Comparative Studies of Regeneration and Single Batch Design for the Properties of Basic Blue-41 Removal Using Porous Clay and Porous Acid-Activated Heterostructures

Abstract

In this investigation, the parent clay mineral montmorillonite (Mnt) was acid activated using sulfuric acid (H₂SO₄) at a specific mass of acid to clay mineral of 0.2 (A-Mnt) prior to the preparation of the porous clay heterostructure precursor. The derived porous acid-activated clay heterostructure (PACH) exhibited properties different from those of the conventional one (PCH). The synthesized materials were characterized using different physiochemical techniques, such as X-ray fluorescence (XRF), powder X-ray diffraction (XRD), thermogravimetric analysis (TA), ²⁹Si MAS-NMR, nitrogen adsorption–desorption, and acidity using cyclohexylamine (CHA) as a probe molecule. The PACH had a surface area of 890 m²/g and an acidity of 0.56 mmol of protons/g. An evaluation of PCH materials was conducted to assess their effectiveness in removing basic blue 41 (BB-41) from aqueous solutions. The removal process was analyzed based on the initial concentration and pH of the BB-41 solution, and the amount of solid used, employing a batch approach. The removal efficiency was found to be greater at higher pH values, specifically between six and nine. Using the Langmuir model, the maximal removal capabilities of the studied materials were determined to be between 274 and 300 mg/g. According to the results of the regeneration tests, PACH materials could still be employed after seven cycles with a 25% efficiency loss and a 50% efficiency loss for PCH materials. Utilizing the Langmuir model equations and mass balance, a single-stage batch design was suggested to estimate the required masses to remove BB-41 at different percentages from a starting concentration of 200 mg/L.

1. Introduction

In the next 30 years, the global population is expected to reach 10 billion. This growth will exacerbate existing challenges, particularly as the world’s freshwater resources remain unevenly distributed [1]. Over the next decade, water scarcity will remain a critical issue, with some regions facing severe shortages, making it one of the most pressing global threats [2]. To combat this growing crisis, there is an increasing demand for alternative water sources [3,4]. Research is therefore essential to explore and utilize non-traditional water sources, such as harvested rainwater and recycled wastewater [5].

Industrial activities significantly contribute to environmental pollution, introducing contaminants such as dyes, polycyclic aromatic hydrocarbons, and heavy metals into water bodies [6,7]. These pollutants, which can infiltrate the food chain, are major drivers of water crises and are closely linked to waterborne diseases [8,9,10]. Addressing these challenges requires a sustainable supply of clean water supported by efficient wastewater treatment technologies for domestic and industrial needs [6]. Hence, the textile industry, one of the most polluting sectors for aquatic ecosystems, discharges large volumes of contaminated effluent into the environment [11]. Consequently, wastewater from this sector undergoes various treatment processes before being reintroduced into the water cycle. These treatments employ chemical, biological, physical, and hybrid methodologies [6,12,13]. Among these, adsorption has proven to be one of the most effective techniques. As a result, ongoing research focuses on identifying alternative adsorbents that are both efficient and environmentally friendly [14].

Clay minerals are still attracting a lot of interest from the scientific and industrial communities. When it comes to the preparation of new materials, clay minerals are often regarded as the top choice due to their environmental compatibility and cost effectiveness. Considerable effort is dedicated to exploring innovative methods for utilizing clays in both their native and modified forms for various applications [15]. This process enhances the properties of clays, including acidity, pore size, surface area, polarity, and other characteristics that influence their effectiveness as catalysts or adsorbents [16,17].

One of the modification processes for clay minerals is acid activation. It consists of treating the clay powder or suspension mainly with inorganic acids under different conditions, such as the concentration of acid, temperature of activation, and reaction time [18,19,20]. Hydrochloric acid (HCl) and sulfuric acid (H₂SO₄) are the main acids used. H₂SO₄ is a strong oxidizing agent, which can help remove impurities and organic matter from the clay surface, further improving its adsorption capacity. Compared to H₂SO₄, HCl does not possess the same oxidizing properties. Nitric acid is also employed. Due to its toxicity, nitric acid is not used in the chemical industry for the production of modified materials. However, it was used as an engineered barrier system for isolating liquid radioactive waste containing nitric acid. In some cases, organic acids were also employed [21]. These days, the combination of their enhanced properties, environmental benefits, and diverse applications, such as catalysts and adsorbents, has made acid-activated clays a subject of significant interest for several international research groups [22,23,24].

Acid-activated clay minerals were used as hosts to prepare different types of materials, such as pillared acid-activated clays [25,26], nanocomposites [27], and organic acid-activated clays [28]. In this regard, preparing the later family of materials requires some attention, especially the type of surfactant used and the source of the starting clay minerals. The common surfactant is cetyltrimethylammonium (C16TMA) salt, either bromide from aqueous solutions [29] or another type of surfactant such as C16TMA hydroxide (C16TMAOH) [30]. The amount of surfactant uptake depends not only on the used surfactant solution but also on the sources of the clay mineral, as well as its physicochemical properties [31].

In the early 1990s, researchers at Mobil Corporation made a significant advancement by discovering novel ordered mesoporous materials, which they designated as MCM-41 (Mobil Composition of Matter No. 41). This innovative approach involved the use of alkylammonium-based surfactants as templates to create ordered and uniform porous silica materials. The materials in question exhibit pore diameters ranging from 1.6 to 10 nanometers, which surpass the dimensions commonly associated with zeolites. [32,33]. Cetyl trimethylammonium surfactant was used as a templating agent, while the silica source was the tetraethyl-orthosilicate (TEOS) [32,33]. Based on this idea, in 1992, a novel way that used a template approach for synthesizing porous layered silicate was introduced by the Pinnavaia group, and the materials were called porous clay heterostructures (PCHs) [34]. The synthesis of porous clay heterostructures leads to the production of silica-based materials characterized by a uniform pore size distribution. However, this process is currently constrained by the exclusive use of tetraethyl orthosilicate (TEOS) as the silica source [35]. Porous clay heterostructures form a class of unique materials in a particular pore size range and act similarly to a bridge between mesoporous materials, pillared clays, and microporous zeolites. The synthesis of such an advanced structure starts with an initial preparation step for the organoclay, which is considered as important as further processing and development. This primary preparation of the organoclay provides the very ground for the complex creation of the porous clay heterostructure and allows the properties to be engineered toward better functionality in applications [36].

