# Facile Preparation of Graphene Oxide-MIL-101(Fe) Composite for the Efficient Capture of Uranium
## 3 Results and Discussion
### U(VI) Adsorption on the GO-MIL-101(Fe)
Considering the dispersibility of the composite and adsorption performance of adsorbents as well as the interference of complex hydrolysis product such as UO\({}_{2}\)(OH)\({}_{2}\) precipitate at higher pH, a pH of 5.5 was used as the condition for the adsorption experiments. Furthermore, both samples presented improved adsorption efficiency towards U(VI) with the adsorbent dosage increasing (Figure 4c). The increasing trend can be attributed to increased adsorption sites introduced as the addition amount of adsorbents become larger. Further increasing the adsorbent dosage to 1 g L\({}^{-1}\) gives rise to an adsorption percentage of 96%. Satisfying the treatment result can be achieved either by increasing the adsorbent addition or the adsorption steps. On the other hand, the dependence of adsorption performance on the ionic strength was investigated to eValuate the application potential of adsorbents. It is shown in Figure 4d that the amount of U(VI) adsorbed on GO-15 MIL-101(Fe) fluctuated slightly in the range of \(\pm\)5% as the concentration of NaNO\({}_{3}\) increased, while that of pure MIL-101(Fe) showed an overall decreasing trend. Therefore, the uptake of U(VI) on GO-15 MIL-101(Fe) is controlled by inner-sphere surface complexation [8; 64]. To eValuate the removal rate of uranyl ions on the MIL-101(Fe) and composite samples, the adsorption percentage of U(VI) under different times was monitored. Figure 4e showed that both GO-15-MIL-101(Fe) and MIL-101(Fe) exhibited a rapid increasing adsorption percentage of U(VI) in the first 30 min and achieved it in 60 min. In comparison with the composite sample, the adsorption efficiency of MIL-101(Fe) had much lower equilibrium adsorption efficiency (~26%). In order to further understand the adsorption process, the adsorption kinetics were simulated by a pseudo-first-order (Equation (3)) [65] and the pseudo-second-order models (Equation (4)) [66]:  
Pseudo-first-order model:  
\[\ln(q_{e}-q_{t})~{}=~{}\ln q_{e}-k_{1}t \tag{3}\]  
Pseudo-second-order model:  
\[\frac{t}{q_{t}}~{}=~{}\frac{1}{k_{2}q_{e}^{2}}+\frac{t}{q_{e}} \tag{4}\]  
In Equations (3) and (4), \(q_{t}\) and \(q_{e}\) are the adsorption amount at time \(t\) and equilibrium, respectively. \(k_{1}\) and \(k_{2}\) correspond to the rate constants for the pseudo-first-order model and pseudo-second-order model, respectively. The kinetic plots of uranyl ions on both MIL-101(Fe) and the composite sample can be better fitted by the pseudo-second-order model with the correlation coefficients (\(R^{2}\)) close to 1 (Table 1). This was also confirmed by the almost linear plots of \(t/q_{t}\) as a function of \(t\) (Figure 4f). Therefore, chemisorption is one of the rate-limiting processes in the adsorption kinetics of U(VI) on GO-15-MIL-101(Fe). Moreover, the diffusion of U(VI) onto the surface of adsorbent also affected the adsorption kinetics. The more rapid adsorption performance of GO15-MIL-101(Fe) in comparison with MIL-101(Fe) may arise from the meso-macroporous structure and enlarged mesopore distribution, which served as an efficient ion diffusion channel. The real uranium-contaminated water always had several coexisting ions which could interfere with the adsorption of uranium on the adsorbents. Therefore, the adsorption percentage of uranyl ions on MIL-101(Fe) and the composite was monitored to eValuate the effect of various cations and anions (Figure 5a,b). According to the statistics shown in Table S3, GO15-MIL-101(Fe) showed an overall improved adsorption performance in comparison with MIL-101-(Fe) under different coexisted anions and cations. Especially, the adsorption of U(VI) on the composite samples was less suppressed