Fabrication of Easy Separable and Reusable Adsorbent Composite based on MIL-125/MIL-53 Binary MOF/CNT/Alginate Microbeads for Aremoval of Tetracycline from Water Bodies

During the turbulent period of COVID-19, the medical staff is exerting great efforts to preserve humanity. However, the tons of pharmaceutical residues especially antibiotics that is being disposing daily into water bodies may be the seed to an even more ferocious pandemic. Thence, it is inevitable to find out effective strategies for removing these noxious pharmaceutical residues from water. We aimed in this investigation to fabricate easy separable composite microbeads for efficient adsorption of tetracycline (TC) drug. Herein, MIL-125/MIL-53 binary metal organic framework (MOF) was synthetized and incorporated with carbon nanotube (CNT) into alginate (Alg) microbeads to form MIL-125/MIL-53/CNT@Alg microbeads matrix. In addition, the adsorption process followed the pseudo-second-order and well-fitted to Freundlich and Langmuir models with a maximum adsorption capacity of 294.12 mg/g at 25 ◦C , while the adsorption process was endothermic, randomness and spontaneous. Besides, reusability test signified that MIL-125/MIL-53/CNT@Alg composite microbeads retained admirable adsorption properties for six consecutive cycles, emphasizing its potentiality for removing of pharmaceutical residues.


Introduction
Presently, the scarcity of drinking water is the major problem that is sweeping the world, menacing humanity with annihilation 1 . During the cruel circumstances of COVID-19, one source of water pollution has reached its peak which is pharmaceutical contaminants. In this regard, antibiotics such as tetracyclines (TCs) have been recommended in new research that they may be able to treat  infection through their anti-inflammatory and anti-apoptotic activities [2][3][4] . However, humans could not completely metabolize TCs and around 50-80% of the applied dosage is secreted via urine 5 . Thence, numerous developing techniques have been used for TCs removal from wastewater including; adsorption 6 , ultrasonic irradiation 7 , photocatalytic degradation 8 , membrane process 9 , and fenton oxidation 10 . Among the mentioned techniques; adsorption has been considered as the most favorable technique for the removal of TCs from wastewater owing to it is simple, economic, low-energy consumption, etc. 11,12 .
Metal organic frameworks (MOFs) is a new brilliant class of crystalline materials that has increasingly drawn a vast consideration owing to its versatile applications 13,14 . Notably, owing to the unique characteristics of MOFs such as water stability, ultrahigh porosity, easy functionalization, thermal stability and high surface area, MOFs have successfully been utilized for the adsorptive removal of heavy metals 15 , pharmaceutical contaminants 16 and synthetic dyes 17 . One of iron-based MOFs that has been exhibiting notable adsorptive behavior is MIL-53 owing to its structure flexibility, stability in water and chemical stability 18 . Furthermore, MIL-125 is one more bright member in MIL-family that possesses promising photo-catalytic and adsorptive behavior due to its photo-reactivity, thermal and chemical stability, etc. 19 .
Although, the individual features of MOFs, there is a huge obstacle to apply them in practical applications which is the difficulty of their recycling as well as their difficult separation from the adsorption mediums. Fabrication of the shaping MOFs like membrane, fiber and beads is considered an effective solution to get rid of MOFs drawback 20,21 . Sodium alginate (Alg); is a water-soluble anionic polysaccharide that is smoothly extracted from brown seaweed 22 . Alginate has acquired huge fame owing to its unique merits such as biodegradability, nontoxicity, strong gelation, biocompatibility, high chemical stability, chelating ability and possession of abundant function groups onto its surface (i.e. hydroxyl and carboxyl) [23][24][25][26] . Therefore, Alg has been considered a premium-supporting host of chemical and biological compounds in several potential applications including pharmaceutical, biomedical and especially in wastewater treatment owing to its ability to capture the cationic ions from the target contaminants whether heavy metals or dyes via ion-exchange mechanism 22,27,28 .
The remarkable features of carbon nanotubes (CNTs) including high mechanical strength, high surface area, low cost and its ability to form strong bonds with other molecules or atoms, make them promising candidates for the adsorptive removal of diverse pollutants from wastewater 29 . Moreover, multi-walled CNTs are higher accessible and lower cost than single-walled CNTs which render multi-walled CNTs more favorable for the potential applications than single-walled CNTs 30 .
Herein, we aimed to fabricate a novel binary MOF/ CNT composite embedded into Alg microbeads to facilitate the separation of MIL-125/MIL-53 binary MOF after the adsorption process as well as ameliorate its efficiency and reusability. The fabricated MIL-125/MIL-53/CNT@Alg composite microbeads were characterized by bountiful tools as well as their adsorption aptitude towards TC was assessed utilizing a batch adsorption technique. The kinetics, isotherms and thermodynamics were adequately studied. Furthermore, to prove the economic viability of the fabricated MIL-125/MIL-53/CNT@Alg composite microbeads, recyclability test was investigated.

