3.2.3 Effect of equilibrium concentration on uranium recovery
To determine the adsorption capacity of AO-Fc, equilibrium adsorption studies were performed in a simulated uranium solution at different initial uranium concentrations. As shown in Figure 8a, the uranium adsorption amount increased rapidly with increasing the initial uranium concentration, but in due course, the absorption capacity reached a plateau, which meant the adsorption capacity had reached the maximum adsorption capacity of the adsorbent. It was attributed, to more opportunities to adsorbent to intact with the higher uranium initial concentration until the maximum adsorption capacity was achieved. Further, the adsorption capacity of AO-Fc was higher than CN-Fc and Fc due to the presence of amidoxime groups. To measure the adsorption amount of adsorbent for uranium, various linear and non-linear isotherm models such as Langmuir isotherm model, Freundlich isotherm model, and Temkin isotherm model were applied to treat the experimental adsorption data at various uranium concentrations (Supporting information).
The fitting results of AO-Fc, CN-Fc and Fc were shown in Figure 8b, Figure S6 and Figure S7 and the values of various parameters are listed in Table 4 and Table S3 and Table S4. From results listed in Table 4 and Table S3 and Table S4, for AO-Fc the Langmuir isotherm model showed a higher regression coefficient (R2=0.998) than Freundlich isotherm(R2=0.944) and Temkin isotherm (R2=0.985), confirming the uranium adsorption onto the surface of AO-Fc followed a homogenous and monolayer adsorption coverage. Moreover, the applicability of the isotherm model was checked by calculating various statistical errors. For AO-Fc the Langmuir isotherm model showed lower error values as compared to Freundlich isotherm and Temkin isotherm, indicating the experimental data were best fitted with the Langmuir isotherm. Also, for AO-Fc the qm (mg/g) calculated from the Langmuir isotherm model was 420±12 mg/g at an equilibrium concentration of (0.5 mg/L), which was compared with the previously reported amidoxime adsorbents till now (Table 5). The results from Table 5 showed higher adsorption capacity for AO-Fc, indicating that it had greater potential to extract uranium from an aqueous solution.
Figure 8 Adsorption isotherm of uranium onto AO-Fc, CN-Fc and Fc, the effect of uranium initial concentration (a) and linear fitting by the Langmuir isotherm (T=303 K; adsorbent dosage = 0.01 mg/L and t = 350 min)
Table 4 Isothermmal parameters for uranium obtained by linear regression