Fabrication of UiO-66/MIL-101(Fe) binary MOF/carboxylated-GO composite for adsorptive removal of methylene blue dye from aqueous solutions

This study provides a novel composite as an efficient adsorbent of cationic methylene blue dye. UiO-66/MIL-101(Fe) binary metal organic framework (MOF) was fabricated using a solvothermal technique. Additionally, the developed binary MOF was modified with carboxylated graphene oxide (GOCOOH) using a post-synthetic technique. The as-fabricated UiO-66/MIL-101(Fe)-GOCOOH composite was analyzed by FTIR, XRD, SEM, BET, TGA, XPS and zeta potential analysis. The adsorption performance of UiO-66/MIL-101(Fe)-GOCOOH composite was examined for its aptitude to adsorb cationic MB dye using a batch technique. The obtained data revealed that, the developed UiO-66/MIL-101(Fe)-GOCOOH composite exhibited higher adsorption capacity compared to UiO-66/MIL-101(Fe) binary MOF. Adsorption isotherms and kinetic studies revealed that MB dye adsorption onto UiO-66/MIL-101(Fe)-GOCOOH composite fitted a Langmuir isotherm model (qm = 448.71 mg g−1) and both pseudo 1st order and pseudo 2nd order kinetic models. An intra-particle diffusion model showed that the adsorption process occurs through three steps. Besides, thermodynamic data reflected that the adsorption of MB onto UiO-66/MIL-101(Fe)-GOCOOH composite is an endothermic and spontaneous process and the adsorption involves both physisorption and chemisorption interactions. The as-fabricated UiO-66/MIL-101(Fe)-GOCOOH composite exhibited good reusability and can be considered as a promising reusable adsorbent for the treatment of dye-containing industrial effluents with high efficiency.


Introduction
A dye is an organic compound constructed from two main components; a chromophore that is responsible for producing the color and an auxochrome that increases the water solubility. 1,2 Undoubtedly, dyes are a double-edged sword, since they are essential for important industries such as textiles, pharmaceuticals, plastics, polymers, reneries, and leather. 3,4 However, the released effluents from these dye-containing industries cause signicant hazards to human health and the aquatic environment. For instance, methylene blue (MB) dye is mostly utilized for coloring cotton, wood and silk, but its discharge into water bodies even in low concentration leads to various harmful impacts such as eye burning, vomiting, cyanosis, convulsions, tachycardia and methemoglobinemia. 3,5 Accordingly, several techniques have been applied for dye removal such as advanced oxidation, 6 membrane separation, 7 electrolysis, 8 catalytic reduction, 9,10 photocatalytic degradation, 11 occulation. 12 Other than the mentioned techniques, visible light-driven photocatalysis and adsorption methods are considered the most promising technologies in the environmental remediation eld owing to their high efficiencies, low cost, minimal harmful by-products and their low energy consumption. [13][14][15][16] Metal-organic frameworks (MOFs) are a blossoming category of hybrid porous materials constructed from the assembly of metal centers with organic linkers. 17 MOFs have gained special interest because of their tunable pore size, large surface area and thermal stability. 18,19 These unique features make MOFs an excellent candidate for industrial applications such as catalysis, 19 drug delivery, 20 gas storage 21 and water treatment. 22 One of the great advances in reticular chemistry was the MIL-101 MOF, since it exhibited high chemical stability, high surface area, remarkable thermal robustness and persistent porosity. 23 Therefore, MIL-101 MOF has been frequently utilized for many applications including gas storage, 24 catalysis, 25 sensing 26 and adsorption. 27 Furthermore, UiO-66 is a terephthalic acid-based MOF having both tetrahedral and octahedral cavities. 28 UiO-66 possesses high thermal and chemical stability, extremely high specic surface area, as well as, excellent adsorption properties. 29 In the last decade, several researches have been reported on the modications of MOFs by diverse techniques such as fabrication of bimetallic MOFs, 30 MOF-based composites, 31 and binary MOFs. 32 Although, binary MOFs have shown superior behavior in several applications such as catalysis 32 and wastewater treatment 28 compared with the pristine MOFs, there are few researches that have been studied the fabrication of binary MOFs and their applications. Additionally, MOF-based composites such as MIL-101(Fe)@PDopa@Fe 3  Graphene oxide (GO), the oxidation product of graphene, contains numbers of oxygen-functional groups including hydroxyls, epoxides, carboxyls and carbonyls which signicantly improve the chemical reactivity of GO compared to the raw graphene. 36 GO has gained great concern due to its features such as large specic surface area, good mechanical characteristics, easy functionalization and its high adsorption capacity for dyes and heavy metals. 37 Further, GO structure has been modied via functionalization processes which resulting in well-dispersion and high stability in aqueous solution in order to enhances its adsorption propertied and widen its applications range. 14,38 For example, adsorption properties of MIL-101(Fe) greatly enhanced aer its modication with GO. 39 Herein, MIL-101(Fe)/UiO-66 binary MOF was synthesized via one-pot synthesis and then further modied by GOCOOH via a post-synthetic step in order to generate extra negatively charged groups and to provide a multi-functional group template with variety of available adsorption sites. The capability of the developed UiO-66/MIL-101(Fe)-GOCOOH composite for MB dye removal was estimated and discussed. Besides, isotherms, kinetics and thermodynamics of the process and reusability of UiO-66/MIL-101(Fe)-GOCOOH composite were evaluated.

