3.2.2 Effect of contact time for uranium adsorption 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 in Figure 6a, 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 the 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 adsorption kinetics and mechanism
various theoretical models including 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 the pseudo-first order model and
pseudo-second order model and their kinetic parameters were shown in
Figure 6b, Figure S4, Figure S5, Table 3, and Table S1. The kinetic
parameters of linear plots of the 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 the pseudo-first order model and pseudo-second order
model were calculated by plotting a graph between qtversus 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-orde model was close to unity than the 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 the
pseudo-second order model fit was investigated by calculating the
p-value, qecal value (Figure S6), 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 the
pseudo-second order46.