Introduction

Dye is the first thing that is recognized in an effluent. With increasing production of dye and its various applications, there is higher production of effluents with high strength (Rafatullah et al. 2010; Ali et al. 2016; Gupta et al. 2005; Yola et al. 2014a, b). Most materials in effluents are toxic, dangerous and cause severe pollutions in the environment (Rafatullah et al. 2010; Mansoorian et al. 2013; Dias and Petit 2015). One of the most common dyes extensively used in the textile industry is methylene blue which is a cationic dye (Vargas et al. 2011). Till now, various methods for dye adsorption are used, these are, photo-catalytic degradation (Ehrampoosh et al. 2010; Sohrabi and Ghavami 2008), sono-chemical process (Ali et al. 2012), adsorption (Umoren et al. 2013; Ali and Gupta 2006; Ali 2010), H2O2 (Masoumbeigi and Rezaee 2015), TiO2 (Deilami and Fallah 2015), photo-fenton oxidation (Gowtham and Pauline 2015), electro-fenton degradation (Xia et al. 2014) and biological method (Cheng et al. 2015). From these, adsorption is one of the most popular processes. If properly designed, it can effectively remove various dye materials (Crini 2006; Ali 2014). Thus, different adsorbents such as biopolymer oak sawdust composite (El-Latif et al. 2010), mesoporous carbon (Álvarez-Torrellas et al. 2015), waste cotton activated carbon (Ekrami et al. 2016), activated carbon and water hyacinth (Kanawade and Gaikwad 2011), natural illitic clay (Amrhar et al. 2015), modified pumice stone (Derakhshan et al. 2013), zeolite material (Fungaro et al. 2010; Ali et al. 2012), can papyrus (Saed et al. 2014), carbon nanotubes (Shahryari et al. 2010), boron enrichment waste (BW) and molasses modified boron enrichment waste (MBW)-based nanoclays (Gupta et al. 2014), blast furnace sludge (Malina and Radenovic 2014), NaOH-modified dead leaves (Gong et al. 2013), Fe@Au bimetallic nanoparticles (Gupta et al. 2014), TiO2 nanoparticles involved boron enrichment waste (Yola et al. 2014a, b), wool fiber and cotton fiber (Khan et al. 2005), and bentonite (Hong et al. 2009) are used for adsorption of methylene blue from water and wastewater. These adsorbents have different advantages and disadvantages. With significant development of science in the past decades, considerable progress has been made in the construction and synthesis of new adsorbents (mesoporous materials) (Ali 2012). Metal-organic framework (MOF) is one of these new adsorbents (Hasan et al. 2013; DeCoste and Peterson 2014). Actually, MOFs are a class of porous materials that are composed of two parts: inorganic (as metal core) and organic ligand (as linker) (Katz et al. 2013; Hasan and Jhung 2015). The advantages of these new adsorbents include: high specific surface area, adjustable size by temperature changes, large pore volume and coordinately saturated or unsaturated site to regulate the adsorption capability (Bakhtiari and Azizian 2015; Huo and Yan 2012). MOFs are used in various applications such as advance oxidation processes: (MIL-53, MIL-100(Fe) and FeII@MIL-100(Fe) (Du et al. 2011; Huanli et al. 2015), fluoride (Uio-66) (Massoudinejad et al. 2016), gas separation (Zr-MOF) (Abid et al. 2013; Barea et al. 2014), 2,4-dichlorophenoxyacetic (Jung et al. 2013), phthalic acid and diethyl phthalate (ZIF-8) (Khan et al. 2015), p-nitrophenol (HKUST-1) (Lin et al. 2014), arsenate (ZIF-8, MIL-53 and F-BTC) (Li et al. 2014; Zhu et al. 2012). However, the application of Uio-66 in the methylene blue dye adsorption from water and wastewater has not been evaluated. Until now, several metal-organic frameworks are used for adsorption of various dyes such as MOF (Co/Ni) (Abbasi et al. 2016), MIL-125(Ti) (Guo et al. 2015), magnetic CU3(BTC)2 (Zhao et al. 2015; Lin et al. 2014), Fe(BTC) (García et al. 2014), iron terephthalate (MOF-235) (Haque et al. 2011) and MIL-100(Fe) Huo and Yan 2012). Therefore, the aim of this study was to synthesize Uio-66 MOFs and show its uses in the adsorption of methylene blue from synthetic samples.

