Characterization of adsorbentWhen the XRD pattern was examined, it was observed that the WJMP material showed crystallinity in regions similar to the sepiolite material as stated in the literature and in Fig. 245,46. Sharp XRD peaks indicate a high degree of crystallinity of WJMP. The marked peaks in XRD and the raw sepiolite peaks in the literature are seen in similar places. The fact that other peaks appear at different angles is thought to be due to the crystal structure being processed and waste being formed. In addition, since the raw material XRD pattern was taken without any pre-treatment, it was observed that it gave a similar pattern with the raw and natural materials in the literature47,48. The obtained data revealed that the waste material we utilized was in its raw form, similar to natural sepiolite clay without any treatment (as it was collected immediately after jewellery processing). Remarkably, its adsorption capacity was found to be comparable to that of sepiolite49.Figure 2While the FT-IR spectrum of raw WJMP (waste material) was obtained, some material peaks were not clearly visible due to the presence of moisture. To address this, the material was dried in an oven at 65 °C and FT-IR spectra were obtained. Similarly, after the adsorption of Cu(II) and MB dye was carried out under optimum conditions, the adsorbent was dried with the same method and FT-IR spectra were recorded. The changes caused by MB dye and Cu(II) ions on the adsorbent were investigated.The FTIR spectrum of the WJMP adsorbent is given in Fig. 3. The spectrum of the adsorbent has characteristic stretching vibrations of hydroxyl groups and OH-bend bands at 3551 and 1652 cm−139,42,43. Also, the characteristic Si–O combination bands at 1202 cm−1 and 1004, 969 and 466 cm−1 (Si–O–Si stretching bands) were seen39,50 and the intense band at 1004 cm−1 with a shoulder at 969 cm−1 is assigned to the Si–O stretching. The bands at 778, 667 and 646 cm−1 correspond to Mg3OH bending vibrations. In addition, a bending band occurs at 468 cm−147,51. In the spectrum of Cu(II) loaded adsorbent, it was observed that the peak at 1416 and 1357 cm−1 was appeared which was not found in raw WJMP spectrum. The increase in the intensity of this peak belonging to the H–OH band in the WJMP crystal may indicate that the adsorbent has weak interactions with the copper ion. At the same time, it can be stated that as a result of adsorption isotherm, Cu(II) ions accumulate on the surface as a result of physical adsorption. In addition, the disappearance of the Si–O peak in 1004 cm−1 supports the explanation of this situation. In case of MB-WJMP spectrum the peak at 1370 cm−1 found. MB dye molecules and adsorbent molecules are interacted each other on adsorbent surface. The existence of the MB molecule on the adsorbent surface induces changes in the bond vibrations and tensile energies of the adsorbent, resulting in a minor shift in the peaks observed in the spectra. MB dye molecules interact with the groups in the WJMP crystal structure, and it is also supported by the Langmuir isotherm52.Figure 3FT-IR spectrum of WJMP, Cu-WJMP, MB-WJMP.N2 adsorption/desorption isotherm (BET) data revealed a BET surface area of 235.3567 m2 g−1 and a total pore volume for pores smaller than 27.226 Å. In addition, the average pore width determined from adsorption was measured to be 20.0884 Å.SEM images at 500× magnification show WJMP, Cu-WJMP and MB-WJMP as shown in Fig. 4. The images reveal that the WJMP example exhibits a layered structure. However, this layered arrangement is disrupted after adsorption with MB and Cu(II). Upon MB adsorption, agglomeration is observed in the images, while the images after Cu(II) adsorption show an increase in crystal particles. In Fig. 4, the images of adsorbent and loaded adsorbent proved the above characterization results. The presence of MB and Cu(II) adsorbed on the WJMP adsorbent can also be seen in the picture from the coloration of the adsorbent.Figure 4SEM images of WJMP, MB-WJMP and Cu(II)-WJMP.In Fig. 5, the images of adsorbent and loaded adsorbent proved the above characterization results. The presence of MB and Cu(II) adsorbed on the WJMP adsorbent can also be seen in the picture from the coloration of the adsorbent.Figure 5Images of of WJMP, Cu-WJMP, MB-WJMP.Optimum studies with classical batch methodBoth dyes and heavy metal adsorption can be significantly impacted by a solution’s pH. This is due to the fact that pH variations may have an impact on the surface charges of the adsorbent and adsorbate molecules53,54. Under constant experimental conditions, the effect of pH of the starting solution on the adsorption of Cu(II) and MB dye was examined. The parameters for removing Cu(II) and MB dye were established by preliminary experiments, which included 100 mg L−1 concentrations of Cu(II) and MB dye solution, adsorbent amounts of 12.5 g L−1 for Cu(II) and 0.5 g L−1 for MB dye, a temperature of 25 °C, and a contact duration of 60 min. Figure 6 illustrates how pH affects Cu(II) and MB dye adsorption.Figure 6Effect of parameters for Cu(II) and for MB dye on WJMP.In the context of heavy metal