Comparative Removal Properties of Sodium Magadiite and Its Protonic Form on Basic-Blue 41 from Contaminated Aqueous Solution

Abstract

Sodium magadiite (Na-Mgd) was hydrothermally prepared and converted to its protonic (H-Mgd) form by reaction with hydrochloric (HCl) solution. The obtained products were studied as adsorbents for basic blue 41 (BB-41) removal from polluted aqueous solution. Na-Mgd and H-Mgd were characterized by different techniques. Powder X-ray (PXRD) diffraction data confirmed a pure Na-Mag phase and its conversion to acidic form (H-Mgd) with shift in d₀₀₁ value from 1.54 nm to 1.12 nm. X-ray fluorescence (XRF) data supported the exchange of Na cations by protons for H-Mag. ²⁹Si magic angle spinning nuclear magnetic resonance (MAS-NMR) indicated a change in the local environment of silicon nucleus when Na-Mgd was treated with HCl solution. The BB-41 removal dyes were investigated throughout the batch process. Effects of selected parameters, for example, the adsorbent dosage, pH of the BB-41 solution, pH of the H-Mag solid, and starting concentration, were explored. The equilibrium data were fitted to the Langmuir and Freundlich isotherm models. The maxima removal capacities of Na-Mgd and H-Mgd were 219 mg/g and 114 mg/g, respectively. The regeneration and reusability tests were performed using initial concentrations of 50 mg/L and 200 mg/L for seven cycles. The efficiency was maintained for 5 to 6 cycles with a decline of 10% using low initial concentration; however, a decline of efficiency to 30 to 50% was achieved when a higher initial concentration was employed after 3 to 4 regeneration tests for Na-Mgd and H-Mgd samples. Adsorber batch design using the Langmuir and Freundlich isotherm parameters was used to predict its performance for commercial usage. The predicted masses of H-Mgd were higher than those of Na-Mgd to treat different effluent volumes contaminated with 200 mg/L of BB-41 dyes at desired removal percentages.

1. Introduction

Significant attention is growing from the technical and scientific communities to the layered silicates due to their interesting properties. One of the members of this family is the Na-Mgd with a general formula of Na₂Si₁₄O₂₉⋅nH₂O, where n varies from 0 to 20, and a cation exchange capacity (CEC) in the range of 100 to 200 meq/100 g [1,2]. It has been shown that they have a high CEC when contrasted to clay minerals and zeolites [3,4]. This material is described as the most pervasive material on earth, is half-abandoned and half-disregarded [5]. Nonetheless, in recent years, it has not been disregarded [6].

Layered silicates have a large number of silanol groups (SiO⁻ or SiOH⁻) covering the interlayer surface, in contrast to layered clays that possess silanol groups only at the edges of their layers [7]. Diverse applications emerged for these materials: they were proposed as catalyst supports for specific reactions [8], precursors to prepare zeolite materials [9], electrical capacitors [5], isolators for anticorrosion and mitigating hydrogen evolution [10], composites [6], and adsorbents for water contaminants such as dyes and metals [11,12,13,14,15,16,17]. Na-Mgd can exist naturally or be prepared in the laboratory. It is synthesized easily by treating a simple mixture of silica source, alkaline solution, and water in hydrothermal Teflon autoclaves at different temperatures and periods of time [16]. Recently, the different parameters that affected the synthesis of Na-Mgd have been reported and discussed in detail; pure Na-Mgd phase is achieved at mild temperatures and in a narrow range of SiO₂/NaOH/H₂O molar ratios [16].

Na-Mgd exhibited a negative charge, and it is used to remove basic dyes with a positive charge once dissolved in water [17,18]. However, for dyes negatively charged (or acidic dyes), the raw Na-Mgd is submitted to specific modification to change its surface charge character from negative to positive. In this regard, the hydrophilic Mgd is transformed to hydrophobic or organophilic silicates by reaction with surfactant solution [11,12]. The removal of Eosin and rhodamine-B dyes is successfully achieved using organomagadiites, and it is closely related to the content of surfactant cations incorporated in the Magd materials [11,12].

The chemical composition of Mgd material can be tuned by exchanging the Na cations with protons during the reaction of Na-Mgd solid with HCl solution at room temperature. The obtained H-Mgd is identified as acidic or protonic magadiite with the general formula of H₂Si₁₄O₂₉·nH₂O [19,20]. The exchange of Na cations is supported by different techniques, and the solid pH is changed from basic to acidic character, which affects the removal properties of the magadiite solids [20].

In case of Methylene blue or basic blue-41 removal properties, the pH is considered one of the major factors that needs to be controlled during the removal process [16,17,20]. The amounts of these dyes removed are mainly related to the cation exchange capacity values and not to the specific surface areas, as is believed for other materials [21].