Materials and methods

Materials

Zirconium chloride (IV) and terephthalic acid (TPA) were obtained from Merk Company. N,N-dimethylformamide (DMF), methanol (CH3OH), methylene blue (C16H18ClN3S), sodium hydroxide (NaOH) and sulfuric acid (H2SO4) were supplied by Sigma-Aldrich. All the reagents and solvents were used as received from commercial suppliers without further purification.

Synthesis and preparation of Uio-66

UiO-66 MOF was synthesized according to previous studies (Shen et al. 2013a, b; Luu et al. 2015; Gao et al. 2016). In usual synthesis, 0.2332 g of ZrCl4 [Zr6O4(OH)4(BDC)6] (Furukawa et al. 2013) and 0.161 g terephthalic acid were dissolved in 50 ml DMF solution. Then, the solution was transferred to a 100-ml Teflon autoclave. The autoclave was sealed and heated in a vacuum oven at 120 °C for 48 h under constant pressure. After cooling, the sample was purified with methanol solution (95%) three times to make sure that the occluded DMF molecules were eliminated. After drying, Uio-66 was obtained under vacuum at 100 °C for 12 h.

General characteristics

The synthesized UiO-66 MOF was characterized by X-ray diffraction, field emission-scanning electron microscopy, energy-dispersive X-ray spectroscopy and Brunauer–Emmett–Teller surface area. Total pore volume of the samples was determined by nitrogen (N2) adsorption isotherms at 77 K.

Adsorption studies

All the experiments of methylene blue adsorption were performed by Uio-66 in batch conditions. The effect of various parameters including pH, initial methylene blue concentration, reaction time, and adsorbent dose was evaluated. Initially, stock solution of methylene blue (1000 mg L−1) was prepared by dissolving 1 g of methylene blue in 1 L of distilled water and then stored under standard conditions. An adsorbent dose was added to 50 ml of methylene blue solution. The solution pH was adjusted by using 0.1 N NaOH and HCl. After the experiments, the remaining adsorbent was separated from the solution by centrifugation (2000 rpm and 15 min). Then, the residual methylene blue concentration was determined by a spectrophotometer (λ = 624 nm) (Li et al. 2014). All the experiments were done at constant temperature of 25 ± 1 °C (Cai et al. 2015). Finally, the amount of methylene blue adsorbed on the Uio-66 was calculated according to Eq. 1 (Reardon and Wang 2000):

$$q_{e} = \frac{{V(C_{0} - C_{e} )}}{m}$$
(1)

where C 0 and C e are the concentration of initial and final methylene blue in the solution (mg L−1), respectively, V is the volume of methylene blue solution (ml), and m is the weight of the adsorbent (g). The removal efficiency of methylene blue was calculated according to Eq. 2 (Bhaumik et al. 2013):

$$R\;(\% ) = \frac{{(C_{0} - C_{t} )}}{{C_{0} }}$$
(2)

where C 0 and C t represent the initial and final methylene blue concentration (mg L−1), respectively.

Results and discussion

X-ray diffraction analysis

X-ray diffraction (XRD) pattern is a useful tool for identification of the atomic and molecular structures of a crystal sample. The as-synthesized Uio-66 was in the form of white powder. The crystallographic structure of synthesized Uio-66 was investigated by X-ray diffraction. The X-ray diffraction pattern of the prepared Uio-66 is shown in Fig. 1. The Uio-66 MOF contained characteristic peaks at 2θ equal to 7° and 8.45° with variation of peak intensity depending on the reaction time (Luu et al. 2015). X-ray diffraction pattern was similar to that of previous studies (Lin et al. 2016; Peterson et al. 2014; Shen et al. 2013a, b). Also, Fig. 2 shows FTIR spectra of the prepared Uio-66.

