Chelating agents are widely used in a variety of environmental and industrial separation processes. These include (i) selective extractants in hydrometallurgy, (ii) metal ion binding functionalities for ion exchange resins, and (iii) high- capacity polymeric ligands for water treatment [ 7 ]. The complexation of metal ions is an acid–base reaction that depends on several parameters including (i) metal ion size and acidity, (ii) ligand molecular architecture and basicity, and (iii) solution physicochemical conditions [ 7 ]. Although macrocyles and their
“open chain” analogues (unidentate and polydentate ligands) have been shown to form stable complexes with a variety of metal ions [ 7 ], their limited binding capacity (i.e., 1:1 complexes in most cases) is a major impediment to their utilization as high-capacity chelating agents for industrial and environmental separations. Their relatively low molecular weights also preclude their eff ective
Figure 11.1 Selected classes of dendritic nanopolymers.
Dendrimers
Hyperbranched polymers
Dendrigraft polymers Core-shell tecto(dendrimers)
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recovery from industrial wastewater streams by low cost membrane-based techniques (e.g., UF). The invention of dendrimers is providing unprecedented opportunities to develop high–capacity, recyclable chelating agents with high molar mass and well-defi ned molecular composition, size, and shape.
Poly(amidoamine) (PAMAM) dendrimers provide good model systems for probing the aqueous coordination chemistry of cations with dendritic nanopolymers. These dendrimers were the fi rst dendrimer family to be synthesized, characterized, and commercialized. PAMAM dendrimers ( Fig. 11.2 ) possess amide, tertiary and primary amine groups arranged in regular “branched upon branched” patterns, which are displayed in geometrically progressive numbers as a function of generation level. This high density of nitrogen ligands in concert with the possibility of attaching various functional groups such as amines, carboxyl, and so on to PAMAM dendrimers make them particularly
Figure 11.2 Structure of G4-NH 2 poly(amidoamine) dendrimer.
NH2 NH2 NH2 NH2
N
N N
N N
N
N
N N
N
N N
N N
N
N N
N N N
N N
N N
N N N
N
N N
N N
N N
N N N N N N
N N
N
N N
N N N
N
N
N N
N
N N N
N N
N
N N
NH O
O O
O O O O
O
O
O
O
O
O
O O O O O
O O O O
O
O O
OO
OO
OO
OO
OO OO
O O
OO O O
O O O
O
O O
O OO
O OO
O O
O
O O
O O
O
O
O O
O
O
O O
O OO O
O OO
O O O OO OO O
O O
O O
O OO O O O O
O O O
OO O
O
O
O
O
O O O
OO
O
O OO
O
OO O
O
O O
O O NH NH NH NH NH NH NH NH NH NH NH NH
NH
NH NH
NH NH
NH
NH NH
NH NH NH
NH NH
NH NH
NH
NH NH
NH NH
NH NH
NH NH
NH NH
NH
NH NH
NH
NH NH
NH
NH NH
NH NH
NH
NH
NH
NH NH
NH
NH
NH
NH NH NH
NH
NH NH
NH NH
NH NH NH
NH
NH NH NH
NH
NH
NH NH NH
NH NH NH NH NH
NH NH NHNH
NH NH
NH NH
NH NH NH NH NH
NH NH NH NH NH NH NH NH NH NH NH NH NH NH NH NH
NH NH NH NHNH
NH NH2 NH2
NH2 NH2 NH2 NH2
NH2 NH2 NH2 NH2 NH2
NH2
NH2 NH2 NH2
NH2 NH2
NH2 NH2
NH2 NH2
NH2 NH2
NH2 NH2 NH2 NH2 NH2 NH2 NH2
NH2 NH2
NH2 NH2
NH2 NH2
NH2 NH2 NH2
NH2 NH2
NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2
N H
NH
N H
N H NH
NH
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attractive as high-capacity chelating agents for cations including transition metals, lanthanides, and actinides [ 8 , 9 ].
Diallo et al. [ 8 , 9 ] have carried out an extensive study of Cu(II) and U(VI) binding to PAMAM dendrimers of diff erent generations and terminal groups.
