Calcium-Rich Biochar Derived from Cactus Feedstock and Its Efficient Adsorption Properties for Industrial Dye

Abstract

The leather industries are large producers of industrial wastewater. The present work investigated the performance of calcium-rich biochar derived from cactus cladodes (CCB) as an adsorbent in the treatment of this wastewater. Biochar samples (CCB500, CCB600, and CCB700) were produced by slow pyrolysis at 500, 600, and 700 °C and tested as adsorbents of Repanil Blue (RB) dye. The tests were carried out under various experimental conditions. The RB dye removal increased with the contact time, adsorbent dose, and concentration. With dye concentrations between 10 and 125 mg/L, removal efficiencies of 80, 80 and 100% were reached using CCB500, CCB600, and CCB700, respectively. The Langmuir model and the pseudo-second-order model described chemisorption with a high correlation factor (R²). The highest adsorption capacity of 56 mg/g was obtained with CCB700 within 5 min. The results strongly suggest that calcium-rich biochars are promising adsorbents for the removal of RB dye from aqueous solutions.

1. Introduction

The leather manufacturing sector is an industrial hotspot that is unfortunately known for its heavy environmental impact. It is responsible for approximately 33 million tonnes per year of solid wastes, the management of which is very challenging. The leather industry also generates massive quantities of wastewater, which contains a complex mixture of contaminants at concentration values over the established regulatory limits. This is also an environmental challenge [1]. Synthetic organic dyes commonly used in leather industry are, indeed, resistant to biodegradation and pose significant health concerns [2,3].

It is estimated that 700,000 tonnes of colour are manufactured each year from around 100,000 commercial dyes and that the industries that produce and use dyes generate around 3000 to 4000 kilotons per year of wastewater worldwide [4]. Given the complexity of these pollutants, it is essential to put in place a reliable and efficient wastewater treatment system. This issue has been investigated by looking into physicochemical methods for the removal of organic dyes from wastewater [1]. Among these, membrane separation, osmosis, biological treatment, ion exchange, coagulation/flocculation, photocatalysis, adsorption, and chemical oxidation have proven to be effective [3,5].

Adsorption has received particular attention as a purification technique owing to its relative simplicity and high effectiveness [5,6]. Nevertheless, some of the disadvantages of commonly used adsorbents include the high cost and low efficiency of the adsorption process. The preparation of adsorbents for wastewater treatment and dye elimination from residual biomass appears to be an eco-friendly and economically attractive solution. Indeed, biochars derived from different biomass feedstocks have recently been proposed as adsorbents thanks to their physical and chemical properties, such as their high surface area, surface chemistry, and morphology [7]. Biochars with a large surface area and numerous active sites often have a high capacity for the adsorption and removal of contaminants. In this respect, biochars have numerous functional groups including hydroxyls, carboxyls, carbonyls, ethers, esters, and amides, as well as trace minerals and metals, which can confer the ability to exchange electrons and a high degree of reactivity. Indeed, Inyang et al. [8] indicated that the carboxyl, carbonyl, lactone, and phenolic groups on the biochar surface affect its ability to eliminate dyes. Song et al. [9] have used cow hair to prepare porous biochars with different methods and operating conditions (carbonization atmospheres, the MgO template method, and KOH activation) for the removal of blue dye from wastewater in the leather industry. They reported that the adsorption capacity of the cow-hair-based char materials prepared with KOH activation was effective for blue dye adsorption, reaching a capacity of 1477 mg/g. Jadhav et al. [10] tested biochar prepared from Musa acuminata stem for the adsorption of Congo red and Brilliant blue leather dyes. They reported a maximum adsorption capacity of 208.33 and 46.33 mg/g for Brilliant blue and Congo red, respectively.

Haddad et al. [11] performed adsorption experiments using biochar from the pyrolysis of tannery fleshing waste for the elimination of Sella Fast Red dye. The maximum dye uptake ranged from 26 mg/g to 39.86 mg/g for biochars prepared at temperatures between 400 and 600 °C. Chagtmi et al. [1] also studied the adsorption of Sella Fast Grey CLL industrial dye using biochar derived from cow bone waste pyrolysis at temperatures of 500–700 °C. The best adsorption capacity of 61.7 mg/g was reached with the biochar produced at 700 °C.

In this study, we aimed to develop new adsorbents for the removal of industrial dyes used in the leather tanning industry in Tunisia. From the perspective of the circular economy, these new adsorbents were produced from the pyrolysis of cactus cladodes (CCs), a biomass, which grows in areas with arid and semi-arid climates [12] and is widespread in Tunisia [13]. CC biochars (CCBs) were prepared by slow pyrolysis in a fixed-bed reactor. This choice was made on account of the relatively low complexity of the pyrolysis technology compared to fast or flash pyrolysis and in order to increase the biochar yield. The temperature of pyrolysis varied, in the range of 500–700 °C. Dye removal tests were carried out, controlling for various parameters (contact time, initial concentration, pH, and dose).

