Iron Modified Opuntia ficus-indica Cladode Powder as a Novel Adsorbent for Dyes Molecules

1  Introduction

In recent years, adsorption has been established as one of the most widely used methods for the removal of dyes from water and wastewater. Its high efficiency, ease of use, and the abundance of various adsorbents make it a sustainable and cost-effective environmental solution [1]. Numerous studies have explored the use of various adsorbents, for dye removal, including activated carbon [2], activated carbon composites such as Peat-AC [3], Moringa oleifera leaves, Borassus flabellifer, Mangifera indica [4], pithecellobium dulce carbon [5], palm fruit bunch, date stones, and maize cobs [6], for dye removal. Opuntia ficus-indica cladodes, fruit pulp and peel mucilage and electrolytes have shown significant potential for the treatment of various types of wastewaters. These materials have exhibited remarkably high maximum sorption capacities and removal percentages for both dyes and metallic species. Biosorption capacities for dyes ranged from 125.4 to 1000 mg/g, whereas those for metallic species varied from 0.31 to 2251.56 mg/g. Although several factors, including local availability and treatment requirements, are crucial for comparing adsorbents [7], the cost aspect is often overlooked [8]. Biomass adsorbents rich in cellulose have demonstrated significant potential for removing micropollutants, including dyes, from aqueous solutions [9–12]. Several studies have demonstrated the potential of lignocellulosic-based materials as effective adsorbents for dyes. The adsorption efficiency can be further enhanced by chemical modification of the biomass waste. This includes oxidation [13], esterification [14], grafting of cationic sites [15], or hybridization with metal oxides [16,17]. In this regard, Opuntia ficus indica cladodes, primarily composed of water, carbohydrates (starch, cellulose, hemicellulose, pectin, and chlorophyll), proteins, lignin, and lipids (carotene), are widely available biomass growing naturally or farmed in arid regions such as north and south Africa, south America among others [18,19]. Over the last decades, research has been initiated to develop new environmentally friendly and cost-effective adsorbents from Opuntia ficus indica cladodes [20–22]. Findings indicate a moderate removal capacity, reaching up to 10 mg/g, which may be attributed to the low specific surface area. This limitation is due to the presence of natural minerals, primarily Ca(C2O4)·H2O (whewellite), covering the adsorbent surface [23,24]. To improve the adsorption capacity, new by-products are being developed by Djobbi et al. (2021) by modifying the surface of natural Opuntia ficus indica. Acid treatment significantly enhanced the adsorption capacity of Opuntia ficus indica for manganese cations in aqueous solutions, increasing the maximum capacity to 42.02 mg/g, substantially higher than the 20.8 mg/g capacity of untreated cactus [25]. Louati et al. (2018) investigated the potential of prickly pear cactus cladodes powder (PPCP) derived from Opuntia ficus indica as an eco-friendly and cost-effective biosorbent to remove Acid Orange 51 (AO51) and Reactive Red 75 (RR75) dyes, widely used in the textile industry [26]. A comparative study of Opuntia ficus indica sourced from Lomé and Marrakech was conducted by Degbe et al. Their experiments, using methylene blue solutions, revealed rapid initial elimination within the first 20 min for both adsorbents [27]. Further experiments were conducted to study the effects of pH, adsorbent mass, and adsorbate concentration on the adsorption process. The study found that at a pH of 12.5, the Marrakech cactus exhibited a maximum elimination of 72.38%, whereas the Lomé cactus reached 71.22% [7,27]. It’s important to note that composites of these materials with iron could be explored for their photocatalytic capabilities, which would extend their application beyond simple adsorption. Therefore, experiments in the dark are necessary to assess the adsorption behavior independently [28].

