# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 3 Applications
### Separation and Extraction
Zirconium hybrid phosphonate frameworks made from 1-phosphomethylproline (H\({}_{3}\)PMP) and 1,4-bis(phosphomethyl) piperazine (BPMP) phosphonic ligands resulted in a pH-sensitive porous framework. This framework was employed for the adsorption and delivery of nucleic acids without further modification/post-treatment. The adsorption of salmon sperm DNA onto the framework followed Langmuir type adsorption. Among the various hybrids created by this method, the ZrBF-2 showed the largest equilibrium monolayer capacity of 238.6 mg/g. The increased uptake was attributed to the presence of uncondensed P-OH groups and pyridinic N-atoms present along the porous framework of the hybrid material [101]. This uptake by the hybrid framework is comparable to salmon sperm DNA uptake of 375 and 110.7 mg/g by magnetic mesoporous silica adsorbents [102; 103]. The mechanism of adsorption followed pseudo-second order kinetics. Moreover, the framework was pH responsive, allowing adsorption-desorption cycles by changes in the pH. The structure exhibited a cationic surface under acidic and neutral pH where the surface could adsorb negatively charged DNA molecules by electrostatic interactions. Subsequently, the release of the nucleic acid was triggered by increasing the pH, changing the surface charge and resulting in desorption. This allowed for the pH controlled loading and unloading of nucleic acids as illustrated by Figure 7 [101]. In another application, zirconium phosphonate frameworks synthesized by zirconium(IV) and phenyl phosphonic acid in the presence of an SDS template were employed as immobilized metal affinity chromatography (IMAC) for the selective enrichment of phosphopeptides. The specificity for phosphopeptides was evaluated with use of a tryptic digest consisting of a mixture of bovine serum albumin (BSA) and bovine-\(\beta\)-casein (B-\(\beta\)-C) as the sample peptides (mass ratio B-\(\beta\)-C:BSA = 1:5). The specificity of the bovine-\(\beta\)-casein adsorption was attributed to the chelation of phosphopeptide to the Zr(IV) metal centers present in the framework [104]. Finally, we shall go through the use of porous metal phosphonates for the solid-phase extraction of lanthanides and actinides. Tin phosphonate synthesized from various polyphosphonic acids (HEDP, ATMP, EDTMP and DTPMP) was employed for radionuclide solid-phase separation. The study consisted of: a batch type sorption of metal ions Cs\({}^{+}\), Sr\({}^{2+}\), Al\({}^{3+}\), Fe\({}^{3+}\), Th\({}^{4+}\), UO\({}_{2}\)\({}^{2+}\), and rare earth elements (REE) (all lanthanides) in 0.1, 1 and 3 M HNO\({}_{3}\) solutions and a competitive sorption kinetics study for the metal ions Eu\({}^{3+}\), Fe\({}^{3+}\), La\({}^{3+}\), Lu\({}^{3+}\), Sr\({}^{2+}\), Th\({}^{4+}\), and UO\({}_{2}\)\({}^{2+}\) in 1M HNO\({}_{3}\) solution. In the 0.1 M HNO\({}_{3}\) solution, tin phosphonate hybrids containing HEDP displayed a high uptake of middle lanthanides (K\({}_{\rm d}>\) 6000 mL/g), whereas the EDTMP displayed a high uptake of late lanthanides (K\({}_{\rm d}>\) 4000 mL/g), showing the importance of the type of phosphonic linker. However, a high uptake of Fe\({}^{3+}\) (K\({}_{\rm d}\) \(\sim\)1000-10,000 mL/g) was observed among all the hybrids in 0.1 M HNO\({}_{3}\) solution. An interesting observation was that all the hybrid materials showed extreme affinity for the Th\({}^{4+}\) radionuclide in all nitric acid solutions (K\({}_{\rm d}\) \(\sim\)127,171 mL/g and 164,399 mL/g for ATMP and EDTMP hybrids). Nevertheless, although these hybrids showed promising capabilities in use as a solid-phase adsorbent, they displayed slow kinetics for the uptake of radionuclides [105]. Mixed hybrids of zirconium phosphonate-phosphate (phosphonate/phosphate ratio from 1.5:1 to 1:8) were evaluated for solid-phase lanthanide extraction from an aqueous mixture of metal ions. In this study, the choice of metal ions were Nd\({}^{3+}\), Sr\({}^{2+}\), and Cs\({}^{+}\).