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Further experiments confirmed that, besides the ethylenediamine groups, many
electronegative groups or atoms including -OH, -SH, and N could contribute to
metal ion adsorption. Moreover, competitive adsorption experiments demon-
strated that the phosphonate-based adsorbents had an innate selective affinity for
the adsorption of one particular ion over the others. A distinct preference of the
PMTP-1 adsorbent for the uptake of Cu
2
+
ions, compared with that of Pb
2
+
and
Cd
2
+
ions, was observed. This phenomenon was observed in the previous ethyl-
enediamine-containing mesoporous silica, which had also a distinct preference for
the uptake of Cu
2
+
ions over Ni
2
+
and Zn
2
+
ions [
18
]. Therefore, it is important
from a technical point of view to select suitable coupling molecules with a specific
structure and to enlarge the surface area and pore volume of the hybrid materials,
and thus to improve the adsorption performance.
Meanwhile, the adsorption characteristics of organic pollutants on mesoporous
metal phosphonate materials were also identified. It was revealed that mesoporous
titanium phosphonate PMTP-1 exhibited excellent adsorption performance for
the cationic dye methylene blue (MB) as target pollutant from aqueous solution
[
19
]. The adsorption equilibrium was achieved after 30 min of contact time, and
the adsorption of MB on PMTP-1 was best fitted to the Langmuir isotherm model
with the maximum monolayer adsorption capacity of 617.28 mg g
−
1
, indicating
that the PMTP-1 could be used as an efficient adsorbent for the removal of textile
dyes from effluents. Results of kinetic studies indicated that the adsorption process
followed the pseudo-second-order model, which suggests that the process might
involve chemisorption.
The adsorption of biomacromolecules such as proteins from solution onto
solid surfaces is also of great scientific importance in many areas, such as biol-
ogy, medicine, biotechnology, and food processing [
20
]. Under pH conditions
close to the isoelectric point, the adsorption of lysozyme on aluminum phospho-
nate hybrid materials was dominated by host-guest hydrophobicity-hydrophobic-
ity interactions [
21
]. Interestingly, unlike inorganic framework adsorbents used for
the adsorption of proteins [
22
], the porous phosphonate hybrid adsorbents had an
organic-inorganic framework, which contains plenty of hydrophobic alkyl groups
inside the framework [
21
]. The hydrophobicity/hydrophilicity could be controlled
at a chemical dimension. So the hydrophobic interactions between the organic
groups inside the channel walls and the nonpolar side chains of the amino acids
on the surface of lysozyme were greatly enhanced, leading to an increased mon-
olayer adsorption capacity. When extra-long hydrophobic alkyl chains (-[CH
2
]
6
−
)
were incorporated, the resultant adsorption ability was higher than for organo-
phosphonates with fewer hydrophobic -CH
2
- groups. Since various biomolecules
exhibit distinct isoelectric points and spatial sizes, the molecules can be effectively
adsorbed and separated by changing the pH and the porosity and pore structures of
metal phosphonates.
Chromatography (e.g., gas and liquid phase) is one of the most power-
ful separative methods in analytical chemistry. PMOs and organically modified
mesoporous silicas have been employed as the stationary phases in reverse-phase
high-performance liquid chromatography (HPLC) in the form of packed columns,
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