These materials are reported to have a variety of applications, including medication delivery, catalysis, and environmental remediation [36,37,38,39]. The two main variables that needed to be considered are the surface area and acidity [39]. Either metal implantation into the silica framework or impregnation can alter the acidity [40,41]. Enhancing the host clay’s acidity before the TEOS and C₁₂amine reaction is the third method. It involves using acid-activated clay instead of pure clay. Furthermore, there is an increase in the surface area of the acid-activated clay mineral, which may contribute to the improvement of the surface area of the resultant PACHs [42,43]. Few investigations have been conducted on the development of organic acid-activated clay as a dye removal agent [44,45]. According to a literature survey, there are only two research papers on porous acid-activated clay heterostructures (PACHs) [46,47], and only one catalytic application is reported for this material [46].

Thus, the purpose of our study is to develop porous acid-activated clay heterostructures (PACHs) utilizing TEOS and C₁₂amine in a typical manner. Initially, a ratio of 0.2 (acid/clay (w/w)) was chosen based on the dry weight of the clay and 98% H₂SO₄. This ratio ensures the preservation of the layered structure [47]. Calcination at 550 °C was used to remove the surfactants. Characterization was carried out using different physiochemical techniques: XRF, XRD, TGA, and ²⁹Si MAS-NMR. The textural properties were also reported in addition to the acidity properties using CHA as a probe molecule. For an effective removal agent of dye pollutants, the high surface area and pore size are considered major requirements. The PCH materials satisfy these requirements. An investigation was conducted on the removal of basic blue 41 (BB-41) dye from synthetic dye solutions. The effects of the starting dye concentration, adsorbent dose, and pH were investigated as removal parameters. Isothermal experiments were conducted to estimate the maximum removal capacities. Moreover, the oxone–cobalt technique was employed to monitor the materials’ regeneration capabilities. Utilizing the parameters from the isotherm model in conjunction with the mass balance equations, a design for a single-batch adsorber stage has been proposed.

2. Materials and Characterization

2.1. Materials and Chemicals

The basic blue 41 (C₂₀H₂₆N₄O₆S₂) dye, oxone, sodium hydroxide (NaOH), cobalt nitrate salt, sulfuric acid H₂SO₄ (98%), and hydrochloric acid (HCl) were purchased from Across Organics company (Loughborough, UK). Tetraethoxide orthosilicate (TEOS), dodecylamine (C₁₂amine), and cetyl trimethylammonum bromide (C16TMABr) surfactants were supplied by Aldrich (St Louis, MO, USA). The montmorillonite (STX-1, Mnt) used in the study was collected from Purdue University’s Source Clays Repository/United States.

These chemicals were purchased as analytical reagent grade and used without further purification. Deionized water was utilized to prepare a 1000 mg L⁻¹ stock BB-41 solution, which was then further diluted as required to obtain the desired dye solution for batch testing.

2.2. Acid Activation of Montmorillonite

A specific mass of Mnt was added to a solution of H₂SO₄ (fixed volume of acid/clay with a mass ratio of 20 mL g⁻¹) and stirred overnight at a temperature of 90 °C [30]. Based on the dry weight of Mt and the H₂SO₄ (98%) solution, the acid/clay ratio of 0.2 (in weight) was determined. After continuous rinsing, the resultant acid-activated clay mineral was separated and washed several times with double distilled water until the SO₄²⁻ ions were eliminated (BaCl₂ test). Finally, the synthesized material was allowed to dry at room temperature. The substance will be known as A-Mnt.

2.3. Preparation of Porous Acid-Activated Clay Heterostructures

The process described elsewhere was followed to create A-Mnt intercalated with C16TMA cations as the initial step [47]. After one hour of stirring, one gram of A-Mnt clay mineral was suspended in 100 milliliters of deionized water. A specified volume of C16TMABr solution (0.5 M) was added to the suspension at 50 °C and incubated for an overnight period, along with a two-fold excess of A-Mnt’s CEC. After filtering and washing with deionized water, the C16A-Mnt clay was allowed to air dry at room temperature.

Neutral amine (C₁₂H₂₅NH₂) and TEOS were reacted with one gram of the produced organic acid-activated clay at a molar ratio of C16A-Mnt clay/C₁₂amine/TEOS of 1/10/75. At room temperature, the mixture was stirred for four hours. Following the reaction, the precursor was filtered, allowed to air-dry overnight, and identified as PACH-pre.

In order to provide a comparison, the identical process was carried out utilizing raw Mnt clay, which is acid-activated clay-free. The sample will be identified as PCH-pre.

The PCH and PACH precursors were calcined in an air environment for six hours at 550 °C, heating at a rate of 1.0 °C per minute. The resulting materials were assigned as PCH-cal and PACH-cal, respectively.

2.4. Batch Removal of BB-41

The parametric effects, such as pH (2 to 11), adsorbent dose (0.050 to 1 g), and initial dye concentrations (50 to 500 mg/L), were tested in a batch removal study [39]. The dye solution for the batch experiments was placed in a 50 mL Erlenmeyer flask with 0.050 g of sample. Using a controlled water bath shaker, dye solution mixing was carried out at 125 rpm at a fixed temperature of 25 °C. At the end of each batch test, the samples were separated using Whatman filter paper (Sigma-Aldrich, St Louis, MO, USA), and the final concentration of BB-41 in the supernatant was evaluated employing a UV spectrophotometer (Cary 100 model, Varian, Victoria, Australia), with a characteristic wavelength of 610 nm.

