Enhanced removal of C.I. direct black 80 by phosphoric acid activated plant biomass supported by DFT insights

Abstract

The world is facing shortage in drinking water due to dryness and pollution. Among the most worrying pollutants are azo dyes, such as C.I. Direct Black 80, used extensively in the textile industry. Their resistance to natural degradation, their toxic potential and their visibility even at low concentrations pose a major environmental challenge. The development of high-performance, long-lasting adsorbent materials derived from unused biomass represents a promising approach to the treatment of polluted water. In this work, two activated carbons (AC1 and AC2) were prepared from discarded Zygophyllum gaetulum stems by chemical activation with phosphoric acid (H₃PO₄). Materials were characterized by scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), thermogravimetric derivative (TGD), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The effects of different operating parameters in batch tests (pH, adsorbent mass, contact time, temperature, initial concentration) were evaluated. Density Functional Theory (DFT) calculations were also performed to elucidate the electronic properties of the dye and identify the key interaction sites involved in the adsorption process. The point of zero charge (pHpzc) was defined as 5.95. Kinetic, isothermal and thermodynamic studies were carried out, as well as adsorption/desorption cycles to assess reusability. The activated carbons presented an amorphous, thermally stable structure enriched with oxygenated functional groups. Optimum conditions were achieved at pH = 3, with a dose of 1 g/L and a contact time of 1 h, enabling removal rates above 96% with corresponding maximum adsorption capacities of 146.95 mg·g⁻¹ for AC1 and 155.95 mg·g⁻¹ for AC2. The pseudo-first-order model and Langmuir isotherm showed the best fits, suggesting a mechanism dominated by physisorption at homogeneous sites. Thermodynamic parameters confirm a spontaneous, exothermic process based on physical interactions. Regeneration tests revealed yields of more 80% after five cycles. The results obtained demonstrate the high potential of activated carbons derived from Zygophyllum gaetulum as effective, regenerable and economically viable biosorbents for the treatment of dye-contaminated water, while at the same time contributing to the valorization of a so far neglected plant biomass.

Introduction

Water is a vital element for the survival of ecosystems, playing a key biological, economic and environmental role¹. However, in the 21 st century, the degradation of its quality and its growing scarcity has become a major challenge². According to current estimates, almost four billion people worldwide suffer from water stress for at least one month of the year. Among the principal sources of hydric pollution, industrial discharges, particularly those from the textile industry, occupy a central place^3,4. The dyes used in textile manufacturing processes are major contaminants, exacerbating the problem of water quality⁵.

Dyes are organic compounds used in a wide range of industries, notably textiles, paper, leather and plastics, primarily for their ability to absorb and reflect specific wavelengths of light, producing visible colors⁶. Chemically, they generally consist of a chromophore backbone, which imparts color, and auxochrome groups, which modify that color and improve water solubility⁷. Worldwide, it is estimated that over 10,000 different dyes are produced, with global annual demand reaching around 1 million tons^5,8. Among these dyes, synthetic dyes, introduced in the 19th century, have gradually replaced natural dyes on account of their stability, wide color range and relatively low production costs⁹. However, this massive production has a considerable environmental impact: around 20% of industrial wastewater comes from the textile industry, and almost 15% of the synthetic dyes used in dyeing processes end up in the environment without prior treatment^10,11.

Among the different classes of dyes, azo dyes are particularly dominant, accounting for over 70% of dyes used worldwide^12,13. Characterized by one or more azo groups (–N = N–), they exhibit exceptional resistance to light, heat, and biological degradation^11,14. While these properties are advantageous for industrial use, they make azo dyes highly persistent once discharged into aquatic systems. Under reducing conditions, they may degrade into aromatic amines, many of which are toxic, carcinogenic, or mutagenic¹⁵. Consequently, there is a pressing need to develop effective treatment methods for dye-contaminated wastewater.

Several physical, chemical, and biological methods have been explored for dye removal¹⁶. Physical methods, such as membrane filtration and adsorption, focus on separating dyes from water^17–19. Chemical processes, including ozonation, electrochemical oxidation, Fenton-based advanced oxidation processes (AOPs), and photolysis, aim to degrade dyes into less harmful compounds through oxidation^20–22. Biological approaches rely on microorganisms to biodegrade some dyes²³. However, each method has limitations: chemical treatments may generate secondary pollutants, while biological methods often fail to degrade complex synthetic dyes such as azo dyes²⁴. Among these, adsorption stands out for its efficiency, simplicity, adaptability, and broad applicability to different types of pollutants. It is based on the attachment of dye molecules to the surface of an adsorbent material through physical or chemical interactions²⁵.

Activated carbon is one of the most widely used adsorbents due to its large surface area (up to 3000 m²/g), high porosity, and the presence of surface functional groups^26,27. Typically produced from renewable precursors such as wood²⁸, coconut shells²⁹, or other plant biomass, it is non-toxic, cost-effective, and regenerable³⁰. Beyond dyes, it can also remove a range of other organic contaminants, including pesticides and pharmaceuticals^31,32. Compared with other processes, adsorption on activated carbon is both an effective and adaptable option for a variety of industrial contexts^33,34.

In this study, we evaluate the performance of activated carbons obtained from the discarded stems of Zygophyllum gaetulum for the elimination of C.I. Direct Black 80, a large and complex azo dye that remains little explored in adsorption studies despite its high toxicity and persistence in industrial effluents. The valorization of this biomass not only prevents waste accumulation but also provides a sustainable alternative to conventional activated carbons, offering both environmental and economical benefits.

The aim is to optimize adsorption conditions by studying the influence of different parameters such as adsorbent mass, pH, temperature, contact time and initial concentration. Kinetic, isotherm and thermodynamic analyses were carried out to understand the mechanisms underlying this adsorption. The results obtained will enable us to explore the potential of this biomass as a sustainable solution for industrial wastewater treatment, while valorizing an abundant plant resource in the arid and semi-arid regions of Morocco.

Materials and methods

Materials

For the present study, the following chemicals were supplied by Sigma-Aldrich: C.I. Direct Black 80 (C₃₆H₂₃N₈Na₃O₁₁S₃), phosphoric acid (H₃PO₄), hydrochloric acid (HCl), and sodium hydroxide (NaOH). With no additional purification, all chemicals used were of reagent grade. Bi-distilled water was used for the preparation of all aqueous solutions. The raw material employed for activated carbons synthesis consisted of discarded Zygophyllum gaetulum stems, collected in June 2024 from the Foum Zguid region, located in the Tata Province in southeastern Morocco (Souss-Massa).

Activated carbon preparation

Two activated carbons, AC1 and AC2, were prepared from the raw material. The stems were first washed thoroughly with tap water to remove impurities, then dried at 90 °C for 24 h in an oven. Once dried, they were ground using a laboratory grinder, and the resulting particles were sieved through a 200 μm stainless steel sieve to obtain a homogeneous particle size.

