Adsorption of cationic dye onto Raphanus seeds: optimization, adsorption kinetics, thermodynamic studies

The effect of pHEvaluate the pH at the point of zero charge (pzc) for the utilized adsorbent. The pH of the solution plays a crucial role in the adsorption process, particularly in the extraction of methylene blue (MB) from an aqueous solution. It influences surface charges, ionization degree, and Adsorbate specification of the adsorbent. Initial parameters for MB adsorption, including a concentration of 10 ppm and a contact time of 90 min, were established to achieve equilibrium. Equilibrium studies, conducted at varying pH values from 2 to 8, aimed to comprehend the pH impact on MB adsorption. The pH of solutions was measured using the HANNA pH 211-Romania pH-meter which clear in Fig. 6.Figure 6pH effect regarding MB adsorption onto Raphanus seeds solid residual (ACRS) (initial concentration = 10 ppm, Raphanus seeds solid residual (ACRS) dose = 0.5 g/250 ml, stirring speed = 300 rpm T = 25 ℃ and time = 90 min).The observed increase in pH correlated with a higher % removal16), attributed to the acidic state of the bioadsorbent. As the pH increases, the adsorbent’s surface becomes negatively charged, leading to an increased adsorption capacity17, reaching its peak at pH  7. Results demonstrate a significant rise in the percentage removal of methylene blue (MB) from 82.7% to 99.4% as the solution’s pH increases from 2 to 7. Beyond pH 7, the % removal of MB decreases with an increase in the solution’s pH, indicating a reduction in adsorption capacity. The maximum removal obtained at pH  7 was 99.4%, as shown in Fig. 7.Figure 7pH effect regarding MB adsorption.Effect of initial concentrationAn investigation into the effect of initial methylene blue MB concentration on its removal onto Raphanus seeds solid residual (ACRS) was performed under controlled conditions. The conditions were maintained at a temperature of 25 °C, a pH of 7, a contact time of 90 min, and an agitation speed of 300 rpm. adsorbent dose = 0.5 g/250 ml for different MB initial concentrations (10,20,30,40and 50 ppm), As depicted in Fig. 8, the percentage removal of methylene blue (MB) decreases with an increase in the initial MB concentration. At low concentrations, MB is adsorbed on the vacant sites of the adsorbent surface and these places are saturated and filled by increasing the concentration. When methylene blue’s initial concentration is low, an abundance of active spots on the adsorbent’s surface become available for dye adsorption. However, if the initial concentration of MB dye is increased, the number of moles of the MB dye is higher than the number of vacant sites. Therefore, the available sites are quickly saturated and the dye removal rate decreases4.Figure 8Effect of % removal of Raphanus seeds solid residual (ACRS) (pH  7, Raphanus seeds solid residual (ACRS) with time at different MB initial concentration dose = 0.5 g/250 ml, stirring speed = 300 rpm T = 25 ℃ and time = 90 min).The result reveals that as the initial concentration of methylene blue (MB) increased from 10 to 50 ppm, the percentage removal of MB decreased from 99.4 to 50.9%, as demonstrated in Fig. 918.Figure 9Effect MB initial concentration on adsorption.The effect of the adsorbent doseThe impact of the adsorbent dose on the removal of methylene blue (MB) was evaluated at an initial MB concentration of 10 ppm, 90 min contact, and temperature of 25 °C. Different adsorbent doses (ACRS = 0.1, 0.3, 0.5, 0.7, and 1 g/250 ml). According to Fig. 10, the percentage removal of methylene blue (MB) increases with an increase in the amount of adsorbent used. This result is expected because a rise in the amount of adsorbent results in increased surface area with available adsorptive sites. at a fixed initial concentration of sorbate i.e. the increased accessibility increases the number of exchangeable sites or the surface area insured which enhanced the uptake of MB18,19,20.Figure 10Effect of the adsorbent dose of Raphanus seeds solid residual (ACRS) on the % removal MB.The outcomes revealed that the uptake or percentage removal of methylene blue (MB) increased