The linearity and precision of the analytical calibration curve for the BB-41 absorbance vs. concentration, which was used to trace the concentration of BB-41, were evaluated using the relative standard deviation (RSD) ± 20% and an accepted standard for calibration R² > 0.995 [9].

2.5. Regeneration Procedure

It is well known that the regeneration process plays a crucial factor in the practical application of any new removal agents. In our evaluation of the synthesized material, we considered this issue. Therefore, after adsorption, the saturated sorbent material was filtered, washed with distilled water, and then regenerated in a solution made up of cobalt nitrate salt and oxone [43]. The Co²⁺ cations acted as a catalyst to decompose the peroxymonosulfate, which proceeded via a radical mechanism. This sulfate radical (SO₄⁻) was used to degrade the adsorbed BB-41 via sulfate radical oxidation. The resulting solid was collected and extensively washed with distilled water to eliminate residual contaminants. Thereafter, the regenerated sorbent was brought into contact with fresh BB-41 dye of Ci = 200 mg/L and stirred continuously for six hours to ensure a sufficient interaction. The same experimental run was repeated with the same sorbent to infer performance in further regeneration cycles.

2.6. Characterization Techniques

These synthesized materials were characterized using various methods and techniques. Using the XRF approach, the chemical composition of the various synthesis steps was ascertained (XRF Bruker model 354). Powder X-ray diffraction (PXRD) data were obtained utilizing a Bruker Advance 8 diffractometer (Karlsruhe, Germany). The analysis of the acid activation process and the synthesis of PCH materials was conducted within the 2θ range of 1° to 15°. The TG analysis was carried out using a model SDT2960 from TA instruments (New Castle, DE, USA). The sample was heated in air at a rate of 10 °C per minute to 800 °C from room temperature. Using a Bruker 400 (Karlsruhe, Germany), ²⁹Si MAS NMR spectra were obtained. Additional information is available elsewhere. The materials’ textural characteristics were ascertained utilizing N₂ adsorption–desorption isotherms at 77 K, which were acquired using a Quantachrome instrument (Denver, CO, USA). The BET method was used to compute the specific surface area (S_BET). At a relative p/p_o = 0.95, the total pore volume (T.P.V.) was taken into consideration. The acidity in terms of the proton concentration was determined by the desorption of the probe molecule CHA as follows: the samples were exposed to liquid CHA at room temperature (RT). Then, they were kept overnight at RT, and finally, they were transferred to an oven set at 80 °C and incubated for two hours [39].

Samples of 50 mg were mixed with 25 mL of 0.01 M NaCl solutions that had been carefully adjusted to pH levels of 2, 4, 6, 8, 10, or 12 using either 0.1 M HCl or NaOH. This method was utilized to accurately determine the point of zero charge (PZC) for the materials under investigation. After agitating the samples employing a shaker apparatus for 24 h at RT, the filtered solutions’ pH values were determined. To assess the PZC, the values of the initial and final pH were graphically plotted [48].

3. Results and Discussion

3.1. XRF Data Analysis

During the acid activation process, the percentage of SiO₂ was increased, accompanied by decreases in the Al₂O₃ and MgO contents, due to the leaching of these metals from the clay layers (

Table 1. The obtained XRF data for the parent clay, acid-activated clay, and related PCH precursors.

Samples	Mnt	A-Mnt	PCH	PACH
SiO2	56	63.9	79.9	84.2
Al2O3	22.4	16.3	5.54	3.21
MgO	3.00	1.59	0.66	0.54
CaO	1.77	0.055	0.014	0.033
Fe2O3	0.884	0.628	0.139	0.467
TiO2	0.236	0.164	0.03	0.055
K2O	0.108	0.084	0.03	0.055
CEC (meq/g)	0.92	0.81	---	---

Notes: --- Not measured.

). The cation exchange capacity of A-Mnt was also reduced from 0.92 meq/g to 0.81 meq/g [49]. The leaching of the magnesium and aluminum metals from the parent clay layers was responsible for the CEC’s reduction. Similar data were reported for other types of clay minerals [43,50].

Meanwhile, The XRF data from the PCH and PACH precursors suggest that the incorporation of the silica source during the synthesis of PCH was attributable to the increased SiO₂ content, which was exchanged into the interlayer spacing at the expense of the exchangeable hydrated cations found in the host clay and C16TMA cations [51].

The PACH material prepared from acid-activated clay exhibited a higher content of SiO₂ (%) compared to the pristine one. This was related to the high initial content of SiO₂ in the starting A-Mnt. In addition, during the preparation of PCH, it was observed that a further reduction in other metal oxides was accompanied by an increase in the silica content [52]. Herein, the XRF analysis indicated the success of the PCH synthesis.

3.2. XRD Data Analysis

The obtained PXRD patterns for the starting clays and their PCH derivatives are presented in Figure 1. The Mnt starting material displayed a sequence of reflections at 2θ = 5.81° with d₀₀₁ = 1.52 nm, 2θ = 17.43° with d₀₀₃ = 0.506 nm, and 2θ = 19.84° with d₁₀₀ = 0.447 nm. According to the JCPDS card (No. 00-003-0010), it is corresponding to Ca-montmorillonite [53].

After acid activation, the reflection intensity of (001) and (003) decreased in intensity due to the loss of layer stacking. The d₀₀₁ of 1.52 nm was maintained. It was reported that during acid activation, the calcium ions (Ca²⁺) were replaced by protons, leading to a decrease in the d₀₀₁ basal spacing from 1.52 nm to 1.11 nm [54]. In the present case, the observed variation could be related to the auto-transformation of the acid-activated clay during washing and drying or to the difference in water content in the interlayer space, as confirmed by the TGA data (see below) [55,56]. After calcination at 500 °C, the d₀₀₁ values of Mnt and A-Mnt shrunk to 1.11 nm due to the release of bound water molecules to the cations in the interlayer spacing (Figure 1) [57].