Notably, CC is a valued source of fibres, proteins, polysaccharides, vitamins, minerals, polyphenols, etc., and has recently been proposed for environmental applications [14,15,16,17,18,19,20], such as the production of activated carbon or coagulants/flocculants in wastewater treatment. Notably, CC has some peculiar characteristics, which make it quite interesting for the scope of the present work and which deserve consideration and further analysis.

As shown in previous work [14] by thermogravimetric analysis, CC shows a peculiar pyrolysis behaviour: the decomposition of cellulose, hemicellulose, and lignin occurs in well-resolved sequential stages, differently from what happens for most lignocellulosic materials, where the intervals of thermal decomposition for different biopolymers considerably overlap. This suggests that in CC, the interconnection between the biopolymers at the plant structural level is limited, which may lead to biochars with interesting microstructures and porosity. The second feature worthy of investigation is the large content of calcium, which could contribute to the reactivity of CC biochar.

The relevance of these features and their relationship with the adsorption capacity of biochars has not yet been fully investigated and will be addressed in the present work.

2. Materials and Methods

2.1. Biomass Preparation and Pyrolysis Experiments

Cactus cladodes (CCs) were collected from the Siliana government in north-western Tunisia. They were firstly cut into small specimens and dried in the open air. The CC biomass was ground and sieved to obtain a particle size of 1–3 mm.

The pyrolysis experiments were carried out using a fixed-bed reactor (Figure S1). Around 300 g of CC biomass was placed in the reactor and pyrolyzed for 2 h and 10 °C/min at 500, 600, and 700 °C. The produced biochars were labelled as CCB500, CCB600, and CCB700.

Pyrolytic biochar yield was determined using the following Equation (1):

(1)$\mathrm{B}\mathrm{i}\mathrm{o}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} (\mathrm{w}\mathrm{t}\mathrm{\%})={\displaystyle \frac{Biochar collected weight}{biomass intial weight }}$

2.2. Biomass and Biochar Characterization

Proximate and ultimate analyses of CC biomass and of the biochars (CCB500, CCB600, and CCB700,) were performed by a thermogravimetric analyzer (TGA Q5000IR, TA Instruments, New Castle, DE, USA) and a LECO CHNSO-932 elemental analyzer (LECO, St. Joseph, MI, USA), respectively.

The mineral composition was analyzed under vacuum conditions using a sequential wavelength-dispersive X-ray fluorescence (WDXRF) spectrometer (Axios 2005, PANalytical, Worcestershire, UK).

FTIR analysis was recorded from 4000 to 400 cm⁻¹ in absorbance mode with an accumulation of 32 scans using a spectrophotometer of the Bruker Vertex 80V series (Bruker, Billerica, MA, USA).

X-ray diffraction analysis was carried out using a PANalytical X’Pert Pro X-ray diffractometer (Philips, Amsterdam, The Netherlands) equipped with Cu Kα radiation (λ = 1.540562 Å) at room temperature. The data were collected from 10 to 80 °C on a 2θ scale [21].

The surface properties were analyzed using CO₂ adsorption measurements at 0 °C and atmospheric pressure in Micromeritics ASAP 2020 (Micromeritics, Norcross, GA, USA). The specific surface area and the micropore volume were calculated according to the Dubinin–Radushkevich method. The morphology of the sample surface was analyzed by scanning electron microscopy (SEM) using a JEOL JSM-6400 (JEOL, Tokyo, Japan).

The values of the point of zero charge (pH_pzc) of the produced biochars were assessed with the addition of biochar (0.2 g) to a series of solutions at different pH values (2–12) using 0.1 M HCl and 0.1 M NaOH.

2.3. Batch Adsorption Experiments

To estimate the Repanil Blue (RB) dye adsorption capacity of the biochars (CCB500, CCB600, and CCB700), batch experiments were carried out. To obtain a concentration of 1000 mg/L, a stock solution of RB dye was prepared by dissolving 1.0 g of the dye in one litre (1 L) of distilled water. Calibration and absorption experiments were carried out using a standard solution diluted to the required concentrations. Tests were first performed to determine the optimal experimental parameters, i.e., the contact time (1–180 min), initial dye concentration (10–125 mg/L), solution pH (2–12) controlled with 0.1 M HCl and 0.1 M NaOH and dose (0.01–0.8 g).

Adsorption tests were carried out in 100 mL glass flasks containing 50 mL of RB dye solution with an initial concentration of 60 mg/L. The flasks were stirred at 400 rpm using a magnetic stirrer and then filtered. Experiments were conducted in triplicate at room temperature. A Perkin Elmer Lambda 25 UV VIS Spectrometer was used at λ = 590 nm.