from 63.6 to 99.4% as the adsorbent dose increased from 0.1 g/250 ml to 1 g/250 ml, as illustrated in Fig. 11Figure 11Effect of adsorbent dose % removal Methylene Blue.The effect of temperatureFigure 12 demonstrates the impact of temperature on the percentage removal of methylene blue (MB) from wastewater using (ACRS). Different temperatures (25, 30, 35, 40, and 45 °C) with an initial concentration of 10 ppm of MB were used in this study in the presence of (1 g/250 ml) for (ACRS). The figure indicated that within 90 min, the removal efficiency increased as the temperature increased because Higher temperatures facilitated elimination of methylene blue (MB) by promoting the adsorption at the coordination sites of the adsorbent. This was due to the speeding up of some previously slow steps and the creation of additional activation sites on the adsorbent surface21,22,23.Figure 12Effect of temperature on % removal of MB on (ACRS), initial MB concentration 10 ppm, adsorbent dose: 1 g/250 ml stirring speed 300 rpm, contact time 90 min and pH  7.The result reveals that the uptake of a Fig. 13 demonstrates that the percentage removal of methylene blue (MB) increased from 97.22 to 99.64% with increasing the temperature from 25 to 45 °C.Figure 13The effect of temperature on % the removal of the MB.RSM analysis for optimization of % removal of the methylene blue from aqueous solutionThe study of the removal of methylene blue was performed using a Box–Behnken factorial design-based response surface methodology (BBD). The initial step involved investigating the influence of individual variables through single parameter experiments. The levels of selected parameters were then used in a series of experiments, the results of which were recorded and analyzed using statistical plans and response procedures. The details of the selected the parameters’ levels are presented in Table 1, while the results of the statistical analysis are outlined in Table 2Table 2 Evaluation of operational factors in the Box–Behnken factorial design for Methylene Blue removal for optimization of Methylene Blue removal percent (%) using activated carbon from Raphanus seeds residual.The results demonstrated that the % of removal of methylene blue in varying degrees of the four variables were different, due to the impact of various variable levels on the % of removal of MB. As indicated by the results in Table 2, the highest MB removal rate was achieved in test experiment 14, reaching a value of 79%. Additionally, the small discrepancy between the actual and predicted MB removal percentages reflects the high model accuracies24,25. As a result, the study employed a quadratic model of analysis of polynomial equations and the second order to establish the connection between the key elements and the response variable, which is the percentage removal of Methylene Blue, as the optimal model. The validation of the model was performed using ANOVA analysis and the results of variance analysis are presented in Table 3. Multivariate Regression Analysis of the experimental results resulted following second-order polynomial equation, represented as Equation (Eq. 3). The equation gotten through analysis of the experiment’s outcomes using multiple regression (Eq. 3) is a second-order polynomial equation that can explain the % removal of Methylene Blue for the significant variables. The linear parameters in the equation are represented by the variables A, B, C, and D.; second-order parameters of the model were A2, B2, C2, and D2, along with the interaction parameters AB, AC, AD, BC, BD, and CD:$${\text{Y}} = { 16}.0{736} – {34}.{\text{313A}} + 0.{\text{4519B}} – {1}.{\text{5233C}} + {31}.{\text{6555D}} + 0.{\text{2123A}}^{{2}} – 0.00{\text{42B}}^{{2}} + 0.0{\text{255C}}^{{2}} – {33}.{35}0{\text{8D}}^{{2}} + 0.{\text{1121AB}} – 0.00{\text{54AC}} + {6}.{\text{2263AD}} – 0.00{\text{82BC}} + 0.{3}0{\text{89BD}} – 0.{\text{3572CD}}$$
(3)
