β‐Cyclodextrin Immobilized in Geophagic Clay Sheets for the Absorption of Dyes From Wastewater Using a Fixed‐Bed Method

ABSTRACT

Industrial wastewater contaminated with synthetic dyes remains a critical environmental concern due to their persistence, toxicity, and poor biodegradability. In this study, β‐cyclodextrin (βCD) functionalized geophagic clay was evaluated as an adsorbent under continuous flow fixed‐bed conditions. Structural characterization confirmed successful βCD immobilization, with SEM/EDS showing carbon‐enriched agglomerated sheets, the FTIR spectra exhibiting new O–C stretching bands at 1152 and 1025 cm⁻¹, and XRD patterns indicating lattice reorganization with enhanced basal reflections and new peaks at 17.9° and 25.1°. Performance testing demonstrated that an optimized 3‐cm bed height achieved 93% RhB removal within 3 min 57 s, while MO removal exceeded 96% at 50 ppm. Regeneration studies showed strong reusability, with the composite retaining over 80% efficiency after five adsorption–desorption cycles. Kinetic modeling revealed that RhB uptake followed the pseudo‐second‐order model (R ² > 0.99), confirming chemisorption dominated by electrostatic attraction and hydroxyl‐mediated hydrogen bonding. In contrast, MO adsorption on βCD clay was well described by both pseudo‐first‐order (R ² = 0.99) and pseudo‐second‐order (R ² = 0.98) models, reflecting a dual mechanism of rapid diffusion‐controlled physisorption and stable chemisorption via cavity inclusion and hydrogen bonding. These findings highlight the synergistic role of βCD functionalization in enhancing adsorption capacity, selectivity, and reusability for industrial dye remediation.

Summary

• β‐Cyclodextrin functionalization of geophagic clay introduced hydroxyl‐rich surfaces and host–guest cavities, enhancing adsorption selectivity.

• Fixed‐bed column studies demonstrated high removal efficiencies: 96% for rhodamine B at 20 ppm and > 96% for methyl orange at 50 ppm using βCD clay composite.

• The mechanism obtained from FTIR spectral comparisons indicated that electrostatic attraction and hydrogen bonding dominate RhB adsorption, while cavity inclusion and hydroxyl interactions govern MO removal.

• Regeneration studies confirmed > 80% efficiency retention after five cycles, significantly outperforming pristine geophagic clay.

• The study demonstrates the use of low‐cost raw materials, reusability, and scalable continuous‐flow operation.

β‐Cyclodextrin‐functionalized geophagic clay was applied in a fixed‐bed column for the removal of rhodamine B (RhB) and methyl orange (MO). RhB interacts via electrostatic attraction and hydrogen bonding, while MO undergoes cavity inclusion complexation and hydrogen bonding within the βCD structure.

1. Introduction

Industrial wastewater discharge containing synthetic dyes from the textile and paper industries has been a focus of research due to the critical environmental challenges they pose when released into water bodies. Mahabeer and Tekere (2021) reported that industrial dye effluents and heavy metal contamination have significantly altered the quality of water and sediment in the Umdloti River, highlighting urgent ecological risks and the need for integrated pollution management (Mahabeer and Tekere 2021). Synthetic dyes are characterized as nonbiodegradable, carcinogenic, and mutagenic, posing environmental and human health risks (Drumond Chequer et al. 2013). The application of conventional adsorbents (activated carbon and nanoparticles) at a large scale is limited by high production costs and challenges with regeneration in resource‐constrained regions (Chen et al. 2026; Hussein et al. 2025; Satyam and Patra 2024).

Natural clays have attracted attention as low‐cost adsorbents due to their layered aluminosilicate structure, high cation‐exchange capacity, and widespread environmental availability (Uddin et al. 2024). Due to the absence of a chemical bond between the clay layers, the capacity of the stacked layers increases, resulting in improved absorption and dissipation of water and other available molecules (Awasthi et al. 2019). Montmorillonite minerals in geophagic clay affect the swelling or shrinking properties of the clay, resulting in large volume changes due to their layered crystal structure, which allows water molecules to enter the interlayer spaces upon hydration (Salah et al. 2025). However, the inherent hydrophilic properties, high negative charge, and structural constraints of bentonite clay result in low selectivity toward complex aromatic dye molecules (Elmoutez et al. 2024; Qiao et al. 2009).

Geophagic clay has versatile uses due to its low cost, abundance, high ion‐exchange capacity, environmentally friendly nature, and wide pH range (Osobamiro et al. 2020). Geophagic clay has been used topically in mud spas (pelotherapy) to absorb toxins from the skin, remove oils, secretions, and contaminants and provide heat to stimulate circulation for the treatment of rheumatism (Baba et al. 2026). However, some people eat geophagic clay to soothe nausea, act as an antiacid, and provide minerals such as iron and calcium (Bonglaisin et al. 2022). A study by Osobamiro has investigated the adsorptive capacity of geophagic clay to remove Cd²⁺ and Pb²⁺ ions from water. The adsorption of Cd²⁺ and Pb²⁺ in clay samples increased with increasing pH, concentration, time, and temperature, resulting in high ion removal (Osobamiro et al. 2020). Dim et al. (2020) investigated the use of acid‐activated kaolin clay for the removal of Cr(VI) and Fe(III) from water. The acid‐modified clay achieved 79% and 90% metal‐ion recovery, and the clay exhibited high metal desorption after adsorption. The study bookmarks clay as an economical method of wastewater treatment, providing an avenue for safe disposal or reuse of the adsorbent (Dim et al. 2020).

Therefore, functionalizing bentonite clay using various methods, such as acid activation (Kahsay et al. 2025), surfactant modification (Andrunik and Bajda 2019), cation exchange (D'Amico et al. 2014), and a polymer coating, increases its adsorption capacity for pollutants. This is facilitated by increasing the surface area, expanding the interlayer spacing, and introducing active binding sites. Cyclodextrin (βCD) is a cyclic oligosaccharide composed of seven glucose units with an inner hydrophobic cavity and a hydrophilic exterior that allows host–guest inclusion complexes with aromatic dyes (Poulson et al. 2022). Despite its molecular recognition capacity, it has a low surface area, poor mechanical stability, and limited reusability (Xu et al. 2023). Modification with β‐cyclodextrin (βCD) increases the spacing between layers and imparts hydrophilicity to clay surfaces by introducing abundant hydroxyl groups (Sadjadi and Koohestani 2021). The hydrophilic surface facilitates interactions with water, making it ideal for adsorbing organic pollutants and nonpolar contaminants (Heydari and Sheibani 2015). Therefore, integrating the structural robustness and ion‐exchange properties of clay with the selective molecular encapsulation of cyclodextrins produces an adsorbent with synergistic properties, combining the high surface area, swelling, and adsorption capacity of natural clays with the unique host–guest complexation and hydrophobic cavities of cyclodextrins (Yakout et al. 2021).