adsorption, under high pH conditions (alkaline conditions), the surface of the adsorbent can become negatively charged, thereby attracting positively charged heavy metal ions, thereby increasing the adsorption capacity. Similarly, in the case of dye adsorption, the pH of the solution can affect both the surface charge of the adsorbent and the dye molecules. For example, at high pH levels, the surface of the adsorbent tends to carry a negative charge, resulting in increased attraction for positively charged dye molecules and, consequently, increased adsorption capacity54.The amount of adsorbent used plays a very important role in determining the adsorption capacity. Typically, increasing the amount of adsorbent results in higher adsorption capacity due to the availability of a larger surface area for adsorption. To investigate the effect of WJMP amount, after optimization from the pH effect results, experiments were performed for Cu(II) at pH 6 and for MB removal at pH 8. The contact time, temperature and adsorbate concentration values used were the same as those in the pH effect optimization experiments. Figure 6 shows the effect of WJMP amount. In the case of heavy metal and dye adsorption, increasing the amount of adsorbent can lead to an increase in available binding sites for metal ions and dye molecules, thereby increasing the adsorption capacity. However, excessive use of the adsorbent may cause saturation of the solution and prevent further adsorption55.The contact time between adsorbent and adsorbate also affects the adsorption capacity. Generally, adsorption increases with time until it reaches equilibrium, after which there is no further adsorption. While the contact time effects were examined at 0.250 g WJMP for Cu(II) removal and 0.030 g for MB removal, at pH 6 for Cu(II) and pH 8 for MB, other parameters were kept consistent with the pH effect optimization. Figure 6 shows the effect of contact time on both adsorbates. If the adsorption capacity is plateaued, it indicates that the adsorbent has reached its maximum adsorption capacity and cannot further remove dye molecules or heavy metal ions. The rate of equilibrium depends on the specific adsorbent-adsorbate system5.Furthermore, increasing the adsorbate concentration generally leads to an increase in adsorption capacity as more adsorbate molecules are available for adsorption. However, the effect of adsorbate concentration on dye and heavy metal adsorption depends on several factors, such as the properties of the adsorbent surface, the chemistry of the adsorbate, and the concentration range considered. In this study, the concentration of Cu(II) and MB dye solution was changed by keeping other optimization parameters constant. The results of the concentration effect of the adsorbate are presented in Fig. 5.Temperature also affects the adsorption of dyes and heavy metals. The effect of temperature on adsorption is complex and varies depending on the specific adsorbent-adsorbate system. Determining the optimum temperature is essential to maximize adsorption efficiency for a given system. In this study, the effect of temperature on dye and heavy metal adsorption was investigated at 25, 35 and 45 °C (Fig. 6) with optimum values determined from the above-mentioned experiments.Isotherm resultsThe Cu(II) and MB adsorption plots on WJMP examined by the three adsorption isotherm models are shown in Fig. 7 and the fitting results are shown in Table 5. It was observed that the most suitable adsorption isotherm model is Langmuir for MB and Freundlich for Cu(II). WJMP adsorbent appears to be favourable to both MB dye and Cu(II) according to the RL value in the table (0 < RL < 1).Figure 7Isotherm models for Cu(II) and MB.Table 5 Isotherm models parameters result of Cu(II) and MB.The Langmuir model’s linear correlation coefficients are clearly greater than 0.99, and the theoretical q value (200.65 mg g−1) is nearly equal to the experimental value (227.27 mg g−1). It suggests that the Langmuir isotherm model is appropriate for describing adsorption behavior, and monolayer adsorption of the MB dye on adsorbent surfaces has been presented. While the predicted maximum adsorption capacity of the adsorbent in Cu(II) removal was 8.44 mg g−1, the Langmuir isotherm calculation revealed that the maximum adsorption capacity of WJMP was 10.6 mg g−1. On the observed adsorbent heterogeneity surface, positively charged Cu(II) adsorption is indicated by the R2 value of Cu(II) metal, which guarantees the appropriateness and heterogeneity of the Freundlich isotherm model with a value of 0 < 1/n < 1. It demonstrates that the adsorption data for the Freundlich isotherm model is superior to that of the Langmuir and Temkin isotherm models. The applicability of the Freundlich isotherm indicates that Cu(II) and WJMP may interact intermolecularly in some situations, and that several sites with various adsorption energies are involved. Consequently, WJMP verifies the multilayer coverage of Cu(II) on the adsorbent surface with a heterogeneous limited saturation limit in adsorption affinity56.Kinetic resultsThe kinetic graphs of the adsorption process