CharacterizationsSEM and TEMThe surface morphology of the modified SCB (SCB@SA) was detected by employing the SEM, as seen in Fig. 2. At low magnification, the SCB@SA appears as large disordered particles(Fig. 2a, b). With further increases in the magnification (250 X) these particles appeared as pleated seams collected together to form the SCB@SA particles. At high magnification (500×) these seams become more clear, Fig. 2c. This surface morphology would provide a suitable surface area for an efficient adsorption process. The TEM images of SCB@SA captured at different magnifications (2 μm, 1 μm, and 200 nm), Fig. 2d, showed that the SCB@SA has a layered structure and many windings which exposed the binding sites efficiently to the pollutants.Figure 2SCB@SA SEM images at different magnifications (a) ×100, (b) ×250, (c) ×500, and (d) SCB@SA TEM images at different scales.FTIRThe FTIR analysis of SCB, SCB@SA, SCB@SA-MB, and SCB@SA-BB was carried out as shown in Fig. 3. SCB showed characteristic bands at 3338 cm−1 corresponding to absorbed water, 2895 cm−1 related to C–H stretching, 1728 cm−1 related to COOH groups, 1599 cm−1 due to water molecule bending and 1033 cm−1 because of C–O stretching. Upon reacting of SCB with diazonium salt to form SCB@SA, the previous bands had small shifts due to the modification process. Moreover, upon adsorption of MB and BB onto SCB@SA biosorbent, the resulting FTIR spectra were highly altered compared to the spectrum of SCB@SA as shown in Fig. 3. These observed results suggested successful adsorption of the dyes on the surface of the used SCB@SA biosorbent.Figure 3FTIR spectra of SCB, SCB@SA, SCB@SA-MB and SCB@SA-BB.XRDThe XRD pattern of the raw SCB shows characteristic wide weak peaks at 2θ ≈ 16o and 22°. After modification with sulfanilic acid the intensities and sharpness of these peaks are enhanced, Fig. 4. Besides, another intense peak at 2θ ≈ 28° and other weak peaks at 2θ ≈ 35°, 38°, 42° and 47° also appeared. These observations cleared the enhancement of the crystallinity due to modification with organic material. These findings are in good agreement with the results reported in the literature24,25.Figure 4XRD patterns of raw SCB and SCB@SA biosorbent.TGAThe TGA patterns of the raw SCB and SCB@SA biosorbent are shown in Fig. 5. The TGA pattern of the raw SCB showed four main pyrolysis stages; (31–58 °C, 5.20%) due to evaporation of surface water, (58–259 °C, 3.37%) attributed to liberation of interlayer water, the main decomposition stage (259–358 °C, 70.72%) due to degradation of cellulosic chains and finally, (259–358 °C, 16.00%) related to residue carbonization. By modification of SCB with the SA, the amount of absorbed surface water was enhanced by 11% and evaporated at a high temperature of 99 °C which is higher than that of raw SCB. Moreover, the degradation percentage was highly reduced from 70.72 to 52% which provides suitable thermal stability. Finally, the last stage showed an increase in the pyrolysis percentage from 16.00 to 29.00%.Figure 5EDSFigure 6 shows the EDS of SCB and SCB@SA biosorbent.The EDS analysis is required to determine different elemental analyses that compose the analyzed materials. Here, the EDS analysis of SCB mainly consists of C and O-atoms. Moreover, after modification of SCB with diazonium salt, new elements appeared in the EDS analysis of SCB@SA biosorbent such as S and Na-atoms which are related to the anilinic acid.Figure 6EDS of raw SCB and SCB@SA biosorbent.Adsorption studiesFirstly, we applied unmodified SCB and diazonium-modified material(SCB@SA) for the adsorption of MB and BB, under similar environmental conditions such as the temperature, initial concentration of MB, pH, and the adsorbent dose. As seen in Fig. 7., the diazonium-modified materials showed enhancement in the adsorption efficiency towards MB and BB compared to the original materials. Therefore, further experiments applied the modified SCB@SA biosorbent for studying the optimum adsorption parameters.Figure 7Comparison between the removal percentage (R%) of MB and BB using raw SCB and SCB@SA biosorbent.Effect of pHThe pH of the solution primarily affects the degree of ionization of the dyes and the surface properties of the adsorbents.The impact of pH on the removal efficiency of MB and BB onto SCB@SA biosorbent was investigated as shown in Fig. 8a. It is clearly noted that the adsorption performance of SCB@SA biosorbent is enhanced with