Investigation the adsorption kinetic and isotherm studies of Remazol Red 5B dye on benzoic acid modified Al2O3/UiO-66 composite

Ratna Ediati; Alvin Romadhoni Putra Hidayat; Terry Denisa Syukrie; Liyana Labiba Zulfa; Miftahul Jannah; Harmami Harmami; Hamzah Fansuri; Badrut Tamam Ibnu Ali

doi:10.1016/j.arabjc.2024.106078

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Review article

01 2024

:18;

106078

doi:

10.1016/j.arabjc.2024.106078

Investigation the adsorption kinetic and isotherm studies of Remazol Red 5B dye on benzoic acid modified Al₂O₃/UiO-66 composite

Ratna Ediati^{^a,⁎}, Alvin Romadhoni Putra Hidayat^{^a,^b}, Terry Denisa Syukrie^{^a}, Liyana Labiba Zulfa^{^a}, Miftahul Jannah^{^a}, Harmami Harmami^{^a}, Hamzah Fansuri^{^a}, Badrut Tamam Ibnu Ali^{^c}

aDepartment of Chemistry, Faculty of Science and Data Analytics, Institut Teknologi Sepuluh Nopember, Jl. Arif Rahman Hakim, Kampus ITS Keputih-Sukolilo, Surabaya 60111, Indonesia

bDepartment of Chemistry, Diponegoro University, Semarang, 50275, Indonesia

cResearch Center for Process and Manufacturing Industry Technology, National Research and Innovation Agency, 625 Building, Technology Energy Cluster, PUSPIPTEK, South Tangerang, Indonesia

⁎Corresponding author. ratna.ediati@its.ac.id (Ratna Ediati)

Received: 2024-5-8, Accepted: 2024-12-2,

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Abstract

This research used a defect engineering approach by introducing a benzoic acid modulator to synthesis Al₂O₃/UiO-66 composites. Adding 5 eq benzoic acid (Al/UiO-66(B5)) produces mesopores with irregular morphology. Increasing benzoic acid to 20 eq (Al/UiO-66(B20)) produces ordered octahedral crystals. Deformation of Al₂O₃/UiO-66 by benzoic acid can effectively remove Remazol Red 5B (RRB) dye from the aqueous solution. The adsorption performance of the synthesized material was investigated in terms of pH, temperature, initial concentration of RRB dye, and contact period. Al/UiO-66(B5) shows the best RRB adsorption capacity performance with Q_max = 333.33 mg/g at 30 °C due to the defects and mesopore structure formed. From 30° to 50 °C, the adsorption capacity decreases, and acidic media are more conducive to the adsorption of RRB dye. The presence of interfering anions such as Cl⁻, NO₃⁻, CO₃²⁻, and SO₄²⁻ can be a limiting factor in adsorption. The kinetics results show that a pseudo second order model describes absorption. The data on chemical and physical adsorption equilibrium on heterogeneous surfaces are well-fitted by the Freundlich isotherm model. Other kinetic models, such as Intraparticle Diffusion and Elovich, as well as other isotherm models, such as Temkin and Scatchard, were also evaluated in this study. RRB adsorption was hypothesized to be spontaneous, feasible and exothermic based on the assessed thermodynamic parameters ΔG°, ΔH° and ΔS°. Therefore, Al₂O₃/UiO-66 with benzoic acid modification can be a promising adsorbent in wastewater treatment. This work can provide new theoretical insights into the modification of composite MOFs for simultaneous removal of RRB dyes.

Keywords

UiO-66

MOF composites

Modulator

Adsorption

Remazol Red B

Kinetic and isotherm

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1 Introduction

The textile industry is characterized by the extensive use of dyes in significant volumes. The wastewater produced by the textile industry has been recognized as a contaminant with mutagenic and carcinogenic properties, posing potential risks to human health when eaten in excessive amounts (Preethi et al., 2006; Sobhanardakani et al., 2017; Yousefzadeh et al., 2024 ). The Remazol Red 5B dye, an anionic dye, is widely used in the textile sector. The removal of dye has been achieved using many methods. Among them, physical processing, namely adsorption, is well recognized as successful due to its versatility and efficiency (Sobhanardakani et al., 2016, 2013; Zandipak and Sobhanardakani, 2016 ).

Metal Organic Frameworks (MOFs) are a kind of porous materials composed of metal ions or metal clusters interconnected by organic ligands, which serve as bridge molecules (Liu et al., 2014; Qiu et al., 2014). MOFs have a substantial surface area ranging from around 1000 to 3000 m²/g, accompanied by pore volumes ranging from 0.2 to 0.8 cm³/g. One notable benefit of MOFs is in their facile synthesis, which may be achieved by several techniques tailored to specific structural and property requirements (Embaby et al., 2018; Rowsell and Yaghi, 2004). UiO-66 was chosen as a dye adsorbent, considering its superior chemical and thermal stability (Wang et al., 2016). UiO-66 consists of a Zr₆O₄(OH)₄ cluster with an organic 1,4-benzene dicarboxylate acid (H₂BDC) linker. The Zr⁴⁺ ion in the Zr group strongly interacts with carboxylate ligands, forming a highly connected framework (Vakili et al., 2018; Wang et al., 2016). Although specific characteristics of MOFs have been widely reported, most MOFs have micropores. The micropores of UiO-66 could cause problems with the adsorption diffusion rates (Kondo et al., 2012). In theory, the pore size of a MOF can be increased by selecting a longer organic linker. In addition, synthesizing long linkers is often too complicated and expensive considering the cost (Wang et al., 2016). Recently, various efforts have been made to improve the active side of UiO-66. The defect approach and adding a metal oxide to MOFs has received great attention because it helps overcome diffusion limitations and produces active sites (Koutsianos et al., 2019; Liu et al., 2023; Ramos-Fernandez et al., 2011; Wang et al., 2016 ).

Alumina is a promising material to composite with UiO-66 because of its large surface area, high adsorption capacity, and excellent thermal stability (Al-Rubayee et al., 2016; Liu et al., 2023). Until now, many studies have been carried out to modify MOFs with alumina including MIL-101/α-Al₂O₃ (Ramos-Fernandez et al., 2011), MOF-5@γ-Al₂O₃ (Liu et al., 2023), Al₂O₃@UiO-66 (Hidayat et al., 2021), HKUST-1/γ-Al₂O₃ (Qin et al., 2016), ZIF-67/Al₂O₃ (Xu et al., 2020), these composites have been reported for various applications, namely heterogeneous catalysts, adsorbents, and gas separation. Liu et al.(Liu et al., 2023) stated that MOF-5@γ-Al₂O₃ and ZIF-8@γ-Al₂O₃ showed selective adsorption behavior for anionic dyes where the adsorption capacity of MOF-5-NO₃@γ-Al₂O₃ for Congo Red was 1388.01 mg/g.

The phenomenon of defect engineering in composites of MOF has garnered increasing interest in recent years. It is primarily due to the controllable manipulation of surface chemistry, surface area, and pore size distribution, which may be achieved by introducing missing linkers or clusters inside the framework (Koutsianos et al., 2019; Shearer et al., 2016; Taddei et al., 2019; Wang et al., 2016 ). Missing linker faults occur when the linker is detached from the structure. The outcome of this situation leads to a need for coordination among two neighboring clusters (Wißmann et al., 2012). The study conducted by Yuan et al.(Yuan et al., 2018) revealed that UiO-66(Zr) with missing-linker defects exhibited enhanced adsorption ability for U(VI). Recent studies have discovered that including monocarboxylic acids may improve the regenerability and crystallinity of MOFs (Vo et al., 2021; Wißmann et al., 2012; Wu et al., 2013 ). This study selects benzoic acid as a rigid modulator to address flaws in MOF composites. Defects are capable of enhancing adsorption capacity by augmenting the number of active sites resulting from impaired frameworks, amplifying porosity and surface area, and generating stable colloidal solutions (Wang et al., 2016; Zhang et al., 2021). According to prior studies, it has been suggested that benzoic acid has the potential to serve as a substitute for the fumaric linker in UiO-66. This substitution significantly enhanced the total surface area, with a rise of 23 % from 1236 to 1515 m²/g (Atzori et al., 2017). Using a benzoic acid organic acid modulator, which has a bigger and more rigid structure compared to acetic acid and formic acid, may result in steric interactions between the modulator coordinating on the cluster and the linker linked to the cluster. The enhancement of reproducibility is facilitated, hence enabling control over crystal size, shape, and degree of crystal aggregation via the promotion of crystallinity synthesis. Inspired by the modulation strategy for producing defects in a MOF, we hypothesized that adding a modulator to the synthesis of UiO-66 could induce crystal defects that are beneficial in the dye adsorption process due to several advantages.