The amount of dye adsorbed at a given time, $\mathrm{t}$, ($\mathrm{q}\mathrm{t}$ = mg/g) by the produced biochars was calculated using Equation (2):

(2)$\mathrm{q}\mathrm{t}={\displaystyle \frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}}}{\mathrm{M}}}\times \mathrm{V}$

where ${\mathrm{C}}_0$ and ${\mathrm{C}}_{\mathrm{t}}$ (mg/L) are the initial and $\mathrm{t}$ time aqueous RB concentrations, respectively; $\mathrm{V}$ is the volume of the aqueous solution; and $\mathrm{M}$ is the mass of the used biochar (g).

2.4. Adsorption Kinetic Study and Isotherms

Langmuir and Freundlich isotherms are the most commonly used models for adsorption isotherms. On the other hand, pseudo-first- and second-order models are also applied to rate constant calculations in sorption processes.

Adsorption equilibrium data for the different biochars were assessed, with the initial concentration in the range 10–125 mg/L and contact times of 15, 15, and 5 min to maintain equilibrium in the dye molecules and the adsorbents.

3. Results and Discussion

3.1. Effects of Pyrolysis Temperatures on Biochar Yield and Characteristics

Figure 1 shows the yield in volatiles and biochar at different pyrolysis temperatures. The results indicate that the yield in biochar decreased from 37.10 wt% to 33.89 wt% when the pyrolysis temperature was increased from 500 to 700 °C. In fact, the biochar still contained residual volatile matter, as shown in Table 1, because pyrolysis was not complete in any of the conditions tested. The thermal treatment was indeed sufficient to accomplish the loss of moisture and hydroxyl groups [22,23], but only the partial degradation of the lignocellulosic components, most likely of the holocellulosic ones [24].

The physicochemical characteristics of the produced biochars (CCB500, CCB600, and CCB700) are listed in Table 1. Compared to the initial feedstock (Table S1), the data on the proximate analysis reveal higher ash and FC contents with increasing pyrolysis temperature, ranging from 37.97 to 45.44 wt% and from 24.74 to 26.58 wt%, respectively. The VM content varies in inverse order and decreases from 35.29 to 26.07 wt%.

The increase in ash content from 500 to 700 °C is a consequence of the increased degree of volatilization of the lignocellulosic components at higher temperatures [25]. During pyrolysis, VM is lost as bio-oil and gas. Even though the FC content increases along with increasing temperature [26], the elemental analysis results indicate a decline in C content with the rising pyrolysis temperature. Also, the O and H content decrease along with the pyrolysis temperature: CCB700 has the lowest C (37.93 wt%), H (0.93 wt%), and O (14.61 wt%) contents. These findings concur with the results obtained by the FTIR spectra, which show the degradation of oxygenated functional groups with increasing temperature (Figure 2a). Due to the large presence of ashes, and of calcium in particular, the decline in C at 700 °C could also be attributed to a loss of carbonate carbon.

The pH values of the different biochars are given in Table 1. It can be observed that the pH increases from 10.26 to 12.45 with the temperature of the biochar preparation. Typically, the pH values of biochar change with the biomass nature and production process [27]. The high pH value (12.45) of CCB700 could be the result of the loss of acidic functional groups and the release of alkaline minerals [28]. At higher temperatures (above 600 °C), the biochar ash content is more crucial in determining the pH [29]. The results obtained in the present work are in line with the literature on biochar of its alkaline nature [23,27]. According to previous studies, biochars with high content of salts, carbonates, Ca, K, and Mg tend to have a high pH and liming capacity [27] and are more suitable for adsorption due to enhanced ion exchange or chemical interactions compared to physical interactions [30,31].

3.2. Fourier Transform Infrared Spectroscopy (FTIR) Analysis

The FTIR spectra of the biochars obtained at various temperatures (CCB500, CCB600, and CCB700) are given in Figure 2a. As the pyrolysis temperature gradually increases, several peaks disappear and other are preserved with different intensities. CaCO₃ peaks dominate the FITR spectra. These are evidenced by the bands around 1427, 874, and 708 cm⁻¹ [32]. A clear change in the CaCO₃ peak intensity can be noticed with the rise in temperature. For CCB500, a broad band at 3480 cm⁻¹ depicts the -OH and -NH stretching of acid groups and aliphatic compounds, which disappears with increasing temperature, reflecting the breakdown of -H bonds from hydroxyl and the subsequent dehydration and disintegration of aliphatic compounds at a high pyrolysis temperature [33]. A stretching vibration of –CH₂ is noticed at 2920 and 2856 cm⁻¹, which is still present for CCB700 at low intensity. Furthermore, the peaks of C-O functional groups are found at ~1060 cm⁻¹ (in CC biomass), suggesting that oxygenated C-O groups remain in the biochar samples. With increasing temperature, a peak at 3641 cm⁻¹ of -OH groups is observed for CCB700, indicating the formation of CaO and Ca(OH)₂ [33]. The FTIR spectra is in good agreement with the results observed in XRF (Table 1) and XRD analyses (Figure 2b), which confirm the richness of the produced biochars in CaCO₃.