an increase in the solution pH value in the range 2–8. This behavior can be explained as follows: in the acidic environment the density of H+ ion in the solution is high; therefore, H+ ions will compete with the cationic dye species for the binding sites. By increasing the pH value, the H+ ions’ density will be reduced and the binding sites will be more ionized (–SO3−Na+)26. This environment will favour the adsorption of the cationic species. At pH below pHpzc (6.60), Fig. 8b, the surface of SCB@SA biosorbent was positively charged and the adsorption of cationic dyes is decreased due to the repulsive forces. At pH > pHpzc, the surface of SCB@SA biosorbent acquired negative charges resulting in increased adsorption due to greater electrostatic attraction.Figure 8(a) pH effect on MB and BB adsorption efficiency onto SCB@SA (T = 25 °C, dose = 0.0075 g, [MB] = [BB] = 100 mg/L; v = 10 mL), (b) The pHpcz of SCB@SA biosorbent.Effect of initial dye concentration and adsorption isothermsIn order to study the effect of initial dye concentration on the removal efficiency, the experiments were carried out at various dye concentrations in the range of 50–400 mg/L at room temperature. It was observed that, the removal efficiency significantly decreased from 99 to 24% and from 99 to 32% for MB and BB, respectively upon increase of the dye concentration in the studied range, Fig. 9a. This phenomenon can be explained as follow: at a low initial concentration of the dye, the number of the dye’s molecules is well fitted with the number of the binding sites on the adsorbent’s surface which results in an improvement of the removal efficiency. With the increase in the initial dye concentration, the number of the dye’s molecules will be increased compared to the limited number of the binding sites leading to a decrease in the dye’s removal efficiency27.Figure 9(a) Effect of initial concentration of MB and BB on the removal percentage (b) Langmuir isotherm model, (c) Freundlich isotherm model for MB and BB adsorption.For describing the mechanism of the adsorption, Langmuir and Freundlich isotherm models were employed to describe whether adsorption proceeded on a homogeneous or heterogeneous active site’s surface. Langmuir, and Freundlich’s isothermal models, which can indicate the maximum adsorption capacity (qe) and binding affinity, were applied in the linear form and the parameters were determined as present in Eqs. (4), (5), and (6), respectively. The dimensionless equilibrium factor (RL) presented in Eq. (7), is an important parameter that is used in adsorbent-sorbate affinity prediction. Its values are explained as follows: if the RL value is found to be greater than 1.0 this means that the investigated material is unsuitable and unfavorable, while if it is found to be (0 < RL < 1), (RL = 0), or (RL = 1) this means the reaction is favorable, irreversible, or linear, respectively The Langmuir and Freundlich isotherm model that can be expressed by equations1:$${{\text{lnq}}}_{e}=\mathrm{ ln}{{\text{q}}}_{m} -\mathrm{ k}{\upvarepsilon }^{2}$$
(4)
$$\frac{{{\text{C}}}_{{\text{e}}}}{{{\text{q}}}_{{\text{e}}}}=\frac{1}{{{\text{k}}}_{{\text{L}}}{{\text{q}}}_{{\text{m}}}}+\frac{{{\text{C}}}_{{\text{e}}}}{{{\text{q}}}_{{\text{m}}}}$$
(5)
$${\text{ln}}{q}_{e}=ln{K}_{f}+\frac{1}{n}ln{C}_{e}$$
(6)
$${R}_{l }= \frac{1}{1+{K}_{l}{C}_{\circ}}$$
(7)
Estimation of the best fitted isotherm modelsMany error functions were employed to investigate the well-fitted kinetic and isotherm models and also to decrease the error distribution between the calculated values from theoretical model correlations and experimental data. Four error functions were utilized including chi-square statistic (χ2), mean square error (MSE), the sum of squares error (SSE), and hybrid fractional error (HYBRID). that are presented in Eqs. (8)–(11), respectively28,29.$${\upchi }^{2}={\sum }_{i=1}^{n}\frac{{({q}_{e i} exp-{q}_{e i}{\text{cal}})}^{2}}{{q}_{e i} exp}$$
(8)
$$MSE=\frac{1}{{N}_{exp}}{\sum }_{i=1}^{n}{({q}_{e i} exp-{q}_{e i}{\text{cal}})}^{2}$$
(9)
$$SSE={\sum }_{i=1}^{n}{({q}_{e i} exp-{q}_{e i}{\text{cal}})}^{2}$$
(10)
$$HYBRID=\frac{100}{{N}_{exp}-{N}_{parameter}}{\sum }_{i=1}^{n}\frac{{q}_{e i} exp-{q}_{e i}{\text{cal}}}{{q}_{e i} exp}$$
(11)