Research studies have shown increasing interest in the pursuit of natural organo‐nanocomposites, such as modifying fly ash zeolite with βCD to improve the removal of the tetracycline antibiotic, achieving 83% removal (Bandura et al. 2022). In another study, βCD‐bentonite clay dendrimer nanocomposites were fabricated for Pd immobilization and the development of a catalyst for the hydrogenation of nitroarenes (Sadjadi and Koohestani 2021). The advantage of βCD clay, when benchmarked against advanced adsorbents such as graphene oxide and biochar, is that βCD organoclay nanocomposites can be fabricated using less energy‐intensive synthesis routes (Koohestani et al. 2021). The fixed‐bed adsorption method further enhances the practical application of the organoclay nanocomposite in the removal of dyes in water. In contrast to batch adsorption, fixed‐bed columns enable continuous treatment, improved mass transfer, and scalable operation. This is a low‐energy‐intensive method, as low pressure and gravitational force can be applied, making it particularly suitable for industrial wastewater management (Malbenia John et al. 2024).

Therefore, the work aims to design and evaluate a sustainable point‐of‐use wastewater system that uses natural materials such as sand and geophagic clay for the absorption of cationic and anionic dyes from wastewater using a fixed‐bed method. Furthermore, geophagic clay will be functionalized with βCD to fabricate a low‐cost and highly effective nanocomposite to improve the efficiency of cationic and anionic dye removal. βCD geophagic nanoclay composites align with the principles of the circular economy by integrating naturally abundant clay with biodegradable modifiers. The low‐cost base of raw materials, abundantly available, and environmentally benign nature enable the production of scalable solutions critical for addressing water and industrial pollution. Their synergistic adsorption mechanisms enhance scalable performance in fixed‐bed columns, and a sustainable material base positions them as a promising solution for industrial wastewater treatment.

2. Methodology

2.1. Materials

Geophagic clay was sourced from Ghana, purified, and sieved to a particle size below 0.85 mm. An analytical‐grade βCD (97%), rhodamine B (RhB) (cationic) (95%), methyl orange (MO) (anionic) (80%), glutaraldehyde (25%), dimethyl sulfoxide (DMSO) (99%), methanol (99%), and ethanol (99%) were purchased from Sigma Aldrich and used without any further modifications. A PVC column was purchased from Osmotech, South Africa.

2.2. Geophagic Bentonite Clay Modification

The clay (picture presented in Figure S1) was first crushed and milled to a fine powder using a mortar and a pestle, then sieved through a 0.85 mm sieve. The bentonite clay was washed with deionized water before use. For modification, geophagic clay was reacted with a binary mixture of dimethyl sulfoxide and water at an 80:20 wt.% ratio. This increased the surface hydroxyl groups, after which βCD was immobilized onto the clay sheets by covalent bonding at 25°C with continuous stirring for 12 h, using glutaraldehyde as the crosslinking agent in an ethanol/water binary mixture (60:40 wt.%) (Martin et al. 2019; Tuncer et al. 2025). The aimed degree of functionality ratio was 1:2 clay to βCD, as this ratio provides the best balance and ensures that the βCD cavities are available for interaction with the dye molecules in aqueous solutions, while the clay provides a mechanically stable, high‐surface‐area carrier (Heydari and Sheibani 2015). A higher degree of functionalization has been reported to distort the hydrophobic cavity required for host–guest complexation, which is the primary mechanism for dye trapping (Song et al. 2021). The resulting composite was thoroughly washed with ethanol and deionized water, then dried at 60°C. A schematic diagram for the modification reaction is presented in Figure 1.

FIGURE 1.

Schematic representation of β‐cyclodextrin immobilization onto geophagic bentonite clay sheets by covalent bonding.

2.3. Characterization

Characterization of the βCD geophagic composite was performed using scanning electron microscopy (SEM) (JOEL JSM–IT300), and the samples were sputter‐coated with gold before analysis. For elemental composition, the samples were analyzed using an energy‐dispersive X‐ray spectrometer (EDS) (Oxford Instruments) coupled to the SEM. The surface functional groups of functionalized clay and clay samples were examined using Fourier transform infrared (FTIR) spectroscopy with a PerkinElmer Frontier FTIR spectrometer (Frontier Optica, Massachusetts, USA) over the 4000‐ to 550‐cm⁻¹ range. X‐ray diffraction (XRD) was conducted using a Rigaku Ultima IV X‐ray diffractometer for crystallinity analysis, and a Zeiss microscope was used to observe the sand particles. The VWR UV‐3100PC UV–Vis Spectrophotometer was used to analyze the concentration of the dye in water. The charge and particle size of clay molecules were measured using the Malvern Zetasizer (Malvern, UK). BET analysis of the GO nanoparticles was conducted using a Quanta‐chrome Autosorb iQ3 Automated gas sorption analyzer (Austria, Anton Paar, GmbH, Graz).

2.4. Batch Adsorption Tests

Batch adsorption tests were performed to assess the effect of different operational factors, including the effect of the adsorbent dosage, contact time (kinetics), the effect of temperature on the adsorption, and the effect of concentration. These experiments were conducted in triplicate, using 50 mL of dye water in a thermos‐shaker at 200 rpm. The temperature was maintained at 25°C for all experiments, except for the thermodynamic experiments, where the temperature was varied to 25°C, 40°C, and 50°C. The samples were centrifuged at 4500 rpm for 5 min, and the clear supernatant was taken for UV analysis. The UV analysis was obtained at λ _max = 465 for MO and λ _max = 555 for RhB. The equilibrium absorption capacity of the adsorbents was calculated using Equation (1):

$$q_e=\frac{\left(C_o-C_e\right)\times V}{m}$$ (1)

q _e is defined as the adsorption capacity at equilibrium (ppm), C _o is the initial concentration of the dye solution (ppm), C _e is the dye concentration at equilibrium (ppm), V is the volume of the solution used during the absorption process, and m is the adsorbent mass used during absorption experiments.

The adsorption kinetics of MO and RhB in pristine clay and βCD functionalized with βCD were evaluated using the pseudo‐first‐order model and pseudo‐second‐order model, as expressed in Equations (2) and (3), respectively:

$$\ln q_e-{\mathit{ln}}_{\mathit{qt}}=\mathit{ln}{q}_e-k_{1}t$$ (2)

In other words, q _e and q _t are defined as the equilibrium and time‐dependent adsorption capacities; k1 is the first‐order rate constant.

$$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e}$$ (3)

where by k ₂ is the second‐order rate constant and t is time.

2.5. Adsorption Column Setup

Clay adsorption experiments were conducted using a custom‐built vertical column (Figure 2). The column was fabricated from a transparent cylindrical PVC tube measuring 25 cm in length and 5 cm in internal diameter. The increase in the height of the geophagic clay bed above 5 cm resulted in low permeability. A packed bed of clay media was confined between two 3 cm sand particles to prevent particle loss while allowing unhindered flow. The sand particles were soaked in a 50‐ppm dye solution and then dried in the oven before use to eliminate adsorption by the sand particles. The clay and sand beds were separated using a fine mesh screen with a pore size of 0.45 mm. Water was introduced through the inlet at the top of the column and was passed downward by gravity through the clay bed before exiting through the outlet port at the base. The downward flow configuration ensured uniform contact between the influent and the clay media, thereby maximizing the adsorption efficiency. The dimensional annotations of the column are indicated in Figure 2 for clarity and reproducibility.