3.3. X-Ray Diffraction (XRD) Analysis

The XRD patterns of biochar samples (CCB500, CCB600, and CCB700) (Figure 2b) reveal the existence of a large proportion of mineral crystals, as well as other inorganic compounds. The peaks due to such mineral phases appear at higher pyrolysis temperatures, reflecting an increase in the mineral content of the biochars, which is also confirmed by the XRF results. It can be seen that the Ca peaks increase with the pyrolysis temperature. The most noticeable changes occur at 700 °C. CaCO₃ transformation into Ca(OH)₂ and CaO is favoured, and several peaks are detected at 2θ of 28.6°, 29.6°, 36.2°, 39.6°, 40.7°, 43.4°, 47.2°, and 48.6°. CaCO₃ transformation is confirmed by the XRF analysis, which provides a value for Ca content in CCB700 60 wt% higher than those in the other produced biochars (CCB500 and CCB600) (Table 1) and the CC biomass (Table S1). These results are in good agreement with previous studies using Ca-rich biomasses, such as spent mushroom [30], crab shell [33], and dairy manure [34], highlighting that the Ca content increases with the pyrolysis temperature.

3.4. Mineral Content

The inorganic composition of biochar samples is given in Table 1. The XRF results reveal the presence of CaO as the major element in the ash, which increases from 53.88 wt% to 60.90 wt%, followed by MgO (from 9.40 wt% to 10.75 wt%). However, the K₂O and Cl decrease with rising pyrolysis temperatures, from 16.13 wt% to 8.16 wt% and from 8.75 wt% to 2.89 wt%, respectively. The ash fraction includes other minerals at low concentrations, in particulars P₂O₅, Na₂O, Al₂O₃, and Fe₂O₃. These findings highlight the fact that CC biochar has rich mineralogical characteristics, which makes it a good candidate for adsorption process. For instance, the presence of these minerals, particularly Ca, is expected to favour chemical interactions, ion exchange, and/or electrostatic adsorption when the selected dye interacts with the adsorbent [30]. Many research studies have focused on Ca-rich biochar for dye removal and have reported a high adsorption efficiency [30,32,33].

3.5. Scanning Electron Microscopic Analysis

The surface morphology of the different biochars (CCB500, CCB600, and CCB700) is illustrated in Figure 3. The SEM images of all biochar samples indicate that the surface is heterogeneous, with an irregular shape, featuring fissures and a multitude of pores that come in different sizes, which increase with the pyrolysis temperature [35]. CCB700 particles, indeed, appear the most porous, with several cracks, which may play a crucial role in the adsorption of target dye molecules. At the surface level, CCB500 appears spongy. Small mineral particles appear to be deposited on the surface and inside the cracks in CCB600 and CCB700 particles. According to the XRD analysis, Ca peaks increase with the pyrolysis temperature (Figure 2b); thus, mineral particles appear to be more abundant in CCB700 than in CCB600.

The importance of large pores and cracks in dye adsorption is evident from the SEM images of CCB600 particles after adsorption, which can be found in the Supplementary Materials. Notably, the particle size and shape are not significantly modified upon adsorption, but at higher magnification, cavities are evidently filled.

3.6. Surface Area Measurement

The surface area and pore volume of biochar have been mentioned as key parameters in the case of its application as an adsorbent. The surface areas and pore volumes of all biochar samples are given in Table 1. It can be observed that both parameters increase with the pyrolysis temperature. In the biochar produced at 700 °C (CCB700), the surface area and pore volume reach 209.64 m²/g and 0.033 cm³/g. However, CCB500 presents a surface area of 95.80 m²/g and a pore volume of 0.013 cm³/g. The faster loss of VM from the CC indeed creates cracks and pores within the resulting biochar structure [25]. Previous studies highlighted an increase in the biochar surface area with the pyrolysis temperature [1,23]. In terms of applications, the biochar surface area and porosity can greatly influence their adsorption capacity [36]. Typically, a higher pyrolysis temperature results in larger pore sizes and therefore a larger surface area, which increases the adsorption capacity [36]. Thus, a high surface area with a large porous structure and abundant functional groups indicates the presence of large number of chemically active sites for target dye adsorption [35].

The surface area value obtained in our study is significantly higher than those reported for other raw materials, such as cactus peels [23], bone waste [1], and leather waste, ref. [11] under slow pyrolysis conditions.