A monoazo-basic dye with a vivid blue color, basic blue-41, is frequently utilized for dyeing acrylic, nylon, silk, cotton, and wool [22]. Additionally, it acts well as a stainer to identify bone marrow, blood, and avian leukocytes [23]. Most likely, it is considered one of the most hazardous materials. It causes irreversible harm to both human and animal eyes, while consumption through the mouth results in a burning feeling and may cause nausea, vomiting, excessive perspiration, mental disorientation, and methemoglobinemia [24]. Coagulation, filtration, adsorption, and other conventional methods are employed to reduce the content of dyes [25,26,27]. However, these methods have not been able to fully eliminate organic contaminants from wastewater, resulting in secondary problems. Often, an additional step is employed in the traditional wastewater treatment methods [27].

There is scarce research on protonic magadiite usage as a removal agent for basic dye, and only one research paper was reported in the literature focused on the usage of H-Mgd to adsorb methylene blue from an aqueous solution [20]. In this study, H-Mgd has attracted attention as a potential precursor for BB-41 removal. Within this realm, the synthesis of this material and characterization were reported in detail using different techniques. The level of BB-41 removal from wastewater is affected by several factors, including adsorbent dose, temperature, pH, treatment time, and initial concentrations (Ci). In this study, some of these factors were investigated, such as Ci values, the material dose, pH solution, and effects of acid treatment. Two isotherm models were tested to determine the maximum removal capacities of Na-Mgd and H-Mgd samples. The regeneration and reusability of these materials are important factors to be considered [28]. In this regard, the samples were submitted to seven consecutive regeneration tests using a benign, friendly method to the environment. The required masses of Mgd materials were estimated by designing a single batch adsorber to treat various effluent volumes with a particular initial concentration.

2. Results and Discussion

2.1. X Ray Diffraction Data

Figure 1a indicates that Na-Mgd is a pure phase, and the reflections coincided well with the ones reported in the literature [29,30,31]. The pattern exhibited typical reflections of (001), (002), and (003), with an average basal spacing of 1.54 nm. The other reflections were indexed to the (020), (021), (202), (022), and (203) planes in good agreement with in agreement with ICDD 42-1350 card [32]. The d₀₀₁ value of 1.54 nm depended on the cation type that existed in the interlayer spacing. Copper Mgd exhibited a d₀₀₁ value of 1.36 nm due to the exchange of hydrated sodium ions by small copper ions [33]. In some cases, the interactions of the cations with ≡Si-O⁻ groups and the variation in the amount of interlayer water affected the d₀₀₁ values [34].

After treatment of Na-Mgd with HCl solution, H-Mgd exhibited a different PXRD pattern with a basal spacing (d₀₀₁) value of 1.21 nm, resulting from the exchange of Na cations by protons. In some cases, a value of 1.32 nm was reported for H-Mgd material [35]. The other reflections were indexed based on the acidic magadiite (ICDD 29-0668) [36]. Changes in shape and intensity of some reflections occurred due to low crystallinity and to a stacking disorder upon proton exchange. Similar data were reported for clay minerals treated with sulfuric acid [21,37].

According to the XRF analysis summarized in

Table 1. Chemical composition of Na-Mgd and H-Mgd samples.

Sample	Na2O (wt%)	SiO2 (wt%)	Total Mass Loss (%) *
Na-Mgd	5.41	94.21	14.6
H-Mgd	~0.00	99.52	5.14

* deduced from TGA data.

, the chemical composition of the Na-Mag sample was close to that reported for similar materials, with some differences due to the synthesis conditions [16]. The exchange of Na cations with protons was successfully achieved after treatment with HCl solution. Significant reduction in Na₂O (% in weight) from 5.41% to almost 0% was observed [38,39], and associated with a decrease in total mass loss between 25 and 800 °C.

2.2. FTIR Data

The FTIR spectrum of the Na-Mgd sample is depicted in Figure 2a. The spectrum exhibited similar shapes reported for similar materials [40,41], with Si-OH⁻ and OH⁻ of hydration water being attributed to the normal OH⁻ stretching at 3666, 3587, and 3440 cm⁻¹, respectively [40]. The deconvolution of this broad band revealed the presence of four bands (Figure 3a). The vibrations linked to hydrogen bonding between free water molecules and Si-OH groups and the hydration water of sodium cations were also observed at 1680 cm⁻¹ and 1635 cm⁻¹, respectively [41].

While the 1680 cm⁻¹ describes doubly hydrogen-bound water molecules, the position at 1635 cm⁻¹ is indicative of water molecules interacting with Na⁺ cations, noticed in swelling clay minerals. The terminal Si-O stretching in Q³ species gave rise to bands at 1081 and 1020 cm⁻¹ (Figure 2a), while the Si-O-Si asymmetric stretching was detected at 1240 and 1163 cm⁻¹ [40]. The deconvolution of the broad band is presented in Figure 4a. Si-O-Si symmetric stretching modes were found at 828 and 790 cm⁻¹. Simple and double ring Si-O-Si bending deformations were attributed to the bands at 632 and 452 cm⁻¹ [40].

