Eco-Friendly Dye Removal: Black Cumin Seed Press Cake for Toluidine Blue Adsorption

Abstract

This research investigates the potential of Seed Press Cake of Nigella sativa (SPCN) as a low-cost, eco-friendly biosorbent for the removal of the cationic dye Toluidine Blue (TB) from aqueous solutions. The physicochemical properties of the material were characterized using Fourier-transform infrared (FTIR) spectroscopy, nitrogen adsorption–desorption isotherms, and scanning electron microscopy (SEM). Adsorption performance was evaluated under varying conditions, with the process best described by the pseudo-second-order kinetic model and the Langmuir isotherm, indicating monolayer adsorption. The maximum adsorption capacity was determined to be 305 mg·g⁻¹ at 20 °C. Thermodynamic analysis revealed that the adsorption is spontaneous, exothermic, and entropy-driven. FTIR analysis indicated that TB interacts with SPCN primarily via physical interactions, including electrostatic attraction, van der Waals forces, and hydrogen bonding, without strong chemical bonding. These findings demonstrate the high potential of black cumin seed waste as a sustainable and efficient biosorbent for dye removal in wastewater treatment.

1. Introduction

Access to safe and clean drinking water is a fundamental requirement for the survival and well-being of all living organisms. Despite its importance, water sources are increasingly being contaminated with harmful substances, including heavy metal ions, synthetic dyes, pharmaceutical residues, and pathogenic microorganisms. These pollutants pose significant risks to human health and ecosystems, contributing to a wide range of diseases and environmental degradation. Removing these pollutants is critical for safeguarding both human health and environmental sustainability. Despite existing regulations intended to ensure the proper disposal of industrial effluents, over 80% of the world’s wastewater is still discharged without adequate treatment [1]. This highlights the urgent need for effective and sustainable water treatment technologies.

Toluidine Blue (TB) is a synthetic dye widely used in biological staining, medical diagnostics, and various industrial applications. However, its release into water bodies poses significant environmental and health risks due to its toxic, non-biodegradable, and potentially carcinogenic nature [2]. The dye can disrupt aquatic ecosystems by blocking sunlight penetration and reducing photosynthetic activity, while also contaminating drinking water sources [3]. Consequently, the removal of toluidine blue from wastewater is essential to mitigate its adverse effects. Efforts to remove toluidine blue often involve adsorption, advanced oxidation, or biodegradation methods. Adsorption, in particular, is a cost-effective and efficient approach that utilizes various materials to capture and remove dyes from water. A variety of adsorbents, such as activated carbon [4], polyurethane foam/zinc oxide nanocomposite [5], nanoparticles [3], manganese/graphene coated titaniferous sand [6], agricultural waste-derived materials [7], and natural zeolites [8], among others, have been successfully employed for the elimination of toluidine blue from aqueous solutions. However, the high cost and non-renewable nature of some adsorbents have driven researchers to explore sustainable and eco-friendly alternatives [9,10].

In recent years, there has been a growing interest in utilizing low-cost natural materials as adsorbents for dye removal. Agricultural by-products, in particular, have gained attention due to their abundance, biodegradability, and potential for waste valorization [11]. The black cumin seed press cake, by-product generated in large quantities during oil extraction, has emerged as a promising candidate. This material offers potential as a cost-effective natural biosorbent, addressing both waste management and water purification challenges [12,13,14]. The black cumin seed by-product, obtained after oil extraction, is rich in protein, fiber, and bioactive compounds [15]. It has been repurposed for various applications, including animal feed, organic fertilizers, and functional food ingredients [16,17]. Additionally, its antioxidant and antimicrobial properties make it valuable in nutraceuticals and natural health products [12,18]. Recent studies have highlighted the potential of black cumin seed by-products as adsorbents for environmental and industrial applications. Due to its porous structure, high content of organic compounds, and surface functional groups such as hydroxyl (OH), carbonyl (C=O), carboxyl (O–C=O), and amines (NH₂), it can effectively adsorb pollutants such as heavy metals, dyes, and organic contaminants from wastewater [12,16,19]. This makes it a low-cost, eco-friendly alternative to synthetic adsorbents, offering a sustainable solution for water purification and pollution control.