The primary objective of this research is to develop novel Fe(OH)x@Cellulose hybrid adsorbents utilizing the low-cost biomass, Opuntia ficus indica. Inspired by the successful application of Fe(OH)3@Cellulose fibers derived from cotton, which exhibited high adsorption capacity (689.65 mg/g) for CR dye [29], this study aims to create a similar system by modifying the surface of Opuntia ficus indica. This approach seeks to combine simplicity, high efficiency, low cost, and environmental friendliness, with the goal of developing a new product with significant potential for industrial applications. The adsorption performance of the modified biomass will be evaluated for NR and CR dye removal, and the structural properties of the synthesized materials will be characterized using XRD, FTIR, XRF and SEM.

2  Materials and Methods

Opuntia ficus-indica cladode powder (OFIC) was prepared and activated for use as an adsorbent. Cladodes were harvested in June from a plantation located in the La Rabta region of Tunis northern Tunisia. The chemicals used in the activation process were sodium hydroxide and iron(III) chloride hexahydrate (Scharlab S. L., Sentmenat, Spain).

2.1 Methods Preparation of Opuntia ficus-indica Cladodes Powder (OFIC)

Opuntia ficus-indica cladode powder was prepared following the method described by Djobbi et al. [25]. After removing the spines, the cladodes were carefully washed with ordinary tap water and rinsed with distilled water before being cut into small, 4 cm cubes. The cubes were dried in an oven at 333 K for 48 h and subsequently ground into a fine powder (40–80 μm) using a laboratory mill (Fig. 1). To remove hemicellulose, lignin, and excess minerals like whewellite and CaCO3 from the cladode surface, the powder was treated with a 15% aqueous sodium hydroxide solution. The suspension was subjected to a 4-h stirring period to facilitate thorough mixing, subsequently filtered, and then rinsed with distilled water until the washings exhibited a neutral pH. The chemically treated powder was dried overnight in an electric oven at 333 K, then ground and stored in clean, dry glass vials for future use (Fig. 1). The untreated material was designated as OFIC, while the sodium hydroxide-treated material was labeled OFIC-NaOH.

Figure 1: Process flowchart illustrating the sequential steps involved in sample material creation

To prepare Fe(OH)x@Cellulose, 0.25 g samples of OFIC were stirred in 500 mL of aqueous FeCl3 solutions (1 g·L−1) at a 1:2 mass ratio for 2 h at room temperature. The resulting solutions were filtered and washed with water until the pH reached neutrality. Analysis of the wash water revealed that 87.7% of the iron (0.151 g) was removed, indicating a retention of 12.3% (0.021 g) on the cellulose. The chemically treated powders, Fe(OH)x@Cellulose and OFIC-NaOH, were dried in an oven at 333 K for 4 h, and their final weights were recorded (Fig. 1).

2.2 Characterization of Raw and Treated OFIC

The crystalline structure of the raw and treated materials was investigated using X-ray diffraction (XRD) on a Bruker D8 diffractometer with Cu Kα1/Kα2 radiation (λ1 = 0.1540 nm, λ2 = 0.1544 nm). Data were collected from 5° to 60° using a Malvern Panalytical X’pert3 Powder diffractometer (Almelo, The Netherlands), equipped with a flat rotating sample holder and a 1D-PIXcell detector. Fourier transform infrared spectroscopy (FTIR) analysis was performed using a Bruker Tensor 27 spectrophotometer (Billerica, MA, USA). KBr pellets were prepared for FTIR measurements, which were carried out within the 400–4000 cm−1 spectral range. This technique allowed for the investigation of vibrational properties and chemical bond information present in the materials. Subsequent analysis of the iron content in the metallic material was carried out by X-ray fluorescence spectrometry “Thermo Scientific Niton FXL model 959 (Waltham, MA, USA)”. The microstructure and morphology of samples were analyzed using Scanning Electron Microscopy (SEM) at 3–5 kV acceleration voltage. Thermogravimetric analysis (TGA) was conducted using a TGA 400 thermogravimetric analyser from Perkin Elmer (PerkinElmer, Shelton, SC, USA) under an airflow in the temperature range of 50°C–800°C at a heating rate of 10°C/min. The ash content was determined from the residue at 800°C, and the moisture content was calculated from the weight loss at 150°C.

