Facile fabrication of amino-functionalized MIL-68(Al) metal–organic framework for effective adsorption of arsenate (As(V))

Characterization of amino-MIL-68(Al) and the effect of amine group in improving MOF properties

Figure 1 shows the morphology of the amino-MIL-68(Al) surface at different magnifications. As can be seen, the MOF surface consists of cumulative amorphous particles. According to Fig. 1f, the particle diameters are in the range of 60 to 80 nm. In Wu et al.’s study, a similar structure was reported for aluminum-based MOFs³⁶. The XRD spectrum of amino-MIL-68(Al) is presented in Fig. 2a. The Sharp characteristic peaks observed at 2θ = 5, 8.8, 9.44, 12.6, 15.25, 17.8, 18.92, 24.9 and 26.8° have been reported in previous studies, indicating that the MIL-68(Al) is well synthesized in the MOF structure^{36,37,38,39,40}. The functional groups of the synthesized amino-MIL-68(Al) can be seen in Fig. 2b. The bands appearing at 990, 1257 and 1337 1/cm belong to the n(C–N) absorption distinctive of aromatic amines. N–H vibration can be seen at bands 1580 and 776 1/cm. The peak at around 1440 1/cm is attributed to the stretching vibration of C=C of 2-aminoterephthalic acid⁴¹. C–H and C=C of the benzene rings can be identified at bands 1123 and 1395 1/cm, respectively. The primary amines –NH₂ on organic linkers can be found at band 3386 1/cm. The band at 3495 1/cm belongs to the vibration of OH group. Finally, the bands at 472, 551, and 608 1/cm belong to the vibrations of the metal center of Al–O⁴².

FESEM images of amino-MIL-68(Al) with different magnifications (a–f).

Experimental and simulated XRD patterns of amino-MIL-68(Al) (a); FTIR spectra of amino-MIL-68(Al) (b).

Figure 3a shows the EDX-mapping analysis of the amino-MIL-68(Al) surface. As can be seen, the synthesized amino-MIL-68(Al) consists of a uniform distribution of the elements carbon (61.81 wt%), oxygen (31.6 wt%), aluminum (4.35 wt.%), and nitrogen (2.25 wt%). The chemical structure of amino-MIL-68(Al) is shown in Fig. 3. Accordingly, its chemical formula and molecular mass are ({text{C}}_{8} {text{H}}_{5} {text{AlNO}}_{5}^{ + }) and 222.11 g/mol, respectively. In addition, the theoretical analysis of the elements indicates that the mass percentages (wt%) of carbon, oxygen, aluminum, nitrogen and hydrogen are 43.26, 36.02, 12.15, 6.31 and 2.27, respectively. As can be seen, the mass ratio of aluminum to nitrogen is almost double, which is consistent with the EDX results. Figures 3b,c show the ADS/DES isotherm of nitrogen and Barrett-Joyner-Halenda (BJH) pore size of amino-MIL-68(Al), respectively. The shape of the isotherm is similar to type II with H3 type hysteresis, suggesting the presence of mesoporous texture with the micropores⁴³. The results also showed that the Brunner−Emmet −Teller (BET) surface area for the synthesized amino-MIL-68(Al) is 1170.9 m²/g, which is much larger than the surface area reported for MIL-88B(Fe) (214 m²/g)⁴⁴, NH₂-MIL-88(Fe) (201 m²/g)⁴⁵ and NH₂-MIL-68(In) (655 m²/g)⁴⁶. The mean pore diameter and total pore volume for amino-MIL-68(Al) were also 2.64 nm and 0.7743 cm³/g, respectively.

EDX-mapping analysis of amino-MIL-68(Al) (a). The ADS/DES isotherm of N₂ on amino-MIL-68(Al) (b), BJH pore size of amino-MIL-68(Al) (c).

Figures 4a–d show the surface morphology of MIL-68(Al) at different magnifications. As can be seen, the surface morphology of the amino-MIL-68(Al) (Fig. 1) is much less porous compared to MIL-68(Al), which reduces the surface area of the MOF in the absence of the amine group. The results of the ADS/DES isotherm of nitrogen, as well as the BJH pore size of MIL-68(Al), are presented in Fig. 4e,f, respectively. As expected, the BET surface area for MIL-68(Al) is 239.98 m²/g, which is less than the amino-MIL-68(Al). In other words, functionalization of MIL-68(Al) with the amine group increases the surface area of MOF by about 5 times, which means increasing the active sites for the absorption of environmental pollutants. In addition, the results showed that the mean pore diameter and total pore volume for MIL-68(Al) were 5.73 nm and 0.3436 cm³/g, respectively.

