Dendritic Anion Hosts: PerchlorateUptake by G5-NH2Poly(propyleneimine) Dendrimer inWater and Model ElectrolyteSolutionsM A M A D O U S . D I A L L O , * , † , ‡

K W E S I F A L C O N E R , ‡

J A M E S H . J O H N S O N , J R . , ‡ A N DW I L L I A M A . G O D D A R D , I I I †

Materials and Process Simulation Center, Beckman Institute139-74, California Institute of Technology, Pasadena,California 91125, and Department of Civil Engineering,Howard University, Washington, DC, 20059

Perchlorate (ClO4-) has emerged as a major groundwater

and surface water contaminant in the United States.Ion exchange (IX) is the most widely used technology fortreating water containing lower concentrations ofperchlorate (<100 ppb). However, a major drawback of IXis the need for frequent regeneration or disposal of theperchlorate-laden resins. As a first step toward thedevelopment of high-capacity, selective and recyclabledendritic ligands for the recovery of perchlorate from aqueoussolutions by dendrimer filtration, we tested the hypothesisthat dendrimers with hydrophobic cavities and positivelycharged internal groups should selectively bind ClO4

- overmore hydrophilic anions such as Cl-, NO3

-, SO42-, and

HCO3-. We measured the uptake of ClO4

- by the fifthgeneration (G5-NH2) poly(propyleneimine) (PPI) dendrimerwith a diaminobutane core and terminal NH2 groups indeonized water and model electrolyte solutions as a functionof (i) anion-dendrimer loading, (ii) solution pH, (iii)background electrolyte concentration, and (iv) reactiontime. The ClO4

- binding capacity of this dendrimer iscomparable to those of perchlorate-selective IX resins.However, its ClO4

- binding kinetics is faster and reachesequilibrium in ∼1 h. Note also that only a high pH (∼9.0)aqueous solution is needed to release more than 90% ofthe bound ClO4

- anions by deprotonation of the dendrimertertiary amine groups. The overall results of this studysuggest that dendritic macromolecules such as the G5-PPI NH2 dendrimer provide ideal building blocks for thedevelopment of high-capacity, selective and recyclable ligandsfor the recovery of anions such as perchlorate fromaqueous solutions by dendrimer enhanced filtration.

IntroductionIn recent years, perchlorate (ClO4

-) has emerged as a majorgroundwater and surface water contaminant in the UnitedStates (1-3). Transition metal complexes (4) and nanoscale

zerovalent iron particles (5) are not very effective at reducingperchlorate in aqueous solutions. Although bioreactors caneffectively reduce perchlorate to chloride under anaerobicconditions (6), regulatory and public acceptance may limittheir utilization in drinking water treatment. Reverse osmosis(RO), nanofiltration (NF), and electrodialysis (ED) are alsonot cost-effective or efficient at recovering perchlorate fromcontaminated water in most cases (7). Ion exchange (IX) isthe most widely used technology for treating water containinglower concentrations of ClO4

- (<100 ppb) (8-10). However,a major drawback of IX is the need for frequent disposal orregeneration of the perchlorate-laden resins. The dendrimerfiltration process developed by Diallo (11-13) could providea viable alternative to IX for the treatment of water con-taminated by anions such as perchlorate once suitable andcost-effective dendritic anions have been identified. As a firststep toward the development of high-capacity, selective andrecyclable dendritic anion hosts for water treatment, weinitially focus on the characterization of perchlorate uptakeby dendrimers in water and electrolyte solutions. Whilesignificant research efforts have been devoted to the devel-opment of selective chelating agents for cation separations(14), anion separations have comparatively received limitedattention (15). Unlike cations, anions have filled orbitals andthus do not covalently bind to ligands in most cases. Anionshave a variety of geometries (e.g., spherical for Cl- andtetrahedral for ClO4

-) [Table S1 in the Supporting Informa-tion]. Thus, shape-selective receptors may be needed toeffectively target anions. The charge-to-radius ratios of anionsare also lower than those of cations. Because of this, anionbinding to ligands through electrostatic interactions tendsto be weaker. Anion binding capacity and selectivity alsodepend on (i) anion hydrophobicity, (ii) solution pH, and(iii) solvent polarity (15).