FIGURE 2.

Schematic of the fixed‐bed adsorption column (25 cm long × 5 cm diameter, 3‐cm clay bed).

2.6. Fixed‐Bed Adsorption Studies

Dye solutions of RhB (cationic) and MO (anionic) were prepared at concentrations ranging from 20 to 50 ppm. The solutions were introduced into the fixed‐bed column at a controlled flow rate of 1 mL/min, and the gravitational force influenced the liquid flow metrics in the column. The effluent samples were collected in sealed glass vials and analyzed using UV–VIS spectrophotometry to determine residual dye concentrations, and the calibration curve is shown in Figure S3. All experiments were conducted in triplicate to ensure reproducibility. The reusability of the adsorbent was assessed through adsorption–desorption cycles. The dyes were desorbed using an ethanol:water binary mixture (4:6 v/v), and the process was repeated three times to evaluate regeneration efficiency. The comparative performance was compared with pristine geophagic clay and βCD functionalized bentonite composites, with adsorption capacity (ppm) and regeneration efficiency as key performance metrics. The ANOVA analysis was performed using Origin, and the results are provided in the Supporting Information (Table S1). The setup is shown in Figure S2, with the operating parameters of the column presented in Table S1.

The removal rate was calculated using the equation:

$$\%\mathit{\text{Removal}}=\left(\frac{C_o-C_t}{C_o}\right)x100$$ (4)

The theoretical empty bed contact time (EBCT) for the column was calculated using Equation (5) (Hadi et al. 2011):

$$\mathit{\text{EBTC}}=\frac{V_b}{Q}=\frac{H}{v}$$ (5)

where V _b is the adsorption bed volume (mm), Q is the volumetric influent flow rate (mm/min), H = depth of bed (cm), and v is the linear flow rate (cm/min).

3. Results and Discussion

3.1. Microscopic View of the Sand Particles

The microscopic images of the sand particles in Figure 3 show quartz particles exhibiting translucent morphology under light microscopy. This is because sand contains quartz (silicon dioxide [SiO₂]), which appears translucent under light. Sand particles are also charged with a slight negative charge due to their composition, resulting in strong electrostatic interactions with cationic dyes (Sharma et al. 2019). In contrast, MO sand was internally colored through physical entrapment rather than being chemically adsorbed on the surface of the sand (Dragan and Dinu 2018).

FIGURE 3.

Microscopic images of quartz sand particles illustrating different dye–sand interactions: (a) clean particles, (b) rhodamine B soaked particles showing electrostatic binding, and (c) methyl orange‐soaked particles exhibiting physical entrapment.

3.2. Zeta Potential and Size of Clay and βCD‐Functionalized Clay

The zeta potential and zeta potential of the clay sample was measured at neutral pH using a Malvern ZetaSizer (Table 1). The average particle size of the pristine clay was measured at 815 nm (±0.0263), whereas βCD clay exhibited a significantly larger dimension of 1257 nm (±0.214). This increase in particle size is consistent with the immobilization of bulky βCD moieties on the clay surface, which not only contributes additional molecular mass but also promotes agglomeration through enhanced hydrogen bonding. In contrast, the zeta potential increased upon functionalization, from −33.5 mV (±0.209) for pristine clay to −14.4 mV (±0.402) for βCD clay. The reduction in surface charge density suggests partial shielding of the clay's native negative sites by βCD molecules, leading to reduced electrostatic repulsion between particles and thereby facilitating aggregation. Collectively, these results confirm that βCD functionalization alters both the colloidal stability and surface chemistry of the clay, producing larger particles. Such modifications are expected to influence the uptake of dye molecules due to the increased particle size and increased zeta potential.

TABLE 1.

Size and zeta potential of clay and βCD functionalized clay.

Parameter	Clay	βCD
Size (nm)	815 ± 0.0263	1257 ± 0.214
Zeta (mV)	−33.5 ± 0.209	−14.4 ± 0.402

3.3. SEM Images/EDS

The SEM micrographs show different morphological features of the pristine geophagic clay (Figure 4a) and the βCD‐functionalized geophagic clay (Figure 4b). The unmodified clay exhibits a heterogeneous surface with smaller, irregularly distributed clay layers, indicative of its layered silicate structure and high surface roughness. In contrast, the βCD functionalized clay exhibits agglomerated clay sheets, indicating that the coating of the clay surfaces with the organic modifier (βCD) and the formation of a more compact morphology. This structural transformation of βCD functionalized clay is in agreement with the grafting of βCD, which introduces hydroxyl‐rich moieties to the clay sheets, thus promoting interparticle aggregation.

FIGURE 4.

The SEM micrographs showing: (a) bentonite clay, (b) βCD functionalized bentonite clay, (c) elemental composition of bentonite clay, and (d) elemental composition of βCD functionalized clay.

The complementary EDS analysis further substantiates the modification by showing elemental differences between bentonite clay and the βCD functionalized bentonite clay. The elemental profile of pristine bentonite (Figure 4c) shows peaks for oxygen, magnesium, and silicon, indicating the aluminosilicate framework of the geophagic bentonite clay. Minor contributions from aluminum, potassium, and iron are also evident, as are traces of gold from sample preparation. Upon functionalization (Figure 4d), the elemental profile of the βCD functionalized bentonite clay shows an increase in carbon content due to the clay lattice and alkane, the ethylene groups, and the functional groups of ether present in the chemical skeleton of βCD (Poulson et al. 2022). The EDS analysis revealed that the degree of functionalization achieved for the βCD modified clay corresponded to a weight ratio of 1:2 (clay:βCD). This indicates that the βCD was successfully incorporated at approximately twice the proportion of the clay matrix, confirming effective surface modification and the formation of an organic–inorganic hybrid structure. The relative reduction in magnesium and the emergence of sulfur traces suggest a partial substitution of exchangeable cations, because geophagic clay reacted with dimethylsulfoxide to enhance the introduction of organic functionalities. The relative reduction in oxygen is due to the resulting ether or ester bonds obtained during crosslinking. Therefore, βCD clay contains less oxygen by mass than the geophagic clay sample, containing the original hydroxyl groups (Tcheumi et al. 2019). Collectively, the SEM and EDS results demonstrate that βCD immobilization alters the surface morphology and chemical composition of bentonite, producing an organo‐clay compound with improved organic chemical properties and a modified surface. In the Supporting information, lower‐magnification (×200) SEM micrographs are presented (Figure S4).