FESEM images of MIL-68(Al) with different magnifications (a–d); The ADS/DES isotherm of N₂ on MIL-68(Al) (e), BJH pore size of MIL-68(Al) (f).

Data analysis, process modeling and optimization

To model As(V) adsorption on amino-MIL-68(Al), experimental design was performed based on CCD method. Table 2 shows the efficiency of As(V) removal in designed experiments. Data analysis showed that a quadratic model is able to predict system response with R² > 0.99. ANOVA results for As(V) adsorption by amino-MIL-68(Al) are shown in Table 3. For the model, F-value and p-value are 2389.17 and < 0.0001, respectively, which confirms that the developed model is statistically significant. However, Lack of Fit is statistically non-significant because the values of these parameters are 1.23 and 0.4341, respectively. These results indicate that the data fit well with the developed quadratic model^47,48. The final model equation is presented in Eq. (3). As can be seen, the proposed model is affected by the linear (A, B, C and D), interaction (AB, AC, AD, BC, BD and CD) and quadratic (A², B², C² and D²) effects of the independent variables. Equation (4) was used to calculate the percentage effect of each parameter on the system response³⁴. The final column in Table 3 shows the percentage of the positive and negative effects of each of the model parameters on the system response. As can be seen, the greatest effects on the system response are related to the linear effects of solution pH (A: − 41.34%), amino-MIL-68(Al) dose (B: + 28.70), As(V) concentration (D: − 14.29%) and reaction time (C: 9.98%), respectively. In addition, the system response is totally 3.89% affected by the interaction effects of the independent variables. Among the quadratic effects, the most important parameter affecting the system response is the amino-MIL-68(Al) dose (B²: − 0.178%).

where Y indicates the system response or As(V) removal efficiency (%). A, B, C and D represent the independent variables defined in Table 1. β_i is also the regression coefficients of the parameters in the model equation based on coded factors.

GA method was utilized to optimize the process and predict the highest As(V) removal efficiency. For this purpose, the equation of the quadratic model was entered into the software as a fitness function, and the independent variables were adjusted to their high and low values (± α)⁴⁹. The software output is shown in Fig. 5. As can be seen, after about 200 generations, the optimal values of the independent variables are predicted. Accordingly, the optimal values for solution pH, amino-MIL-68(Al) dose, reaction time and As(V) concentration were 3, 0.4 (g/L), 80 min and 2.5 mg/L, respectively. For these laboratory conditions, the predicted removal efficiency for As(V) adsorption on amino-MIL-68(Al) was about 99.45%. To evaluate the accuracy of the model, three adsorption experiments were performed under optimal conditions, which showed that the average experimental removal of As(V) (99.87%) is very close to the predicted removal of As(V).

Output of GA method for optimization of independent variables in As(V) adsorption process on amino-MIL-68(Al). MATLAB R2013a software was used to create this figure (https://www.mathworks.com/products/matlab.html).

Interaction of independent variables on the efficiency of As(V) removal

The effect of the interaction of solution pH and MOF dose on As(V) removal efficiency is shown in Fig. 6. Clearly, with decreasing pH in the range of 3 to 11 and with increasing MOF dose in the range of 0.05 to 0.4 g/L, the As(V) removal efficiency is significantly improved. So that at solution pH of 11 and MOF dose of 0.05 g/L, the As(V) removal efficiency is about 9.5%. However, at a solution pH of 3 and in the presence of 0.4 g/L MOF the efficiency of As(V) removal by the model is predicted to be about 99.5%. In addition, as can be seen in the presence of 1 g/L of MOF, the efficiency of As(V) removal at pHs of 5, 7, 9 and 11 is 88.4, 76.9, 68.2 and 54.2%, respectively.

Interaction of pH and MOF dose on system response (As(V) = 2.5 mg/L, Time = 80 min). Design-Expert v13 software was used to create this figure (https://www.statease.com/docs/v13/).