4. Adsorption Study

4.1. Effect of Contact Time on the Removal of RB

As one of the key parameters, the contact time greatly influences the adsorption process. It is shown that RB dye adsorption onto CC biochar increases with the contact time, until an equilibrium is reached. The sorption capacity of RB using the CCB500, CCB600, and CCB700 biochars at various contact times (0–90 min) and an initial RB concentration of 60 mg/L was studied, and the results are illustrated in Figure 4. The RB adsorption capacity over the first minute is very fast. The adsorption capacity is high due to the efficient collision of the RB dye molecules in the aqueous solution. RB dye removal of 76.2, 79.1, and 100% is recorded for CCB500 after 15 min, for CCB600 after 15 min, and for CCB700 after only 5 min, respectively. After this time, no significant variation is observed as the sorption capacity of the RB dye reaches saturation. The presence of unoccupied adsorption sites makes it relatively simple for dye molecules to interact with the adsorbent surface in the initial step. However, at equilibrium, the adsorption sites are occupied by dye molecules, which leads to a repulsive force on the incoming dye molecules [37].

4.2. Effect of pH on the Removal of RB

The adsorption of RB dye onto CC biochars is greatly influenced by the pH of the solution. Thus, the charge in the adsorbent surface, the degree of ionization, and the nature of the dye solution are affected by the pH. The pH regulates the electrostatic interactions between the functional groups available on the CC biochar surfaces and the RB dye solution. The effect of the initial pH on the removal of RB from an aqueous solution (60 mg/L RB amount) is investigated through the change in pH from 2 to 12 at room temperature (27 ± 2 °C). It can be seen that a removal of 99.5, 100, and 100% is obtained with CCB500, CCB600, and CCB700, respectively, at pH 2 (Figure 4).

The RB dye is negatively charged (sulfonate-rich); thus, the positively charged acidic solution favours its adsorption due to the high electrostatic attraction forces between adsorbent and adsorbate. Further, the adsorption capacity is high at a low pH and decreases with an increasing pH due to the repulsive interactions between negative sulfonate groups and negatively charged biochars (CCB500 and CCB600) at a high basic pH.

Furthermore, the adsorbent’s surface is positively charged at a low pH, which attracts RB dye (anionic dye). Under acidic conditions, the functional groups on the surface of the adsorbent become polarized, adding electrostatic interactions to -H bonding and the Van der Walls force [38]. In addition, as the biochar surface is negatively charged under alkaline conditions (pH > 7), the anionic RB molecules are repelled, resulting in a decrease in dye removal efficiency [39]. It can be seen, however, that the adsorption performance of CCB700 remains good for high pH values, probably due to its high surface area, mineral composition, and high pore volume [11].

The influence of the RB dye solution pH on adsorption efficiency could also be investigated by looking at the pH_PZC. As shown in Figure 5, the determination of the pH_PZC at the intersection of two plots reveals that the adsorbent surface has a zero charge at a pH of 11, 11.35, and 12.53 for CCB500, CCB600, and CCB700, respectively. In this way, the surface sites of the CC biochars are positively charged with the loose protons at pH values below pH_PZC (pH < pH_PZC), which favours the uptake of anionic dyes due to an electrostatic attraction force [31,35]. These results are in line with the pyrolysis temperature, which increases the pH_PZC and biochar mineral content (Table 1), in particular, the Ca concentration.

4.3. Effect of Initial Concentration on the Removal of RB

The initial adsorbate concentration is critical in the adsorption process. Figure 4 shows the effect of the initial concentration of RB on the adsorption capacity of CC biochars by varying the initial concentration from 10 to 125 mg/L. As can be seen, the adsorption process is strongly affected by the solution initial concentration. Thus, for CCB500 and CCB600, the RB removal efficiency for each adsorbent increases with an increasing initial concentration from 10 mg/L to 110 mg/L and decreases for concentration values above 110 mg/L. This reduction in RB removal can be attributed to the saturation of the active sites of the CC biochar or to the increase in electrostatic repulsive forces between the surface of the CC biochar and the RB dye. Moreover, the rate at which dye molecules migrate from the solution to the adsorbent surface decreases with an increasing initial concentration [40]. Notably, only for CCB700 does the removal efficiency increase beyond a 110 mg/L dye concentration. This is largely due to its high surface area (209.64 m²/g) and high total pore volume (0.033 cm³/g). This feature also indicates a greater affinity, attributed to the presence of more functionalities on its surface [41].

4.4. Effect of Adsorbent Dosage on the Removal of RB

Figure 4 illustrates the effect of adsorbent dosage on the adsorption of RB dye using CC biochars by plotting the percentage adsorption of RB as a function of the adsorbent dosage in the 0.01–0.8 g range. The adsorbent dose is a key parameter affecting the removal efficiency [40].

The results show that the removal efficiency increases from 59.94 to 68.50 wt% for CCB500, from 70.34 to 100 wt% for CCB600, with the increment of the adsorbent dose from 0.01 to 0.8 g, and from 59.83 to 100 wt% for CCB700 for an increase from 0.002 to 0.08 g. Generally, an increasing biochar dose means more active sites, as well as more surface area being available for interactions between the adsorbent and the dye. The same trend was observed in a previous study using biochars derived from different biomasses in order to remove organic and inorganic pollutants [38,42,43,44].