This study proposes a novel approach for introducing defects in forming Al₂O₃/UiO-66 using benzoic acid modulation. This modulation aims to enhance the accessibility of active sites, hence facilitating increased adsorption of Remazol Red 5B. This study investigates the impact of benzoic acid, when present at a specific concentration, on the adsorption capabilities of Al₂O₃/UiO-66 towards Remazol Red 5B. The X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) analysis, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) techniques were used to investigate the surface morphology and physicochemical characteristics of both pure Al₂O₃/UiO-66 and Al₂O₃/UiO-66 modified with benzoic acid. In addition, an examination was conducted on the adsorption efficacy of Remazol Red B, focusing on several batch adsorption parameters, including contact duration, starting concentration, pH, and dosage. The study also investigated the competitive influence of anions, including sulfate, chloride, carbonate, and nitrate, in aqueous solutions to assess their effect on the adsorption capacity of Remazol Red 5B. The strategy of adding benzoic acid as a modulator in the synthesis of Al₂O₃/UiO-66 composite is thus an innovative step for the development of MOF composite fabrication as an adsorbent in the research field. Furthermore, the method's ease of use and high stability in water present a significant opportunity for its development as a dye adsorbent on an industrial scale.

2 Experimental

2.1

2.1 Chemicals

Alumina (Al₂O_3, 99 %), benzoic acid (C₆H₅COOH, 99 %), terephthalic acid (H₂BDC, 98 %) were purchased by Sigma-Aldrich. Zirconium chloride (ZrCl₄, 98 %), N,N-dimethylformamide (DMF, 99 %), chloroform (CHCl₃, 99.9 %), Remazol Red 5B (C₁₈H₁₄N₂Na₂O₁₀S₃, 99 %) were pruchased by Merck and demineralised water. All compounds were analytically pure and used unpurified.

2.2

2.2 Synthesis of UiO-66

ZrCl₄ (4.5 mmol) and H₂BDC (4.5 mmol) were combined with 45 mL of DMF and stirred for 15 min (Ediati et al., 2021). After that, place the mixture in an unbreakable laboratory bottle and stir for 30 min. Solvothermal for 24 h at 120 °C. After removing it from the oven, allow the closed laboratory bottle to cool to room temperature. The material was then separated by centrifuging at 3000 rpm, washing three times with DMF and chloroform, and drying for 24 h in a vacuum at 80 °C.

2.3

2.3 Synthesis of Al₂O₃/UiO-66

Al₂O₃/UiO-66 was created by dissolving ZrCl₄ and H₂BDC in N,N-dimethylformamide (DMF) in a laboratory bottle. Al₂O₃ was added after dissolving ZrCl₄ in a DMF solution. Solvothermal for 24 h at 120 °C in the oven. Chloroform and DMF were used to wash the white residue once it cooled. For 24 h, the material was dried in a vacuum at 80 °C. Al/UiO-66 is the designation given to the solids formed by drying at a 0.5 M ratio of Al to Zr (Hidayat et al., 2021).

2.4

2.4 Synthesis of Al₂O₃/UiO-66 by benzoic acid

At ambient temperature, ZrCl₄, H₂BDC, and benzoic acid (5, 10, 20, 30, and 50 meq of ZrCl4) were mixed in 45 mL of DMF. The next phases are the same as those mentioned in 2.2 by UiO-66. The solids produced were designated Al/UiO-66(B5), Al/UiO-66(B10), Al/UiO-66(B20), Al/UiO-66(B30), and Al/UiO-66(B50). The synthesis procedure for this stage is depicted in Fig. 1.

Schematic illustration of the Al/UiO-66(Bx) synthesis methods.

2.5

2.5 Characterization

Several characterization techniques, such as X-ray diffraction, Fourier Transform Infra-Red, scanning electron microscopy, thermogravimetric analysis, and nitrogen adsorption–desorption isotherms, are used to conduct a comprehensive analysis of the physicochemical properties of Al/UiO-66 and Al/UiO-66(Bx). The manufactured substances were analyzed using an X-ray diffractometer for characterization. The laser utilized in the test had a radiation wavelength of 1.5406 and was a Cu K laser running at 40 kV and 30 mA. Fourier transform infrared spectroscopy (FTIR) was utilized to learn more about the molecular structures of MOFs and the functional groups they contain. The Shimadzu 8400S infrared spectrophotometer was used for the FTIR study, and readings were collected between 400 and 4000 cm⁻¹. After the adsorption investigations were finished, the presence of Remazol Red 5B was checked using FTIR. Scanning Electron Microscopy and EDAX Advanced Microanalysis Solutions were used for SEM-EDX characterization to determine the particle morphology of Al/UiO-66 and Al/UiO-66(Bx) surfaces. The Hitachi HT-7700 transmission electron microscope was employed to conduct transmission electron microscopy (TEM) measurements of the samples. Quantifying the surface area and assessing the porosity of the sample was accomplished with the Gas Sorption Instrument (Quantachrome Autosorb-1). Using the Perkin Elmer Pyris 1 Analyzer, a Thermogravimetry Analysis (TGA) was performed on a sample that weighed about 10 mg. After securing the specimen in a holder, heat it in airflow at a continuous rate of 10 °C per minute throughout a range of 30 to 900 °C. Using a Thermo Scientific GENESIS 10S UV–Vis Spectrophotometer, we calculated the Remazol Red 5B concentration in the sample solution.

2.6

2.6 Test point of zero charge (pH_pzc)

The pH_pzc experiment was conducted in a 50 mL beaker containing 20 mL of a 0.1 M NaCl buffer without adding dye. The method of calculating pH_pzc by adding salt (NaCl) is comparable to recent investigations by Usman et al. (2022) (Usman et al., 2022). After adding 0.1 M HCl and 0.1 M NaOH, the pH of each beaker was modified to a range of 1 to 12. After adding 0.1 g of adsorbent, each beaker was sealed with aluminium foil. It was stirred for 24 h at 100 rpm. Measurement of the solution's ultimate pH was conducted using a pH meter. pH_pzc curve was constructed by plotting the initial pH and the ΔpH value, which indicates the numerical difference between the initial and final pH values. The adsorbent's surface charge (pH_pzc) at pH = 0 is calculated by identifying the point where the resulting curve intersects with the x-axis (Usman and Khan, 2022). The adsorbent's surface becomes positively charged when the pH value is above the critical pH (pH_pzc). The surface acquires A negative charge when the pH value is below pH_pzc (Nhi et al., 2021; Rao et al., 2016).

2.7

2.7 Adsorption experiments

To test Al/UiO-66 and Al/UiO-66(Bx), Remazol Red 5B (RRB) adsorption contact time was determined. Each sample (10 mg) was added to a beaker with 20 ml of 100 mg/L RRB solution at ambient temperature and pH (7) for kinetic experiments. After mixing at room temperature with a magnetic stirrer at 350 rpm at pre-selected intervals, the mixtures were centrifuged for 10 min at 2500 rpm. For adsorption isotherms, samples at a fixed dosage (10 mg) and pH 7 were introduced to beakers with RRB concentrations from 100 to 400 mg/L for 25 min before collecting the final aliquot. Adsorbent dose and pH were also examined. In the dosage test, 5–20 mg of adsorbent was added into an RRB solution with a predetermined concentration and pH and agitated for 25 min to achieve the final sample concentration. In the pH variation experiment, the optimal dosage from the previous experiment was added to a 100 mg/L RRB solution at pH 2–13 and swirled for 25 min to obtain a final aliquot for analysis. To study anion competition, 100 mg/L RRB and 20 ppm carbonate, chloride, sulfate, and nitrate were dissolved in water before adding the adsorbent. After stirring for 25 min at 350 rpm, the final aliquot was examined at 520 nm using a UV–Vis spectrophotometer. Adsorbent regeneration was completed by putting 10 mg into 20 mL of 100 mg/L RRB solution and stirring for 25 min. After centrifugation, the saturated adsorbent was soaked in methanol and agitated for 90 min. The residual RRB concentration of the permeate was determined, and the absorbed adsorbent was washed in ethanol three times and vacuum filtered after 80 °C overnight drying. The regenerated adsorbent underwent four RRB adsorption cycles. Eq. (1) calculates the adsorbent's equilibrium adsorption capacity, Qe, (mg/g).