The PXRD data indicated that the PCH-pre exhibited a d₀₀₁ of about 3.92 nm, with a higher intensity of the first reflection of PCH-pre compared to PACH-pre (Figure 1). This value confirmed that PCH synthesis was successful and comparable for similar materials [58,59]. Multiple reflections were absent in the PXRD pattern, aligning with previously reported data on conventional PCHs. [58,59]. It appears that the basal spacing of the initial organoclays had no impact on the d₀₀₁ value of the PCH precursor. The organoclay displayed a d₀₀₁ value of 2.1 nm, while the acid organoclay displayed a value of 3.8 nm.

The basal spacing dropped to 3.60 nm during calcination at 550 °C as a result of the C₁₂amine surfactant burning, the framework’s dehydroxylation, and the silica channels’ cross-linking to the clay sheets (Figure 1) [52]. In several cases, the initial reflection was not noticed following the calcination of PCH materials made from various clays, indicating virtually no crystallinity. The result suggests that the strong interaction between the templates and the silica framework causes a structural collapse of the mesoporous silica tubes during calcination [47], thus confirming that the molecularly organized silicate-–surfactant mesophases exhibit instability upon the removal of their templates [60].

The changes in the crystallite size were calculated using the Debbye and Scherrer equation from the (001) reflection of Mnt and A-Mnt samples. The crystallite size increased from 26.4 nm to 35.2 nm after acid activation. However, after calcination at 500 °C, the crystallite sizes were reduced to 25.1 nm and 26.5 nm, respectively. This decrease was related to the sintering process during calcination and an increase in the quantity of the stacking fault.

3.3. TGA Data Analysis

The TGA/DTA technique was employed to investigate the thermal stability of the starting montmorillonites and PCH precursors. The TGA of Mnt showed a 24% overall mass loss across a number of temperature ranges (Figure 2). The dehydration in the first range happened in two stages: between 25 and 100 °C and between 100 and 200 °C. These temperatures correspond to the loss of water molecules that were physically adsorbed on the surface and that bound to the cations in the interlayer region, respectively. Two maximum temperatures of loss were shown by the DTG curve at 70 and 137 °C (Figure 2). Dehydroxylation of the clay layers, with a DTG peak at 653 °C, was responsible for the 6.8% mass loss in the 500–850 °C range that showed a linear trend with respect to temperature [29].

In the case of acid-activated clay (A-Mnt), the intensity of the peak related to the loss of water attached to Ca²⁺ cations at 137 °C was significantly decreased due to the replacement of Ca²⁺ ions with protons, with a mass loss of 15%. A shift of the dehydration mass loss of water molecules to lower temperatures from 137 °C to 92 °C occurred due to the ease of the loss of dehydration water compared to Mnt water (Figure 2) [45]. Even though the PXRD data indicated the same position of 1.54 nm, the loss of water occurred in a different manner. The percentage of the mass loss related to silicate layers dehydroxylation decreased in intensity, and the corresponding DTG peak became broad and shifted from 652 °C to 612 °C as a result of the elimination of some hydroxyls during the acid treatment and easier loss of the remaining hydroxyls after acid activation (Figure 2) [30].

Compared to the initial Mnt clay, the PCH-pre’s TGA showed distinct features. A total of 20% more mass was lost as a result of surfactant and co-surfactant molecule breakdown, which happened in numerous stages over the course of 100–400 °C and resulted in two DTG peaks at 204 and 334 °C. With a low-intensity DTG peak at 50 °C, the mass loss of water molecules was further decreased by the presence of C₁₂amine molecules, from 11% to 5%. In the temperature range of 420 to 780 °C, a consistent mass loss of 8% was observed. This loss can be attributed to the complete elimination of all carbon species, as well as the dehydroxylation of clay sheets and silica species, which are associated with the corresponding DTG peak at 652 °C (Figure 3) [61,62].

The PACH-pre derived from acid-activated clay behaved similarly to PCH-pre at lower temperatures, with an increase in the mass loss of water molecules (Figure 2). The presence of Al or Zr in the silica framework, or the acid sites on the surface of the montmorillonite layers, which are known to be catalytically active in the hydrocarbon cracking process, promoted the degradation of aliphatic amines [52,63]. The lack of a discernible shift in the DTG peaks in the case of PACH-pre may suggest that the initial A-Mnt acidity was insufficient to have a discernible effect. Comparatively speaking, PACH-pre contained fewer surfactants and co-surfactants than PCH-pre.

3.4. ²⁹Si NMR Data Analysis

The ²⁹Si MAS NMR technique was employed to identify the different environments of Si atoms and their changes in the parent clay minerals and derivatives of PCH materials. The features are presented in Figure 3. The starting Mnt clay mineral exhibited a strong resonance band at −95 ppm associated with Q³ for the SiO₄ groups crosslinked in the tetrahedral sheet, with an additional broad band with low intensity at −110 ppm related to Q⁴ Si species resulting from the presence of quartz phase as impurities [39,45]. Following acid activation, there was a drop in the strength of the band at −94 ppm and an increase in that at −110 ppm due to some structural collapse (Figure 3). This resulted in the generation of protonated three-dimensional silica, which is the primary building block of this phase and is what causes clay mesopores to form [49,64].