Fig. 1
figure 1

a X-ray diffraction pattern of the prepared Uio-66, b FTIR spectra of the prepared Uio-66

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Fig. 2
figure 2

Field emission-SEM image of the prepared Uio-66 MOFs

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Field emission-scanning electron microscopy (FE-SEM)

Figure 2 shows the SEM morphology of as-synthesized Uio-66. The field emission-SEM image showed that the sizes of Uio-66 crystals were between 153 and 213 nm. In similar studies, the prepared Uio-66 MOFs crystals sizes were between 200 and 500 nm (Lin et al. 2015).

The nitrogen adsorption–desorption isotherms

Figure 3 shows N2 adsorption–desorption isotherms and BJH pore size distributions of Uio-66 MOFs. Adsorption–desorption isotherm of Uio-66 was similar to type I. Table 1 shows some properties of the prepared Uio-66 MOFs.

Fig. 3
figure 3

N2 adsorption–desorption isotherm and BJH pore size distributions of Uio-66

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Table 1 Some properties of Uio-66 for BET experiments at 77 K
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Energy-dispersive X-ray spectroscopy (EDAX)

Energy-dispersive X-ray spectroscopy is an analytical technique used for the elemental analysis or chemical composition of one sample (Masoumbeigi and Rezaee 2015). In this work, energy-dispersive X-ray spectroscopy technique was utilized to check the chemical composition of the synthesized Uio-66. The results showed that percentages of C, O, Zr, and Cl compounds were 46.84, 23.65, 26.64, and 2.87%, respectively. As shown in the results, the ingredients of as-synthesized Uio-66 were the same with the raw materials used before that without any impurity.

Effect of pH

pH is one of the most important parameters that influence dye adsorption, because of change in the surface charges of adsorbent and the degree of ionization of the target pollutants (Guo et al. 2015; Lin et al. 2014). So, the initial pH of the methylene blue solution is a significant factor. In this work, methylene blue adsorption by Uio-66 MOF was studied in various pHs and at 298 °K. To evaluate the effect of pH on methylene blue adsorption, various pHs in the range of 3–11 were used. In the adsorption experiments, a 0.4 g of Uio-66 MOF was added to 50 ml volume of methylene blue solution with initial concentration of 30 mg L−1. Figure 4 shows methylene blue adsorption at various pHs. Increasing pH from 3 to 11 led to methylene blue adsorption. At pH of 3 and 9, methylene blue adsorption was 10.3 and 80.96%, respectively. Uio-66 with pH = 9 can remove large amount of methylene blue. In Lin study, methylene blue adsorption efficiency increased with increase in pH (Lin et al. 2014). Methylene blue dye is a cationic dye. At low pH, H+ concentration is high, so, Uio-66 surface charge is positive which in turn reduces the methylene blue adsorption. By increasing pH, OH− concentration gradually increased and methylene blue adsorption on Uio-66 increased (Weng and Pan 2007; Gürses et al. 2006). Isoelectric point of Uio-66 was equal to 6. At pH higher than isoelectric point, methylene blue adsorption on Uio-66 was favorable. Maximum adsorption was achieved at pH 9. As a result, pH 9 was selected for further experiment.

Fig. 4
figure 4

The effect of pH on methylene blue adsorption by Uio-66 MOF

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Effect of initial concentration of methylene blue dye

In order to evaluate the effect of initial concentration, 10–50 mg L−1 methylene blue concentration was used at contact time of 60 min. Figure 5 shows the effect of initial methylene blue concentration on adsorption by Uio-66. With increasing methylene blue concentration, adsorption efficiency decreased. In methylene blue equal to 10 mg L−1, methylene blue dye was completely adsorbed after 300 min contact time. In lower concentrations, methylene blue molecules were adsorbed on Uio-66 adsorbent surface, rapidly, but Uio-66 surface became saturated gradually with increase in methylene blue. Finally, the adsorption decreased because of the repulsion among methylene blue molecules (García et al. 2014; Zvinowanda et al. 2009).