The silicate framework’s bands remained the same when protons were substituted for the sodium cations. The FTIR spectra of H-Mgd and Na-Mgd were comparable (Figure 2b). The bands varied in relative intensity and form. Because H⁺ cations are less hydrophilic than Na⁺ cations [41,42], the bands connected to OH vibrations at 3630 cm⁻¹ and 1637 cm⁻¹ were less strong (Figure 3b). The deformation water band at 1680 cm⁻¹ vanished for the H-Mgd. The presence of protons, which are less hydrated than the Na cations, could be the reason. More pronounced bands related to silanol groups in the 3200–3750 cm⁻¹ interval were recorded (Figure 2b and Figure 3b). Two single broad bands at 1184 cm⁻¹ and 1070 cm⁻¹ were also noticed (Figure 4b). The sodium replacement with protons resulted in an improvement of the 3442 cm⁻¹ band with an additional one at 715 cm⁻¹. The difference is associated with the structure of the H-Mgd and with the dehydroxylation of the magadiite silicate layers [42].

2.3. ²⁹Si MAS NMR Data

²⁹Si-MAS NMR was used to analyze the connectivity of the silicon atoms in the Na-Mgd structure. Na-Mgd spectrum is presented in Figure 5a. There were two signals in the Q⁴ area (−112 and −114 ppm) assigned to [Si(4OSi)] in various chemical environments, as well as a signal in the Q³ region (−100 ppm) attributable to [Si(3OSi) (OH⁻)] and [Si(3OSi) (O-Na⁺)] [43,44]. Different Si-O-Si binding angles between Si(OSi)₄ can be the source of two signals relating to Q⁴ sites. In some cases, a third resonance peak at −110 ppm was reported; it has been proposed that the three Q⁴ sites may differ by their average Si-O-Si bond angles [44]. In this study, it was difficult to detect and could overlap. However, after deconvolution of the spectrum, this peak was easily detected (Figure S1). The presence of this peak depended on the synthesis conditions [16,43].

The H-Mgd sample exhibited a similar spectrum to Na-Mgd (Figure 5b) with some variation in intensity and shape of specific resonance peaks [44]. The intensity of the band at −101 ppm decreased in intensity, and the strong band centered at −111 ppm became broad, with the band at −114 ppm detected as a shoulder. The deconvolution of the broad peak revealed the existence of three peaks (Figure S2). Similar data were reported for other H-Mgd prepared from different silica sources and conditions [42]. A little shift in the resonance peaks was also noted, associated with slight variations in the Si-O-Si angles as a result of changes in local restrictions after HCl treatment [44]. The Q⁴/Q³ ratio (in terms of area) was estimated to be 2 and 3 for Na-Mgd and H-Mgd, respectively. The increase in the Q⁴/Q³ ratio confirmed the acid treatment of Na-Mgd and exchange of Na cations with protons.

2.4. TGA Data

The TGA/DTG features of Na-Mgd reveal three distinct mass loss steps in the range of 25 to 800 °C. The first step from 25 °C to 200 °C (Figure 6a, left and right) was related to the removal of water molecules from different environments [16] as indicated by FTIR data. The initial 5.86% step, which took place between 25 and 100 °C, was attributed to the desorption of the physisorbed water from the Na-Mgd’s surface and was accompanied by a DTG peak at 85 °C. The loss of interlayer water molecules and those bonded to the Na cations was linked to the second step, at a rate of 5.57% and occurred between 100 °C and 200 °C [16,30] and was associated with two DTG peaks at 115 and 127 °C, respectively. The basal spacing (d₀₀₁) is reduced to 1.14 nm in this range (Figure S3). The silanol dehydroxylation, which forms siloxane linkages, was attributed to the continuous decrease in mass loss of about 0.70% at temperatures over 200 °C, and a weak DTG peak at 280 °C was observed. For temperatures above 300 °C to 800 °C, a constant and undefinable mass loss of 1% was also noted; this was ascribed to the full dehydroxylation of the layered structure [16], and a broad DTG peak was detected during this stage at 580 °C. The PXRD data indicated that Na-Mgd was converted to an amorphous silica phase at 500 °C, and then to a crystalline quartz phase at temperatures higher than 800 °C [16].

H-Mgd showed a distinct feature from Na-Mgd with an initial mass loss of 2.8% below 300 °C, associated with the removal of water from the H-Mgd’s surface (Figure 6b, left). The exchange of Na cations by protons was linked to a decrease in the amounts of intercalated water, as evidenced by the reduction in the total mass loss in the 25–200 °C range [30]. The removal of OH groups from the structure was associated with the mass loss over 300 °C (Figure 6b, left). The DTG curve was very different (Figure 6b, right); it showed a single peak at 356 °C, and a broad peak with low intensity at 670 °C. The total mass loss between 25 °C and 800 °C was 14.6% and 5.14% for Na-Mgd and H-Mgd, respectively (Table 1).

2.5. SEM Micrographs Data

The SEM monographs of different materials are shown in Figure 7. Na-Mgd exhibited a particle morphology composed of silicate layers intergrown to form spherical structures resembling rosettes (Figure 7a,b). After HCl treatment, the morphology of cauliflower was almost lost, which was related to the decrease in the crystallinity detected by powder XRD (Figure 7c,d) [21,45,46].