To make it easier to distinguish between the silica network created by TEOS polymerization and the parent montmorillonite clay, NMR analysis was also conducted on the silica material made from a combination of C16TMA, C₁₂amine, and TEOS without the addition of Mnt. The final product showed two ²⁹Si resonance bands at −100 and −110 ppm (Figure 3), which were identified as the Q³ Si(OSi)₃OH and Q⁴ Si(OSi)₄ centers in the formed silica framework [65]. Three signals were detected in the ²⁹Si MAS NMR spectra of the PCH-pre and PACH-pre. These signals occurred at −94, −99, and −110 ppm. Comparing the spectra revealed that the −94 ppm resonance originated from the framework Q³ silicon sites in Mnt clay, whereas the −99 and −110 ppm signals may be attributed to Q³ and Q⁴ Si nuclei present in the silica species formed from the C16TMA/C₁₂amine/TEOS system [62,65].

The latter peak shifted slightly to −97 ppm during calcination at 550 °C, suggesting an alteration of the Q³ sites’ environment in the synthesized calcined clay mineral (Figure 4). In addition to this resonance, the Q³ and Q⁴ sites of the TEOS-derived silica network, which are associated with surface silanol and bulk Si sites, respectively, exhibited contributions of −102 ppm and −110 ppm. [62,65]. A comparison with the spectrum of the precursor demonstrated that the crosslinking events preceding thermal treatment resulted in an increased strength of the Q⁴ resonance relative to the other components of the NMR signal [39]. ²⁹Si MAS NMR results for PACH-cal were comparable to those of PCH-cal, with slight decreases in intensity and the degree of broadening of the characteristic bands.

3.5. The Textural Properties

Table 2. The PACH and PCH materials’ textural properties and acidity, which were obtained from different clays after calcination at 550 °C.

Samples	SBET (m2 g−1)	T.P.V. (mL g−1)	A.P.D. (nm)	Acidity *
Mnt	95	0.07	10.9	0.51
A-Mnt	150	0.11	6.4	0.61
PCH-pre	197	0.246	4.98	---
PACH-pre	271	0.269	3.97	---
PCH-cal	740	0.49	2.64	0.58
PACH-cal	890	0.57	2.56	0.56

Notes: * in terms of mmol of protons per gram of samples; --- not measured, S_BET stands for specific surface area, T.P.V. for total pore volume, and A.P.D. for average pore diameter.

summarizes the estimated total pore volumes (T.P.V.s), specific surface areas (S_BETs), and average pore diameter (A.P.D.s) of starting clays and their PCH counterparts. The S_BET of the Mnt clay was 90 m²/g; this value increased to 150 m²/g with acid activation. Comparable information was provided for other acid-activated clays of various origins [30,66,67]. The creation of an amorphous silica phase during the dissolution of the clay layers was the cause of the increase in S_BET. In several instances, the surface areas rose to a certain point during acid activation before declining. The surface area rose and did reach a maximum for the same Mnt clay, with a value of 270 m²/g [43].

After the reaction with C₁₂amine and TEOS, PCH-pre and PACH-pre (without calcination) exhibited reasonable specific surface areas of 197 and 271 m²/g, respectively. Similar values were reported for other precursors [39]. The values were higher than the starting Mnt and A-Mnt and could indicate that the precursors provide reasonable adsorption sites for the nitrogen molecules either on the surface or inside the pores (Table 2).

After being calcined at 550 °C (surfactant-free PCH), the S_BET of PCH-cal increased significantly and exhibited a surface area value of 740 m²/g, which is greater than those found in reports for comparable materials made with various clay types and origins [37,68,69]. The improvement of the S_BET resulted in the removal of the surfactants that made smoothly the complete accessibility of the pores. In contrast, the PACH-cal revealed an 890 m²/g value. In fact, we expected this result because the initial clay had a higher S_BET, which could have an impact on the final value (Table 2). The measured value was less than the stated value (915–950 m²/g) for PACHs [46]. This variation could be assigned to the employed clay composition, which influences the thermal stability of the observed PACHs. In this regard, several research groups reported comparable findings for pillared acid-activated clays [25,70].

Furthermore, PCH-cal and PACH-cal had pore volumes (T.P.V.s) that were much higher than those of the starting clays, and some documented similar materials in the literature [37,68,69,70]. PACH-cal showed a general increase in pore volume when compared to its PCH-cal counterpart. Despite the fact that the initial A-Mnt had a larger pore volume than the non-acid-activated clay (Mnt), it appears that there was an appreciable influence on the PACH pore volume in our situation. These were the same data as those stated in the reference [46]. Assuming the cylindrical shape of the pores, the mean diameter of the pore can be calculated based on the formula d = 4V_p/S_p (where the total pore volume is V_P and the total specific surface area is S_p). Interestingly, the A.P.D. decreased for the acid-activated clay (A-Mnt) from 10.9 nm to 6.4 nm compared to the pristine Mnt. The decrease in A.P.D. could indicate the formation of some mesopores during acid activation (Table 2) [43,67]. The A.P.D values for different PCH calcined materials continued to decrease and reached values in the range of 2.56 to 2.64 nm; however, they were still in the mesoporous range [71,72].

3.6. Acidity Measurements

Using cyclohexylamine thermally programmed desorption, the acidity of the calcined samples was determined. The technique uses thermogravimetric analysis (TGA) to quantify the amount of acid sites—primarily proton sites—that can interact with the base after being heated to 80 °C. As indicated in Table 2, the acid content (mmol of protons/gram of samples) was determined by desorption mass loss of the base from acid sites (290–415 °C), suggesting that one mole of CHA equates to one mole of protons, as reported in Section 3.4.

Table 2 presents the measured acidity values, and it counts the number of acid sites that are both strong enough and easily accessible to interact with CHA.

In comparison to the pristine clay (Mnt), the acid-activated clay (A-Mnt) exhibited a higher acidity of 0.61 mmol/g, which is in agreement with the reported data [25,26]. The PCH-cal material had a greater acidity level of 0.58 mmol of H⁺/g. The resulting PACH-cal had a lower acidity than the acid-activated clay (A-Mnt), although it was still lower than PCH-cal. This value was less than that of other PACHs, as documented in the literature [46,47]. The difference could be related to the different origins and types of clay minerals used. After the surfactant was removed, the protons were released to balance the clay layer charge, and the acidity of PCH materials rose [72,73]. As previously stated [47], it stands to reason that increasing the clay sheets’ acidity will also increase the Pachs’ acidity. In our instance, however, the disintegration of the layered structure prevented the cyclohexylamine from reaching the acid sites, which had an impact on the PACHs’ acidity values.

