Fabrication of easy separable and reusable MIL-125(Ti)/MIL-53(Fe) binary MOF/CNT/Alginate composite microbeads for tetracycline removal from water bodies

In this investigation, we aimed to fabricate easy separable composite microbeads for efficient adsorption of tetracycline (TC) drug. MIL-125(Ti)/MIL-53(Fe) binary metal organic framework (MOF) was synthetized and incorporated with carbon nanotube (CNT) into alginate (Alg) microbeads to form MIL-125(Ti)/MIL-53(Fe)/CNT@Alg composite microbeads. Various tools including FTIR, XRD, SEM, BET, Zeta potential and XPS were applied to characterize the composite microbeads. It was found that the specific surface area of MIL-125(Ti)/MIL-53(Fe)/CNT@Alg microbeads was 273.77 m2/g. The results revealed that the adsorption of TC augmented with rising CNT proportion up to 15 wt% in the 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 and pH 6. Furthermore, thermodynamic study clarified that the TC adsorption process was endothermic, random and spontaneous. Besides, reusability test signified that MIL-125(Ti)/MIL-53(Fe)/CNT@Alg composite microbeads retained superb adsorption properties for six consecutive cycles, emphasizing its potentiality for removing of pharmaceutical residues.

Presently, the scarcity of drinking water is the major problem that is sweeping the world, menacing humanity with annihilation 1,2 . 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 3 . In this regard, antibiotics such as tetracyclines (TCs) have been recommended in new research that they may be able to treat COVID-19 infection through their anti-inflammatory and anti-apoptotic activities [4][5][6] . However, humans could not completely metabolize TCs and around 50-80% of the applied dosage is secreted via urine 7 . Thence, numerous developing techniques have been used for TCs removal from wastewater including; adsorption 8,9 , ultrasonic irradiation 10 , photocatalytic degradation [11][12][13] , membrane process 14 , and fenton oxidation 15 . 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. 16,17 .
Metal organic frameworks (MOFs) is a new brilliant class of crystalline materials that has increasingly drawn a vast consideration owing to its versatile applications 18,19 . Notably, owing to the unique characteristics of MOFs chased from Alpha Chemika (India). Titanium isoproproxide (TBOT), sodium alginate (NaC 6 H 7 O 6 ; medium viscosity) and N,N dimethyl formamide (DMF) were obtained from Shanghai Chemical Reagent (China). Tetracycline and 1,4-benzene dicarboxylic acid (BDC) were bought from Loba Chemie Ltd (India). Ethanol, ammonium solution (NH 4 OH) and dimethyl sulfoxide (DMSO) were provided by Ninghai Jiahe (China).  was fabricated using a modified procedure reported by Yang et al. 39 , Typically, 1.990 g BDC was dissolved into 50 mL DMF and then 2.7 mL TBOT was slowly added. The reaction solution was transferred into a 100 mL autoclave and heated at 140 °C for 20 h. The resultant white solid was separated by centrifugation, washed with DMF and methanol and dried in oven at 80 °C for 24 h.  was fabricated according to the previous reported procedure by Yu et al., with slight modifications 23 . Exactly, 0.679 g FeCl 3 .6H 2 O and 0.415 g BDC were dissolved in 50 mL DMF and then kept under mechanical stirring for 15 min. The reaction mixture was transferred into a 100 mL autoclave and heated at 140 °C for 20 h. Finally, the yellow product was separated by centrifugation, washed with DMF and methanol and dried in oven at 80 °C for 24 h. Furthermore, X-ray diffractometer (XRD-MAC Science M03XHF) was used to distinguish the crystal phase. The surface morphology was identified by a Scanning Electron Microscope (SEM-Hitachi-S4800). 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(Ti)/ MIL-53(Fe)/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(Ti)/MIL-53(Fe)/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 °C. 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; (1)  43 . In addition, there is no characteristic peak to Alg owing to its amorphous phase 28 . This result is consistent with the study by Eltaweil et al. 2 .  44,45 . Whereas, the bands at 1101 and 1291 cm −1 are assigned to C-H and C-O, respectively 46 . Besides, the two bands at 1385 and 1581 cm −1 are ascribed to the vibration of the carboxyl group of BDC that coordinates to the metal centers (i.e. Ti and Fe) 39 . 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 47,48 . In the Alg spectrum (Fig. 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 49,50 . 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 CO 2 group 51 . Figure 3D clarifies the main characteristic bands of the pristine components, suggesting the successful combination between them.   Effect of the adsorption conditions. Figure   Effect of adsorbent dose. Figure 8A points out the impact of the increase in the dose of MIL-125(Ti)/MIL-53(Fe)/CNT@Alg composite microbeads on the removal (%) and adsorption capacity of TC. As expected, the increment in the adsorbent dose from 0.01 to 0.08 g results in an increasing in the removal (%) from 43.08 to 86.33% and a dropping in the adsorption capacity from 58.02 to 13.62 mg/g. This behavior may be interpreted by the increment in the adsorbent dose leads to an increase in the provided active sites that renders the removal (%) goes up. Contrariwise, the adsorption capacity dwindles due to the particles aggregation 25 .
Effect of initial TC concentration. Figure 8B demonstrates an increase in the adsorption capacity from 61.52 to 258.10 mg/g with the increase in the TC initial concentration from 50 to 300 mg/L which most likely due to the increase in the driving force of TC molecules towards MIL-125(Ti)/MIL-53(Fe)/CNT@Alg composite micro- where q e and q m the equilibrium adsorption capacity and the monolayer adsorption capacity, respectively. C e is the residual concentraion of TC at equilibrium and K L is Langmuir constant. K F and n are Freundlich constants. B = RT b , b is Temkin constant related to heat of adsorption and A is the equilibrium binding constant. T is the absolute temperature and R is the gas constant (8.314 J/mol.k). ε = RTLn 1 + 1 C e is the Polanyi potential. K ad is a constant related to mean free energy of adsorption per mole of adsorbate and q s is the saturation adsorption capacity.
According to the R 2 values (Table S1) Kinetic study. To deduce the adsorption mechanism of TC onto binary MIL-125(Ti)/MIL-53(Fe) MOF/ CNT@Alg composite microbeads, the experimental data were thoroughly modeled by pseudo-first-order, pseudo-second-order and Elovich model (Fig. 9A-C). Equations 7-10 symbolize the linear forms of these kinetic models 62 .
where q e represents the amount of TC that adsorbs onto MIL-125(Ti)/MIL-53(Fe)/CNT@Alg composite microbeads at equilibrium, while q t expresses the amount of TC adsorption at time t. k 1 is the rate constant of pseudofirst-order and k 2 is the rate constant pseudo-second-order. α and β are Elovich coefficients that represent the initial adsorption rate and the desorption coefficient, respectively, also relate to the extent of surface coverage and activation energy for chemisorption.
To find out the suitable kinetic model that fits the experimental data there are two main criteria; R 2 of the suitable kinetic model should be higher than R 2 of the other applied models as well as the presence of an analogy between q exp and q cal from the suitable model. Accordingly, pseudo-second-order is the most suitable model to represent the adsorption of TC onto MIL-125(Ti)/MIL-53(Fe)/CNT@Alg composite microbeads (Table S2). It (7) pseudo − first − order : ln(q e − q t ) = lnq e − k 1 (t) (8) pseudo − second − order : t/q t = 1/k 2 q 2 e + 1/q e (t)  (Table 1) prove the spontaneity of this adsorption process.
where K e is the thermodynamic equilibrium constant. C Ae is the TC concentration onto the surface of MIL-125(Ti)/MIL-53(Fe)/CNT@Alg composite microbeads, while C e is the concentration of TC in solution at equilibrium. T is the adsorption temperature and R is gas constant. Figure S2 represents Van't Hoff Plot that elucidates ∆S° and ∆H° from the intercept and slope, respectively. The positive value of ∆S° and ∆H° indicates that the adsorption of TC onto the surface of MIL-125(Ti)/MIL-53(Fe)/CNT@Alg composite microbeads randomness and endothermic, respectively.

