Synthesis of MIL-101(Cr)/Sulfasalazine (Cr-TA@SSZ) hybrid and its use as a novel adsorbent for adsorptive removal of organic pollutants from wastewaters

doi:10.21203/rs.3.rs-1438090/v1

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Synthesis of MIL-101(Cr)/Sulfasalazine (Cr-TA@SSZ) hybrid and its use as a novel adsorbent for adsorptive removal of organic pollutants from wastewaters

https://doi.org/10.21203/rs.3.rs-1438090/v1

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Metal-organic frameworks (MOFs) are considered strong adsorbents to the removal of organic pollutants due to their unique characteristics. In this work, a new type of metal-organic porous material MIL-101(Cr)/Sulfasalazine (Cr-TA@SSZ) hybrid successfully synthesized by a hydrothermal approach for the first time. The synthesized Cr-TA@SSZ and MIL-101(Cr) adsorbents were applied for adsorption of terephthalic acid (TA), para-toluic acid (p-tol), and benzoic acid (BA). The Cr-TA@SSZ and MIL-101(Cr) were characterized by the general tests including X-ray diffraction (XRD), Fourier transform infrared (FTIR), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), Scanning electron microscope(SEM), thermal gravimetric analysis (TGA), differential thermal analysis (DTA), zeta potential, and Elemental analysis (EDX). Based on the above analyses, it was concluded that Cr-TA@SSZ has a different composition and network structure to the MIL-101(Cr). The formula for new MOF (Cr-TA@SSZ) proposed as: Cr₃F (H₂O)₂O[C₆H₄(CO₂)₂][C₆H₃N(OH)(CO₂)], 2.5H_2.

The experiments for evaluating the effect of the different parameters such as pH, initial concentration, contact time, and temperature on the removal of the terephthalic acid (TA), para-toluic acid (p-tol), and benzoic acid (BA) were carried out in batch mode. The isotherm, kinetic and thermodynamic models were also analyzed for the adsorption of TA, p-tol, and BA. Equilibrium adsorption was evaluated employing Langmuir, Freundlich, Temkin, and Redlich–Peterson equations, in which Langmuir and Redlich–Peterson models were in good agreement with the experimental results. Maximum adsorption capacity (q₀) of the Cr-TA@SSZ for terephthalic acid (TA), para-toluic acid (p-tol), and benzoic acid (BA) were obtained 2208.4 mg.g^− 1, 1241.2 mg.g^− 1, and 1009.5 mg.g^− 1, respectively while for MIL-101(Cr) were obtained 1692.0 mg.g^− 1, 952.4 mg.g^− 1, and 769.2 mg.g^− 1 respectively. The Cr-TA@SSZ was found to be more efficient in the removal of terephthalic acid (TA), para-toluic acid (p-tol), and benzoic acid (BA) from water than the MIL-101(Cr). Also, the results showed that a pseudo-second-order kinetic model with a higher correlation coefficient (R² > 0.99) matched well for the adsorption of terephthalic acid (TA), para-toluic acid (p-tol), and benzoic acid (BA) onto MIL-101(Cr) and Cr-TA@SSZ. The thermodynamic parameters such as a change in Gibbs free energy (ΔG), enthalpy (∆H), and entropy (ΔS) were determined and the negative values of ΔG indicated that the process of removal was spontaneous at all values of temperatures. Further, the values of ∆H indicated the exothermic nature of the removal process. Moreover, adsorption experiments using industrial wastewater from a TA production plant showed that Cr-TA@SSZ‌ can be used as a promising adsorbent in the adsorptive removal of organic pollutants from wastewaters. This MOF was able to remove 40% COD from the concentrated phase (equivalent to 13000 ppm) and remove 77.3% COD from the diluted phase (equivalent to 4250 ppm) wastewater.

Metal-organic framework (MOF)

MIL-101(Cr)/Sulfasalazine (Cr-TA@SSZ)

MIL-101(Cr)

Adsorption

Annually, large quantities of organic compounds enter the water and the environment through various industrial wastewaters such as petrochemical, paper making, plastics, textile, and leather industries. The presence of organic compounds in water and wastewater endangers the ecosystem and human health. Therefore, the reduction of hazardous pollutants in water and wastewater must be considered [1–3].

Among the organic compounds, aromatic materials derivatives are widely used in industries such as petrochemical companies, feed industrial units, food preservatives, and pharmaceutical industries [4, 5]. For example, the wastewater of purified terephthalic acid (PTA) manufacturing plants include p-toluic acid (p-Tol), benzoic acid (BA), 4-carboxybenzaldehyde (4-CBA), phthalic acid (PA), and terephthalic acid (TA) [6–8], which aromatic compounds make up to 75% of the COD (chemical oxygen demand) of the wastewaters that should be removed and reach to the standard level of water and wastewater [6].

In this regard, there are various technologies to remove the cyclic pollutants such as ozonation, catalytic ozonation, photocatalyst, cooling membrane, separation, and adsorption [7, 8]. Each of them has advantages and disadvantages. However, among these methods, the adsorption process was more used because of the inexpensive process, simple control, and the possibility of adsorbent recovery [9–11]. The most common adsorbents for this purpose are silica gel, activated carbon, zeolite, organoclay, carbon nanotubes, and other nanoadsorbent [12].

The Metal-organic frameworks (MOFs), known as coordination polymers, are a class of highly ordered porous crystalline hybrid materials consisting of metal clusters and multifunctional organic linkers [13] that have wide applications in different areas, including adsorption, separation, sensing, catalysis, and proton conduction, as well as in some emerging technologies, e.g., drug delivery, light-harvesting, photocatalysis, biocatalysis, and nanofluids [14–20].

The particular interest in MOF materials for various purposes such as adsorption is due to the easy tunability of their pore size and shape from a microporous to a mesoporous scale by changing the connectivity of the inorganic moiety and the nature of the organic linkers [18], different topology with a different molecular structure such as linear, T&Y shaped, square-planar, square-pyramidal, octahedral, trigonal-bipyramidal, trigonal-prismatic, pentagonal-bipyramidal and other special properties such as thermal and chemical stability, high surface area and functionality [13, 21, 22]. It should be noted that in the synthesis of a metal-organic framework, the proper selection of metal bands or organic ligands can bring us closer to our desired purpose and application. Also, choosing the type of organic compound directly affects the pore size distribution, and the type of metal, due to its inherent characteristics and empty orbitals, will have a unique effect on the adsorption process [18, 23].

Chromium (Cr) is one of the most preferred metals for the manufacture of MOFs due to its strong and regular bonds. Furthermore, Chromium, due to its unique properties between other transition metals. The 4s and 3d orbitals of this element produce oxidation states of + 2 to + 6 by electron displacement, of which the + 3 oxidation states are the most stable and the + 4 and + 5 states are relatively rare. Chromium in the + 6 oxidation state is a strong oxidizer that makes it prone to a large number of bonds. So, it seems that this metallic element can play an important role in creating stable and resistant 3D clusters. On the other hand, Chromium salts are easily accessible and almost inexpensive as raw materials for the synthesis of MOFs [24]. MIL-101(Cr) is a three-dimensional porous MOF based on Chromium terephthalate, which has various applications. The powerful structure, high surface area, and special characteristics led to many studies on various methods of synthesis, functionalization, encapsulation, and its compositing with various compounds to modify and use them in processes such as adsorption, catalytic processes, gas storage, drug release, etc. [25].