Indeed, for CCB500, the dye removal efficiency increases with adsorbent dose until 0.025 g and then decreases to reach constant efficiency. This behaviour could be explained by the fact that the high adsorbent accumulation and high availability of active sites lead to a decrease in adsorption capacity. At a constant dye concentration and with increasing amounts of adsorbent, dye removal is limited [45].

5. Adsorption Kinetics

In order to select the optimum operating conditions for the large-scale batch process, kinetic models were used to study the sorption mechanism and potential rate-controlling steps. Pseudo-first- and pseudo-second-order models were used to describe the adsorption kinetics of the tested biochars. The kinetics parameters for each model are given in Table 2. It can be clearly observed from Table 2 that the experimental data are well fitted by a pseudo-second-order kinetic model, with a good agreement between experimental and calculated ${\mathrm{q}}_{\mathrm{e}}$ values and correlation coefficient (R²) values in the range of 0.995–0.999. In contrast, the pseudo-first-order model shows a weak fit to the experimental data, with correlation coefficients (R²) ranging from 0.32 to 0.82 and a significant difference between the experimental values (${\mathrm{q}}_{\mathrm{e}}$) and the calculated values (${\mathrm{q}}_{\mathrm{e}\mathrm{I}}$), indicating that the latter is not suitable for describing the RB dye adsorption process.

Compared with the pseudo-first-order model, the pseudo-second-order model is more appropriate for describing the kinetic behaviour of adsorption and for describing the mechanism of the RB adsorption. This finding confirms the hypothesis that adsorption is largely a chemisorption process between the adsorbate and the adsorbent, entailing ion exchange and covalent bonding between the biochar and the RB dye and including electronic forces between the functional groups [42]. These findings are in line with previous investigations [1,11].

6. Adsorption Isotherms

To analyze adsorption isotherms, the Langmuir and Freundlich models were employed (Table 2). Based on the adsorption isotherms results, the Langmuir model shows better adaptation for the RB dye on the surface of the CC biochars with a good regression coefficient (0.989–0.999) compared to the Freundlich model, where the R² values remained in the range of 0.327–0.827. According to the Langmuir model, RB dye adsorption behaviour on CC biochars is largely influenced by monolayer surface coverage (homogeneous surface), which involves physical and chemical adsorption processes. Previous studies on dye adsorption onto biochars derived from different biomass feedstocks confirm this observation [37,43]. Furthermore, the Langmuir model assumes that a higher adsorption site is initially occupied and then the adsorption capacity of the adsorbent decreases with the increase in the coverage of the adsorption site [46].

The maximum adsorption capacitiesobtained by the Langmuir model for CCB500, CCB600, and CCB700 are equal to 39.37, 51.28, and 56.18 mg/g, respectively (Table 3). This is consistent with previous investigations, which show that the Langmuir isotherm is the best-fitting model, indicating homogeneous sites and monolayer adsorption [40]. ${\mathrm{K}}_{\mathrm{L}}$ represents the equilibrium constant for the adsorption process, reflecting how strongly the dye molecules bind to the biochar surface [47,48]. The calculated ${\mathrm{K}}_{\mathrm{L}}$ parameter for CCB500, CCB600, and CCB700 is 0.07, 0.12, and 1.58, respectively. Higher ${\mathrm{K}}_{\mathrm{L}}$ values indicate stronger interactions [49]. These results indicate that CCB700 presents a higher affinity for RB dye than CCB500 and CCB600, which is related to its high surface area, mineral composition, and high pore volume.

Table 3 shows the favourability degree $\left({\mathrm{R}}_{\mathrm{L}}\right)$ values calculated for different initial concentrations of RB dye. It can be seen that ${\mathrm{R}}_{\mathrm{L}}$ values range between 0 and 1 for CC biochars, and the lowest ${\mathrm{R}}_{\mathrm{L}}$ value is obtained for CCB700, demonstrating the significantly greater adsorption efficiency of biochar produced at high temperatures. The favourability degree is generally linked to the irreversibility of the system, supplying a qualitative assessment of interactions between CCB– and RB dyemolecules. The degrees are between 0 and 1, which indicate a favorable adsorption process [50].

The findings for the physical characterization, kinetics, and isothermal modelling were used to infer the adsorption mechanism. As discussed above, the adsorption process of RB dye onto CC biochar seems to be mainly dominated by chemisorption. Thus, the sorption mechanism depends upon various factors, such as porosity, specific surface area, biochar functional groups, and pH value [3]. In general, the sorption process involves a combination of diverse types of interactions between adsorbent and sorbate including complexation, ion exchange, electrostatic interaction, and precipitation, which are the key mechanisms implicated in the sorption of dyes [35,51].