where the response variable is represented by “Y”, which is the amount of Methylene Blue removed, and “A”, “B”, “C”, and “D” are coding values for the significant test variables, including pH, time (min), the concentration of Methylene Blue (in parts per million), and the adsorbent dose (in grams per 250 ml), respectively.Table 3 ANOVA results for % removal of Methylene Blue.The importance of the model was assessed using the F-value and p-value parameters. a significant F-value and a small p-value (less than 0.05) suggest that the model accurately predicts the experimental results. In this case, the F-value was 16.21027 and the p-value was 0.000003, indicating a completely significant model. Table 3 revealed that pH, contact time (minutes), initial methylene blue concentration (ppm), and adsorbent dose (g/250 ml) were significant factors affecting the % removal of methylene blue, with initial concentration being the most effective factor.The predictor variables have been plotted between the X and Y axes, with the response variable displayed on the Z axis, in order to identify the important impact of interactions among the various predictors (Fig. 14A–F) on % removal of MB using activated carbon from Raphanus seeds residual. The response surface’s 2D contour plot have been generated in three dimensions for the pairwise combinations of the four factors (AB, AC, AD, BC, BD, CD), with the third factor kept at its central point level (0). As illustrated in Fig. 2E, the interactive effect of the dose and concentration resulted in the highest response.Figure 14Response surface methodology of % removal of MB using activated carbon from Raphanus seeds residual: (A) time (min)/pH, (B) effect of concentration of MB (ppm)/pH, (C) adsorbent dose (g/250 ml)/ pH, (D) effect of concentration of MB (ppm)/time(min), (E) adsorbent dose (g/250 ml)/time (min), (F) adsorbent dose (g/250 ml)/concentration of MB (ppm).Figure 14A, D, E demonstrates some variables (effect of contact time, pH, Conc. of MB, and Adsorbent dose on % removal of MB using activated carbon from Raphanus seeds residual which increased with lengthening the time from 0 to 90 min due to increasing the time, networks of the adsorbent where fast spread at the early stage and then reach to equilibrium after 90 min. So, it was much easier for MB molecules at the first time to penetrate inside the activated carbon from Raphanus seeds residual and include the adsorption sites showed an initial increase with time, however, with further progression, the adsorbent’s surface-active groups began to decrease and the percentage removal of MB stabilized26.CharacterizationThe raw seeds, known as the residual solid from Raphanus seeds (ACRS), serve as a crucial component in the study. The characterization of these raw seeds is imperative for understanding their properties and potential applications.The physical and chemical properties of ACRS were analyzed to provide comprehensive insights into its composition. Commonly assessed characteristics include morphology, particle size, surface area, elemental composition, and structural features. Techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and elemental analysis are commonly employed for characterization. Morphological studies using SEM reveal the surface topography, size, and shape of the ACRS particles. XRD provides information about the crystalline structure, while FTIR helps identify functional groups present in the material. Elemental analysis assists in determining the elemental composition, providing data on the presence of key elements.This characterization is essential for establishing a baseline understanding of the raw seeds’ properties, which, in turn, contributes to the interpretation of the adsorption process and the effectiveness of ACRS as an adsorbent.Surface studyThe surface area calculated by BET equation of the sample was 9.25 m2/g. The surface morphology studied by SEM (Fig. 15) shows that the sample is nonporous. It obvious, the structure of the sample has a small surface area. This is attributed to the simple and inexpensive preparation method that matches the environmental conditions, which is the direct burning of the sample at 400 degrees. The observed surface area is likely due to the small voids formed by the aggregation of small pieces of solid matter.Figure 15Scanning electron micrograph (SEM).X-ray diffraction analysis (XRD)Figure 16 presents the X-ray diffraction (XRD) pattern obtained from the radish seeds residual sample, which was exposed to Cu Kα radiation (λ = 1.54 Å) with a 2θ scanning range of 5 to 80 degrees. The broad peak observed at 2θ = 21.10 suggests that the radish seeds residual powder is amorphous in nature27. Additionally, the XRD pattern reveals that the crystalline structure of the radish seeds residual sample is disordered.Figure 16XRD pattern of Raphanus seeds residual powder.Fourier transform infrared spectraThe FTIR spectrum of the sample has been recorded between 4000 and 400 cm−1 at resolution (0.1 cm−1)28. Figure 17 displays the FT-IR spectra of the natural radish seeds residual, with the vibrational assignments presented in Table 1. The broad band with intense intensity at 3414 cm−1 is a result of the hydroxyl (OH) group’s stretching vibrations. The bands for the aliphatic C–H stretching occur between 2851 and 2920 cm−1. The stretching of the alkyne group is observed at 2207 cm−1, and the stretching of the carbonyl group is seen at 1615 cm−1. The stretching mode of the C=O group can be found at 1112 cm−1. FTIR spectral data indicate the presence of flavonoids and polyphenols in Radish Seed extract (Table 4). All of the aforementioned groups in the FT-IR analysis facilitate MB adsorption due to the formation of a physical bond.Figure 17FT‒IR spectra of natural Raphanus seeds residual.Table 4 FT‒IR assignments of natural Raphanus seeds residual.Adsorption of kineticsPseudo-first order modelThe pseudo-first-order equation explains adsorption rate. Lagergren (1898) proposed a model of pseudo–first order kinetic which was given by Eq. (2):$$\text{Ln }\left({\text{q}}_{\text{e}}-{\text{q}}_{\text{t}}\right)=\text{Ln}\left({\text{q}}_{\text{e}}\right)-{\text{k}}_{1}\text{t }$$
(4)
the form in Eq. (4) with boundary conditions of t = 0, qt = 0 and t = t$${\text{q}}_{\text{t}}=\frac{({\text{C}}_{0}-{\text{C}}_{\text{t}})\times \text{v}}{\text{m}}$$
(5)
$${\text{q}}_{\text{e}}=\frac{({\text{C}}_{0}-{\text{C}}_{\text{e}})\times \text{v}}{\text{m}}$$
(6)
The plot of time (t) versus the natural logarithm of (qe − qt) in Fig. 18 displays a linear relationship. The parameters qe and k1 can be calculated from the intercept and slope of the graph, respectively29.Figure 18Pseudo-first order kinetic fit for MB plots for the adsorption onto Raphanus seeds solid residual (ACRS) at (initial concentration = 10 ppm, Raphanus seeds solid residual (ACRS) dose = 0.5 g/250 ml, stirring speed = 300 rpm, T = 301 K and time = 90 min, pH  7).Pseudo-second order modelThe pseudo-second order kinetic rate can be determined using the following Eq. (7):$$\frac{{dq}_{t}}{dt}={k}_{2}{\left({q}_{e}-{q}_{t}\right)}^{2}$$
(7)
where k2 indicates the rate constant for the pseudo second order adsorption.(mg g−1 min−1). The Eq. (8) can be derived from Eq. (7) variables first be separated, then integrated, under the conditions (qt = 0 at t = 0 and qt = qe at t = t) yielding a linear expression that describes the adsorption kinetics in a linearized integral form20:$$\frac{\text{t}}{{\text{q}}_{\text{t}}}=\frac{1}{{\text{k}}_{2}{\text{q}}_{\text{e}}^{2}}+\frac{1}{{\text{q}}_{\text{e}}}\text{t}$$
(8)
The integral form represented by Eq. (8) that indicated the ratio of time over the adsorbed amount of MB (t/qt) should be a linear function of time, as demonstrated in Fig. 19. Corresponding correlation coefficient values (R2) indicate that the pseudo-first-order model is better obeyed than the pseudo-second-order model. This is because the R2 value for the pseudo-first-order model is slightly higher than the R2 value for the pseudo-second-order model.Figure 19Pseudo-second order kinetic for the adsorption of MB onto Raphanus seeds solid residual (ACRS) at (initial conc. = 10 ppm, Raphanus seeds solid residual (ACRS) dose = 0.5 g/250 ml, stirring speed = 300 rpm, T = 301 K and time = 90 min, pH  7).Weber and Morris modelThe Weber-Morris model, which is also known as the model of intra-particle diffusion, is of significance in the field of liquid systems as it determines the rate of adsorption. Equation (9) provides a general representation of the kinetics involved in this model, where the intercept is a direct function of mass transfer across the boundary layer and the exponent is expected to have a value of 0.5$${q}_{t}= {K}_{m}{t}^{0.5}+C$$