The sieved stem powder (40 g) was then impregnated with 85% phosphoric acid (H₃PO₄), in two powder/acid mass ratios of 1:2 (w/w), referred to as AC1, and 1:3 (w/w), referred to as AC2. Impregnation lasted 24 h at room temperature. Impregnated samples were dried at 110 °C, then subjected to thermal activation in an open-air atmosphere, with a temperature rise of 10 °C/min until 600 °C was reached, a temperature maintained for 2 h to ensure complete carbonization.

After carbonization, the samples were washed with distilled water until a neutral pH was reached, thus eliminating any residual acid. The washed samples were dried at 110 °C for a further 24 h. Finally, the activated carbon obtained was sieved through a 200 μm sieve to refine the final granulomere and optimize the material’s specific surface area.

Physicochemical properties of activated carbon

The moisture, volatile matter and ash content of the activated carbons were determined using standardized methods.

Moisture content was assessed by drying a 1 g sample (𝑚_𝑖) at 100 °C to a constant mass, enabling quantification of the fraction of water physically adsorbed. Moisture content (%) was calculated according to the following equation:

m_i : Initial sample mass.

m_f : Final mass after drying.

The results obtained indicate a moisture content of 7%, suggesting a low residual water content in the material, making it favorable for use in adsorption.

Volatile matter content was determined by subjecting the dried sample of mass m₂ to calcination at 1000 °C for 3 h in a muffle furnace under open-air atmosphere. After cooling in a desiccator, the sample was weighed (m₃), and the volatile matter content (%) was calculated according to the formula :

With a volatile matter content of 21.6%, the activated carbon shows advanced carbonization and a porous structure suitable for adsorption.

In order to determine the ash content, indicative of the residual mineral fraction after complete combustion, the sample was calcined at 750 °C for 3 h in a muffle furnace. The mass of the sample after drying at 100 °C (𝑚_𝑠) was used as a reference for this calculation, while the mass of the residue after calcination (m_c) was measured after cooling in a desiccator. The ash content (%) was determined according to the following equation:

Activated carbon has an ash content of 5.7%, reflecting the low proportion of inorganic residues after carbonization. This value preserves the material’s porosity and optimizes accessibility to adsorption sites, reducing the risk of pore clogging.

Characterization

Several analytical techniques were employed to study in depth the properties of activated carbon from our biomass.

Fourier transform infrared (FTIR) spectroscopy was used to identify the functional groups present on the surface of the material. Spectra were obtained with an FTIR spectrometer (Bruker Alpha Platinum-ATR) in the wavelength range between 4000 and 400 cm-¹ using the KBr pellet technique.

The crystalline structure of activated carbon was studied by X-ray diffraction (XRD), enabling the nature and organization of the material’s crystalline phases to be identified. X-ray diffraction was used to examine the crystalline structure of the samples in a 2θ angle range from 5° to 70°, using a Shimadzu XRD-6100 diffractometer.

To evaluate the thermal stability and decomposition stages of the materials, TGA, DTG and DSC analyses were carried out using a TGA/DSC instrument (Setaram Labsys™ Evo). Measurements were carried out between 20 and 700 °C, with a heating rate of 10 °C/min, under a controlled air flow of 45 mL/min.

Surface morphology was examined by scanning electron microscopy (SEM), to observe the porous arrangement of the activated carbon. Complementary elemental analysis was carried out by energy dispersive X-ray spectroscopy (EDX), with both analyses performed using the JEOL JSM-7600 F microscope.

Dye characteristics

An anionic azo dye, C.I. Direct Black 80 (DB80) (Fig. 1), was chosen as the model pollutant for evaluating activated carbon adsorption performance. Table 1 summarizes its main physico-chemical characteristics.

Fig. 1.

Chemical structure of DB80 dye.

Table 1.

Principal physico-chemical characteristics of DB80 dye.

Dye	Class	Family	Chemical formula	Solubility	Molar mass (g/mol)	λmax (nm)	CAS No
C.I. Direct Black 80	Direct dye	Azoic	C36H23N8Na3O11S3	Soluble in water	908.8	599.8	8003-69-8

Batch adsorption tests

To evaluate the adsorption capacity of two types of activated carbon, prepared in two proportions, adsorption experiments were carried out in batch mode: AC1 (1:2) and AC2 (1:3). An aqueous solution of 100 mg/L C.I. Direct Black 80, prepared with bidistilled water, was used for all tests. All experiments were performed in Erlenmeyer flasks containing 25 ml of dye solution and a defined quantity of activated carbon.

Various adsorption parameters were investigated to better understand the performance of the two activated carbon ratios. Adsorbent mass was tested over a range from 0.25 to 5 g/L, and the effect of contact time was examined for durations between 5 and 240 min, with a constant adsorbent mass of 1 g/L. Similarly, the impact of initial dye concentration was analyzed over a range from 5 to 250 mg/L, again with an activated carbon mass of 1 g/L. The pH of the solution was adjusted between 3 and 11 using H₂SO₄ (0.1 M) and NaOH (0.1 M). All tests were carried out with constant stirring at 300 rpm on an orbital shaker (Edmund Buhler GmbH).

The samples were then centrifuged at 1000 rpm for 10 min to separate the adsorbent from the supernatant. The remaining dye concentrations were then analyzed by UV-Vis spectrophotometry (SP-UV1100, DLAB) at a wavelength of 599.8 nm.

The dye removal rate was determined according to the following equation:

where C_i and C_f (mg/L) are the initial and final concentrations of the dye in solution, respectively.

The amount of dye adsorbed at a given time qₜ (mg/g) and at equilibrium qₑ (mg/g) was calculated according to :

where Cₜ and Cₑ (mg/L) are respectively the concentrations of the dye at an instant t and at equilibrium.

V (L) is the volume of the solution, and m (g) corresponds to the mass of adsorbent.

Adsorption kinetics

To understand the mechanisms involved in the adsorption of C.I. Direct Black 80 by activated carbons AC1 and AC2, it is essential to study the adsorption kinetics. The non-linear equations of the pseudo-first-order, pseudo-second-order and Elovich models were applied to the experimental data to identify the most appropriate kinetic model.

According to Lagergren’s pseudo-first-order model, the adsorption rate is proportional to the number of active sites still available³⁵. The non-linear form of the equation is as follows:

where:

k₁ (min⁻¹) = Pseudo-first-order rate constant.

t (min) = Contact time.

As for the pseudo-second-order model, developed by Ho and McKay, it implies that adsorption is controlled by a chemical process involving interactions between the dye and the adsorbent’s active sites³⁶. The non-linear expression is as follows:

where:

k₂ (g.mg⁻¹.min⁻¹) is the second-first-order rate constant.