The EDX analysis revealed that three elements existed mainly in Na-Mgd (O, Na, and Si). The O element is the major element, in addition to the Si and Na elements. The content of Na element was in the range of 3.77%. However, the H-Mgd sample contained a lower percentage of Na, close to zero %; this fact was related to exchange of Na cations with protons from the HCl solution, and confirmed the data presented in Table 1.

2.6. Textural Properties

The textural properties of Na-Mgd and H-Mgd are summarized in

Table 2. Textural properties of Na-Mgd and H-Mgd samples.

Sample	A.P.D (nm)	T.P.V. (cm3/g)	SBET (m2/g)
Na-Mgd	29	0.196	26
H-Mgd	25.8	0.263	40

. The nitrogen isotherm adsorption data revealed that the Na-Mgd exhibited an isotherm type IV characteristic of non-porous material, with a specific surface area (S_BET) of 29 m²/g. The low value is typical of layered solids. Comparable data were stated for similar materials [16,46,47]. Some values of 19 m²/g to 39 m²/g were reported by some authors and could be affected by the synthesis conditions. However, a slight increase in the S_BET value was observed for H-Mgd; it reached 40 m²/g. The shape of the isotherm was similar to that reported for other acidic magadiites. However, in some cases, an enhancement of the S_BET was reported and attained a value of 54 m²/g [48]. The total pore volume (T.P.V.) of H-Mgd (0.263 cm³/g) was higher than Na-Mgd (0.196 cm³/g) with a decrease in average pore diameter (A.P.D) from 29 nm to 25.8 nm. The A.P.D. is related to voids between the magadiite particles, and the variation could be assigned to the reorganization of the particles during the acid treatment as revealed by the SEM technique.

3. Factors Affecting the Removal of BB-41

The Equations (S1) and (S2) are used to estimate the removal percentage (R%) and removed amount (q_e, mg/g),

3.1. Initial Concentration Impact

The concentration of contaminants in aqueous solution plays a significant role in their removal onto solid surfaces [49]. The impact of the initial concentration of the BB-41 dye on H-Mgd removal efficiency was studied and presented in Figure 8 left. The data revealed that as the BB-41 concentration increased, the removal percentage decreased from 100% to 20%. At the beginning of the process, a large number of active sites were available for removal. However, as the concentration of the BB-41 dye increased, these active sites on the removal solid became saturated, which resulted in fewer active sites left for removal assistance. In simpler terms, the mass of the solid was unchanged, implying the number of removal sites was kept constant. At lower concentrations, there were more active sites than dye molecules. However, as the concentration increased, the number of dye molecules did too, and the sites became saturated, leading to a decline in BB-41 removal efficiency. This phenomenon has been noted in previous studies and is in line with comparable findings [16,50,51]. On the other hand, the BB-41 removal amount (mg/g) increased with BB-41’s C_i values, owing to the induction of driving forces (by initial concentration) to overcome the resistance to the mass transfer of BB-41 between aqueous and solid phases. The increase of C_i also enhanced the interaction between the removal agent and BB-41. The Na-Mgd sample exhibited similar behavior with the increase in the removal efficiency and the removed amount as reported in a previous study [16].

3.2. Effect of Dosage

The used adsorbent plays a crucial role in the efficiency of removing pollutants from a contaminated solution. It represents the number of adsorbent sites that affected the removal process [52]. Figure 8 right presents the impact of H-Mgd dosage on the BB-41 removal properties. It increased from 26% to 60% as the H-Mgd dose was raised from 0.025 g to 0.2 g. After adding more H-Mgd with masses that are higher than 0.4 g, the removal percentage reached 72%, and it was not effectively improved by employing more H-Mgd samples. Comparable data were reported for Na-Mgd and other layered silicates [16,53,54].

For a fixed volume of BB-41 solution, the number of BB-41 molecules remained constant, and as more H-Mgd mass was added, further removal sites were offered, and the removal percentage was improved. However, the maximum amount removed of 104 mg/g was obtained at the minimum dose (0.025 g). As the H-Mgd mass rose, the amount removed decreased from 104 mg/g to 7.2 mg/g because of the split in the flux or the concentration gradient between the solute concentration in the solution and the solute concentration on the adsorbent surface [55].

Furthermore, as Equation (S2) shows, the utilized mass and q_e (mg/g) have an inverse relationship. This means that a high concentration of adsorbent lowers the number of adsorption sites per unit mass, which reduces the quantity of removal.

3.3. Effect of pH Environment

3.3.1. Effect of BB-41 pH Solution

An important consideration is the dye solution’s pH. It affected the chemistry of the dye solution in terms of the degree of ionization and the charge of the used material’s surface [56].