Figure 11.3(a) and (b) shows the eff ects of metal ion dendrimer loading and solution pH on the extent of binding (EOB; i.e., number of moles of bound metal ions per mole of dendrimer] and fractional binding (FB) of Cu(II) in aqueous solutions of a G4-NH 2 EDA core PAMAM dendrimer. The tertiary amine groups of this dendrimer have a pKa of 6.30–6.85 [ 9 ]. Conversely, the pKa of its primary amine groups is 9.0–10.2 [ 9 ]. At pH 9, the EOB of Cu(II) increases linearly with metal ion dendrimer loading within the range of tested
Figure 11.3 (a) Extent of binding of Cu(II) in aqueous solutions of G4-NH 2
poly(amidoamine) (PAMAM) dendrimer at room temperature [ 9 ]. (b) Fractional binding of Cu(II) in aqueous solutions of G4-NH 2 PAMAM dendrimer at room temperature [ 9 ].
(a)
Extent of binding (mol/mol)
160 140 120 100 80 60 40 20 0
G4-NH2 pH 7 replicate 1
EOBmax2=74.0
EOBmax1=12.0 G4-NH2 pH 5
G4-NH2 pH 9
G4-NH2 pH 7 replicate 3 G4-NH2 pH 7 replicate 2
Metal ion dendrimer loading (mol/mol)
0 20 40 60 80 100 120 140 160
(b)
Fractional binding (%) 50 100 150
0
G4-NH2 pH 7 replicate 1
G4-NH2 pH 5 G4-NH2 pH 9
G4-NH2 pH 7 replicate 3 G4-NH2 pH 7 replicate 2
Metal ion dendrimer loading (mol/mol)
0 20 40 60 80 100 120 140
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metal ion dendrimer loadings. In all cases, 100 percent of the Cu(II) ions are bound to the dendrimers. This behavior is attributed to the low extent of protonation of the dendrimer amine groups. When these groups become fully protonated at pH 5.0, no binding of Cu(II) is observed ( Fig. 11.3(a) ). A more complex metal ion uptake behavior is observed at pH 7.0. In this case, the EOB of Cu(II) in aqueous solutions of the G4-NH 2 PAMAM dendrimer go through a series of two distinct binding steps as metal ion dendrimer loading increases ( Fig. 11.3(a) ). A more detailed discussion of Cu(II) coordination with PAMAM dendrimers is given elsewhere [ 9 ].
Figure 11.4(a) and (b) highlights the binding of U(VI) to G4-NH 2 PAMAM dendrimer in deionized water and NaCl solutions [ 8 ]. At pH 7.0 and 9.0, the
Figure 11.4 (a) Extent of binding of U(VI) in aqueous solutions of G4-NH 2
poly(amidoamine) (PAMAM) dendrimer at room temperature [ 8 ]. (b) Eff ect of NaCl on the extent of binding of U(VI) in aqueous solutions of G4-NH 2 PAMAM dendrimer at room temperature [ 8 ].
(a)
Metal ion dendrimer loading (mol/mol)
0 50 100 150 200 250
0 50 100 150 200 250
Extent of binding (mol/mol)
pH 7.0 (replicate 1)
pH 3.0 pH 5.0 pH 9.0
pH 7.0 (replicate 2)
(b)
Metal ion dendrimer loading (mol/mol)
0 50 100 150 200 250
0 50 100 150 200 250
Extent of binding (mol/mol)
pH 7.0 G4-NH2
pH 3.0 and 1.0M NaCl pH 3.0 and 0.1M NaCl pH 7.0 and 1.0M NaCl pH 7.0 and 0.1M NaCl
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G4-NH 2 PAMAM dendrimer can bind up to 220 U(VI) ions without reaching saturation. The uranyl FB is greater than 92 percent in all cases. At pH 3.0, Fig. 11.4(a) also shows signifi cant binding of U(VI) to the G4-NH 2 PAMAM dendrimer (with FB approximately 76–87 percent and EOB up to 180) even though its tertiary and primary amine groups are fully protonated in this case.
Note that no binding of Cu(II) by the dendrimer was observed at pH 5.0 ( Fig. 11.3(a) ). This strongly suggests that uranyl complexation by the G4-NH 2 PAMAM dendrimer at pH 3.0 and 5.0 involves the deprotonation of its amine groups followed by coordination with the UO 2 2+ metal ion. Diallo et al. [ 8 ] were able to suppress the uptake of U(VI) by the G4-NH 2 PAMAM in aqueous solutions containing at least 0.1 M (5.8 g/L) of sodium chloride at pH 3.0 ( Fig. 11.4(b) ). The overall results of the metal binding experiments strongly suggest that dendritic nanopolymers such as PAMAM can serve as high- capacity, selective, and recyclable chelating ligands for transition metal ions (e.g., Cu(II)) and actinides (e.g., U(VI)) [ 8 , 9 ].