Boosting the adsorptive and photocatalytic performance of MIL-101(Fe) against methylene blue dye through a thermal post-synthesis modification

Photocatalytic degradation under ultra-low powered light is a viable advanced oxidation process technique against extensive emerging contaminants. As a new and remarkable class of nanoporous materials, metal-organic frameworks (MOFs), attract interest for the supreme adsorptive and photocatalytic functionalities. An outstanding MOF, MIL-101(Fe) chosen as a photocatalyst template for the synthesis of α-Fe2O3 by a simple thermal modification to improve the structural properties toward methylene blue (MB) eradication. Octahedron-like α-Fe2O3 photocatalyst (Modified MIL-101(Fe), M-MIL-101(Fe)) was superior in dispersion and separation properties in aqueous medium. Moreover, the adsorptive and catalytic performance was increased for modified form by ~ 7.3% and ~ 17.1% compared to pristine MIL-101(Fe), respectively. Synergistic improvement of MB removal achieved by simultaneous adsorption/degradation under 5-W LED irradiation. Parametric study indicated an 18.1% and 44.5% improvement in MB removal was observed by increasing pH from 4 to 10, and M-MIL-101(Fe) dose from 0.2 to 1 g L−1, respectively. MB removal followed the pseudo-second-order kinetics model and the process efficiency dropped by 38% as MB concentration increased from 5 to 20 mg L−1. Radical trapping tests revealed the significant role of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{OH}}^{.}$$\end{document}OH. and electron radicals as the major participants in dye degradation. A significant loss in the efficiency of M-MIL-101(Fe) was observed in the reusability tests that is good to study further. In conclusion, a simple thermal post-synthesis modification on MIL-101(Fe) improved its structural, catalytic, and adsorptive properties against MB.

Instruments.The UV-vis diffuse reflectance spectroscopy (UV-vis DRS) was carried out in a DRS S_4100 SCINCO spectrophotometer.The photoluminescence (PL) spectra of MIL-101(Fe) and M-MIL-101(Fe) were recorded using Perkinelmer LS 45 spectrometer at room temperature.The FTIR spectra of MIL-101(Fe) and M-MIL-101(Fe) were evaluated in the range of 400-4000 cm −1 by a KBr pellet on a Thermo Nicolet-Avatar 370 Spectrometer at room temperature.The X-ray diffraction (XRD) patterns of the synthesized materials were recorded by a Bruker/D8 Advanced diffractometer by using a CuKα (λ = 0.15406 nm) radiation.The surface morphology and energy-dispersive X-ray spectroscopy (EDS) of prepared materials were investigated through the field-emission scanning electron microscopy (FE-SEM, MIRA3 TESCAN, Czech Republic).

Preparation of modified MIL-101(Fe). The orange MIL-101(Fe) powder was transferred to an open
cruiser where heated at 320 °C for 20 min.The orange powder turns brown after the calcination in air.The schematic illustration for the preparation of MIL-101(Fe) and M-MIL-101(Fe) is shown in Fig. 1.
Adsorption/photocatalytic studies.Adsorption and photocatalytic degradation of prepared MIL-101(Fe) and M-MIL-101(Fe) were determined in batch mode system using MB as the model contaminant.The experimental study consists of preliminary tests to evaluate the removal efficiency of pristine MIL-101(Fe) and M-MIL-101(Fe) against MB.The adsorptive properties and photocatalytic degradation performance of the materials were assessed by conducting the process under the dark and light irradiation, respectively.
A simple LED module was used to supply light using 5-W LED lamps.Using LED module have the advantage of energy-efficient, durable, low-cost, and available source of light for lab studies and real treatment units.
Once determined which MIL-101(Fe) or M-MIL-101(Fe) was more efficient in MB removal, the study continued to assess the effect of environmental condition on the process.One factor at a time (OFAT) method was opted to determine the significance of pH, contact time, catalyst dose, and MB concentration on the process.The levels of studied variables in photocatalytic degradation of the dye are presented in Table 1.
The degradation efficiency of MB was calculated using Eq. ( 1) by the difference between the initial ( C 0 ) and final concentration ( C ) as measured by spectrophotometer at 665 nm: (1)  propose the mechanism of pollutant destruction.In scavenger test, a chemical agent that block specific radical added to the photocatalytic system and the system efficiency then analyzes for significance of active species.In this study, AgNO 3, IPA, AA, and KI with a concentration of 2 mmol L −1 used to block electron ( e − ), hydroxyl radical ( OH • ), superoxide radical ( O •− 2 ), and hole ( h + ), respectively.Additional experiments for the reusability of M-MIL-101(Fe), kinetic of MB removal, and structural stability of photocatalyst accomplished that discussed in detail in the following sections.