The BB-41 dye exhibited superior removal in a basic environment due to its cationic characteristic (Figure 9). At basic pH levels, an increase in OH⁻ ions deprotonated the surface and resulting in a strong electrostatic bond between the negatively charged adsorbent surface and the positively charged BB-41 molecules. This fact is highlighted in Figure 9, where a diminution pH level reduced the BB-41 removal efficiency. However, a lower pH value decreased the negatively charged sites, resulting in electrostatic repulsion between BB-41 molecules and the H-Mgd surface, thereby hindering the BB-41 removal. At higher pH values greater than 10.5, the BB-41 dye was unstable and a brown precipitate was formed, and further studies could not be carried out [16].

It was reported that the point zero charge values determine the charge of the adsorbent’s surface [57]. The latter determines the pH values at which the surface acquires a negative or positive charge, and it is known as pH_pzc. The pH_pzc reported for Na-Mgd was close to 8 [16], and a higher value of 8.8 was also reported [17]. After the acid treatment, the pH_pzc of H-Mgd decreased to 3.7. This value was in the range of 3.6 to 4.5, reported for similar materials [13]. The decrease in pH_pzc indicated a change in the surface charge. Similar reduction was also noted for clay minerals treated with an acid solution [58]. The pH_pzc is closely related to the change in acidic or basic properties of the materials as reported for carbon materials [59].

At pH values lower than pH_pzc, the surface of the adsorbent acquires a positive charge and thus leads to an electrostatic repulsion between the BB-41 cations and the H-Mag surface. While at pH values higher than pH_pzc, the surface acquires a negative charge, which thus affects the percentage of BB-41 removal by enhancing the electrostatic force between the BB-41 and the negatively charged surface of H-Mgd.

3.3.2. Effect of H-Mgd pH Suspension

In this part, the pH solution of BB-41 was not altered. However, the H-Mgd solid was treated with different volumes of NaOH (0.1 M) to change the acidic to basic character of H-Mgd. The BB-41 solution exhibited a pH of about 5.4. By adding pristine H-Mgd solid, the pH of the suspension BB-41 and H-Mgd was reduced to 3, which affected the low removal properties of BB-41. However, when adding H-Mgd soaked in alkaline solution, the final pH of the BB-41 and H-Mgd suspension was increased to 8, resulting in an improvement of BB-41 removal efficiency to 100%. Further treatment with NaOH did not lead to an improvement in removal efficiency because it had already reached 100%. The PXRD data indicated that the H-Mgd was converted to Na-Mgd when treated with 30 mL of NaOH solution (0.1 M), and the d₀₀₁ value was expanded from 1.21 nm to 1.56 nm, consistent with the data published in the literature [60]. However, when H-Mgd was treated with less volume of NaOH solution, no change in PXRD pattern was observed, which indicated the chemical stability of the H-Mgd sample. In this case, one has to be careful when treating the H-Mgd solid with NaOH solution. However, when H-Mgd was treated with NaCl solution, the PXRD pattern indicated no change in the d₀₀₁ value of 1.21 nm, independently of the added volume of NaCl solution (0.1 M).

3.4. Effect of Acid Treatment of Na-Mgd

The acid treatment of Na-Mgd had no effect on the removal efficiency; all dye molecules were eliminated at a lower C_i of 100 mg/L, and 100% removal percentage was attained. However, H-Mgd’s removal efficiency values decreased and were below those of Na-Mgd at C_i values higher than 200 mg/L. Similar data were reported using acid-activated clay minerals [37]. In this instance, S_BET was not the most essential issue in improving H-Mgd’s removal capabilities. As previously reported, the decrease in H-Mgd’s pH solid might have contributed to this performance. The electrostatic interaction between the surface and the positively charged dye species was reduced as the surface was more positively charged.

3.5. Adsorption Isotherms Data

Adsorption isotherms are essential to comprehend the distribution process of BB-41 dye in the liquid phase when it reaches the equilibrium conditions. Freundlich and Langmuir equations were utilized to elucidate the adsorption isotherm data. Equations (S3) and (S4) displayed the formula for the isotherm models under examination. In monolayer adsorption, where the majority of the adsorption sites have the same affinities for the pollutant, the Langmuir isotherm model is typically employed [61]. When the adsorption site has a particular binding energy, heterogeneous and multilayer adsorption processes are expressed by the Freundlich isotherm model [62]. The maximal removal capacities of Na-Mgd and H-Mgd samples were estimated for comparison purposes.

Table 3. Estimated isotherm parameters for BB-41 removal by Na-Mgd and H-Mgd samples.

Samples	Langmuir			Freundlich
	qmax (mg/g)	KL (L/mg)	R2	1/n	KF (L/mg)	R2
Na-Mgd	219	0.112	0.9998	0.1866	66.68	0.9819
	(222) *	(0.0681)	(0.9172)	(0.2149)	(57.59)	(0.9387)
H-Mgd	114	0.0123	0.9891	0.3193	12.97	0.9891
	(113.3)	(0.0119)	(0.9878)	(0.3045)	(14.36)	(0.9878)

* values between parentheses deduced after nonlinear fitting operation.

summarizes the estimated parameters associated with the linearized and nonlinear isotherms.

