JBCS

INTRODUCTION

Considerable quantities of hazardous substances are released into the environment through both domestic and industrial activities, resulting in significant detrimental effects on human health and ecosystems.¹ These substances encompass heavy metals, antibiotics, chemicals employed in industries, additives present in food products, veterinary drugs, pharmaceuticals, and personal care items,^2-4 all of which pose grave risks to both humans and the natural world. According to recent statistical trends, it is projected that the global population will exceed 10 billion individuals by 2050.^5,6 Approximately 30% of the world’s population lacks access to potable water.⁷ Specifically, the utilization of antibiotics significantly contributes to environmental contamination due to their potent bactericidal or bacteriostatic properties. These substances find extensive application in medical, agricultural, and aquacultural sectors. Mulchandani et al.⁸ recent investigation reveals an estimated annual antibiotic consumption of approximately 107,500 tons in livestock worldwide by 2030, representing a notable increase from less than 100,000 tons recorded in 2020. Notably, Asia (particularly China) exhibits the highest rates of antibiotic usage. Furthermore, there is an anticipated additional surge of around 8% in antibiotic consumption between the years 2020 and 2030.

The inadequate management of these pollutants has led to extensive water pollution, exacerbating the global issue of water scarcity.⁹ It is evident that finding efficient and cost-effective approaches to eliminate these contaminants presents a significant challenge. Various technologies, including filtration, chemical precipitation, and coagulation/flocculation, have been employed for their removal; however, they are associated with drawbacks such as complex equipment requirements and expensive maintenance expenses.¹⁰ Therefore, it remains crucial to explore effective strategies for eradicating these pollutants. Physical, biological, and chemical techniques are commonly utilized for successful control of these harmful substances.^11-14 Adsorption technology has gained widespread acceptance due to its straightforward operation and resistance against toxins or poisons while offering opportunities for reuse along with effective removal capabilities against various types of pollutants.^15-20

The adsorption process is influenced by various factors, such as surface area, pore volume, and functionality.^1-3 Metal-organic frameworks (MOFs) have emerged as highly efficient adsorbents due to their ability to easily customize pore size and structure. These frameworks are formed by connecting inorganic metal atoms (as nodes) with organic ligands (as linkers). They possess a large surface area, numerous active sites, adjustable functional groups, and the capability of charge separation under normal light conditions. MOFs are extensively studied both domestically and internationally due to their diverse variations in coordination bonds. The utilization of MOFs has become an exciting field within coordination chemistry.^21,22 Given these exceptional characteristics exhibited by MOFs, they find widespread applications in various areas including separation,²³ sensing,²⁴ catalysis,²⁵ storage,²⁶ etc. Consequently, many scientists specializing in materials and chemistry have redirected their focus towards exploring the potential applications of MOFs.^27-29

Considering the aforementioned factors, this study focuses on utilizing 2,5-dihydroxyterephthalic acid as an organic ligand and ferrous sulfate heptahydrate as a metal precursor to synthesize Fe-MOFs for the purpose of antibiotic elimination. The results demonstrate that Fe-MOFs exhibit significant efficacy in norfloxacin eradication.

 

EXPERIMENTAL

Materials and instruments employed in the experiment

The chemical substances, namely 2,5-dihydroxyterephthalic acid, ferrous sulfate heptahydrate (FeSO₄^•7H₂O), N, N-dimethylformamide (DMF), and norfloxacin were acquired from Aladdin Technology Co., Ltd.

To investigate the structure of Zn/Fe-MOFs materials, we utilized a range of advanced instruments for material characterization. The crystal structure analysis was conducted using the TD-3300 XRD diffractometer manufactured by Dandong Tongda Technology Co., Ltd. Microscopic morphology observations were performed using the JSM-6700F mode field emission scanning electron microscope provided by Nippon Electronics. The chemical bonding state and thermal stability of the materials were determined utilizing the IRAffinity-1 infrared spectrometer and DTG-60 differential thermogravimetric synchronization analyzer from Shimadzu Company from Japan, respectively. Additionally, we measured the concentration of levofloxacin hydrochloride and Congo Red at different time intervals using the UV-8000S double beam UV-visible spectrophotometer supplied by Shanghai Meisei Instrument Co., LTD.

