The optimization of adsorption parameters
With the data obtained as a result of the experimental design made according to Tables 1, the regression process was carried out using the least squares method. According to the polynomial equation obtained as a result of this four-variable two-level design, the following equation provides us with the response surface graphs.
y = b0 + b1X1 + b2X2 + b3X3 + b4X4 + b12X1X2 + b13X1X3 + b14X1X4 + b23X2X3 + b24 X2X4 + b34X3X4
where Y is the predicted reaction efficiency. The constant term is b0. The respective effect
coefficient is b and X1, X2, X3 X4 stands for initial pH, temperature, initial Sr concentration and contact time, respectively. Y = f(X) equation for removing Sr(II) for MW_S3 and MC_S3 is given in Eqs. (3) and 4, respectively.
y = + 31,23 + 0,76 X1 + 0,30 X2 + 9,83 X3 − 0,18 X4 – 0,25 X1X2 + 0,56 X1X3 – 0,58 X1X4 + 0,10 X2X3 – 0,081 X2X4 – 2,25x10− 3 X3X4 (3)
y = + 28,26 + 1,32 X1 – 0,31 X2 + 8,92 X3 + 1,58 X4 + 1,45 X1X2 + 0,31 X1X3 – 0,39 X1X4 + 0,68 X2X3 – 0,39 X2X4 + 2,00 X3X4 (4)
The coded experimental data points along with the predicted and observed responses are given in Table 2.
Table 2. Experimental data points used in FFD statistical design and observed and predicted
values for Sr (II) uptake capacity of MW-S3 and MC_S3.
ANOVA (analysis of variance) analysis, which is within the 95% confidence interval, was examined together with the full factorial experimental design, and as a result, the compatibility of the model with the experimental findings was checked. A significance F value less than 0.05 (with a 95% confidence interval) indicates that the model is statistically significant [16]. The significance F value of the model resulted as P < 0.05 and the model F value as 169.81 for MW_S3 and 17.19 for MC_S3, indicating that the regression showed a statistically high value. The correlation coefficient (R2) value resulted as 99% for MW_S3 and 97% for MC_S3, indicating a high agreement between the observed values and the predicted values (Tables 3 and 4).
Table 3. ANOVA table of the regression model in strontium uptake of MW_S3 nanobiocomposite.
Table 4. ANOVA table of the regression model in strontium uptake of MC_S3 nanobiocomposite.
The statistical significance evaluation of the coefficients obtained as a result of the regression analysis was made with “P” values. As a result of the low “P” value, the significance value of the relevant coefficient increases [17]. As a result of the evaluations made on the results shown in Tables 3 and 4, pH and concentration, which are among the main effects, lead to an increasing and significant change in the biosorption process with MW_S3; time and temperature did not cause a statistically significant change. Likewise, in the biosorption process with MC_S3, it was observed that there was no statistically significant change in the concentration increasing direction, while pH, time and temperature did not cause a statistically significant change. When the absolute values of the coefficients are examined, it is seen that the effect order is the highest concentration (9.83) and (8.92), for MW_S3 and MC_S3 respectively.
1. For MW_S3, when the coefficients of the interactive effects were examined, it was seen that the bilateral interactions did not show statistical significance (as P > 0.05).
2. For MC_S3, when the coefficients of the interactive effects were examined, it was seen that the interaction effects of the concentration and time were significant (P < 0.05) and had an increasing effect, while the other bilateral interactions were not statistically significant.
Effect Of Initial Sr(Ii) Concentration
As a result of regression analysis, the effect of initial Sr(II) concentration on Sr(II) biosorption on both nanobiocomposites was found to be significant (P < 0.05). The coefficient of the concentration being greater than zero indicates that the folded effect of the concentration on adsorption is positive (+ 9.83 and + 8.92) for MW_S3 and MC_S3, respectively. In Figs. 1 and 2, it is seen that the biosorption of Sr (II) on MW_S3 and MC_S3 nanobiocomposites increases with increasing concentration in the range of 25–75 ppm.
Figure 1. Concentration increase graph of Sr(II) biosorption for MW_S3 in the range of 25–75 ppm.
Figure 2. Concentration increase graph of Sr(II) biosorption for MC_S3 in the range of 25–75 ppm.
Ph Effect
As a result of the regression analysis, it was observed that pH was statistically significant (P < 0.05) and had an increasing effect for MW_S3 (Fig. 3). The effect of solution pH on Sr (II) biosorption on MC_S3 was not found significant in the determined operating ranges (Figure is not shown) (P>0.05).
Figure 3. pH effect of biosorption of Sr(II) on MW_S3.
Effect Of Temperature
As a result of the regression analysis, the effect of temperature on Sr(II) biosorption on MW_S3 and MC_S3 was not found significant in the determined operating range (25–35°C) (P>0.05). Therefore the figures are not given.
Effect Of Contact Time
As a result of regression analysis, the effect of shaking time on Sr (II) biosorption on MW_S3 and MC_S3 was not found significant (P > 0.05) in the range of 30–60 minutes.
