3.2.2 Effect of contact time for uranium recovery capacity
To investigate the adsorption kinetics, the influence of contact on
uranium adsorption onto adsorbent was analyzed for the contact time
ranges of 5 to 450 min. As shown from Figure 7a, AO-Fc showed adsorption
amount increasing rapidly from 5 to 80 min, indicating strong uranium
adsorption from the initial uranium concentration solution. However,
after 80 min the adsorption rate was decreased due to a decrease in
diffusive resistance and active sites on the surface of AO-Fc during the
adsorption process of uranium by amidoxime groups. After 350 min, there
was no significant change in adsorption amount of AO-Fc, confirming the
absorption process achieved the equilibrium of adsorption at 350 min.
CN-Fc and Fc showed lower adsorption capacity in comparison to AO-Fc due
to the presence of low chelating sites and no significant adsorption
capacity was increased after 150 min equilibrium of adsorption
indicating equilibrium of adsorption was achieved. It is worth to report
the longer equilibrium time and slower diffusion were attributed due to
the low surface area and internal microporous structure of AO-Fc. To
analyze the uranium adsorption mechanism, the adsorption kinetics of the
adsorbent was studied by applying linear pseudo first order (LPFO) and
pseudo second order kinetics(LPSO) and non-linear pseudo first order
(NLPFO), and pseudo second order kinetics(NLPSO) (supporting
information).
Both linear and non-linear plots of pseudo first order model and pseudo
second order model and their kinetic parameters were shown in Figure 7b,
Figure S3, Figure S4, Table 3 and Table S1. The kinetic parameters of
linear plots of pseudo first order model and pseudo second order model
were obtained by easily plotting a graph between ln
(qe-qt) versus t and t/qt versus t and
the relevant parameters for studying the uranium adsorption were listed
in Table 3 and Table S1. The kinetic parameters of non-linear plots of
pseudo first order model and pseudo second order model were calculated
by plotting a graph between qt versus t and the relevant
parameters were shown in Table S2. From the results of both linear and
non-linear model fit (Table 3, Table S1 and Table S2), the correlation
coefficient (R2) of the pseudo second order model was
close to unity than pseudo first order model, confirming the pseudo
second order was better to discuss the adsorption behavior over entire
adsorption process. The pseudo second kinetic model is based on the
assumption that the rate-limiting step follows the chemisorption process
through sharing valance electrons between uranium and the chelating
site. Also, the validity of pseudo second order model fit was
investigated by calculating p-value, qecal value (Figure
S5) and statistical errors(supporting information), indicating the
experimental data were best fitted with the pseudo second order.
According to the theoretical calculation of Azizian et al. the
adsorption experiment was carried out at a high initial concentration of
solute >250 mg/L, the adsorption data were best fitted with
pseudo first order rather than pseudo second order44.
Figure 7 Adsorption kinetics of uranium onto AO-Fc, CN-Fc and Fc, the
effect of contact time (a) and linear fitting by the pseudo second order
(b) (T = 303 K and adsorbent dosage = 0.01 mg/L)
Table 3 Kinetic parameters for uranium obtained by linear regression.