(1)

Q e = \frac{(C o -C e) x V}{W}

using the following notation: C_o = starting concentration (mg/L), C_e = final concentration (mg/L), V = sample volume (L), and W = adsorbent dosage (g). The equilibrium adsorption duration, rate constant, and equilibrium capacity of the materials were determined by adsorption kinetics experiments. The pseudo first order and pseudo second order models in Eqs. (2) and (3) are used to get a better understanding of the adsorption dynamics (Elhussein et al., 2020; Ho et al., 2000).

(2)

ln (Q_{e} - Q_{t}) = ln Q_{e} - K_{1} t

(3)

\frac{t}{Q_{t}} = \frac{1}{K_{2} {Q_{e}}^{2}} + \frac{t}{Q_{e}}

where K₁ and K₂ are the rate constants for pseudo first order and pseudo second order (1/min and g/mg/min), and Q_e and Q_t indicate the adsorption capacity (mg/g) under equilibrium circumstances and at a period time (t).

The process of intra-particle diffusion is described by the Weber-Morris model (Hou et al., 2021). According to Mazaheri et al. (2015) and Zhang et al. (2016), the following equations (Eqs. (4) and (5)) may be used in the Weber-Morris and Elovich kinetic model to investigate the adsorption mechanism and regulate the overall adsorption rate (Mazaheri et al., 2015; Zhang et al., 2016):

(4)

Q_{t} = k_{id} t^{1 / 2} + C

(5)

Q_{t} = \frac{1}{β} l n (α β) + \frac{1}{β} \ln t

The intra-particle diffusion rate constant, denoted as Kid (mg/g min^1/2), and the thickness of the boundary layer, indicated by C (mg/g), are defined by Mazaheri et al. (2015) (Mazaheri et al., 2015). Meanwhile, the Elovich model includes the adsorption rate (mg/g/h) and desorption coefficient (g/mg) (Zhang et al., 2016).

The Langmuir and Freundlich isotherm models, expressed in Eqs. (6) and (7), respectively, are used to examine the adsorption of adsorbate on the surface of the adsorbent (Hanif et al., 2020; Hidayat et al., 2024).

(6)

\frac{1}{Q_{e}} = \frac{1}{Q_{m}} + \frac{1}{Q_{m} \times K_{L} \times C_{e}}

(7)

ln Q_{e} = ln K_{F} + \frac{1}{n} ln C_{e}

where Q_m, K_L, K_F represents the maximum adsorption capacity, the Langmuir constant, and the Freundlich constant, respectively.

In the process of adsorption, the Scatchard model analysis is frequently employed to assess the types of interactions between adsorbates and adsorbents. The Scatchard model is frequently employed to collect data regarding the affinity distribution of a homogeneous or heterogeneous binding site. Eq. (8) defines the Scatchard model.

(8)

\frac{Q e}{Ce} = Q_{s} b+ Q_{e} b

The isotherm adsorption parameter (Qs) (mg/g) and the Scatchard constant (b) (L/mg) are the respective values. The adsorbent will only demonstrate one type of bond if a straight line is drawn between Qe/Ce and Qe. Nevertheless, the presence of a deviation from linearity indicates that the adsorbent has a heterogeneous surface or more than one type of bond (J, 2017).

Eq. (9) was used to compute the Temkin model of the isotherm (Prabakaran et al., 2022).

(9)

Q_{e} = \frac{RT}{B} l n K_{T} + \frac{RT}{B} \ln C_{e}

where R is the gas constant (8.314 J/mol K) and T is the Kelvin temperature. The adsorption heat constant (B) (kJ/mol) and the Temkin isotherm equilibrium constant (KT) (L/mg) are the respective values.

Adsorption thermodynamic studies calculated Gibbs free energy, enthalpy, and entropy to better understand RRB adsorption. Experiments were done at 30, 40, and 50 °C. By employing Eq. (10), the activation energy Ea (kJ/mol) and Arrhenius constant A (g/mg min) were computed via the Arrhenius plot (Zhang et al., 2019).

(10)

l n K_{2} = \ln A - \frac{Ea}{RT}

Eqs. (11) and (12) of the Van't Hoff equation computed the adsorption thermodynamic parameters (Santoso et al., 2021; Zhang et al., 2019):

(11)

l n K_{d} = \frac{Δ S^{°}}{R} - \frac{Δ H^{°}}{RT}

(12)

Δ G^{°} = - R T \ln K_{d}

where denoted as ΔS°, ΔH°, and ΔG° represent the variations in standard entropy (J/mol K), enthalpy (kJ/mol), and Gibbs free energy, respectively. T and R are the adsorption temperature (K) and gas constant (8.3145 J/mol K), respectively. Kd = (Qe/Ce) 1000. (Santoso et al., 2021).

3 Results and discussion

Fig. 2 illustrates a schematic depiction of the synthesis procedure for the Al₂O₃/UiO-66 material, which is controlled by benzoic acid. Including the rigid modulator, benzoic acid may lead to steric hindrance between the modulator, which coordinates with the metal cluster, and the ligand attached to the metal cluster. The rigid modulator, benzoic acid, can impede the organization of metal clusters, hence facilitating the inclusion of components with greater bulk. Modulators may also induce structural deformation due to the repulsive interaction between the sizable phenyl ring of the benzoic acid with the ligand molecule. The synthesis process of benzoic acid-modulated Al₂O₃/UiO-66 involves many phases. The first stage consists of creating the Zr cluster and subsequent deprotonation of the ligand or modulator. Zirconium clusters undergo growth on the Al₂O₃ surface and engage in chemical reactions with ligand molecules. The modulator and the ligand engage in a competitive interaction for the Zr coordination site, displacing the ligand coordination bond. Alternatively, the modulator may form a coordination bond with the metal group alongside the ligand. This process results in inhibition of the nucleation stage and slowing of crystal growth. In solvothermal synthesis, modulators can alter the pH of the process. They do this by competing with the base that is released and adding extra protons, which, therefore, hinders the deprotonation of the ligand (Forgan, 2020; Wang et al., 2016; Wißmann et al., 2012 ). At a lower pH level, the deprotonation process exhibits a slower rate, leading to an extended duration of nucleation. Consequently, this phenomenon gives rise to the formation of bigger particles and a wider range of particle sizes (Forgan, 2020). The coordination between zirconium ions and modulator molecules remains intact even in crystal formation, without substituting the modulator molecules. The modulator has one carboxylic group, enabling coordination with one metal cluster. Thus, defects in the framework can be generated, which not only increase the pore size by merging adjacent pores but also produce coordinatively unsaturated Zr sites (Vakili et al., 2018; Wißmann et al., 2012).

Schematic illustration of Al/UiO-66(B5) and Al/UiO-66(B20) particle formation.

3.1

3.1 Material characterization

3.1.1

3.1.1 XRD

XRD analysis was used to validate the crystalline arrangement of MOF. Fig. 3, the XRD patterns for UiO-66, Al/UiO-66, and Al/UiO-66(Bx) match the patterns reported by the literature, indicating the successful synthesis of UiO-66, Al/UiO-66, and Al/UiO- 66(Bx) (Ediati et al., 2021; Hidayat et al., 2021; Lv et al., 2019 ). Moreover, the diffractogram analysis of the UiO-66 material, when combined with Al₂O₃, did not exhibit any noteworthy alterations. Similarly, the diffractogram of the synthesized Al/UiO-66 material did not reveal the characteristic peak associated with Al₂O₃. Including the benzoic acid modulator in quantities of 5 and 10 eq resulted in reduced intensity due to hindered nucleation. However, introducing benzoic acid in insufficient levels did not effectively regulate crystal size to achieve bigger and more uniform crystals. In contrast, the XRD patterns obtained from the materials Al/UiO-66(B20), Al/UiO-66(B30), and Al/UiO-66(B50) exhibit a notable augmentation in intensity. This phenomenon may be attributed to the modulation process at these specific ratios, which effectively hinders the development of intergrown crystals (Bentz et al., 2019). In addition, it should be noted that including a greater amount of benzoic acid modulator leads to a reduction in the breadth of the two characteristic peaks shown in the diffraction pattern. This finding suggests that the addition of a benzoic acid modulator leads to an increase in the regularity of crystal formation (Schaate et al., 2011; Wißmann et al., 2012).