Dendritic macromolecules, which include random hy-perbranched polymers, dendrigraft polymers, dendrons, anddendrimers are emerging as ideal platforms for the develop-ment of advanced materials and processes for water treat-ment (11-13). Dendrimers consist of three covalently bondedcomponents: a core, interior branch cells, and terminalbranch cells. Tomalia and co-workers (16) were among thefirst investigators to establish that dendrimers can encap-sulate anions. They used 13C NMR spin-relaxation (T1)measurements to show that G4 and G5 poly(amidoamine)(PAMAM) dendrimers with methyl ester terminal groups canencapsulate organic anions such as acetyl salicylic and 2,4-dichlorophenoxyacetic acids in chloroform. Subsequently,Twyman et al. (17) showed that PAMAM dendrimers canbind benzoate. Birnbaum et al. (18) have found that PAMAMdendrimers provide ideal building blocks for the synthesisof ligands for anions such as AsO4

3-, CrO42-, and HPO4

2-.Astruc and co-workers (19) reported the synthesis andcharacterization of dendrimers with ferrocene groups thatcan bind H2PO4

-, HSO4-, and adenosine 5′-triphosphate

(ATP) in dichloromethane. Stephan et al. (20) have alsodescribed the synthesis of propylene(imine) dendrimers withurea terminal groups that can bind pertechnate and per-rhenate in chloroform.

Table S1 shows that ClO4- has a larger ionic radius and

hydration free energy than most anions present in ground-water and surface water. Thus, dendrimers with hydrophobiccavities and positively charged internal groups are expectedto selectively bind ClO4

- over more hydrophilic anions suchas Cl-, NO3

-, SO42-, and HCO3

-. To test this hypothesis, weinitially focused on the fifth generation (G5-NH2) poly-(propylene) (PPI) dendrimer with a diaminobutane (DAB)

* Corresponding author phone: 626 395 2730; fax: 626 585 0918;e-mail: diallo@wag.caltech.edu.

† California Institute of Technology.‡ Howard University.

Environ. Sci. Technol. 2007, 41, 6521-6527

10.1021/es0710959 CCC: $37.00 2007 American Chemical Society VOL. 41, NO. 18, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 6521Published on Web 08/11/2007

core and terminal NH2 groups (Figure S1 of SupportingInformation). This is a water-soluble dendrimer with 64terminal NH2 groups and 62 internal tertiary amine groups(Table 1) that can encapsulate hydrophobic organic com-pounds such as Rose Bengal and pyrene (21-23). In thisstudy, we measured the uptake of perchlorate by a G5-NH2

PPI dendrimer in deionized water and electrolyte solutions.The model electrolyte solutions consisted of mixtures ofdeionized water + monovalent and divalent anions includingCl-, NO3

-, HCO3-, and SO4

2-. Selected measurements ofperchlorate uptake were also carried out in (i) aqueoussolutions of a G4-NH2 PAMAM dendrimer with ethylenediamine core (Figure S1) and (ii) mixtures of G5-PPI andG4-NH2 PAMAM dendrimers with monovalent and divalentanions. The G4-NH2 PAMAM dendrimer has 64 terminal NH2

groups, 64 internal amide groups, and 62 internal tertiaryamine groups (Table 1). The overall results of these experi-ments suggest that dendritic macromolecules such as theG5-PPI NH2 dendrimer provide ideal building blocks for thedevelopment of high-capacity, selective and recyclableligands for the recovery of perchlorate from aqueous solutionsby dendrimer filtration (11-13).