Fig. 5
figure 5

The effect of initial methylene blue concentration versus contact time

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Effect of Uio-66 dose

Figure 6 shows the effect of Uio-66 dose and removal efficiency of methylene blue dye. It was observed that methylene blue adsorption increased with the increase in Uio-66 dose until an equilibrium dose was reached. Optimum dose was 0.4 g L−1. With increase in the Uio-66 dose, methylene blue adsorption was not significant. Adsorption efficiency of methylene blue in dosages of 0.4 and 0.5 g L−1 were 72.5 and 75%, respectively. As a result, the optimum dose was 0.4 g L−1. Based on the finding of other researchers, the increase in methylene blue adsorption with Uio-66 dose can be caused by high adsorbent surface and availability of more adsorption sites (Maleki et al. 2015). With dose higher than optimum (0.4 g L−1) and with increase in Uio-66 dose, adsorption capacity of adsorbent was almost constant. This phenomenon can be due to overlapping or aggregation of adsorption sites of Uio-66 that eventually lead to decrease in total Uio-66 surface area (Maleki et al. 2015; Zhang et al. 2014).

Fig. 6
figure 6

The effect of adsorbent dose of Uio-66 on methylene blue adsorption

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Adsorption kinetics and adsorption isotherms

In order to study the mechanisms of methylene blue adsorption on Uio-66, kinetic models like pseudo-first-order and pseudo-second-order kinetic models were used. The pseudo-first-order kinetic model expression of Lagergren is expressed in Eq. 3:

$$\frac{{dq_{t} }}{dt} = k_{1} (q_{e} - q_{t} )^{2}$$
(3)

where q e and q t are the amounts of methylene blue adsorbent on the Uio-66 at equilibrium and at time t, respectively, and k 1 is the rate constant (min−1). The linear form of pseudo-first-order is expressed as Eq. 4 (Baskan and Pala 2011; Mohsenibandpei et al. 2016)

$$\log (q_{e} - q_{t} ) = \log q_{e} - \frac{{k_{1} }}{2.303}t$$
(4)

where k 1 and q e are obtained from the slope and intercept of the linear plots of log (q e  − q t ) versus t, respectively. Pseudo-second-order kinetics model is expressed as Eq. 5 (Kragović et al. 2013; Mohsenibandpei et al. 2016):

$$\frac{{dq_{t} }}{dt} = k_{2} (q_{e} - q_{t} )^{2}$$
(5)

where q t and q e are sorbent methylene blue at time t and equilibrium (mg L−1), respectively, and k 2 is the constant of pseudo-second-order sorption of methylene blue (g mg−1 min−1). The linear form of pseudo-second-order is expressed in Eq. 6:

$$\frac{t}{{q_{t} }} = \frac{1}{{k_{2} q_{e}^{2} }} + \frac{t}{{q_{e} }}$$
(6)

where k 2 and q e are obtained from the slope and intercept of the linear plots of log(q e  − q t ) versus t, respectively. To conduct adsorption kinetic experiments, 0.4 g of Uio-66 adsorbent was added to 50 ml of methylene blue solution at initial concentration of 10–50 mg L−1. Figure 7 shows the effect of initial methylene blue concentration and contact time on methylene blue adsorption on Uio-66 MOF. Initially, methylene blue dye was adsorbed rapidly on Uio-66. Adsorption equilibrium of methylene blue was performed after 200 min. After this time, methylene blue adsorption dose did not significantly change. Calculated constants of kinetic models are given in Table 2. Coefficient of determination (R 2) was used to determine the best kinetic model (Bakhtiari and Azizian 2015). Coefficient of determination of the used kinetic models is given in Table 2. As shown in Table 2, pseudo-first-order (PFR) kinetic model had the highest square R. As a result, the PFR model was a fit model to describe the adsorption kinetic of methylene blue on Uio-66.