Synthesis of carboxylated graphene oxide (GOCOOH)
GO was prepared via Hummers' method with a slight modication. 40 In brief, 2 g graphite and 1 g sodium nitrate were dissolved into concentrated H 2 SO 4 (100 mL) under stirring at 5 C. Then, 10 g potassium permanganate was added and the mixture was stirred for a further 1 h. The solution temperature was raised to 40 C and kept for 30 min under stirring. There-aer, 100 mL deionized water was poured into the mixture and then the temperature was raised to 90 C for 2 h. The termination of the reaction was made by adding 280 mL deionized water and 30 mL hydrogen peroxide. Finally, the brown product was separated, washed three times with HCl (10%) then distilled water and dried in oven at 50 C overnight. In order to prepare GOCOOH, GO was soaked in 100 mL deionized water (2 mg mL À1 ) and sonicated for 30 min, then NaOH (5 g) and Cl-CH 2 COOH (5 g) were added to the suspension and followed by sonication for further 2 h. Thereaer, the solution was neutralized using HCl (10%). The obtained black solid was collected by centrifugation then washed with deionized water and methanol. At last, the GOCOOH was dried for 12 h at 50 C.

Synthesis of UiO-66/MIL-101(Fe) binary MOF
UiO-66/MIL-101(Fe) binary MOF was prepared in one-pot synthesis according to the reported solvothermal method 28 with slight modications. In brief, 0.079 g FeCl 3 $6H 2 O was dissolved in 7.5 mL DMF solution then mixed with H 2 BDC solution (0.1626 g, 7.5 mL DMF), then stirred at 70 C for 4 h and nominated as solution A. Subsequently, in a similar manner solution B has been prepared by dissolving 0.097 g ZrOCl 2 and 0.05 g H 2 BDC into DMF followed by stirring at 70 C for 4 h. Aer that, solutions A and B were transferred into 150 mL Teon sealed-autoclave and kept at 130 C for 20 h. Aer reaction completion, the obtained solid particles were centrifuged and washed with DMF and methanol then dried at 90 C for 24 h.
where, C 0 , C t (mg L À1 ) symbolize MB dye concentration at zero and t time, respectively. V (L) MB volume and m (g) symbolizes the UiO-66/MIL-101(Fe)-GOCOOH composite mass. Fig. 1 represents a schematic diagram for the fabrication of UiO-66/MIL-101(Fe)-GOCOOH composite and laboratory images for MB before and aer adsorption.

Reusability
To check the reusability of the synthesized UiO-66/MIL-101(Fe)-GOCOOH composite, a series of seven successive adsorptiondesorption cycles was performed. Aer complete adsorption of MB dye, UiO-66/MIL-101(Fe)-GOCOOH composite was easily separated by centrifugation, washed with ethanol (99%) as a desorption medium, dried in air oven at 60 C for 3 h and then tested for the next adsorption run.  Fig. 2. All spectra showed a broad band between 3000-3500 cm À1 which is assigned to -OH stretching. FTIR spectrum of GO ( Fig. 2A) showed two peaks at 1045 and 1385 cm À1 which are assigned to epoxy C-O and C-OH stretching, respectively. 42 Further, the peaks at 1612 and 1724 cm À1 are corresponding to the stretching vibration of C]C and C]O, respectively. 1 Moreover, in GOCOOH spectrum (Fig. 2B) the peak at 1039 cm À1 is corresponding to C-O stretching and the peak at 1714 cm À1 is corresponding to C]O stretching, while the two peaks at 1353 and 1577 cm À1 are corresponding to asymmetric and symmetric vibrating bands of COOH. 43 Besides, FTIR spectrum of UiO-66/MIL-101(Fe) binary MOF (Fig. 2C) shows a peak at 486 cm À1 which is ascribed to the vibration mode of Zr-O, while the peak at 549 cm À1 is assigned to Fe-O vibration. 44 The peaks at 737 and 862 cm À1 are related to the bending vibration of aromatic C-H of benzene ring and the peak at 1120 cm À1 is attributed to C-C bond. Additionally, the symmetric stretching band at 1390 cm À1 and the two asymmetric stretching bands at 1525 and 1691 cm À1 are corresponding to carboxyl group. 45 Also, the interaction between Zr and Fe ions with the de-protonated carboxyl group was conrmed by the peaks at 1525 and 1691 cm À1 , respectively. 46,47 Upon incorporation of GOCOOH into UiO-66/MIL-101(Fe) binary MOF, all peaks intensities decreased with the appearance of a new peak at 1060 cm À1 which is corresponding to C-H of GOCOOH (Fig. 2D). This conrms the successful combination between UiO-66/MIL-101(Fe) binary MOF and GOCOOH. Aer MB adsorption (Fig. 2E), new peaks at 1384 and 1327 cm À1 were noticed which are related to the aromatic rings of MB dye. Further, the C-H bond vibrations of MB dye at 874, and 1240 cm À1 were also observed. These new peaks and the variation in peaks intensity conrm the adsorption of MB dye onto UiO-66/MIL-101(Fe)-GOCOOH composite.