Ferey et al. [26] for the first time introduced and developed MIL-101(Cr). Haque et al. [11] have employed chemically modified MIL-101(Cr) for the adsorptive removal of methyl orange from water. They concluded that the adsorption capacity of PED-MIL-101(Cr) is 194 mg/g, which is more than 16 times that of activated carbon. Jhung et al. [27, 28] have synthesized MIL-101(Cr) by a microwave method. The results indicated that the MIL-101(Cr) has a larger capacity than SBA-15, HZSM-5 zeolite, and activated carbon in the adsorption of benzene. Chang et al. [27] reported the benzene adsorption equilibrium on MIL-101(Cr) and revealed that this material had an intricate structure and demonstrates huge, rapid adsorption of benzene, making it a good candidate for the removal of some harmful organic contaminants. Zhao et al. [29] studied the adsorption and diffusion of benzene on Chromium-based metal-organic framework MIL-101(Cr) synthesized by microwave irradiation. Results showed that the maximum amount adsorbed of benzene on the synthesized MIL-101(Cr) was up to 16.5 mmol.g^− 1 at 288 K and 56.0 mbar. Diffusion coefficients of benzene within the MIL-101(Cr) were in the range of (4.25–4.76) ×10^− 9 cm².s^− 1 in 288–318 K with a lower activation energy of 2.41 kJ.mol^− 1. Furthermore, a large number of studies have been conducted on the functionalization of MOFs with different organic or inorganic compounds to optimize the capability of MOF for applications such as adsorption or catalytic processes [30–32]. Functionalization is usually performed with amine, nitro, or sulfur compounds such as ethylenediamine (ED), diethylenetriamine (DETA), aminoproxytrialkoxylan (ASP), nitric acid, sulfuric acid, etc. under special conditions. In some cases, functionalization may reduce the BET area of the final structure compared to the original MOF structure, but due to the proper and uniform distribution of the active phase within the cavities and internal surfaces, the performance of the porous material was increased. On the other hand, selecting an appropriate auxiliary ligand during the synthesis process, which has a synergistic effect with MOF, can lead to a more active surface or larger pore size to improve the adsorption process.

However, it seems that less study and research has been done on the integration of two or more metals or organic ligands to create a new structure with higher capabilities than the original network. Despite the good performance of MOFs, there is a need for structures and networks with superior capabilities, especially for purposes such as catalyst, adsorption, and storage [33].

In this work, for the first time, MIL-101(Cr) was modified by Sulfasalazine as an auxiliary ligand. MIL-101(Cr)/Sulfasalazine (Cr-TA@SSZ) hybrid synthesized by the hydrothermal method for removal of terephthalic acid (TA), para-toluic acid (p-tol), and benzoic acid (BA) from water. X-ray diffraction (XRD), Fourier transform infrared (FTIR), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), Scanning electron microscope(SEM), thermal gravimetric analysis (TGA), differential thermal analysis (DTA), zeta potential, and Elemental analysis (EDX) analyses were used for characterization of adsorbents. Adsorption experiments were performed to investigate the effects of some important parameters such as pH, contact time, temperature, and the initial concentration of BA, TA, and p-tol on the adsorptive behavior of MOFs. Also, the isotherm models, adsorption kinetics, and thermodynamics of BA, TA, and p-tol adsorption processes were evaluated. Finally, the application of Cr-TA@SSZ and MIL-101(Cr), as adsorbents to real wastewater was also compared.

2.1. Material and reagents

For the synthesis of Cr-TA@SSZ and MIL-101(Cr), Sulfasalazine (C₁₈H₁₄N₄O₅S, 98%), Chromium (III) nitrate nonahydrate [Cr(NO₃)₃·9H₂O, 98%], terephthalic acid (TA, 99%), hydrofluoric acid (HF, 40%), and hydrochloric acid (HCL, 37%) were used. Ethanol (dried, 99.98%) and N, N, dimethylformamide (DMF, 99%) were used for the purification of the synthesized MOFs. Benzoic acid (BA, 99%), p-toluic acid (p-tol, 99%), and terephthalic acid (TA, 99%) were applied for adsorption experiments. All chemicals were of analytical grade, purchased from the Merck Company except Sulfasalazine from Sigma-Aldrich Company.

2.2. Synthesis of MOFs

2.2.1. Synthesis of MIL-101(Cr)

MIL-101(Cr) was synthesized according to the method described in our previous works [6, 34] where 2 g of Chromium nitrate (5 mmol), 0.83 g of TPA (5 mmol), 0.25 ml HF, and 25 ml ultrapure water were well mixed for 30 min followed by ultra-sonication for 15 min. The mixture was then transferred into a 50 ml Teflon-lined stainless steel autoclave, sealed, heated to 220 ^oC for 8 h, and subsequently cooled to room temperature. The green suspension of MIL-101(Cr) was then filtered using a Buchner funnel with a glass filter to remove the re-crystallized needle-shaped colorless TPA which remained on the filter and the MIL-101(Cr) suspension which passed through it. The suspension was again filtered by soft filter paper (2–3 µm) to recover the green-colored products. To remove the unreacted TPA, the as-synthesized MIL-101(Cr) was further washed (twice) by 30 ml DMF at 70 ^oC for 8 h with continuous stirring. After filtration, the synthesized MIL-101(Cr) was washed (twice) by 30 ml dried ethanol for 8 h to remove the DMF and additional water and was subsequently filtered again. The purified MIL-101(Cr) was dried overnight at 150°C and stored in a desiccator after cooling to room temperature.

2.2.2. Synthesis of MIL-101(Cr)/Sulfasalazine (Cr-TA/SSZ)

For the synthesis of Cr-TA@SSZ‌, the same method as the synthesis of MIL-101(Cr) was used, except that the Sulfasalazine was added to the mixture with a molar ratio of 0.38 to Chromium nitrate. Also, the amount of ultrapure water and terephthalic acid added to the mixture was about 1.5 times that of the previous method. Briefly, Chromium (III) nitrate nonahydrate (2 g), terephthalic acid (1.25 g), Sulfasalazine (0.75 g), and 0.25 ml HF (equivalent to 5 drops) were added to 40 ml ultrapure water. The mixture was well mixed for 30 min followed by ultra-sonication for 15 min and then transferred into a 50 mL Teflon lined autoclave reactor and heated at 220°C for 8 h (in this case, the molar ratio of the mixture will be 1 [Cr nit.]; 1.5 [TPA]; 2 [HF]; 0.38 [SSZ]). The reaction product was then cooled to room temperature by natural convection. Then, as in the previous method, the green suspension was collected by soft filter paper (2–3 µm) after passing through the Buchner funnel and glass filter. To remove the unreacted terephthalic acid molecules, the as-synthesized Cr-TA@SSZ was added to 30 mL of dimethylformaldehyde (DMF) and placed under the stirring at 70°C for 8 h. This process was repeated twice. After filtration, the product was rewashed and filtered again under stirring with 30 mL of dried ethanol for 8 h to remove DMF and extra water from the Cr-TA@SSZ structure. Finally, the purified Cr-TA@SSZ was dried overnight at 150°C and, to avoid adsorbing moisture was stored in a desiccator after cooling to room temperature.

2.3. Characterizations

Powder X-ray diffraction (XRD) patterns of the samples were obtained in an XPert Pro diffractometer (Panalytical Corp., Netherlands) using Cu Kα radiation and a wavelength of 1.79 Å to determine the crystalline phases of the MOFs. Fourier transform infrared (FT-IR) spectra for the MIL-101(Cr), and Cr-TA@SSZ were recorded using Bruker-vector-22 (Germany) with a range of 400–4000 cm^− 1. Pretreatment was performed by grinding the samples powder together with KBr in an agate mortar and then by pressing them into flakes by a tablet machine. Textural properties of the adsorbents were determined using a Micromeritics ASAP 2010 nitrogen sorption instrument. For these measurements, the powder was degassed and activated under reduced pressure at 105°C for 3 h. The morphology of the prepared samples was obtained by transmission electron microscopy (TEM) images that were taken at 80 kV by a JEM 1230 (JEOLCorp., Japan) and scanning electron microscope (SEM) measurements were performed from a ZEISS DSM960A electron microscope. The thermal stability of MIL-101(Cr) and Cr-TA@SSZ samples were investigated by thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) with Mettler Toledo (Australia). The samples were heated under N₂ atmosphere from room temperature to 800 ºC at 10 ºC/min. A dynamic backlight scattering technique (Malvern ZS Nano series) was applied to measure the zeta potential (pζ) of the particles. The pζ-pH curves were obtained using the MPT-2 autotitrator in conjunction with the Malvern Zetasizer Nano. The autotitrator was connected to a sulphide resistant pH probe which was inserted into the sample measurement cell. Sample pH adjustment was determined by the addition of either 0.1M HCl or 0.05 M NaOH. Finally, the elemental analysis of MIL-101(Cr) and Cr-TA@SSZ samples were carried out by energy dispersive X-ray analysis (EDX, Oxford-Inca).