Synthesis of MIL-125/MIL-53 binary MOF
MIL-125/MIL-53 binary MOF was fabricated as follows; exactly, 0.679 g FeCl3.6H2O and 0.415 g BDC were dissolved in 50 mL DMF and then the reaction mixture was kept under continuous stirring at 60 ºC for 2 h. In another container, 1.990 g BDC was dissolved in 50 mL DMF and then 2.7 mL TBOT was added drop by drop under vigorous stirring followed by stirring at 60 ºC for 2 h. Next, the two solutions were mixed, then transferred to 500 mL autoclave and heated at 140 ºC for 20 h. finally, the obtained powder was collected by centrifugation, washed and dried at 100 ºC for 24 h.

Characterization
MIL-125/MIL-53/CNT@Alg composite microbeads were thoroughly characterized by; Fourier Transform Infrared spectra (FTIR-Tensor II, Bruker) to investigate the chemical composition of the synthesized microbeads. Furthermore, X-ray diffractometer (XRD-MAC Science M03XHF) was used to distinguish the crystal phase. The morphology was defined by a Scanning Electron Microscope (SEM-Hitachi-S4800), while the thermal stability of the developed composite microbeads was identified by Thermogravimetric Analysis (TGA-Shimadzu-50). Besides, X-ray photoelectron spectroscopy (XPS-Thermo scientific-ESCALAB-250Xi VG) was employed to clarify the elemental compositions of the adsorbent. The specific surface area of composite microbeads was measured by Bruner-Emmett-Teller method (BET-Beckman coulter SA3100), while their surface charges were determined by Zeta potential (ZP-Malvern-UK).

Batch experiment
The key parameters that affect the efficiency of the TC adsorption onto MIL-125/MIL-53/CNT@Alg composite microbeads were studied precisely using batch mode. During the whole adsorption experiments TC solution was wrapped with aluminum foil to prevent photodegradation of TC. For specifying the optimum pH, 20 mg of dry adsorbent microbeads were soaked into 25 mL TC solution at pH range from 2 to 10 and stirred for 60 min under agitation rate 150 rpm. While, for investigating the effect of adsorbent dose, various doses of MIL-125/MIL-53/CNT@Alg composite microbeads at range from 10 to 80 mg were added to TC solution at the identified optimum pH. Furthermore, the TC adsorption isotherm was studied at an initial concentration range from 50 to 300 mg/L. Besides, the thermodynamic study was executed at a temperature range from 25 to 55 ºC. At a set time, the un-adsorbed TC concentration was evaluated by withdrawing a sample and measured using spectrophotometry at 354 nm. The removal (%) and adsorption capacity (q) were computed by the following equations; Where, C0 and Ct symbolize the TC initial concentration and its concentration at certain time, respectively. While, V and w symbolize the TC solution volume and the weight of MIL-125/MIL-53/CNT@Alg composite microbeads, respectively.