3.4. BET Surface Analysis for Geophagic and βCD Clay

The nitrogen physisorption parameters for the pristine geophagic clay and βCD functionalized clay are summarized in Table 2. The pristine geophagic clay sample exhibited a high surface area of 30.465 m²/g and a total pore volume of 0.0683 cm³/g. The observed average pore diameter (4.4809 nm) confirms a mesoporous structural clay sheet network. Upon functionalization, the specific surface area decreased approximately by 81% to 5.68 m²/g. Simultaneously, the total pore volume decreased by 84.8% to 0.0104 cm³/g. The observed decrease in both surface area and total pore volume is attributed to the steric hindrance effect of the bulky macrocyclic βCD molecules. The hindrance effect is further corroborated by the reduction of the average pore volume to 3.66 nm due to organic moieties clogging the interlayer spaces and external mesopores of the clay sheets. The decline in surface area is concurrent with the results obtained by Koohestani et al. (2021), which showed that the BET results indicated a decrease in the specific surface area of Bent‐Gu‐CD from 40.62 to 6 m²/g compared with the pristine clay (Koohestani et al. 2021). Conversely, the BET energy constant (C) increased from 68.54 to 175.03 for geophagic clay and βCD clay, respectively. The observed significant increase enhances the affinity between nitrogen gas and the βCD clay sample. This is due to the hydrophobic cavities and hydrophilic surface groups of the βCD framework, resulting in a change of surface energy distribution, yielding highly active adsorption sites. Therefore, the BET energy constant indicates stronger adsorbate–adsorbent interactions, meaning nitrogen molecules bind more tightly to the surface. This often suggests the presence of more polar sites or stronger surface heterogeneity (Lowell et al. 2004; Shi et al. 2021). The nitrogen adsorption–desorption isotherms of pristine clay and βCD functionalized clay are shown in Figure S5.

TABLE 2.

BET results for βCD functionalized clay in comparison with those of pristine geophagic clay.

Parameter	Geophagic clay	βCD clay
Specific surface area (a s, BET)	30.465 m2/g	5.6756 m2/g
Monolayer absorption volume (V m)	6.9995 cm3/g	1.3040 cm3/g
BET constant	68.536	175.03
Total pore volume (p/p o)	0.0683 cm3/g	0.0104 cm3/g
Average pore volume (slit)	4.4809 nm	3.6646 nm

3.5. Functional Group Analysis Using FTIR

The FTIR spectra in Figure 5 show the different chemical functionalities present in the pristine geophagic clay, βCD, and the hybrid βCD geophagic clay composite. In the pristine clay spectrum, characteristic absorption bands are observed at 3694 and 3621 cm⁻¹, corresponding to the stretching vibrations of structural OH groups associated with Al–OH and Si–OH units in the aluminosilicate network (Kovalchuk et al. 2026). Additional peaks at 850 and 754 cm⁻¹ are attributed to Si–O bending vibrations, confirming the layered silicate framework of natural clays (Andrunik and Bajda 2019). In contrast, the spectrum of pure βCD exhibits a broad band at 3280 cm⁻¹, attributed to hydroxyl (OH) groups, along with strong absorptions at 1152 and 1023 cm⁻¹ corresponding to CO–C stretching vibrations of glycosidic linkages in the cyclic oligosaccharide (Sulaiman et al. 2021).

FIGURE 5.

The FTIR spectra of geophagic clay, βCD, and βCD clay composite. (a) Full spectra, (b) extended spectra between 1800 and 500 cm⁻¹.

Upon functionalization, the spectrum of the βCD clay composite integrates chemical moieties of both βCD and geophagic clay, demonstrating the successful immobilization of βCD in clay sheets. The broad –OH stretching band remains prominent, reflecting contributions from clay hydroxyls and hydroxyl‐rich cyclodextrin. New absorption bands emerge at 1152 and 1025 cm⁻¹, which can be assigned to O–C stretching vibrations of the βCD moieties that interact with the clay surface. The persistence of the absorption bands at 1638 cm⁻¹ (H–O–H bending of adsorbed water) and 754 cm⁻¹ (Si–O bending) confirms the retention of the aluminosilicate structure, while the new absorption bands at 1152 and 1025 cm⁻¹ highlight the organic modification. Collectively, these spectral changes confirm the chemical grafting of βCD to bentonite, resulting in an organoclay compound with dual functionalities.

3.6. XRD

The XRD patterns of geophagic clay and βCD functionalized clay (Figure 6) appear to have dominant characteristic peaks associated with bentonite (2θ = 7°, 12°, 20.8°, 21.9°, 26.6°, 27.7°, 36°, 38°, 50°, 62°, and 69°) (Sadjadi and Koohestani 2021). The characteristic reflections of montmorillonite, with basal (001) peaks appearing at 7°, are accompanied by strong Si–O–related peaks around 20.8° and 27.7°. The enhanced reflections at 7° and 12° in the βCD‐functionalized bentonite XRD pattern indicate structural modifications within the montmorillonite lattice. The basal peak (001) near 7° indicates interlayer expansion due to βCD insertion, while the intensified reflection at 12° suggests improved ordering and stabilization of the silicate framework. The emergence of new characteristic peaks at 17.9° and 25.1° in the XRD spectrum of βCD functionalized geophagic clay confirms the structural reorganization of the montmorillonite network. The characteristic peak at 17.9° is attributed to the improved ordering of the Si–O tetrahedral sheets and the higher order basal planes. In comparison, the peak at 25.1° reflects improved framework crystallinity and hybridization effects arising from βCD grafting. There was a change in the base area spacing region 2θ = 5°–15° and high peak pronunciation on the βCD spectra, confirming the expansion of the interlayer galleries due to DMSO exfoliation, allowing the immobilization of the βCD onto the clay sheets. The enhanced intensity in the XRD spectra of βCD clay, particularly around 27.7° and 31.7°, suggests stronger ordering or surface functionalization effects rather than interlayer intercalation, consistent with the steric hindrances of βCD. This was also observed by Tcheumi et al. (2019), whereby smectite‐type clay was functionalized using βCD.

FIGURE 6.

XRD of bentonite clay, β‐cyclodextrin, and β‐cyclodextrin‐geophagic clay composite.

3.7. Factors Affecting Batch Absorption Experiments

3.7.1. The Effect of Absorbent Dose on the Absorption of MO and RhB

The effect of adsorbent dosage on adsorption capacity was investigated using 0.5, 0.8, and 1.2 g, with an initial dye concentration of 50 ppm, a volume of 50 mL, and a temperature of 295 K. The investigation of adsorbent dosage effects on the RhB and MO adsorption exhibits different adsorption characteristics in the different dye solutions. Figure 7a shows the adsorption of RhB in pristine clay and βCD functionalized clay. For RhB molecules, the q _e values of the adsorption in pristine clay were low at 0.5 and 0.8 g of adsorbent dosage. This means that at low adsorbent doses, the adsorbent did not have enough adsorption sites for RhB molecules. However, adsorption on βCD functionalized clay consistently maintained higher q _e values compared with pristine clay. Thus, demonstrating that functionalization improves the binding affinity of the RhB molecules on the active adsorption sites, therefore resulting in strong electrostatic interactions that facilitate RhB adsorption on βCD clay.

FIGURE 7.

Effect of the adsorbent dosage on the uptake of (a) RhB molecules and (b) MO molecules on both clay and βCD functionalized clay.

In contrast, MO exhibited a significant decrease in q _e values with increasing dosage on pristine clay due to weaker electrostatic interactions between the anionic MO molecules and clay surfaces. Thus, this resulted in less efficient use of the adsorption sites at low doses. βCD clay significantly mitigated this limitation, resulting in high q _e values in comparison with pristine clay. The improvement can be attributed to the synergistic contributions of the host‐cavity inclusion, hydrogen bonding, and van der Waals forces of the host‐cavity, providing multiple binding pathways of the MO molecules to the adsorption sites of the βCD functionalized clay.