High-performance liquid chromatography (HPLC) was applied to determine the concentration of TA, p-tol, and BA concentrations before and after the adsorption experiment. HPLC was performed with a waters model 600E instrument equipped with a 150 mm long C18 column and a UV detector with a wavelength set at 230 nm. The mobile phase consisted of 21% (by volume) acetonitrile, 0.1% trifluoroacetic acid, and 78.9% HPLC-grade water.

2.4. Adsorption assay

To determine the superior adsorbent, the adsorption assay was applied. Before comparing the adsorption performance of two adsorbents in a real industrial effluent, the adsorption rate of the three organic compounds (TA, p-tol, and BA) was investigated in the laboratory artificial effluent. For this purpose, the solutions of TA, p-tol, and BA with desired concentrations and pHs were prepared using deionized water and ammonia (or caustic). Then, the solution was measured before and after the experiment by HPLC‌ (High-performance liquid chromatography). In this way, before each experiment, the adsorbents were dried overnight under vacuum at 100 ^oC. Then, an exact amount of the adsorbents (7.0 mg) was placed in the aqueous solution (20 ml) with desired concentration (in the range of 100 to 3000 mg/l). The solutions containing the adsorbent were kept well-mixed using magnetic stirring for a specified time (15 min to 24 h) at a constant temperature (25^oC). At the end of the experiment time, the solution was filtered with a soft paper filter and the filtrate was analyzed to determine the remaining pollutant concentration.

To determine the effect of pH on the adsorption capacity, the pH value of the solutions was adjusted with the addition of 0.05 M NaOH or 0.1 M HCl aqueous solutions. The amount of TA, p-tol or BA adsorbed on the adsorbents was calculated by the following mass balance:

\({q}_{t}=\frac{{(C}_{0}-{C}_{t})}{m}\times v\) (1)

\({q}_{e}=\frac{{(C}_{0}-{C}_{e})}{m}\times v\) (2)

Where \({q}_{t}\) (mg.g^− 1) and\({ q}_{e}\) (mg.g^− 1) represents the adsorption capacity at time t and equilibrium adsorption capacity, C₀ (mg.L^− 1) and C_t (mg.L^− 1) are the initial concentration and concentration of adsorbate at time t, ν (L) and m (g) are the volume of the solution and the mass of the adsorbent, respectively. In addition, the removal percentage of organic contents was measured by the following equation:

\(\%Removal=\left(\frac{{C}_{0}-{C}_{e}}{{C}_{0}}\right)\times 100 \left(\%\right)\) (3)

Where C_e is equilibrium concentrations of organic pollutants in wastewater solution (mg.L^− 1).

2.4.1. Adsorption isotherms

To evaluate the adsorption process, the adsorption isotherms which relate the equilibrium mass of adsorbed material in the adsorbate phase (q_e) to its equilibrium concentration in the solution phase C_e are usually considered. Adsorption isotherms such as Langmuir, Freundlich, Temkin and Redlich- Peterson were used to fit the experimental data.

The Langmuir isotherm presumes that adsorption is monolayer and the adsorbent surface is homogenous which means that all the adsorbent sites have the same affinity for the adsorbate [1]. The linear form of the Langmuir isotherm is given as follows:

\(\frac{{\text{C}}_{\text{e}}}{{\text{q}}_{\text{e}}}=\frac{{\text{C}}_{\text{e}}}{{\text{q}}_{\text{o}}}+\frac{1}{{\text{q}}_{\text{o}}{\text{K}}_{\text{L}}}\) (4)

Where q_o is the maximum adsorption capacity (mg.g^− 1) and K_L is the Langmuir adsorption constant related to adsorption energy (L.mg^− 1). To understand whether the adsorption is favorable or not, the following parameter (R_L) is introduced. Indeed, the less the R_L, the more favorable the adsorption [1, 35].

\({\text{R}}_{\text{L}}=\frac{1}{1+{\text{K}}_{\text{L}}{\text{C}}_{0}}\) (5)

Freundlich isothermal model is assuming that the adsorption process takes place on a heterogeneous surface as a multilayer. The linear form of the Freundlich adsorption isotherm is given by the following equation:

\(ln{q}_{e}=ln{K}_{F}+\left(\frac{1}{n}\right)ln{C}_{e}\) (6)

Where K_F (L.mg^− 1) and n are Freundlich adsorption constants, the value of 1/n represents the surface heterogeneity. When it is greater than unity, the adsorption is favorable.

K_F can be defined as the adsorption or distribution coefficient and it represents the number of organic contaminants adsorbed onto adsorbents for a unit equilibrium concentration [36, 37]

Another isotherm considered in this paper is the non-linear Temkin isotherm which is taking into account the effects of indirect adsorbate/adsorbate interactions on the adsorption process. Furthermore, the isotherm assumes that the heat of adsorption should decrease linearly with the surface coverage. The linear form of Temkin is given as follows:

\({q}_{e}=BlnA+Bln{C}_{e}\) (7)

where B = RT/b_T is a constant related to the heat of sorption (J.mol^− 1), b_T is the Temkin isotherm constant, A is the Temkin isotherm equilibrium binding constant (L.g^− 1), R is the universal gas constant, and T is the absolute temperature [38].

Non-linear three parametric Redlich-Peterson isotherm which is a combination of Langmuir and Freundlich isotherms is generally described as follows:

\({q}_{e}=\frac{{K}_{R}{C}_{e}}{1+{a}_{R}{C}_{e}^{\beta }}\) (8)

Where K_R (L.g^− 1), a_R (L.mg^− 1) are the Redlich-Peterson (R-P) isotherm constants and β is the R-P isotherm exponent which can be used for both homogeneous and heterogeneous systems [38]. At the limit state, the R-P isotherm tends to the Freundlich isotherm at high concentrations (as the exponent β tends to zero). It also fits well with the Langmuir isotherm at low concentrations (as the β values are all close to one) [6, 38].

2.4.2. Adsorption kinetics

To study the adsorption of adsorbates onto adsorbents and to interpret the results, experimental data obtained were fitted with different kinetic models such as the pseudo-first-order, the pseudo-second-order, and an intraparticle diffusion model. These models can be expressed as:

Pseudo-first order model \(ln\left({q}_{e}-{q}_{t}\right)={ln}{q}_{e}-{k}_{1}t\) (9)

Pseudo-second order model \(\frac{t}{{q}_{t}}=\frac{1}{{k}_{2}{{q}_{e}}^{2}}+\frac{1}{{q}_{e}}t\) (10)

Intra-particle diffusion model \({q}_{t}={k}_{d}{t}^{0.5}+F\) (11)

where q_e and q_t (mg.g^− 1) are the uptakes of adsorbates at equilibrium and at time t (min), respectively, while k₁ (min^− 1) is the constant of the pseudo-first-order kinetic model, k₂ (g.mg^− 1 min^− 1) the rate constant of the second-order equation, k_d (mg.g^− 1 min^− 1/2) is the intraparticle diffusion rate constant and F (mg.g^− 1) is a constant related to the thickness of the boundary layer. These parameters were obtained by a least-squares optimization technique from MATLAB [37].