of Cu(II) and MB dye on WJMP were given in Fig. 8. The results obtained from the graphs drawn according to the PSO, PFO and Elovich kinetic models were given in Table 6. According to these results, the kinetics of both adsorbates on the adsorbent were evaluated according to the R2 values, which were compatible with the PSO kinetic model.Figure 8Kinetic models for Cu(II), MB.Table 6 Kinetic model parameters of Cu(II) and MB on WJMP.Fitting the pseudo-second-order model yields theoretical qe values that are closer to the experimental values than the other models, and the correlation coefficient R2 is greater than 0.9900. This suggests that the pseudo-second-order kinetic model can accurately capture adsorption kinetics. Adsorbates can often be transported from bulk solution to adsorbent via a diffusion process.According to the kinetic and isotherm data we acquired, along with the outcomes of our experimental optimization, we have compared our results for the removal of Cu(II) and MB from aqueous solutions with findings from existing literature that employed a similar adsorbent. The comparison is summarized in Table 7. It is evident from the table that comparable investigations have reported higher adsorption capacities for the dye compared to Cu(II).Table 7 Comparison of the adsorption capacities of similar adsorbents in the literature.Thermodynamic resultsThe above equations can be used to compute the thermodynamic parameters, enthalpy change (ΔH), Gibbs free energy change (ΔG), and entropy change (ΔS). Table 8 displays the thermodynamic results. The values ΔH and ΔS can be estimated from the slopes and intercepts of the graph of lnkc versus 1/T, the results obtained for WJMP are − 5.832 kJ mol−1 for Cu(II) and − 10.544 kJ mol−1 for MB dye at 293 K. Therefore, the negative value of ΔH showed that the adsorption process was exothermic.Table 8 Thermodynamic parameters of Cu(II) and MB on WJMP.When ΔG is negative, it means that the adsorption procedure for WJMP is spontaneous, and as the temperature rises, so does the reaction’s degree of spontaneity61. ΔG values are more negative for Cu(II) adsorption, suggesting that this adsorption process is more spontaneous. ΔG values more negative than 40 kJ mol−1 imply charge sharing or transfer from the adsorbent surface to the metal ion to form a coordinate bond, but ΔG values up to 20 kJ mol−1 are compatible with the electrostatic interaction between the adsorption sites and the metal ion62,63.RSM optimizationThe predictive quality of the models using for optimizing Cu(II) and MB dye adsorption processes was verified ANOVA, R2, R2adj, R2pred. Factor’s effects and interactions important for Cu(II) (except for Co) and MB dye adsorption on WJMP had low P values (P < 0.05). The regression coefficients of the proposed models (R2adj) were another important evaluation criterion. The results for the reduced models were shown in Table 9 for Cu(II) and in Table 10 for MB dye. For the obtained reduced models, R2adj values for R was 99.91% for MB dye and 95.15% for Cu(II). It meant that max ~ 5% of the total variation cannot be explained by the models and it can be indicated a high agreement of actual and predicted values. According to the ANOVA results, the relationship between the response (removal percentage) and the significant effects and interactions of the factors were shown with polynomial models for Cu(II) and MB dye. According to the regression equations in coded units, positive and negative signs of coefficients indicate the synergistic and antagonistic effects of factors on the response. pH was the factor with the greatest synergistic effect for Cu(II) and MB dye.Table 9 Results of the ANOVA for the suggested Cu(II) removal model (%).Table 10 Results of the ANOVA for the suggested MB dye removal (%).The quadratic equation obtained from the model was graphically represented by a two-dimensional (2D) contour plot and a three-dimensional (3D) response surface (Figs. 8 and 9).Figure 9Response surface and Contour plots based on R (%) for Cu (II) and MB dye removal.From RSM, the optimum removal of Cu(II) was found to be 95.5% at 6.0 pH, 0.40 g WJMP amount and 35 mg L−1 and initial Cu(II) concentration. For MB dye adsorption onto WJMP, the optimum removal percentage was 97.8% at 8.0 pH, 0.030 g WJMP amount and 50 mg L−1 initial MB concentration. Experimentally obtained removal percentage were 97.2% ± 0.5 for MB dye 96.2% ± 1.0 for Cu(II) with standard deviation and (N = 2).Response surface and contour plots can provide valuable insights into the behaviour of a process and help identify the optimum conditions for achieving the desired response. When the results obtained from the classical method and the surface response and contour graphs were examined, it was determined that the percent of MB dye removal and adsorbent capacity of WJMP adsorbent was more successful than those obtained for Cu(II).Response surface and contour plots were generated to evaluate the Cu(II) and MB dye removal percentage shown in Fig. 9. These charts provide valuable information about the relationship between variables and the efficiency of the removal process.