Results and discussion
Characterization.The optical properties of the MIL-101(Fe) and M-MIL-101(Fe) were examined by UVvis DRS (Fig. 2a).The MIL-101(Fe) shows high light reflectance in the range of 550-800 nm.For M-MIL-101(Fe), the light reflectance strongly decreased in this range, indicating a high visible light absorption.The band gap of samples was calculated by the Kubelka-Munk equation.From the DRS, the Kubelka-Munk function is proportional to the extinction coefficient (α) and R is the reflectance 50 : Then, the band gap energies of the samples were calculated by Eq. ( 3): where hν, A, and E g are the energy, proportionality constant, and band gap energy, respectively.In this equation, n can be 2 or ½ for direct and indirect semiconductors, respectively.
As illustrated in Fig. 2b, MIL-101(Fe) shows the band gap of 2.8 eV, while M-MIL-101(Fe) (Fig. 2c) shows lower bandgap of 1.8 eV, which is beneficial for improvements of photocatalytic performances.
It is well documented that the recombination of photogenerated electron-hole pairs reduced the photocatalytic activity as it prevents e − /h + to involve in redox reactions to form active radicals 16 .A lower PL emission intensity reflects the less recombination rate of the charge carriers on the surface 17 .The PL spectra of MIL-101(Fe) and M-MIL-101(Fe) are presented in Fig. 2d.As shown, M-MIL-101(Fe) exhibits a lower PL peak intensity compared to MIL-101(Fe), indicating the stronger separation rate of the electron-hole pairs 38 .
The FTIR analysis in the range of 400-4000 cm −1 was applied to determine the functional groups for MIL-101(Fe) and M-MIL-101(Fe) as shown in Fig. 2e.The broad vibrational band at about 3200-3700 cm -1 for both samples corresponds to the stretching mode of hydroxyl group 51,52 .In the spectrum of MIL-101(Fe), the absorption bands at 2935.90 and 2840.00 cm −1 are correspond to the C-H asymmetric and symmetric stretching vibrations, respectively.The band at 1661.10 cm −1 arises from the vibration of C=O bond.The bands at 1391.34 and 1598.80 cm −1 corresponds to the symmetrical and asymmetrical vibrations of the carboxylic group (-COO-), respectively.The bands at 749.86, 1017.80, and 1157.23 cm −1 are assigned to the C-H bending vibration, while the band at 1502.68 cm −1 showed the presence of C=C vibration in the benzene ring.The band at 554.25 cm −1 related to the Fe-O stretching vibration 36,53 .In M-MIL-101(Fe) spectrum, the bands at 553.92 and 477.48 cm -1 are assigned to the stretching vibration of Fe-O in α-Fe 2 O 3 54 .The week bands at 1393.89 and 1550.93 cm −1 can be attributed to presence of a portion of MIL-101(Fe) in α-Fe 2 O 3 .
The surface morphology of MIL-101(Fe) and M-MIL-101(Fe) was investigated by FE-SEM and shown in Fig. 3a and b, respectively.As shown in Fig. 3a, the prepared MIL-101(Fe) exhibited an octahedral structure with a smooth surface and clear edges.After calcination in air, the octahedron-like α-Fe 2 O 3 was successfully synthesized.Furthermore, the elemental mapping shows an almost uniform distribution of elements on M-MIL-101(Fe) surface in Fig. 3c-f.The EDX spectrum of M-MIL-101(Fe) (Fig. 3g) confirmed the presence of C, O, and Fe.