Given the limited studies on low-cost and environmentally friendly adsorbents for toluidine blue removal, the present work investigates the efficiency of SPCN as a biosorbent. To the best of our knowledge, black cumin seed press cake has not yet been explored for the adsorption of toluidine blue, leaving a gap in the current understanding of its potential. Therefore, this study aims to evaluate its adsorption performance and highlight its promise as a sustainable and cost-effective material for wastewater treatment.

2. Materials and Methods

Black cumin seed press cake was provided by Nigella OOD (Dimitrovgrad, Bulgaria). Subsequently, oil residues were removed by Soxhlet extraction with petroleum ether (Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany). The extraction was performed for 1.5 h at a solid-to-liquid ratio of 1:2, and the solvent was evaporated under reflux at approximately 70 °C.

The porous structure of studied material was investigated by low-temperature (−196 °C) nitrogen adsorption using a Quantachrome NOVA 1200 apparatus.

Infrared spectra of the sample before and after TB adsorption were recorded using Thermo Nicolet iS50 FTIR Spectrometer. The analyzed samples (1 to 2 mg) were mixed with spectroscopic grade KBr (0.1 to 0.2 g) in an agate mortar. The mixture was pressed in a hydraulic press for 5 min at 5000–10,000 kg per cm². The obtained spectra were subjected to the built-in baseline correction in the OMNIC 9.16.1.188 package.

To examine the morphology of the samples, scanning electron microscopy (SEM) was employed using the JEOL JSM 6390 microscope in secondary electron image modus (SEI) at suitable magnifications. Prior to analysis, the dried samples were sputter-coated with a thin gold layer to improve conductivity and enhance image resolution. Elemental composition was determined using an Oxford INCA energy-dispersive X-ray (EDX) spectroscopy system.

The pH drift method was employed to determine the point of zero charge (pHpzc) of SPCN. A 0.1 M KNO₃ solution was adjusted to an initial pH of 2–12 using 0.1 M HCl or 0.1 M NaOH, and 0.200 g of the biosorbent was then added to 50 mL aliquots. The suspensions were shaken at 25 °C for 24 h, followed by filtration. Final pH values were measured with a calibrated pH meter, and the pH_pzc was taken as the point where ΔpH = 0.

Adsorption experiments were conducted in 50 mL conical flasks, each containing a measured amount of adsorbent and 15 mL of toluidine blue solution. The mixtures were agitated on a rotary shaker for predetermined time intervals and centrifuged to separate the solid and liquid phases upon reaching equilibrium. The supernatant was analyzed using a Thermo Evolution 160 UV–Vis spectrophotometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) at the maximum absorption wavelength (λ_max = 632 nm). TB concentrations were determined from a calibration curve linear over the range 2–10 mg L⁻¹. Samples with higher concentrations were appropriately diluted to fall within this range, ensuring accurate quantification.

All chemicals used in the study were of analytical grade. Batch adsorption experiments were conducted to investigate the influence of various parameters, including contact time, pH, initial dye concentration, and temperature, on the adsorption process. The equilibrium adsorption capacity (Q_e, expressed in mg g⁻¹) was determined using the following equation:

$$Q_e=(C_0-C_e)\times V/m$$

(1)

where C₀ and C_e represent the initial and equilibrium concentrations of TB (mg L⁻¹), respectively. V stands for the solution volume (L), and m corresponds to the mass of the sorbent (g). It is essential to note that each experiment was conducted in duplicate to ensure accuracy and reliability.

3. Results and Discussion

3.1. Texture Parameters

The nitrogen adsorption–desorption isotherm and pore size distribution of the studied biomaterial are presented in Figure 1.

The nitrogen adsorption–desorption isotherm displays a Type IV isotherm, as classified by IUPAC, accompanied by a distinct hysteresis loop, which confirms the presence of mesoporous structures. The hysteresis loop is characteristic of the H3 type, suggesting the existence of aggregated or slit-like mesopores, with potential contributions from micropores. A steep increase in volume at high relative pressure (p/p₀) signifies capillary condensation within the mesopores, further supporting the material’s mesoporous nature. The pore size distribution curve reveals a dominant peak at smaller pore diameters, highlighting the prevalence of micropores and small mesopores. Additionally, the rapid decline in dV/dD values at larger pore diameters indicates a minimal contribution from macropores, confirming that the material is predominantly mesoporous with some microporous features.