The Elovich model is well suited to systems with heterogeneous active sites and complex interactions³⁷. Its non-linear form is given by:

where:

α (mg.g⁻¹.min⁻¹) = Initial adsorption coefficient.

β (g.mg⁻¹) = Parameter related to the activation energy of adsorption.

These three models were fitted to the experimental data using non-linear regression. Their fit was assessed using coefficients of determination (R²) and root mean square error (RMSE). The model with the best correlation coefficient and lowest error was selected as the most representative of the adsorption mechanism of activated carbons AC1 and AC2.

Adsorption isotherms

The adsorption isotherms study provides a clearer understanding of the interaction between C.I. Direct Black 80 dye and activated carbons AC1 (1:2) and AC2 (1:3), along with the mode of binding of adsorbed molecules to the adsorbent surface. To model the adsorption equilibrium, non-linear equations were applied to the experimental data in different classical models.

The Langmuir model assumes that adsorption occurs on a homogeneous surface with a monomolecular layer and a finite number of active sites³⁸. This model’s non-linear equation is given by:

where:

q_m (mg/g) = Maximum absorption capacity.

K_L (L/mg) = Langmuir’s constant.

In addition, the separation factor (R_L) was calculated from the Langmuir constant (K_L) to evaluate the favorability of the adsorption process, according to the following equation:

where C₀ the initial dye concentration (mg·L⁻¹).

The Freundlich model describes adsorption on a heterogeneous surface with interactions between adsorbed molecules and a logarithmic distribution of sorption energies³⁹. This equation is given by:

K_F ((mg/g)(L/mg)^1/n) : Freundlich adsorption constant.

n: Heterogeneity factor.

The Dubinin-Radushkevich (D-R) model is used to distinguish between physical and chemical adsorption by evaluating the average adsorption energy⁴⁰. Its associated non-linear equation is as follows:

B (mol²/Kj²) = Dubinin-Radushkevich isotherm constant dependent on sorption energy.

The average adsorption energy (E) is expressed by:

In Temkin’s model, it is assumed that the adsorption energy of molecules decreases linearly with increasing surface coverage, due to interactions between adsorbate and adsorbent³⁸. Its expression is given by:

b (J/mol) = Adsorption constant.

K_t (L/mg) = Temkin isotherm constant.

Desorption studies

The regeneration and reusability efficiency of activated carbons AC1 (1:2) and AC2 (1:3) was evaluated through successive adsorption/desorption cycles of C.I. Direct Black 80. After each adsorption cycle, the saturated adsorbent was recovered by vacuum filtration, then immersed in a solution of NaOH (0.1 M) to promote desorption of the adsorbed dye.

The desorption process was carried out by stirring the adsorbent with 25 mL of the desorbing solution at 300 rpm at room temperature, for 2 h, using an orbital shaker (Edmund Buhler GmbH). Following this step, the adsorbent was filtered under vacuum, rinsed thoroughly with distilled water to remove alkaline residues, then dried at 70 °C for 12 h before being reused for a new adsorption cycle.

This protocol was repeated over several cycles to assess the regeneration capacity of the activated carbons and their reusability. After each desorption, the concentration of dye released into solution was determined by UV-Vis spectrophotometry (SP-UV1100, DLAB) at 599.8 nm.

The desorption rate (DE%) was determined according to the following equation⁴¹:

C_d (mg/L) = Dye concentration in the desorption solution.

V_d (L) = Volume of the desorption solution.

q_e (mg/g) = Amount of dye adsorbed by the adsorbent.

m (g) = Mass of adsorbent used.

Computational methodology (DFT approach)

Adsorption studies can be greatly benefited from computational methods, particularly DFT and semi-empirical approaches, which offer valuable insights into the interactions between pollutants and adsorbent materials (activated carbon), which are highly beneficial. The DMol3 computer code, which is part of Materials Studio 6.0, was utilized in this work to apply density functional theory (DFT). The molecular structure was refined by utilizing the primary two digital plus polarization (DNP) ensemble alongside the Generalized Gradient Approximation (GGA) approach⁴². The DMol3 ensemble’s other parameters were adjusted to the highest quality settings in order to achieve the most stable adsorption geometry possible⁴³. By using this hybrid methodology, it is possible to predict chemical reactivities, electronic properties, reactive sites, and elucidate adsorption mechanisms, which ultimately leads to more effective strategies for preventing water pollution. The chemical reactivity of DB80 was assessed using a set of quantum chemical descriptors, which included HOMO–LUMO energies, energy gap (ΔEgap), ionization potential (IP), electronegativity (χ), electron affinity (EA), chemical softness (σ), and the electron transfer fraction (ΔN₁₁₀). The Eqs. 1–8 are used to calculate these parameters, which provide essential insights into the molecule’s electronic structure and reactivity profile. The structure of this pollutant’s electrophilic and nucleophilic sites was identified by estimates of Fukui indices (Fukui (+) and Fukui (-)) using Hirshfeld population analysis. These parameters were determined by utilizing Eqs. 7 and 8⁴⁴. In addition, pollutants and activated carbon interacted through the use of reduced density gradient (RDG) and non-covalent interaction (NCI) isosurfaces, with visual tools like VMD, Multiwfn and Gnuplot⁴⁵.

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The electron concentrations on atom k that correspond to N + 1, and N − 1 are denoted by the symbols qk (N + 1), qk (N), and qk (N-1). Systems with N + 1, N, and N-1 electrons, respectively.

Results and discussion

Characterization of activated carbons

Fourier-transform infrared spectroscopy(FTIR)

The FTIR spectra of activated carbon before and after adsorption of DB 80 reveals the functional groups present on the surface of the material (Fig. 2). Before dye adsorption (Fig. 2-a), several characteristic bands were observed. The broad band at 3178 cm-¹ is attributed to the elongation vibrations of hydroxyl groups (-OH), reflecting a significant number of these groups likely to interact by hydrogen bonding⁴⁶. The band at 1936 cm-¹ corresponds to the vibrations of carbonyl groups (C = O) conjugated to aromatic rings, suggesting the presence of oxidized sites⁴⁷. The peak at 1543 cm-¹ is associated with elongation vibrations of aromatic C = C bonds, characteristic of a developed carbon structure. Bands at 1173 cm-¹ and 1057 cm-¹ are attributed to C-O vibrations, suggesting the presence of ether or ester groups.

Fig. 2.

FTIR spectra of activated carbons before and after adsorption of C.I. Direct Black 80: (a) AC1, (b) AC2.

