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