MIL-101(Fe) and M-MIL-101(Fe) removal efficiency.
To compare the dye elimination efficiency, adsorption and photo assisted catalytic degradation of MB were performed for the synthesized materials.A fixed dose (0.5 g L −1 ) of MIL-101(Fe) or M-MIL-101(Fe) added to the dye solutions (10 mg L −1 ) while the process completed for 90 min under LED light irradiation (catalytic degradation) or in the dark environment (adsorption).The difference between the adsorption + catalysis removal columns for the materials in Fig. 4 reflected the catalytic degradation of M-MIL-101(Fe) that is almost negligible for MIL-101(Fe).As seen, M-MIL-101(Fe) exhibited a superior adsorptive and catalytic performance against MB compared to pristine MIL-101(Fe).The adsorptive and catalytic removal efficiencies for M-MIL-101(Fe) were ~ 7.3% and 17.1% more than MIL-101(Fe), respectively.In addition to a supreme removal property, M-MIL-101(Fe) takes advantage of a good dispersibility and separability in water medium.Interestingly, M-MIL-101(Fe) could easily separate by magnetic field.Moreover, M-MIL-101(Fe) separate easier to produce a clear solution by centrifugation as it is denser than MIL-101(Fe).
Influence of operating parameters on MB degradation.To evaluate the photocatalytic performance of M-MIL-101(Fe), various parameters such as pH, catalyst dosage, and initial dye concentration were The effect of catalyst dose on the dye degradation was also studied in the range of 0.2-1 g L −1 and the results are shown in Fig. 5b.The photodegradation efficiency of MB improved significantly from 52.9 to 97.4% with an increase in the photocatalyst dosage from 0.2 to 1 g L −1 .This could be attributed to the increase in active sites for   MB adsorption and also generating more active species during the irradiation.A dose of 0.5 g L −1 was chosen for the following experimental steps.The pollutant concentration also is a significant parameter that affects process efficiency.Thus, MB removal in the concentrations of 5 and 10 mg L −1 were investigated.Figure 5c indicated the degradation efficincy was decreased by the dye concentration.A lower light penetration and decrease in the energy of photons reach the photocatalyst surface, leading to a decrease in the generated oxidizing radicals is a possible cause of drop in MB removal at the elevated concentrations.
Kinetic study.The kinetic of dye degradation was examined by two kinetic models i.e. the pseudo-first order and pseudo-second order.The non-linear forms of pseudo-first order and pseudo-second order kinetic models are given in Eqs. ( 4) and ( 5), respectively 56 : where k 1 and k 2 are the rate constants for pseudo-first order and the pseudo-second order, respectively.C 0 , C, and t are the initial concentration of MB, the concentration of MB at the time t, and reaction time, respectively.
Figures 6a and b shows the pseudo-first order and pseudo-second order models fitted on MB degradation data.The kinetic constants of k 1 and k 2 and statistical parameters i.e., the coefficient of determination (R 2 ), adjusted R-square R 2 adj , and the residual sum of square (RSS) for MB degradation by M-MIL-101(Fe) are outlined in Table 2. From the Table 2, it is clearly seen that the R 2 and R 2 adj values for pseudo-second order model are higher than the pseudo-first order model that indicate the degradation of MB obeys the pseudo-second order model.Furthermore, the smaller value of RSS for the pseudo-second order model than the other model confirms the process obeys this kinetic model.

Radical trapping tests.
To determine the mechanism of MB photocatalytic degradation using M-MIL-101(Fe), radical trapping of major active oxidation species was performed.AgNO 3 , IPA, AA, and KI were applied as the scavengers of electron ( e − ), hydroxyl radical ( OH • ), superoxide radical ( O •− 2 ), and hole ( h + ), respectively.As shown in Fig. 7, the MB removal efficiency was significantly reduced to 55.6 and 41.7% by adding AgNO 3 and IPA into the reaction system, respectively.These results reveal that OH • and electron radicals play a significant role in the photocatalytic degradation of MB.However, the AA and KI have a negligible effect on the removal efficiency, indicating that O •− 2 and h + are not the main active species for the photocatalytic degradation of MB.

MB removal from synthetic wastewater.
To study the dye removal in real condition in the presence of co-existing ions, dye degradation tests accomplished in a water spiked with MB.The characteristics of water matrix are presented in Table S1. Figure 8 compares MB removal percentage for dye solutions prepared by DIW (control) with those in real samples.Interestingly, the presence of co-existing species improved the removal efficacy and MB removal increased by 10.2% in real sample compared to the solutions prepared by DIW.The presence of coexisting ions in synthetic and real wastewater can significantly affect the photocatalytic removal of contaminants.The type and concentration of coexisting ions, as well as the type of catalyst, can influence the photocatalytic activity.Some earlier reports indicated an improvement in photocatalytic performance in the presence of coexisting ions.For instance, Gao et al. 57 indicated the photocatalytic degradation of carbamazepine improved in the presence of Ca 2+ , and Mg 2+ .However, HCO 3− , Cl − , and NO 3 − suppressed the removal due to the quenching effects they have on OH .and h + .In another study, SO 4 2-showed the highest inhibiting impact on MB removal, while chloride ions increased the degradation of MB by CuO-Cu 2 O nanocomposite 58 .
Reusability study.The stability and reusability of photocatalyst play a significant role in the operation of real treatment units.The recyclability of M-MIL-101(Fe) for photocatalytic degradation of MB was evaluated in two successive cycles as shown in Fig. 9.For that, the M-MIL-101(Fe) photocatalyst for each cycle was reused for the next cycle after washing with DIW and dring at 70 °C for 8 h.According to Fig. 9, the degradation efficiency   spectra recorded for fresh and exhausted material.In addition, the FE-SEM images of exhausted M-MIL-101(Fe) with different magnifications are shown in Fig. 10c and d.As can be seen, the physical morphology of some crystals affected by consecutive use-reuse cycles.Overally, the characteristic study of the exhausted M-MIL-101(Fe) indicated that in spite of some physical changes in morphology, the crystalline structure and surface functional groups have no significant changes during the photocatalytic process, confirming the good stability and reusability of the photocatalyst.

Conclusion
AOPs are promising techniques for non-selective abatement of pollutants and attract attentions against emerging contaminants such as dyes.Photocatalytic degradation is an AOP technique with economical and practical advantages, especially when the process operated under low powered light.A particular member of metal organic frameworks

Table 2 .
The kinetic constants and statistical parameters for MB degradation.