After adsorption of DB 80, subtle but significant changes were observed. The band at 3174 cm-¹ becomes less intense, suggesting the involvement of hydroxyl groups in hydrogen bonding with the dye’s anionic groups. A slight attenuation of the band at 1932 cm-¹ indicates a possible change in the environment of the C = O groups following adsorption. The band at 1548 cm-¹ shows a subtle variation, possibly related to the formation of π-π interactions between the aromatic rings of the dye and those of the activated carbon. Finally, the bands at 1171 cm-¹ and 1061 cm-¹, corresponding to C-O vibrations, show a decrease in intensity, testifying to their probable involvement in dipolar or electrostatic interactions. Although the overall appearance of the spectra before and after adsorption remains quite similar, these subtle changes nevertheless provide evidence of the involvement of –OH, C = O, and C–O groups in the adsorption mechanism.

The FTIR spectra of AC2 (Fig. 2-b) exhibited broadly similar bands to those of AC1, notably the O–H stretching around 3176 cm⁻¹, the C = O band at 1934 cm⁻¹, the aromatic C = C vibration at 1546 cm⁻¹, and the C–O bands at 1172 and 1062 cm⁻¹. However, before adsorption, the O–H band appeared less intense and less distinct than in AC1, which can be attributed to the higher activation ratio (1:3) promoting dehydration and condensation of the carbon matrix, thereby reducing the number of free hydroxyl groups and favoring the formation of additional oxygenated functionalities such as C–O and P–O.

After adsorption of DB80, AC2 exhibited more pronounced changes than AC1. The O–H band underwent strong attenuation, confirming hydrogen bonding with the dye’s sulfonate groups. The C = O band also decreased, indicating its involvement in interactions, while the aromatic C = C vibration showed clear variations reflecting stronger π–π interactions. The C–O bands displayed a marked reduction, confirming their contribution to dipolar and electrostatic interactions.

Overall, these results indicate that although AC1 and AC2 share similar spectral features, AC2 shows a more pronounced functional involvement after adsorption, consistent with its comparatively higher degree of activation and superior performance in DB80 removal. The observed spectral modifications highlight the role of –OH, C = O, and C–O groups in the adsorption mechanism, mainly through hydrogen bonding, electrostatic forces, and π–π interactions between the aromatic rings of the dye and those of the activated carbon.

X-Ray diffraction analysis (XRD)

Figure 3 illustrates the X-ray diffraction spectrum of activated carbons AC1 and AC2 before and after adsorption of DB80. In all cases, a broad diffuse band is observed between 2θ ≈ 20° and 30°, confirming the predominantly amorphous nature of the carbon matrix, which is characteristic of biomass-derived activated carbons⁴⁸. Superimposed on this halo, sharper diffraction peaks appear in the range 2θ = 25–50°, which can be attributed to the (002) and (100) planes of graphitic domains as well as to residual mineral phases resulting from phosphoric acid activation⁴⁹. After adsorption, no new peaks or peak shifts were detected, indicating that the crystalline structure of the activated carbons remains unchanged. Only minor variations in peak intensity and background signal are observed, likely due to the deposition of DB80 molecules on the carbon surface. These results demonstrate that adsorption occurs primarily through surface interactions without altering the structural framework of the adsorbents, in agreement with FTIR findings highlighting the involvement of functional groups in the retention of DB80.

Fig. 3.

XRD diffractograms of activated carbons before and after adsorption of C.I. Direct Black 80: (a) AC1 before, (b) AC1 after, (c) AC2 before, (d) AC2 after.

Thermal properties

Thermal analysis by TGA, DTG and DSC has enabled us to gain a better understanding of the thermal stability and degradation behavior of the prepared activated carbon. As shown in Fig. 4, three main stages of mass loss were identified for both AC1 and AC2, with very similar profiles. The first, observed up to around 150 °C, represents a mass loss of around 11% for AC1 and 10% for AC2. This is attributed to the evaporation of physically adsorbed water and weakly bound volatile compounds present on the carbon surface. This phenomenon is supported by a sharp peak on the DTG curve and a clear endothermic signal on the DSC curve.

Fig. 4.

(a) TGA/DTG analysis of AC1; (b) TGA/DSC analysis of AC1; (c) TGA/DTG analysis of AC2; (d) TGA/DSC analysis of AC2.

Between 150 and 350 °C, a second mass loss estimated at 9% for AC1 and 10% for AC2, is observed. This could be explained by the progressive degradation of hemicelluloses, known for their low thermal stability, as well as by the onset of decomposition of certain fragile fractions of the material’s structure. This interpretation is reinforced by a low-amplitude DTG peak and a moderate response on the DSC curve.

Finally, a third major phase is recorded between 350 and 850 °C, reflecting a mass loss of around 33% for AC1 and 36% for AC2. It seems to correspond to the slower decomposition of residual cellulose and more stable lignocellulosic fractions. The multiple peaks observed on the DTG curve testify to the complexity of the processes involved. The DSC curve also shows a spread exothermic peak, suggesting the progressive formation of more ordered carbon structures.

These results are consistent with those reported in the literature for lignocellulosic biomasses, where the succession of mass losses successively reflects the volatilization of light compounds, the degradation of hemicelluloses, then the slower degradation of cellulose and lignin^50–52.

Morphological analysis and elemental composition (SEM/EDX)

Scanning electron microscopy (SEM) images before and after adsorption reveal morphological structures characteristic of activated carbons. SEM images obtained before adsorption reveal a heterogeneous morphology characterized by rough surfaces and the presence of large cavities (around 20 μm), cracks and open canals, which indicate the formation of macroporous channels rather than a fully developed porous network (Fig. 5). These macroporous openings, typically formed during phosphoric acid activation, can facilitate fluid circulation and enhance mass transfer during adsorption. Comparing the two materials, AC2 has a more pronounced porosity and more fragmented structures than AC1, a direct consequence of the higher ratio of phosphoric acid used during activation. While SEM mainly provides information on external morphology, the development of micro- and mesopores is generally associated with H₃PO₄ activation, which helps increase specific surface area and the availability of active adsorption sites^53,54.

Fig. 5.

SEM images and EDX spectra of activated carbon before adsorption (a) AC1; (b) AC2.

After adsorption, the SEM image of sample AC1 shows a smooth, uniformly covered surface with the dye, with fine deposits distributed on the walls and in the microcavities (Fig. 6). This morphology reflects homogeneous and efficient adsorption, testifying to the good interaction between the material and the adsorbed molecules. For sample AC2 after adsorption, the surface is characterized by visible accumulations and a rougher texture, with massive deposits in deep cavities. The observed morphology highlights significant adsorption both on the surface and in the internal structure of the material, underlining its high adsorption potential.

Fig. 6.

SEM images and EDX spectra of activated carbon after adsorption of C.I. Direct Black 80 (a) AC1; (b) AC2.

The EDS analysis after adsorption confirms the presence of nitrogen and sulfur, characteristic of the dye, accompanied by a variation in the atomic percentages of carbon and mineral elements. All of which confirms the high capacity of the prepared materials to capture dye organic molecules.