2.3 Thermodynamics of Adsorption Processes

The following equation was used to determine the equilibrium adsorption capacity Qe (mg/g) of dyes onto OFIC and Fe(OH)x@Cellulose [27,30].

Qe=C0−Cem ∗ V(1)

where C0 (mg/L) is the initial dye concentration, Ce (mg/L) is the equilibrium dye concentration, V (L) is the solution volume and m (g) is the adsorbent mass.

To optimize dye removal processes, a fundamental understanding of dye-adsorbent interactions is crucial. Adsorption isotherms provide valuable insights into this relationship. The models applied to describe the dye adsorption onto the biosorbent (Fe(OH)x@Cellulose) involve the Langmuir, Freundlich, and Temkin equations [3,31]. Langmuir’s model describes adsorption as a process in which the molecules form a single layer on the surface of the adsorbent. It requires that the adsorbent has a fixed number of adsorption sites, each possessing identical binding energies for the molecules, while neglecting any interaction among the adsorbed species. The Langmuir isotherm can be illustrated by a linear equation:

CeQe=1Qmax×KL+CeQmax(2)

where Qmax (mg/g) is the monolayer saturation adsorption capacity and KL (L/mg) is the Langmuir constant linked to adsorption ability. The essential characteristics of the Langmuir isotherm can be expressed by a unitless constant called the separation factor (RL) also known as the equilibrium parameter, defined by Islam and Mostafa [32], as following:

RL=11+KL×Ci(3)

using the Langmuir constant (K) and the initial concentration (C0), we can determine the adsorption behavior. If the calculated RL value is greater than 1, the adsorption is unfavorable and linear. If it’s between 0 and 1, it’s favorable. And if it’s zero, the adsorption is irreversible.

In contrast to Langmuir’s assumption of a constant adsorption enthalpy, Freundlich proposes a logarithmic relationship between enthalpy and surface coverage expressed as follows:

Ln Qe=LnKF+1nLnCe(4)

n and KF (mg/g) are the Freundlich parameters.

Unlike Langmuir and Freundlich, the Temkin model accounts for changes in adsorption energy due to surface heterogeneity or intermolecular interactions. This relationship is expressed linearly as follows:

Qe=B LnKT+B LnCe(5)

The Temkin model includes the parameters, B (J/mol), representing the heat of adsorption, and K (L/g), representing the equilibrium constant at the highest binding energy.

2.4 Adsorption Kinetics Models

To elucidate the adsorption mechanisms of NR and CR by Fe(OH)x@Cellulose, the pseudo-first-order, pseudo-second-order, and Elovich kinetic models were employed [33].

2.4.1 Pseudo-First Order

The pseudo-first-order Lagergren model is a mathematical equation that describes the rate of adsorption in liquid-solid systems, based on the amount of adsorbate taken up:

Ln(Qe−Qt)=LnQe−k1t2.303(6)

The pseudo-first order kinetic model uses Qe (mg/g) for equilibrium adsorption, Qt (mg/g) for adsorption at time t, and k1 (min–1) for the rate constant.

2.4.2 Pseudo-Second Order

The pseudo-second-order adsorption kinetics can be modelled using Blanchard’s equation, which is given by:

1Qt=1kb×Qe2+tQe(7)

Blanchard’s model includes Qe (mg/g) as the equilibrium adsorption, Qt (mg/g) as the adsorption at time t, and kb (mg/g·min) as the pseudo-second-order rate constant.

2.4.3 Elovich Equation

The Elovich model is widely used for describing activated chemical adsorption, particularly in systems with heterogeneous adsorption sites. The Elovich model is represented by the equation below:

Qt=1bLn(ab)+1bLnt(8)

The Elovich equation includes the parameters Qt (mg/g), a (mg/g), and b (g/mg), representing adsorption at time t, initial adsorption rate, and desorption constant, respectively.