Analysis of the nitrogen adsorption–desorption isotherm reveals that the material has a specific surface area of 3 m²/g, a total pore volume of 0.01 cm³/g, and an average pore diameter of 15.6 nm.

3.2. FTIR Study

In Figure 2, the FTIR spectra of pure toluidine blue, black cumin seed press cake (SPCN), and SPCN after TB adsorption are presented, including a full-range spectrum (500–4000 cm⁻¹) and an expanded view (500–2000 cm⁻¹) to clearly display all relevant peaks.

The FTIR spectrum of toluidine blue exhibits its characteristic vibrational frequencies, consistent with the literature reports. These include prominent peaks attributable to aromatic C=C stretching, amine bending vibrations, and vibrations associated with sulfonic acid and amino functional groups, which serve as reference bands for identifying changes in interaction after dye adsorption [20].

The FTIR spectrum of SPCN before dye adsorption shows the following characteristic peaks: A broad absorption band around 3340 cm⁻¹, corresponding to O–H stretching vibrations, typical of hydroxyl groups from alcohols and phenols. The broadness of the band suggests strong hydrogen bonding, likely due to the polysaccharide or polyphenolic components of SPCN. A small band at 2932 cm⁻¹, assigned to C–H stretching vibrations of aliphatic –CH₃ and –CH(CH₃)– groups. These are indicative of methyl and isopropyl fragments, consistent with the molecular structure of thymoquinone, the major bioactive compound in cumin seed [21]. A strong and sharp peak at 1657 cm⁻¹, associated with C=O stretching vibrations, possibly arising from conjugated carbonyl groups or the quinoid structure of thymoquinone [22]. A distinct peak at 1543 cm⁻¹, corresponding to N–H bending or amide II-type vibrations, indicating the presence of amide or amino groups, possibly from residual proteins or structural modifications in SPCN. A peak around 1450 cm⁻¹, attributed to C–H bending (scissoring). Minor bands at 1139 cm−¹ and 1056 cm−¹, indicating C–O stretching and =C–H bending modes, respectively, which suggest oxygenated aromatic functionalities [23].

After TB adsorption, the FTIR spectrum of SPCN exhibits significant shifts and new bands, indicating interactions between SPCN and toluidine blue: The O–H stretching band shifts from 3340 cm⁻¹ to 3512 cm⁻¹, suggesting disruption or weakening of hydrogen bonding. This may be due to the incorporation of the dye, which alters the hydrogen bonding network through steric hindrance or electronic redistribution [24]. The C=O stretching band shifts from 1657 cm⁻¹ to 1687 cm⁻¹, implying a change in electron density around the carbonyl group. This shift is consistent with complex formation between the carbonyl oxygen and a cationic group—likely the dimethylaminium moiety of toluidine blue—leading to weakened conjugation and thus a higher stretching frequency [25]. A red shift in the NH₂ bending vibration, from 1543 cm⁻¹ to 1525 cm⁻¹, may be attributed to hydrogen bonding between the dye’s amine groups and the hydroxyl/carbonyl functionalities of SPCN. This interaction stabilizes the amine group and lowers its vibrational energy [26,27,28].

In addition, after TB adsorption, the FTIR spectrum of SPCN displays new bands at 1608, 1329, 1240, 1179, 1025, 877, 825, and 663 cm⁻¹, which are characteristic of toluidine blue. The appearance of these bands confirms the successful adsorption of the dye onto SPCN. They correspond to various vibrational modes, including aromatic ring stretching, C–N stretching, and out-of-plane C–H bending in substituted aromatic rings.

These spectral shifts suggest that hydroxyl groups participate in hydrogen bonding with the amine functionalities of TB, while carbonyl groups interact with the cationic dimethylaminium moiety of the dye through electrostatic attraction. In addition, the observed changes in the N–H band indicate possible stabilization of the dye’s amine groups via interactions with oxygenated sites on SPCN.