Zero-charge point

pH variations have a significant influence on the surface charge of adsorbent materials, thus having a direct impact on their affinity for the ionic species present in the solution. This determines the adsorbent’s ability to interact with anions or cations, depending on the acid-base conditions⁵⁵. The zero charge point (pHpzc) of the prepared activated carbons was determined to better understand the adsorption mechanism. The pHpzc is defined as the pH at which the net charge on the surface of the material becomes zero. For AC1 and AC2 studied, the values were determined to be 5.95 and 6.12, respectively (Fig. 7). Below this pH, the adsorbent surface shows a positive charge, resulting from the protonation of acidic functional groups, thus favoring the electrostatic attraction of anionic species such as sulfonated dyes. Conversely, above this value, the surface becomes predominantly negative due to the deprotonation of oxygenated groups, leading to a phenomenon of repulsion towards anions and a greater affinity for cations.

Fig. 7.

Determination of pHpzc of AC1 (a) and AC2 (b).

The relatively low pHpzc value confirms the presence of numerous acidic functions on the surface of our activated carbons, contributing to its effectiveness in adsorbing anionic pollutants in acidic environments.

Effect of pH of dye solution

The pH of the solution plays a major role in the adsorption process, influencing both the ionization of functional groups present on the activated carbon surface and the ionic form of the dye⁵⁶. Figure 8 shows the effect of initial pH on the rate of removal of C.I. Direct Black 80 dye by the two adsorbents (AC1 and AC2). Tests were carried out in a pH range of 3 to 11, while sulfuric acid H₂SO₄ (0.1 M) and sodium hydroxide NaOH (0.1 M) were used for solution calibration.

Fig. 8.

Influence of initial solution pH on the percentage removal of C.I. Direct Black 80 by activated carbons AC1 and AC2 (conditions: C₀ = 100 mgL-¹, m = 1 g-L-¹, contact time = 60 min, T = 25 °C).

It is clearly observed that adsorption efficiency decreases with increasing pH. At acid pH (pH = 3), removal rates reach maximum values of 98.7% for AC1 and 99.11% for AC2. This high efficiency is explained by the strong protonation of acidic functional groups (-OH, -COOH, etc.) present on the carbon surface, making the overall surface positively charged. Such a charge favors electrostatic attraction with the Direct Black 80 dye, which is anionic in nature, notably comprising sulfonate groups (-SO₃⁻).

As pH increases above the zero-charge point (pHpzc = 5.95), the carbon surface becomes progressively negative due to deprotonation of the acid groups. This leads to electrostatic repulsion between the surface and the dye molecules, reducing affinity and hence adsorption. At pH 11, retention rates drop to around 77% for AC2 and 76% for AC1.

Interestingly, although both materials follow the same path, AC2 shows a slightly higher efficiency throughout the pH range, which could be attributed to its more porous structure and more pronounced acid activation, favoring a greater number of available active sites.

These results suggest that electrostatic interactions play an important role in the adsorption process, particularly at acidic pH. However, the elevated retention observed under basic conditions could indicate the involvement of other types of interactions, such as hydrogen bonds or π-π interactions between the dye’s aromatic rings and the surface’s carbon structures.

Effect of activated carbons dose

The progressive increase in adsorbent dose (AC1 and AC2) leads to a clear enhancement of the dye removal rate, as shown in Fig. 9. At low doses (0.1–1 g/L), the rapid rise in removal capacity, exceeding 95% from 1 g/L, highlights the high affinity of both materials for dye molecules, reflecting high availability of active sites. This behavior testifies to the high intrinsic efficiency of the prepared carbons, even at low concentrations. Above 1 g/L, a quasi-stagnation in efficiency is observed, indicating that all the dye molecules present in solution find sufficient sites to be adsorbed. This apparent saturation zone suggests that increasing the dose beyond this value brings only a small gain in efficiency, probably due to particle capping or agglomeration, limiting accessibility to internal sites. AC2 has a slight advantage across all concentrations, which corroborates SEM observations showing a more porous and structured surface, a direct consequence of more extensive chemical activation. This structure not only facilitates better diffusion of molecules into the matrix but also allows greater exposure of adsorption sites. The evolution of the removal rate with increasing dose, marked by a rapid rise followed by a plateau, supports the idea that the optimum dose is around 1 g/L providing an effective compromise between efficacy and quantity of material, which is of particular interest for real-scale applications. These results are in line with other studies on the adsorption of dyes by biomaterials, where increasing the dose generally leads to an improvement in elimination up to a threshold value, beyond which the effect becomes negligible^7,57,58.

Fig. 9.

Influence of adsorbents dose on the percentage removal of C.I. Direct Black 80 (conditions: C₀ = 100 mg-L-¹, contact time = 60 min, T = 25 °C).

Adsorption kinetics

In order to understand the interaction mechanism between the dye and the adsorbent surfaces, a kinetic study was carried out for the two materials prepared. This analysis enables us to determine not only the rate of adsorption, but also the nature of the phenomena involved, whether physical or chemical. For this purpose, experimental data were modelled using the pseudo-first-order, pseudo-second-order and Elovich equations (Figs. 10 and 11).

Fig. 10.

Non-linear fitting of kinetic models for the adsorption of C.I. Direct Black 80 dye on AC1 (C₀ = 100 mg·L⁻¹, T = 25 °C, V = 25 mL, adsorbent dosage = 1 g·L⁻¹).

Fig. 11.

Non-linear fitting of kinetic models for the adsorption of C.I. Direct Black 80 dye on AC2 (C₀ = 100 mg·L⁻¹, T = 25 °C, V = 25 mL, adsorbent dosage = 1 g·L⁻¹).

The results show that, for both materials, the pseudo-first-order model provides the best fits to the experimental data, with very high coefficients of determination (R² >0.999) and low RMSE values (1.53 for AC1 and 1.997 for AC2) (Table 2). This well-fitted model suggests that adsorption follows a kinetic pattern governed by the availability of active sites, where the rate is proportional to the number of vacant sites, characteristic of a rapid physisorption process.

Table 2.

Adsorption kinetics: parameters for AC1 and AC2.