The Langmuir model provided a superior fit, as evidenced by higher correlation coefficients (R²), for both Na-Mgd and H-Mgd. This suggests that BB-41 removal is driven by monolayer coverage on a homogenous surface with a finite number of binding sites. The Freundlich model exhibited a lower correlation, indicating that multilayer removal on a heterogeneous surface is not the dominant mechanism for Na-Mgd and H-Mgd samples.

The q_max value of the H-Mgd sample was 114 mg/g, which was less than that of its Na-Mgd counterpart (219 mg/g). This result suggests that the acid treatment had an effect on H-Mgd’s ability to be removed. Similar data were reported when methylene blue dye was investigated. K_L value for Na-Mgd was higher than that of H-Mgd, indicating a good affinity between the Na-Mgd’s removal surface and BB-41 dye molecules. The superior q_max value of Na-Mgd compared to H-Mgd can be associated with the basic character of the silanol groups available for removal. Indeed, a suspension of Na-Mgd in water has a pH value of 8.5, while a value of 3.7 was obtained for H-Mag suspension.

The removal of BB-41 dye occurred via cation exchange reaction between the Na cations and the BB-41 dye, as reported in previous studies when high amounts of BB-41 were removed [37,63]. At lower amounts, the removal occurred on the external surface of Na-Mgd. These facts were deduced from the PXRD data before and after the removal process. However, in case of the H-Mgd sample and at neutral pH, the removal process did not involve a cation exchange reaction, and only adsorption on the surface occurred as revealed by PXRD data. By increasing the pH of the BB-41 solution, two processes took place. The first involved the exchange of protons in H-Mgd by Na cations from NaOH solution, which resulted in the regeneration of the resulting Na-Mgd and an increase in removal efficiency. In the current situation, the specific surface area was not a crucial cause as reported for acid-activated clays [37,63].

Table 4. Removal efficiency of selected silicates and aluminosilicate materials.

Samples	qmax (mg/g)	References
Na-Mgd	219	This study
H-Magd	114	This study
Montmorillonite (Mt)	55	[37]
Saudi Local clays	50–70	[58]
Brick wastes	60–70	[51]
zeolite	39–70	[54]
Mn modified diatomite	77	[64]
Acid-activated PGs *	43–88	[37]
Base acid-activated PGs +	92–110	[37]

* APG corresponds to acid-activated PG clays, ⁺ BAPG corresponds to NaOH-treated acid-activated clays.

reports the q_max of BB-41 by other layered silicates and aluminosilicate materials. The compared materials were limited to layered silicate and aluminosilicates; other materials were also used in the literature, but they had different chemical compositions and structures. The data indicated that the Na-Mgd and H-Mgd samples exhibited great potential as removal agents for BB-41 from aqueous media. The q_max of 219 mg/g was higher than that of several adsorbent materials, and H-Mgd showed a reasonable q_max of 114 mg/g, still higher than other types of materials, and revealed the potential usage of H-Mgd.

3.6. Regeneration Tests

The regeneration of the adsorbent is one of the major economic concerns, because it lowers operating cost and solves the problems related to the disposal of the spent adsorbent [65,66]. Na-Mgd could be regenerated and reclaimed efficiently for the removal of BB-41 without conceding the removal percentage using a lower initial concentration of BB-41 of 50 mg/L. The removal percentage was maintained after five tests of regeneration. Similar data were reported for H-Mgd; the initial removal percentage was maintained for six cycles.

On the other hand, the removal percentage was maintained only for four recycle tests using a C_i solution of 200 mg/L, with a reduction of 20%; this reduction continued to increase up to 35% after seven regeneration tests for the H-Mgd sample (Figure 10). Na-Mgd lost almost 20% of its effectiveness during the first two cycles. During the third run, the percentage elimination decreased steadily, ranging from 60% to 50%. However, 50% of efficacy was still sustained after the seventh run (Figure 10) [16].

The difference could be related to the strong interactions of BB-41 molecules with the removal sites on Na-Mgd that made their destruction difficult, especially when they were intercalated between the silicate layers. The PXRD pattern exhibited a similar d₀₀₁ of 1.98 nm after regeneration tests and indicated the stability of the studied Na-Mgd material. In case of H-Mgd, the removal of BB-41 at natural pH did not involve changes in d₀₀₁ values, and indicated that the destruction of the removed BB-41 dyes was relatively easy due to the facile accessibility of the sulfate radicals, especially for the BB-41 molecules adsorbed on the surface of the H-Mgd sample.

3.7. Single Batch Adsorber Design

This part is considered as a real application of the estimated isotherm parameters to design a single batch adsorber, and to predict the necessary masses of adsorbent to treat different volumes of effluent for target removal percentages. This design will help to reduce the costs, especially for expensive materials, and the environmental impact of such processes [67].

With a solution volume (V) and an adsorbent amount (M), the mass balance for the batch adsorber can be expressed in Equation (1) as follows [68]:

q_oM + C_oV = q_eM + C_eV

(1)

After rearrangement, Equation (2) will be as follows:

M(q_e − q_o) = V(C_o − C_e)

(2)

The relationship between volume (V) and mass (M) is written as in Equation (3):

$${\displaystyle \frac{\mathrm{M}}{\mathrm{V}}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}}$$

(3)

By substitution of q_e either by the Langmuir or Freundlich models, Equation (4) and Equation (5) are obtained, respectively.

