# Selective Extraction of REEs Thanks to One-Pot Silica Hybrid Materials
## 3 Results and Discussion
### Extraction Properties  
The extraction behavior of the screened SHMs (Figure 2) was first evaluated in batch extractions. Neodymium (Nd) extraction efficiencies were determined from the capacity or adsorption ability \(Q_{c}\) (mmol/g or mg/g) of the materials from solution assuming that equilibrium was reached after 24 h of contact at 25 \({}^{\circ}\)C. For each SHM, pH 1, 2, and 4 at a ligand (functional group) to metal ratio (L/M) of around 10 was used. The results are displayed in Figure 3.  
Figure 3: Nd extraction (\(Q_{c}\)) of the functionalized silica hybrid materials (SHMs). Initial conditions: pH = 1, 2, or 4, adjusted with HNO\({}_{3}\); [Nd] = 0.1 mmol/L (14.5 ppm)/volume 10 mL; functionalized materials, 5–7 mg: 0.015 mmol of functional group. Contact for 24 h at 25 \({}^{\circ}\)C.  
Figure 2: Silsesquioxanes investigated in this study.  
The results show different extraction behaviors of the screened materials depending on their functional groups and the initial solution pH. Negligible amounts of adsorption are observed for the materials SHM-1, SHM-2, and SHM-5 regardless of pH. SHM-3 and SHM-4 show pH-dependent extraction behavior. Concerning this pH dependence, SHM-3 (carboxylic acid) only shows extraction at pH 4, while SHM-4 (diglycolamide) shows a decreasing extraction performance with increasing pH.  
The low \(Q_{e}\) observed for SHM-1, SHM-2, and SHM-5 suggests that the triazole group alone is insufficient for the extraction of significant amounts of Nd under these conditions. To go further, in agreement with literature [36], it is valid to assume the non-participation of the triazole group. Therefore, the difference of extraction performance for all materials is assumed to be due to the different functional groups. This allows the discussion of the effects of the individual functional groups.  
The amine group (SHM-2) does not extract Nd under the tested conditions, which is probably due to the protonation of the complex forming a free electron pair (pK\({}_{\text{A}}\approx\) 10.6). Considering that the carboxylic acid (SHM-3) shows extraction only at pH 4, the driving force seems to be the ion exchange between the proton and the Nd-ion. This extraction mechanism is known for carboxylic acids, but is unspecific towards REEs [37]. The situation is more complicated for the diglycolic acid derivatives (SHM-4 and SHM-5).  
The replacement of the diethyl amide by a methyl ester leads to a loss of the extraction performance towards Nd. In agreement with [PERSON] et al. [14], this is explicable by the electron donor capacity of the oxygen atoms in the diglycolic structure. Indeed, for SHM-4 and SHM-5, the first (secondary amide) and second (ether) oxygen participate in the complexation and are identical in SHM-4 and SHM-5. The third oxygen (amide in SHM-4, ester in SHM-5) has a different propensity to contribute its free electrons for metal complexation depending on the functionality. Generally, amides show better complexation than esters. In the present case, it is not only better, but exclusive. The tendency towards better performance at lower pH is a general feature of non-ionic extraction mechanisms like observed for neutral ligands, such as with diglycolamides in liquid-liquid extraction. There, increasing the feed nitric acid concentration causes an increase in the distribution ratio [38; 39].  
Considering the results, it is possible to desorb Nd by adjusting the pH of the solution. To this end, the recovered solid phase after extraction at pH 1 of SHM-4 loaded with Nd was contacted with pure water (Milli-Q) at pH 6.5 at a volume to solid ratio similar to that for the extraction (V = 10 mL, m(SHM-4) = 7 mg). As expected from the decreasing extraction performance at increasing pH, a large quantity of Nd was stripped (80%). The corresponding data are shown in Table 1.