Preparation of Fe-MOFs materials

It was dissolved 0.0713 g of 2,5-dihydroxyterephthalic acid in a beaker containing 5 mL of DMF and the solution was thoroughly mixed. In another container, it was continuously stirred to blend 0.1 g of ferrous sulfate heptahydrate into 10 mL of N, N-dimethylformamide until complete dissolution was achieved. The two solutions were thoroughly combined before transferring them to the reactor vessel. The mixture was then subjected to a temperature of 120 ºC for a duration of 12 h in an oven, ensuring sufficient time for complete reaction occurrence. After cooling down to room temperature, any impurities on the material’s surface were removed through filtration and rinsed off using a small quantity of DMF solution. The filtration process was repeated and then the reactants were dried at a temperature set at 60 ºC for approximately 8 h to obtain Fe-MOFs materials.

Removal of norfloxacin

To assess the adsorption capacity of Fe-MOFs materials on norfloxacin, solutions with varying concentrations of norfloxacin were prepared. Different amounts of Fe-MOFs materials were added to these solutions and gently agitated under natural lighting conditions. Prior to introducing the Fe-MOFs material, the initial solution was extracted using a needle tube. Subsequently, samples were collected at 30-min intervals and their concentrations were measured using an ultraviolet-visible spectrophotometer for further analysis.

The adsorption impact (q_e) of Fe-MOFs material on norfloxacin can be described as follows:

where C₀ (ppm), C_e (ppm), m (mg), and V (mL) represent the initial concentration of the norfloxacin solution, the equilibrium concentration after adsorption, the mass of Fe-MOFs material employed, and the total volume of the solution.

 

RESULTS AND DISCUSSION

Characterization of Fe-MOFs materials

The structure of Fe-MOFs was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and nitrogen adsorption-desorption isotherms. Figure 1 depicts the XRD pattern of the MOFs material, revealing a prominent peak indicative of its exceptional crystallinity. SEM imaging in Figure 2 showcases the distinctive morphology and well-dispersed crystal structure of the Fe-MOFs material. The adsorption efficiency of norfloxacin is influenced by the stability of the MOFs material. Thermogravimetric analysis in Figure 3 demonstrates three distinct stages for the Fe-MOFs material: (i) residual solvent molecules are primarily lost at an external temperature of 414 ºC; (ii) between 414 to 617 ºC, approximately 43.48% initial mass remains with a gradual decrease mainly attributed to metal ion oxidation; (iii) from 618 to 723 ºC, further mass loss occurs due to frame destruction. These findings indicate that Fe-MOFs material maintains stability up to temperatures as high as 613 ºC. In conclusion, synthetic reactions involving Fe-MOFs yield relatively stable structures formed by metal ions and organic ligands. The benzene ring in Figure 4 exhibits a prominent peak at 807 cm^-1, attributed to para-substitution. The absorption peak observed at 1118 cm^-1 corresponds to the C-O bond. Furthermore, coordination of metal ions with carboxyl groups results in the formation of characteristic peaks at 1551 and 1416 cm^-1.^30-32

 

 

 

 

 

 

 

 

The Fe-MOFs exhibit a distinct N₂ adsorption-desorption isotherm, as illustrated in Figure 5. The material demonstrates significant porosity with a measured Brunauer-Emmett-Teller (BET) surface area of 129 m² g^-1. Furthermore, both the adsorption and desorption processes reveal an average pore size of approximately 9.25 and 9.26 nm, respectively, indicating the presence of mesoporous characteristics within this substance.

 

 

Removal of norfloxacin by Fe-MOFs

Influence of reaction concentration and Fe-MOFs mass on the adsorption of norfloxacin

The research investigated the impact of both the quality of Fe-MOFs and the concentration of norfloxacin solution on the efficiency of norfloxacin removal, a contaminant. Figure 6 illustrates a gradual decline in removal effectiveness with increasing concentration of norfloxacin solution. Furthermore, an increase in the quantity of Fe-MOFs material leads to a decrease in adsorption rate. Each set of experiments was repeated more than 5 times, and the optimal outcome demonstrated a norfloxacin concentration of 10 ppm, with a maximum adsorption capacity reaching 166 mg g^-1 upon addition of 10 mg Fe-MOFs material. However, increasing the mass of Fe-MOFs reduces the removal rate due to a reduction in active sites and subsequent decline in adsorption capacity for norfloxacin.