Regeneration study.
To assert the viability of our study, the recyclability of the fabricated MIL-125(Ti)/ MIL-53(Fe)/CNT@Alg composite microbeads was examined for six consecutive adsorption/desorption cycles. Figure 9D 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(Ti)/MIL-53(Fe)/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(Ti)/MIL-53(Fe)/CNT@Alg composite microbeads possess a superb adsorption capacity toward TC compared with other MOFs-, carbon materials-or alginate beads-based adsorbents (Table 2). 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.
Possible mechanisms for the TC adsorption. Based on ZP measurements and the experimental results of the impact of pH on the TC adsorption aptitude, the adsorption of TC onto MIL-125(Ti)/MIL-53(Fe)/CNT@ Alg composite microbeads is not controlled by the electrostatic interaction. Consequently, it is expected that there are other mechanisms that dominate the adsorption process such as; Pore filling effects since the length, width and height of the three-dimensional TC molecules are 1.23, 0.84 and 0.67 nm, respectively, while the average pore size of the microbeads is 2.145 nm. So, the pores of the microbeads are loose enough to penetrate the TC molecules 76 . Besides, π-π interaction between the aromatic rings in MIL-125(Ti)/MIL-53(Fe)/CNT@ Alg (π-electron donor) and TC molecules (π-electron acceptor) 7,43 . In addition, Coordination bonds between the unsaturated metals (i.e., Ti and Fe) and TC as well as hydrophobic interaction especially the presence of CNT increases the hydrophobicity of the microbeads 77,78 . Although, many studies suggested H-bonding as one of the controlling mechanisms on the TC adsorption, it is difficult to be the main adsorption interaction. The H-bonding interaction between water molecules and functional groups is much stronger than that between TC and the functional groups of MIL-125(Ti)/MIL-53(Fe)/CNT@Alg composite microbeads 78 .

Conclusion
All in all, this study presented the fabrication of MIL-125(Ti)/MIL-53(Fe)/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  www.nature.com/scientificreports/ models and kinetic studies led us to conclude that the adsorption of TC onto MIL-125(Ti)/MIL-53(Fe)/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.