Experimental SectionMaterials. Reagent-grade sodium perchlorate (NaClO4),sodium chloride (NaCl), sodium nitrate (NaNO3), sodiumbicarbonate (NaHCO3), and sodium sulfate (Na2SO4) fromSigma-Aldrich were used, respectively, as sources of ClO4

-,Cl-, NO3

-, HCO3-, and SO4

2-. G5-NH2 PPI and G4-NH2

PAMAM dendrimers in methanol solutions were purchasedfrom Sigma-Aldrich and used as received. Table 1 givesselected properties of the PPI and PAMAM dendrimersevaluated in this study. The pKa of the dendrimer tertiaryand primary amine groups (pKNT and pKNH2) were taken fromrefs 24 and 25, respectively. Their extents of protonation(Table 2) were calculated using the Henderson-Hasselbachequation (24-25). Diallo et al. (25) have shown that thissimplified methodology gives results comparable in accuracyto those obtained using theoretically derived dendrimerprotonation models (26-27). The hydrodynamic radii (RH)of the G5-NH2 PPI and G4-NH2 PAMAM dendrimers weretaken from refs 28 and 29, respectively.

Methods and Procedures. Batch experiments were carriedout in deionized water and model electrolyte solutions tomeasure the extent of binding (EOB) of ClO4

- as a functionof (i) anion-dendrimer loading, (ii) solution pH, and (iii)reaction time using the procedures described by Diallo et al.(25). The binding assay procedure consisted of (i) mixingand equilibrating aqueous solutions of ClO4

- + dendrimerat room temperature, (ii) separating the perchlorate-dendrimer solutions from the aqueous solutions by ultra-filtration (UF), and (iii) and measuring the ClO4

- concen-trations of the initial solutions and filtrates. Diallo et al. (25)have found that the pH of dendrimer complexes can becontrolled (within 0.1-0.2 pH unit) by addition of concen-trated acid (e.g., HNO3) or base (e.g., NaOH). The concentra-tions of perchlorate in the solutions [ClO4

-]o and filtrates[ClO4

-]f were measured using a Dionex DX120 ion chro-matograph with conductivity suppression and detection (3).The ClO4

- detection limit was ∼4 ppb. The concentration ofbound perchlorate [ClO4

-]b (mol/L) was expressed as

The EOB (moles of bound perchlorate per mole of dendrimer),the concentration of dendrimer [Cd] (mol/L) in solution andthe fractional binding [FB] (%) were expressed as

where md (g) is the mass of dendrimer in solution, Vs (L) isthe solution volume, and Mwd (g/mol) is the dendrimer molarmass (Table 2).

Results and DiscussionFigures 1 and 2 show the effects of anion-dendrimer loadingand solution pH on the EOB and FB of ClO4

- to a G5-NH2

PPI dendrimer in deonized water at room temperature andreaction time of 1 h. In these experiments, we varied themolar ratio of perchlorate to dendrimer to prepare solutionswith a given anion-dendrimer loading. Three replicatemeasurements were carried out in each set of experiments.To highlight the reproducibility of the data while preservingthe clarity of the figures, we only plot duplicate measure-ments. Figure 1 shows that the EOB of perchlorate goesthrough a series of distinct binding steps at pH 4.0 as anion-dendrimer loading increases. First, we observe a gradualincrease of the EOB to a maximum of ∼6.0 mol of boundClO4

- per mol of dendrimer when the anion-dendrimerloading is ∼11.0. This is a followed by another increase of theEOB [∼9.0 for an anion-dendrimer loading of ∼32.0] and aslight decrease to a value of ∼7.0 when the anion-dendrimer

TABLE 1. Selected Properties of the Dendrimers Evaluated in This Study

dendrimer Mwtha NNT

b NNH2c Namide

d pKNTe pKNH2

f RHg

G5-NH2 PPI 7168 62 64 0 6.10 9.75 1.98G4-NH2 PAMAM 14215 62 64 64 6.30-6.85 9.23-10.0 2.50

a Mwth: Theoretical molecular weight. b NNT: Number of tertiary amine groups. c NNH2: Number of primary amine groups. d Namide: Number ofamide groups. e pKNT : pKa of dendrimer tertiary amine groups. f pKNH2: pKa of dendrimer primary amine groups. The pKa of the tertiary and primaryamine groups (pKNT and pKNH2) of the dendrimers were taken from refs 24 and 25, respectively. g RH: dendrimer hydrodynamic radius. The RH

of the G5-NH2 PPI and G4-NH2 PAMAM dendrimers are taken from refs 28 an29, respectively.