The standard thermodynamic parameters, enthalpy (ΔH°), entropy (ΔS°), and Gibbs free energy (ΔG°), for the adsorption of dyes onto Fe(OH)x@Cellulose can be calculated using the given equations:

ΔG∘=−RTLnKc(9)

Kc=QeCe(10)

The relationship between ΔG°, ΔH°, and ΔS° is given by:

ΔG∘=ΔH∘−TΔS∘(11)

R is the gas constant with a value of 8.314 J/mol·K and T is the absolute temperature (K). Substituting Eq. (10) into Eq. (12) gives:

LnKc=−ΔH∘RT+ΔS∘R(12)

understanding the thermodynamic properties of an adsorption process is crucial for determining its feasibility and elucidating the binding mechanisms between dyes and adsorbents. Physisorption, a process driven by weak Van der Waals interactions, is characterized by a low heat of adsorption (20–40 kJ/mol), rapid kinetics, and reversibility [34].

3  Results and Discussion

3.1 XRD Characterization

The X-ray diffractograms of the raw material (OFIC) and the two modified materials (OFIC-NaOH and Fe(OH)x@Cellulose) are depicted in Fig. 2.

Figure 2: XRD patterns of: (a) OFIC, (b) OFIC-NaOH and (c) Fe(OH)x@Cellulose

The X-ray powder diffractogram (Fig. 2) of the fibers obtained through alkali treatment (Fig. 2b) shows three resolved peaks at approximately 2θ = 14.8°, 21.2°, and 29.3°. These peaks correspond to the (11¯0), (110), and (200) planes, characteristic of a cellulose I-type crystalline structure [25,35]. The prominent crystalline peaks indicative of calcium oxalate monohydrate (whewellite) at 14°–15° and 24°–25° (2θ), along with those at 29°–30°, 39°–40°, and 45°–50° (2θ), characteristic of calcium carbonate (CaCO3), observed in the diffractogram of the raw sample, disappear in diffractograms Fig. 2b,c. This indicates the removal of carbonates and whewellites following the alkaline iron(III) chloride treatments [24,25]. The XRD analysis (Fig. 2) revealed that the cellulose obtained from iron-treated biomass (Fe(OH)x@Cellulose) displayed two diffraction peaks at 2θ = 14.8° and 21.2°, characteristic of cellulose I-type. A comparison of the diffractograms, using HighScore software and the ICDD PDF4+ database, shows that the cellulose is of type Iβ. However, following iron treatment, the cellulose fibers exhibit a higher amorphous content. Additionally, some whewellite peaks that remain at very low intensities in the OFIC-NaOH spectrum are nearly eliminated in the Fe(OH)x@Cellulose spectrum.

3.2 FT-IR Measurements

As depicted in Fig. 3, the treated biomass displayed comparable spectral features to the native biomass, suggesting that no chemical alterations occurred during the treatment process. FTIR spectra of OFIC, OFIC-NaOH, and Fe(OH)x@Cellulose all displayed broad bands in the 3000–3500 cm−1 region. These bands are characteristic of O-H stretching vibrations and are indicate hydrogen bonding interactions within the molecular structures of all three samples [25,28,36,37]. The “fingerprint” region (1800–400 cm−1) (Fig. 3) showed distinct bands at 1438, 1158, 1107, and 890 cm−1, characteristic of native cellulose Iβ [36,38]. An absorption band at 1650 cm−1 suggests the presence of deprotonated carboxylate groups, as evidenced by the asymmetric stretching of the C=O double bond [39]. The broad bands observed around 1030 and 1060 cm−1 can be attributed to polysaccharides’ C-O-C or -OH vibrations [40]. The attenuation of the intensity of bands observed at 1630, 1320, and 778 cm−1, which correspond to asymmetric and symmetric carboxylate stretching and OCO deformation modes, respectively, suggests the removal of calcium oxalate monohydrate (whewellite) by FeCl3 and alkaline treatments of OFIC. The observed bands near 1150 and 1160 cm−1 are assigned to antisymmetric stretching vibrations of the C-O-C and vibrations of C-O glycosidic bond, respectively [41,42].