Fig. 3

XRD peak profile of the synthesized materials.

3.1.2

3.1.2 FTIR

Infrared spectrum analysis was used to characterize further the chemical structures of UiO-66, Al/UiO-66, and Al/UiO- 66(Bx). As shown in Fig. 4, the synthesized UiO-66 and Al/UiO-66 exhibit an aromatic C = C absorption band at a wave number of 1581 cm⁻¹ due to the benzene structure of the organic ligand. In the wave number range of 3379 – 3387 cm⁻¹, the absorption band in the O–H stretching vibration region of the carboxylate is visible, whereas the C–O stretching bond of the C–OOH carboxylate group is visible at 1402 cm⁻¹. The wavenumber range 2928 – 2929 cm⁻¹ exhibits an absorption band in the C–H stretching vibration region. The absorption band of the peak of C = O shifts from 1700–1658 cm⁻¹ due to a deprotonation process in which the C = O bond forms a coordination bond with the central metal in O⁻ (Abid et al., 2012). Moreover, the stretching vibration of Zr–O occurs at a wave number of 663 cm⁻¹ (Vakili et al., 2018). Altering Al₂O₃ does not result in the emergence of additional absorption peaks at distinct wavenumbers. It illustrates that adding Al₂O₃ does not impact the crystal structure of UiO-66. However, adding 5 eq of the benzoic acid modulator caused a peak shift, indicating a change in structure after adding the modulator. Due to the different C = C bond length of the benzoate acid containing organic C = C ligands, the aromatic C = C vibration peak of the benzene structure in the organic ligands of UiO-66 and Al/UiO-66 is shifted to 1554 cm⁻¹ following the addition of the benzoic acid modulator. The peak shift for each type of bond can be shown in Table 1.

FT-IR spectra of the prepared UiO-66 and Al/UiO-66(Bx).

Table 1 Absorption bands of synthesized materials and functional groups.

No.	Characteristics of binding	Wavenumber (cm⁻¹)			Ref
No.	Characteristics of binding	UiO-66	Al/UiO-66	Al/UiO-66(B5)	Ref
1	Zr-O stretching vibration	663	663	659	(Ediati et al., 2021; Hidayat et al., 2021; Putra Hidayat et al., 2023 )
2	Stretching vibration of Al-O	−	600 and 746	605 and 746	(Fan et al., 2023; Li et al., 2019)
3	C-O stretching vibration	1402	1402	1406	(Ediati et al., 2021; Hidayat et al., 2021)
4	Aromatic vibration C = C	1581	1581	1554	(Hidayat et al., 2021; Putra Hidayat et al., 2023b)
5	Stretching vibration of C = O	1658	1658	1654	(Hidayat et al., 2021)
6	OH stretching vibration	3387	3379	3421	(Hidayat et al., 2021)

3.1.3

3.1.3 SEM-EDX and TEM

The SEM images depicting the morphology of UiO-66, Al/UiO-66, and Al/UiO-66(Bx) are shown in Fig. 5. Both UiO-66 and Al/UiO-66 are characterized by small particle sizes and agglomeration (Hidayat et al., 2021; Mohammadi et al., 2017). In addition, there was no discernible difference in particle size between UiO-66 and Al/UiO-66, indicating that Al₂O₃ modification did not affect particle size. The obtained UiO-66 crystals had a particle size of approximately 87 nm. Fig. 5a–e depicts the development of overgrown aggregates from very small crystals to individual and octahedral-shaped crystals as the size of the sample increases. In general, modulators during Al/UiO-66 synthesis influence the number of nuclei, crystal size, and crystallization rate. At low modulator concentrations, its presence generates amorphous precipitates (5 and 10 eq) and inhibits the growth rate, forming more crystalline products. (Fig. 5c) The surface morphology of the synthesized Al/UiO-66(B5) consists of very small crystals. This result is consistent with the XRD characterization of the Al/UiO-66(B5) material, demonstrating a decrease in intensity and the broadest diffraction peak compared to other synthesized materials and adding 5 eq of benzoic acid modulator per mole of ZrCl₄ results in less controlled crystal growth characterized by very small crystal particle diameters, inconsistent size distribution, and agglomeration. Fig. 5d and e depicts the surface morphology of Al/UiO-66(B20). The resultant synthesis comprises octahedral crystals larger than Al/UiO-66(B5). Increasing the concentration of benzoic acid intensifies the competition between ligands and modulator molecules, thereby reducing the number of nuclei and promoting the growth of larger crystals (Schaate et al., 2011a; Vakili et al., 2018).

SEM images of (a) UiO-66, (b) Al/UiO-66, (c) Al/UiO-66(B5), (d-e) Al/UiO-66(B20), (f) EDX mapping of Al/UiO-66, (g) Al/UiO-66(B5).

This study used energy-dispersive X-ray spectroscopy to analyze the chemical composition of the produced UiO-66, Al/UiO-66, and Al/UiO-66(Bx) materials. The UiO-66 material consists exclusively of C, O, and Zr atoms, with no impurities. The presence of Al elements in the EDX spectra and mapping (Fig. 5f and g) provides evidence for the inclusion of Al₂O₃ in the synthesis of Al/UiO-66 and Al/UiO-66(B5). Table 2 shows the percentage distribution of elements for each material synthesized.

Table 2 Distribution of elemental weights within the composition.

Materials	Elements	percentage (%)
UiO-66	Zr	19.18
	O	19.61
	C	61.21
Al/UiO-66	Zr	32.74
	O	16.89
	C	49.35
	Al	1.02
Al/UiO-66(B5)	Zr	30.81
	O	16.66
	C	51.35
	Al	1.18

Fig. 6 illustrates the TEM analysis results of the UiO-66, Al/UiO-66, and Al/UiO-66(B20) materials. The synthesized UiO-66 is of high purity, as evidenced by its small particle with a spherical aggregate morphology, which is not observed by other crystal phases (Ediati et al., 2021). The TEM image of UiO-66 exhibits a particle size distribution that is relatively homogeneous and has decent dispersibility. Al/UiO-66 is composed of aggregated particles identical to UiO-66. Al/UiO-66 and UiO-66 have identical morphologies, suggesting that UiO-66 effectively encapsulated Al₂O₃. Nevertheless, the TEM image of Al/UiO-66 demonstrated a dark colour in the central region, suggesting that Al₂O₃ had permeated the UiO-66 matrix (Xu et al., 2019). Compared to the TEM image of UiO-66 without adding Al₂O₃, the TEM image of UiO-66 with Al₂O₃ exhibited a greater area of black colour. Nevertheless, Al/UiO-66(B20) underwent a morphological transformation, exhibiting a smooth surface and an octahedral morphology. The spherical aggregate morphology was no longer present.

TEM images of (a) UiO-66, (b) Al/UiO-66, (c) Al/UiO-66(B20).

3.1.4

3.1.4 TGA

UiO-66, Al/UiO-66, and Al/UiO-66(Bx) TGA curves are shown in Fig. 7. The three materials UiO-66, Al/UiO-66, and Al/UiO-66(Bx) are heated at 10 °C/minute in air. In the 30–900℃ temperature range, there are three basic mass reduction phases. The mass drop in stage 1 at 30–100 ℃ was due to water and chloroform loss. The evaporated water is physically adsorbed on the surface of UiO-66 (Zhang et al., 2021). Meanwhile, the chloroform that evaporates is the remaining chloroform resulting from the washing process during synthesis. Not much different from UiO-66, the synthesized Al/UiO-66 thermogram shows three stages of mass reduction caused by heating the material. The mass reduction in stage 1 of 0.45 % occurred in the 30–100 °C temperature range. However, the amount of mass reduction in stage 1 is not much, and this occurs because the remaining water and chloroform contained in Al/UiO-66 are very small. The mass decrease in stage 2 appeared in the temperature range of 130–280 °C, which indicates the decomposition of the N,N-dimethylformamide (DMF) solvent. It is by research by Orefuwa et al.(Orefuwa et al., 2012) which says that DMF decomposition occurs in the 153–155 °C temperature range. The additional mass decrease occurring at around 270 °C can be ascribed to the decomposition of most of the benzoic acid modulator and some of the ligands in the decomposed framework. The final mass decrease in the temperature range of 300–600 ℃ is attributed to the decomposition of benzendicaboxylate (BDC) ligands. Damage to the UiO-66 structure results from benzene changing to the gas phase at 540 °C, breaking the link between it and the carboxyl group in the ligand that makes up the framework (Cavka et al., 2008). Damage to the UiO-66 framework will cause the formation of metal oxide, namely ZrO₂. The difference between the decrease in mass of Al/UiO-66 and Al/UiO-66(B5) is caused by several benzoic acid molecules trapped in UiO-66. It is necessary to break down the framework in order to free the many benzoic acid molecules that have been bound after heating (Schaate et al., 2011a). The results imply that Al/UiO-66(B5) has lower thermal stability than UiO-66 and Al/UiO-66. Apart from that, the effect of defects in the rigid modulator of benzoic acid, which causes crystallinity to decrease, also contributes to the decrease in thermal stability.