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

(4)

And

$${\displaystyle \frac{\mathrm{M}}{\mathrm{V}}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{K}}_{\mathrm{F}}·{\mathrm{C}}_{\mathrm{e}}^{1/\mathrm{n}}}}$$

(5)

Figure 11 left depicts the masses needed of Na-Mgd to decrease the initial C_i of 200 mg/L to targeted removal percentages of 50%, 60%, 70%, 80%, 90%, and 95% and using different effluent volumes (L).

Two common tendencies were noticed: as the effluent’s volume and the removal percentage increased, the anticipated masses of Na-Mgd continued to increase. As a matter of fact, it was anticipated that 5.0 g, 6.1 g, 7.4 g, 9.0 g, 12.0 g, and 16.5 g of Na-Mgd were required in order to reach the target concentrations of 50%, 60%, 70%, 80%, 90%, and 95% for a specific volume of 10 L, respectively.

However, in order to reduce 10 L of solution from 200 mg/L to 100 mg/L, 80 mg/L, 60 mg/L, 40 mg/L, 20 mg/L, and 10 mg/L, respectively, more masses of H-Mgd were needed, such as 16.2 g, 21.6 g, 29.5 g, 43.5 g, 82.1 g, and 156.5 g (Figure 11 right). The lower maximum removal capacity (q_max) of the H-Mgd sample as opposed to the Na-Mgd sample was the reason for the difference between the q_max magnitude of the two samples. Comparable behaviors can be observed for the BB-41 dyes with different materials [16,37,53].

In particular, the necessary mass nearly doubled from 82.1 g to 156.5 g for the H-Mgd sample and from 12.0 g to 16.5 g for the Na-Mgd sample when the removal percentage is expected to increase from 90% to 95% (a 5% increase).

A minor increase in the required masses of Na-Mgd and H-Mgd samples was observed when applying the nonlinear parameters for the Langmuir model compared to the linear parameters model. Figure S4 presents an example of these calculations related to the Na-Mgd sample.

The Langmuir and Freundlich models were utilized to examine the impact of the models on the design process. The required masses for H-Mgd were lower when the Freundlich model was applied compared to those obtained from the Langmuir model (Figure S5). A decrease of more than 50% in H-Mgd’s mass was noticed when a reduction of 95% of the initial concentration of 200 mg/L was targeted to treat 10 L of effluent (Figure 12). The needed masses were found to be underestimated by the Freundlich model and overestimated by the Langmuir model. The discrepancy between the two models was related to the shape of the isotherm curves across the investigated concentration range [69].

During the design phase, the impact of starting concentrations is also taken into account. Using both Na-Mgd and H-Mgd materials, the anticipated masses increased for higher initial concentration levels for a fixed removal percentage of 95% and an effluent volume of 10 L. Figure 13 illustrates the variation in the Na-Mgd and H-Mgd masses with the starting initial concentrations; more masses were needed for higher initial concentrations. For various C_i of 200 mg/L, 300 mg/L, 400 mg/L, 500 mg/L, 600 mg/L, and 700 mg/L, the required Na-Mgd masses were 16.4 g, 20.7 g, 25.1 g, 29.4 g, 33.8 g, and 38.1 g, respectively. As expected, employing the H-Mgd sample, with lower removal capability than Na-Mgd, the needed amounts were increased to 152.3 g, 160.7 g, 169.1 g, 177.5 g, 186.0 g, and 190.3 g for the same procedure (Figure 13).

4. Materials and Characterization

4.1. Materials

Hydrochloric acid, sodium hydroxide, and HS-40% Ludox were provided by Aldrich. The basic blue 41 (BB-41) has a molecular formula of C₂₀H₂₆N₄O₆S₂ with a molar mass of 482.57 g/mol, and was provided by Aldrich. Oxone and cobalt nitrate salt were supplied by Across Organics. All the chemicals were used as obtained.

4.2. Na-Mgd and H-Mgd Preparation

A total of 4.8 g of solid was dissolved in 105 g of distilled water, and then 45 g of Ludox AS-40% colloidal silica was added to the alkaline solution drop by drop while stirring for more than half an hour. Na₂O/5SiO₂/122H₂O was the molar composition of the resultant mixture, which was then stirred for an additional hour at ambient temperature [26]. The mixture was moved inside a Teflon liner autoclave and placed in a static oven at 150 °C for 48 h. After this period, the autoclave was immediately quenched in an ice bath, filtered, and rinsed with distilled water to bring its pH down to almost 7. The solid was air-dried at room temperature. The sample was identified as Na-Mgd.

The transformation of Na-Mgd to H-Mgd was achieved by treating 1 g of Na-Mgd in a solution of 0.1 M HCl for 4 h, followed by filtration and washing repeatedly, and finally, drying at ambient temperature [11].