Figure 3: FT-IR spectra of: (a) Fe(OH)x@Cellulose, (b) OFIC-NaOH and (c) OFIC

3.3 X-Ray Fluorescence Analysis

X-ray fluorescence spectroscopy of the Fe(OH)x@Cellulose treated biomass material revealed a 18.2% iron content, indicating significant iron incorporation. This directly impacts the material’s adsorption capabilities. The substantial integration rate increases the material’s active surface area, providing more binding sites for dye molecules. Furthermore, the presence of iron modifies the cellulose’s physicochemical properties, enhancing interaction with adsorbable pollutants [43,44].

3.4 Morphology

The morphology of the neat and modified fibers was studied using scanning electron microscopy (SEM). Fig. 4 illustrates SEM micrographs of OFIC, alkaline-treated OFIC, and Fe(OH)x@Cellulose. The raw OFIC surface presents a smooth, compact appearance with minimal visible channels [45]. The OFIC-NaOH and Fe(OH)x@Cellulose samples (Fig. 4c,e) displayed smoother morphological structures with a greater degree of softness compared to OFIC. The fibrous components observed in both the raw material and the treated biomasses were of similar dimensions, ranging from 2 to 11 μm. The modified biomass adsorbents exhibited distinct surface features, characterized by rough surfaces and numerous extended pores, indicating the creation of larger exposure zones and channels for pollutant uptake [46,47].

Figure 4: SEM images of (a, b) OFIC, (c, d) OFIC-NaOH and (e, f) Fe(OH)x@Cellulose

The raw biomass displayed whewellite crystals, ranging in size from 0.3 to 1 μm, distributed across its entire surface (Fig. 4b). Calcium oxalate crystals were not discernible on the surface of either OFIC-NaOH (Fig. 4d) or Fe(OH)x@Cellulose (Fig. 4f) [24]. The microcrystals observed on the surface of Fe(OH)x@Cellulose (Fig. 4f) are likely attributed to the formation of magnetite [48,49].

4  Thermogravimetric Analysis (TGA)

TGA of the various samples (Fig. 5) was conducted to assess their thermal stability and changes in char content following different chemical treatments. The thermogravimetric analysis revealed multiple weight loss, depending on the treatment performed Table 1. For the neat OFIC, an initial weight loss at 100°C of about 12% is observed, corresponding to the evaporation of adsorbed water. This was followed by a second loss of about 5% at 150°C, attributed to the removal of water from oxalate monohydrate. Subsequently, a three-step degradation was identified, associated with the thermal degradation of lignocellulosic material in the biomass. The weight loss at 700°C presumably results from the decarbonation of oxalate contained in the OFIC, giving rise to a char content of about 20%, corresponding to mineral material contained in the biomass [50].

Figure 5: TG curves of: (a) OFIC, (b) OFIC-NaOH and (c) Fe(OH)x@Cellulose

OFIC-NaOH exhibited, after the first loss of adsorbed water at 100°C, a steep loss of about 50% occurred at 220°C, followed a second event at 420°C and a third event assigned to the thermal degradation of the lignocellulosic material. The weight loss at 700°C was much lower than that of neat OFIC, which is presumably due to the removal of a high fraction of oxalate and mineral material. This explains the decrease in char content to around 10% in OFIC-NaOH. In The iron-treated sample (Fe(OH)x@Cellulose), after the initial weight loss due to water evaporation, a sharp weight loss of more than 85% was observed at about 230°C. This indicates that the thermal degradation of all the lignocellulosic material occurring in one step. The FeCl3 treatment significantly reduced the mineral content of the biomass, leading to the lowest ash content (around 5%) observed among the samples [48,49,51–53].