2.4.3. Adsorption thermodynamics

Thermodynamic behavior of adsorbates onto adsorbents also was evaluated by the thermodynamic parameters including the changes in Gibbs free energy (\(\varDelta G\)), changes in enthalpy (\(\varDelta H\)), and changes in entropy (\(\varDelta S\)). These parameters are calculated from the following equations:

\(\varDelta G=-RTln{K}_{L}\) (12)

\(ln{K}_{L}=\frac{\varDelta S}{R}-\frac{\varDelta H}{RT}\) (13)

where K_L(L.mg^− 1) is the Langmuir isotherm constant. The enthalpy change (ΔH) and entropy change (ΔS) of adsorption were obtained from the van’t Hoff plot of ln (K_L) against 1/T from the slope and intercept [39].

3.1. Characterization of the adsorbents

The X-ray diffraction patterns of MIL-101(Cr) and Cr-TA@SSZ are presented in Fig. 1. The most important diffraction peaks of MIL-101(Cr) occur at 3, 5, 6, and 9 degrees [26], indicating the proper crystalline structure of synthesis MIL-101(Cr). Fortunately, the reported results of the XRD spectrum of our work for MIL-101(Cr) have been matched well with the XRD pattern of MIL-101(Cr) previously published by other researchers [6, 26]. Two small peaks in the range of 2 and 17 degrees are also related to the presence of terephthalic acid in its structure [6].

The differences in the XRD pattern of Cr-TA@SSZ compared to MIL-101(Cr) could indicate the formation of a new composition. Usually, the peaks below 2θ = 5^o confirm the mesoporous structure of this porous material. The additional peak at the 2-degree angle for Cr-TA@SSZ indicates that it is more mesoporous than the MIL-101(Cr). Also, the existence of three new peaks at 9.5, 11, and 17 degrees can be a sign of creating a new structure [40].

It should be noted that in the XRD spectrum of Sulfasalazine available in various sources [41] there are peaks at angles of 5, 9.5, 11, and 17 that the presence of new peaks in these ranges for the Cr-TA@SSZ can be justified the presence of part of Sulfasalazine in its structure [41]. Of course, it seems that the crystalline structure of this compound is similar to MIL-101(Cr) which has not been created a fundamental difference in their XRD peaks [6, 34].

The FTIR spectra for MIL-101(Cr) and Cr-TA@SSZ are shown in Fig. 2. Due to the presence of similar compounds and functional groups, most of the peaks in Cr-TA@SSZ are similar to MIL-101(Cr). The peaks near 577 cm^− 1 are most likely related to in-plane and out-of-plane bending modes of COO-groups [34]. The Peaks at 1400 and 1703 cm^− 1 can be assigned to the carboxylic –COOH groups [40]. The peaks in 749 and 1018 cm^− 1 are also related to the vibration of benzene rings [29]. Peaks of the 2027 cm^− 1 and 2800–2950 cm^− 1 wavelength are related to C–H tensile band and the peaks in the range of 3413 cm^− 1 wavelength can represent the existence of O–H, N–H, or R-OH phenolic ring [34]. In the FT-IR spectroscopy of Cr-TA@SSZ which is given in Figure 2, there is a strong peak in the range of 1621 cm^− 1 wavelength that can prove the presence of N-H-bonds in this compound [41].

Nitrogen adsorption − desorption isotherm curves of MIL-101(Cr) and Cr-TA@SSZ are shown in Fig. 3. As illustrated in Fig. 3, the adsorption isotherm of MIL-101(Cr) are based on type I of IUPAC isotherm, which indicates the microporosity of the mentioned adsorbents and adsorption/desorption isotherm for Cr-TA@SSZ is similar to type IV of IUPAC isotherm representing the mesoporosity of this adsorbent. The results of ASAP analysis show that the BET surface area, the Langmuir surface area, the average pore diameter, the total pore volume, and the V_micro/V_tot of MIL-101(Cr) are 2390 m².g^− 1, 3301 m².g^− 1, 2.23 nm, 1.32 m³.g^− 1, and 0.74 respectively while for Cr-TA@SSZ are equal to 3120 m².g^− 1, 4952 m².g^− 1, 4.3 nm, 1.73 m³.g^− 1, and 0.28 respectively. Thus, the new compound Cr-TA@SSZ has a higher mesoporous volume than MIL-101(Cr), indicating that the Sulfasalazine auxiliary ligand can modify the size of the adsorbent pores. The adsorption analysis of two MOFs revealed that there was an increase in the total surface area, total pore diameter, and total pore volume for the Cr-TA@SSZ in comparison with the MIL-101(Cr), and thus, any improved performance achieved with the hybrid material Cr-TA@SSZ can be explained by an increase in the surface area or changes in the pore structure.

The BET results obtained in this investigation are in good agreement with those reported in the literature [42, 43]. The improvement in adsorption performance for Cr-TA@SSZ hybrid is known to occur when the combination of two complementary materials MIL-101(Cr) and Sulfasalazine can result in a synergistic effect on important properties [40].

Zetasizer is used to measure the crystals size and particles size distribution of two compounds MIL-101(Cr) and Cr-TA@SSZ by Malvern ZS nano series and represented in Fig. 4. According to the diagram, the size of Cr-TA@SSZ crystals is approximately half that of MIL-101(Cr) crystals. This result was also predictable due to the increase in XRD peak width of Cr-TA@SSZ in the range of 8–12 degree angles.

To investigate the surface morphology and the structure of the samples, MIL-101(Cr) and Cr-TA@SSZ samples were characterized by scanning electron microscopy (see Fig. 5a and Fig. 5b) and transmission electron microscopy (see Fig. 5c and Fig. 5d). From Fig. 5a and Fig. 5c, it can be seen that MIL-101(Cr) possesses a typical octahedral structure with perfect cubic symmetry. According to Fig. 5c and Fig. 5d, it can be concluded that after modification of MIL-101(Cr) with Sulfasalazine, the particle size of Cr-TA@SSZ tends to be smaller and more uniform. The results conform to the consequences of XRD and zetasizer analysis.

The TGA and DTA profiles of synthesized MIL-101(Cr) and Cr-TA@SSZ are shown in Fig. 6, both MOFs have desirable thermal stability. The starting point of the compound structure destruction is the beginning of its intense weight loss in the TGA chart or the beginning of an intense endothermic peak in the DTA chart. According to Fig. 6, MIL-101(Cr) and Cr-TA@SSZ are stable until reaching temperatures of 350°C. The initial weight losses occur at 100°C is related to the surface evaporation process and corresponds to the removal of water or ethanol molecules [6]. The second weight loss occurred between 150°C–200°C might be the result of the release of DMF from the pore of adsorbents[34]. The peak in the 380 and 450°C is related to the separation of structural water and various organic compounds from the framework structure [44]. The big weight loss that took place between 352 and 555°C was attributed to the destruction of the framework structure [6, 45]. According to the TGA analysis, Cr-TA@SSZ appears to have better network stability due to a total weight loss of about 56%, which is about 7% less than the MIL-101(Cr).

The zeta potential of the adsorbent is one of the important parameters to determine the behavior and the amount of adsorption. The zero point charge (pH_PZC) is defined as the point where the zeta potential at the adsorbent surface is zero. As shown in Fig. 7, the zero point charge of Cr-TA@SSZ is about pH = 6, while this point for MIL-101(Cr) is about pH = 9.5. Also, the zeta potential of the MIL-101(Cr) which was modified by the Sulfasalazine auxiliary ligands changed to lower values than the MIL-101(Cr) in all pH ranges due to adsorption of anionic functional groups on the surface MIL-101(Cr). So, it can be concluded that the Cr-TA@SSZ has a different composition and framework but is close to the MIL-101(Cr).

In order to find the chemical formula of Cr-TA@SSZ and to estimate its network structure, Elemental analysis (EDX) is required. The elemental analysis of the two compounds was performed and shown in Table 1. According to Table 1, the amount of Chromium, hydrogen, and fluorine in Cr-TA@SSZ‌ is higher than in MIL-101(Cr) while it contains less carbon and oxygen. Also, according to the results of elemental analysis, nitrogen is present in the Cr-TA@SSZ framework structure. The amount of 0.51 wt.% sulfur (S) is also due to the remaining impurities of Sulfasalazine (Sulfapyridine) in the structure [46].

