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Water Treatment by Dendrimer-Enhanced Filtration
CHAPTER
15
Water Treatment by Dendrimer-Enhanced Filtration: Principles and Applications
Mamadou S. Diallo1,2 1
Materials and Process Simulation Center, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA 2 Department of Civil Engineering, Howard University, Washington, DC, USA
15.1 Introduction ...................................................................................................227 15.2 Dendrimers as recyclable ligands for cations...................................................229 15.3 Dendrimers as recyclable ligands for anions....................................................233 15.4 Dendrimer-enhanced filtration: overview and applications ................................235 15.5 Summary and outlook .....................................................................................238 Acknowledgments ...................................................................................................238 References .............................................................................................................239
15.1 Introduction The availability of clean water has emerged as one of the most serious problems facing the global economy in the twenty-first century. Water treatment systems typically involve a series of coupled processes, each designed to remove one or more different substances in the source water, with the particular treatment process being based on the molecular size and properties of the target contaminants. Membrane processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) are emerging as key components of water treatment, reuse, and desalination systems throughout the world [1 3]. RO is very effective at retaining dissolved inorganic and small organic molecules. NF can effectively remove hardness (e.g., Ca(II)) and natural organic matter. However, high pressures (100 1,000 psi) are required to operate both RO and NF membranes. Conversely, UF and MF membranes require lower pressure Street, Sustich, Duncan and Savage. Nanotechnology Applications for Clean Water, 2nd Edition. © 2014 Elsevier Inc. All rights reserved. DOI: http://dx.doi.org/10.1016/B978-1-4557-3116-9.00015-9
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(5 60 psi) but unfortunately cannot retain dissolved ions and organic solutes. Advances in nanochemistry such as the invention of dendritic nanopolymers are providing unprecedented opportunities to develop enhanced UF and MF processes for recovering dissolved ions from aqueous solutions. Dendritic nanopolymers are highly branched 3D globular nanoparticles with controlled composition and architecture and sizes in the range of 1 100 nm [4]. They consist of three components: a core, interior branch cells, and terminal branch cells. Dendritic nanopolymers include hyperbranched polymers, dendrigraft polymers, dendronized linear polymers, dendrimers, and many other supramolecular assemblies such as core-shell tecto(dendrimers) and dendrimer-like star polymers (Fig. 15.1) [4]. They exhibit a number of critical physicochemical properties that make them attractive as separation and reaction media for water purification. Dendritic nanopolymers can encapsulate a broad range of solutes in water including cations (e.g., copper, silver, gold, and uranium), anions (e.g., perchlorate, nitrate, and phosphate), and organic compounds (e.g., pharmaceuticals and pesticides) [4 6]. Dendritic nanopolymers can serve as nanoscale reactors and catalysts [4]. They can also bind and deactivate bacteria and viruses [4]. Their globular shape and large size makes them easier to filter than linear polymers [5,6]. Diallo and coworkers are exploiting these unique properties of dendritic nanopolymers to Dendrimers
Dendrigraft polymers
FIGURE 15.1 Selected classes of dendritic nanopolymers.
Hyperbranched polymers
Core-shell tecto(dendrimers)
15.2 Dendrimers as recyclable ligands for cations
develop enhanced UF and MF processes for recovering dissolved cations and anions from aqueous solutions [5,6]. This chapter gives an overview of the principles and applications of dendrimer-enhanced filtration (DEF) [5,6]. The use of DEF in the treatment of industrial wastewater contaminated by heavy metals (e.g., copper) and radionuclides (e.g., uranium) and the remediation of groundwater contaminated by anions (e.g., perchlorate) are highlighted.
15.2 Dendrimers as recyclable ligands for cations 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 effective 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 welldefined 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 first dendrimer family to be synthesized, characterized, and commercialized. PAMAM dendrimers (Fig. 15.2) 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 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 different generations and terminal groups. Figure 15.3A and B shows the effects 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-NH2 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
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NH2 NH2 NH2
N
NH NH O O
NH2 NH2
NH NH O O
NH2
NH O O NH2 NH
NH
N
NH O NH2 NH O
NH O O NH
N
OO
N
NH2
O O NH NH
NH2
NH2
N
N
N
N N
NH
NH
O
NH
N
O
N
N
O O NH NH
NH
NH
NH2
NH2
NH2
O
O
O NH
O
NH2
N
O O NH NH
OO NH NH2
NH
NH
NH O ONH
NH2 NH2
NH O O NH
NH2
OO
NH NH2
NH2
NH2 NH
NH OO
NH2
NH NH2 O O NH NH2
O NH O NH
N N
NH2
NH2
NH
N
NH
N
O N
NH O
N
O NH
NH
N
NH2
N O
O
N
N
NH O O NH
NH N
NH
O O NH NH2 NH
N
N H
N
O NH
N
O NH
O NH
NH
NH2
O
NH
NH
N
NH
O NH
O
N
O NH O
N
O
O
O
NH O
NH
O O NH
N
NH O
NH OO
NH
N H
N N
O N NH
O
NH2
NH O ONH
O NH
N H
NH
NH
O
O NH
O O NH
N
N
O NH
N
O
NH O
N
NH2
N
O
NH
O
O N
NH2 N O O H NH NH2
N H O
N NH
NH O O
NH O O NH2 NH
NH2 N H
NH O
O
O NH
NH O
NH
N
NH2
O
N
N
O
O
N
NH2
O NH
N NH
N
N NH
N
NH2
NH2 NH O NH2 NH O O NH N O NH2 N NH O NH2 O N NH NH NH NH2 NH O NH2 O NH OO N N O NH NH2 NH NH NH N OO O NH2 O N NH NH
N N
NH2 NH2 NH
NH O
NH O
NH O
NH
N
N
NH NH O O NH2
NH O O
N
O
NH NH O O
NH
O O
O
NH2
NH2
NH NH O O
NH NH
NH
NH
NH2
NH2 NH2
NH2 NH2
NH2
NH2 NH2
NH2
NH2 NH2
NH2
FIGURE 15.2 Structure of G4-NH2 poly(amidoamine) dendrimer.
ion dendrimer loading within the range of tested 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. 15.3A). 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-NH2 PAMAM dendrimer go through a series of two distinct binding steps as metal ion dendrimer loading increases (Fig. 15.3A). A more detailed discussion of Cu(II) coordination with PAMAM dendrimers is given elsewhere [9]. Figure 15.4A and B highlights the binding of U(VI) to G4-NH2 PAMAM dendrimer in deionized water and NaCl solutions [8]. At pH 7.0 and 9.0, the G4-NH2 PAMAM dendrimer can bind up to 220 U(VI) ions without reaching saturation.