Therefore, the comparative analysis indicates that the mechanism of RhB adsorption is dominated by strong site‐specific electrostatic interactions. In contrast, MO adsorption is based on diffusion and secondary binding forces, resulting in a dosage‐dependent adsorption mechanism. The improved performance of βCD clay (Figure 7b) during the adsorption of both MO and RhB molecules confirms that functionalization improves site accessibility and introduces selective binding mechanisms that counteract the dependence on electrostatic interactions. Importantly, the contrast between RhB and MO highlights the versatility of βCD functionalized clays in achieving high q _e values for cationic dyes such as RhB, and significantly improving the adsorption efficiency for anionic dyes such as MO.

3.7.2. The Effect of Contact Time During Adsorption

The effect of contact time on the adsorption kinetics was investigated using 1.2 g, with an initial dye concentration of 50 ppm, a volume of 50 mL, and a temperature of 295 K. The adsorption kinetics of MO and RhB onto pristine clay and βCD functionalized clay were investigated to study the role of contact time in the removal efficiency and equilibrium capacity (Figure 8). For MO adsorption on pristine clay, the percentage of removal gradually increased from 41.3% ± 1.62% to 66.2% ± 0.346% in 180 min, with the equilibrium adsorption capacity (q _e ) rising from 0.862 ± 0.03 to 1.38 ± 0.007 mg/g. This slow uptake suggests that the adsorption is reliant on a diffusion‐controlled mechanism regulated by weaker electrostatic interactions between MO molecules and the clay surface (Khraisheh et al. 2002). In contrast, RhB adsorption on clay achieved equilibrium rapidly, with a high removal efficiency reaching 98.6% and q _e showing marginal increases (2.03 ± 0.004 to 2.05 ± 0.0009 mg/g), indicating strong affinity and rapid site saturation.

FIGURE 8.

Effect of contact time (kinetics): (a) adsorption of MO on clay, (b) adsorption of RhB on clay, (c) adsorption of MO on βCD functionalized clay, and (d) adsorption of RhB on βCD functionalized clay.

Functionalization with βCD significantly improved both the adsorption capacity (q _e ) and kinetics. MO adsorption on βCD clay exhibited a rapid increase in the removal efficiency of 69.04% ± 0.336% to 96.4% ± 0.0516%, with q _e rising from 1.45 ± 0.007 to 2.01 ± 0.001 mg/g. The enhanced performance can be attributed to host–guest inclusion interactions between MO molecules and the hydrophobic cavity of βCD, as well as additional hydrogen bonding and van der Waals forces (Sulaiman et al. 2021). Similarly, RhB adsorption on βCD clay achieved high removal of RhB molecules (99.3% ± 0.037%) in 180 min, with q _e increasing from 2.005 ± 0.001 to 2.08 ± 0.0004 mg/g. The rapid achievement of equilibrium highlights the synergistic effect of cavity encapsulation and electrostatic attraction, which accelerated the adsorption of RhB molecules on the βCD clay compared with pristine clay.

3.7.3. Adsorption Thermodynamics

The adsorption thermodynamics of MO and RhB in pristine clay and βCD functionalized clay at different temperatures (T = 20°C, 40°C, and 60°C) for 1 h are illustrated in Figure 9. The effect of temperature on the adsorption capacity (q _e ) of MO and RhB onto pristine clay and βCD functionalized clay provides information on the adsorption mechanism of dyes under different temperature conditions. For MO (Figure 9a), the q _e values increased consistently with increasing temperature. This can be attributed to the fact that higher temperatures weaken the electrostatic forces between the dye molecules and the active sites on the clay surface. However, βCD clay exhibited enhanced adsorption capacities compared with pristine clay throughout the range. The temperature dependence suggests that MO adsorption is endothermic, thereby increasing thermal energy, which enhances the diffusion of dye into pores and encourages host–guest inclusion within the βCD cavities. The enhanced q _e values for βCD clay highlight the role of functionalization in reducing diffusion barriers and introducing additional binding forces such as hydrogen bonding and van der Waals interactions, which become more effective at higher temperatures.

FIGURE 9.

Temperature‐dependent adsorption capacity (q _e ) of clay and βCD clay for (a) methyl orange (MO) and (b) rhodamine B (RhB).

In contrast, in RhB adsorption (Figure 9b), the variations in q _e with increasing temperature are insignificant, and the differences in adsorption capacity between clay and βCD clay were low. This weak temperature dependence indicates that RhB adsorption is temperature independent, consistent with a process dominated by strong electrostatic interactions between the cationic dye and negatively charged clay surfaces. The consistent q _e values suggest that adsorption occurs primarily at external sites with minimal contribution from intraparticle diffusion or thermally activated binding. The improvement observed with βCD clay reflects the limited role of host–guest inclusion for RhB compared with MO, as the bulky cationic structure of RhB limits the encapsulation of RhB molecules within the βCD cavity.

Therefore, MO adsorption is governed by a diffusion‐controlled mechanism, and cavity encapsulation can be attributed to the enhanced adsorption capacity in the βCD functionalized clay. In contrast, RhB adsorption remains governed by strong site‐specific electrostatic interactions that are relatively independent of temperature changes. The observed improved performance of βCD clay for MO highlights the importance of functionalization in the tailoring of adsorbent materials for dyes with weaker natural affinities to clay surfaces.

3.7.4. Kinetic Modeling

Kinetic constants are presented in the Supporting Information. The plot of ln ($q_e-q_t$) versus time provides insight into the suitability of this model for describing the adsorption process based on the linearity. In Figure 10a, the regression plot shows low linearity, with fitted $q_e$ values deviating significantly from experimental data. This suggests that MO adsorption on pristine clay is not governed by pseudo‐first‐order kinetics. The low slope (k ₁) reflects a slow adsorption rate, which is indicative of diffusion‐controlled adsorption and access to active sites. Functionalizing the clay with βCD improves the fit (Figure 10b), resulting in a higher correlation coefficient, and with the fitted q _e and experimental $q_e$ showing high linearity. The enhanced slope indicates an increase in the adsorption of MO on the βCD clay, attributing to the host–guest inclusion within βCD cavities and hydrogen bonding interactions.

FIGURE 10.

Pseudo‐first‐order kinetic graphs for the adsorption of MO and RhB in clay and β‐cyclodextrin functionalized clay: (a) MO on clay, (b) MO on βCD clay, (c) RhB in clay, and (d) RhB on βCD clay.