3.3. SEM Analysis

In Figure 3, the SEM images show the surface morphology of SPCN before and after TB adsorption. The material exhibits a rough and porous structure with irregular morphology (Figure 3a). After TB adsorption, noticeable changes in surface appearance are observed, including increased coverage and deposition of dye molecules. The inset image highlights that the pores appear more occupied; this visual effect is due to dye molecules adsorbing onto the pore walls and covering the rough surfaces, rather than physically blocking the large pores. Such surface coverage accounts for the observed morphological changes in the SEM images, even though the molecular dimensions of TB are much smaller than the pore size (~50 µm).

The corresponding EDX spectra further confirm this observation (Figure 3c,d). Raw SPCN shows only a very low sulfur content (0.15 wt.%), whereas SPCN after TB adsorption exhibits a pronounced sulfur peak (3.56 wt.%). Since sulfur is a characteristic element of the toluidine blue molecule, this increase provides direct evidence of dye uptake on the biosorbent surface. Additionally, the relative increase in carbon and decrease in oxygen content suggest effective coverage of the SPCN surface by the organic dye molecules. These combined SEM-EDX results provide both morphological and elemental confirmation of successful adsorption.

3.4. Adsorption Studies

3.4.1. Effect of Contact Time and Adsorption Kinetics

The effect of contact time on TB adsorption was investigated at pH 7 with an initial dye concentration of 1000 mg L−¹ and 50 mg of biosorbent. As shown in Figure 4, adsorption increased rapidly and reached equilibrium within approximately 30 min, demonstrating a fast adsorption rate. A contact time of 2 h was used in all experiments to ensure complete adsorption under varying conditions of pH, temperature, and initial dye concentration.

To better understand the adsorption mechanism, three kinetic models were applied to the experimental data: the pseudo-first-order, pseudo-second-order, and intra-particle diffusion models. These models are represented by the following equations:

Pseudo-first-order model:

$$\log \left(Q_e-Q_t\right)=\log \left(Q_e\right)-(k_1/2.303)t$$

(2)

Pseudo-second-order model:

$$(t/Q_t)=(1/k_2{Q}_e)+(1/Q_e)t$$

(3)

Intraparticle diffusion model:

$$Q_t=k_{id}{t}^{1/2}+C$$

(4)

In these equations, Q_e and Q_t represent the amounts of TB adsorbed at equilibrium and at time t, respectively. The terms k₁, k₂, and k_id denote the rate constants for each model, while C is a constant related to the boundary layer thickness. The calculated rate constants and correlation coefficients (r²) are summarized in

Table 1. Kinetic parameters and isotherm constants for TB adsorption.

Pseudo-First Order Constants			Pseudo-Second Order Constants			Intra-Particle Diffusion Constants
Q0 (mg g−1)	kl (min−1)	r2	Qe (mg g−1)	k2 (g mg−1 min−1)	r2	kid (mg g−1 min−1/2)	C (mg g−1)	r2
158.46	0.042	0.8574	244.50	0.371	0.9996	2.393	206.15	0.9353
Langmuir Parameters			Freundlich Parameters			Dubinin-Radushkevich Parameters
Q0 (mg g−1)	KL (L mg−1)	r2	kF (mg1−n Ln g−1)	n (L mg−1)	r2	Qm (mg g−1)	E (kJ mol−1)	r2
304.88	0.019	0.9996	59.68	4.16	0.8690	270.70	0.04	0.9422

.

Although the pseudo-first-order model exhibits a considerable correlation coefficient, it is substantially less accurate than the pseudo-second-order model. The calculated equilibrium adsorption capacity from the pseudo-first-order model is significantly lower than the experimental value, further confirming its inadequacy in describing the process. In contrast, the pseudo-second-order model shows a correlation coefficient very close to 1, and its calculated equilibrium adsorption capacity aligns well with the experimental value, indicating an excellent fit. This model assumes that the rate-limiting step involves chemical adsorption, characterized by the sharing or exchange of electrons between toluidine blue and the biosorbent [29]. The initial adsorption rate, h = k₂Q_e², derived from the pseudo-second-order model, was calculated as 2.21 × 10⁴ mg·g⁻¹·min⁻¹, confirming the very rapid uptake of TB and supporting the short equilibrium time observed experimentally.

