The adsorbent material used in this study is activated carbon made from neemseed cake. The neemseed cakes were washed several times with tap water before being rinsed with distilled water; they were then placed in an oven at 105˚C for one hour. After drying, the samples obtained were carbonized in an oven at 400˚C. A mass of the charcoal obtained was brought into contact with orthophosphoric acid in a 200 mL beaker with a mass ratio equal to 1. The mixture was stirred for one hour, then neutralized with sodium carbonate (Na₂CO₃). The mixture thus obtained was rinsed with distilled water to remove traces of remaining acid or base and then placed in an oven at 105˚C for one hour. The material obtained constitutes activated carbon based on neem seed cake (Figure 1).

(a) (b)

Figure 1. (a) Neem seed cakes after washing; (b) Activated carbon obtained from neem cake.

All experiments were carried out in 100 mL beakers. For each test, a certain mass of activated carbon was added to the crystal violet. The effectiveness of the activated carbon is determined by its adsorption capacity, denoted as q_t and expressed in (mg/g), or by its adsorption rate, denoted as R and expressed in (%), given respectively by the following relationships.

With:

C₀: initial solute concentration (mg/L);

C_e: residual concentration of the solute at equilibrium (mg/L);

V: volume of the solution (L);

m: mass of the adsorbent (g).

The adsorption experiments of crystal violet onto activated carbon derived from neem seed cakes were conducted using UV-visible spectrophotometry to determine the dye concentration before and after adsorption, at a maximum absorbance wavelength of 585 nm. A comprehensive set of parameters was investigated to optimize the adsorption process and understand the underlying mechanisms. These parameters include:

Effect of particle size of the activated carbon, which influences the available surface area and porosity; Effect of adsorbent mass, to evaluate the relationship between the quantity of material and the adsorption efficiency; Effect of contact time, to assess the kinetics of adsorption and determine the equilibrium time;Effect of stirring speed, which affects external mass transfer and the homogeneity of the system;Effect of solution volume, to analyze the dilution effect and adsorption capacity per unit volume;Effect of initial dye concentration, to understand the influence of the concentration gradient and site saturation; Effect of pH, given its impact on the ionization state of the adsorbate and the surface charge of the adsorbent; Effect of temperature to evaluate the thermodynamic behavior of the adsorption process (endothermic or exothermic nature).

3.1.1. Effect of Coal Particle Size

Five types of absorbent particle sizes were used to study their effects on the adsorption of crystal violet (CV). The results obtained are shown in Figure 2.

Figure 2. Evolution of the adsorption rate as a function of particle size: m = 2 g, V = 50 mL, Ts = 60 min, [CV] = 5 mg/L, Ω = 750 rpm.

The results obtained show that the adsorption rate increases slightly with increasing particle size (from 87.6% to 89.9%) for a size range from 160 to 500 µm. Beyond 500 µm, a reduction in the rate was observed, down to 69.96%. The best rates are therefore obtained with small particle sizes. This is due to the fact that small particles have much larger specific surface areas than large particles ([38]; [31]).

3.1.2. Effect of Adsorbent Mass

Figure 3 describes the evolution of the adsorption efficiency of crystal violet and the adsorption capacity of the material as a function of the mass of the adsorbent.

Figure 3. Effect of mass on the adsorption of crystal violet: [CV] = 5 mg/L, V = 50 mL, Ts = 60 min, Ω = 750 rpm.

The adsorption rate of crystal violet increases significantly with the mass of activated carbon. In fact, the rate increases until it reaches a value of 90.40%, from which it stabilizes. This increase from 44.53% to 90.40% for masses ranging from 0.5 to 2 g could be attributable to the increase in the specific surface area of the adsorbent and the availability of a greater number of adsorption sites. Beyond 2 g, the number of accessible free sites becomes stable ([1]; [7]; [29]).

3.1.3. Effect of Contact Time

Figure 4 represents the evolution of the adsorption capacity of the absorption material and that of the removal rate of crystal violet as a function of contact time.

Figure 4. Evolution of the adsorption rate as a function of contact time: [CV] = 5 mg/L, V = 50 mL, m = 2 g, Ω = 750 rpm, particle size = 500 μm.

The results obtained in this study show that activated carbon based on neem seed cake is an effective adsorbent for eliminating CV dye from aqueous solutions because, in a very short time (20 minutes), the rate is around 80%. Indeed, it increases rapidly during the first 40 minutes depending on the contact time. Beyond this value, it remains constant. This can be explained by molecular diffusion ([33]; [36]; [9]) towards the adsorption sites until an adsorption equilibrium is reached, where all sites become occupied.