In addition, the existence of certain residual carbonaceous materials in the calcined PACHs may have caused the gray tint, which could have masked the acid sites. Similar outcomes were noted for pillared acid-activated saponite (at a 0.2 acid/clay ratio), where the pillaring process resulted in a decrease in acidity [25].

The DTG features of the desorption of cyclohexylamine indicate that the peak for A-Mnt shifted to higher temperatures, indicating a change in the strength of acidity compared to the Mnt raw clay [47]. In the case of PCH-cal, the feature was different and two DTG peaks were detected at 209 °C and 347 °C, which were associated with different acid sites with different strengths [47,72,73]. However, PACH-cal exhibited similar features, and the intensity of the second DTG peak was enhanced, resulting from the change in the strong acid site population after acid activation, and was not enough to affect the overall acidity [47].

3.7. Influence of BB-41 Removal Parameters

The removal efficiency of a solid material refers to its ability to extract a specified percentage or proportion of solute from a solution relative to a given mass of that material. In this study, all materials were calcined at a temperature of 550 °C prior to use, unless otherwise noted.

3.7.1. Effect of pH

The pH of the solution is a key factor in the removal process of textile dyes, influencing dye ionization, the solid surface charge, and overall removal mechanisms [74,75,76]. Optimizing pH conditions is essential for enhancing dye removal efficiency in wastewater treatment processes. In this investigation, batch experiments were carried out at varying pH values of the original BB-41 solution, while maintaining constant values for the adsorbent dose of 1 g/L, the overnight contact period, and the starting concentration of 200 mg/L.

The results using PCH-cal presented in Figure 4 show that the percentage of BB-41 removal significantly increased from 45% to 93% as the pH of the solution increased from two to six. A removal of 100% was achieved at pH 7.5. An additional increase in the pH of the solution above 10.5 caused the formation of brown precipitate in the BB-41 solution [52], and no removal study was carried out. In a highly basic medium (pH > 9), the concentration of (OH⁻) in the medium becomes very high, leading it to react with the positively charged cationic dye molecules. As a result, the hydroxide ions could neutralize the dye charge and form insoluble salts as a precipitate. Moreover, dye molecules may undergo hydrolytic cleavage of their reactive groups or other structural modifications, forming less soluble hydrolyzed byproducts. A similar behavior was observed for methylene blue dye (a type of basic dye) and was related to the formation of hydrolytic decomposition products [76].

In the case of PACH-cal, a similar trend was noticed; however, the maximum removal percentage was shifted to a higher pH value close to 8.5. Basic blue-41 is a cationic dye due to the nitrogen group in the molecule, which has a positive charge and influences the dye’s interaction with sorbents.

Prior to the acid treatment, the natural Mnt clay had pH_pzc of 7.8. The pH_pzc values of montmorillonites are in the range of 4.5 to 8.5, depending on the origin of the clay minerals [45,77,78,79]. The acid-activated clay’s pzc value of 4.5 seems to be lower than that of the natural clay (of 7.8). The result is not surprising because the acid treatment makes clays more acidic, which lowers their pzc [77,78]. Following acid treatment, the pHpzc of A-Mnt dropped by 42% from the starting value.

For the PCH materials, the pH_PZC was 4.9 and 5.7 for the PACH-cal and PCH-cal materials, respectively. Similar values were reported for other PCH materials [52,63,80]. The pH_pzc did not depend on the clay and its activation, but it depended on the metal inserted in the silica framework of the related pHC materials [39,63]. As expected, when the pH was higher than the pH_pzc, the PCH surface became more negatively charged due to deprotonation, which led to increased attraction of dye molecules and improved their BB-41 removal properties [39]. When the solution pH is lower than the pH_pzc of the adsorbent, more positive charge density is expected to bind the surface of the adsorbent, thereby decreasing the removal efficiency of basic BB-41 dyes [63]. Related reports testify to the remarkable removal efficiency of BB-41 in alkaline environments [81].

3.7.2. Effect of the Initial Concentration

The concentration of dye is a critical factor that influences the removal efficiency in the sorption process, which is essential for the effective operation of adsorption systems utilized in wastewater treatment [82,83]. Recent research has demonstrated that variations in the dye concentration can significantly affect the efficacy of dye removal. These insights are instrumental in optimizing sorption processes, thereby enhancing overall environmental management strategies.

As the initial BB-41 concentration increased, the removal efficiency of PACH-cal in terms of mg/g typically showed a trend of increasing. However, the removal percentage in terms of % decreased in effectiveness (Figure 5). The increase in the removal capacity was due to the high driving force of mass transfer in the initially high concentration of dye.

The data displayed in Figure 5 demonstrated that the quantity removed increased from 50 to 100 mg of BB-41 per gram of the solid by changing the starting concentration of the BB-41 solution from 50 to 100 mg/L. On the other hand, with a lower starting concentration of BB-41, the percentage removed was higher than the efficiency reached when higher initial concentrations were employed. The BB-41 elimination yield decreased to 58% for a C_i of 500 mg/L, despite being determined to be 100% for an initial concentration of 50 mg/L. This happens because there are more dye molecules than available active sites on the sorbent material at greater concentrations.

As a result, the removal sites get saturated, which lowers the removal process’s overall efficiency. These findings suggest that when BB-41 concentrations in the solution rise, there is more competition for adsorption sites, leading to the involvement of energetically less favorable sites [82,83]. A similar behavior was noticed for the PCH-cal, with a decrease in the removal percentage and efficiency.