Models	Parameter	Value for AC1	Value for AC2
Pseudo-first order	Qe (exp) (mg.g−1)	95.85	96.79
	Qe (cal) (mg.g−1)	96.56 ± 0.69	97.48 ± 0.88
	K1 (min−1) R2 SSE RMSE	0.08714 ± 0.00493 0.999 21.09 1.53	0.10006 ± 0.00779 0.999 35.94 1.99
Pseudo-second order	Qe (exp) (mg.g−1)	95.85	96.79
	Qe (cal) (mg.g−1)	100.78 ± 1.72	101.38 ± 1.41
	K2 (g·mg⁻¹·min⁻¹)	0.000173 ± 0.000019	0.00204 ± 0.00019
	R2 SSE RMSE	0.989 96.52 3.28	0.992 69.23 2.77
Elovich	α (mg.g−1 min−1)	270.73 ± 196.34	229.47 ± 204.14
	β (g.mg − 1) R2 SSE RMSE	0.083 ± 0.01 0.966 299.69 5.77	0.08286 ± 0.01 0.946 480.81 7.31

This behavior reflects a mechanism dominated by physical interactions, such as Van der Waals forces or hydrogen bonds, between the materials’ surface functional groups (hydroxyls, carbonyls, carboxyls) and the dye’s sulfonate groups. These observations are in line with the results of the FTIR analysis, which highlighted the involvement of these functions in the interaction with the dye molecules. In addition, SEM observations evidenced the presence of interconnected macroporous channels, which facilitate fluid circulation and enhance external mass transfer. Such structural features promote rapid diffusion of dye molecules toward accessible active sites, thereby accelerating the overall adsorption kinetics.

In contrast, the pseudo-second-order model provided lower correlations and systematically overestimated the adsorption capacities, indicating that chemisorption is not the dominant rate-limiting mechanism. The Elovich model, which usually reflects surface heterogeneity, showed the weakest agreement with the experimental data, especially at intermediate and long contact times. These deviations, also visible on the kinetic curves, further demonstrate that neither chemisorption nor strong heterogeneity effects play a significant role in this system. Although the pseudo-first-order model provides the best fit to the experimental data, the slightly lower-performance results of the pseudo-second-order model may suggest the existence of some more complex or varied interactions, probably related to the nature of the biomass-derived carbonaceous material. So, even if physisorption remains the predominant mechanism, a certain diversity of adsorption sites could also play a role in the overall kinetics of the process.

Overall, both materials exhibit very rapid and efficient adsorption kinetics, well described by the pseudo-first-order model. Their performances are comparable, although slight variations are observed. These differences could be linked to the porous structure, the distribution of active sites or internal diffusion, all of which influence the rate of adsorption. Such fast and efficient kinetics are particularly advantageous for aqueous depollution applications, where treatment time needs to be reduced. These results are consistent with those reported in the literature on lignocellulosic biosorbents, where physical interactions generally dominate the adsorption process for anionic dyes^59–61.

Adsorption isotherms

In order to assess the adsorption capacity and better understand the equilibrium interactions between C.I. Direct Black 80 dye and the prepared activated carbons (AC1 and AC2), several isotherm models were applied to the experimental data, namely Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R). These models allow for characterization of the adsorption mechanism, the distribution of active sites, and the nature of adsorbate–adsorbent interactions. The fitting results, including the calculated isotherm parameters, are presented in Table 3 (AC1 and AC2), while the non-linear regression plots are illustrated in Fig. 12 (AC1) and Fig. 13 (AC2).

Table 3.

Adsorptions isotherms: parameters for AC1 and AC2.

Isotherm	Parameter	Value for AC1	Value for AC2
Langmuir	Qm (mg.g−1)	146.95 ± 14.31	155.94 ± 14.90
	KL (L.mg−1)	0.0124 ± 0.0032	0.0114 ± 0.0028
	R2	0.996	0.996
	SSE	388.25	359.27
	RSME	5.94	5.52
Freundlich	KF ((mg/g)(L/mg)1/n)	5.6767 ± 2.3301	5.4476 ± 2.1850
	1/n	0.5494 ± 0.2721	0.566 ± 0.252
	R2	0.926	0.933
	SSE	1350.85	1310.83
	RSME	11.08	10.93
Dubinin–Radushkevich	Kad (mol2/kJ2)	1278.99 ± 290.19	1484.45 ± 364.45
	qd (mg/g)	102.47 ± 5.27	107.31 ± 5.72
	R2	0.955	0.955
	SSE	805.92	867.19
	RSME	8.56	8.89
Temkin	KT (L.g−1)	0.0369 ± 0.0164	0.03312 ± 0.01394
	b (J.mol−1)	49.741 ± 10.453	53.792 ± 10.988
	R2	0.964	0.964
	SSE	742.51	701.68
	RSME	8.22	7.98

Fig. 12.

Non-linear fitting of isotherms models for the adsorption of C.I. Direct Black 80 dye on AC1 (t = 60 min, T = 25 °C, V = 25 mL, adsorbent dosage = 1 g·L⁻¹).

Fig. 13.

Non-linear fitting of isotherms models for the adsorption of C.I. Direct Black 80 dye on AC2 (t = 60 min, T = 25 °C, V = 25 mL, adsorbent dosage = 1 g·L⁻¹).

The Langmuir model provided the best fit, with coefficients of determination (R²) above 0.995 for both materials, low statistical errors (SSE and RMSE), and maximum adsorption capacities reaching 146.95 mg·g⁻¹ for AC1 and 155.95 mg·g⁻¹ for AC2. This excellent agreement suggests a monolayer adsorption mechanism on a relatively homogeneous surface, consistent with a uniform distribution of active sites. Also, the Langmuir constant (K_L) was used to calculate the separation factor (R_L) to assess the favorability of adsorption. For an initial dye concentration of 100 mg·L⁻¹, the R_L values were 0.446 for AC1 and 0.467 for AC2. As these values lie between 0 and 1, the adsorption of C.I. Direct Black 80 on both activated carbons is considered favorable. The slightly better performance of AC2 may be attributed to its more developed porosity and enhanced surface accessibility, as evidenced by scanning electron microscopy (SEM) observations.

C.I. Direct Black 80 is an anionic dye containing several sulfonate groups (–SO₃⁻), which promote electrostatic interactions with functional groups present on the surface of the activated carbons. Although the carbonization process degrades a large portion of the original organic constituents, residual oxygenated functional groups originating from cellulose, hemicellulose, and, to a lesser extent, lignin remain. These groups (such as hydroxyl, carbonyl, and carboxyl) can contribute to hydrogen bonding or Van der Waals interactions with dye molecules. Such interactions are typically observed in lignocellulosic-based materials, where polar surface functionalities play a key role in the retention of charged species^62,63.

The Freundlich model, which describes multilayer adsorption on heterogeneous surfaces, showed a less accurate fit, particularly for AC1, suggesting that the adsorption sites are predominantly homogeneous. Nonetheless, the presence of a limited number of sites with variable energy cannot be excluded, potentially due to the structural complexity inherited from the original biomass. Temkin’s model, which considers adsorbate–adsorbate repulsive interactions, provided a moderate fit, indicating that such interactions are not predominant in the studied system.

The Dubinin–Radushkevich model was used to estimate the mean adsorption energy, yielding very low values (< 0.8 kJ·mol⁻¹; E = 0.0183 kJ·mol⁻¹ for AC1 and E = 0.0184 kJ·mol⁻¹ for AC2), confirming that the adsorption process is dominated by physisorption mechanisms. These values are consistent with weak Van der Waals forces and hydrogen bonding, supported by the surface chemistry of the adsorbents, which is rich in polar groups derived from cellulose and hemicellulose.