The pH of the solution is one of the operational factors that affects the efficiency of the adsorption process by affecting the properties of the adsorbent surface and the distribution of the dominant species of As(V)⁵⁰. As(V) is mainly present in the form of ({text{H}}_{2} {text{AsO}}_{4}^{ – }) in aqueous solutions with a pH in the range of 3–6. However, with increasing pH (pH > 7), the predominant forms will be ({text{HAsO}}_{4}^{2 – }) and ({text{AsO}}_{4}^{3 – })⁵¹. The zeta potential of amino-MIL-68(Al) at pHs of 3, 5, 7, 9, and 11 was measured to be + 11.8, + 9.5, + 4.6, + 1.2, and − 2.3 mV, respectively. Accordingly, the pHzpc for the amino-MIL-68(Al) was determined at 9.2. In other words, in a sample solution with a pH greater than 9.2, the amino-MIL-68(Al) surface charge has a negative state. Accordingly, the amino-MIL-68(Al) efficiency for adsorption of As(V) anionic species at high pH (pH > 9.2) is limited. However, the decrement in As(V) removal efficiency with increasing solution pH in the range of 3 to 9 can be related to the decrease of surface potential of amino-MIL-68(Al), which reduces the electrostatic attraction between the As(V) anions and the MOF surface⁵². Accordingly, the optimal pH in the present study was predicted to be 3, which is consistent with the results of some previous studies. Vu et al. have studied the effect of pH in the range of 3 to 11 on the removal efficiency of As(V) with MIL-53(Fe). In their study, the most As(V) removal was reported at pH 5, 3, 9, and 11, respectively⁵³. In the study of Wang et al., the highest As(V) adsorption on UiO-66 was obtained at pH between 1 and 3¹³. In addition, in the study of Wu et al., The highest As(V) removal efficiency on MIL-88A microrods was observed at pH 3 and 5⁵⁴. As can be seen in Fig. 6, at a solution pH of 3, the As(V) removal efficiency in the presence of MOF doses of 0.05, 0.1375, 0.225, 0.3125 and 0.4 g/L is about 61.2, 74.5, 83, 92.1 and 99.5%, respectively. Clearly, as the adsorbent dose increases, the number of available adsorption sites increases, resulting in improved removal efficiencies⁵⁵. Improvement of contaminant removal efficiency by increasing the adsorbent dose has been reported by other scholars^50,56.

The interaction of reaction time and As(V) concentration on As(V) removal efficiency is shown in Fig. 7. As can be seen, increasing the reaction time and decreasing the As(V) concentration improve the removal efficiency. Accordingly, the efficiency of As(V) removal at the initial concentration of 50 mg/L after 10 and 80 min of the reaction is about 45.7 and 85.2%, respectively. On the other hand, the removal efficiency of As(V) at initial concentrations of 2.5 and 50 mg/L after 80 min of reaction is about 99.5 and 85.2%, respectively. At a constant dose of adsorbent, the removal efficiency at high concentrations of As(V) is reduced due to the limited active adsorption sites at the surface of MOF. In this regard, the results reported by other researchers are consistent with our study^50,54.

Interaction of reaction time and As(V) concentration on system response (pH = 3, MOF dose = 0.4 g/L). Design-Expert v13 software was used to create this figure (https://www.statease.com/docs/v13/).

Adsorption isotherms and kinetics

The study of adsorption isotherms can reveal valuable information about the adsorption capacity and behavior of an adsorbent in interaction with an adsorbate⁵⁷. For this purpose, under optimal conditions, the experimental data were evaluated with Freundlich, Langmuir, Temkin and Dublin–Radushkevich isotherm models. The Langmuir isotherm describes the monolayer adsorption on the homogeneous surface of the adsorbent. The Freundlich isotherm assumes that adsorption is not monolayer and describes equilibrium at heterogeneous surfaces⁵⁸. The nonlinear and linear models of the Langmuir isotherm are presented in Eqs. (5 and (6). For Freundlich isotherm, nonlinear and linear models can be seen in Eqs. (7) and (8).

where qe is the mg of As(V) adsorbed per g of amino-MIL-68(Al) (mg/g). C_e indicates the equilibrium concentration of As(V) (mg/L). q_max represents the maximum adsorption capacity (mg/g) and K_L represents the Langmuir equilibrium constant (l/mg). K_f and 1/n show the adsorption capacity (l/mg) and adsorption intensity, respectively. K_f and n are determined from the nonlinear graph qe versus Ce and the linear graph log q_e versus log C_e.