15.2 Dendrimers as recyclable ligands for cations
(A)
Extent of binding (mol/mol)
160 G4-NH2 pH 7 replicate 1 G4-NH2 pH 7 replicate 2 G4-NH2 pH 7 replicate 3 G4-NH2 pH 9 G4-NH2 pH 5
140 120 100
EOBmax2=74.0
80 60 40 20
EOBmax1=12.0
0 0
20
40 60 80 100 120 Metal ion dendrimer loading (mol/mol)
140
160
Fractional binding (%)
(B) G4-NH2 pH 7 replicate 1 G4-NH2 pH 7 replicate 2 G4-NH2 pH 7 replicate 3 G4-NH2 pH 9 G4-NH2 pH 5
150
100
50
0 0
20
40 60 80 100 120 Metal ion dendrimer loading (mol/mol)
140
FIGURE 15.3 (A) Extent of binding of Cu(II) in aqueous solutions of G4-NH2 poly(amidoamine) (PAMAM) dendrimer at room temperature [9]. (B) Fractional binding of Cu(II) in aqueous solutions of G4-NH2 PAMAM dendrimer at room temperature [9].
The uranyl FB is greater than 92 percent in all cases. At pH 3.0, Fig. 15.4A also shows significant binding of U(VI) to the G4-NH2 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. 15.3A). This strongly suggests that uranyl complexation by the G4-NH2 PAMAM dendrimer at pH 3.0 and 5.0 involves the deprotonation of its amine groups followed by coordination with
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CHAPTER 15 Water Treatment by Dendrimer-Enhanced Filtration
(A)
Extent of binding (mol/mol)
250 pH 7.0 (replicate 1) pH 7.0 (replicate 2) pH 9.0 pH 5.0 pH 3.0
200 150 100 50 0 0
50 100 150 200 Metal ion dendrimer loading (mol/mol)
250
(B) 250 Extent of binding (mol/mol)
232
pH 7.0 pH 7.0 and 0.1M NaCl pH 7.0 and 1.0M NaCl pH 3.0 and 0.1M NaCl pH 3.0 and 1.0M NaCl
200 150
G4-NH2
100 50 0 0
50
100
150
200
250
Metal ion dendrimer loading (mol/mol)
FIGURE 15.4 (A) Extent of binding of U(VI) in aqueous solutions of G4-NH2 poly(amidoamine) (PAMAM) dendrimer at room temperature [8]. (B) Effect of NaCl on the extent of binding of U(VI) in aqueous solutions of G4-NH2 PAMAM dendrimer at room temperature [8].
the UO221 metal ion. Diallo et al. [8] were able to suppress the uptake of U(VI) by the G4-NH2 PAMAM in aqueous solutions containing at least 0.1 M (5.8 g/L) of sodium chloride at pH 3.0 (Fig. 15.4B). 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].
15.3 Dendrimers as recyclable ligands for anions
15.3 Dendrimers as recyclable ligands for anions Anions have emerged as major water contaminants throughout the world. In the United States, the discharge of anions such as perchlorate (ClO4 ) and nitrate (NO3 ) into publicly owned treatment works, surface water, groundwater, and coastal water systems is having a major impact on water quality. Although significant research efforts have been devoted to the design and synthesis of selective chelating agents for cation separations [7], anion separations have comparatively received limited attention [10,11]. The design of selective ligands for anions is a challenging undertaking. Unlike cations, anions have filled orbitals and thus cannot covalently bind to ligands [10,11]. Anions have a variety of geometries (e.g., trigonal for NO3 and tetrahedral for ClO4 ) and are sensitive to solution pH in many cases [3,4]. Thus, shape-selective and pH-responsive receptors may be needed to effectively target anions. The charge-to-radius ratios of anions are also lower than those of cations. Thus, anion binding to ligands through electrostatic interactions tends to be weaker than cation binding. Anion binding and selectivity also depend on (i) anion hydrophobicity and (ii) solvent polarity [3,4]. As a first step toward the development of high-capacity, selective, and recyclable dendritic ligands for anions such as perchlorate, Diallo et al. [12] tested the hypothesis that dendrimers with hydrophobic cavities and positively charged internal groups should selectively bind ClO4 over more hydrophilic anions such as Cl , NO3 , SO42 , and HCO3 . They measured the uptake of ClO4 by the fifth generation (G5-NH2) poly(propyleneimine) (PPI) dendrimer with a diamobutane core and terminal NH2 groups (Fig. 15.5) in deonized water and model electrolyte solutions as a function of (i) anion dendrimer loading, (ii) solution pH, (iii) background electrolyte concentration, and (iv) reaction time [12]. Figure 15.6A and B shows the effects of anion dendrimer loading and solution pH on the EOB and FB of ClO4 to a G5-NH2 PPI dendrimer in aqueous solutions at room temperature and reaction time of 1 hour. The pKa of the dendrimer tertiary and primary amine groups are, respectively, equal to 6.10 and 9.75 [12]. At pH 4.0, 99 percent of the tertiary amine groups of the G5-NH2 PAMAM are protonated [12]. As shown in Fig. 15.6B, 98 percent of ClO4 are bound to the dendrimer in this case when the anion dendrimer loading is approximately 0.31. The corresponding EOB is approximately 9.0. The FB of ClO4 in aqueous solutions of the G5-NH2 PPI dendrimer is approximately 53 percent at pH 7.0 with an anion dendrimer loading of 0.31. In this case, only 11 percent of the tertiary amine groups of the dendrimer are protonated, whereas 99 percent of its primary amine groups remain protonated. Note that the maximum EOB of ClO4 (approximately 2.5 at anion dendrimer loading of 32.0) decreases by a factor of 4 at pH 7.0 compared to that at pH 4.0. The maximum EOB of ClO4 (approximately 2.20) is also smaller at pH 9.0 and anion dendrimer loading of 32.0 (Fig. 15.6A) even though approximately
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CHAPTER 15 Water Treatment by Dendrimer-Enhanced Filtration
H3N H3 N H3N H3N
H3 N
NH3 NH 3 NH3 H3N NH3 NH3
H3N
N
N
N
N
N
N
N N
N
NH3
N
N
H3N
NH3
N N
N
H3N
NH3 N
N
H3N H3N
NH3 NH3 N
N
N
N
NH3 NH3 NH3
N
N
N
N
H3N
NH3
N
N
H3N H3N
N
N
NH3 NH3
NH3 N
NH3
N
NH3
N
N
N
NH3
N
H3N N
N
N
H3N
N
N
N
H3N
NH3
N
N
H3N
N
N
N
N
N
N H3N
N
H3N
N
N
N
H3N
H3N
NH3 NH3
N N
N
NH3 NH3
H3N
H3N
NH3
N
N H3N H3N H3N
NH3
N
N N
NH3
N
N
H3N
NH3
NH3 NH3 NH3 NH3
NH3
NH3
NH3
FIGURE 15.5 Structure of G5-NH2 poly(propyleneimine) dendrimer [8].