3.7.3. Effect of the Dose

The solid dose is instrumental in optimizing dye removal processes. It is essential to identify the optimal dose to prevent inefficiencies and unnecessary expenditures while also ensuring the effective and cost-efficient removal of dyes from wastewater [84].

The results illustrated in Figure 6 indicate that increasing the PACH-cal dose to 2.0 g/L significantly enhanced the removal efficiency of BB-41 from 37% to 93%. Furthermore, at higher PACH-cal doses exceeding 4.0 g/L, the removal efficiency reached a remarkable 100%. Increasing the dosage of the sorbent typically enhances the removal efficiency of textile dyes from aqueous solutions. This improvement can be attributed to the greater availability of sorption sites and surface area for the dye molecules. Additionally, a higher sorbent dose facilitates more effective dye removal, stemming from an enhanced sorption capacity and improved interactions between the dye molecules and the sorbent surface. However, the amount removed was reduced as the amount of added solid increased. Similar data were reported for other solids, as there was an inverse proportional relationship between the amount removed and the used mass [63,84].

3.7.4. Effect of Acid Activation

The raw Mnt clay removed about 57 mg/g of BB-41, as reported previously [63]; this value was enhanced from 57 to 125 mg/g by the pillaring process using alumina species [39]. The acid-activated Mnt clay (AMnt) exhibited a higher surface area of 155 m²/g than the raw Mnt clay, and the amount removed was increased to 71 mg/g. This fact indicated that the acid activation process had activated some removal sites and increased the available surface area for the removal of BB-41, as reported for an acid-activated local clay treated at a specific acid/clay ratio below 0.3 [43]. Above this value, the surface area continued to increase; however, in the meantime, the properties of BB-41 removal were reduced due to the destruction of the removal sites. In this instance, the removal properties were not improved by the increased surface area. Reported findings for additional silicate minerals were comparable [67].

3.7.5. Adsorption Model

Adsorption isotherm models play crucial roles in assessing equilibrium data in adsorption processes [85]. To achieve this, the relationship between the removed BB-41 dye on the PCH surface was established using the Langmuir isotherm model. The Langmuir isotherm model assumes that monolayer sorption takes place on the adsorbent surface, with only one pollutant substrate being adsorbed at a single adsorption active site [86]. The Langmuir model assumes a homogeneous adsorbent surface with comparable, energetically similar adsorption active sites.

The linear equation of the Langmuir model is the most commonly employed equation for comparison purposes of the maximum removal capacities of different materials. The parameters deduced from the Langmuir equation are presented in

Table 3. Langmuir parameters deduced from the linear equation for the different materials.

Samples	qmax (mg/g)	KL (L/g)	R2	PCC *	Γmax, mg/m2
Mnt	57	0.0289	0.9854	0.9993	0.633
A-Mnt	73	0.0518	0.9996	0.9996	0.4866
PCH-cal	243	0.0804	0.9997	0.9993	0.3283
PACH-cal	298	0.2673	0.9998	0.9996	0.3348

Notes: * stands for Pearson’s correlation coefficient.

.

The R² coefficients were close to one and confirmed the suitability of this model for the removal of BB-41 dye with the investigated materials. The linear equation of the Langmuir model showed a substantial correlation between C_e and C_e/q_e, as indicated by Pearson’s correlation coefficients, which were near one [39].

The parent acid-activated clay (AMnt) showed a removal of 73 mg/g, which was higher than the parent clay at 57 mg/g. This difference could be related to the increase in the specific surface area of the different materials, as reported in Section 3.5.

The data indicated that the maximum amount removed was achieved for PACH-cal at about 300 mg/g. Meanwhile, PCH-cal exhibited a value of 243 mg/g.

The investigation of the various materials revealed that the maximum adsorption capacity (q_max) values were consistently higher than the experimental values. This observation may suggest that the monolayer coverage of the adsorbents was not complete (see Table 3) [39]. In the case of BB-41, the estimated K_L parameter, which relates to the adsorption energy of the dye, was found to range from 0.26 L/g to 0.011 L/g. This range indicates that BB-41 exhibits a higher affinity for adsorption [84]. Furthermore, the removal of BB-41 using montmorillonite (Mt) resulted in the lowest Langmuir constant (K_L), which signified a reduced affinity of the dye for the surface of the material.

The maximum removal capacity (q_max) values (expressed in mg/g) were converted to maximum surface concentrations (Γ_max, expressed in mg/m²), as detailed in Table 3. Under the specified experimental conditions, the maximum surface concentration for the PCH-cal material was recorded at 0.311 mg/m², while the PACH-cal materials exhibited a maximum surface concentration of 0.354 mg/m². It is noteworthy that, in general, the maximum surface concentration tends to decrease as the PCH materials are derived. Furthermore, a three-dimensional computational analysis indicated that the BB-41 dye presented a planar configuration, with dimensions measuring 1.716 nm in length, 0.663 nm in width, and 0.665 nm in thickness. These measurements are consistent with previously reported values in the literature [87]. The calculated monolayer capacity for BB-41 was determined to be 0.618 mg/g, reflecting its planar area. The results indicate that, within the current experimental framework, the q_max values for all materials studied did not exceed the capacity of a monolayer of BB-41 molecules. This observation is consistent with both the theoretical and experimental q_max values previously outlined. As previously observed for other materials, the adsorption of dye molecules may take place through diffusion into the pores of used PCH materials [63]. Indeed, it was reported that the adsorbents’ pores with diameters of 1.3–1.8 times greater than the solute molecules’ diameters allow the molecules to diffuse into them [63]. The BB-41 molecules disperse easily in these pores because of the size of the molecules and the pores of PCH materials. Nevertheless, as reported in other studies for layered silicate materials [88,89,90], BB-41 molecules were intercalated between the layers of these materials with higher initial concentrations.