Table 1

Elemental analysis (EDX) of MIL-101(Cr) and Cr-TA@SSZ‌.
S (wt%)	F (wt%)	O (wt%)	N (wt%)	H (wt%)	C (wt%)	Cr (wt%)	Material
0.51	2.79	32.80	2.10	3.10	32.20	26.5	Cr-TA@SSZ
-	2.64	33.38	-	2.23	40.06	21.7	MIL-101(Cr)
UOP 864	F Analyzer	O Analyzer	ASTM D5291			ICP	Analysis method

As it turns out, the Sulfasalazine molecule is made up of two smaller molecules, 5-aminosalicylic acid (5-ASA) and Sulfapyridine (SPD). Referring to the previous works [41, 46–49], the active ingredient in this compound appears to be 5-aminosalicylic acid. As shown in Fig. 8, after entering the reaction medium, Sulfasalazine decomposes at the N = N double bond site, and the resulting 5-ASA molecule, due to its greater affinity, participates in the reaction to form a new structure. Due to the presence of nitrogen, OH‌, and a carboxyl group in the 5-ASA and the good activity of this substance at low pHs, this compound can be a suitable auxiliary ligand to improve the adsorbent structure of this work.

So, referring to the previously discussed tables and diagrams and the results of elemental analysis, the proposed formula of the new structure Cr-TA@SSZ was obtained as follows:

Cr₃F (H₂O)₂O[C₆H₄(CO₂)₂][C₆H₃N(OH)(CO₂)], 2.5H₂O

For this compound, molecular weight, MW = 586 gr.mol^− 1, and bulk specific density relative to MIL-101(Cr), sp.gr = 0.6 were obtained. Finally, the 2-D structure of the Cr-TA@SSZ network was proposed as shown in Fig. 9. The R in Fig. 9 is an input –COO connection or output –COO connection from repeated network units. As shown in Fig. 9, the Cr-TA@SSZ framework has two types of trigonal and decagonal windows to construct the mesoporous cages in the 3D network which this type of molecular arrangement ultimately leads to a higher average pore diameter than MIL-101(Cr) (which has two types of pentagonal and hexagonal windows). According to the ASAP analysis, as mentioned above, the average pore diameter of Cr-TA@SSZ is twice the diameter of pores in MIL-101(Cr), approximately. Also, the SEM and TEM images are shown in Fig. 5 confirm the regular and almost circular crystal-shaped proposed structure of this compound. Based on these results, it is expected that Cr-TA@SSZ due to higher mesoporous porosity, higher surface area and volume, a higher percentage of Chromium in its framework structure (which leads to more active metal sites), and having a functional group -OH in the network (which can play an important role in creating hydrogen bonds with guest molecules) can be more successful in adsorbing various compounds, especially organic compounds, than MIL-101(Cr).

3.2. Effect of pH on the adsorption of benzoic acid (BA), terephthalic acid (TA), and p-toluic acid (p-tol)

The pH of the medium is an important parameter to control the adsorption of organic compounds from the aqueous solution on the adsorbent surface. Changes in pH often affect the zeta potential of the adsorbents as well as the degree of ionization of the adsorbents resulting in a change in electrostatic interaction between adsorbent-adsorbate. This is because opposite electrostatic charges are absorbed by each other, while equal electrostatic charges repel each other [50].

To investigate the pH effect on adsorption of terephthalic acid (TA), p-toluic acid (p-tol), and benzoic acid (BA) on both adsorbents MIL-101(Cr) and Cr-TA@SSZ, first, the uptake of three substances (TA, p-tol, and BA) with a constant concentration of 2000 ppm, at ambient temperature and fixed adsorption time of 24 hours was examined. The result of this experiment is shown in Fig. 10 and Fig. 11.

As shown in Fig. 10 and Fig. 11 the MIL-101(Cr) and Cr-TA@SSZ exhibited similar sorption trends for TA‌, p-tol, and BA in the whole pH range, but their adsorption capacities were different. The adsorption rate decreases with the increasing pH of the solution. Comparing Figs. 10 and 11 with Fig. 7, it can be concluded that the chemical nature of the surface of the adsorbents has a great effect on the adsorption rate of these organic compounds [6].

As we know, TA, p-tol, and BA are the weak acids with pKa 3.51 and 4.82 for TA, 4.36 for p-tol, and 4.19 for BA, respectively (at room temperature)[6, 51]. At pH level below pKa, the acid-base interaction (the molecular form of the adsorbent) determines the amount of adsorption, while at higher pH, due to the deprotonation of the solute, the ionized form of the adsorbents has a greater effect on the quality and quantity of adsorption (the electrostatic interaction is more effective) [52].

Therefore, the molecular and ionized forms of the adsorbents behave differently on the adsorbent surface. In Fig. 12 the diagrams of the charge distribution and the concentration distribution of the three desired organic solutes based on pH are shown.

At higher pH, due to the increase in OH^- ions as well as the increase in the degree of ionization of the adsorbents, a repulsion occurs between the negative surface of the adsorbent and the ionized adsorbate, resulting in a decrease in the electrostatic interaction of the anionic molecules TA, p -tol or BA with adsorbent surface and reduced adsorption capacity [52].

It should be noted that because TA and p-tol tend to precipitate at pH below their pKa (molecular form), to prevent errors, experiments were performed for these two substances from pH = 5. As result, TA‌ and p-tol at pH = 5 and BA at pH = 2 ~ 5 have the highest adsorption rate for the Cr-TA@SSZ, while the highest uptake for MIL-101(Cr) was obtained for all three solutes at pH = 5.

3.3. Effect of initial concentration of benzoic acid (BA), terephthalic acid (TA), and p-toluic acid (p-tol) onto MIL-101(Cr) and Cr-TA@SSZ

The initial concentrations of benzoic acid (BA), terephthalic acid (TA), and p-toluic acid (p-tol) affect removal efficiency indirectly by either increasing or decreasing the availability of binding sites on the adsorbent [50]. Generally, an increase in the initial concentration of adsorbates in the solution will cause the adsorption sites on the adsorbent surface to become saturated, which eventually leads to a decrease in the removal efficiency. Indeed, at the low initial concentration of the organic solution, the amount of adsorbent active sites is higher in comparison with the total number of organic pollutants. So, the organic contents could interact with the adsorbent easier to occupy the active sites [1]. As a result, their removal from the solution would be simpler. The results of the percentage removal of benzoic acid (BA), terephthalic acid (TA), and p-toluic acid (p-tol) onto Cr-TA@SSZ and MIL-101(Cr) are presented in Fig. 13a and Fig. 13b. As shown in Fig. 13, the Cr-TA@SSZ has the highest percentage removal of benzoic acid, terephthalic acid (TA), and p-toluic acid (p-tol) at an initial concentration of 100 ppm (81%, 90%, and 87% respectively), while the highest percentage removal for the MIL-101(Cr) are 70%, 84%, and 72% respectively. As a result, it can be concluded that the Cr-TA@SSZ are more efficient than the MIL-101(Cr) in the percentage removals of mentioned adsorbates.