The Temkin isotherm describes the process on a heterogeneous surface with adsorption sites with the same bond energy. Equations (9) and (10) describe the nonlinear and linear models of this isotherm³¹. The Dublin–Radushkevich isotherm describes the adsorption process on the heterogeneous surfaces. However, unlike the Freundlich isotherm, the absorption energy dissipation in this isotherm is linear⁵⁹. The nonlinear and linear models of the Dublin–Radushkevich isotherm are presented in Eqs. (11) and (12).

where B represents the Temkin isotherm constant (J/mol). K_T is the maximum bond energy (l/mg). R and T are also related to gas constant (8.314 J/K mol) and temperature (K), respectively. q_max is the monolayer adsorption capacity in the Dublin–Radushkevich isotherm (mg/g). β also represents the adsorption energy constant in this isotherm. ε is the Polanyi potential calculated by Eq. (13). In the Dublin–Radushkevich isotherm, the most probable free adsorption energy (E, J/mol) is calculated by Eq. (14). E < 8 and 8 < E < 16 kJ/mol show a physical nature and chemical nature, respectively.

The nonlinear form of isotherm models is plotted in Fig. 8a. Table 4 also presents the values of different parameters and coefficients for the studied models. The results show that the Langmuir isotherm (R² = 0.9998) describes the experimental data better than other models. Accordingly, the adsorption of As(V) on amino-MIL-68(Al) is homogeneous monolayer process^14,60. The maximum adsorption capacity of As(V) by the Langmuir isotherm was obtained to be 74.29 mg/g, which is higher than the reported values for ZrO₂-sawdust (12 mg/g)⁶¹, organic biochar (16.2 mg/g)⁶², CuO nanoparticles (22.6 mg/g)⁶³, and Fe₃O₄-RGO-MnO₂ (12.22 mg/g)⁶⁴. In addition, the maximum adsorption capacity of As(V) on the synthesized amino-MIL-68(Al) compared to other MOFs including MIL-53(Fe) (21.27 mg/g)⁵³, MIL-100(Fe) (110 mg/g)⁶⁵, Fe-BTC (12.29 mg/g)⁶⁶, Fe₃O₄@MIL-101(Cr) (80 mg/g)⁶⁷, ZIF-8 (90.92 mg/g)⁶⁸, Co-MOF (96.1 mg/g)⁶⁹, Fe₃O₄@UiO-66 (73.2 mg/g)⁷⁰, MOF-808 (24.8 mg/g)³⁵, UiO-66 (68 mg/g)⁶⁰ and UiO-66-(SH)2 (10 mg/g)⁷¹ is an acceptable value.

Nonlinear isotherm models (pH = 3) (a), and nonlinear adsorption kinetic models (pH = 3, As(V) = 50 mg/L) (b) for As(V) adsorption on amino-MIL-68(Al).

Determining the reaction rate and its mechanism depends on conducting kinetic studies. Different types of models have been developed to describe the kinetics of adsorption process. In the present study, the adsorption kinetic of As(V) on amino-MIL-68(Al) were studied and fitted with three different kinetic models including pseudo-first order⁷², pseudo-second order⁷³, and intraparticle diffusion models⁷⁴. The nonlinear equations of these kinetic models are shown in Eqs. (15) to (17), respectively.

where q_t (mg/g) indicates the absorption capacity at time t. q_e (mg/g) is also related to the absorption capacity at equilibrium time. k₁ (1/min) and k₂ (g/mg.min) are the rates constant of adsorption for pseudo-first-order and second-first-order kinetic models, respectively. k_i (mg/g.min^0/5) expresses the rate constant of intraparticle diffusion kinetic model.

Figure 8b and Table 5 show the data obtained from the study of adsorption kinetic in the nonlinear form of the models. As can be seen, the experimental data are well consistent with the pseudo-second-order nonlinear model (R² = 0.9822), which indicates that the chemical adsorption mechanism dominates the adsorption process⁶⁰. The results of recent studies show that As(V) adsorption on MOFs (Fe/Mg-MIL-88B(n)⁷⁵, Fe–Co MOF-74⁵², Fe/Al-BDC-NH₂⁷⁶ and UiO-66/PAN membrane⁷⁷) is generally well described by pseudo-second-order kinetic model.