TGA thermograms of UiO-66, Al/UiO-66, and Al/UiO-66(B5).

3.1.5

3.1.5 Analysis of N₂ adsorption–desorption

The surface area of the synthetic material was evaluated through nitrogen adsorption–desorption experiments. All synthesized materials exhibit a type IV N₂ adsorption–desorption isothermal pattern with various hysteresis loops, as depicted in Fig. 8. At a relative pressure (P/P₀) of 0.81–1, UiO-66, Al/UiO-66, and Al/UiO-66(B10) exhibit an H1-type hysteresis loop, indicating that the mesopores are formed from intercrystalline. On the other hand, Al/UiO-66(B5) has a larger hysteresis loop shape than other materials, indicating that it has a greater distribution of meso-sized pores. It is due to the addition of 5 eq of benzoic acid modulator during the synthesis process, which can lead to the formation of meso pore diameters due to defects in the framework (Mouchaham et al., 2018). In contrast to other materials, Al/UiO-66(B20) exhibits an H3-type hysteresis loop characterized by hysteresis over a broad P/P₀ range (Fan et al., 2015). Table 3 displays the surface area, pore diameter, and pore volume of UiO-66, Al/UiO-66, and Al/UiO-66(Bx). Al₂O₃ modification can decrease the specific surface area of the UiO-66. Thus, it may be concluded that the presence of Al₂O₃ in the MOF pore structure causes pore blockage. In addition, the addition of the rigid modulator benzoic acid can reduce the specific surface area because the extra benzene ring from benzoic acid occupies a portion of the pore volume, and an amount of the benzoic acid cannot be removed during the washing process because it is trapped in the UiO-66 framework (Schaate et al., 2011). However, both the volume and average diameter of apertures increased. It can be caused by defects in the substructure of the material that create larger pore diameters. The presence of defects in the framework can result in the fusion of adjacent pores and the formation of Zr sites with insufficient coordination (Zhang et al., 2022). A larger pore size permits dye molecules to diffuse into the pores and interact with the active site on the adsorbent, thereby enhancing the adsorption efficacy of the adsorbent material (Kondo et al., 2012).

(a) N2 adsorption–desorption isotherms, (b) UiO-66, Al/UiO-66, and Al/UiO-66(Bx) pore size distributions.

Table 3 Textural properties of synthesized materials.

Materials	Specific surface area (m²/g)	Total pore Volume		Pore size(nm)
Materials	Specific surface area (m²/g)	Micropore volume (cm³/g)	Mesopore volume (cm³/g)	Pore size(nm)
UiO-66	826	0.29	0.30	3.39
Al/UiO-66	764	0.36	0.39	3.92
Al/UiO-66(B5)	418	0.18	0.46	5.41
Al/UiO-66(B10)	439	0.20	0.37	5.09
Al/UiO-66(B20)	450	0.19	0.20	3.49

It is evident that adding 5 mmol eq of benzoic acid modulator results in a decrease in surface area but also increases pore size and mesopore volume in the process. On the other hand, the presence of more benzoic acid led to a decrease in pore size and mesopore volume, but it also resulted in an increase in surface area. This condition explains why adding a higher concentration of benzoic acid does not have a positive effect on the synthesis of Al₂O₃/UiO-66. However, the addition of a benzoic acid modulator at 5 mmol eq has a significant effect. This could be due to the presence of more defects and coordinative unsaturated sites, which could potentially enhance the color dye absorption process of the substance (Sun et al., 2022). The decrease in surface area at 5 mmol eq is caused by benzoic acid, which creates unbonded bonding ends. Meanwhile, increasing the mmol eq modulator causes the bonding ends to bond with each other, resulting in a larger surface area. In addition, the decrease in pore size and mesopore volume with the addition of the modulator results from the filling of benzoic acid into the Al₂O₃/UiO-66 framework, which may cause pore clogging (Zhang et al., 2024).

3.2

3.2 Adsorption performance of organic dyes

3.2.1

3.2.1 Adsorption kinetics

Fig. 9 illustrates the adsorption of RRB from an aqueous solution onto a synthesized material with 100 ppm concentration as a function of contact time. With increasing contact duration from 5 to 25 min, there is a substantial increase in adsorption capacity. It demonstrates that as the contact time increases, more adsorbent particles interact with the active groups on the dye, resulting in a greater adsorption capacity of the adsorbent. This state will persist until saturation or the optimum contact time is reached. The adsorption capacity starts to become constant at a contact time of 25 to 35 min, indicating that the adsorbent has reached a saturated condition and cannot adsorb any more dye. If the adsorbent is saturated with dye, desorption will occur. Based on the description above, the optimum adsorption time for RRB dye is 25 min.

Influence of contact period on adsorption RRB at room temperature and initial concentration of 100 ppm.

The adsorption rate of RRB dye in water by the synthesized adsorbent material was measured by conducting adsorption kinetics experiments using adsorption findings obtained at different contact periods. The determination of adsorption kinetics is achieved using pseudo first order and pseudo second order reaction kinetics equations. The appropriateness of the adsorption kinetics model for each adsorbent may be determined by evaluating the compatibility of the experimental data and the correlation coefficient (R²). Figs. 10 and 11 show the plots of pseudo first order and pseudo second order adsorption kinetics at different concentrations and temperatures. Pseudo first order models are often used to represent adsorption mechanisms associated with physisorption and diffusion, while pseudo second order models are created explicitly for chemisorption processes (Lv et al., 2019; Zhang et al., 2021). In Tables 4 and 5, the kinetic parameters of RRB adsorption are detailed. All correlation coefficient (R²) values for the pseudo second order model were discovered to be greater than those for the pseudo first order model. Based on Table 5, the rate of initial adsorption diminishes explicitly as the temperature rises. At constant temperature, however, K₂ increases in proportion to the initial concentration of RRB (Table 4) (Chen et al., 2012; Nanthamathee and Dechatiwongse, 2021). The high K₁ value (0.18162 min⁻¹) indicates that the RRB adsorption rate on Al/UiO-66(B5) is faster than in other samples. These findings demonstrate that chemisorption predominates in the adsorption of RRB via the synthesized material, as the pseudo second order model predicted. In other words, the adsorption capacity is mainly determined by the number of active sites, whereas the adsorption rate is affected by chemisorption (Lv et al., 2019). Compared with all samples, Al/UiO-66(B5) had the highest RRB adsorption capacity, increasing by 33.96 % from 104.27 to 139.69 mg/g when compared with UiO-66. The enhanced adsorption of RRB by Al/UiO-66(B5) may be due to the introduction of mesopores and a more significant pore diameter. The Al/UiO-66(B5) sample was analyzed for its mesopore volume, and it was shown that the adsorption capacity of RRB increased notably as the mesopore volume increased. The adsorption capacity of RRB is seen to positively correlate with the increase in mesopore volume (Wang et al., 2021).

RRB adsorption on synthesized materials: pseudo first and pseudo second order kinetics fits at 30 °C and starting concentrations of 100, 150, and 200 ppm.

RRB adsorption on synthetic materials: pseudo first and pseudo second order kinetics at 40 and 50 °C.

Table 4 Kinetic parameters as a function of the starting concentration of RRB.