TABLE 2. Extents of Protonation of the Dendrimers Evaluatedin This Studya

G5-NH2 PPI pH 4.0 pH 7.0 pH 9.0 pH 11.0

RNT 0.99 0.11 0.00 0.00RNH2 0.99 0.99 0.81 0.00

G4-NH2 PAMAM pH 4.0 pH 7.0 pH 9.0 pH 11.0

RNT 0.99 0.17-0.41 0.00 0.00RNH2 0.99 0.99 0.63-0.91 0.02-0.09

a The extents of protonation of the tertiary and primary amine groups(RNT and RNH2) of the PPI and PAMAM dendrimers were calculated usingthe Henderson-Hasselbach equation (24, 25): log Ri/1 - Ri ) pKa

i - pH;where i ) NT and NH2.

[ClO4-]b ) [ClO4

-]o - [ClO4-]f (1)

EOB )[ClO4

-]b

Cd(2)

Cd )md

VsMwd(3)

FB ) 100 × [ClO4-]b

[ClO4-]o

(4)

6522 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 18, 2007

loading is ∼64.0. The FB of perchlorate also exhibits a seriesof distinct binding steps in aqueous solutions of the G5-NH2

PPI dendrimer (Figure 2). At pH 4.0 and low anion-dendrimerloading (∼0.17-1.29), the FB is greater than 90% in all cases.Note that the FB sharply decreases and levels off around30% followed by a slight decrease as anion-dendrimerloading increases. Because each successive perchloratebinding step involves an initial increase of solute binding

followed by a second increase (Figures 1 and 2), we attributethis behavior to the presence of dendrimer “sites” withdifferent binding capacity and affinity for ClO4

-. As the sitesnear the dendrimer surface become filled, the ClO4

- guestsdiffuse inside the dendritic host to occupy its internal andconfined cavities. This mode of guest uptake by a macro-molecular host is commonly referred to as topologicaltrapping in the supramolecular chemistry literature (21). Notethat a dendrimer often undergoes conformational rear-rangements as it accommodates guest molecules (21). Thus,the slight decrease of the EOB of ClO4

- to a value of ∼7.0when the anion-dendrimer loading is ∼64.0 could beattributed to the release of “loosely” bound guests as theG5-NH2 PPI dendrimer rearranges itself to provide a favorableenvironment for perchlorate anions.

Two potential driving forces for perchlorate uptake bythe G5-NH2 PPI dendrimer are (i) electrostatic interactionsbetween ClO4

- anions and protonated amine groups of thedendrimer and (ii) hydrophobic partitioning in the dendrimerinterior. Figure 2 illustrates the role of the electrostaticinteractions between ClO4

- and the dendrimer protonatedtertiary amine groups as a key driving force of perchlorateuptake. At pH 4.0, 99% of the dendrimer tertiary amine groupsare protonated (Table 2). As shown in Figure 2, 98% of ClO4

-

are bound to the dendrimer at pH 4.0 when the anion-dendrimer loading is ∼0.31. Conversely, the FB of ClO4

- inaqueous solutions of the G5-NH2 PPI dendrimer is ∼53% atpH 7.0 and anion-dendrimer loading of 0.31. In this case,only 11% of the tertiary amine groups of the dendrimer areprotonated; whereas 99% of its primary amine groups remainprotonated (Table 2). Note that the maximum EOB of ClO4

-

(∼2.5 at anion-dendrimer loading of 32.0) decreases by afactor of 4 at pH 7.0 compared to that at pH 4.0. The maximumEOB of ClO4

- (∼2.20) is also smaller at pH 9.0 and anion-dendrimer loading of 32.0 (Figure 1) even though ∼81% ofthe dendrimer NH2 groups remain protonated (Table 2). Thissuggests that electrostatic interactions between ClO4

- andprotonated NH2 groups of the dendrimer do not have asignificant effect on perchlorate uptake. Figure 1 also showssome binding of ClO4

- [with a maximum EOB ∼1.29 at anion-dendrimer loading of 32.0] at pH 11.0 when both the tertiaryand primary amines of the G5-NH2 PPI dendrimer areunprotonated (Table 2). We attribute this “residual” bindingto the topological trapping of ClO4

- anions inside thehydrophobic cavities of the PPI dendrimer. This hypothesisis consistent with the results of fluorescence spectroscopicstudies of pyrene binding to a G5-NH2 PPI dendrimer inaqueous solutions carried out by Pistolis and Malliaris (23).They reported that polarity of the interior of a G5-NH2 PPIdendrimer is comparable to that of hexane; which has adielectric constant εhexane ) 2.0 (23). Thus, a hydrophobicguest such as pyrene is “pushed deeper and deeper insidethe dendrimer” where it becomes shielded from the waterphase (23).