Thermodynamic studies

Thermodynamic analysis (Table 4) is an essential complement to the study of the adsorption mechanism, providing information on the nature of the interactions involved, the spontaneity of the process and its behavior towards temperature. Standard thermodynamic parameters, such as Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°), were calculated from the adsorption equilibrium constant K_ad, obtained from the Langmuir parameter K_L, converted to L/mol according to the following equation⁶²:

Table 4.

Thermodynamic parameters for C.I. Direct black 80 adsorption on AC1.

Dye	T (K)	ΔG◦ (kJ/mol)	ΔH◦ (kJ/mol)	ΔS◦ (J/mol)	R 2
C.I. Direct Black 80	300	−24,9182	−32,4825	−25,0251	0,998
	310	−24,8275
	320	−24,4488
	330	−24,2035

where M represents the molar mass of C.I.Direct Black 80 (g/mol).

Then, 𝐾ad was determined according to equation :

𝛾

Activity coefficient determined based on ionic strength of the solution.

C_ref

Reference molar concentration.

According to the usual conditions, the reference concentration (𝐶_ref) is generally set at 1 mol/L, making 𝐾_ad identical to the Langmuir constant 𝐾_𝐿. However, this assumption can be adjusted according to the specific properties of the adsorbate. When the solubility of the dye in water is less than 1 mol/L, as is the case for C.I. Direct Black 80, whose solubility is around 30 g/L (i.e. 0.033 mol/L), it makes more sense to use this value as the reference concentration. Thus, 𝐶_ref has been replaced by the saturation concentration to more accurately reflect the system’s behavior. The activity factor (𝛾) was calculated using the Davis relationship⁶⁴, to adjust for the effect of the solution’s ionic strength on the adsorption equilibrium.

The standard free energy (ΔG°) was determined for each temperature from the following equation:

In parallel, the Van’t Hoff equation was used to determine ΔH° and ΔS°, from the slope and y-intercept of the straight line obtained by plotting ln 𝐾_ad versus 1/𝑇 (Fig. 14):

Fig. 14.

Plot: ln(K) vs. 1/T for determination of thermodynamic parameters of C.I. Direct Black 80 adsorption on AC1.

ΔG° : Standard Gibbs free energy (J/mol).

K_ad : Equilibrium constant of adsorption.

ΔH° : Standard enthalpy (J/mol).

R : Perfect gas constant (8.314 J/K.mol).

T : Absolute temperature (in Kelvin).

ΔS° : Standard entropy (J/K.mol).

The results are shown in Table 4. Negative ΔG° values, ranging from − 24.20 to −24.91 kJ/mol, indicate that the adsorption process is thermodynamically favorable at all studied temperatures. The moderate increase in ΔG° with temperature suggests that adsorption is less favorable at higher temperatures, which is typical of an exothermic process.

The negative enthalpy (ΔH° = −32.48 kJ/mol) in Table 4, confirms this exothermic character, and the low value of this enthalpy (< 40 kJ/mol) indicates that the mechanism is mainly physical, involving Van der Waals-type interactions and hydrogen bonds. Finally, the negative entropy (ΔS° = −25.02 J/mol-K) reflects a decrease in disorder at the solid-liquid interface, probably linked to the organization of the dye molecules as they attach to the active sites. This structuring is consistent with observations made by FTIR analysis, which showed the involvement of polar groups in the interaction.

These results confirm that the adsorption of C.I. Direct Black 80 on AC1 carbon is a physical, spontaneous and exothermic phenomenon, governed by weak but specific interactions between the surface functions of the biosorbent and the sulfonate groups of the dye.

Desorption studies

The desorption efficiency of the C.I. Direct Black 80 dye using a 0.1 M NaOH solution was evaluated over five consecutive cycles for both activated carbons (Fig. 15). The alkalinity of the solution induced deprotonation of the surface functional groups, conferring a negative charge on the adsorbent and promoting electrostatic repulsion towards the sulfonate groups of the dye, facilitating its release.

Fig. 15.

Desorption of C.I. Direct Black 80 from AC1 and AC2.

The results show a high level of desorption from the first cycle, with efficiency dropping from 93% to 80% for AC1, and from 94% to 82% for AC2 by the fifth cycle. This slight decrease over the cycles reflects good material stability and efficient regeneration of the active sites. The slightly higher performance of AC2 could be linked to a more developed porous structure, favoring better accessibility to adsorbed molecules.

These results confirm the value of using mild basic solutions such as NaOH to restore adsorption capacity without altering the structure of the material, in line with previous work on the desorption of anionic dyes by biosorbents⁶⁵.

Quantum chemical analysis of DB80 reactivity and interaction sites

DFT techniques with GGA functions provide a definitive model for understanding the global reactivity and potential active sites of the molecule DB80. By using this approach, it is possible to gain insight into the molecule’s electron acceptance and release capacities, which are crucial for comprehending the adsorption mechanism on the surface studied. Figure 16 shows the optimized molecular structures and boundary orbitals, HOMO, LUMO, Electrostatic Surface Potential (ESP) structure, and Fukui function indices. The DB80’s strong adsorption capacity and chemical reactivity are highlighted by the quantum chemical parameters presented in Table 5. The high (–5.062 eV) and positive indicate its ability to donate electrons, while the presence of lone pairs on oxygen, suffer and nitrogen atoms facilitates interaction with the metal surface. The DB80’s ability to absorb surface is further confirmed by its low hardness (0.649 eV) and high chemical softness value (1.540 eV). Furthermore, its low (–3.764 eV) indicates that it can accept electrons effectively, leading to better electrostatic interactions. Finally, the narrow (1.298 eV) and high electronegativity ( = 4.413 eV) underline the DB80’s enhanced reactivity, making it a promising corrosion inhibitor. Fukui function indices are a significant factor in determining the most reactive sites of a molecule and their donor and acceptor characteristics (Fig. 16) and Table 5. The findings reveal that atoms such as N(13), N(25), and N(24) have higher values, which suggest that they are the most probable centers for nucleophilic attack. On the other hand, atoms such as C(6), N(12), and N(17) have higher values, suggesting they have a stronger affinity for electrophilic interaction on the AC surface, thus demonstrating their crucial role in the adsorption process. The Mulliken charge distribution offers valuable information about the electronic structure and reactive tendencies of the investigated molecule. Among the carbon atoms, C(31) exhibits the largest positive charge (+ 0.308), suggesting its susceptibility to nucleophilic attack. In contrast, O(52) (–0.784) and O(53) (–0.819) display the most pronounced negative charges, indicating their potential as strong electron donors and possible coordination sites. Nitrogen atoms, including N(12) (–0.294) and N(17) (–0.164), also carry negative charges, which points to their capacity for proton acceptance through hydrogen bonding. Conversely, hydrogen atoms such as H(64) (+ 0.247), H(65) (+ 0.245), and H(66) (+ 0.245) represent electron-deficient sites with a propensity for electrophilic interactions. These findings are corroborated by the electrostatic potential (ESP) map (Fig. 16), which delineates regions of electrophilic and nucleophilic reactivity. Negative ESP regions (red areas) are primarily located around heteroatoms such as nitrogen and sulfonate groups, thereby increasing polarity and promoting adsorption onto activated carbon surfaces^66,67. Positive ESP regions (blue areas), on the other hand, are associated with electron-deficient domains near electrophilic centers, including hydrogen atoms bound to electronegative atoms or protonated functionalities^68–70. The complementary Mulliken and ESP analyses highlight the crucial role of donor–acceptor distribution and local electronic features in dictating the adsorption behavior of DB80, providing a clear rationale for its strong affinity toward activated biomass surfaces.