Materials	Dye Concentration (ppm)	Pseudo first order			Pseudo second order			Qe,exp (mg/g)
Materials	Dye Concentration (ppm)	Q_e (mg/g)	K₁ (min⁻¹)	R²	Q_e (mg/g)	K₂ (g mg⁻¹ min)	R²	Qe,exp (mg/g)
UiO-66	100	79.753	0.08512	0.9412	131.579	0.00083	0.9772	101.146
	150	124.527	0.15559	0.9199	136.986	0.00096	0.9858	108.438
	200	124.537	0.15590	0.9199	142.857	0.00107	0.9897	116.771
Al/UiO-66	100	80.720	0.13308	0.8181	119.048	0.00139	0.9881	104.271
	150	92.481	0.13760	0.7902	131.579	0.00141	0.9822	113.646
	200	112.730	0.13760	0.7958	138.889	0.00142	0.9883	121.979
Al/UiO-66(B5)	100	97.397	0.18162	0.7369	161.290	0.00100	0.9834	139.688
	150	157.707	0.15170	0.8690	169.492	0.00102	0.9874	140.729
	200	190.776	0.16190	0.8495	178.571	0.00105	0.9864	154.271
Al/UiO-66(B10)	100	97.397	0.14430	0.82972	129.870	0.00070	0.9835	98.021
	150	112.314	0.15170	0.8795	123.456	0.00110	0.9896	100.104
	200	152.012	0.17561	0.8911	131.579	0.00119	0.9896	109.479
Al/UiO-66(B20)	100	51.243	0.08006	0.89943	90.909	0.00079	0.9620	64.688
	150	68.820	0.13880	0.8610	89.286	0.00140	0.9766	69.896
	200	58.903	0.13690	0.7916	96.154	0.00159	0.9843	77.187

Table 5 Kinetic parameters as a function of the temperature changes.

Materials	Temperature (°C)	Pseudo first order			Pseudo second order			Qe,exp (mg/g)
Materials	Temperature (°C)	Q_e (mg/g)	K₁ (min⁻¹)	R²	Q_e (mg/g)	K₂ (g mg⁻¹ min)	R²	Qe,exp (mg/g)
UiO-66	30	98.563	0.08512	0.9412	131.579	0.00083	0.9772	101.146
	40	88.482	0.14580	0.8496	125.000	0.00080	0.9623	91.771
	50	79.753	0.15200	0.8162	114.943	0.00074	0.9664	83.438
Al/UiO-66	30	83.254	0.13308	0.8181	119.048	0.00139	0.9881	104.271
	40	80.640	0.14130	0.9075	111.111	0.00124	0.9881	86.563
	50	80.720	0.14130	0.9806	105.263	0.00108	0.9848	78.229
Al/UiO-66(B5)	30	150.831	0.18162	0.7369	161.290	0.00100	0.9834	139.688
	40	150.731	0.15670	0.8733	153.846	0.00092	0.9830	123.021
	50	97.397	0.15800	0.8733	147.059	0.00086	0.9795	114.688
Al/UiO-66(B10)	30	106.517	0.14430	0.8297	129.870	0.00070	0.9835	98.021
	40	104.517	0.15260	0.9022	128.205	0.00055	0.9792	81.354
	50	97.397	0.15360	0.9022	126.582	0.00040	0.9609	73.021
Al/UiO-66(B20)	30	91.241	0.08006	0.8994	102.041	0.00079	0.9620	64.688
	40	51.243	0.14140	0.8824	90.909	0.00037	0.9294	51.146
	50	23.656	0.10790	0.8249	56.180	0.00111	0.9733	32.396

The intraparticle diffusion model is employed to forecast the rate of the controlling phase, which is primarily determined by the diffusion of the surface or pore. Throughout the entire time period, the intraparticle diffusion plot of Q_t against t^0.5, as illustrated in Fig. 12a, never displays a straight line. The adsorption of RRB was a three-step process. The adsorption that took place on the adsorbent's outer surface was the reason for the more significant increase in the initial phase. The rate limit was the intraparticle diffusion during the second step of sequential adsorption. The third step suggested that the active site of the adsorbent surface was nearly entirely covered with RRB molecules. The solution's concentration of adsorbate was so low that intraparticle diffusion began to slow down (Lin et al., 2016). These findings indicate that intraparticle diffusion was not the sole dominant mechanism for RRB adsorption (de Franco et al., 2017). The intraparticle diffusion model's increased C parameter value results in a more significant impact on the boundary layer as a result of the boundary layer's expansion. The Elovich model, which is another adsorption kinetics model, was also evaluated. Fig. 12b illustrates the Elovich adsorption kinetic model diagram. Table 6 contains the parameter values of the Elovich model and the intraparticle diffusion model. The chemisorption process is described by the Elovich model, which is appropriate for systems with heterogeneous adsorbent surfaces. Consequently, it is applicable to a wide range of RRB adsorption systems (Wu et al., 2009). The surface area and the chemisorption activation energy are described by the parameters of the Elovich model. The activation energy and surface area are subject to variation under the presumption that the site distribution is heterogeneous (Pintor et al., 2018). The Elovich model was more appropriate for the adsorption process than the intraparticle diffusion model, as evidenced by the R² value of approximately 0.931 to 0.971.

(a) Plot of Intraparticle Difussion and (b) Elovich model at 30 °C and initial concentration of 100 ppm.

Table 6 Intraparticle Difussion and Elovich parameters at 30 °C and initial concentration of 100 ppm.

Materials	Intraparticle difussion			Elovich
Materials	k_id (g/mg min^1/2)	C (mg/g)	R²	α (g mg⁻¹h⁻¹)	β (g/mg)	R²
UiO-66	15.939	15.856	0.883	1643.017	0.032	0.941
Al/UiO-66	11.439	36.870	0.974	4120.913	0.046	0.954
Al/UiO-66(B5)	15.486	49.803	0.968	5708.411	0.034	0.931
Al/UiO-66(B10)	15.444	11.591	0.962	1466.523	0.034	0.971
Al/UiO-66(B20)	10.909	2.692	0.971	808.184	0.048	0.961

3.2.2

3.2.2 Adsorption isotherm

As the concentration of the RRB solution rises, the adsorption capacity of the RRB dye also increases. A substantial rise in concentration was observed between 100 and 300 mg/L. This result could be attributed to a robust propelling force, as the interactions between the RRB and the adsorbent are enhanced as the concentration rises. Because the adsorbate has saturated the adsorbent between 350 and 400 mg/L, the adsorption capacity becomes constant. The optimum concentration for adsorption of RRB dye, as shown in Fig. 13, is 350 mg/L. Likewise, changes in Al/UiO-66(B5) concentration during the contact time demonstrated a higher adsorption capacity than alternative adsorbent materials. Increasing the active site through modification with benzoic acid is advantageous because it can enlarge the pores and mesopore volume, making adsorbate molecules and the adsorption space more accessible. The bond between the metal center and the organic connector is severed due to the introduction of benzoic acid, which also facilitates the adsorption of anionic dyes by creating more Zr-OH active sites. (Zhang et al., 2021).

Influence of starting concentration on adsorption RRB.

Adsorption isotherm plots were performed at starting concentrations ranging from 100 to 400 mg/L, at temperatures of 30, 40, and 50 °C, and with a contact period of 25 min to get insight into the adsorption process. The Langmuir and Freundlich isotherm models are the most popular ones. Table 7 displays the isotherm parameters and R² values for the two models. The findings favor the Freundlich model above the Langmuir model in this case. Adsorption occurs on heterogeneous adsorbent surfaces with varying adsorption energies, leading to varied adsorption powers, as shown by the Freundlich model established R² value of more than 0.90 (Guo et al., 2014). Table 7 additionally includes Freundlich isotherm parameters like K_F and n. The adsorption feasibility is proportional to the n value; a n number greater than one is desirable. Table 7 shows that all adsorbents have n values greater than one (Nanthamathee and Dechatiwongse, 2021). Fig. 14 and 53 provide plots of RRB adsorption isotherms at various temperatures. The adsorption capacity of RRB adsorbed on the adsorbent decreases with increasing temperature, suggesting that adsorption is an exothermic process. (Wang et al., 2021). Based on the Langmuir isotherm model, the maximum adsorption capacity of Al/UiO-66(B5) was 333.33 mg/g Fig. 15.

Table 7 Langmuir and Freundlich parameters at varied temperatures.

