# Selective Extraction of REEs Thanks to One-Pot Silica Hybrid Materials
## 3 Results and Discussion
### Mechanism of the Nd Extraction by SHM-4  
The isotherm for adsorption of the Nd onto SHM-4 at 25 \({}^{\circ}\)C was prepared from nitric acid solutions (pH 1) at different Nd concentrations (1-250 mg/L). Other parameters were kept constant (\(V/m=10\)). The Langmuir isotherm results are illustrated in Figure 4.  
Figure 4 shows the uptake capacity (black) and the modified uptake capacity (red). The uptake capacity shows a saturation at Nd concentrations above 0.007 mmol/L. Fitting the modified uptake capacity (red) with the Langmuir equation \(Q_{e}=Q_{max}\frac{L\ C_{e}}{1+L\ C_{e}}\) leads to a solid R\({}^{2}\) value (0.9962).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **[C] (g/L)** & **V (mL)** & \(C_{i}\) **(ppm)** & \(C_{f}\) **(ppm)** & **\%Efficiency** \\ \hline
**Extraction** & & 14.53 & 8.49 & 42\% \\
**Stripping** & 0.7 & 13.50 & 6.04 & 4.98 & 82\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Recovery of Nd from the loaded extraction of solid phase SHM-4. Extraction condition: [Nd] = 0.1 mmol/L (14.5 ppm) pH = 1 + SHM-4 (7 mg: 0.015 mmol of functional group). Contact for 24 h at 25 \({}^{\circ}\)C. Stripping condition: Nd-loaded SHM-4: 7 mg + Milli-Q water pH: 6.5 (volume 10 mL). Contact for 24 h at 25 \({}^{\circ}\)C.  
This result indicates the formation of a monolayer where all Nd adsorption sites are equal in energy. From this, it can be concluded that all sites where Nd is adsorbed are similar in energy and a monolayer is formed. For comparison, the Freundlich isotherm \(Q_{e}=K_{F}C_{e}{}^{1/n}\) was also investigated (Figure S3). It would suggest the energetic heterogeneity of the adsorption sites. Plotting \(\mathrm{Ln}(Q_{e})=\mathrm{f}(\mathrm{Ln}(C_{e})\), an unsatisfactory \(\mathrm{R}^{2}\) value of 0.94 is obtained, excluding this behavior.  
From this, a maximum extraction capacity \(\mathrm{Q}_{\mathrm{max}}\) for Nd was determined to be 0.168 mmol/g (24.25 mg/g). Compared to the literature, this value is superior to the values found by [PERSON] et al. [16], who investigated diglycolamide-functionalized mesoporous silica (200 mg/g), and is similar to the studies of [PERSON] et al., who investigated diglycol amic acid functional groups (\(\mathrm{Q}_{\mathrm{max}}=16\) mg/g) [13].  
Small amounts of adsorbed nitrogen were measured for the materials from N\({}_{2}\) adsorption/desorption measurements. However, this is typical of the investigated materials and irrelevant for the accessibility of functional groups, as shown by the extraction results and in the literature [40]. To investigate the presence of Nd in SHM more closely, SEM-EDX analyses were performed. Figure 5 shows the resulting images.  
Figure 4: Isotherm for adsorption of neodymium on SHM-4 at 25 \({}^{\circ}\)C and linear regression of the Langmuir model (\(C\)/\(Qe=\mathrm{f}(C\)\(e\)): y = 5.95(\(\pm\)0.13)x + 0.1001(\(\pm\)0.0009); \(\mathrm{R}^{2}=0.996\). Initial pH 1.0 adjusted with HNO\({}_{3}\); [Nd] = 1-250 ppm/volume, 10 mL SHM-4: 5 mg.  
Figure 5: SEM-EDX mapping of SHM-4 after contacting solution with Nd: (**a**) SHM-4 morphology; (**b**) spatial distribution of Nd.  
At the observed scale, an even distribution of Nd in SHM-4 can be assumed after the extraction (Figure 5b). This indicates that, after 24 h of contact, the material was effectively filled with the solution and that the equilibrium was reached. Moreover, from the EDX measurements, the elemental composition of materials (Figure S4) was determined. A molar ratio of Nd/Si = 0.103 was estimated, which is close to the ratio determined from Q\({}_{\text{max}}\) (Nd/Si = 0.080 \(\pm\) 0.005) assuming a molecular weight of SHM-4 of about 481 \(\pm\) 30 g mol\({}^{-1}\).