To test the hypothesis that the G5-NH2 PPI dendrimerprovides a more favorable environment for the partitioningof ClO4

- anions than the more hydrophilic PAMAM den-drimers, we measured perchlorate uptake in aqueous solu-tions of a G4-NH2 PAMAM dendrimer at pH 4.0 and 9.0. Thisdendrimer has the same number of tertiary amine andprimary groups as the G5-NH2 PPI dendrimer (Table 1). Theextents of protonation of the tertiary and primary aminegroups of the PPI and PAMAM dendrimers are comparable(Table 2). However, the G4-NH2 PAMAM dendrimer is morehydrophilic. Indeed, it has 64 additional internal amidegroups that can interact with water through hydrogenbonding. Molecular dynamics simulations of Gx-NH2 PAMAMdendrimers with explicit water molecules carried out byGoddard and co-workers (30) showed extensive waterpenetration in the interior of a G4-NH2 PAMAM dendrimer.

FIGURE 1. Extent of binding of perchlorate to G5-NH2 PPI dendrimerin deionized water at room temperature and reaction time of 1 h.The initial perchlorate concentration in solution is 1000 ppb(0.01 mM).

FIGURE 2. Fractional binding of perchlorate to G5-NH2 PPI dendrimerin deionized water at room temperature and reaction time of 1 h.The initial perchlorate concentration in solution is 1000 ppb(0.01 mM).

VOL. 41, NO. 18, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 6523

Niu et al. (27) reported an estimated value of εdendrimer ) 23for the internal dielectric constant of a G4-NH2 PAMAMdendrimer thereby suggesting that its polarity is similar tothat of ethanol, which has a dielectric constant εethanol ) 24(31). Figure 3 shows the EOB of perchlorate to the G4-NH2

PAMAM dendrimer in deonized water. The correspondingFB curve (Figure S2) is given in the Supporting Information.In this case, the EOB curve also goes through a series ofdistinct binding steps at pH 4.0 with a maximum EOB ∼2.3.This value is smaller (by a factor of 4.5) than the EOB ofClO4

- to the PPI dendrimer in deionized water even thoughthe tertiary amine groups of the PAMAM dendrimer are fullyprotonated at pH 4.0 (Table 2). Similarly, at pH 9.0, themaximum EOB of ClO4

- (∼0.9) in aqueous solutions of thePAMAM dendrimer is also smaller (by a factor of ∼2.4). Theseresults are consistent with our hypothesis that the hydro-phobic G5-NH2 PPI dendrimer provides a more favorableenvironment for ClO4

- than the more hydrophilic G4-NH2

PAMAM dendrimer.To assess the effects of competing anions on perchlorate

uptake by the G5-NH2 PPI dendrimer, we measured the EOBof ClO4

- in two model electrolyte solutions at pH 4.0 and 9.0(Figure 4). The low background electrolyte (Electrolyte 1)consisted of a solution of 0.1 mM NaCl, 0.3 mM NaHCO3, 0.1mM NaNO3, and 0.1 mM Na2SO4 with an initial molar ratioof SO4

2- to ClO4- equal to 10.0. The high background

electrolyte (Electrolyte 2) was similar to that used by Gu etal. (8) in their recent study of ClO4

- and U(VI) sorption ontoIX resins with quaternary alkyl ammonium groups. Itconsisted of a solution of 1.0 mM NaCl, 3.0 mM NaHCO3, 1.0mM NaNO3, and 1.0 mM Na2SO4 with an initial molar ratioof SO4