Materials	Temperature (°C)	Langmuir			Freundlich
Materials	Temperature (°C)	Q_max (mg/g)	K_L (L/mg)	R²	n	K_F (mg/g(L/mg)^1/n)	R²
UiO-66	30	158.730	0.0262	0.7776	3.4662	28.9422	0.9057
	40	156.250	0.0221	0.7934	3.1969	24.0997	0.9112
	50	149.254	0.0169	0.8189	2.7809	17.0560	0.9200
Al/UiO-66	30	144.928	0.0336	0.7207	4.1494	35.0018	0.8702
	40	138.889	0.0290	0.7366	3.8183	29.6185	0.8762
	50	131.579	0.0222	0.7627	3.3080	21.5958	0.8858
Al/UiO-66(B5)	30	333.333	0.0243	0.9513	2.7601	39.6345	0.9889
	40	322.581	0.0196	0.9626	2.5681	33.4115	0.9887
	50	312.500	0.0142	0.9769	2.2868	24.6555	0.9884
Al/UiO-66(B10)	30	243.902	0.0116	0.9476	2.2341	15.1545	0.9816
	40	232.558	0.0098	0.9579	2.0903	12.2338	0.9819
	50	232.558	0.0070	0.9716	1.8372	7.9106	0.9842
Al/UiO-66(B20)	30	140.845	0.0034	0.9011	1.5921	1.9977	0.8752
	40	138.889	0.0026	0.9885	1.4486	1.2117	0.9810
	50	84.034	0.0016	0.9240	0.7831	0.0484	0.9613

Fitting the Langmuir model of adsorption isotherm to RRB at different temperatures.

Fitting the Freundlich model of adsorption isotherm to RRB at different temperatures.

Additional isotherm models, specifically Temkin and Scatchard, were also included in the plot, as depicted in Fig. 16. The parameters for the Temkin and Scatchard isotherm models can be found in Table 8. The Temkin isotherm explains the relationship between the adsorbent and adsorbate by considering that the heat of adsorption decreases as the interaction between them takes place during the adsorption process (Batool et al., 2018). According to the Temkin isotherm, a positive value of B suggests that the adsorption reaction is exothermic (Zhang et al., 2010). An analysis was conducted using the Scatchard adsorption isotherm to determine the active site of the adsorbent and estimate the number of sites and their affinity for the adsorption of RRB (Öztürk et al., 2020; Yang et al., 2019).

(a) Fitting the Temkin and (b) Scatchard model of adsorption isotherm to RRB at 30 °C.

Table 8 Temkin and Scatchard parameters at 30 °C.

Materials	Temkin			Scatchard
Materials	B (mg/g)	K_T (L/g)	R²	b (L/mg)	Qs (mg/g)	R²
UiO-66	35.827	0.217	0.865	0.015	183.071	0.627
Al/UiO-66	28.727	0.712	0.834	0.019	165.037	0.583
Al/UiO-66(B5)	74.748	0.182	0.971	0.019	337.495	0.827
Al/UiO-66(B10)	61.767	0.072	0.951	0.009	262.091	0.829
Al/UiO-66(B20)	33.638	0.028	0.826	0.002	202.200	0.645

3.2.3

3.2.3 Adsorption thermodynamics

The adsorption thermodynamic changes of UiO-66 and Al/UiO-66(B5) for RRB organic dyes were explored, and thermodynamic equations were utilized to quantify the adsorption thermodynamic parameters. Fig. 17 depicts the Arrhenius and Van't Hoff plot. Based on the data from the Arrhenius plot, the activation energy (E_a) is −6.140 kJ/mol. Physical interactions are likely responsible for the RRB adsorption to Al/UiO-66(B5), as shown by the low adsorption activation energy (Karaer and Kaya, 2016). Adsorption with an activation energy on the order of less than 40 kJ/mol is categorized as physical adsorption (Santoso et al., 2021). Table 9 lists the adsorption thermodynamic parameters. Table 9 shows that the adsorption of RRB by UiO-66 and Al/UiO-66(B5) is a spontaneous exothermic reaction since both ΔH° and ΔG° are negative. It agrees with experimental adsorption isotherm data shown in Table 7. In general, physisorption is indicated by values of ΔG° between −20 and 0 kJ/mol, whereas chemisorption is meant by values of G° between −400 and −40 kJ/mol. Adsorption in this investigation occurs by physisorption and chemisorption, as shown by the adsorption free energy (ΔG°) values of −40 to −20 kJ/mol (Wang et al., 2021; Zhang et al., 2021). A positive ΔS°value shows that the RRB adsorption process is becoming more random (Wang et al., 2021).

(a) Arrhenius and (b) Van’t Hoff plot of adsorption of RRB on Al/UiO-66 and UiO-66(B5).

Table 9 Parameters of RRB adsorption thermodynamics.

Materials	Ea (kJ/mol)	A (g/mg min)	ΔH° (kJ/mol)	ΔS° (J/mol K)	ΔG° (kJ/mol)			R²
Materials	Ea (kJ/mol)	A (g/mg min)	ΔH° (kJ/mol)	ΔS° (J/mol K)	303 K	313 K	323 K	R²
UiO-66	$-$ 4.648	0.00013	$-$ 13.736	17.435	$-$ 18.084	$-$ 19.284	$-$ 19.328	0.9725
Al/UiO-66(B5)	$-$ 6.140	0.00009	$-$ 14.926	20.795	$-$ 21.189	$-$ 21.516	$-$ 21.599	0.9787

3.2.4

3.2.4 Influence of pH, co-existing ions and dosage

A critical factor affecting dye adsorption is pH, which can change the adsorbent surface charge and the pollution ionization amount (Mohammadi et al., 2017). The adsorption of RRB by UiO-66 and Al/UiO-66(B5) at different pH levels at 30 °C was investigated in the tests. A pH range of 1–11 was employed to assess the influence of pH on RRB adsorption. Adsorption was carried out by adding 10 mg of adsorbent into 20 mL of methylene blue solution with an initial concentration of 100 mg/L. Fig. 18a illustrates the effect of pH on the adsorption capacity of RRB by UiO-66 and Al/UiO-66(B5). As the pH decreases, the adsorption capacity increases gradually. Lower pH levels result in higher adsorption capacities. Upon further examination, this trend can be attributed to the fact that Al/UiO-66(B5) was used as the adsorbent to remove RRB during the test. The pHpzc test results in Fig. 19 show that the surface charges of Al/UiO-66(B5) and UiO-66 decrease with increasing pH, indicating the presence of negative surface charges. The pHpzc value or isoelectric point of UiO-66 was 6.5 in this study, indicating that the adsorbent surface produced a positive effect at pH levels below 6.5 and a negative impact at pH levels above 6.5 (Rao et al., 2016). The pHpzc value of UiO-66 was 6.4 in a previous study (Sun et al., 2019). At pH below 6.5, the surfaces of UiO-66 and Al/UiO-66(B5) have higher positive charges, which facilitates the electrostatic attraction and interaction between the adsorbent and the anionic dye RRB. The adsorption of RRB on Al/UiO-66(B5) is favorable at pH levels lower than the isoelectric point. This indicates the importance of pH in adsorption by changing the contact force between the adsorbate and the adsorbent (Song et al., 2020; Wang et al., 2021).

(a) Influence of initial pH, (b) influence of adsorbent dose, (c) influence of co-exisiting anions of RRB adsorption capacity on UiO-66 and Al/UiO-66(B5).

Point of zero charge (pHPZC) for UiO-66 and Al/UiO-66(B5).

Fig. 18b illustrates the impact of varying doses on the adsorption of RRB dye by UiO-66 and Al/UiO-66(B5). The experiment revealed that the adsorption of RRB positively correlated with the dosage of UiO-66 and Al/UiO-66(B5) until it reached the point of equilibrium. The increased adsorption capacity of RRB may be due to the bigger surface area of the adsorbent and the higher quantity of adsorption sites (Maleki et al., 2015). The study determined that the most effective dosage was 10 mg. When the adsorbent was exposed to a dosage higher than the ideal amount (10 mg), its adsorption capacity remained relatively steady and may have even reduced. This behavior can arise from the overlapping or clustering of Al/UiO-66(B5) adsorption sites, leading to a reduction in the overall surface area of Al/UiO-66(B5) (Mohammadi et al., 2017).

It is crucial to recognize that the wastewater discharged from the textile industry often includes inorganic anions, which might hinder the adsorption process (Banerjee et al., 2019). Investigating the impact of inorganic anions on the adsorption capacity of RRB has significant importance. The presence of additional anions in the environment may hinder the optimum functioning of the adsorbent by competing with dyes. Fig. 18c illustrates the impact of co-existing ions on several inorganic anions, namely Cl⁻, NO₃⁻, CO₃²⁻, and SO₄²⁻. These specific anions were chosen as competitive ions in the conducted adsorption studies. Cl⁻ ions have the most negligible impact on the adsorption efficiency of UiO-66 and Al/UiO-66(B5), whereas CO₃²⁻ and NO₃⁻ ions exhibit a somewhat higher degree of interference. Nevertheless, the presence of the SO₄²⁻ anion significantly impacted the adsorption efficiency of RRB. The strong attraction shown by Al/UiO-66(B5) towards SO₄²⁻ ions might be attributed to the creation of a complex between the open metal site of cluster zirconium and SO₄²⁻ ions (Hall and Bollini, 2019; Wang et al., 2021). Therefore, research on anions demonstrates that including sulphate ions (SO₄²⁻) in aqueous solutions might impede the treatment process when using Al/UiO-66(B5) as an adsorbent.