Results and discussion
3.1.2. TGA. Thermal behaviors of UiO-66/MIL-101(Fe) binary MOF and UiO-66/MIL-101(Fe)-GOCOOH composite were studied using TGA. It is clear from TG curves (Fig. 3A), that both samples show three stages of weight loss. The rst one between 30 and 100 C is due to the vaporization of adsorbed water, while the second stage between 100 and 320 C is attributed to the elimination of DMF molecules. The third weight loss starts at 450 C is ascribed to the burning of the organic ligand which led to decomposition of both binary MOF and GOCOOH-binary MOF composite. 48 Further, it was found that UiO-66/MIL-101(Fe)-GOCOOH composite has a total weight loss of 52.7% indicating its higher thermal stability than UiO-66/MIL-101(Fe) binary MOF which has a total weight loss of 56.5%.   This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 19008-19019 | 19011 with no distinct peaks for GOCOOH which could be attributed to its good distribution in the sample.   increasing the GOCOOH mass ratio to 3 (0.5 : 1.5) is most likely due to the pore blocking affect, resulting from the high content of GOCOOH in the composite. A similar conclusion was reported by Y. Cao and co worker. 51 Based on these results, the composite of equal mass ratio (UiO-66/MIL-101(Fe)-GOCOOH) was chosen for the subsequent adsorption experiments.
3.2.1. Effect of initial MB dye concentration. As noticed from Fig. 7B, increasing MB initial concentration from 50 to 300 mg L À1 led to increasing the uptake amount (q e ) of MB dye onto UiO-66/MIL-101(Fe)-GOCOOH composite from 99.51 to 439.36 mg g À1 , this behavior is basically due to increasing the driving force that outdo the mass transfer resistance of dye molecules from bulk to the UiO-66/MIL-101(Fe)-GOCOOH surface. Otherwise, a decrease in the removal (%) from 99.5 to 68.4% was observed on increasing initial MB dye concentration   which can be explained by the deciency of active sites needed for high MB concentration. 52 3.2.2. Effect of pH. pH medium is a crucial parameter in adsorption of dyes because it controls the sign, the magnitude of the adsorbent surface charge as well as the ionization extent of dye molecules. 53 Fig. 7C demonstrated that the adsorption capacity and the removal (%) of MB dye are signicantly increased with raising the pH from 3 up to 11. This behavior can be explained as follow; increasing pH of MB dye solution led to an increase in the magnitude of the negative charges on UiO-66/MIL-101(Fe)-GOCOOH composite surface, which in turn increase the electrostatic attraction between the negatively charged binary MOF-GOCOOH composite and the positively charged MB. Thus, the removal (%) and the adsorption capacity increase accordingly.