where, n is the number of included observations. The subscript cal refers to theoretically calculated data while exp subscript represent experimental data.Table 1 provides the fitted parameter values for the adsorption isotherm models. Because of the higher correlation coefficient (R2 ≥ 0.999) and the lower error functions, the equilibrium adsorption isotherms of MB and BB for a single-dye system fit Langmuir isotherm model. This indicated that Langmuir isotherm fitted well the experimental data for both the two-model dye and both dyes prefer to adsorb on the adsorbent as monolayer on equally energetic binding sites. Moreover, the RL values obtained for both dyes lie between 0 and 1, indicating that MB and BB adsorption by SCB@SA biosorbent is a favorable process.Table 1 Langmuir and Freundlich isotherm models for SCB@SA-MB and SCB@SA-BB.Effect of shaking time and kinetic studiesThe effect of the shaking time on the removal efficiency of MB and BB dyes was investigated and plotted in Fig. 10a. We observed that the removal efficiency markedly increased with further passage of the time till it reached equilibrium at 120 min. Thereafter, the removal efficiency attained a constant curve with a further increase in the shaking time. The rapid adsorption kinetics denotes strong surface complexation among the dye species and sulfonated binding sites30.Figure 10(a) Effect of shaking time on the removal percentage of MB and BB using SCB@SA biosorbent, (b) Pseudo-1st-order for MB and BB adsorption, and (c) pseudo-2nd-order for MB and BB adsorption onto SCB@SA biosorbent.The kinetic behavior and adsorption mechanism can be analyzed by applying the adsorption kinetic models (pseudo-first-order and pseudo-second-order) to demonstrate the experimental data. The linear formulas of pseudo-first order and pseudo-second order are presented in Eqs. (12) and (13), respectively.The kinetic parameters obtained from the slope and intercept (Fig. 10b, c) were calculated and listed in Table 2. The correlation coefficients (R2 = 0.999) cross-ponding to the pseudo-second-order for both studied dyes are larger than those for the pseudo-first-order. Moreover, the calculated values of adsorption capacities for both two dyes are highly close to those obtained from experiments. This proved that the pseudo-second order kinetics model was fitted with experimental data.. Besides, the error functions’values of the pseudo-2nd-order are significantly lower than those of the pseudo-1st-order, implying that the adsorption of MB and BB onto SCB@SA biosorbent fits the pseudo-2nd-order kinetic model in a chemosorption manner28.Table 2 Pseudo-first-order and pseudo-second-order kinetic model parameters for adsorption of MB and BB-dye from aqueous solution.$${\text{log}}({{\text{q}}}_{\mathrm{e }}- {{\text{q}}}_{{\text{t}}})={\text{log}}{q}_{e}+\frac{{{\text{K}}}_{{\text{ads}}}{\text{t}}}{2.303}$$
(12)
$$\frac{{\text{t}}}{{q}_{t} }=\frac{1}{{K}_{2 }{q}_{e}^{2}}+\frac{t}{{q}_{e}}$$
(13)
Effect of biosorbent doseThe effect of an biosorbent dose (2.5–20 mg) on the removal efficiency of MB and BB was studied as shown in Fig. 11. It was clearly noted that the adsorption performance of both MB and BB dyes was improved with the increase in the amount of biosorbent used in range (2.5 to 7.5 mg). Thereafter, the adsorption percent remained constant with a further increase in the dose to 20 mg. In this situation, with increase in the adsorbent dose, the number of binding sites is increased . As a result, the removal of both studied dyes were enhanced till the number of binding sites fitted the number of the dye species31. Thereafter, any further increase in the adsorbent dose will not show an increase in the adsorption efficiency.Figure 11Effect of biosorbent dose on MB and BB removal percentage.Effect of temperature and thermodynamic studiesThe adsorption process is generally impacted by the solution’s temperature. Here, the effect of solution’s temperature in the range (25–45 °C) on the adsorption efficiency of MB and BB dye is presented in Fig. 12a. We observed that the adsorption percent is declined with a further increase in the temperature above 25 °C. This decrease can be attributed to the increase in the adsorption solution’s temperature, which leads to a reduction in the thickness of the boundary layer, due to the high tendency of the dye species to emit from the adsorbent surface to the solution phase, as a result the adsorption efficiency decrease32.Figure 12(a) Effect of temperature on MB and BB removal percentage and (b) Plot of ln KC versus (1/T) absolute temperature for the adsorption of MB and BB on the SCB@SA surface.The thermodynamic factors such as Gibbs free energy change ΔGο (J mol−1), enthalpy change ΔHo (J mol-1) and entropy change ΔSο (J mol-1 K-1) were measured employing the equations:$${\Delta {\text{G}}}^{{\text{o}}}= -\mathrm{RT LnK}$$
