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.