 

 

To validate theoretical predictions with practical observations regarding norfloxacin removal, we describe below the kinetic model for pollutant elimination based on existing experimental diagrams and data.

In the provided Equations 2 and 3, C_t (ppm), C₀ (ppm), k₁ (L min^-1), k₂ (g mg^-1 min^-1) and t (min) represent the concentration of norfloxacin at a specific time point, initial concentration of norfloxacin, rate constant for the kinetic reaction, and duration of the reaction at time t, respectively. q_t (mg g^-1) and q_e (mg g^-1) denote the concentration of adsorbent material at time t and equilibrium state, respectively.

The results obtained from assessing Fe-MOF’s efficacy in removing norfloxacin are presented in Figures 7-10, Tables 1 and 2, with correlation coefficients (R²) surpassing 0.99. The adsorption strength of the adsorbent to the adsorbate increases with a higher adsorption correlation coefficient, indicating that the adsorbent can effectively immobilize the adsorbate on its surface during the process of adsorption, thereby leading to enhanced separation or purification outcomes. The reaction rate constant k can serve as an indicator of the reaction’s velocity, with a higher value indicating an accelerated rate and a lower value suggesting a decelerated rate. This suggests that the utilization of a quasi-second-order kinetic model can effectively elucidate the adsorption process by Fe-MOFs, thereby providing substantial evidence for the theoretical validity and practical applicability of Fe-MOFs in norfloxacin removal.

 

 

 

 

 

 

 

 

 

 

 

 

The intra-particle diffusion model holds significant importance in investigating the diffusion properties of particles in various environments, thereby facilitating a comprehensive understanding of internal diffusion processes within particles, examining particle-environment interactions, and comprehending particle transport characteristics across diverse settings. By delving into the intricacies of the intra-particle diffusion model, we can unravel the underlying mechanisms and governing laws that dictate material transfer within particles. This comprehensive study provides a robust theoretical foundation and practical guidance for industrial processes such as particle transportation and processing. The formula representing the intra-particle diffusion model is provided below.

The results obtained from the intra-particle diffusion model are presented in Figure 11 and Table 3. The higher intercept values indicate a greater influence of surface diffusion compared to intra-particle diffusion. Significant R² values suggest that the rate-limiting step in this process is external surface adsorption. Deviation of the line from its origin and a gradual adsorption stage with low R² imply the presence of additional factors constraining the rate, beyond intrapellular diffusion. This observation provides valuable insights into comprehending the underlying mechanism governing this adsorption process.

 

 

 

 

The influence of pH on the adsorption behavior of norfloxacin

To investigate the impact of different pH levels on the efficiency of norfloxacin removal, we evaluated the adsorption performance of Fe-MOFs material using norfloxacin solutions with varying initial pH values (pH = 6, 8, 10, and 12) at a concentration level of 15 ppm. As illustrated in Figure 12, it was observed that within the pH range from 6 to 10, there was a decrease in norfloxacin adsorption as the solution’s pH decreased. Specifically, at a pH value of 10, the measured adsorbed capacity for norfloxacin was approximately found to be 197.7 mg g^-1. Conversely, when the solution’s pH dropped to 6, there was a reduction in adsorption capacity to around 70.7 mg g^-1. Interestingly enough, upon increasing the solution’s pH to reach a value of 12, a slight decline in adsorption capacity to approximately 183.7 mg g^-1 was noted. The primary factors contributing to this outcome are the concentration of the solution and material electrification, which result in varying concentrations of hydrogen ions in norfloxacin solutions with different strengths. Additionally, the charge of Fe-MOFs tested was measured at 3.26 mV. Therefore, besides any structural irregularities in Fe-MOFs, there exists a correlation between their charge and the observed results.³³

 

 

To investigate the adsorption of norfloxacin by Fe-MOFs, theoretical exploration was conducted using Langmuir and Freundlich isotherms. The respective formulas are as follows:

The adsorbent’s capacity for equilibrium adsorption (q_e) and saturated adsorption (q_m) is measured in milligrams per gram (mg g^-1), while the concentration of norfloxacin at equilibrium (C_e) is expressed in milligrams per liter (mg L^-1). Additionally, the adsorption constant is denoted as k_L.