Effect Of Initial Concentration Of Strontium And Shaking Time
Evaluating the results obtained, it was found that the combined effects of the parameters on the biosorption of Sr(II) on MW_S3 adsorbent were not significant, but the combined effects of concentration and time were only significant on the biosorption of Sr(II) on MC_S3 adsorbent.
As a result of the regression analysis, the effect of the initial concentration of strontium and the shaking time on the Sr (II) biosorption on MW_S3 was not found significant (P>0.05).
The fact that the coefficient of concentration and agitation time is greater than zero (+ 2.00) shows that the effect of this double interaction is positive on biosorption. As the main effect, if we consider the coefficients of concentration and time; the concentration has a coefficient of + 8.92, and the agitation time has a coefficient of + 1.58. The fact that the coefficient of the concentration is larger and positive and the coefficient of the duration is also positive, the cumulative effect of the bilateral interaction in the examined interval shows a positive feature (Fig. 4).
Figure 4. The effect of initial strontium concentration and shaking time on the biosorption of Sr(II) on MC_S3.
As a result, it was concluded that the changes within the limits of 25–75 mg/L and 30–60 minutes had statistically significant effects on adsorption (p<0.005).
Apart from the bilateral interactions given regarding the adsorption of Sr (II) on both nanobiocomposites, other bilateral interactions were not found significant (P>0.005) and therefore graphs related to these were not given.
Characterization Of Sr(Ii) Loaded Adsorbent By Xps Analysis
After Sr(II) biosorption on the nanobiocomposite, XPS analysis was used to determine the chemical composition and structure of the starch stabilized material. The XPS spectrum of the Sr(II) charged-magnetic nanobiocomposite is shown in Fig. 5. The magnetic starch nanobiocomposite adsorbent shows binding energy peaks of approximately 285, 530 and 711 eV, which are attributed to C1s, O1s and Fe2p electrons, respectively. The energy value observed around 711 eV for the Fe atom is due to the Fe3O4 structure showing the 2p orbital. In addition, a peak at 134 eV attributable to Sr(II) coordination was detected, proving that Sr(II) is retained on the magnetic nanobiocomposite.
Figure 5. XPS spectrum of Sr(II) loaded starch based magnetic nanobiocomposite.
Isotherm Studies
Adsorption isotherms are used to characterize the adsorption process. If it is necessary to define the adsorption isotherm, we can state that it is the graphs that make sense of the equilibrium state between the amount of the substance being adsorbed on the adsorbent (qe, mg/g) and the concentrations of the substance increasing without being adsorbed in the solution (Ce, mg/L) in a constant temperature and pH environment. For the formation of these isotherms, the solutions prepared at different concentrations with a known amount of adsorber must reach equilibrium [18].
For this purpose, Freundlich, Langmuir and D-R (Dubinin-Radushkevich) isotherms were studied in the range of 10–125 mg/L strontium concentration. With the support of these isotherms, the surface properties of the adsorbent, the affinity between the adsorbent and the adsorbed, and the properties such as the maximum adsorption capacity and adsorption energy can be examined and information about the mechanism of the adsorption process can be obtained.
These isotherms are given in Fig. 6–8. The data of the adsorption isotherms are shown in Table 6.10. The equation of the Langmuir isotherm is given in Eq. 5.
$$\frac{{C}_{e}}{{q}_{e}}= \frac{1}{{Q}_{o}{b}_{L}}+\frac{{C}_{e}}{{Q}_{o}}$$
5
The slope of the Ce/qe versus Ce graph drawn according to this equation gives the value 1/Qo and its extrapolation (intercept) 1/QobL (Fig. 6). From this, the isotherm constants Qo and bL are calculated. With the balance parameter RL calculated from here, it is possible to comment on the suitability of the system.
$${R}_{L}=\frac{ 1}{1+{b}_{L}{C}_{o}}$$
6
Figure 6. Langmuir isotherm in the biosorption of Sr(II) on MW_S3 (a) and MC_S3 (b).
In the biosorption processes performed in accordance with the Freundlich model, the adsorbent material is located heterogeneously on the adsorbent surface. The linearized form of the equation of the Freundlich model is as follows:
$$\text{log}{q}_{e}=\text{log}{K}_{F}+\frac{1}{{n}_{F}}\text{log}{C}_{e}$$
7
1/nF from the slope of the graph of lnCe versus lnqe drawn according to the equation, and lnKF from the intersection point are found (Fig. 7). From here, the nF and KF values are calculated. According to the Freundlich model, the more heterogeneous the surface, the closer the 1/n heterogeneity effect will be to zero. If the n adsorption intensity constant value found here is between 1 and 10, it is understood that the adsorption is efficient.
Figure 7. Freundlich isotherm in the adsorption of Sr(II) on MW_S3 (a) and MC_S3 (b).