3.1.4. Effect of Stirring Speed

The results of the variation in the adsorption rate of crystal violet by CA as a function of the stirring speed are shown in the following Figure 5.

Figure 5. Effect of stirring speed on the adsorption of crystal violet: [CV] = 5 mg/L, V = 50 mL, m = 2 g, Ts = 60 min, particle size = 500 µm.

Increasing the stirring speed leads to a slight decrease in the adsorption rate. The rate values found are approximately equal to 90%. This means that it has little influence on the adsorption rate. This small reduction observed could be due to the fact that for very high stirring speeds ([41]), the crystal violet molecules will have difficulty attaching to the activated carbon, but also due to the agglomeration of the support particles ([6]; [13]; [5]).

3.1.5. Effect of Solution Volume

The results of the volume variation on the retention of crystal violet by the CA are represented in the following Figure 6:

Figure 6. Evolution of the adsorption rate as a function of the volume of the solution. [CV] = 5 mg/L, m = 2 g, Ts = 60 min, Ω = 750 rpm, particle size = 500 µm.

These results reveal that the adsorption rate increases from 88.93% to 99.04% for volumes ranging from 50 to 200 mL. This increase is much greater with the 100 and 150 mL volumes. This can be explained by the fact that the increase in volume causes a dispersion of the activated carbon particles in solution ([8]; [45]), facilitating their encounter with the crystal violet molecules.

3.1.6. Effect of Initial Concentration

The results of the variation in the adsorption rate of crystal violet by CA as a function of the initial concentration are shown in Figure 7.

It shows that an increase in the initial concentration of CV leads to an increase in the quantity adsorbed by CA. Indeed, if the initial concentration of the crystal violet becomes greater, there will consequently be more molecules that will diffuse towards the sites of the particles of the support, so retention becomes greater. Saturation is reached for a concentration of 8 g/L, corresponding to an adsorption rate of 93.53%. This can therefore be explained by the occupation of all the adsorption sites of this CA ([19]; [17]). Moreover, the adsorption mechanism of crystal violet (CV) onto activated carbon involves electrostatic interactions, π–π stacking, and van der Waals forces. At low concentrations, the active sites are readily accessible. At higher concentrations, competition between molecules increases until saturation is reached. Adsorption then reaches a plateau corresponding to the maximum capacity of the material ([24]).

Figure 7. Effect of initial concentration on the adsorption of crystal violet: V = 50 mL, m = 2 g, contact time = 60 min, speed = 750 rpm, particle size = 500 μm.

3.1.7. Influence of the pH of the Solution on Adsorption

Figure 8 shows the test results on the effect of pH versus CV removal in solution on the absorption material.

Figure 8. Evolution of the adsorption rate as a function of the pH of the solution. [CV] = 5 mg/L, V = 50 mL, m = 2 g, contact time = 60 min, speed = 750 rpm, particle size = 500 µm.

Acidic conditions (pH < 4.16) resulted in adsorption rates ranging from 72.08% to 90%. This behavior could be attributed to structural modifications of the crystal violet molecules in strongly acidic media, as suggested by the visible change in color of the solution during the process. At higher pH values (between 4.16 and 8), similar trends were observed, although the adsorption efficiency varied depending on the degree of ionization of the dye and the surface charge of the adsorbent ([36]; [18]; [25]). These findings confirm that pH has a direct and significant impact on the adsorption capacity. Consequently, optimal removal of crystal violet should preferably occur under near-neutral pH conditions, where the dye remains stable and the surface charge of the activated carbon favors adsorption ([15]; [43]).

3.1.8. Influence of Temperature

The effect of temperature on the adsorption properties of the dye (CV) on activated carbon is shown in Figure 9.

Figure 9. Evolution of the adsorption rate as a function of temperature: [CV] = 5 mg/L, V = 50 mL, m = 2 g, contact time = 60 min, speed = 750 rpm, particle size = 500 µm.

The adsorption rate exhibits only slight variations with temperature, suggesting that temperature has a limited influence on the retention of crystal violet under the conditions tested. This weak dependence may indicate that the adsorption process is not strongly thermally activated and that physical interactions dominate the adsorption mechanism ([46]; [14]).

3.2.1. Freundlich Modeling

The adsorption results (Figure 10) are modeled here based on Freundlich assumptions. The model equation is ([22]).