The intra-particle diffusion model also plays a role, as indicated by its relatively high k_id value (2.393 mg g⁻¹ min^−1/2) and r² value (0.9353). However, the lower r² compared to the pseudo-second-order model suggests that intra-particle diffusion is not the sole rate-limiting step. The higher value of C further implies that boundary layer resistance significantly influences the adsorption process, indicating that other mechanisms, such as boundary layer diffusion, may also contribute.

3.4.2. Effect of pH

The effect of initial pH on the adsorption capacity of toluidine blue (1000 mg L⁻¹) onto black cumin seed by-product was investigated using 50 mg of biosorbent, as illustrated in Figure 5a.

The adsorption capacity remains relatively stable across the investigated pH range, with a slight increase observed at neutral pH (~6–7) and minor decreases at both lower (pH 3–4) and higher (pH 8–9) values.

The point of zero charge (pH_pzc) of SPCN was determined to be 6.6 (see Figure 5b), which aligns with the observed adsorption trend. At pH values below the pH_pzc, the SPCN surface is predominantly protonated, leading to electrostatic repulsion with cationic TB molecules and a slight reduction in adsorption. Near the pH_pzc (pH ~ 6–7), the surface charge balance favors stronger interactions, resulting in the highest adsorption capacity. Under alkaline conditions (pH > pH_pzc), the surface becomes negatively charged, which in principle favors electrostatic attraction with TB; however, the slight decrease at pH 9 may be due to dye aggregation, changes in speciation, or competitive effects from hydroxyl ions (OH⁻). Overall, maximum adsorption occurs around pH 6–7, indicating optimal conditions for TB uptake.

The relatively small variation in adsorption across the pH range suggests that electrostatic interactions, while relevant, are not the dominant mechanism. Instead, hydrogen bonding, hydrophobic interactions, and π–π stacking likely contribute substantially to adsorption, maintaining stability even when electrostatic effects are reduced or altered. This combination of mechanisms enhances the robustness and versatility of SPCN as a biosorbent, a feature particularly advantageous for practical wastewater treatment, where pH conditions can vary considerably.

3.4.3. Adsorption Isotherm

The experimental isotherm data for toluidine blue (initial concentrations of 500–2000 mg L⁻¹) obtained using 50 mg of biosorbent, together with the calculated model isotherm constants and correlation coefficients, are presented in Figure 6 and Table 1. The data in Figure 6 indicate a Type I (Langmuir-type) isotherm, characteristic of monolayer adsorption on a surface with a finite number of identical sites. The steep slope at lower Ce values reflects the strong affinity between toluidine blue and the black cumin seed–derived adsorbent, likely due to the abundance of active sites available at the initial stages of adsorption. Although SPCN exhibits a very low BET surface area (3 m² g⁻¹) and a total pore volume of only 0.01 cm³ g⁻¹, it demonstrates a remarkably high maximum adsorption capacity for toluidine blue. This suggests that, for lignocellulosic biosorbents, adsorption efficiency is governed primarily by surface chemistry rather than porosity. FTIR analysis confirmed the presence of hydroxyl, carbonyl, amine, and aromatic functional groups, which provide multiple binding sites for electrostatic attraction, hydrogen bonding, and π–π interactions with cationic dyes. Furthermore, the material contains transport macropores that facilitate rapid dye diffusion, contributing to the fast adsorption kinetics observed in this study.

Adsorption isotherms illustrate the relationship between the quantity of adsorbate on the adsorbent surface and its equilibrium concentration in the solution. These models provide insights into the interaction mechanisms between adsorbates and adsorbents and enable the prediction of adsorption capacity. In this work, the equilibrium data for TB dye adsorption were evaluated using three widely used isotherm models: Langmuir, Freundlich, and Dubinin–Radushkevich. Each of these models, as described in [30], is based on distinct theoretical principles and is expressed by the following equations:

• Langmuir model:

  $$C_e/Q_e=1/K_L{Q}_0+C_e/Q_0$$

  (5)
• Freundlich model:

  $$ln{Q}_e=ln{k}_F+(1/n)ln{C}_e$$

  (6)
• Dubinin–Radushkevich model:

  $$ln{Q}_e=ln{Q}_m-\beta {\epsilon }^2$$

  (7)

where C_e is the dye concentration in the equilibrium solution (mg L⁻¹); Q_e is the amount of dye adsorbed (mg) per unit mass of adsorbent (g); Q₀ and Q_m are the maximum adsorption capacity (mg g⁻¹); K_L and k_F are the Langmuir and Freundlich constants, β is the adsorption energy constant (mol² J⁻²), and ε is the Polanyi potential, E is the mean adsorption energy.

