm model, the adsorp - tion of DOC by Zr - MOFs proceeded via multilayer adsorp - tion. The aforementioned results show that Zr - MOFs have good application prospects for removing DOC from wastewater.


Introduction
Antibiotics have been widely used in human and veterinary medicine to prevent and treat infectious diseases.
However, due to poor metabolism in animals and humans, about 30-90% of administered antibiotics are excreted into the aquatic environment after treatment.Many antibiotics cannot be fully used and therefore enter nature, which may produce drug-resistant bacteria [1].Doxycycline hydrochloride (DOC) readily dissolves in water [2].Sewage treatment methods do not adequately remove antibiotics from water and sometimes produce secondary pollution.Therefore, many different technologies have emerged to remove DOC, including microbial degradation, phytoremediation, constructed wetland, microbial fuel cell enhanced biodegradation, constructed wetland-coupled microbial fuel cell degradation, and adsorption methods [3][4][5][6].Among them, the degradation rate of the microbial degradation method and phytoremediation method is low.The constructed wetland method requires a large land area and long operation times.The enhanced biodegradation method of microbial fuel cells is restricted by many factors, such as solution concentration, electrode material, aeration rate, ionic strength, and external resistance and temperature, so the accuracy of the obtained data is low [7][8][9].The removal rate using the constructed wetland-coupled microbial fuel cell degradation method is greatly affected by influent co-matrix and antibiotic concentration.Its development is also constrained by immature technology and high costs [10].The adsorption method in this article has a simple procedure, is safe, uses simple equipment, has a narrow pH change, and uses less organic solvent to remove pollutants.However, it suffers from poor selectivity and unstable adsorbent performance.

Experimental reagents and instruments
The ligand 2-amino-terephthalic acid, metal-derived zirconium acetate, and DOC were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

Preparation of Zr-MOFs
The compound was synthesized according to the method previously described [16,20].2-Amino-terephthalic acid (0.3623 g) was dissolved in 15 mL of N,N-dimethylformamide (DMF).Then, 3.305 mL of zirconium acetate was mixed in a reaction kettle and then placed in a constant-temperature drying box set to 150°C.After reacting for 14 h, it was taken out and cooled to room temperature.The products were transferred to a centrifugal pipe and then centrifuged.Then, crystals were transferred to a beaker, magnetically stirred, and then washed three times with DMF and water.Centrifugation was repeated after each washing to remove unreacted starting matter, and the sample was put into a drying oven.

Removal of antibiotics by Zr-MOFs
Different masses of Zr-MOF samples obtained by the hot solvent method (20, 30, 40, and 50 mg) were added to 200 mL of DOC solutions with different concentrations (20, 30, 40, and 50 mg•L −1 ).Then, they were stirred under natural light.Samples were taken every 30 min, and changes in antibiotic concentration in the solution at 269 nm were analyzed using a UV spectrometer.Then, by analyzing the relationship between time and concentration, the adsorption capacity and removal rate of DOC by Zr-MOFs were calculated.The adsorption capacity and removal rate of DOC were calculated using the following two formulas: where C 0 is the initial concentration of DOC, C e is the concentration at adsorption equilibrium, C t is the concentration at time t, V is the volume of solution, and m is the mass of Zr-MOFs.

Structure of Zr-MOFs
The infrared spectrum in Figure 1 has strong absorption peaks at 1,581 and 1,375 cm −1 .Strong absorption peaks also appeared between 1,620-1,550 and 1,420-1,300 cm −1 , which indicate that carboxylic acids reacted with the metal salt [31,32].In Figure 2, the XRD patterns of the materials show that Zr-MOF had a poor crystallinity and small particle size.Figure 3 shows that Zr-MOFs had a better morphology and dispersion.This was mainly because intermolecular interactions between organic ligands were weakened, and the deprotonation of organic ligands was enhanced, which promoted the growth of crystals in the solvent.
Figure 4 shows that the Brunauer-Emmett-Teller surface area was 405.7776 m 2 •g −1 .The adsorption average pore diameter was 3.61059 nm, indicating the mesoporous nature of the material.As can be seen from Figure 3, the Zr-MOFs displayed type IV of isotherms with H3 hysteresis loops, indicating the mesoporous properties of the sample.As shown in Figure 5, the thermogravimetric curves of the Zr-MOFs were characterized by three different stages: (1) a mass loss of 12.3% at about 90°C, which was dominated by residual solvent molecules; (2) a mass loss of 13% between 91°C and 275°C, which was mainly due to the oxidation of metal ions; and (3) a mass loss that began at 275°C and ended at 500°C, leaving a residual amount of 16% [33].This corresponded to the destruction of the framework, indicating that the material was stable below 275°C.In most cases, Zr-MOFs generated via reactions between metal ions and organic ligands have more stable structures [34].