2- to ClO4- equal to 100.0. Figure 4 compares the uptake

of perchlorate by the G5-NH2 PPI dendrimer in deonizedwater and electrolytes. The corresponding FB curve (FigureS3) is given in the Supporting Information. In the lowbackground electrolyte solution, the maximum EOB (∼6.0)decreases by a factor of 1.5 compared to that observed indeonized water (∼9.0) at pH 4.0. Note that perchlorate uptakeby the G5-NH2 PPI dendrimer is still appreciable even though

the molar concentration of the divalent SO42- anions in

solution is 10 times higher than that of the ClO4- anions. At

pH 4.0, Figure 4 shows a large decrease of the maximumEOB of perchlorate (∼2.0) in the high background electrolytesolution compared to that observed in deionized water (∼9.0)at anion-dendrimer loading of ∼32.0. At pH 9.0, themaximum EOB of perchlorate in deionized water (∼2.20) iscomparable to that in the low background electrolyte solution(∼1.95). However, it is ∼1.78 times larger than the measuredEOB of perchlorate (∼1.10) in the high background electrolytesolution. Recall that the initial molar ratio of SO4

2- to ClO4-

in the high background electrolyte solution is equal to 100.Thus, the presence of “excess” amounts of SO4

2- anions insolution has a significant impact on perchlorate uptake bythe G5-NH2 PPI dendrimer.

The preferential removal of divalent anions (e.g., SO42-)

over monovalent anions (e.g., ClO4-) has been observed in

IX resins with quaternary alkyl ammonium groups (8-9, 32).Barron and Fritz (32) coined the term “electroselectivity” toexplain the preference for divalent anions over monovalentions in these systems. We carried out measurements of ClO4

-

uptake by a mixture of G5-PPI and G4-NH2 PAMAM den-drimers in the high background electrolyte solution to gaininsight into SO4

2- electroselectivity in aqueous solutions ofdendrimers + anions. A key objective of these experimentswas to “suppress” sulfate electroselectivity by addition of ahydrophilic dendrimer with protonated amine groups.Indeed, we expect the hydrophilic G4-NH2 PAMAM den-drimer to provide a more favorable environment for SO4

2-

than the more hydrophobic G5-NH2 PPI dendrimer. To testthis hypothesis, we varied the molar ratio of SO4

2- to PAMAMdendrimer to prepare solutions with sulfate-dendrimerloadings equal to those of perchlorate-dendrimer loadingsused in previous experiments (Figures 1, 3, and 4). For the

FIGURE 3. Extent of binding of perchlorate to G5-NH2 PPI and G4-NH2 PAMAM dendrimers in deionized water at room temperatureand reaction time of 1 h. The initial perchlorate concentration insolution is 1000 ppb (0.01 mM).

FIGURE 4. Extent of binding of perchlorate to G5-NH2 PPI dendrimerin deionized water and model electrolyte solutions at roomtemperature and reaction time of 1 h. The initial perchlorateconcentration in solution is 1000 ppb (0.01 mM). Electrolyte 1 consistsof an aqueous solution containing 0.1 mM NaCl, 0.3 mM NaHCO3,0.1 mM NaNO3, and 0.1 mM Na2SO4. Electrolyte 2 consists of anaqueous solution containing 1.0 mM NaCl, 3.0 mM NaHCO3, 1.0 mMNaNO3, and 1.0 mM Na2SO4.

6524 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 18, 2007

G5-NH2 PPI dendrimer, the molar ratios of ClO4- to dendrimer

were also equal to those in previous experiments (Figures 1,3, and 4). Figure 5 shows that addition of the G4-NH2 PAMAM

dendrimer in the high background electrolyte solution“suppresses” the effect of SO4

2- on perchlorate uptake bythe G5-NH2 PPI dendrimer. Note that the EOB and FB ofClO4

- in deionized water (Figure 5 and Figure S4 of SupportingInformation) are equal to those in the high backgroundelectrolyte solution when the G4-NH2 PAMAM dendrimer isadded to the solution. Conversely, the addition of the PAMAMdendrimer has no effect on perchlorate uptake in the highbackground electrolyte solution (Figures 5 and S4) at pH 9.0when the tertiary amine groups of the G4-NH2 PAMAMdendrimer are neutral (Table 2). In this case, the EOB andFB of ClO4