Adsorption isotherm models
To investigate the nature of the interaction between the MB and the synthesized UiO-66/MIL-101(Fe)-GOCOOH composite, Langmuir and Freundlich isotherm models have been applied. The linear forms of these models are expressed by eqn (3) and (4). 56 where, q e and C e are MB uptake amount and MB concentration at equilibrium, b is Langmuir constant, q m is the theoretical maximum MB uptake, k F and n are Freundlich constants. The isotherm plots (Fig. S1 †) and their derived parameters (Table 1) were clearly reected that the experimental data best ts the Langmuir (R 2 ¼ 0.997) than Freundlich model (R 2 ¼ 0.859). Furthermore, the theoretical value of q m (448.71 mg g À1 ) that was determined from intercept of the Langmuir plot is much close to the experimental value (439.36 mg g À1 ). Another Langmuir model-derived parameter is the dimensionless separation factor R L (eqn (5)) that considered a credible indicator of the adsorption favorability. 57 where, C 0 is the initial MB dye concentration and b is Langmuir constant. From R L values (Table S1 †

Adsorption kinetics
In order to demonstrate the rate of mechanism controlling the MB dye adsorption onto UiO-66/MIL-101(Fe)-GOCOOH composite, kinetics of adsorption was inspected by both pseudo 1 st order and pseudo 2 nd order kinetic models. The forms of these models are dened by the eqn (6) and (7), respectively. Besides, the possibility of dye particles diffusion into the UiO-66/MIL-101(Fe)-GOCOOH composite pores is elucidated by the intra-particle diffusion model presented by eqn (8).
ln(q e À q t ) ¼ ln q e À k 1 t (6) where, q t and q e are amount of MB dye uptakes at time t and at equilibrium, respectively, k 1 and k 2 are the rate constants of pseudo 1 st order and pseudo 2 nd order, respectively. K p is the intra-particle diffusion rate constant. The values of intercept C provide a notion about the thickness of boundary layer. Adsorption kinetics curves and kinetic parameters at 25 C are represented in Fig. 8(A and B) and Table 2, respectively. The determination coefficients revealed that the adsorption kinetics is well depicted by both pseudo 1 st (R 2 values exceeded 0.96) and pseudo 2 nd order (R 2 values exceeded 0.99). However, the pseudo 2 nd order model has a higher conformity of the calculated and the experimental adsorption capacities. This means that the pseudo 2 nd order model provides more in-depth and rigorous reection of the adsorption process of MB dye onto UiO-66/MIL-101(Fe)-GOCOOH composite than the pseudo 1 st order model does.
Intra-particle diffusion plots (Fig. 8C) showed that the MB dye adsorption onto UiO-66/MIL-101(Fe)-GOCOOH occurs throughout three steps (Fig. 2S †). Furthermore, it was observed from the intra-particle diffusion rate constants values (Table 3) that the rate of 1 st step > 2 nd step > 3 rd step which can be assigned to the variation in the MB dye diffusion rate during the three steps as following; in the 1 st step (K p,1 ), MB dye molecules rapidly migrate from the bulk to UiO-66/MIL-101(Fe)-GOCOOH composite surface until the outer surface of the composite become saturated. Then in the 2 nd step (K p,2 ), MB dye molecules enter the pores of UiO-66/MIL-101(Fe)-GOCOOH composite with an increase in the resistance of diffusion. Finally, in the 3 rd step (K p,3 ) MB dye molecules slowly diffuse into the pores of UiO-66/MIL-101(Fe)-GOCOOH composite up to reach equilibrium. Moreover, Fig. 8C showed that rising the initial concentration of MB dye led to an increase in the slope and intercept of all the three steps, which could be explained by the fact that the intra-particle diffusion was developed base on Fick's Law. An increase in the concentration gradient led to more rapid diffusion and faster adsorption. Also, this increase in the rate constants values for the three steps could be assigned to the increasing in the driving force of MB dye molecules that resulting from increase the initial concentration of MB dye. 59,60 Moreover, it is clear from Fig. 8C that plots for all studied concentration do not pass through the origin (C s 0), con-rming that the intra-particle diffusion is not the only rate controlling step. 61

Adsorption thermodynamics
Adsorption thermodynamics is another important section for understanding and realizing the nature and the mechanism of the MB dye adsorption process. For thermodynamic studies, the adsorption of MB dye onto the synthesized UiO-66/MIL-101(Fe)-GOCOOH composite was performed at different temperatures and the thermodynamic parameters; change in free energy (DG ), change in enthalpy (DH ) and change in entropy (DS ) were computed from the eqn (9) and (10).
where, K e (C Ae /C e ) is the thermodynamic equilibrium constant, C Ae is the concentration of MB onto the UiO-66/MIL-101(Fe)-GOCOOH composite surface (mg L À1 ), C e is the concentration of MB in solution at equilibrium (mg L À1 ), R is gas constant and T is adsorption temperature. DH and DS values that have been computed from the slope and intercept of van't Hoff plot (Fig. 8D). The positive DH value reects the endothermic nature of the adsorption of MB onto UiO-66/MIL-101(Fe)-GOCOOH composite. It has been reported that, physical adsorption is predominant when DH value is lower than 25 kJ mol À1 . However, chemical adsorption is predominant for DH value ranging from 40 to 200 kJ mol À1 . 62 In this research, DH is 38.53 kJ mol À1 which reveals that there is a chemical adsorption co-exists with the physical adsorption between UiO-66/MIL-101(Fe)-GOCOOH composite and MB dye molecules. Further, the positive DS value reveals that the MB dye adsorption onto the synthesized UiO-66/MIL-101(Fe)-GOCOOH composite is accompanied with high randomness at solid/solution interface. Also, the negative values of DG (Table 4) conrm that MB dye adsorption onto the synthesized binary MOF-GOCOOH composite is spontaneous process.