In Figure 10c,d, the adsorption of RhB in pristine clay and the adsorption of RhB on βCD functionalized clay are presented, respectively. Despite a stronger electrostatic attraction between cationic RhB and negatively charged clay surfaces, the regression plots reveal low linearity. The slope is steeper than in MO systems, indicating faster adsorption, but the poor correlation coefficient suggests that the pseudo‐first‐order model does not adequately capture the adsorption dynamics. Although adsorption is rapid for the uptake of RhB onto βCD clay, the linearity of the plots remains low. The calculated $q_e$ values do not align with experimental data, indicating that even in the presence of synergistic effects (electrostatic binding plus cavity encapsulation), the pseudo‐first‐order model is not the best model to explain the adsorption kinetics. The adsorption kinetics constants shown in Table S2 indicate that the adsorption is based on the pseudo‐second‐order model with R ² values approaching.

The adsorption kinetics of MO and RhB in pristine clay and βCD functionalized clay were further analyzed using the pseudo‐second order model, as shown in Figure 11. The linearity of the t/q _t versus time plot in Figure 11a is high compared to the pseudo‐first‐order model. The calculated $q_e$ values show closer agreement with experimental data, suggesting that MO adsorption on clay is better described by pseudo‐second‐order kinetics. This indicates that chemisorption interactions between MO molecules and βCD clay surface sites play a significant role. However, in Figure 11b, the linear fit correlation was lower (0.98) for MO adsorption on βCD clay, suggesting that while chemisorption is dominant, additional interactions (host–guest inclusion, hydrogen bonding) introduce complexity that reduces the precision of the model. The pseudo‐second‐order model provided an excellent fit (R ² = 0.999) as shown in Figure 11c, higher than the pseudo‐first‐order model for RhB adsorption at the clay active sites. This confirms that RhB adsorption is governed by chemisorption, with rapid kinetics facilitated by electrostatic attraction between cationic RhB and negatively charged clay surfaces. Figure 11d shows high linearity with R ² = 0.999. The observed high linearity and agreement between calculated and experimental $q_e$ highlight the synergistic effects of electrostatic binding and cavity encapsulation. This system exemplifies the suitability of the pseudo‐second‐order model for describing the adsorption of RhB onto the βCD functionalized clay.

FIGURE 11.

Pseudo‐second‐order kinetic graphs for the adsorption of MO and RhB in clay and β‐cyclodextrin functionalized clay: (a) MO in clay, (b) MO on βCD clay, (c) RhB in clay, and (d) RhB on βCD clay.

3.8. Effective Bed Height

Comparing the performance of 2‐, 3‐, and 5‐cm clay geophagic beds, the results indicate that shorter bed heights (2 cm) yield lower removal due to insufficient contact time, resulting in lower removal percentages (Figure 12). The experimental conditions were standardized to maintain a feed concentration of 20 ppm, an ambient temperature of 25°C ± 1°C, and a constant flow rate of 1 mL/min. However, the 5‐cm beds did not show permeation for 2 h, as the experiment relied on a gravity‐driven water flow in the column, thereby restricting the practical application of deeper bed heights. Therefore, the 3‐cm bed height (46.8 g) represented a balanced configuration that maintained manageable flow resistance and resulted in 93% removal with a 20‐ppm RhB dye solution and a contact time of 3 min 57 s. However, the theoretical EBTC was calculated to be 58.09 min. The theoretical EBCT is higher than the actual or effective experimental contact time due to bed void fraction, flow channeling, and physical media volume displacement (Hasson et al. 2017). The 2‐cm bed removed only 73% of the 20‐ppm RhB dye solution after 2 min and 4 s of contact time. The 2‐cm bed achieved a 73% removal, confirming limited adsorption sites and an insufficient mass transfer zone. This is due to the restricted development of a proper mass transfer zone through the 2‐cm clay bed, which reduced the contact time between the dye solution and the geophagic clay surface.

FIGURE 12.

The effect of geophagic clay bed height on the removal (%) of dye molecules.

3.9. Fixed‐Bed Experiments Using Clay and βCD Clay for the Removal of MO and RhB

The removal efficiency of anionic dyes on geophagic clay differs significantly from that of cationic dyes due to the negatively charged surface of the clay and exchangeable anions (Figure 13a). The negative surface charge arising from the silicate structure of geophagic clay repels anionic dye molecules, resulting in low dye molecule uptake compared with that of cationic dyes. Therefore, at low concentrations, the removal efficiency is low due to electrostatic repulsion between the clay surface and the dye molecules, resulting in a decrease in the removal efficiency (Mouhsine et al. 2025; Rawat and Ahammed 2026). The small number of dye molecules in solution further reduces the likelihood of overcoming this repulsion, resulting in the underutilization of available sites. Therefore, it results in the removal % of 73.5% ± 0.8% at the lowest dye concentration (20 ppm). In contrast, at higher concentrations, the increased number of dye molecules enhances the driving force for mass transfer, overcoming the repulsive interactions. This stronger concentration gradient facilitates high adsorption, and secondary mechanisms, such as van der Waals forces, hydrogen bonding, or multilayer adsorption, may further improve the removal efficiency (Rawat and Ahammed 2024). At the highest concentration (50 ppm), the removal % was 94.6% ± 0.03%. The batch adsorption experiments provided insight into the kinetics of MO adsorption on clay, whereby the pseudo‐first‐order model yielded low correlation. In contrast, the pseudo‐second‐order model showed an excellent correlation with the calculated q _e . This confirms that chemisorption, involving stronger binding interactions such as hydrogen bonding and site‐specific adsorption, plays a dominant role in overcoming electrostatic repulsion at higher concentrations.

FIGURE 13.

Showing (a) the effect of initial concentration on the removal (%) of MO molecules using clay and βCD functionalized clay, and (b) the effect of initial concentration on the removal (%) of RhB molecules using clay and βCD functionalized clay.

Geophagic clay acts as an effective adsorbent for cationic dyes due to its negatively charged surfaces, high surface area, and layered silicate structure. In a fixed‐bed system, the electrostatic attraction between clay sheets and cationic dye molecules primarily governs RhB absorption (Figure 13b). The abundant active sites on the clay surface enable strong electrostatic attraction and hydrogen bonding, resulting in rapid uptake and high percentage removal. As concentration increases, these active sites become saturated, and multilayer adsorption or weaker van der Waals interactions govern the adsorption of the molecules. Therefore, this results in a decrease in removal efficiency. At low influent concentrations, the column maintains high removal efficiency and longer operational lifespan, consistent with chemisorption predicted by pseudo‐second‐order kinetics. At higher concentrations, site saturation accelerates the breakthrough and reduces the percentage of removal, as shown in Figure 13b. Subsequently, as the concentration increased, the percentage of removal of dye molecules decreased. Higher concentrations increased the absolute uptake of dye per unit mass of geophagic clay, while simultaneously reducing the percentage removal efficiency and resulting in a short operational lifespan of the column.

The presence of hydroxyl (–OH) groups introduced through βCD functionalization increases the adsorption of RhB onto βCD geophagic clay composite. The batch adsorption experiments showed that pseudo‐second‐order kinetics best explain the adsorption of RhB onto βCD functionalized clay. βCD imparts hydroxyl groups to geophagic clay, which function as hydrogen‐bond donors and acceptors, enabling secondary bonding interactions with the amino and carboxyl groups in the RhB structure. This hydrogen bonding complements the electrostatic attraction between the negatively charged clay layers and the cationic RhB molecules, thereby stabilizing adsorption complexes and improving removal efficiency. However, due to steric hindrance from the bulky xanthene ring of RhB, the removal efficiency decreases with increasing concentration. Therefore, hydroxyl‐mediated hydrogen bonding becomes the dominant mechanism of adsorption rather than inclusion within the available βCD cavities (Liu et al. 2025). This decline is attributed to the saturation of adsorption sites on the clay bed, limiting further uptake at higher solute loads. These findings suggest that RhB molecules, due to their bulkier structure and steric hindrance, interact less effectively with βCD cavities, leading to reduced complexation efficiency at elevated concentrations.