The following equation is used to find the constants of the Dubinin-Radushkevich model used to determine the pore structure and pore volume of the adsorbent:
$$\text{ln }{C}_{ads}=\text{ln}{X}_{m}-\beta {\epsilon }^{2}$$
8
$$e=RTln\left(\frac{1}{1+{C}_{e}}\right) \left(9\right)$$
$$E=\frac{1}{\sqrt{-2\beta }}$$
10
Here, Cads (mol/g) is the amount of solute sorbed per unit weights of solid, Xm (mol/g or mg/g) is the sorption capacity, β (mol/J)2 a constant related to energy and Ɛ is the D-R isotherm constant. This approach is generally used to distinguish whether the physical and chemical adsorption of metal ions is by the mean free energy per molecule of adsorbate, E, which can be calculated by the relationship given in.
Together with the average adsorption energy (E), we can make predictions about the adsorption mechanism. If the E value is between 8–16 kJ/mol, the adsorption process is characterized by ion exchange. If E < 8 kJ/mol, the adsorption is physical, if 20 < E < 40kJ/mol, the adsorption process is estimated to be chemical [19].
Figure 8. D-R isotherm in the adsorption of Sr(II) on MW_S3 (a) and MC_S3 (b).
Table 5. Isotherm and correlation constants for Sr(II) ion biosorption.
According to the R2 correlation values given in Table 5, it was observed that the Sr(II) ion biosorption of MW_S3 and MC_S3 adsorbents was compatible with Langmuir and mostly the D-R isotherm model. As seen in Table 5, the Langmuir and D-R model is suitable for describing the strontium biosorption equilibrium with MW_S3 and MC_S3 nanobiocomposites. Accordingly, although adsorption conforms to the monolayer isotherm model, it is known that according to the D-R isotherm model, adsorption follows the pore-filling mechanism and provides information about adsorption energy and nanobiocomposite porosity [20].
It was observed that the RL values of the Langmuir model were less than one. It has been determined that the systems with equilibrium parameter values less than zero are suitable.
According to the D-R isotherm, Qo, the maximum sorption capacity was calculated as 0.0237 mol/g for MW_S3 and 0.0206 mol/g for MC_S3. The adsorption energy E value was calculated as 6.84 kJ/mol for MW_S3 and 6.97 kJ/mol for MC_S3. These results show that the biosorption mechanism takes place in the direction of physical adsorption in both adsorbents.
Adsorption Thermodynamics
In order to determine the thermodynamic model of the adsorption process, ΔG° standard Gibbs free energy, ΔH° standard enthalpy and ΔS° standard entropy values were used. The following equations were used to calculate these parameters.
$$ln{K}_{d}=\frac{?{S}^{o}}{R}-\frac{?{H}^{o}}{RT}$$
11
$$?{G}^{o}=?{H}^{o}-T?{S}^{o}$$
12
$${K}_{d}=\frac{{\complement }_{i}-{\complement }_{e}}{{\complement }_{e}}x\frac{V}{W}$$
13
Experiments on the investigation of biosorption thermodynamics were carried out in the range of 20-40°C and with 50 mg/L strontium solutions. As a result of the thermodynamic investigation of Sr(II) biosorption on MW_S3 nanobiocomposite, ΔH°<0 indicates that the equilibrium for adsorption is an exothermic reaction, while ΔG°<0 indicates that the adsorption process occurs spontaneously (voluntarily). The fact that the ΔG° value has smaller negative values with the increase in temperature indicates that the adsorption process proceeds spontaneously in the low temperature environment and strontium ions voluntarily adhere to the adsorbent. A negative ΔS° value indicates that the adsorption process is regulated through the formation of a complex between adsorbate and adsorbent. In addition, a negative ΔS° value reflects that no significant changes occur in the internal structures of the adsorbent during the adsorption process [21]. As a result of the thermodynamic investigation of Sr (II) adsorption on MC_S3 nanobiocomposite, ΔH°>0 indicates that the equilibrium for adsorption is an endothermic reaction, while ΔG°<0 indicates that the adsorption process occurs spontaneously (voluntarily). The fact that the ΔG° value has larger negative values with increasing temperature indicates that the adsorption process proceeds spontaneously at high temperatures and strontium ions voluntarily attach to the adsorbent. The fact that ΔS°>0 indicates that the disorder at the solid-liquid interface increases during adsorption. In addition, the positive entropy shows the interest of strontium ions in the adsorbent [22].
Table 6. Thermodynamic parameters of Sr(II) adsorption.
In order to have an idea about the adsorption mechanism, the size of the enthalpy (H) and free energy change is used. Generally, the magnitude of ΔH° is less than 20 kJ/mol for absolute physical adsorption, while this value is in the range of 80–200 kJ/mol for chemical adsorption.
In general, the absolute magnitude of the change in Gibbs free energy for physisorption is between − 20 and 0 kJ/mol, and chemisorption is in the range of -80 to -400 kJ/mol [23]. In this study, the results found for MW_S3 nanobiocomposite were between − 18.9129 and − 18.4793 kJ/mol, while for MC_S3 nanobiocomposite it was between − 17.9545 and − 20.1507 kJ/mol. According to the results obtained, the biosorption process that takes place at 20–40 °C has a physical character [24]. Finally, it can be concluded that while the Sr(II) biosorption with MW_S3 nanobiocomposite has physical adsorption character, the Sr(II) biosorption with MC_S3 nanobiocomposite is governed by the combined control of several mechanisms [25].