Fig. 7
figure 7

Effect of initial methylene blue concentration and contact time on Uio-66 MOF

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Table 2 Calculated constants of kinetic models for the adsorption of methylene blue on Uio-66
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Adsorption isotherms were used to describe the adsorption of methylene blue and mechanisms of adsorption on Uio-66. The adsorption of methylene blue on Uio-66 MOF was studied with five concentrations (10, 20, 30, 40, and 50 mg L−1), various contact times (30–500 min) at temperature and stirring speed of 298 °K and 300 rpm, respectively. In this research, the equilibrium data of Uio-66 were fitted to the Langmuir (I, II, III, and IV) and Fraundlich isotherms. The Fraundlich isotherm model is expressed in Eq. 7 (Camacho et al. 2011):

$$q_{e} = K_{f} C_{e}^{1/n}$$
(7)

where q e is the amount of methylene blue adsorbed at equilibrium, C e is the equilibrium concentration of methylene blue in the solution, and Kf and n are the Fraundlich constants. The linear form of the Fraundlich isotherm model is expressed in Eq. 8 (Lee and Tiwari 2013):

$$\log q_{e} = \frac{1}{n}\log C_{e} + \log K_{f}$$
(8)

where q e and C e are the amount adsorbent of methylene blue (mg g−1) and sorption concentration (mg L−1) at equilibrium, respectively. KF and 1/n are the Fraundlich constants denoting adsorption capacity and adsorption intensity or surface heterogeneity, respectively. The Langmuir isotherm model is expressed in Eq. 9 (Jayakumar et al. 2015):

$$q_{e} = \frac{{q_{m} K_{L} C_{e} }}{{1 + K_{L} C_{e} }}$$
(9)

where q e and q m are amount of adsorbed methylene blue per unit mass of Uio-66 and maximum sorption capacity of the Uio-66 (mg g−1), respectively. C e is equilibrium methylene blue concentration, and KL is Langmuir isotherm constant (dm3 g−1). The linear form of the Langmuir isotherm model is expressed as Eq. 10 (Jayakumar et al. 2015):

$$\frac{{C_{e} }}{q} = \frac{1}{{q_{0} b}} + \frac{{C_{e} }}{{q_{e} }}$$
(10)

where q is the amount of methylene blue adsorbed per unit weight of Uio-66 (mg g−1) at equilibrium, C e is the equilibrium methylene blue concentration (mg L−1), q 0 is the Langmuir monolayer adsorption capacity, and b is the Langmuir constant (L g−1). Calculated constants of kinetic models are given in Table 3. Coefficient of determination (R2) was used to determine the best isotherm model (Huo and Yan 2012). R squared or coefficient of determination is a number that indicates the proportion of the variance in the dependent variable which is predictable from the independent variable (Steel and Torrie 1960). Coefficient of determination of the used isotherm models is shown in Table 3. As illustrated in Table 3, Langmuir I model has the highest R 2 (0.9993). As a result, the Langmuir I isotherm was the best fit model to describe the adsorption isotherm of methylene blue on Uio-66. According to Langmuir I model, the maximum adsorption capacity (q m ) (for initial methylene blue concentration, 10–50 mg L−1) of Uio-66 for methylene blue adsorption was equal to 64.5 mg g−1.

Table 3 Calculated constants of isotherm models for the adsorption of methylene blue onto Uio-66
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Regeneration studies

One of the most important issues in the adsorption processes is reuse of the used adsorbent. In this work, Uio-66 was regenerated with methanol. To do so, 0.4 of Uio-66 was added to 50 ml of methylene blue solution (initial concentration = 20 mg L−1). After completion of methylene blue adsorption process, the used Uio-66 was separated. For the regeneration of the used Uio-66, methanol was applied with high purity of 95%. After washing with methanol, the adsorbent was dried in an oven at 100 °C for 10 h. Regeneration process was performed with 5 cycles. The results showed that Uio-66 MOFs can be easily regenerated with methanol and methylene blue adsorption efficiency which is almost unchanged after multiple times.

Conclusion

In this research, Uio-66 was synthesized using hydrothermal method. It was used for the adsorption of methylene blue dye from synthetic samples. Uio-66 structure was characterized by using X-ray diffraction, Fourier transform infrared spectroscopy spectra, field emission-scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Brunauer–Emmett–Teller. Adsorption isotherm of methylene blue was described based on Langmuir model that represents the monolayer adsorption onto Uio-66. The best kinetic model for methylene blue adsorption is a more suitable pseudo-first-order model. The maximum adsorption capacity of Uio-66 MOFs was 90 mg g−1 at an initial concentration of 50 mg L−1. The Uio-66 MOFs is a promising adsorbent in the adsorption of methylene blue from aqueous solution.