4.3. Basic Blue-41 Removal Process

Batch adsorption was used to remove BB-41 in duplicate. First, a 1 L stock solution of 1000 mg/L of BB-41 is made. The prepared stock solution was used throughout the study to prepare the required concentrations by dilution. A series of 10 mL of BB-41 solution with various initial concentrations between 25 mg/L to 1000 mg/L was added to separate sealed glass tubes, and 100 mg of Na-Mgd or H-Mgd were suspended in these tubes under shaking in a water bath with a controlled temperature of 25 °C for overnight. After centrifugation, the supernatant’s concentrations were measured at 617 nm using carry 100 UV/VIS spectrometer to determine the removed amount (mg/g) and the removal percentage (%) [26].

The parameters that affected the removal properties were optimized. One parameter at a time must be changed during the optimization process while the others remain constant. Investigation included the effects of the solution’s pH (over a range of 3 to 11), the dosage of Na-Mgd or H-Mgd (ranging from 2 to 5 g/L in a 10 mL solution), and the starting dye concentration (which varied from 25 to 1000 mg/L).

4.4. Regeneration Tests

Oxone and cobalt nitrate solution were used to examine the regeneration of Na-Mgd and H-Mgd samples. Two different C values of 50 mg/L and 200 mg/L were used. The samples were first treated for six hours in a 50 mg/L or 200 mg/L solution of each concentration, and then separated by centrifugation and rinsed with distilled water. A total of 12 milliliters of a solution made up of 12 milligrams of oxone and cobalt nitrate was added to the spent samples for 30 min. Following separation and washing, the resultant samples were treated again with 50 milliliters of fresh BB-41 solutions, and then subjected to the same procedure for seven cycles [26].

4.5. Characterization

Different techniques were used to identify the success of the Na-Mag and H-Mag synthesis. The powder XRD (PXRD) technique was the essential tool to identify the different magadiite phases. An XRD diffractometer (Advance D8 from Bruker, Billerica, MA, USA) was employed and equipped with a Cu-Kα radiation source (λ = 1.5406 Å) operated at 40 kV and 40 mA. X-ray fluorescence (XRF, S4 explorer from Bruker) was used to confirm the presence of Na in Na-Mgd and in H-Mgd. Using pressed KBr pellets and an Shimadzu FTIR spectrometer (Kyoto, Japan), 64 scans were acquired to generate infrared (FTIR) spectra in the 4000–400 cm⁻¹ range at a resolution of 4 cm⁻¹. The mass-loss calculations were performed under a nitrogen flow on a TA instrument (SDT2960 model) (New Castle, DE, USA), with a heating rate of 5 °C/min, between 25 °C and 800 °C. Scanning electron microscopy (SEM) from Jeol (model JSM-6700F) (Tokyo, Japan) was employed to examine the morphology of the samples. A Bruker 400 spectrometer set to ²⁹Si NMR frequency 78 MHz was used to collect ²⁹Si MAS NMR spectra. To measure N₂ adsorption isotherms, the Autosorb 6 instrument from Quantachrome (Boynton Beach, FL, USA) was used after a degassing temperature of 120 °C. The S_BET was estimated using the BET method, the T.P.V. is determined at relative pressure (P/P_o) at 0.95, and the A.P.D. is estimated following the relation A.P.D. = 4 × (T.P.V)/S_BET. The Variant Cary 100 UV-Vis spectrophotometer (Palo Alto, CA, USA) was used to measure the dye concentration.

5. Conclusions

Na-Mgd and H-Mgd materials were successfully synthesized. The exchange of Na cations was easily attained in HCl solution with a reduction in d₀₀₁ basal spacing from 1.54 nm to 1.12 nm. Both layered silicates acted as reliable and alternative removal agents for BB-41 dyes. Na-Mgd has the aptitude to intercalate the BB-41 between the interlayer spacing of the matrix using higher initial concentrations. However, this fact occurred when the removal of BB-41 occurred at a higher pH range from 8 to 10, with a first step of exchange of protons by Na cations, and then the exchange of Na cations with BB-41 dye cations. However, at lower pH values, the removal occurred at the surface of the matrix for both materials. High determination coefficient values (R² > 0.999) suggested that, of the two equilibrium isotherm models used, the Langmuir isotherm best fitted the experimental data over the entire concentration range. For Na-Mgd and H-Mgd, the maximum removal capacities (q_max) were 222 and 108 mg/g, respectively. When seven regeneration cycles were tested, the regeneration results depended on the initial concentrations used. Good reusability was maintained for H-Mgd after four recycle tests, and then it gradually reduced with the increase in cycle number.

During single batch adsorber design, as the treated effluent volumes and the removal percentages increased, additional masses were needed. The Freundlich model underestimated the required masses, while the Langmuir model overestimated them. This study also underlined that selecting the appropriate isotherm is essential to reaching high removal percentages with minimum masses.

Thermodynamic and kinetic studies will be undertaken in the future to understand the removal mechanisms and to reduce the required time for efficient removal percentages.