- in the mixtures of the PPI and PAMAM dendrimersare lower than those in deionized water. Note they are equalto the EOB and FB of ClO4

- in the high background electrolytesolution. This suggests that the suppression of the effect ofsulfate on perchlorate uptake by the PPI dendrimer in thehigh background electrolyte solution involves SO4

2- bindingto the protonated tertiary amine groups of the G4-NH2

PAMAM dendrimer. These results also provide indirectevidence that (i) the G5-NH2 PPI dendrimer selectively bindsClO4

- over more hydrophilic anions such as Cl-, NO3-, SO4

2-,and HCO3

-, and (ii) the SO42- anions bound to the G4-PAMAM

dendrimer can be released by deprotonation of its tertiaryamine groups. However, more direct measurements areneeded to further corroborate these indirect observations.Finally, we would like to point out that perchlorate uptakeby the G5-NH2 PPI dendrimer in aqueous solutions reachesequilibrium in ∼1 h (Figure 6 and Figure S5 of SupportingInformation).

Environmental ImplicationsAs stated in the Introduction, ion exchange (IX) is the mostwidely used process for treating water contaminated by lowerconcentration of ClO4

- (<100 ppb) (8-10). Two commonlyused IX resins for perchlorate removal are (i) nonselectiveresins and (ii) selective resins. Resin regeneration/reuse andbrine management/disposal are two critical issues that havea major impact on the efficiency, cost effectiveness, andenvironmental acceptability of IX systems used to treat watercontaminated by ClO4

-. The nonselective resins requirefrequent regenerations with brine (∼6-12 wt % NaClsolution) due to their low ClO4

- sorption capacity/selectivity(9). This generates wastes (brine + ClO4

-) that need to bepretreated prior to disposal (9). Conversely, the ClO4

- selectiveresins do not require frequent regenerations. However, evenconcentrated brine solutions (∼12 wt %) are not effective atregenerating the resins in most cases because of their strongbinding affinity for ClO4

-. Because of this, spent ClO4-

selective resins are incinerated in most cases following asingle use. Although Gu et al. (9) have shown that a solutionof 1.0 M FeCl3 + 4.0 M HCl could regenerate ClO4

- selectiveresins, the treatment of the regenerant solutions involvesthe use of a flow-through reactor operating at high tem-perature (190-204 °C) to reduce perchlorate using concen-trated solutions of ferrous iron and/or nontoxic organicreducing agents. Dendrimer filtration (11-13) could providea viable alternative to IX for the treatment of ClO4

- con-taminated water once suitable dendritic ligands have beenidentified. This process combines functionalized dendriticnanomaterials with membrane-based separation technolo-gies such as ultrafiltration (UF). The G5-NH2 PPI dendrimer(Figure S1) has a molar mass of 7168 Da and a hydrodynamicradius (Rh) of 1.98 nm (21-22). Thus, it can be effectivelyseparated from aqueous solutions by UF (11). As shown inFigure 1, the maximum EOB of ClO4

- in aqueous solutionsat pH 4.0 is ∼9.0 for an anion-dendrimer loading of ∼32.0.On a mass basis, this corresponds to a binding capacity of125 mg of ClO4

- per g of dendrimer. Gu et al. (9) havemeasured the sorption of ClO4

- onto nonselective andselective IX in aqueous solutions and model electrolytes at

FIGURE 5. Extent of binding of perchlorate to G5-NH2 PPI dendrimerin deionized water, electrolyte, and mixture of electrolyte + G4-NH2 PAMAM dendrimer at room temperature and reaction time of1 h. The initial perchlorate concentration in solution is 1000 ppb(0.01 mM). The electrolyte consists of an aqueous solution containing1.0 mM NaCl, 3.0 mM NaHCO3, 1.0 mM NaNO3, and 1.0 mM Na2SO4.

FIGURE 6. Extent of binding of perchlorate to G5-NH2 PPI dendrimerin deionized water and electrolyte at room temperature as a functionof reaction time. The initial perchlorate concentration in solutionis 1000 ppb (0.01 mM). The electrolyte consists of an aqueous solutioncontaining 1.0 mM NaCl, 3.0 mM NaHCO3, 1.0 mM NaNO3, and 1.0mM Na2SO4.