81 percent of the dendrimer NH2 groups remain protonated. This suggests that electrostatic interactions between ClO4 and protonated NH2 groups of the dendrimer do not have a significant effect on perchlorate uptake. Figure 15.6A also shows some binding of ClO4 (with a maximum EOB of approximately 1.29 at anion dendrimer loading of 32.0) at pH 11.0 when both the tertiary and primary amines of the G5-NH2 PPI dendrimer are unprotonated. A more detailed discussion of the mechanisms of perchlorate binding to the G-NH2 PPI dendrimer is given elsewhere [12]. The overall results of the anion binding experiments suggest that dendritic macromolecules such as the G5-PPI NH2 PPI dendrimer provide ideal building blocks for the development of high capacity, selective, and recyclable ligands for anions such as ClO4 .
15.4 Dendrimer-enhanced filtration: overview and applications
(A) pH 4.0 pH 7.0 pH 9.0 pH 11.0
Extent of binding (mol/mol)
14 12 10 8 6 4 2 0 0
20
40
60
80
Perchlorate–dendrimer loading (mol/mol) (B)
Fractional binding (%)
100 80 pH 4.0 pH 7.0 pH 9.0 pH 11.0
60 40 20 0 0
20
40
60
80
Anion–dendrimer loading (mol/mol)
FIGURE 15.6 (A) Extent of binding of ClO4 in aqueous solutions of G5-NH2 PPI dendrimer at room temperature. (B) Fractional binding of ClO4 in aqueous solutions of G5-NH2 PPI dendrimer at room temperature. Data for (A) and (B) taken from [12].
15.4 Dendrimer-enhanced filtration: overview and applications As stated in the introduction, the invention of dendritic nanopolymers is providing unprecedented opportunities to develop enhanced UF and MF processes for recovering dissolved ions from aqueous solutions. The DEF process developed by Diallo [5,6] is structured around three unit operations (Fig. 15.7): (i) treatment
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CHAPTER 15 Water Treatment by Dendrimer-Enhanced Filtration
Dendrimer solution
Contaminated water
Clean water recovery unit
Dendrimer recovery unit
UF/MF system
UF/MF system
Treatment unit
Clean water
Concentrated contaminant solution
FIGURE 15.7 Water treatment by dendrimer enhanced filtration. Adapted from [5]. UF: Ultrafiltration; MF: Microfiltration.
unit; (ii) clean water recovery unit; and (iii) dendrimer recovery unit. In the treatment unit, contaminated water is mixed with a solution of functionalized dendritic nanopolymers to carry out the specific reactions of interest (e.g., cation and anion binding). Following completion of the reaction, the complexes of nanopolymers 1 bound contaminants are sent to the clean water recovery unit where they are filtered using UF or MF to recover the clean water. The resulting concentrated solution of nanopolymers 1 target substance is subsequently sent to the recovery unit. This system consists of an UF or MF unit in which the bound target substance is separated from the nanopolymers by, for example, changing the acidity (i.e., pH) of the solution. Finally, the recovered concentrated solution of contaminants is collected for disposal or subsequent processing whereas the nanopolymers are recycled [5,6]. The key novel feature of the proposed DEF process is the combination of dendritic polymers with multiple chemical functionalities with the well-established technology of UF and MF. This allows for the development of a new generation of water treatment processes that are flexible, reconfigurable, and scalable [5]. The flexibility of DEF is illustrated by its modular design approach. DEF systems can be designed to be “hardware invariant” and thus reconfigurable in most cases by simply changing the “dendrimer formulation” and process conditions for the targeted contaminants [5]. Because DEF is a membrane-based process, it is scalable and could be used to develop small and mobile water treatment systems as well as large and fixed treatment systems. As a proof-of-concept study, Diallo et al. [6] have combined bench scale measurements of metal ion binding to Gx-NH2 PAMAM dendrimers with dead-end UF experiments to assess the feasibility of using DEF to recover Cu(II) from aqueous solutions. On a mass basis, the Cu(II) binding capacities of the Gx-NH2 PAMAM
15.4 Dendrimer-enhanced filtration: overview and applications
(A)
Cu(II) retention (%)
120
Regenerated cellulose
100 80 G4-NH2 pH =7.0 G4-NH2 pH =7.0 G4-NH2 pH =4.0 G4-NH2 pH =4.0
60 40
10 kD 5 kD 10 kD 5 kD
20 0 0
1
2 3 Filtration time (hours)
4
5
4
5
(B)
Normalized permeate flux
1.2
Regenerated cellulose UF membrane
1.0 0.8 pH =7.0 10kD pH =7.0 5kD pH =4.0 10kD pH =4.0 5kD
0.6 0.4 0.2 0.0 0
1
2 3 Filtration time (hours)
FIGURE 15.8 (A) Retention of Cu(II) complexes with G4-NH2 poly(amidoamine) (PAMAM) dendrimer by regenerated cellulose ultrafiltration (UF) membranes [6]. (B) Normalized permeate flux of aqueous solutions of Cu(II) 1 G4-NH2 PAMAM dendrimer through regenerated cellulose membranes [6].