5  Adsorption Properties of OFIC and Fe(OH)@Cellulose Modified Biomass

5.1 Effect of Contact Time

Reaching equilibrium in adsorption systems is essential, and contact time is a crucial parameter that plays a key role in defining the rate at which this equilibrium is established.

To determine the optimal contact time, the evolution of dye removal rates was studied. The curves presented in Fig. 6 demonstrate that both dyes exhibited enhanced adsorption by the treated cladode powder compared to the untreated material. Investigating the adsorption rate variation over time revealed that NR adsorption onto Fe(OH)x@Cellulose increased gradually (Fig. 6a), reaching a maximum of over 70% after two hours. The adsorption kinetic is quite rapid, reaching more than 65% within the first 20 min. Subsequently, the rate slowed significantly, suggesting saturation of the adsorbent’s surface sites. Continued, although slower, adsorption likely occurred through diffusion of the dye into the adsorbent’s pores. The treated material Fe(OH)x@Cellulose demonstrates exceptional CR dye removal efficiency (Fig. 6b). Rapid initial adsorption is observed with a 93.7% removal rate within 5 min, highlighting its swift initial action. Moreover, sustained high performance is evident, as it maintains a 94.15% removal rate after one hour, signifying consistent and effective dye removal. The high polarity of CR probably explains the different optimal contact times due to increased interactions with polar materials [47].

Figure 6: Effect of contact time on the sorption efficiency of dyes ((a): NR; (b): CR) by the OFIC and the Fe(OH)x@Cellulose at room temperature (20 ± 5°C) madsorbent = 150 mg; C0 = 100 mg·L−1 and pH ≈ 6.5

5.2 Adsorption Isotherms

Experimental adsorption data for NR and CR dyes onto Fe(OH)x@Cellulose were analyzed using the Langmuir, Freundlich, and Temkin isotherm models. The results indicated that the adsorption of both NR and CR is best described by the Langmuir model, suggesting homogeneous adsorption sites and monolayer coverage. This conclusion was supported by high R2 values of 0.980 for NR and 0.973 for CR. These findings provide valuable insights into the adsorption mechanisms and can inform the design of more efficient adsorption processes. Thermodynamic parameters obtained from the isotherm models are tabulated in Table 2.

The high regression coefficients confirm that the Langmuir model accurately represents the experimental isotherms (Figs. 7 and 8), suggesting monolayer adsorption within the interlayer pores. The Langmuir constant and monolayer capacity values effectively characterize the adsorption behavior of the dyes.

Figure 7: (a) Langmuir isotherme, (b) Freundlich isotherm and (c) Temkin isotherm, for NR adsorption on Fe(OH)x@Cellulose

Figure 8: (a) Langmuir isotherme, (b) Freundlich isotherm and (c) Temkin isotherm, for CR adsorption on Fe(OH)x@Cellulose

The adsorption of NR and CR onto Fe(OH)x@Cellulose highlights distinct mechanisms. NR, with its amine (-NH2) and azo (-N=N-) groups, exhibits an adsorption mechanism based on strong hydrogen bonding with the hydroxyl (-OH) groups of cellulose. However, the formation of rigid complexes limits its adsorption capacity (Qm= 40.55 mg/g). CR, with its sulfonate groups (-SO3−) and aromatic rings, exhibits an adsorption mechanism dominated by hydrophobic interactions and water release, explaining its high adsorption capacity (Qm= 89.29 mg/g).

The separation factor RL was calculated for various dye concentrations and is presented in Table 3. As RL values for both CR and NR adsorption lie between 0 and 1, the adsorption process is favorable, indicating that Fe(OH)x@Cellulose is an effective bio-sorbent for removing these dyes from aqueous solution [25,54]. A comparison of the dye adsorption capacities of OFIC and OFIC-Cell with those found in similar research is summarized in Table 4.