3.4. Effect of contact time

Figure 14a and Fig. 14b show the diagram drawn between the amount of terephthalic acid (TA) and p-toluic acid (p-tol), and benzoic acid (BA) adsorbed (q_t) versus time, t, at a constant initial concentration of 2000 mg/l. The time variation diagram shows that the removal of mentioned solutes is rapid in the initial stages but when it approaches equilibrium, it slows down gradually. This may be due to the availability of vacant surface sites during the preliminary stage of adsorption, and after a certain time, the vacant sites get occupied by adsorbates molecules which lead to creating a repulsive force between the adsorbate on the adsorbent surface and in the bulk phase. The attainment of equilibrium takes place after agitating the solutions containing the adsorbent up to 1440 min and once equilibrium was attained, the percentage of adsorption of adsorbates did not show any appreciable change with time. This suggests that after equilibrium was attained, further treatment did not provide more removal. In batch adsorption, the rate of removal of the adsorbate from aqueous solutions was controlled mainly by the transport of solutes molecules from the surrounding sites to the interior sites of the adsorbent particles [52]. The equilibrium capacities of Cr-TA@SSZ for terephthalic acid, p-toluic acid, and benzoic acid were obtained 1824 mg.g^− 1, 1091 mg.g^− 1, and 885 mg.g^− 1 respectively while for MIL-101(Cr) were obtained 1420 mg.g^− 1, 816 mg.g^− 1, and 708 m. g^− 1 respectively.

3.5. Adsorption isotherms

The experimental data in a wide range of concentrations (100–2500 ppm) for the removal of benzoic acid (BA), terephthalic acid (TA), and p-toluic acid (p-tol) were tested with different isotherm models Langmuir, Freundlich, Temkin, and Redlich–Peterson (R-P). Figure 15a and Fig. 15b presented profiles of different isotherms at 25°C, for Cr-TA@SSZ adsorbent (the most efficient adsorbent) and the linear fit of the data was also presented by the Langmuir model.

The calculated isotherm constants of each model for both adsorbents MIL-101(Cr) and Cr-TA@SSZ were calculated and presented in Table 2.

Table 2

Parameters of different adsorption isotherm models.
Adsorbate	Adsorbent	Langmuir parameters			Freundlich Parameters			Temkin parameters			Redlich–Peterson parameters
Adsorbate	Adsorbent	q_o(mg/g)	K_L(l/mg)	R²	K_F (mg/g(l/mg)^1/n)	n	R²	A(l/g)	B(J/mol)	R²	a_R(l/mg)^−β	K_R (l/g)	β	R²
TA	MIL-101(Cr)	1692.0	2.91×10^− 3	0.97	33.11	1.89	0.95	0.054	303.03	0.90	0.0020	3.78	1	0.97
TA	Cr-TA@SSZ	2208.4	3.62×10^− 3	0.98	37.42	1.79	0.97	0.070	391.08	0.90	0.0028	6.69	1	0.98
BA	MIL-101(Cr)	769.2	6.29×10^− 3	0.99	42.36	2.60	0.94	0.087	139.51	0.96	0.0041	3.28	0.99	0.99
BA	Cr-TA@SSZ	1009.5	3.97×10^− 3	0.98	42.55	2.43	0.96	0.080	173.71	0.96	0.1138	13.73	0.73	0.99
p-tol	MIL-101(Cr)	952.4	3.40×10^− 3	0.99	28.47	2.19	0.96	0.053	175.67	0.98	0.025	5.30	0.94	0.99
p-tol	Cr-TA@SSZ	1241.2	4.04×10^− 3	0.98	48.04	2.35	0.98	0.166	280.39	0.94	0.1294	16.49	0.71	0.98

According to Table 2, the maximum adsorption capacities (q₀) of the Cr-TA@SSZ for TA, p-tol, and BA are 2208.4 mg·g^− 1, 1241.2 mg·g^− 1, and 1009.5 mg·g^− 1, respectively while for MIL-101(Cr) the maximum adsorption capacities are 1692.0 mg·g^− 1, 952.4 mg·g^− 1, and 769.2 mg·g^− 1 respectively. Furthermore, the Cr-TA@SSZ exhibits a higher adsorption capacity than MIL-101(Cr) in adsorption of TA, p-tol, and BA. This phenomenon can be explained by the larger specific surface and pore size of Cr-TA@SSZ compared to MIL-101(Cr) and also according to EDX analysis higher Chromium metal in Cr-TA@SSZ‌ network than MIL-101(Cr) which leads to increased electrostatic interaction between the active sites of Cr-TA@SSZ‌ and the organic solutes.

The values of R_L (separation factor of the Langmuir isotherm) for the entire range of initial concentrations of TA, p-tol, and BA are depicted in Fig. 16. The results indicate that the adsorption of TA, p-tol, and BA over two adsorbents MIL-101(Cr) and Cr-TA@SSZ were favorable) 0 < R_L <1(. Also, as shown in Fig. 16, it seems that the uptake of TA and p-tol by Cr-TA@SSZ‌ is more favorable than MIL-101(Cr). This is because the R_L value for Cr-TA@SSZ is smaller than MIL-101(Cr). But in the case of BA, it is the opposite and the R_L value for Cr-TA@SSZ compared to the MIL-101(Cr) is larger. This means that BA‌ has a higher tendency to be absorbed by the MIL-101(Cr).

According to Table 2, the β parameter in the Redlich-Peterson (R-P) model for all samples is close to one, it can be concluded that the adsorption behavior of the mentioned systems follows the Langmuir monolayer model. Therefore, to study the thermodynamics of these systems, the constants and parameters of Langmuir can be used. It should be noted that lower values of β for Cr-TA@SSZ in the adsorption of p-tol and BA‌ could be due to the greater mesoporous porosity of this adsorbent and its tendency to adsorb multilayer, especially at high concentrations [53].

3.6. Adsorption Kinetics

To study the kinetic behavior of TA, p-tol, and BA by MIL-101(Cr) and Cr-TA@SSZ, three models of Lagergren pseudo-first-order, pseudo-second-order, and intraparticle diffusion model have been used. Adsorbates were tested at three concentrations of 500, 1000, and 2000 ppm for a period of 15-1440 minutes. The adsorption results of each adsorbate and the fit of their results with different models are shown in Fig. 17a to Fig. 17f and the calculated constants of the three kinetic equations along with R² values are presented in Table 3.

Table 3

Kinetic parameters for TA, p-tol‌, and BA adsorption by MIL-101(Cr) and Cr-TA@SSZ‌ at various initial concentrations (T = 25 ⁰C).
Adsorbate		Adsorbents	Pseudo-first order			Pseudo-second order				Intra-particle diffusion
Adsorbate	C_o (mg/l)	Adsorbents	q_e,cal (mg/g)	k₁×10³ (min^− 1)	R²	q_e,cal (mg/g)	k₂×10⁵ (g/(mg min))	R²	q_e,exp (mg/g)	k_d (mg/g.min^1/2)	F (mg/g)	R² (Linear reg.)
BA	500	MIL-101(Cr)	236.8	5.38	0.91	366.3	5.19	0.99	351.4	2.18	275.0	0.84
	500	Cr-TA@SSZ	329.1	5.34	0.92	458.7	2.79	0.99	431.4	3.74	302.3	0.56
	1000	MIL-101(Cr)	461.7	7.11	0.98	602.4	3.06	0.99	577.1	3.91	443.0	0.70
	1000	Cr-TA@SSZ	353.3	6.45	0.84	606.1	3.83	0.99	584.3	4.70	492.0	0.54
	2000	MIL-101(Cr)	490.0	4.85	0.91	740.7	2.30	0.99	708.0	5.41	517.9	0.83
	2000	Cr-TA@SSZ	652.6	5.04	0.93	925.9	1.66	0.99	885.7	8.61	588.4	0.57
TA	500	MIL-101(Cr)	225.5	5.06	0.78	467.3	7.53	0.99	457.1	1.86	391.2	0.68
	500	Cr-TA@SSZ	330.9	4.56	0.83	552.5	3.77	0.99	535.7	4.07	393.0	0.53
	1000	MIL-101(Cr)	301.4	4.90	0.67	793.7	6.62	0.99	785.7	2.18	707.1	0.70
	1000	Cr-TA@SSZ	506.2	5.17	0.79	1014.9	3.24	0.99	992.9	4.33	840.5	0.60
	2000	MIL-101(Cr)	431.0	6.51	0.64	1435.1	5.35	0.99	1420.1	3.42	1369.4	0.70
	2000	Cr-TA@SSZ	786.8	6.39	0.74	1854.2	2.61	0.99	1824.3	5.19	1646.1	0.54
p-tol	500	MIL-101(Cr)	290.0	5.58	0.96	403.2	4.13	0.99	385.7	3.32	271.4	0.71
	500	Cr-TA@SSZ	405.1	5.42	0.99	487.8	2.16	0.99	454.3	6.01	248.2	0.66
	1000	MIL-101(Cr)	304.6	5.44	0.76	588.2	4.48	0.99	570	2.02	498.5	0.67
	1000	Cr-TA@SSZ	480.1	6.08	0.92	729.9	2.80	0.99	700	4.57	543.4	0.52
	2000	MIL-101(Cr)	469.6	5.59	0.82	840.3	2.92	0.99	816.3	3.27	701.2	0.77
	2000	Cr-TA@SSZ	770.4	6.45	0.97	1128.3	2.08	0.99	1091.4	7.38	838.6	0.71