dendrimers are much larger and more sensitive to solution pH than those of linear polymers with amine groups [6]. Separation of the dendrimer Cu(II) complexes from solutions can be achieved simply by UF (Fig. 15.8A). The metal ion laden dendrimers can then be regenerated by decreasing the solution pH to 4.0 5.0 [6], thus enabling the recovery of the bound Cu(II) and recycling of the dendrimer. Dendritic nanopolymers such the Gx-NH2 PAMAM dendrimers have also much
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CHAPTER 15 Water Treatment by Dendrimer-Enhanced Filtration
less tendency to pass through the pores of UF membranes than do linear polymers of similar molar mass because of their much smaller polydispersity and globular shape [6]. As shown in Fig. 15.8B, the Gx-NH2 EDA core PAMAM have also a very low tendency to foul the commercially available regenerated cellulose (RC) membranes. Dendritic nanopolymers have also much smaller intrinsic viscosities than linear polymers with the same molar mass because of their globular shape [6]. Thus, comparatively smaller operating pressure, energy consumption, and loss of ligands by shear-induced mechanical breakdown could be achieved with dendritic polymers in cross-flow UF systems typically used industrial water treatment [2]. These unique properties of the Gx-NH2 EDA core PAMAM dendrimers make DEF (Fig. 15.7) an attractive process for recovering metal ions such as Cu(II) from contaminated water. Other applications of DEF including the recovery of ClO4 and U (VI) from aqueous solutions are discussed elsewhere [8,12].
15.5 Summary and outlook Dendritic nanopolymers are among the most chemically and structurally diverse classes of nanomaterials available to date. These “soft” nanoparticles, with sizes in the range of 1 100 nm, can serve as high-capacity, recyclable ligands for cations and anions. The DEF process exploits these unique properties of dendritic nanopolymers to develop a new generation of low-pressure filtration processes for treating water contaminated by toxic metal ions (e.g., Cu(II) and U(VI)) and oxyanions (e.g., ClO4 ). A start-up company (Aqua Nano Technologies) has been set up in California to commercialize the DEF technology. Initial target applications include: 1. Recovery of perchlorate from contaminated groundwater. 2. Recovery of uranium from in situ mining leach solutions and contaminated groundwater. 3. Recovery of metal ions (e.g., copper, silver, nickel, and zinc) from industrial wastewater.
Acknowledgments I thank the US National Science Foundation (NSF Grants CTS-0086727, CTS-0329436, and NIRT CBET-0506951) and the US Environmental Protection Agency (NCER STAR Grant R829626) for funding my research on the use of dendritic nanopolymers as functional materials for water purification. Partial funding for this research was also provided by the Department of Energy (Cooperative Agreement EW15254), the W.M. Keck Foundation, and the National Water Research Institute (Research Project Agreement NO 05-TT-004).
References
References [1] US Bureau of Reclamation and Sandia National Laboratories, Desalination and Water Purification Technology Roadmap: A Report of the Executive Committee, Water Purification Research and Development Program Report No. 95, US Department of Interior, Bureau of Reclamation, January 2003. [2] L.J. Zeman, A.L. Zydney, Microfiltration and Ultrafiltration: Principles and Applications, Marcell Dekker, New York, 1996. [3] N. Savage, M.S. Diallo, Nanomaterials and water purification, J. Nanopart. Res. 7 (4 5) (2005) 331 342. [4] J.M.J. Frechet, D.A. Tomalia (Eds.), Dendrimers and other Dendritic Polymers, John Wiley & Sons, New York, 2001. [5] M.S. Diallo,Water treatment by dendrimer enhanced filtration, US Patent Application, US 1006/0021938 A1, 2006. [6] M.S. Diallo, S. Christie, P. Swaminathan, J.H. Johnson Jr., W.A. Goddard III, Dendrimer enhanced ultrafiltration. 1. Recovery of Cu(II) from aqueous solutions using Gx-NH2 PAMAM dendrimers with ethylene diamine core, Environ. Sci. Technol. 39 (5) (2005) 1366 1377. [7] A.E. Martell, R.D. Hancock, Metal Complexes in Aqueous Solutions, Plenum Press, New York, 1996. [8] M.S. Diallo, A. Wondwossen, J.H. Johnson Jr., W.A. Goddard III, Dendritic chelating agents. 2. U(VI) binding to poly(amidoamine) and poly(propyleneimine) dendrimers in aqueous solutions, Environ. Sci. Technol. 42 (2008) 1572 1579. [9] M.S. Diallo, S. Christie, P. Swaminathan, L. Balogh, X. Shi, W. Um, et al., Dendritic chelating agents. 1. Cu(II) binding to ethylene diamine core poly(amidoamine) dendrimers in aqueous solutions, Langmuir 20 (7) (2004) 2640 2651. [10] P.D. Beer, P.A. Gale, Anion recognition and sensing: the state of the art and future perspectives, Angew. Chem. Int. Ed. Engl. 40 (2001) 486 516. [11] K. Gloe, H. Stephan, M. Grotjahn, Where is the anion extraction going? Chem. Eng. Technol. 26 (2003) 1107. [12] M.S. Diallo, K. Falconer, J.H. Johnson Jr., W.A. Goddard III, Dendritic anion hosts: perchlorate binding to G5-NH2 poly(propyleneimine) dendrimer in aqueous solutions, Environ. Sci. Technol. 41 (18) (2007) 6521 6527.