(9)
where: km is the intra- particle diffusion rate constant (mg/g min1/2).The plot of qt versus t1/2 in Fig. 20 demonstrates a straight line with a slope (km) and an intercept (C). The value of C represents an approximation of the boundary layer thickness, with a greater value indicating a thicker boundary layer.Figure 20The Weber and Morris model for MB adsorption onto Raphanus seeds solid residual (ACRS) at (initial concentration = 10 ppm, Raphanus seeds solid residual (ACRS) dose = 0.5 g/250 ml, stirring speed = 300 rpm, T = 301 K and time = 90 min, pH  7).In conclusion, the kinetic modeling of methylene blue (MB) adsorption onto Raphanus seeds solid residual (ACRS) was investigated using various kinetic models, namely the pseudo-first order, pseudo-second order, and Weber-Morris models.For the pseudo-first-order model, Lagergren’s kinetic equation was applied, revealing a linear relationship between the natural logarithm of (qe − qt) and time (t). The calculated parameters qe and k1 were obtained from the intercept and slope, respectively. However, this model may not be the most accurate representation, as indicated by the correlation coefficient (R2) values.The pseudo-second-order model, described by the equation dq_t/dt = k2(qe − qt)2, was employed to assess the kinetic rate. The linearized integral form demonstrated that the pseudo-first-order model better adheres to the experimental data, with higher R2 values compared to the pseudo-second-order model.Furthermore, the Weber-Morris model, focusing on intra-particle diffusion, was utilized. The linear plot of qt versus t1/2 revealed valuable insights into the mass transfer mechanism. The slope (km) and intercept (C) provided information about the intra-particle diffusion rate constant and boundary layer thickness, respectively.Overall, the comparison of these kinetic models helps elucidate the dominant mechanisms governing the adsorption process. The pseudo-first-order model demonstrated a closer fit to the experimental data, suggesting its appropriateness for describing the kinetics of MB adsorption onto ACRS.The adsorption of MB onto ACRS was studied under conditions of initial concentration of 10 ppm, contact time of 90 min, agitation rate of 300 rpm, and temperature of 25 °C. The results of the kinetics models and related parameters were calculated and presented in Table 5.Table 5 Kinetic models parameters and the other parameters for adsorption of (MB) onto Raphanus seeds solid residual (ACRS) at initial concentration of (ACRS).Thermodynamic parametersThe adsorption equilibrium data obtained at different temperatures were used to evaluate the important thermodynamic properties, including the standard Gibbs free energy (ΔG°), standard enthalpy change (ΔH°), and standard entropy change (ΔS°). The standard Gibbs free energy of the MB adsorption process was calculated using Eq. (8).30$$\Delta {G}^{0}=-RTln{K}_{e}$$
(10)
The adsorption equilibrium constant (Ke) can be determined for any temperature using Eq. (11).$${K}_{e}=\frac{{q}_{e}}{{C}_{e}}$$
(11)
where Ce (mg/L) represents the equilibrium concentration of MB in the solution, R stands for the gas constant (8.314 J/mol·K), T denotes the absolute temperature in Kelvin, and qe (mg/g) represents the amount of MB adsorbed from the solution at equilibrium.$$\text{Ln }{\text{K}}_{\text{e}}= -\left(\frac{{\Delta \text{H}}^{0}}{\text{RT}}\right)+\left(\frac{{\Delta \text{S}}^{0}}{\text{R}}\right)$$
(12)