3.7.6. Regeneration Studies

Research on regeneration and reuse is vital for gaining insights into environmental impacts and evaluating the efficacy of the PCH removal agent. Different methods were proposed in the literature, from chemical to physical methods [82,91]. The desorption method was also reported [92]. The pH level plays a critical role in the desorption process. An increase in pH can significantly enhance the desorption of anionic dyes, whereas a decrease in pH promotes the desorption of cationic dyes [93]. In this study, a different method was proposed, and it consisted of using an oxone and cobalt solution to break down the removed BB-41 on the surface and in the pores of PCHs.

The pH of the solution used for the regeneration tests was about 5.5. Desorption could not be postulated because the resulting solution was not blue, and the BB-41 dye molecules were completely destroyed.

Regeneration was examined using seven cycles. Figure 7 indicates that the PACH-cal material retained 90% of its initial removal percentage value with a reduction of 10% after the fourth regeneration recycles. This value continued to diminish gradually to 35% after seven regenerations. However, in the case of the PCH-cal material, the initial value was reduced by 25% even at the third regeneration cycle. This value kept decreasing to 50% after the seven tests. The latter value was not achieved for PACH-cal even after eight tests. Similar data were reported for PCH materials doped with aluminum or zirconium [39,63].

These tests revealed that the BB-41 molecules were strongly bound to the removal sites in case of the PACH-cal material compared to PCH-cal or to the inactivation of the removal sites in PACH-cal. This fact might indicate that the destruction of the removed BB-41 molecules was easily achieved in the case of PACH-cal during the regeneration studies due to the easy access of oxone and Co²⁺ cations necessary for their destruction. In addition, this efficiency of the process could be activated by the acid sites present in the used material; indeed, the PCH-cal exhibited higher acidity compared to PACH-cal, as reported in Table 3. This data revealed that the studied materials had good regeneration properties.

After each regeneration test, the PXRD measurement indicated that a slight decrease in the intensity of the major reflection was observed. The data confirmed that the structure was maintained after these seven runs and the PCH structure was not altered due to the mild conditions of the regeneration tests.

3.7.7. Single-Batch Design

This design is a crucial first step toward applying the lab bench inquiry’s findings to a wider scale and eventually developing an industrial wastewater treatment system [39]. It will help reduce the cost of the materials as well if they are expensive. Single-stage batch adsorption is the most often utilized method by researchers to evaluate their adsorbents for commercial applications [94].

For a total dye solution with a particular volume V (L), the masses required to lower the initial BB-41 dye concentration of C_o (mg/L) to C_t (mg/) are estimated using the mass balance equation and the removal adsorption isotherms. The amount of BB-41 removed ranges from q_o (mg/g) to q_t (mg/g), where q_o the amount removed at time = 0 s, and it is equal to 0 mg/g. The amount of additional removal solid was M (g). Under equilibrium, the mass balance for the BB-41 dye in the one-stage process is expressed as follows:

$$\mathrm{V}\left({\mathrm{C}}_0-{\mathrm{C}}_1\right)=\mathrm{M}\left({\mathrm{q}}_0-{\mathrm{q}}_1\right)={\mathrm{M}\mathrm{q}}_1$$

(1)

Since the experimental data fitted the Langmuir model, the q_e expression is substituted in Equation (1), leading to Equation (2) [63]:

$${\displaystyle \frac{\mathrm{M}}{\mathrm{V}}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_1}{{\mathrm{q}}_1}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_1}{\frac{{\mathrm{q}}_{\mathrm{m}}{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_1}{1 + {\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_1}}}$$

(2)

The predicted quantities of PCH-cal and PACH-cal needed to remove 50%, 60%, 70%, 80%, and 90% of the original dye concentration of 200 ppm are shown as a function of different solution volumes in Figure 8 and derived from Equation (2). Generally, the required masses increased with the desired percentages removed and the treated volumes of the effluents, as reported for other materials [63,94].

For example, the predicted minimum masses of PACH-cal to treat a 10 L effluent volume at an initial concentration of 200 mg/L of BB-41 were 3.5, 4.2, 5.0, 5.9, and 7.2 g to reach specific BB-41 removal percentages of 50%, 60%, 70%, 80%, and 90%, respectively. These values increased to 7.8, 10.4, 14.2, 21.1, and 40.1 g using PCH-cal under the same conditions. The difference was due to the maximum removal capacity (q_max) of PACH-cal compared to PCH-cal material and confirmed by the data reported in previous works [39,63]. Similar data were reported for other organomagadiites and organoclays and [88,95].

These plots could be extended to different conditions, such as the removal of 100% or the effects of temperatures and contact times.

4. Conclusions

The parent properties of the Mnt clay mineral were modified by an acid activation process using a sulfuric acid solution at a specific ratio of acid/clay of 0.2 (in weight). The raw clay’s acidity and surface area both increased as a result of acid activation.

Attempts were made to synthesize a porous clay heterostructure from acid-activated clay. The PACH-cal material has been successfully synthesized from both parent and acid-activated clay. This material demonstrates remarkable properties, including a high surface area of 880 m²/g and an acidity value of 0.56 moles of H⁺/g. The synthesized sorbent materials were used in the removal of BB-41 from an aqueous solution. The most successful removal with a maximum amount of 247 to 300 mg/g was found at a pH of 8, which may be attributed to the porosity and acidity, besides the greater surface area. The majority of the experimental data aligns with a monolayer isotherm of the Langmuir type. The materials under investigation were subjected to regeneration after three to four consecutive cycles. Modeling of the equilibrium data, combined with the single-stage batch design, suggests that approximately 90% of the initial concentration of 200 mg/L can be effectively eliminated using a reduced quantity of PACH-cal (8 g) for a treated volume of 10 L. The PACH-cal material is considered a promising candidate as a remedial agent for the environment. To reduce the time and energy needed to eliminate the surfactants from the precursor, an extraction method will be investigated in the future, and a dual-batch design will be proposed