5.3 Adsorption Kinetics

The kinetic study of NR and CR dye adsorption onto biomass was investigated. Pseudo-first-order, pseudo-second-order, and Elovich models were applied (Figs. 9 and 10). The obtained results indicated that the adsorption kinetics followed a pseudo-second-order model. The correlation coefficients for this model were high, reaching 0.999 and 0.989 for NR and CR adsorption onto Fe(OH)x@Cellulose, respectively.

Figure 9: (a) Pseudo-first-order, (b) Pseudo-second-order and (c) Elovich models, for NR on Fe(OH)x@Cellulose adsorbent

Figure 10: (a) Pseudo-first-order, (b) Pseudo-second-order and (c) Elovich models for CR on Fe(OH)x@Cellulose adsorbent

The calculated parameters for the different models are listed in Table 5. The Elovich model plot revealed a relatively high correlation coefficient for NR adsorption but a lower one for CR adsorption. This finding aligns with the ΔH values, suggesting physisorption for CR dye [6,59].

5.4 Effect of Temperature

Temperature is a critical factor affecting the adsorption process, as it directly influences the rate of molecule adsorption. The adsorption of NR and CR dyes by treated Fe(OH)x@Cellulose biomass was studied over wide temperature range from 25°C to 80°C (298 to 353 K). While NR removal rates exceeded 74% at 298 K, CR dye removal rates increased from 41% to 54% as the temperature increased from 305 to 348 K. These contrasting trends suggest distinct adsorption mechanisms for the two dyes (Fig. 11). Notably, NR removal efficiency decreased with rising temperatures, indicating an exothermic adsorption process. Conversely, the increase in CR removal rate with temperature points to an endothermic adsorption mechanism [32,60].

Figure 11: Effect of temperature on removal rate of dyes ((a): NR; (b): CR) by the Fe(OH)x@Cellulose

5.5 Thermodynamics Studies

The impact of temperature on dye adsorption by Fe(OH)x@Cellulose was investigated through thermodynamic analysis. The thermodynamic parameters ΔH°, ΔS° and ΔG° were calculated and grouped in Table 6. For CR, the ΔG° values were negative across the entire temperature range, indicating a spontaneous adsorption process. In contrast, for NR, the ΔG° was negative up 303 K, and became positive above this temperature, suggesting that adsorption becomes less favorable as temperature increases. A linear relationship between ln KF and 1/T (Fig. 12) allowed for the determination of ΔH° and ΔS°. The calculated adsorption enthalpies for NR and CR were −67.153 and 3994 kJ·mol−1, respectively. These results suggest that the adsorption of NR is exothermic, whereas that of CR is endothermic. The adsorption behavior of NR and CR on Fe(OH)x@Cellulose higlights the complex interplay of intermolecular forces and thermodynamic parameters. NR’s strong affinity to Fe(OH)x@Cellulose, as evidenced by its negative adsorption enthalpy, is attributed to favorable hydrogen bonding between their respective functional groups, specifically the hydroxyl groups (-OH) of cellulose and the amine (-NH2) and azo (-N=N-) groups of NR. However, the adsorption process is somewhat hindered by a decrease in system disorder, as indicated by the negative entropy value (−220.32 J·mol−1·K−1). Conversely, CR’s adsorption, although endothermic due to factors like electrostatic repulsion between the sulfonate groups (-SO3−) of CR and the slightly negative cellulose surface, and desolvation, is ultimately promoted by an increase in system disorder (positive entropy of 26.18 J·mol−1·K−1), likely caused by the release of water molecules and hydrophobic interactions involving the aromatic rings of CR and cellulose [61,62].

Figure 12: The Van’t Hoff plots for the adsorption of: (a) NR and (b) CR onto Fe(OH)x@Cellulose

6  Conclusion

Raw and treated biomass of the Opuntia ficus indica cladodes were characterized using FTIR, SEM, and XRD spectroscopy. The Fe(OH)x@Cellulose produced after treating OFIC proved to be an effective biosorbent for the dyes studied in this work, making it a promising candidate for wastewater treatment. Thermal decomposition, as indicated by TGA analysis, confirmed the material’s thermal stability.