Equation (12) displays ΔS° and ΔH°, which were obtained from the intercept and slope, respectively, of the Van’t Hoff plot of 1/T versus ln(keq) for MB, as depicted in Fig. 21.Figure 21Van̕ t Hoff̓ s plot of the natural logarithm of the adsorption equilibrium constant at initial conc = 10 ppm, pH  7, Raphanus seeds solid residual (ACRS) dose = 0.5 g /250 ml and 300 rpm, contact time 90 min at different temperature.Table 6 presents the values of ΔG°, ΔH°, and ΔS°, which illustrate the energetic properties of the divalent MB exchange. The positive ΔH° value suggests a high energy demand for the process, while the positive ΔS° value indicates a favorable interaction between the adsorbate and adsorbent. On the other hand, a negative ΔG° value implies that the adsorption process is feasible and spontaneous31,32,33,34.Table 6 Thermodynamic characterization of MB adsorption at concentration 10 ppm, remove by (ACRS).Isothermal modelsAnalysis of equilibrium data is a fundamental aspect in the evaluation of the maximum adsorption capacity of adsorbents. This analysis plays a crucial role in the determination of this capacity. Furthermore, it is critical to formulate an equation that accurately captures the experimental results, as this equation can then be utilized for design purposes. The Freundlich and Langmuir equations are the most commonly used models for the representation of adsorption equilibrium in an adsorption system29,35.Langmuir adsorption isothermThe assumption of homogeneous surface adsorption for the solute molecule MB was made, implying that the process occurs via monolayer adsorption without any interaction between the adsorbed species. The Langmuir equation is mathematically represented as Eq. (13)30,36:$$\frac{{\text{C}}_{\text{e}}}{{\text{q}}_{\text{e}}}=\frac{1}{{\text{q}}_{\text{max}}\times \text{b}}+\frac{{\text{C}}_{\text{e}}}{{\text{q}}_{\text{max}}}$$
(13)
In the context of adsorption, The term “qe” represents the equilibrium concentration of the adsorbate (methylene blue, MB) on the adsorbent, expressed in parts per million (ppm). “Ce” represents the equilibrium concentration of MB in the solution, while “qmax” represents the maximum achievable amount of MB that can adsorb onto the surface of the adsorbent, forming a monolayer. “b” is known as the Langmuir constant. The relationship between Ce and qe can be visualized through a plot, which is shown in Fig. 22. This plot demonstrates that the adsorption of MB adheres to the Langmuir isotherm model, as evidenced by the linear relationship between Ce/qe and Ce. The slope of this plot corresponds to the reciprocal of qmax (1/qmax), while the intercept corresponds to the reciprocal of qmax times b (1/qmax.b). The Langmuir isotherm can be characterized by a dimensionless constant referred to as the separation factor or equilibrium parameter, RL.Figure 22Langmuir adsorption isotherm for MB adsorption for solution of initial MB concentration = 10 ppm at pH  7, Raphanus seeds solid residual (ACRS) dose 0.5 g /250 ml at 300 rpm, contact time 90 min at temperature = 25 ℃.$${R}_{l}= \frac{1}{1+b\times {C}_{0}}$$
(14)
Freundlich adsorption isothermThe Freundlich adsorption isotherm is a commonly employed mathematical model for fitting experimental data over a wide range of concentrations. This isotherm accounts for both surface heterogeneity and the exponential distribution of active sites and their energies. The Freundlich model is represented in a nonlinear fashion as follows37:$${\text{q}}_{\text{e}}={\text{K}}_{\text{f}} {\left({\text{C}}_{\text{e}}\right)}^{1/n}$$
(15)
The linear from of Freundlich model is expressed as follows:$$\text{log}{q}_{e}=\text{log}{K}_{f}+\left(\frac{1}{n}\right)\text{log}{C}_{e}$$
(16)
where “Kf” is the Freundlich constant representing the adsorption capacity and “n” is a constant related to the sorption intensity, which varies based on the heterogeneity of the adsorbent. A plot of log qe versus log Ce results in a linear relationship with a slope (1/n) and an intercept (log Kf), as depicted in Fig. 2338,39.Figure 23The Freundlich isotherm for MB adsorption was determined using an initial MB concentration of 10 ppm, at a pH of 7, with an ACRS dose of 0.5 g per 250 ml at a stirring speed of 300 rpm, a contact time of 90 min, and a temperature of 25 ℃.Table 7 provides a comparison between the Langmuir and Freundlich models. In the Langmuir model, the maximum monolayer sorption capacity qmax decreased with increasing doses of (ACRS), however in the Freundlich model.Table 7 Langmuir isotherm constants and Freundlich isotherm constants in case of methylene blue.