The Langmuir model provides the best fit for the adsorption process, as indicated by the high r² value (0.9996). This suggests that adsorption occurs primarily as a monolayer on a homogeneous surface. The Freundlich model also offers a reasonable fit, but its lower r² value (0.8690) and high n value (4.16) imply some degree of surface heterogeneity. The Dubinin–Radushkevich (D-R) model further supports the conclusion that the adsorption process is predominantly physical, as the calculated energy value (below 8 kJ mol⁻¹) typically corresponds to weak van der Waals interactions.

The experimental maximum adsorption capacity of SPCN was found to be 288.14 mg g⁻¹, closely matching the theoretical value predicted by the Langmuir model (304.88 mg g⁻¹), confirming the biosorbent’s strong potential for cationic dye removal. As summarized in

Table 2. Comparison of the adsorption capacity of SPCN with other adsorbents for TB.

Adsorbent	Qmax (mg g−1)	References
Manganese/graphene-coated titaniferous sand	5.32	[6]
Limestone	9.71	[31]
Clay-biochar composite of bentonite and sweet sorghum bagasse	9.94	[32]
Lemna minor	26.24	[2]
Starch Sulfate	26.56	[33]
Gypsum	28	[34]
Zeolite	38.53	[8]
Melon husk	42.02	[7]
Magnetic Bacillus niacini nano-biosorbent	82.88	[35]
Polyurethane foam/zinc oxide nanocomposite	91.7	[5]
Carboxymethylcellulose magnetic composite	100.0	[3]
Activated carbon	123.5	[4]
Protein-immobilized nanofiber membranes	435.4	[36]
Graphene oxide/bentonite composites	458.7	[37]
Black Cumin Seed Press Cake (SPCN)	304.88	Present study

, SPCN shows a significantly higher adsorption capacity than most natural and low-cost biosorbents reported in the literature. Although some engineered materials, such as graphene oxide/bentonite composites (458.7 mg g⁻¹) and protein-immobilized nanofiber membranes (435.4 mg g⁻¹), exhibit higher performance, these involve costly precursors or sophisticated fabrication techniques, which may hinder their large-scale application. By contrast, SPCN is an agricultural by-product requiring minimal processing, making it a promising low-cost and sustainable adsorbent with adsorption performance comparable to advanced materials but without the associated complexity and cost.

3.4.4. Effect of Temperature and Thermodynamic Studies

Temperature plays a crucial role in the adsorption process by influencing the kinetic energy of dye molecules. To assess its effect on TB removal, experiments were conducted at 293 K, 303 K, and 333 K. The results showed that TB adsorption decreased with rising temperature, confirming an exothermic process. The distribution coefficient (K_d) and thermodynamic parameters are described by the following equations:

$$K_d=Q_e/C_e$$

(8)

$$ln{K}_d=∆S^0/R-∆H^0/RT$$

(9)

In these formulas, K_d represents the dimensionless equilibrium constant, R is the gas constant (J mol⁻¹K⁻¹), and T is the temperature (K). By plotting lnK_d against 1/T (Figure 7), the enthalpy change (ΔH⁰) can be determined from the slope, while the entropy change (ΔS⁰) is derived from the intercept.

Thermodynamic parameters (ΔG⁰, ΔH⁰, and ΔS⁰) provide valuable insight into the nature and mechanism of TB adsorption on SPCN. The negative ΔG⁰ values at all temperatures (−13.86, −14.54, and −15.21 kJ mol⁻¹ at 293 K, 313 K, and 333 K, respectively) indicate that the process is spontaneous and thermodynamically favorable. The negative ΔH⁰ value (−3.85 kJ mol⁻¹) confirms the exothermic nature of adsorption, meaning energy is released when toluidine blue binds to SPCN. The relatively low magnitude of ΔH⁰ suggests that physical adsorption—such as van der Waals forces or hydrogen bonding—plays a significant role in the adsorption process. These findings are consistent with the adsorption energy (E) value obtained. The positive ΔS⁰ value (33.64 J mol⁻¹ K⁻¹) indicates increased randomness at the solid-liquid interface. This suggests that toluidine blue molecules are more freely distributed in the adsorbed state, likely due to the displacement of water or solvent molecules from the adsorbent surface. The high affinity of SPCN for toluidine blue further supports this observation.