DOC removal using Zr-MOFs
To study the ability of Zr-MOFs to remove DOC, this experiment controlled the amount of Zr-MOFs, DOC-HCl, and reaction time.As shown in Figure 6, when the concentration of DOC was 30 ppm and the concentration of Zr-MOF was 50 mg, the removal rate reached 84.4% after 5 h.After three cycles, the removal rate of Zr-MOFs was 19.7%, indicating reasonable reusability (Figure 7).It can also be seen from the figure that when the concentration of DOC was unchanged, the removal rate gradually increased after adding Zr-MOF.When the dosage of Zr-MOF was constant, the removal rate increased upon decreasing the DOC concentration.When the mass of Zr-MOF was 30 mg, and the concentration of DOC was 50 mg•L −1 , the maximum adsorption capacity was 148.7 mg•g −1 .
To verify the consistency between theory and experimental practice, the first-order and second-order kinetics were simulated to study the removal of DOC by Zr-MOF.The results are shown in Figures 8 and 9 and Table 1.The formulas are as follows [35][36][37]:  As shown in Figures 8 and 9 and Table 1, the R 2 value of the second-order kinetics model was better than that of the first-order model.The simulation results were in agreement with the experimental results.The adsorption of DOC by Zr-MOFs occurred mainly via chemisorption.
The experimental results were also analyzed by the Langmuir and Freundlich isotherm models [38,39].When the concentration of DOC adsorbed by was 20 ppm, the parameters obtained by Langmuir and Freundlich models are shown in Figure 10 and Table 2.The R 2 values were 0.99162 and 0.99787, respectively.The data showed that the Freundlich model was more consistent with the experimental adsorption data of DOC, indicating that the adsorption of DOC by Zr-MOFs proceeded via multilayer adsorption.
To explore the effect of pH on DOC adsorption, 30 mg of Zr-MOF was added to 50 ppm DOC solutions with different pH values.As shown in Figure 11, the adsorption capacity of DOC increased with the pH.The zeta potential of Zr-MOFs was −9.98 mV, which might be attributed to the strong electrostatic interactions between DOC molecules and Zr-MOF adsorbent surfaces.The maximum adsorption capacity was obtained at pH 6 and 10.
To verify the influence of temperature, 30 mg of Zr-MOFs was added to 50 ppm DOC solutions at different temperatures and compared with ambient normal temperature.The adsorption capacity slightly decreased upon increasing the temperature, and the adsorption effect was the best at ambient temperature.
To gain further insight into the mechanism of DOC adsorption, the thermodynamic equilibrium constant (K 0 ) and Gibbs free energy change (ΔG 0 ), enthalpy change (ΔH 0 ), and entropy change (ΔS 0 ) were determined using the following equations: where K 0 is the Langmuir adsorption constant (L•mol −1 ) and R is the gas constant (8.314J•mol −1 •K −1 ).A linear plot of ln K 0 versus 1/T was obtained as shown in Figure 8. ΔH 0 and ΔS 0 were calculated from (−slope × R) and (intercept × R) of the van't Hoff plot as shown in Figure 9 and Table 3.
As shown in Figure 12 and Table 3, the negative ΔG 0 and ΔH 0 values indicated that the adsorption of DOC over MOFs was spontaneous and exothermic.Enthalpy changes due to chemisorption fall in the range of 84 and 420 kJ•mol −1 , while physical absorption tends to occur below 84 kJ•mol −1 [40].Thus, the adsorption of DOC over MOFs may have occurred by physisorption.Moreover, the entropy change ΔS 0 was positive, which revealed that the process increased the randomness because the number of desorbed water molecules was larger than that of adsorbed DOC molecules [41].
To study whether the removal of DOC by Zr-MOFs was affected by light, 30 mg of Zr-MOFs was added to 50 ppm DOC, and the results were compared under natural lighting and catalysis by a xenon lamp, as shown in Figure 13.After comparison and sampling within the same time period, the amount of DOC adsorbed under xenon lamp irradiation was 34.5 mg•g −1 less than that under natural light.
A comparison of the adsorption capacity of DOC by other materials is shown in Table 4. Zr-MOF had the best adsorption capacity for DOC.The adsorption of DOC by    Table 3: Thermodynamic parameters of DOC adsorption onto MOFs

Conclusion
A hot solvent method was used to prepare Zr-MOFs, which were characterized using XRD, scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and differential thermal-thermogravimetric analysis.Zr-MOFs were used to remove the antibiotic DOC, and the results showed that the best removal effect was with pH = 6 and pH = 10; simulated kinetics showed that DOC removal by Zr-MOFs followed the second-order kinetics model, with an R 2 > 0.99.The Langmuir and Freundlich isotherm model analysis showed that the adsorption mechanism was consistent with multilayer adsorption.It can be seen that DOC can be removed by Zr-MOFs and may have practical applications.

Figure 7 :
Figure 7: Reusability of Zr-MOFs for the removal of DOC.

Figure 11 :
Figure 11: Effect of pH on the adsorption amount of DOC.
occurred via a combination of chemisorption and physical adsorption, which led to a better removal effect.Zr-MOFs and DOC could form hydrogen bonds.Zr-MOFs are porous materials, which allow them to absorb DOC.Zr-MOFs may contain unreacted groups that could adsorb DOC via electrostatic adsorption.Because both Zr-MOFs and DOC have benzene rings, the two molecules may be bound together by π-π stacking.Due to the aforementioned effects, Zr-MOFs could remove DOC[46][47][48][49][50].

Figure 12 :
Figure 12: Van't Hoff plots used to determine the ΔH and ΔS of DOC adsorption over Zr-MOFs.

Table 1 :
Kinetic parameters for the adsorption of DOC over Zr-MOF

Table 2 :
Adsorption isotherm parameters of DOC onto MOFs at room temperature

Table 4 :
Comparison of the adsorption capacity of different adsorbents for DOC removal