(14)
$$\mathrm{Ln K}= \frac{\Delta {{\text{S}}}^{{\text{o}}}}{{\text{R}}}- \frac{\Delta {{\text{H}}}^{{\text{o}}}}{{\text{RT}}}$$
(15)
where R is the universal gas constant (8.314 J/mol K) and T is temperature (K).The values of ΔHo and ΔSo were calculated from the slope and intercept from Fig. 12b and are  listed in Table 3. As can be seen, the estimated negative ΔGo values prove that the MB and BB adsorption on SCB@SA biosorbent is spontaneous in addition to the thermodynamic feasibility of the MB and BB adsorption under the investigated temperature range. Moreover, the ΔHo negative charge indicates that the MB and BB adsorption process was exothermic. Besides, the -ve charge of the ΔSo signifies that MB and BB adsorption results in lower randomness and alignment increase. It was noticed that the adsorption of MB and BB is decreased as the temperature increases1.Table 3 Thermodynamic parameters for adsorption of MB and MB using SCB@SA biosorbent.Desorption/reusability studiesIn order to investigated the reusability of SCB@SA biosorbent, 5 cycles of MB and BB adsorption–desorption were carried out at the optimum conditions. Several eluents were investigated and it was found that NaOH (0.1 mol/L) is the best among them. Figure 13 shows the obtained data of the repeated five cycles of MB and BB adsorption–desorption using NaOH (0.1 mol/L) as eluent.Figure 13Repeated 5 cycles of MB and BB adsorption–desorption.Removal of BB and MB in binary systemFigure 14 represents the removal of BB and MB dyes in the binary system. It was observed that two distinct characteristic peaks for BB and MB clearly appeared at wavelength 475 nm and 666 nm, respectively. As the organic pollutants (dyes) are not present in real polluted water in individual forms, it is very necessary to apply the SCB@SA biosorbent for potential removal of the investigated dyes in the binary system at different time intervals. For the adsorption experiment, 0.0075 g of SCB@SA biosorbent was added to the binary system solution (75 ppm BB and 50 ppm MB) at pH 7. Then the mixture was shaken at 150 rpm. The equilibrium concentration of the adsorbates was calculated from UV–Vis data. The adsorption efficiency of BB and MB was calculated by employing Eq. (2). As it can be noticed from Fig.14, the absorbance, which is an indication of the dye’s concentration, is markedly reduced with increasing time of adsorption denoting the efficient use of SCB@SA biosorbent for the removal of the investigated dyes in the binary system. Figure 14UV data of BB and MB mix after adsorption by SCB@SA at different time intervals.Plausible mechanism of adsorptionThe adsorption mechanism of MB and BB onto the SCB@SA biosorbent was expected by taking into consideration the SO
−3
functional groups on the SCB@SA surface. In a pH basic medium, the SCB@SA adsorbs MB and BB via electrostatic interactions between the negatively charged oxygen on the SCB@SA biosorbent and the positively charged groups of the positively charged dyes, as illustrated in Fig. 15. Also, hydrogen bonding had a vital role in the MB and BB adsorption process via the interaction between the SCB@SA hydrogen atoms and nitrogen atoms in the structure of the MB and BB. Additionally, the adsorption of MB and BB can be attributed to the n–π and π–π interactions. The –π-π interactions occurred through the interaction of the aromatic rings of SCB@SA biosorbent with the aromatic rings of MB and BB dyes. While the n–π interactions occurred by the electron-donating groups (the nitrogen and oxygen groups of the SCB@SA) biosorbent interacting with the aromatic rings of MB and BB.Figure 15Plausible mechanism of adsorption of MB and BB onto SCB@SA biosorbent.Performance of SCB@SA biosorbentTable 4 represents a comparison between the SCB@SA biosorbent performances with other cited adsorbents for remediation of MB and BB taking the sorption capacity, pH and time of adsorption into consideration. Eventually, the SCB@SA biosorbent has high capacities and efficiencies for both MB and BB recovery compared to other mentioned adsorbents as found in Table 4.Table 4 Comparison of adsorption capacity of MB and BB onto SCB@SA biosorbent with previously reported studies.