The obtained results show the superiority of the pseudo-second-order models in justifying the kinetic behavior of the systems. According to Table 3, the value of R² in the pseudo-second-order model is higher than the other two models (R² > 0.99). On the other hand, the equilibrium adsorption capacity (q_e) obtained by this model is closer to the experimental results than the other models. According to Table 3, although there is a large difference between the experimental and calculated adsorption capacity values when the intraparticle diffusion model was applied, however, the result of this model is an important tool to determine the adsorption mechanism and analyze the type of adsorbates penetration within the adsorbents. The intraparticle diffusion model, in addition to demonstrating the amount and role of each adsorption step, can determine the phase controlling the adsorption rate.

In general, the adsorption of a molecule into a porous adsorbent involves three successive steps: 1- The transfer of the adsorbed molecule from the dissolved film to the adsorbent surface (film penetration). 2- Transfer from the surface into the adsorbent pores (intraparticle penetration) and, 3- Adsorption of the guest molecule on the inner surface of the pores. As we know, slower stages (stages with higher mass transfer resistance) determine the overall rate of adsorption [54]. The third stage is usually overlooked because it is faster than the previous two stages. According to the intra-particle diffusion model, a plot of the amount of adsorbed (q_t) versus the square root of time (t^1/2) should be linear, and if these lines pass through the origin, then the intraparticle diffusion is the only rate-controlling stage [54]. However, if the data exhibit a multi-linear plot, then two or more steps influence the sorption process. The steeper slope of the graph corresponds to the faster reaction phase (adsorption on the inner surface of the pores). The second part of the diagram deals with the slower phase (intraparticle diffusion) which controls the adsorption rate [36]. Data presented in Table 3 indicate that as the concentration of adsorbates in the solution increases, the diffusion constant k_d and the film diffusion index F in both MIL-101(Cr) and Cr-TA@SSZ adsorbents increase. This is due to the increase in the difference in chemical potential outside and inside the adsorbents at higher concentrations [55]. The mesoporosity of the adsorbents is also important in better penetration of the adsorbents, especially at high concentrations. According to the results, at a concentration of C_o=2000 ppm the intraparticle diffusion rate constant k_d for all three adsorbates on Cr-TA@SSZ is greater than MIL-101(Cr)‌, which can be due to the larger pore size and more inclination of the active site Cr-TA@SSZ to uptake organic compounds.

3.7. Adsorption thermodynamics

The removal of TA, p-tol and BA by MIL-101(Cr) and Cr-TA@SSZ adsorbents also was evaluated at different temperatures 25, 35, and 45 ^oC for the determination of thermodynamic parameters. According to equations (12) and (13), the enthalpy change (ΔH) and entropy change (ΔS) of adsorption was obtained from the van't Hoff plot of ln(K_L) against 1/T (Fig. 18) and are reported in Table 4. The estimated values of Gibbs free energy (ΔG) of the adsorption at different temperatures 25, 35, and 45 ^oC were negative indicating that the adsorption process was spontaneous at all the studied temperatures [11]. As can be seen, with increasing temperature, the Gibbs free energy changes tend to zero and the Langmuir isotherm constant becomes smaller. Therefore, it can be concluded that the adsorption process becomes weaker with increasing temperature (moves towards equilibrium) and the adsorption will be exothermic. This claim was also confirmed by the negative ∆H in all experiments [11].

It should be noted that in adsorption processes, the chemical and physical adsorption of adsorbents can severely affect the adsorption capacity and removal percentage of solutes. In general, when the values of ΔG are between 0 and − 20 kJ.mol^− 1, physical adsorption is occurring with Van der Waals forces playing a dominant role, resulting in the adsorption effect being small and desorption occurring easily [56]. When the values of ΔG are between − 80 and − 400 kJ.mol^− 1 or ∆H ranges from 40 to 120 kJ.mol^− 1, chemical adsorption is occurring with chemical bonding playing a dominant role, resulting in larger adsorption energy and a higher tendency for irreversible adsorption [57]. Also, unlike chemical adsorption, in physical adsorption, the bond between the organic compound and the active sites of the adsorbent weakens with increasing temperature, which results in negative enthalpy changes. In this study, the values of ΔG were between − 1.53 and − 4.55 kJ.mol^− 1 and ΔH were between − 4.09 and − 22.16 kJ.mol^− 1 indicating that the adsorption of TA, p-tol, and BA onto MIL-101(Cr) and Cr-TA@SSZ occurred by physical adsorption and confirming that the adsorption of TA, p-tol and BA adsorption solutes by adsorbents is feasible and spontaneous.

The negative values of ΔS with increasing temperature suggested the decrease in randomness at the solid/solution interface during the adsorption of TA, p-tol, and BA adsorption onto MIL-101(Cr) and Cr-TA@SSZ adsorbents. The negative entropy of the adsorption and immobilization of TA, p-tol, and BA adsorption onto MIL-101(Cr) and Cr-TA@SSZ adsorbents surface may be attributed to the decrease in the degree of freedom of solutes molecules.

Also, according to the results in Table 4, it seems that from a thermodynamic point of view, Cr-TA@SSZ‌ is stronger than MIL-101(Cr) in adsorbing TA‌ and p-tol and is weaker in adsorbing BA.

This MOF‌, despite having a higher adsorption capacity than MIL-101(Cr)‌, for reasons such as the presence of OH-functional groups in its network, its larger pore size than MIL-101(Cr), and its low zeta potential compared to MIL-101(Cr) (which reduces electrostatic interaction, especially at low concentrations), has a longer equilibrium time than MIL-101(Cr), especially in BA adsorption. This result is similar to the result stated in the patent of Markus Schubert et al. regarding the Al-TA@SSZ adsorbent [46].

Table 4. Adsorption isotherm and thermodynamic parameters for TA, p-tol‌ , and BA adsorption by MIL-101(Cr) and Cr-TA@SSZ at different temperatures.