Our findings indicate that the adsorption of both dyes onto the Fe(OH)x@Cellulose surface is best described by the Langmuir adsorption isotherm, suggesting monolayer coverage. This implies that the adsorption process is primarily driven by the formation of a single layer of dye molecules on the available active sites of the Fe(OH)x@Cellulose. Furthermore, the adsorption process is well-described by the pseudo-second-order kinetic model. Thermodynamic analysis reveals that the adsorption of both dyes is spontaneous under standard conditions, as demonstrated by the negative Gibbs free energy values. However, the thermodynamic parameters offer valuable insights into the nature of the adsorption mechanisms for each dye. For NR adsorption, the negative enthalpy and entropy values suggest an exothermic process driven by a decrease in system disorder at the adsorbent surface. This implies that the NR adsorption is favored at lower temperatures and involves a reduction in the degree of freedom within the system. Conversely, for CR adsorption, the positive entropy value suggests an increase in randomness during the adsorption process, which is characteristic of an endothermic process. This indicates that the adsorption of CR is favored at higher temperatures and involves an increase in the degree of freedom of the system.

Detailed characterization and mechanistic insights provided by this study, coupled with the demonstrated effectiveness of Fe(OH)x@Cellulose as a biosorbent for NR and CR dyes, establish a strong foundation for future research and development. This work highlights the potential for practical application in industrial wastewater treatment, prompting further exploration into optimization and implementation. Specifically, future investigations should focus on scaling up the synthesis process to ensure consistent material properties, fine-tuning synthesis parameters to enhance adsorption capacity and stability, evaluating the reusability and regeneration potential of the adsorbent, assessing performance in complex, real-world industrial wastewater matrices, developing strategies for integrating the biosorbent into existing or novel treatment systems; and examining the long-term stability and environmental impacts of spent adsorbents. Crucially, further studies should also investigate the performance of the material under varying pH conditions. This is essential for understanding its behavior in diverse industrial effluents and for optimizing its application in real-world scenarios. Exploring the pH-dependent adsorption behavior will provide valuable insights into the material’s selectivity and efficiency for different dye contaminants, leading to the development of customized depollution techniques. By pursuing these research avenues, we can translate the promising findings of this study into tangible advancements in sustainable wastewater treatment.

Acknowledgement: The authors would like to express their sincere gratitude to Dr. Hanen Salhi for his valuable contributions and dedicated work throughout this project. They also extend their sincere thanks to their colleague, Prof. Nejib Mekni, a faculty member of ISTMT at El Manar University, for his expert review of the English language in the manuscript and their sincere gratitude to Rammeeza BiBi Roheeman for her invaluable contribution in reviewing the article’s English. The Ministry of Higher Education and Scientic Research of Tunisia is gratefully acknowledged.

Funding Statement: The authors received no specific funding for this study.

Author Contributions: The synthesis, structural characterisation by powder XRD, FTIR, MEB and adsorption measurements by UV spectroscopy: Mehrzia Krimi, Alma Jandoubi, Nabil Nasri and Rached Ben Hassen. Draft manuscript preparation: Mehrzia Krimi and Rached Ben Hassen. Thermogravimetric study and drafting of this section: Sami Bouf. Writing–review and editing: Mehrzia Krimi and Rached Ben Hassen. Supervision: Rached Ben Hassen and Sami Boufi. All authors reviewed the results and approved the final version of the manuscript.

Availability of Data and Materials: The data that support the findings of this study are available from the corresponding author, Mehrzia Krimi, upon reasonable request.

Ethics Approval: Not applicable.

Conflicts of Interest: The authors declare no conflicts of interest to report regarding the present study.

Abbreviations

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