In the present study, the adsorption capacity of Ca-rich cactus biochar for RB dye is significantly higher than that of adsorbents prepared from various feedstocks reported in previous studies [1,31,44]. For instance, Chagtmi et al. [1] found a maximum adsorption capacity for removing SFG CLL dye of around 49.0, 50.8, and 61.7 mg/g for biochar prepared from cow bone waste at 500, 600, and 700 °C, respectively. Likewise, Jellali et al. [47] tested Ca-rich biochars produced from the co-pyrolysis of poultry manure, date palm fronds, and waste marble powder for amoxicillin (AMX) removal. The highest AMX removal capacity was obtained for Ca-rich biochar prepared at 900 °C (56.2 mg/g) compared to biochars prepared at 700 °C (14.6 mg/g) and 800 °C (46.8 mg/g). The characteristics of the biochar, including its aromaticity and mineral content could be behind the difference in adsorption performance.

Figure 6a compares the FTIR spectra of biochar before (CCB700) and after (CCB700-RB) adsorption tests, in order to highlight possible interactions. A remarkable difference can be noticed in the chemical features. As shown in the Supplementary Materials, the RB dye contains several functionalities: C=O, S=O, NH₂, and NH groups, while, as illustrated in Figure 2a, the FTIR spectrum of CCB700 reveals the existence of CaO and CaCO₃ at 1427, 874, and 708 cm⁻¹. After adsorption, these peaks weaken or disappear, suggesting that surface complexes can be generated between the Ca-rich biochar and the RB dye molecules via two possible mechanisms: (i) electrostatic attraction (CO₃²⁻) and (ii) ion exchange (Ca²⁺) [32,47]. On the other hand, the peak at 1613 cm⁻¹ is significantly intensified, which shows that the RB dye has been successfully adsorbed by the CCB700. A similar trend for anionic Remazol Brilliant Blue R adsorption from aqueous solution using various crop residue waste-based biochars was reported by Singh et al. [31]. The authors highlighted the importance of the biochar mineral matter content, which was among the factors behind differences in the dye adsorption performance. For example, the positive and high correlation between biochar Ca²⁺ concentration and ${\mathrm{Q}}_{\mathrm{m}}$ implies the possibility of metal bonding or cation–π interaction.

The influence of pH on adsorption efficiency may also be relevant to the mechanism between the adsorbent and the adsorbate [52]. In this study, the adsorption of the RB dye is optimal at an acidic pH2. The surface groups of the adsorbent are therefore protonated and electrostatically attracted to the negatively charged sulphonic acid group (-SO₃H) of the anionic RB dye. This indicates significant electrostatic interactions between the RB dye and the CC biochar.

Regarding the chemical structure of the RB dye and the CC biochar, it can be concluded that the coexisting ions play an important role in the dye removal by the tested biochars, in particular, the presence of Na⁺ on the RB surface [53]. The presence of NaCl promotes the dimerization of RB molecules due to the increase in intermolecular forces such as Van der Waals forces, ion–dipole interactions, dipole–dipole interactions, and dispersion forces resulting from π-electron delocalization [54].

7. Conclusions

The present study investigated the production of biochars from cactus cladodes (CCs) and their use as efficient, economic, and green materials for the adsorption of the industrial RB dye used in the leather industry. The findings highlight the richness of the CC biochar, with different functional groups, high porosity, and a high surface area, as well as a high content of mineral matter, in particular, Ca.

The adsorption performance increases with the temperature of biochar preparation. The char prepared at 700 °C is the most efficient adsorbent for the removal of RB dye from an aqueous solution.

According to the Langmuir isotherm, the maximum adsorption capacity of RB is 56.18 mg/g and the dye is adsorbed as a monolayer on the adsorbent. The kinetic adsorption data for RB dye follow the pseudo-second-order model.

The pH has a significant impact on the adsorption process. The findings show that the adsorption process involves not only physical forces but also chemical and electrostatic attraction between the biochar surface and the dye molecule. The mineral content and the functional groups available on the surface of this biochar are also important to its performance as an adsorbent.

We suggest that more research be conducted to determine the feasibility of using biochar to remove dyes from real wastewaters. On the other hand, Ca-rich biochar loaded with RB dye can be regenerated by desorption and reused in water pollution treatment. However, further exploration of both desorption methods and the field of regeneration and reuse is required.