The findings depicted in Figure 13 and Table 4 reveal that the Langmuir isotherm and Freundlich isotherm models exhibit high nonlinear correlation coefficients of 0.99633 and 0.99767, respectively. It is worth noting that there exists minimal disparity between the correlation coefficients derived from both models. The data presented in Figure 13 and Table 4 demonstrate the effective applicability of both the Langmuir isotherm and Freundlich isotherm models in describing the adsorption behavior of norfloxacin by Fe-MOFs. This discovery provides corroborative evidence for the adsorption mechanism involved in norfloxacin elimination. Moreover, previous studies¹¹ have indicated that Fe-MOFs contribute to the adsorption process of norfloxacin through both physical and chemical mechanisms.

 

 

 

 

To demonstrate the efficacy of Fe-MOFs in norfloxacin removal, a comparative analysis presented in Table 5 reveals that Fe-MOFs exhibit superior advantages over other adsorbents. The rationale behind this observation is as follows: firstly, the porous structure of Fe-MOFs enables efficient infiltration of norfloxacin into its interior. Secondly, both Fe-MOFs’ ligands and norfloxacin contain benzene rings, facilitating their adsorption through π-π bond interactions. Thirdly, hydrogen bonding potential exists between the surface of Fe-MOFs and norfloxacin, thereby enhancing their adsorption capacity. Fourthly, due to its charge properties, Fe-MOFs possess a zeta potential value of 3.26 mV; similarly, the surface functional group of norfloxacin also carries a slight charge. Consequently, electrostatic forces play a significant role in effectively capturing norfloxacin by Fe-MOFs.^34,35 Collectively, these aforementioned factors contribute to the exceptional performance demonstrated by Fe-MOFs in removing norfloxacin from solutions.

 

 

In order to assess the influence of temperature on the adsorption characteristics of norfloxacin by Fe-MOFs, a solution containing 15 ppm concentration of norfloxacin was treated with 20 mg of Fe-MOFs material. The impact of different temperatures (T = 15, 25, 35 ºC) on the interaction between Fe-MOFs and norfloxacin during the adsorption process was investigated. The experimental procedure can be summarized as follows:

For this study, we employed the ideal gas constant (R = 8.314 J mol^-1 K^-1) and Langmuir adsorption constant (K₀ in L mol^-1). To determine the values of ΔH⁰ (enthalpy) and ΔS⁰ (entropy), a linear regression analysis was performed by plotting lnK₀ against 1/T using a van’t Hoff plot. Specifically, ΔH⁰ was calculated as the negative slope multiplied by R, while ΔS⁰ was obtained from the intercept multiplied by R.

According to the existing literature,²³ it has been documented that enthalpy changes ranging from 84 to 420 kJ mol^-1 indicate chemical adsorption, while an enthalpy change below 84 kJ mol^-1 suggests physical adsorption. The experimental results presented in Figure 14 and Table 6 provide compelling evidence supporting the endothermic and spontaneous adsorption of norfloxacin by Fe-MOFs. Therefore, it can be inferred that the predominant mechanism underlying norfloxacin’s adsorption onto Fe-MOFs is primarily of a physical nature. Consequently, alterations in entropy and enthalpy significantly influence norfloxacin’s adsorptive behavior on Fe-MOFs.³⁹ Physical interactions serve as the main driving force behind the process of adsorption.

 

 

 

 

CONCLUSIONS

In summary, Fe-MOFs were successfully synthesized using a hydrothermal technique and characterized through XRD, SEM, and TG analyses. These characterized Fe-MOFs were utilized for the efficient elimination of norfloxacin. The outcomes demonstrated that at pH 10, the adsorption capacity of norfloxacin reached 197.7 mg g^-1. Examination of pseudo-first-order kinetics, pseudo-second-order kinetics, and adsorption isotherm indicated that the removal process of norfloxacin by Fe-MOFs followed pseudo-second-order kinetics and Langmuir adsorption isotherm models. Moreover, the results indicate that Fe-MOFs exhibit improved adsorption capabilities towards norfloxacin due to the combined effects of physical and chemical adsorption mechanisms. These findings highlight the promising potential of Fe-MOFs as highly effective materials for applications in removing antibiotics.

 

ACKNOWLEDGMENTS

This work was supported by Doctoral Fund of Anshun University (asxybsjj202103).

 

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Associate Editor handled this article: Cassiana C. Montagner