Fig. 16.

The Optimized geometry, along with the HOMO–LUMO orbitals, Mulliken charge, ESP distribution, and Fukui function indices of the molecule DB80.

Table 5.

The quantum chemical parameters of the DB80.

Parameters  (eV)	EHOMO	ELUMO	EA		ΔEgap		σ
DB80	−5.062	−3.764	5.062	3.764	1.298	0.649	1.540	4.413	6.831

Visualization of non-covalent interactions (RDG and NCI analysis)

To gain a more detailed understanding of the experimental observations and to fully analyze the interactions between the DB80 and the surface, NCI plots and Reduced Density Gradient (RDG) analyses were conducted. These investigations were performed using the Forcite module of Materials Studio, while the RDG and NCI isosurfaces were generated with the aid of Multiwfn and visualized using Gnuplot and VMD. Analyzing these diagrams can provide insight into the weak forces between the DB80 molecules and the AC surface, including steric hindrance, van der Waals forces, and hydrogen bonding. The RDG isosurfaces and scatter plot display the values of the sign(λ2)ρ function using different colors, which effectively reveals the types and strengths of NCI⁴⁵. Electron density (ρ) plays a significant role in determining the strength of these interactions, while the sign of λ2 aids in identifying interactions that are bonding (λ2 < 0) and non-bonding (λ2 > 0). Blue represents hydrogen bond interactions that have a larger negative sign(λ2)ρ, while green represents Van der Waals effects that have a smaller sign(λ2)ρ, and red represents steric repulsion interactions that have a more positive sign(λ2)ρ⁷¹. As shown in Fig. 17, the intense blue areas confirm the formation of strong hydrogen bonds between DB80 and the AC surface, while red regions suggest steric effects. Thus, the adsorption of DB80 can be attributed to a synergistic contribution of hydrogen bonding and van der Waals forces, in agreement with the experimental findings.

Fig. 17.

NCI and RDG analyses reveal optimized and most stable adsorption geometries for the DB80 on the AC surface.

Comparative study

Table 6 reports a comparison of the adsorption capacities of various activated carbons derived from different biomasses. Materials AC1 and AC2, derived from Zygophyllum gaetulum, have capacities of 146.95 and 155.94 mg/g respectively. These results, obtained under optimized conditions, show that this biosorbent is competitive and deserves to be considered as a potential alternative for the treatment of dyed wastewater.

Table 6.

Comparative study of dye adsorption performance on activated carbons derived from plant sources.

No	Activated Carbon source	Dose (g.L−1)	Time (min)	C0 (mg.L−1)	qm (mg/g)	Reference
1	Brewing cereals residues	0.5	420	35	63.1		57
2	Waste coffee grounds	0.3	40	20	151.51		72
3	Monguba shell	0.1	60	100	50.00		73
4	Sawdust	0.3	120	30	75.81		74
5	Zygophyllum Gaetulum (AC1)	1	60	100	146.95		Current study
6	Zygophyllum Gaetulum (AC2)	1	60	100	155.94		Current study

For comparison, in our previous study using the raw plant material as a biosorbent, a maximum capacity of 163.23 mg·g⁻¹ was obtained at a lower initial concentration (50 mg·L⁻¹) and a biosorbent dosage of 20 g·L⁻¹, with equilibrium reached within 2 h⁶⁰. In contrast, the activated carbons achieved comparable removal efficiency using a much lower adsorbent dosage (1 g·L⁻¹) and a shorter equilibrium time (1 h). This clearly highlights the added value of chemical activation in significantly reducing both the required contact time and the adsorbent mass, while maintaining high performance. Overall, this transformation of an abundant plant waste into activated carbon offers a highly ecological and economically viable approach, enabling more efficient, faster, and resource saving dye removal key factors for sustainable large scale wastewater treatment applications.

Conclusion

This study highlighted the effectiveness of activated carbons prepared from the discarded stems of Zygophyllum gaetulum, an unvalued plant biomass, for the removal of the anionic azo dye C.I. Direct Black 80 in aqueous solution. Chemical activation with H₃PO₄ produced a material with a well-developed porous structure enriched in oxygenated functional groups, as confirmed by XRD, SEM, FTIR, TGA, DTG and DSC analyses, which also revealed its amorphous nature, high surface reactivity and good thermal stability.

High performance was achieved under cost-effective operating conditions, with an optimum dose of 1 g allowing over 96% removal, and a contact time of just one hour required to achieve over 95% efficiency. These results underline the practical value of these materials for rapid, low-cost treatment. Fitting the experimental data to the pseudo-first-order model and Langmuir isotherm indicates an adsorption mechanism dominated by physisorption on homogeneous sites. Thermodynamic analysis confirms a spontaneous, exothermic process, consistent with the surface interactions identified. These results are in line with the surface characterizations, which revealed functional groups and structural features suitable for weak physical interactions, such as hydrogen bonding and Van der Waals forces.

In addition, desorption cycles demonstrated good adsorbent regenerability, with efficiency maintained above 80% after five reuses.

Finally, DFT calculations provided valuable insights into the electronic properties and reactive sites of the dye molecule, supporting the proposed adsorption mechanism and confirming the predominance of weak physical interactions.

These results position AC1 and AC2 as sustainable, high-performance and economically viable solutions for industrial water treatment, while at the same time valorizing a hitherto untapped plant biomass.

Author contributions

C. Haoufazane and S. Jodeh: Writing – original draft; **R. Tihmmou and R. Salghi** : Visualization, Project administration, Investigation; B.E. Kartah: Writing – review & editing, Supervision, Software, Project administration. K. Azzaoui: Resources. B. Hammouti: Formal analysis. F. Zaaboul: Methodology. H. El Monfalouti: Formal analysis.

Funding

NA.

Data availability

The data presented in this study are available upon request from the corresponding author.

Declarations

Competing interests

The authors declare no competing interests.

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.