Adsorbate	Adsorbent	Temperature (^◦C)	K_L (l/mg)	q₀ (mg/g)	R²	∆G (kJ/mol)	∆H (kJ/mol)	∆S (J/mol.K)
BA	MIL-101(Cr)	25	6.29×10^-3	769.2	0.999	-4.55	-11.27	-22.65
		35	5.19 ×10^-3	740.7	0.999	-4.22
		45	4.73×10^-3	704.2	0.998	-4.11
	Cr-TA@SSZ	25	3.97×10^-3	1009.1	0.983	-3.42	-4.09
		35	3.78×10^-3	925.9	0.979	-3.41		-2.23
		45	3.58×10^-3	854.7	0.984	-3.37
p-tol	MIL-101(Cr)	25	3.40×10^-3	952.4	0.994	-3.03	-15.52	-41.91
		35	2.76×10^-3	877.2	0.987	-2.60
		45	2.29×10^-3	833.3	0.975	-2.20
	Cr-TA@SSZ	25	4.04×10^-3	1241.2	0.980	-3.46	-4.78	-4.45
		35	3.76×10^-3	1176.5	0.976	-3.39
		45	3.58×10^-3	1090.5	0.983	-3.37
TA	MIL-101(Cr)	25	2.91×10^-3	1692	0.967	-2.65	-19.24	-55.44
		35	2.46×10^-3	1666.7	0.969	-2.30
		45	1.78×10^-3	1584.8	0.951	-1.53
	Cr-TA@SSZ	25	3.62×10^-3	2208.4	0.976	-3.19	-22.16	-63.51
		35	2.87×10^-3	2178.6	0.969	-2.70
		45	2.06×10^-3	2020.2	0.951	-1.91

3.8. Application of the MIL-101(Cr) and Cr-TA@SSZ to a real wastewater sample

Two samples were obtained from the wastewater streams of the PTA (purified terephthalic acid) production plant of STPC[1] Company (Bandar-Mahshahr, Iran). The first sample was taken from the WW stream of CTA (crude terephthalic acid) section (concentrated WW stream of catalyst recovery and washing units) and the second sample was taken from the PTA+CTA wastewater stream (more diluted WW before the anaerobic digestion section of the treatment unit). These two samples were used as an adsorption medium for investigating the effect of the MIL-101(Cr) and Cr-TA@SSZ in TA, p-tol, and BA removal from real wastewater. The characteristics of the collected two samples used in this study are presented in Table 5 indicating that in addition to TA, p-tol, and BA there were significant quantities of other organic compounds including the total solids (TS) that contained a variety of likely multi-cyclic organic compounds named high and low boiling byproducts. The wastewater adsorption experiments were performed with similar amounts of each adsorbent (0.1 g) in 20 ml of the WW. The WW containing the adsorbent was mixed well with magnetic stirring and maintained for 24 h at a constant temperature of 25 ^oC. Subsequently, the solutions were separated from the adsorbents with a slow flow cellulose filter paper (2–3 μm) and COD (chemical oxygen demand) removal from the WW were determined by the following equations:

\(\text{C}\text{O}\text{D}=\left(\frac{{\text{C}\text{O}\text{D}}_{0}-{\text{C}\text{O}\text{D}}_{\text{e}}}{{\text{C}\text{O}\text{D}}_{0}}\right)\times 100 \left(\text{%}\right)\) (14)

Where COD_o and COD_e (mg/l) are the initial and final calculated COD of WW after 24 h, respectively. The adsorption amount of each organic compound by adsorbents in both concentrated and diluted flow, as well as the percentage of COD removal by them, is shown in Fig. 19. As can be seen in Fig. 19, Cr-TA@SSZ‌, like the laboratory results, has well adsorbed organic compounds from the industrial effluent and in all cases has shown its superiority over MIL-101(Cr)‌. This MOF was able to remove 40% COD from the concentrated phase (equivalent to 13000 ppm) and remove 77.3% COD from the diluted phase (equivalent to 4250 ppm) wastewater.

Table 5

Characteristics of the industrial raw purified terephthalic acid wastewater samples used in this study.
Stream	Item	Amount	Unit
CTA WW (stream 1)	pH	4.5	-
	Benzoic acid	1600	mg/l
	Terephthalic acid	2050	mg/l
	p-toluic acid	50	mg/l
	Bromine(Br⁻)	155	mg/l
	Cobalt (Co)	29	mg/l
	Manganese(Mn)	24	mg/l
	Total solid (TS( (after liquid vaporization of the sample)	2.3	wt.%
	COD (chemical oxygen demand)	32900	mg/l
PTA + CTA WW (stream 2)	pH	4.5	-
	Benzoic acid	300	mg/l
	Terephthalic acid	850	mg/l
	p-toluic acid	350	mg/l
	Bromine(Br⁻)	55	mg/l
	Cobalt (Co)	12	mg/l
	Manganese(Mn)	10	mg/l
	Total solid (TS) (after liquid vaporization of the sample)	0.25	wt.%
	COD (chemical oxygen demand)	6000	mg/l

[1] Shahid Tondgooyan Petrochemical Company.

In summary, for the first time, MIL-101(Cr)/Sulfasalazine (Cr-TA@SSZ) was synthesized by a hydrothermal approach as a novel adsorbent for removal of TA, p-tol, and BA from aqueous solutions, and the results compared with MIL-101(Cr). The Cr-TA@SSZ and MIL-101(Cr) were characterized by applying the general analyses including X-ray diffraction (XRD), Fourier transform infrared (FTIR), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), Scanning electron microscopy(SEM), thermal gravimetric analysis (TGA), differential thermal analysis (DTA), zeta potential, and Elemental analysis (EDX). Based on the results, it was expected that Cr-TA@SSZ due to higher mesoporous porosity, higher surface area and volume, the higher percentage of Chromium in Cr-TA@SSZ framework structure (which leads to more active metal sites) and having a functional group -OH in the network (which can play an important role in creating hydrogen bonds with guest molecules) can be more successful in adsorbing various compounds, especially organic compounds, than MIL-101(Cr) (Cr). Characterization of two adsorbents revealed that Cr-TA@SSZ has a different composition and network structure to MIL-101(Cr) (Cr). The formula for Cr-TA@SSZ is proposed as:

Cr₃F (H₂O)₂O[C₆H₄(CO₂)₂][C₆H₃N(OH)(CO₂)], 2.5H_2.

The batch adsorption of TA, p-tol, and BA onto the MIL-101(Cr) and Cr-TA@SSZ were tested at a variety of pH, initial concentrations, contact time, and temperatures. The isotherms of TA, p-tol, and BA adsorption onto the MIL-101(Cr) and Cr-TA@SSZ can be well described by Langmuir and Redlich–Peterson isotherms, and the adsorption kinetics of TA, p-tol, and BA can be well-fitted with the pseudo-second-order model(R² > 0.99). The maximum adsorption capacities (q₀) of the Cr-TA@SSZ for TA, p-tol, and BA were 2208.4 mg·g^− 1, 1241.2 mg·g^− 1, and 1009.5 mg·g^− 1, respectively while for MIL-101(Cr) were 1692.0 mg·g^− 1, 952.4 mg·g^− 1, and 769.2 mg·g^− 1 respectively. The Cr-TA@SSZ was found to be more efficient in the removal of TA, p-tol, and BA from water than the MIL-101(Cr). In addition, thermodynamic parameters were calculated and their values indicated that the process of removal was spontaneous and exothermic and the randomness decreases with the adsorption of TA, p-tol, and BA. Moreover, adsorption experiments using industrial wastewater from a TA production plant showed that Cr-TA@SSZ‌ can be used as a promising adsorbent in the adsorptive removal of organic pollutants from wastewaters. This MOF was able to remove 40% of COD from the concentrated phase (equivalent to 13000 ppm) and to remove 77.3% of COD from the diluted phase (equivalent to 4250 ppm) wastewater.

Author Contribution

The authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by somayeh tourani and alireza behvandi. The first draft of the manuscript was written by somayeh tourani and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Conflict of Intrest

The authors declare that they have no conflict of interest.

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No competing interests reported.

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Synthesis of MIL-101(Cr)/Sulfasalazine (Cr-TA@SSZ) hybrid and its use as a novel adsorbent for adsorptive removal of organic pollutants from wastewaters

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Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Material and reagents

2.2. Synthesis of MOFs

2.2.1. Synthesis of MIL-101(Cr)

2.2.2. Synthesis of MIL-101(Cr)/Sulfasalazine (Cr-TA/SSZ)

2.3. Characterizations

2.4. Adsorption assay

2.4.1. Adsorption isotherms

2.4.2. Adsorption kinetics

2.4.3. Adsorption thermodynamics

3. Results And Discussion

3.1. Characterization of the adsorbents

3.4. Effect of contact time

3.5. Adsorption isotherms

3.6. Adsorption Kinetics

3.7. Adsorption thermodynamics

3.8. Application of the MIL-101(Cr) and Cr-TA@SSZ to a real wastewater sample

4. Conclusion

Declarations

References

Additional Declarations

Status:

Version 1