# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 3 Applications
### Separation and Extraction  
A summary of the application of porous metal phosphonates in separation and extraction are displayed in Table 1. An example is the mesoporous hybrid system based on aluminum with BHMT (N-based pentaphosphonic) and ATMP (N-based triphosphonic) phosphonate moieties in the presence of F127 as a surfactant, providing a hierarchical meso/macroporous texture (type-II isotherm) with inter-particle mesoporous and surface areas of 128 and 154 m\({}^{2}\)/g, respectively. They were evaluated for the extraction of copper ions from an aqueous solution (10-50 mg/L). The adsorption of Cu\({}^{2+}\) ions (10 mg/L) resulted in a K\({}_{\rm d}\) (distribution coefficient) of 277,485 and 17,281 mL/g for BHMT and ATMP aluminum phosphonate hybrids, respectively, at a pH \(\sim\)6-8. However, with higher Cu\({}^{2+}\) concentrations (50 mg/L) the K\({}_{\rm d}\) value decreased significantly to 6370 mL/g and 2744 mL/g for BHMT and ATMP aluminum frameworks, respectively. The adsorption was attributed to the copper chelation to nitrogen and RPO\({}_{3}\)\({}^{2-}\) groups present in the hybrid framework and their K\({}_{\rm d}\) (at lower Cu\({}^{2+}\) concentration) was comparable to mesoporous silica [92; 93]. Next to metal adsorption, the same materials were applied for the adsorption of large size molecules such as proteins, in this example lysozyme. The hybrid frameworks followed Langmuir type adsorption with a monolayer capacity of 11.8 umol/g (pH = 11), 10.42 umol/g (pH = 12), 9.23 umol/g (pH = 9.6), and 8.24 umol/g (pH = 6.5). The maximum adsorption capacity was observed at the isoelectric point of the protein (pI = 11). Therefore, the hydrophobic interactions between the protein molecules and the alkyl chains lining the pores of the hybrid BHMT-aluminum phosphonate are promising for development of adsorbents for large molecule-like proteins [94]. However, these hybrid materials display lower uptake in comparison to mesoporous silica (\(>\)100 umol/g) such as SBA-15 and PMOs [95; 96; 97; 98]. A hybrid framework based on zirconium phosphonate created from zirconium(IV) precursors and ATMP has been evaluated as cation-exchange adsorbent. This hybrid material was chosen for the separation of three groups of metals; transition metals (Cu\({}^{2+}\), Zn\({}^{2+}\), Ni\({}^{2+}\) and Co\({}^{2+}\)), heavy metals (Pb\({}^{2+}\), Hg\({}^{2+}\), and Cd\({}^{2+}\)) and f-block metals (Ce\({}^{3+}\) and Th\({}^{4+}\)). The preferential order of adsorption amongst the transition metals were Cu\({}^{2+}\) \(>\) Ni\({}^{2+}\)\(>\) Co\({}^{2+}\)\(>\) Zn\({}^{2+}\) (K\({}_{\rm d,Cu}\) = 9879.7, K\({}_{\rm d,Ni}\) = 15500.0, K\({}_{\rm d,Co}\) = 600.0, and K\({}_{\rm d,Zn}\) = 584.6 mL/g), for heavy metals Pb\({}^{2+}\)\(>\) Cd\({}^{2+}\)\(>\) Hg\({}^{2+}\) (K\({}_{\rm d,Pb}\) = 1261.5, K\({}_{\rm d,Cd}\) = 1037.5, and K\({}_{\rm d,Hg}\) = 56.4 mL/g), and for f-block metals Ce\({}^{3+}\)\(>\) Th\({}^{4+}\) (K\({}_{\rm d,Ce}\) = 2000.0, and K\({}_{\rm d,Th}\) = 114.7 mL/g) in aqueous solution. However, in acetic acid, HNO\({}_{3}\), and ammonium nitrate solutions, the distribution coefficients were reduced due to ion competitions. Amongst the metal ions studied, only Cu\({}^{2+}\) (in aqueous, 0.02 and 0.2 M acetic acid) and Ce\({}^{3+}\) (in 0.02 M acetic acid) showed complete sorption with a high separation factor (SF) between Cu\({}^{2+}\)/Zn\({}^{2+}\) (16.9) and Ce\({}^{3+}\)/Th\({}^{4+}\) (17.54) [99]. Similarly, hybrid manganese phosphonate synthesized from MnCl\({}_{2}\) and EDTMP was evaluated for metal ion uptake and protein adsorption. The optimal pH for the Cu\({}^{2+}\) uptake was \(\sim\)6 and the adsorption followed Langmuir behavior with a maximum monolayer capacity of 79.5 mg/g at 20 \({}^{\circ}\)C. This value is in the same order of magnitude as that of mesoporous silica adsorbents (\(\sim\)24.8 mg/g) and hybrid titanium phosphonates (also made with EDTMP showed \(\sim\)28.8 mg/g uptake) [100].