In contrast, MO removal efficiency increased with concentration, from approximately 94% at 20 ppm to over 96% at 50 ppm. This trend highlights the stronger affinity of βCD functionalized clay for MO, due to the planar aromatic structure of MO, which fits more readily into the βCD inclusion cavities (Zhao et al. 2016). Therefore, the removal efficiency is not compromised, suggesting that the clay bed retains sufficient adsorption capacity for MO even at elevated concentrations. This can be attributed to the synergistic action of cavity inclusion and hydroxyl interactions, which play a crucial role in removing MO molecules from the solution (Li et al. 2021; Zhao et al. 2016). This shows a heterogeneous removal mechanism enabled by βCD functionalization on the surface of geophagic clay, which improves surface hydrophilicity, enhances hydrogen bonding, and facilitates interaction between the βCD cavity and MO molecules, as MO adsorption dynamics are structurally dependent on the βCD functionalized geophagic clay. The dual mechanism is concurrent with the observed batch adsorption kinetics, whereby the pseudo‐first‐order model provided an excellent correlation coefficient of R ² = 0.99. This indicates that diffusion‐controlled physisorption plays a significant role in the initial adsorption process, consistent with the improved uptake at lower concentrations. However, the adsorption process also showed a strong correlation to the pseudo‐second order (R ² = 0.98), confirming the contribution of chemisorption mechanisms.

3.10. Regeneration Studies

The experimental conditions were standardized to maintain a feed concentration of 50 ppm, a constant solution pH, an ambient temperature of 25°C ± 1°C, and a constant flow rate of 1 mL/min. Figure 14 presents the regeneration cycles of geophagic clay and βCD functionalized clay, investigated over five adsorption–desorption cycles for RhB (Figure 14a) and MO (Figure 14b). For both dyes, the βCD compound shows higher removal efficiency than pristine bentonite, retaining a higher fraction of its initial adsorption capacity across five adsorption–desorption cycles. In the case of pristine clay, adsorption primarily relies on cation exchange and surface interactions, which are prone to irreversible binding of dye molecules. Over five cycles, these saturated active sites cannot be fully regenerated. Simultaneously, a gradual loss in mechanical stability and leaching of fine particles reduces the effective adsorbent mass, resulting in the observed decline in efficiency. The βCD geophagic clay shows improved stability in the removal of dyes due to the hydroxyl groups and βCD cavities, allowing reversible inclusion and hydrogen bonding aimed for host–guest interactions. Evidently, five cycles resulted in partial degradation of the βCD moieties and minor weight loss due to incomplete desorption and gradual loss of the βCD clay composite. As a result, although the βCD clay composite retains a higher fraction of its initial adsorption capacity compared with pristine clay, both materials exhibit a gradual decline in removal efficiency with each cycle, reflecting the cumulative effects of site loss and reduced adsorbent mass.

FIGURE 14.

Regeneration of the geophagic clay bed in comparison to that of βCD‐geophagic composite: (a) showing the removal efficiency for RhB molecules and (b) showing the removal efficiency for MO molecules.

3.11. Long‐Term Operation

Figure 15 shows comparative removal efficiencies of RhB and MO over time using βCD geophagic clay composite. The comparative removal efficiency, expressed as the dimensionless ratio $\frac{C_t}{C_0}$, where C _t is the concentration of dye at time (t) and C ₀ is the initial dye concentration, whereby the ratio of effluent to influent was plotted against time. The C _t /C ₀ curve (RhB) indicates rapid adsorption and high removal efficiency during the initial time frame. In contrast, the C _t /C ₀ curve (MO) demonstrates a more gradual reduction, reflecting slower adsorption kinetics and comparatively lower removal efficiency under identical conditions. This divergence highlights the dye‐specific affinity of the βCD functionalized clay bed, where hydroxyl‐mediated hydrogen bonding facilitates RhB retention. Conversely, the removal of MO is governed by electrostatic attraction and steric compatibility with the hydrophobic cavity of βCD. The sharper trend of RhB removal suggests stronger binding interactions, whereas slower MO removal highlights the role of molecular structure in host–guest interactions within βCD's inclusion cavities.

FIGURE 15.

Comparative removal kinetics of rhodamine B (RhB) and methyl orange (MO) in a β‐cyclodextrin functionalized with β‐cyclodextrin.

Figure 16a shows UV spectra correlating to the removal of RhB by the βCD functionalized geophagic clay bed functionalized with βCD functionalized with βCD. The absorbance spectra show a progressive decrease in peak intensity at 554 nm as contact time increases from 30 to 180 min. The initial spectrum at 50 ppm shows the highest UV absorbance corresponding to the initial concentration of the dye solution. The continuous decline in peak height reflects a reduction in RhB concentration in the solution, confirming strong host–guest and hydrogen bonding interactions between RhB molecules and βCD functionalized bentonite. The UV spectra at 30 and 60 min show low UV absorbance, indicating effective RhB uptake by the adsorbent due to the high availability of adsorption sites.

FIGURE 16.

UV Spectra of the dye solution showing the initial concentration (CO) at 50 ppm and the residual concentration (C _t ) at 30 to 180 min. (a) RhB dye solution and (b) MO dye solution.

In Figure 16b, the absorbance spectra of the MO show a main peak around 467 nm that gradually decreases over time. The initial 50 ppm solution shows the highest absorbance, while subsequent spectra at 30–180 min show reduced concentration due to the removal of MO molecules. Compared with RhB, MO removal shows lower absolute absorbance values (absorbance = 0.5). This suggests that MO adsorption is mainly governed by electrostatic attraction and steric compatibility with the βCD cavities, resulting in efficient but slightly slower removal kinetics compared with RhB removal. The UV spectra at 30 and 60 min show low UV absorbance, indicating effective MO uptake by the adsorbent due to the high availability of the adsorption sites.