Figure 10. Freundlich model for CV adsorption by activated carbon.

The modeling is shown in the figure below:

3.2.2. Langmuir Modeling

We also used the Langmuir isotherm to analyze the results. The Langmuir equation is:

The Langmuir linear is:

The modeling of the experimental results by the isotherm, exploited in its linear form, is represented in Figure 11.

Figure 11. Langmuir model for CV adsorption by activated carbon.

The values of the Langmuir and Freundlich parameters, as well as the correlation coefficients, are mentioned in Table 1.

Table 1. Parameter values of the Langmuir and Freundlich models.

According to the graphic representation of each isotherm and the correlation factors, as well as the constants of each model, we noticed that the Freundlich isotherm seems to be best suited to the modeling of the adsorption isotherm of crystal violet by the adsorbent material (linear regression coefficient close to 1).

The kinetic data obtained during the adsorption process of crystal violet on the adsorbent material were analyzed by two kinetic models: pseudo-first order kinetics and pseudo-second order kinetics. The plotting linear equations are given respectively by Figure 12 and Figure 13.

Figure 12. Plot of the linear form of the pseudo-first-order kinetic model.

Figure 13. Plot of the linear form of the pseudo-second-order kinetic model.

Table 2 groups together the values of the rate constants k₁ and k₂, the adsorption capacities, and the determination coefficients of the two kinetic models.

Table 2. Kinetic parameters of the adsorption of crystal violet by the adsorbent.

These results show that the adsorption of crystal violet by activated carbon based on neem seed cake follows a pseudo-second-order kinetic law because the coefficient of determination R² given by the second-order model is much higher than that of the pseudo-first-order. Furthermore, the adsorption capacity calculated from the pseudo-second-order kinetic model (0.1179 mg/g) is in good agreement with the adsorption capacity determined experimentally (0.1131 mg/g).

The thermodynamic study informs us about the feasibility and spontaneous nature of the adsorption process. Parameters such as free energy (ΔG˚), enthalpy change (ΔH˚), and entropy change (ΔS˚) can be estimated from equilibrium constants at different temperatures. They were determined from the experimental data, in order to evaluate and understand whether the adsorption processes studied were spontaneous or not; exothermic or endothermic. We have the following formulas:

K c : The distribution constant of the solute in the adsorbent and in the solution;

q e : The quantity adsorbed at equilibrium (mg/g);

C e : Solute concentration;

R: the ideal gas constant (8.314 J·mol⁻¹·K⁻¹);

T: absolute temperature (K).

The plot of ln K c versus 1/T (Figure 14) is a line with slope (ΔH˚/R) and ordinate at the origin (ΔS˚/R).

Figure 14. Representation of ln(K_c) as a function of temperature (1/T).

Table 3. Thermodynamic parameters of the adsorption of crystal violet on CA based on neem seed cakes.

The thermodynamic parameters, namely ΔH˚, ΔS˚, and ΔG˚, were determined from the experimental results obtained at different temperatures (Table 3).

The negative value of ΔH˚ shows that the adsorption of crystal violet on the adsorbent is an endothermic process. It is a physical type of adsorption because ΔH˚ < 40 kJ/mol. The negative value of the entropy change indicates that crystal violet is more organized at the solid–liquid interface than in the liquid phase. The positive values of ΔG˚ (free enthalpy) indicate that the adsorption of crystal violet on the adsorbent is a non-spontaneous process. Increasing the temperature does not favor the adsorption of this dye.

Figure 15 presents a comparative analysis of copper adsorption using two different masses of activated carbon (2 g and 4 g), each calcined at two distinct temperatures (400˚C and 700˚C). All experiments were conducted under identical conditions.

Figure 15. Effect of calcination temperature on adsorption Rate (%).

Activated carbon at 700˚C has better adsorption rates (97.37% and 97.53%) than that calcined at 400˚C (86.69% and 94.52%) for the two masses of carbon (2 and 4 g, respectively). This could be explained by the fact that for the carbon calcined at 400˚C, there remained a quantity of degradable organic matter. As the calcination temperature increases, this organic matter degrades more and more, leading to the existence of a significant number of active sites on the carbon. We can therefore conclude that the calcination temperature of the activated carbon influences the adsorption of copper.

Domestic wastewater is treated with activated carbon based on neem seed cake following pretreatment and decantation over a few days. The COD decreased from 750 to 296 mg/L, representing a reduction rate of 60.5%.