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Water Treatment by Dendrimer-Enhanced Filtration: Principles and Applications
Mamadou S. Diallo1,2 1
Materials and Process Simulation Center, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA 2 Department of Civil Engineering, Howard University, Washington, DC, USA
15.1 Introduction ...................................................................................................227 15.2 Dendrimers as recyclable ligands for cations...................................................229 15.3 Dendrimers as recyclable ligands for anions....................................................233 15.4 Dendrimer-enhanced filtration: overview and applications ................................235 15.5 Summary and outlook .....................................................................................238 Acknowledgments ...................................................................................................238 References .............................................................................................................239
15.1 Introduction The availability of clean water has emerged as one of the most serious problems facing the global economy in the twenty-first century. Water treatment systems typically involve a series of coupled processes, each designed to remove one or more different substances in the source water, with the particular treatment process being based on the molecular size and properties of the target contaminants. Membrane processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) are emerging as key components of water treatment, reuse, and desalination systems throughout the world [1 3]. RO is very effective at retaining dissolved inorganic and small organic molecules. NF can effectively remove hardness (e.g., Ca(II)) and natural organic matter. However, high pressures (100 1,000 psi) are required to operate both RO and NF membranes. Conversely, UF and MF membranes require lower pressure Street, Sustich, Duncan and Savage. Nanotechnology Applications for Clean Water, 2nd Edition. © 2014 Elsevier Inc. All rights reserved. DOI: http://dx.doi.org/10.1016/B978-1-4557-3116-9.00015-9
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(5 60 psi) but unfortunately cannot retain dissolved ions and organic solutes. Advances in nanochemistry such as the invention of dendritic nanopolymers are providing unprecedented opportunities to develop enhanced UF and MF processes for recovering dissolved ions from aqueous solutions. Dendritic nanopolymers are highly branched 3D globular nanoparticles with controlled composition and architecture and sizes in the range of 1 100 nm [4]. They consist of three components: a core, interior branch cells, and terminal branch cells. Dendritic nanopolymers include hyperbranched polymers, dendrigraft polymers, dendronized linear polymers, dendrimers, and many other supramolecular assemblies such as core-shell tecto(dendrimers) and dendrimer-like star polymers (Fig. 15.1) [4]. They exhibit a number of critical physicochemical properties that make them attractive as separation and reaction media for water purification. Dendritic nanopolymers can encapsulate a broad range of solutes in water including cations (e.g., copper, silver, gold, and uranium), anions (e.g., perchlorate, nitrate, and phosphate), and organic compounds (e.g., pharmaceuticals and pesticides) [4 6]. Dendritic nanopolymers can serve as nanoscale reactors and catalysts [4]. They can also bind and deactivate bacteria and viruses [4]. Their globular shape and large size makes them easier to filter than linear polymers [5,6]. Diallo and coworkers are exploiting these unique properties of dendritic nanopolymers to Dendrimers
Dendrigraft polymers
FIGURE 15.1 Selected classes of dendritic nanopolymers.
Hyperbranched polymers
Core-shell tecto(dendrimers)
15.2 Dendrimers as recyclable ligands for cations
develop enhanced UF and MF processes for recovering dissolved cations and anions from aqueous solutions [5,6]. This chapter gives an overview of the principles and applications of dendrimer-enhanced filtration (DEF) [5,6]. The use of DEF in the treatment of industrial wastewater contaminated by heavy metals (e.g., copper) and radionuclides (e.g., uranium) and the remediation of groundwater contaminated by anions (e.g., perchlorate) are highlighted.
15.2 Dendrimers as recyclable ligands for cations 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 effective 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 welldefined 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 first dendrimer family to be synthesized, characterized, and commercialized. PAMAM dendrimers (Fig. 15.2) 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 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 different generations and terminal groups. Figure 15.3A and B shows the effects 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-NH2 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
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NH2 NH2 NH2
N
NH NH O O
NH2 NH2
NH NH O O
NH2
NH O O NH2 NH
NH
N
NH O NH2 NH O
NH O O NH
N
OO
N
NH2
O O NH NH
NH2
NH2
N
N
N
N N
NH
NH
O
NH
N
O
N
N
O O NH NH
NH
NH
NH2
NH2
NH2
O
O
O NH
O
NH2
N
O O NH NH
OO NH NH2
NH
NH
NH O ONH
NH2 NH2
NH O O NH
NH2
OO
NH NH2
NH2
NH2 NH
NH OO
NH2
NH NH2 O O NH NH2
O NH O NH
N N
NH2
NH2
NH
N
NH
N
O N
NH O
N
O NH
NH
N
NH2
N O
O
N
N
NH O O NH
NH N
NH
O O NH NH2 NH
N
N H
N
O NH
N
O NH
O NH
NH
NH2
O
NH
NH
N
NH
O NH
O
N
O NH O
N
O
O
O
NH O
NH
O O NH
N
NH O
NH OO
NH
N H
N N
O N NH
O
NH2
NH O ONH
O NH
N H
NH
NH
O
O NH
O O NH
N
N
O NH
N
O
NH O
N
NH2
N
O
NH
O
O N
NH2 N O O H NH NH2
N H O
N NH
NH O O
NH O O NH2 NH
NH2 N H
NH O
O
O NH
NH O
NH
N
NH2
O
N
N
O
O
N
NH2
O NH
N NH
N
N NH
N
NH2
NH2 NH O NH2 NH O O NH N O NH2 N NH O NH2 O N NH NH NH NH2 NH O NH2 O NH OO N N O NH NH2 NH NH NH N OO O NH2 O N NH NH
N N
NH2 NH2 NH
NH O
NH O
NH O
NH
N
N
NH NH O O NH2
NH O O
N
O
NH NH O O
NH
O O
O
NH2
NH2
NH NH O O
NH NH
NH
NH
NH2
NH2 NH2
NH2 NH2
NH2
NH2 NH2
NH2
NH2 NH2
NH2
FIGURE 15.2 Structure of G4-NH2 poly(amidoamine) dendrimer.
ion dendrimer loading within the range of tested 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. 15.3A). 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-NH2 PAMAM dendrimer go through a series of two distinct binding steps as metal ion dendrimer loading increases (Fig. 15.3A). A more detailed discussion of Cu(II) coordination with PAMAM dendrimers is given elsewhere [9]. Figure 15.4A and B highlights the binding of U(VI) to G4-NH2 PAMAM dendrimer in deionized water and NaCl solutions [8]. At pH 7.0 and 9.0, the G4-NH2 PAMAM dendrimer can bind up to 220 U(VI) ions without reaching saturation.