Mechanism of MB dye adsorption
Adsorption process of MB dye onto UiO-66/MIL-101(Fe)-GOCOOH composite is sophisticated since there are several co-existed interactions including electrostatic interactions, p-p stacking, hydrogen bonds and n-p conjugations, etc. FTIR spectra (Fig. 2E) demonstrated that the variation of peaks intensity and appearance of new peaks which strongly suggested that the removal mechanism of MB dye onto UiO-66/ MIL-101(Fe)-GOCOOH composite involves electrostatic interactions between the MB dye and adsorbent functional groups which is a good agreement with pH results. 63 Moreover, Zr-O and Fe-O centers represent good possibilities for n-p conjugation. This conjugation is conrmed by variation in intensity and the shi of the peaks in the range 440-530 cm À1 . Further, kinetics data suggested the presence of physical and chemical interactions, where the adsorption of MB dye over UiO-66/MIL-101(Fe)-GOCOOH composite obeys both the pseudo 1 st order as well as the pseudo 2 nd order models. Moreover, p-p stacking mechanism is expected predicted between aromatic rings of both MB and UiO-66/MIL-101(Fe)-GOCOOH composite (originating from H 2 BDC and/or GOCOOH). In addition, the present nitrogen atoms in the structure of MB dye are expected to form hydrogen bonds with -OH groups of UiO-66/MIL-101(Fe)-GOCOOH. 64 The possible adsorption mechanism of MB dye on the surface of UiO-66/MIL-101(Fe)-GOCOOH composite includes electrostatic interactions, p-p stacking, hydrogen bonds and n-p conjugations.

Reusability
Reusability of UiO-66/MIL-101(Fe)-GOCOOH composite in MB adsorption was investigated for 7 cycles. The removal efficiency and adsorption capacity of MB onto UiO-66/MIL-101(Fe)-GOCOOH composite is represented in Fig. 9. A removal efficiency of 71.04% and an adsorption capacity of 142.07 mg g À1 were obtained aer seven cycles. The reusability study indicated that UiO-66/MIL-101(Fe)-GOCOOH composite can be utilized as a re-usable adsorbent for dye removal.  adsorbents. The high adsorption capacity could be a result of the successful combination between GOCOOH and UiO-66/MIL-101(Fe). Besides, the high surface area, the existence of unsaturated bonds in the fabricated composite and the generated extra negative charges on the adsorbent surface provide strong attraction forces with the positively charged MB dye molecules. In addition, the present synergetic effect of both binary UiO-66/ MIL-101(Fe) MOF and GOCOOH as well as the well-dispersion of GOCOOH in the composite matrix signicantly improved the adsorption process. It could be concluded that the formation of the composite allows the exceptional adsorption features of both UiO-66/MIL-101(Fe) MOF and GOCOOH to be combined which reects positively on the adsorption process (i.e. boosting the adsorption capacity). Accordingly, UiO-66/MIL-101(Fe)-GOCOOH composite could be suggested as an efficient and reusable candidate for removing cationic dyes from their aqueous solutions.

Conclusion
A novel UiO-66/MIL-101(Fe) binary MOF-carboxylated graphene (GOCOOH) composite was fabricated and its ability for the adsorption of cationic MB dye was evaluated. Results claried that incorporation of GOCOOH greatly increase the negative surface charge of UiO-66/MIL-101(Fe)-GOCOOH composite than the pristine UiO-66/MIL-101(Fe) binary MOF. The fabricated composite shows a superior adsorption performance for adsorbing of the cationic methylene blue dye. Moreover, the maximum adsorption capacity of MB dye onto UiO-66/MIL-101(Fe)-GOCOOH composite was found to be 448.71 mg g À1 . Further, the recyclability test indicates a good capability of UiO-66/MIL-101(Fe)-GOCOOH to reuse for many times with no signicant decrease in the adsorption capacity, conrming the application potential of our synthesized composite.

Conflicts of interest
There are no conicts to declare.