3.12. Benchmarking of Results

Benchmarking against other functionalized clay composites demonstrates the advantages of βCD modification for dye removal (Table 3). Acid‐activated bentonite clay typically achieves high initial efficiencies (> 90%) for dyes such as Bemacid Blue and Congo Red, with regeneration stability declining rapidly after four cycles due to structural degradation (Kahsay et al. 2025). The investigation of biopolymer coatings in bentonite clay, such as chitosan‐bentonite hybrid composite (CBC), for the removal of anionic (amaranth red [E 123]) and cationic (methylene blue [MB]). The authors observed that the composite's removal efficiency remains stable over three cycles, and the decline is due to mechanical stability (Dotto et al. 2016). Surfactant‐modified organo‐bentonites (e.g., CTAB functionalization) deliver very high adsorption of anionic dyes (acid red [AR]) and acid fuchsin (AF), obtaining a removal efficiency of 94% for both AR and AF dyes, maintaining a regeneration performance above 70% for three cycles due to irreversible surfactant loss (Zhu et al. 2023). Poly(methacrylic acid‐co‐acrylamide) and poly(methacrylic acid‐co‐acrylamide)/bentonite showed improved adsorption capacity for methyl violet, achieving 95% removal of MV at 45°C (Safarzadeh et al. 2022). Montmorillonite clay modified with iron oxide (MtMIO) was investigated for MB removal, with a maximum adsorption capacity of 71.12 mg g⁻¹ at 333 K, corresponding to removal efficiencies exceeding 90% under optimized conditions. The composite retained over 80% of its initial removal efficiency for the first three cycles, confirming structural stability and effective regeneration (Cottet et al. 2014). In contrast, the synthesized βCD geophagic clay composite evaluated in this study combines high removal efficiency (85% for RhB and > 90% for MO) with superior regeneration stability (> 80% retained after five cycles), underscoring the role of cyclodextrin cavities and hydroxyl groups in facilitating reversible host–guest inclusion and hydrogen bonding. Collectively, these comparisons highlight that βCD‐functionalized clay offers a balanced advantage in efficiency, selectivity, and sustainability, positioning it as a competitive and practical adsorbent for dye remediation relative to other geophagic clay‐based composites.

TABLE 3.

Benchmarking performance of the βCD geophagic clay sheets with other clay‐modified adsorbents for dye removal.

Composite	Modification	Target dyes	Removal efficiency	Regeneration cycles	Reference
βCD geophagic (current work)	Grafting β‐cyclodextrin onto geophagic clay	Rhodamine B, methyl orange	> 85% RhB, > 90% MO	> 80% retained after 5 cycles	This study
Acid‐activated bentonite	Hydrochloric acid treatment (8 N)	Bemacid blue, Congo red	> 90% BB 85% CR, > 90% BB > 85% CR	> 70% after 4 cycles	(Kahsay et al. 2025)
Chitosan‐bentonite composite	Biopolymer coating with chitosan	Amaranth red, methylene blue	> 88% E 123, > 40% MB	Declines after 3 cycles	(Dotto et al. 2016)
Organo bentonite (CTAB modified)	Cetyltrimethylammonium bromide surfactant	Methyl violet	> 95% MV	N/A	(Safarzadeh et al. 2022)
Natural clay	Calcination at 500°C	Malachite green dye	88.6% (MG)	5 cycles	(Patra et al. 2026)
Montmorillonite clay modified with iron oxide	Incorporating iron oxide coatings into the clay matrix	Methylene blue	> 90% MB	> 80%	(Cottet et al. 2014)

3.13. Mechanism

The enlarged view of the FTIR spectra in Figure 17b reveals distinct mechanistic pathways for the adsorption of RhB and MO onto βCD functionalized geophagic clay. For RhB adsorbed βCD spectra, attenuation of the native βCD hydroxyl (1174 cm⁻¹) and ether (1199 cm⁻¹) bands indicates strong hydrogen bonding and cavity encapsulation of the dye's xanthene ring, while suppression of the Si–O stretching band at 1040 cm⁻¹ reflects disturbance of the clay lattice. The RhB adsorption is dominated by hydrogen bonding, accompanied by structural reorganization of the clay framework. In contrast, MO adsorption produces a different spectral fingerprint. The disappearance of the 1199 cm⁻¹ ether band and the suppression of the 1040 cm⁻¹ Si–O vibration indicate the consumption of βCD backbone functionalities and the disruption of the clay lattice. This is consistent with the ionic binding of sulfonate groups to exchangeable cations, and the loss of host signals indicates that MO is encapsulated in the hydrophobic cavities of βCD. The selective loss of these functional group signals demonstrates that adsorption of the dye onto the βCD clay compound suppresses functionality, confirming a dual mechanism: βCD encapsulation in the hydrophobic cavity, and reorganization of the geophagic clay framework during dye removal.

FIGURE 17.

FTIR spectra of β‐cyclodextrin functionalized geophagic clay (βCD clay), RhB‐adsorbed βDC‐clay, and MO‐adsorbed βDC‐clay (a) showing full spectra and (b) enlarged view of the spectra (1500–900 cm⁻¹).

4. Conclusions

This study investigates the potential of βCD functionalized geophagic clay as a robust and sustainable adsorbent for removing cationic and anionic dyes under continuous‐flow and fixed‐bed conditions. Structural characterization proved the successful immobilization of βCD, whereby SEM/EDS showed agglomerated carbon‐enriched clay sheets; FTIR spectra revealed new O–C stretching bands at 1152 and 1025 cm⁻¹; and XRD patterns indicated reorganization of the lattice, with basal reflections showing more intense and new peaks at 17.9° and 25.1°. βCD functionalization imparted hydroxyl‐rich surfaces and host–guest inclusion cavities, which, in synergy, enhanced dye uptake, as confirmed by FTIR spectra. RhB adsorption on βCD functionalized geophagic clay involves electrostatic attraction between negatively charged clay layers and cationic dye molecules; hydrogen bonding mediated by the hydroxyl groups of the amino and carboxyl groups enhances this process. In contrast, MO removal is facilitated by the inclusion cavity of βCD and secondary hydrogen bonding, which overcomes electrostatic repulsion and provides high efficiency even at high concentrations. The optimized bed height of 3 cm achieved 93% RhB removal within 3 min 57 s, while MO removal was > 96% at 50 ppm, highlighting the dependence of the adsorption process on the structure. Reusability tests confirmed the adsorbent's composite retained > 80% of its original adsorption capacity after five adsorption–desorption cycles. Thus, βCyclodextrin functionalized geophagic clay has emerged as an affordable and scalable solution in the context of a circular economy for pre‐treatment of industrial wastewater, owing to its synergistic adsorption mechanisms involving electrostatic attraction, hydrogen bonding, and cavity inclusion.

Author Contributions

Nomcebo P. Khumalo: conceptualization, investigation, writing – original draft, methodology, validation, visualization, writing – review and editing, formal analysis, data curation. Mukuna P. Mubiayi: conceptualization, visualization, writing – review and editing, formal analysis, data curation, methodology. Bhekie B. Mamba: writing – review and editing, project administration, supervision, resources, conceptualization, funding acquisition.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Figure S1: Showing the clay sample received from Ghana and the crushed sample, which was used for the experiments.

Figure S2: Clay bed filtration setup.

Table S1: The operating parameters of the column.

Figure S3: Calibration curve of (a) Rhodamine B standards and (b) Methyl orange.

Figure S4: The SEM micrographs for (a) geophagic clay, (b) βCD functionalised geophagic clay.

Figure S5: Nitrogen adsorption–desorption isotherms of (a) pristine clay and (b) βCD functionalised clay.

Table S2: kinetic constant of the adsorption process.

Table S3: Summary of ANOVA statistics.

Data Availability Statement

The data supporting the findings of this study are available from the corresponding author upon reasonable request.