15.2 Dendrimers as recyclable ligands for cations
(A)
Extent of binding (mol/mol)
160 G4-NH2 pH 7 replicate 1 G4-NH2 pH 7 replicate 2 G4-NH2 pH 7 replicate 3 G4-NH2 pH 9 G4-NH2 pH 5
140 120 100
EOBmax2=74.0
80 60 40 20
EOBmax1=12.0
0 0
20
40 60 80 100 120 Metal ion dendrimer loading (mol/mol)
140
160
Fractional binding (%)
(B) G4-NH2 pH 7 replicate 1 G4-NH2 pH 7 replicate 2 G4-NH2 pH 7 replicate 3 G4-NH2 pH 9 G4-NH2 pH 5
150
100
50
0 0
20
40 60 80 100 120 Metal ion dendrimer loading (mol/mol)
140
FIGURE 15.3 (A) Extent of binding of Cu(II) in aqueous solutions of G4-NH2 poly(amidoamine) (PAMAM) dendrimer at room temperature [9]. (B) Fractional binding of Cu(II) in aqueous solutions of G4-NH2 PAMAM dendrimer at room temperature [9].
The uranyl FB is greater than 92 percent in all cases. At pH 3.0, Fig. 15.4A also shows significant binding of U(VI) to the G4-NH2 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. 15.3A). This strongly suggests that uranyl complexation by the G4-NH2 PAMAM dendrimer at pH 3.0 and 5.0 involves the deprotonation of its amine groups followed by coordination with
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(A)
Extent of binding (mol/mol)
250 pH 7.0 (replicate 1) pH 7.0 (replicate 2) pH 9.0 pH 5.0 pH 3.0
200 150 100 50 0 0
50 100 150 200 Metal ion dendrimer loading (mol/mol)
250
(B) 250 Extent of binding (mol/mol)
232
pH 7.0 pH 7.0 and 0.1M NaCl pH 7.0 and 1.0M NaCl pH 3.0 and 0.1M NaCl pH 3.0 and 1.0M NaCl
200 150
G4-NH2
100 50 0 0
50
100
150
200
250
Metal ion dendrimer loading (mol/mol)
FIGURE 15.4 (A) Extent of binding of U(VI) in aqueous solutions of G4-NH2 poly(amidoamine) (PAMAM) dendrimer at room temperature [8]. (B) Effect of NaCl on the extent of binding of U(VI) in aqueous solutions of G4-NH2 PAMAM dendrimer at room temperature [8].
the UO221 metal ion. Diallo et al. [8] were able to suppress the uptake of U(VI) by the G4-NH2 PAMAM in aqueous solutions containing at least 0.1 M (5.8 g/L) of sodium chloride at pH 3.0 (Fig. 15.4B). 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].
15.3 Dendrimers as recyclable ligands for anions
15.3 Dendrimers as recyclable ligands for anions Anions have emerged as major water contaminants throughout the world. In the United States, the discharge of anions such as perchlorate (ClO4 ) and nitrate (NO3 ) into publicly owned treatment works, surface water, groundwater, and coastal water systems is having a major impact on water quality. Although significant research efforts have been devoted to the design and synthesis of selective chelating agents for cation separations [7], anion separations have comparatively received limited attention [10,11]. The design of selective ligands for anions is a challenging undertaking. Unlike cations, anions have filled orbitals and thus cannot covalently bind to ligands [10,11]. Anions have a variety of geometries (e.g., trigonal for NO3 and tetrahedral for ClO4 ) and are sensitive to solution pH in many cases [3,4]. Thus, shape-selective and pH-responsive receptors may be needed to effectively target anions. The charge-to-radius ratios of anions are also lower than those of cations. Thus, anion binding to ligands through electrostatic interactions tends to be weaker than cation binding. Anion binding and selectivity also depend on (i) anion hydrophobicity and (ii) solvent polarity [3,4]. As a first step toward the development of high-capacity, selective, and recyclable dendritic ligands for anions such as perchlorate, Diallo et al. [12] tested the hypothesis that dendrimers with hydrophobic cavities and positively charged internal groups should selectively bind ClO4 over more hydrophilic anions such as Cl , NO3 , SO42 , and HCO3 . They measured the uptake of ClO4 by the fifth generation (G5-NH2) poly(propyleneimine) (PPI) dendrimer with a diamobutane core and terminal NH2 groups (Fig. 15.5) in deonized water and model electrolyte solutions as a function of (i) anion dendrimer loading, (ii) solution pH, (iii) background electrolyte concentration, and (iv) reaction time [12]. Figure 15.6A and B shows the effects of anion dendrimer loading and solution pH on the EOB and FB of ClO4 to a G5-NH2 PPI dendrimer in aqueous solutions at room temperature and reaction time of 1 hour. The pKa of the dendrimer tertiary and primary amine groups are, respectively, equal to 6.10 and 9.75 [12]. At pH 4.0, 99 percent of the tertiary amine groups of the G5-NH2 PAMAM are protonated [12]. As shown in Fig. 15.6B, 98 percent of ClO4 are bound to the dendrimer in this case when the anion dendrimer loading is approximately 0.31. The corresponding EOB is approximately 9.0. The FB of ClO4 in aqueous solutions of the G5-NH2 PPI dendrimer is approximately 53 percent at pH 7.0 with an anion dendrimer loading of 0.31. In this case, only 11 percent of the tertiary amine groups of the dendrimer are protonated, whereas 99 percent of its primary amine groups remain protonated. Note that the maximum EOB of ClO4 (approximately 2.5 at anion dendrimer loading of 32.0) decreases by a factor of 4 at pH 7.0 compared to that at pH 4.0. The maximum EOB of ClO4 (approximately 2.20) is also smaller at pH 9.0 and anion dendrimer loading of 32.0 (Fig. 15.6A) even though approximately
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H3N H3 N H3N H3N
H3 N
NH3 NH 3 NH3 H3N NH3 NH3
H3N
N
N
N
N
N
N
N N
N
NH3
N
N
H3N
NH3
N N
N
H3N
NH3 N
N
H3N H3N
NH3 NH3 N
N
N
N
NH3 NH3 NH3
N
N
N
N
H3N
NH3
N
N
H3N H3N
N
N
NH3 NH3
NH3 N
NH3
N
NH3
N
N
N
NH3
N
H3N N
N
N
H3N
N
N
N
H3N
NH3
N
N
H3N
N
N
N
N
N
N H3N
N
H3N
N
N
N
H3N
H3N
NH3 NH3
N N
N
NH3 NH3
H3N
H3N
NH3
N
N H3N H3N H3N
NH3
N
N N
NH3
N
N
H3N
NH3
NH3 NH3 NH3 NH3
NH3
NH3
NH3
FIGURE 15.5 Structure of G5-NH2 poly(propyleneimine) dendrimer [8].