3.5. Desorption Studies

The desorption of pre-adsorbed toluidine blue from black cumin seed by-product was investigated to evaluate the potential for biosorbent regeneration. For adsorption–desorption (reusability) experiments, a TB concentration of 1000 mg L⁻¹ was used to assess SPCN performance under relatively high dye loading conditions. Batch desorption experiments were carried out using four different desorbing solutions: a 50:50 mixture of ethanol and 1 M HCl, a 50:50 mixture of ethanol and 1 M NaCl, and 1 M HCl and 1 M NaCl individually. Pre-adsorbed SPCN (50 mg) was added to 15 mL of each desorbing solution and stirred at room temperature for 24 h. The solutions were then filtered, and the concentration of desorbed dye was measured to assess desorption efficiency.

The results, summarized in

Table 3. Desorption of TB.

Desorbing Agent	Desorption, %
1M HCl:ethanol (50:50)	86.6
1M NaCl:ethanol (50:50)	77.0
1M HCl	85.4
1M NaCl	76.0

, indicate that the highest desorption efficiency (87%) was achieved using the ethanol and 1M HCl mixture. The high desorption percentage suggests that TB adsorption is predominantly governed by physical interactions, such as van der Waals forces or hydrogen bonding, rather than chemical bonding, which aligns with previous findings. Additionally, the data reveal that ethanol has a negligible role in the desorption process, as mixtures containing it did not significantly outperform HCl or NaCl alone. These findings highlight the potential for efficient biosorbent regeneration, making black cumin seeds particularly suitable for wastewater treatment applications.

In this context, the ability of a biosorbent to maintain its adsorption capacity over multiple cycles is a key factor for practical applications. Figure 8 illustrates the performance of SPCN during three consecutive adsorption–desorption cycles, using ethanol/1 M HCl (50:50) as the regenerating eluent. The results indicate only a slight decrease in adsorption efficiency (from 82.1% to 78.6%) and desorption efficiency (from 86.6% to 83.2%) over the cycles. Such minimal loss in performance confirms that SPCN can be effectively regenerated and reused for at least three cycles without significant deterioration. This reusability highlights the reversible adsorption mechanism of SPCN, which is a major advantage for sustainable dye removal. Overall, these findings suggest that SPCN is a promising biosorbent with potential for practical application in wastewater purification.

4. Conclusions

This study evaluated the efficiency of Nigella sativa seed press cake (SPCN) as a biosorbent for removing toluidine blue (TB), a cationic dye, from aqueous solutions. The effects of pH, contact time, initial dye concentration, and temperature on the adsorption process were systematically investigated. The adsorption efficiency remained consistently high across a wide pH range, indicating the material’s practical versatility.

Kinetic and isotherm modeling showed that the adsorption process followed the pseudo-second-order kinetic model and was best described by the Langmuir isotherm, suggesting monolayer coverage on a homogenous surface. The maximum adsorption capacity of SPCN was found to be 305 mg·g⁻¹, surpassing many comparable low-cost adsorbents. Thermodynamic parameters (negative ΔG⁰ and ΔH⁰) confirmed that the adsorption is spontaneous and exothermic. The relatively low magnitude of ΔH⁰, together with high desorption efficiency, indicates that the process is predominantly driven by reversible physical interactions.

FTIR analysis demonstrated the cooperative role of hydroxyl, carbonyl, amine, and aromatic functional groups in adsorption through hydrogen bonding, electrostatic interactions, and π–π stacking. Importantly, despite its low BET surface area (3 m²·g⁻¹) and minimal pore volume (0.01 cm³·g⁻¹), SPCN achieved high adsorption capacity, highlighting that adsorption efficiency in lignocellulosic biosorbents is primarily governed by surface chemistry rather than porosity.

Overall, SPCN is a renewable, cost-effective, and reusable biosorbent with strong potential for large-scale application in the removal of cationic dyes from wastewater, offering performance comparable to advanced engineered materials but without their associated cost or complexity.