Adsorption thermodynamic

Thermodynamic parameters provide useful information about whether the reactions are endothermic or exothermic, whether the processes are spontaneous or not, and the entropy changes in the process. Thermodynamic parameters were determined using Eqs. (18) to (20)⁷⁸.

where ΔG° is the standard Gibbs free energy (kJ/mol), K_c is the distribution coefficient, which was calculated by Eq. (20). T also represents the absolute temperature of the solution (K). ΔS° (J/mol. K) and ΔH° (kJ/mol) are the entropy and enthalpy parameters. In Eq. (20), q_e and C_e represent the adsorption capacity (mg/g) and As(V) concentration (mg/L) in equilibrium, respectively. The thermodynamic parameters of As(V) adsorption on amino-MIL-68(Al) are presented in Table 6. As can be seen, the value of ΔG° becomes more negative with increasing temperature in the range of 25 to 50 °C. Accordingly, As(V) adsorption on prepared MOF is a spontaneous process that is improved by higher temperatures⁷⁶. Given the positive value of ΔH°, it is clear that the nature of the As(V) adsorption on the amino-MIL-68(Al) is an endothermic process⁷⁹. In other words, higher temperatures accelerate mass-transfer and process kinetics, resulting in an improved adsorption process. Also, a positive value of ΔS° means that chaos increases at the solid–liquid interface. In such a situation ion exchange occurs during As(V) uptake⁸⁰.

Comparison of efficiency and reusability of amino-MIL-68(Al) and MIL-68(Al) for As(V) removal

The results of Sect. “Characterization of amino-MIL-68(Al) and the effect of amine group in improving MOF properties” showed that the functionalization of MIL-68(Al) with the amine group (-NH₂) was able to significantly improve the porosity and surface area of the MOF. Therefore, to confirm the effect of the amine group in improving the absorption of As(V), the removal efficiency and reusability of amino-MIL-68(Al) and MIL-68(Al) were compared for ten consecutive reuse cycles under optimal conditions. MOF regeneration was performed after each reuse round using 0.01 M nitric acid solution^65,75. As shown in Fig. 9a, the removal efficiency of As(V) with amino-MIL-68(Al) is significantly higher compared to MIL-68(Al). Accordingly, in the first round of use, the removal efficiency of As(V) with amino-MIL-68(Al) and MIL-68(Al) was 99.8% and 74.4%, respectively. The Functionalization of MOFs with the amine group not only increases the number of electron-rich nitrogen sites and the positive charge in the MOF structure but also strengthens hydrogen bonds and increases the adsorption rate⁴⁶. In this regard, Haque et al., reported that NH₂-MIL-101(Al) has a higher adsorption capacity to remove methylene blue than MIL-101(Al)⁸¹. In addition, the results showed that amino-MIL-68(Al) has more reusability compared to MIL-68(Al) so that the removal efficiency of As(V) with amino-MIL-68(Al) and MIL-68(Al) decreased by about 19.6% and 29.2% after ten reuse cycles, respectively. These results confirm that functionalization of MIL-68 (Al) with the amine group not only increases the surface area of MOF but also improves As(V) removal efficiency and MOF reusability.

As(V) removal efficiency by amino-MIL-68(Al) and MIL-68(Al) for ten consecutive reuse cycles under optimal conditions (a). Interventional effect of anions on As(V) removal efficiency by amino-MIL-68(Al) (The oncentration of anions = 1 mM) (b).

Effect of interfering anions on As(V) removal efficiency

Natural waters and industrial wastewater are always contaminated with a variety of ion-interfering species that dramatically affect the efficiency of the adsorption process. In this study, the effect of chloride, nitrate, sulfate, and phosphate anions at a constant concentration of 1 mM and under optimal laboratory conditions (pH = 3, MOF dose = 0.4 g/L, reaction time = 80 min and As(V) concentration = 2.5 mg/L) was investigated on the efficiency of As(V) removal, the results of which are presented in Fig. 9b. As can be seen, the presence of chloride, nitrate, and sulfate anions in the samples solution does not have much interference with As(V) adsorption on MOF. However, in the presence of phosphate, a significant reduction is observed in the As(V) removal efficiency. Phosphate competes with As(V) for active sites at the MOF surface, resulting in inhibition of As(V) adsorption. The high intervening effect of phosphate with As(V) adsorption can be attributed to similar physicochemical properties of these two elements⁶⁰.