81 percent of the dendrimer NH2 groups remain protonated. This suggests that electrostatic interactions between ClO4 and protonated NH2 groups of the dendrimer do not have a significant effect on perchlorate uptake. Figure 15.6A also shows some binding of ClO4 (with a maximum EOB of approximately 1.29 at anion dendrimer loading of 32.0) at pH 11.0 when both the tertiary and primary amines of the G5-NH2 PPI dendrimer are unprotonated. A more detailed discussion of the mechanisms of perchlorate binding to the G-NH2 PPI dendrimer is given elsewhere [12]. The overall results of the anion binding experiments suggest that dendritic macromolecules such as the G5-PPI NH2 PPI dendrimer provide ideal building blocks for the development of high capacity, selective, and recyclable ligands for anions such as ClO4 .
15.4 Dendrimer-enhanced filtration: overview and applications
(A) pH 4.0 pH 7.0 pH 9.0 pH 11.0
Extent of binding (mol/mol)
14 12 10 8 6 4 2 0 0
20
40
60
80
Perchlorate–dendrimer loading (mol/mol) (B)
Fractional binding (%)
100 80 pH 4.0 pH 7.0 pH 9.0 pH 11.0
60 40 20 0 0
20
40
60
80
Anion–dendrimer loading (mol/mol)
FIGURE 15.6 (A) Extent of binding of ClO4 in aqueous solutions of G5-NH2 PPI dendrimer at room temperature. (B) Fractional binding of ClO4 in aqueous solutions of G5-NH2 PPI dendrimer at room temperature. Data for (A) and (B) taken from [12].
15.4 Dendrimer-enhanced filtration: overview and applications As stated in the introduction, the invention of dendritic nanopolymers is providing unprecedented opportunities to develop enhanced UF and MF processes for recovering dissolved ions from aqueous solutions. The DEF process developed by Diallo [5,6] is structured around three unit operations (Fig. 15.7): (i) treatment
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CHAPTER 15 Water Treatment by Dendrimer-Enhanced Filtration
Dendrimer solution
Contaminated water
Clean water recovery unit
Dendrimer recovery unit
UF/MF system
UF/MF system
Treatment unit
Clean water
Concentrated contaminant solution
FIGURE 15.7 Water treatment by dendrimer enhanced filtration. Adapted from [5]. UF: Ultrafiltration; MF: Microfiltration.
unit; (ii) clean water recovery unit; and (iii) dendrimer recovery unit. In the treatment unit, contaminated water is mixed with a solution of functionalized dendritic nanopolymers to carry out the specific reactions of interest (e.g., cation and anion binding). Following completion of the reaction, the complexes of nanopolymers 1 bound contaminants are sent to the clean water recovery unit where they are filtered using UF or MF to recover the clean water. The resulting concentrated solution of nanopolymers 1 target substance is subsequently sent to the recovery unit. This system consists of an UF or MF unit in which the bound target substance is separated from the nanopolymers by, for example, changing the acidity (i.e., pH) of the solution. Finally, the recovered concentrated solution of contaminants is collected for disposal or subsequent processing whereas the nanopolymers are recycled [5,6]. The key novel feature of the proposed DEF process is the combination of dendritic polymers with multiple chemical functionalities with the well-established technology of UF and MF. This allows for the development of a new generation of water treatment processes that are flexible, reconfigurable, and scalable [5]. The flexibility of DEF is illustrated by its modular design approach. DEF systems can be designed to be “hardware invariant” and thus reconfigurable in most cases by simply changing the “dendrimer formulation” and process conditions for the targeted contaminants [5]. Because DEF is a membrane-based process, it is scalable and could be used to develop small and mobile water treatment systems as well as large and fixed treatment systems. As a proof-of-concept study, Diallo et al. [6] have combined bench scale measurements of metal ion binding to Gx-NH2 PAMAM dendrimers with dead-end UF experiments to assess the feasibility of using DEF to recover Cu(II) from aqueous solutions. On a mass basis, the Cu(II) binding capacities of the Gx-NH2 PAMAM
15.4 Dendrimer-enhanced filtration: overview and applications
(A)
Cu(II) retention (%)
120
Regenerated cellulose
100 80 G4-NH2 pH =7.0 G4-NH2 pH =7.0 G4-NH2 pH =4.0 G4-NH2 pH =4.0
60 40
10 kD 5 kD 10 kD 5 kD
20 0 0
1
2 3 Filtration time (hours)
4
5
4
5
(B)
Normalized permeate flux
1.2
Regenerated cellulose UF membrane
1.0 0.8 pH =7.0 10kD pH =7.0 5kD pH =4.0 10kD pH =4.0 5kD
0.6 0.4 0.2 0.0 0
1
2 3 Filtration time (hours)
FIGURE 15.8 (A) Retention of Cu(II) complexes with G4-NH2 poly(amidoamine) (PAMAM) dendrimer by regenerated cellulose ultrafiltration (UF) membranes [6]. (B) Normalized permeate flux of aqueous solutions of Cu(II) 1 G4-NH2 PAMAM dendrimer through regenerated cellulose membranes [6].