Author Contributions

Writing—original draft preparation, conceptualization, methodology, software, data curation, A.M., R.C. and F.C.; writing—review and editing, validation, visualization, O.S. and B.A.; supervision, A.B.H.T. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

CC	Cactus cladode biomass
CCB	Cactus cladode-derived biochar
CCB500	Biochar produced from cactus cladodes at 500 °C
CCB600	Biochar produced from cactus cladodes at 600 °C
CCB700	Biochar produced from cactus cladodes at 700 °C
RB	Repanil Blue industrial dye
C	Carbon content (wt%)
H	Hydrogen content (wt%)
O	Oxygen content (wt%)
N	Nitrogen content (wt%)
S	Sulphur content (wt%)
VM	Volatile matter (wt%)
FC	Fixed carbon (wt%)
pHPZC	pH at point of zero charge
${\mathrm{C}}_0$	Initial aqueous RB concentration (mg/L)
${\mathrm{C}}_{\mathrm{t}}$	t time aqueous RB concentration (mg/L)
${\mathrm{K}}_{\mathrm{L}}$	Equilibrium constant for the adsorption process
$\mathrm{M}$	Mass of the used biochar (g)
$\mathrm{n}$	Freundlich exponent related to adsorption intensity
$\mathrm{q}\mathrm{t}$	Amount of dye adsorbed (mg/g)
${\mathrm{Q}}_{\mathrm{m}}$	Maximum amount of adsorption (mg/g)
${\mathrm{R}}_{\mathrm{L}}$	Dimensionless constant separation factor
R2	Correlation coefficient
$\mathrm{t}$	Time (min)
$\mathrm{V}$	Volume of the aqueous solution (L)

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Influence of pyrolysis temperature on biochar yield distribution.

Figure 2. (a) FTIR and (b) XRD spectra of the biochar samples.

Figure 3. SEM images of the biochar samples at 5 and 100 µm.

Figure 3. SEM images of the biochar samples at 5 and 100 µm.

Figure 4. Effect of contact time, initial concentration, dose, and pH of biochar on RB removal.

Figure 5. pHPZC of the studied biochar samples.

Figure 6. (a) FTIR and (b) XRD spectra of CCB700 after RB dye removal (pH 2).

Table 1

Physicochemical properties of the biochar samples.

	CCB500	CCB600	CCB700
Proximate analysis (wt%)
Moisture	2.01	1.35	1.91
VM	35.29	30.01	26.07
FC	24.74	25.11	26.58
Ash	37.97	43.52	45.44
Ultimate analysis (wt%)
C	41.55	38.07	37.91
H	1.33	1.16	0.93
N	1.61	1.13	1.09
S	-	-	-
O *	17.53	16.12	14.61
Morphological properties
Surface area (m2/g)	95.80	143.56	209.64
Total pore volume (cm3/g)	0.013	0.03	0.033
Physicochemical properties
pH	10.26	11.12	12.45
pHPZC	11.00	11.35	12.53
Mineral content (wt%)
CaO	53.88	56.12	60.90
K2O	16.13	13.57	8.16
MgO	9.40	9.57	10.75
P2O5	1.24	1.11	1.30
Fe2O3	0.09	0.05	0.10
Na2O	0.72	0.13	0.27
SO3	1.73	1.68	1.98
Cl	8.75	6.34	2.98
SiO2	0.55	1.02	0.93
Al2O3	0.29	1.19	0.94

⁽*⁾ by difference.

Table 2

Experimental and calculated equilibrium values and best-fit adsorption kinetic parameters.

	Pseudo-First-Order				Pseudo-Second-Order
	K1	qe Experimental (mg/g)	qeI Calculated (mg/g)	R2	K2	qe Experimental (mg/g)	qeII Calculated (mg/g)	R2
CCB500	0.15	21.93	1.89	0.079	0.03	21.93	21.9	0.995
CCB600	0.12	23.33	0.128	0.005	0.05	23.33	23.3	0.993
CCB700	0.15	30.25	3.02	0.099	0.02	30.25	30.3	0.999

(q_e) is the experimental adsorption capacity at equilibrium. (qe, I) is the calculated adsorption capacity at equilibrium by the pseudo-first-order model. (qe, II) is the calculated adsorption capacity at equilibrium by the pseudo-second-order model.

Table 3

Best-fit kinetic parameters for the Freundlich and Langmuir adsorption models and statistical indicators of the level of fit.

Sample	Langmuir				Freundlich
	${\mathbf{K}}_{\mathbf{L}}$	${\mathbf{Q}}_{\mathbf{m}}$ (mg/g)	${\mathbf{R}}_{\mathbf{L}}$	R2	${\mathbf{K}}_{\mathbf{f}}$ (mg/g)	${\mathbf{B}}_{\mathbf{f}}$	$\mathbf{n}$	R2
CCB500	0.0708	39.3701	0.33	0.9984	9.0148	0.9852	1.0150	0.8270
CCB600	0.1174	51.2821	0.14	0.9896	9.0463	0.9537	1.0485	0.6106
CCB700	1.5752	56.1798	0.13	0.9999	9.6268	0.3732	2.6790	0.3275

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app15020894/s1, Table S1: Physicochemical properties of cactus cladodes sample; Figure S1: Schematic of slow pyrolysis reactor for biochar production; Figure S2: (a) FTIR and (b) XRD spectra of cactus cladodes powder; Figure S3: FTIR spectrum of RB dye

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