Reusability study
Undoubtedly, regeneration behavior is one of the main criteria for choosing an adsorbent. Consequently, the reusability study was implemented for five successive   31 . Figure 3B depicts the distinguishing bands of CNT at 1650, 2330 and 2675 cm -1 which are attributed to C=C, the formed H-bond and C-H 36,37 . In the Alg spectrum (Figure 3C), the band at 799 cm -1 is related to C-H vibration of pyranose, while the band at around 2916 cm -1 is ascribed to C-H stretching vibration. Besides, the band at 1019 cm -1 is ascribed to C-O stretching and the band around 3250 cm -1 belongs to OH stretching vibration 38,39 . In addition, the belonging peaks to asymmetric and symmetric COOgroup emerged at 1401 and 1592 cm -1 , respectively. Besides, the observed band at 2330 cm -1 is assigned to CO2 group 40 . Figure 3D clarifies the main characteristic bands of the pristine components, suggesting the successful combination between them.

Effect of pH
The pH influence on the adsorption behavior of MIL-125/MIL-53/CNT@Alg composite microbeads towards TC was scrutinized at a pH range from 2 to 10. In fact, TC molecule exists in multi forms, depending on the solution pH at which TC is cationic (TCH3 + ) at pH < 3.3, zwitterionic (TCH2 0 ) at 3.3 < pH < 7.7 and anionic (TCHor TC 2-) at pH > 7.7 12 . Figure

Isotherm study
For interpreting the nature of interaction between TC and MIL-125/MIL-53/CNT@Alg composite micrbeads, the experimental data were comprehensively analyzed by; Langmuir, Freundlich, Temkin and Dubinin-Radushkevich (D-R) isotherm models (Figure 9). The linear form of These models are expressed as follows 50,51 ; Langmuir equation:

Kinetic study
To deduce the adsorption mechanism of TC onto binary MIL-125/MIL-53 MOF/CNT@Alg composite microbeads, the experimental data were thoroughly modeled by pseudo-first-order, pseudo-second-order and Elovich model (Figure 10A-C). Equations 7-10 symbolize the linear forms of these kinetic models 50 .

Regeneration study
To assert the viability of our study, the recyclability of the fabricated MIL-125/MIL-53/CNT@Alg composite microbeads was examined for six consecutive adsorption/desorption cycles. Figure 10D depicts an inconsiderable decrease in the removal (%) and the adsorption capacity from 65.10 % and 42.02 mg/g to 53.50 % and 35.22 mg/g, respectively, confirming the good recyclability of MIL-125/MIL-53/CNT@Alg composite microbeads that renders us recommend our novel composite microbeads as a promising candidate for the removal of TC from an aqueous solution.

Comparison study
To sum, MIL-125/MIL-53/CNT@Alg composite microbeads possess a superb adsorption capacity toward TC compared with other MOFs-, carbon materials-or alginate beads-based adsorbents ( Table 4). This finding suggests that the fabricated composite beads may be utilized in actual wastewater treatment taking into consideration the advantage of their easy separation and remarkable renewability.

Conclusion
All in all, this study presented the fabrication of MIL-125/MIL-53/CNT@Alg composite microbeads for removing of tetracycline drug residue from aqueous solutions. The formulated adsorbent composite was proved its chemical structure, thermal stability and surface morphology, while batch adsorption experiments were conducted to evaluate its aptitude for adsorption of TC under several optimization conditions. The results clarified that incorporation of CNC into the composite matrix played a significant role in the adsorption process, since the removal (%) of TC was increased with increasing CNC quantity up to 15w%. A sequence of adsorption isotherm models and kinetic studies led us to conclude that the adsorption of TC onto MIL-125/MIL-53/CNT@Alg composite microbeads process was fitted to Freundlich and Langmuir isotherm model with a maximum adsorption capacity of 294.12 mg/g at 25 •C,.and followed the pseudo-second-order kinetic model, spontaneous. The results of thermodynamic studies clarified that the adsorption process could be described as spontaneous, endothermic and randomness process. Reusability studies confirmed that the developed adsorbent exhibited a superior recycling capability even after sex repeated cycles with good performance for adsorption of TC. Thus, the fabricated adsorbent composite has some operational benefits such as easy separation, decent adsorption performance and better reusability, suggesting its applicability for removing TC residue from aquatic mediums.