# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 3 Applications
### Separation and Extraction
The Nd\({}^{3+}\) ions were always adsorbed onto the hybrid materials \(>\)98% at all phosphate ratios in the presence of Sr\({}^{2+}\) and Cs\({}^{+}\) ions. The uptake capacity increased with higher phosphate amounts for Nd\({}^{3+}\) but at very high phosphate amounts, the uptake of Sr\({}^{2+}\) and Cs\({}^{+}\) ions increased, and led to a lower separation factor (SF) with respect to Nd\({}^{3+}\) ions [106]. In the same study, hybrid tin phosphonate-phosphate were compared with a Zr(IV) phosphonate-phosphate system. The Zr-hybrid showed clear affinity only towards M\({}^{3+}\) ions, whereas the Sn-hybrid showed affinity to M\({}^{3+}\) and slightly for M\({}^{2+}\) ions. Thus, showing the importance of the choice of metal and phosphonate/phosphate ratio while creating hybrid material used for the solid-phase extraction of lanthanides [106]. In another study, Zr/Sn-phosphonates containing no inorganic phosphate were also evaluated for selective separation. These were subjected to the lanthanide extraction of Nd\({}^{3+}\), Eu\({}^{3+}\), and Ce\({}^{3+}\) in the presence of Cs\({}^{+}\) and Sr\({}^{2+}\) at pH = 3. Additionally, here, the Zr-based hybrid phosphonates showed the preferential uptake of only +3 metal ions, whereas the Sn-based hybrids had a slight affinity towards Sr\({}^{2+}\) along with +3 metal ions. These hybrids were also evaluated for the removal of Am\({}^{3+}\) from spent nuclear fuel. Both Zr and Sn hybrids showed \(>\)99% removal of Am\({}^{3+}\) from the solution (pH = 3) with  
Figure 7: (**A**)—illustrative scheme for ZrBF’s biofunctionality, pH sensitivity, and functionalizability; (**B**)—adsorption kinetics of ZrBF-n (n = 1, 2, 4 and 8) for DNA. Insets—linear fit according to the pseudo-first order rate equation (a) and pseudo-second order rate equation (b), respectively. Reprinted with permission from [101]. Copyright (2013), American Chemical Society. \(\mathrm{K_{d}}\) of 78,000 and 30,000 mL/g, respectively. To evaluate the radiolytic stability of these hybrid materials, the sorbents were subjected to an irradiation with a Co-60 gamma source \(3.18\times 10^{6}\) Gray at a dose of \(1.617\times 10^{5}\) rad/hr. The hybrid materials retained their structure as evidenced by XRD and showed a similar uptake of Am\({}^{3+}\) ions after being irradiated [107]. Zirconium phosphonate were synthesized hydrothermally, using the benzene-1,3,5-triphosphonic acid (BTP) ligand to obtain both amorphous and crystalline (in the presence of HF) frameworks. These materials were evaluated for metal separation from a mixed solution containing the following ions: Cs\({}^{+}\), Sr\({}^{2+}\), La\({}^{3+}\), Ce\({}^{3+}\), Pr\({}^{3+}\), Nd\({}^{3+}\), Gd\({}^{3+}\), Dy\({}^{3+}\), Ho\({}^{3+}\), Co\({}^{2+}\) and Th\({}^{4+}\) with an ionic strength of 1 mmol/L and the nominal concentration of individual ions -20 mg/L in 0.1 M HNO\({}_{3}\) solution. Figure 8, shows the extraction of the different ions as a function of ionic radii of the metal ions by the hybrid Zr-phosphonate in function of the phosphate/zirconium ratio in the material varying from 0.5 (low) to 0.8 (high). The order of the selectivity of ions followed: Th\({}^{4+}\) > Ln\({}^{3+}\) > Co\({}^{2+}\), Sr\({}^{2+}\) > Cs\({}^{+}\), and the selectivity was dependent on the cation valence and polarizability. With higher molar amounts of phosphonic ligands, the \(\mathrm{K_{d}}\) increased, suggesting that the binding of the cations is achieved via the phosphonic acid groups distributed in the framework. A competitive kinetics study showed that at equilibrium (concentration of metal ions > 1 mg/L), Dy\({}^{3+}\) and Ho\({}^{3+}\) were strongly extracted in mixed cation solution containing the rest of the (REE) ions along with Cs\({}^{+}\), Sr\({}^{2+}\) and Co\({}^{2+}\). The authors highlighted in the same work that the materials that were amorphous and free of fluoride performed with enhanced selectivity and better kinetics than the crystalline materials in highly acidic solutions.