dendrimers are much larger and more sensitive to solution pH than those of linear polymers with amine groups [6]. Separation of the dendrimer Cu(II) complexes from solutions can be achieved simply by UF (Fig. 15.8A). The metal ion laden dendrimers can then be regenerated by decreasing the solution pH to 4.0 5.0 [6], thus enabling the recovery of the bound Cu(II) and recycling of the dendrimer. Dendritic nanopolymers such the Gx-NH2 PAMAM dendrimers have also much
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less tendency to pass through the pores of UF membranes than do linear polymers of similar molar mass because of their much smaller polydispersity and globular shape [6]. As shown in Fig. 15.8B, the Gx-NH2 EDA core PAMAM have also a very low tendency to foul the commercially available regenerated cellulose (RC) membranes. Dendritic nanopolymers have also much smaller intrinsic viscosities than linear polymers with the same molar mass because of their globular shape [6]. Thus, comparatively smaller operating pressure, energy consumption, and loss of ligands by shear-induced mechanical breakdown could be achieved with dendritic polymers in cross-flow UF systems typically used industrial water treatment [2]. These unique properties of the Gx-NH2 EDA core PAMAM dendrimers make DEF (Fig. 15.7) an attractive process for recovering metal ions such as Cu(II) from contaminated water. Other applications of DEF including the recovery of ClO4 and U (VI) from aqueous solutions are discussed elsewhere [8,12].
15.5 Summary and outlook Dendritic nanopolymers are among the most chemically and structurally diverse classes of nanomaterials available to date. These “soft” nanoparticles, with sizes in the range of 1 100 nm, can serve as high-capacity, recyclable ligands for cations and anions. The DEF process exploits these unique properties of dendritic nanopolymers to develop a new generation of low-pressure filtration processes for treating water contaminated by toxic metal ions (e.g., Cu(II) and U(VI)) and oxyanions (e.g., ClO4 ). A start-up company (Aqua Nano Technologies) has been set up in California to commercialize the DEF technology. Initial target applications include: 1. Recovery of perchlorate from contaminated groundwater. 2. Recovery of uranium from in situ mining leach solutions and contaminated groundwater. 3. Recovery of metal ions (e.g., copper, silver, nickel, and zinc) from industrial wastewater.
Acknowledgments I thank the US National Science Foundation (NSF Grants CTS-0086727, CTS-0329436, and NIRT CBET-0506951) and the US Environmental Protection Agency (NCER STAR Grant R829626) for funding my research on the use of dendritic nanopolymers as functional materials for water purification. Partial funding for this research was also provided by the Department of Energy (Cooperative Agreement EW15254), the W.M. Keck Foundation, and the National Water Research Institute (Research Project Agreement NO 05-TT-004).
References
References [1] US Bureau of Reclamation and Sandia National Laboratories, Desalination and Water Purification Technology Roadmap: A Report of the Executive Committee, Water Purification Research and Development Program Report No. 95, US Department of Interior, Bureau of Reclamation, January 2003. [2] L.J. Zeman, A.L. Zydney, Microfiltration and Ultrafiltration: Principles and Applications, Marcell Dekker, New York, 1996. [3] N. Savage, M.S. Diallo, Nanomaterials and water purification, J. Nanopart. Res. 7 (4 5) (2005) 331 342. [4] J.M.J. Frechet, D.A. Tomalia (Eds.), Dendrimers and other Dendritic Polymers, John Wiley & Sons, New York, 2001. [5] M.S. Diallo,Water treatment by dendrimer enhanced filtration, US Patent Application, US 1006/0021938 A1, 2006. [6] M.S. Diallo, S. Christie, P. Swaminathan, J.H. Johnson Jr., W.A. Goddard III, Dendrimer enhanced ultrafiltration. 1. Recovery of Cu(II) from aqueous solutions using Gx-NH2 PAMAM dendrimers with ethylene diamine core, Environ. Sci. Technol. 39 (5) (2005) 1366 1377. [7] A.E. Martell, R.D. Hancock, Metal Complexes in Aqueous Solutions, Plenum Press, New York, 1996. [8] M.S. Diallo, A. Wondwossen, J.H. Johnson Jr., W.A. Goddard III, Dendritic chelating agents. 2. U(VI) binding to poly(amidoamine) and poly(propyleneimine) dendrimers in aqueous solutions, Environ. Sci. Technol. 42 (2008) 1572 1579. [9] M.S. Diallo, S. Christie, P. Swaminathan, L. Balogh, X. Shi, W. Um, et al., Dendritic chelating agents. 1. Cu(II) binding to ethylene diamine core poly(amidoamine) dendrimers in aqueous solutions, Langmuir 20 (7) (2004) 2640 2651. [10] P.D. Beer, P.A. Gale, Anion recognition and sensing: the state of the art and future perspectives, Angew. Chem. Int. Ed. Engl. 40 (2001) 486 516. [11] K. Gloe, H. Stephan, M. Grotjahn, Where is the anion extraction going? Chem. Eng. Technol. 26 (2003) 1107. [12] M.S. Diallo, K. Falconer, J.H. Johnson Jr., W.A. Goddard III, Dendritic anion hosts: perchlorate binding to G5-NH2 poly(propyleneimine) dendrimer in aqueous solutions, Environ. Sci. Technol. 41 (18) (2007) 6521 6527.
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