3.2.5

3.2.5 Proposed adsorption mechanism

FTIR and XRD analyses were performed before and after adsorption to establish the RRB adsorption mechanism. In Fig. 20a, the RRB shows an absorption band at 1629 cm⁻¹ in the stretching vibration of –N = N–. An absorption band in O–H stretching vibration coupled with N–H is seen at 3446 cm⁻¹, and a sulfonate group S = O is seen at 1211 and 1379 cm⁻¹ (Waghmode et al., 2012). RRB deposited on the surface of Al/UiO-66(B5) adds functional groups following adsorption. The metal core clings to the material due to a sulfonate group S = O (–SO₃⁻) that links to it at 1213 cm⁻¹ (Wang et al., 2016). In addition, Sanmuga Priya et al.(Sanmuga Priya et al., 2015) found an absorption band in the stretching vibration of –N = N– at 1637 cm⁻¹. The XRD spectrum (Fig. 20b) shows that UiO-66 and Al/UiO-66(B5) peak locations do not change after adsorption, indicating that their crystal structures do not change (Molavi et al., 2018). After adsorbing, the peak intensity of UiO-66 and Al/UiO-66(B5) faded. It might be because RRB molecules engage with the adsorbents active site (Zhu et al., 2015).

Fig. 20

(a) FT-IR spectra of Al/UiO-66(B5) prior to and subsequent to adsorption, (b) XRD peak profile of UiO-66 and Al/UiO-66(B5) prior to and subsequent to adsorption, (c) UiO-66 and Al/UiO-66(B5) zeta potentials.

As illustrated in Fig. 20c, Al/UiO-66(B5) exhibits a positive surface charge due to a charge imbalance caused by the coordination action between Zr₆O₄(OH)₄ and 1,4-benzene dicarboxylic acid, which attracts hydrogen bonding (Embaby et al., 2018; Wang et al., 2021). The sequential protonation of the Zr₆O₄(OH)₄ node to [Zr₆(OH)₈]⁴⁺ increases the zeta potential of Al/UiO-66(B5) significantly (Wang et al., 2015). The positively charged protonated surface has more OH bond interaction sites and higher electrostatic affinity with the dye sulfonate groups (Fig. 21), increasing adsorption capacity. RRB binds to UiO-66 through anion exchange of the sulfonate group of dye with a hydroxide bridge at Zr₆O₄(OH)₄ node (Embaby et al., 2018). The hydroxyl groups on the MOF nodes and the sulfonate groups on the dye form hydrogen bonds that bind negatively charged RRB in neutral conditions. The RRB adsorption method by Al/UiO-66(B5) involves physical and chemical adsorption. See Fig. 21 for the adsorption scheme. RRB adsorption by Al/UiO-66(B5) involves various mechanisms, such as π-π stacking, n-π bond interactions, electrostatic interactions of negative SO₃⁻ ions, and hydrogen bonds and covalent bonds under acid-base complexation (Khan et al., 2013). In addition, the active side of Al₂O₃ forms Al–OH₂⁺ at neutral pH, which has a positive charge and attracts anionic dyes electrostatically, and benzoic acid as a modulator increases the porosity material due to framework defects.

Proposed mechanism of adsorption RRB dye onto Al/UiO-66(B5).

3.2.6

3.2.6 Regeneration

In dye adsorption applications, adsorbent regeneration is crucial; therefore, the efficacy of the Al/UiO-66(B5) adsorption cycle for RRB adsorption was evaluated. Al/UiO-66(B5) was desorbed in this investigation utilizing a 0.1 M HCl and methanol solution after RRB adsorption. Following adsorption, Al/UiO-66(B5) was rinsed for 30 min with a solution containing methanol and 0.1 M HCl while being stirred (Ediati et al., 2021; Zhao et al., 2020). The sample was then desiccated before conducting recycling experiments using identical conditions. The adsorption capacity of recycled Al/UiO-66(B5) for RRB exhibits a marginal decrease as the recycling quantity increases, as illustrated in Fig. 22. RRB demonstrated an adsorption capacity of 84.53 mg/g after four regeneration cycles. It could be because RRB molecules continue to occupy certain active sites following desorption (Zhang et al., 2021). These outcomes suggest satisfactory adsorbent cycling capability and stability for removing RRB dye. The findings indicated that Al/UiO-66(B5) can be recycled for RRB removal due to its high stability.

3.2.7

3.2.7 Economic analysis of cost production

Cost analysis of Al₂O₃/UiO-66 composite synthesis with benzoic acid modulator is needed as a first step to prepare for industrial scale. The cost of composite synthesis is comprised of two components: the cost of utilising the synthesis precursor and the cost of the electricity source employed (Ediati et al., 2023). The cost of synthesis precursors includes all the solvent precursors used to wash the synthesized material, with a total of US$45.84/2g. Meanwhile, the electricity cost comes from the use of hot plates and ovens during the solvothermal and drying processes, with a total of US$0.42/2g. Thus, the total production cost of Al₂O₃/UiO-66 composite with benzoic acid modulator is US$46.26/2g. The maximum RRB adsorption capacity was 333.33 mg/g. Thus, compared to the commercial price of UiO-66 (US$216/2g), the Al₂O₃/UiO-66 composite with benzoic acid modulator is much more economical, but still has high adsorption performance.

4 Conclusions

The Al₂O₃/UiO-66 composite was successfully synthesized by solvothermal method and benzoic acid modification. The synergistic effect between Al₂O₃ modulation and benzoic acid plays an important role in the formation of defects, mesopores, and the enhancement of active sites to improve the adsorption performance. The resulting synergistic effect was proven by Scanning Electron Microscopy (SEM) and N₂ physisorption characterization. Deformation of Al₂O₃/UiO-66 by benzoic acid can effectively remove Remazol Red 5B (RRB) dye from aqueous solution. At a temperature of 30 °C, the Al/UiO-66(B5) adsorbent achieved an optimum adsorption capacity of 286.35 mg/g. The reaction kinetics show that the adsorbent follows pseudo-second-order kinetics. The presence of coexisting anions (Cl⁻, NO₃⁻, CO₃²⁻, and SO₄²⁻) can disrupt the RRB adsorption process by the adsorbent. Thermodynamic studies revealed exothermic and spontaneous adsorption processes across the investigated temperature range. Furthermore, the composite demonstrated easy regeneration using 0.1 M HCl and methanol, with an adsorption capacity of 84.53 mg/g and up to four adsorption–desorption cycles. Therefore, the development of Al₂O₃/UiO-66 composites with benzoic acid modification is a promising innovation to produce adsorbents with high adsorption capacities and thus reduce the presence of dye waste in the environment.

CRediT authorship contribution statement

Ratna Ediati: Writing – review & editing, Supervision, Methodology, Conceptualization. Alvin Romadhoni Putra Hidayat: Writing – review & editing, Resources, Methodology, Formal analysis. Terry Denisa Syukrie: Writing – original draft, Methodology, Investigation, Conceptualization. Liyana Labiba Zulfa: Writing – review & editing, Formal analysis. Miftahul Jannah: Resources, Formal analysis. Harmami Harmami: Writing – review & editing, Data curation. Hamzah Fansuri: Writing – review & editing, Supervision. Badrut Tamam Ibnu Ali: Writing – review & editing, Investigation.

Acknowledgements

The authors gratefully acknowledge financial support from the Institut Teknologi Sepuluh Nopember (ITS), Indonesia, for this work, under a project scheme of the Publication Writing and IPR Incentive Program (PPHKI) 2024.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Introduction

Experimental

Chemicals
Synthesis of UiO-66
Synthesis of Al2O3/UiO-66
Synthesis of Al2O3/UiO-66 by benzoic acid
Characterization
Test point of zero charge (pHpzc)
Adsorption experiments

Results and discussion

Material characterization
Adsorption performance of organic dyes

Conclusions

CRediT authorship contribution statement

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