VOL. 41, NO. 18, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 6525

pH 8.5. They obtained sorption capacity <1.0 mg/g for anonselective resin with trimethyl ammonium groups (PuroliteA850). Conversely, they reported ClO4

- sorption capacity of75 mg/g for a selective bifunctional polystyrene resin withethyl/hexyl ammonium groups. In both cases, the equilibriumconcentration of ClO4

- in water is equal to 200 ppb. Thus,the ClO4

- binding capacity of the G5-NH2 PPI dendrimer iscomparable to those of selective IX resins with trialkylammonium groups that have been evaluated by Gu et al. (9).

The presence of competing divalent anions such as SO42-

can greatly limit the efficiency and longevity of IX resinsused to recover perchlorate from contaminated water (8, 9).Figure 4 shows that SO4

2- anions can greatly reduce theperchlorate binding capacity of the G5-NH2 PPI dendrimerin electrolyte solutions. However, the addition of a morehydrophilic dendrimer such a G4-NH2 PAMAM dendrimercan “suppress” the effect of SO4

2- anions on the ClO4- binding

capacity and selectivity of the G5-NH2 PPI dendrimer inelectrolytes (Figure 6). This suggests that mixtures of den-drimers can be used to effectively recover perchlorate fromaqueous solutions containing high concentrations of divalentanions such as SO4

2- (32). Because perchlorate uptake bythe G5-NH2 PPI dendrimer occurs in a homogeneous liquid-liquid process, its kinetics is relatively fast and reachesequilibrium in ∼1 h (Figure 7). Conversely, IX is a hetero-geneous solid-liquid process that involves ion diffusionthrough the resin pores followed by an exchange reaction.Not surprisingly, ClO4

- binding to the IX resins requires atleast 24 h before it reaches equilibrium in most cases (8, 9).Finally, we would like to point out the significant decreaseof the EOB and FB ClO4

- in aqueous solutions of the G5-NH2

PPI dendrimer at pH 9.0 and 11.0. Note that most perchlorate-selective IX resins contain sites that can be exchanged withClO4

- at any pH. Thus, they cannot be regenerated by a simplechange of solution pH. Conversely, only a high pH (∼10-11.0) aqueous solution will be needed to release more than90% to the G5-NH2 PPI dendrimer by deprotonation of itstertiary amine groups. The overall results of this study suggestthat dendritic macromolecules such as the G5-NH2 PPIdendrimer provide ideal building blocks for the developmentof high-capacity, selective and recyclable ligands for per-chlorate. Additional experiments and computer-aided mo-lecular design studies are underway to develop high-capacity,selective and recyclable dendritic anion hosts (33) for recoveryof anions such as ClO4

- from aqueous solutions by dendrimerfiltration (11-13).

AcknowledgmentsThis work was carried out in the Materials Process SimulationCenter of the Division of Chemistry and Chemical Engineer-ing at the California Institute of Technology and in theDepartment of Civil Engineering at Howard University.Funding for this work was provided by the National ScienceFoundation (NIRT CBET Award # 0506951). Supplementalfunding for this research was provided to Howard Universityby the Department of Energy (DOE Massie Chair Grant DE-FG02-94EW11423). We thank Prof. Charles Glass and Ms.Lily Wan of the Department of Civil Engineering at HowardUniversity for their assistance with the ion chromatographyanalyses.

Supporting Information AvailableTable S-1 and Figures S-1-S-5. This material is available freeof charge via the Internet at http://pubs.acs.org.

Literature Cited(1) Hogue, C. Rocket-Fueled River. Chem. Eng. News 2003, 81,

37-46.

(2) Pontius, F. W.; Damian, P.; Eaton, A. D. Regulating perchloratein drinking water. In Perchlorate in the Environment; Urbansky,T. E., Ed.; Kluver Academic: New York, 2000.

(3) Mosier-Boss, P. A. Recent developments in perchlorate detec-tion. In Perchlorate Environmental Occurrence, Interactions andTreatment; Gu, B., Coates, J. D., Eds.; Springer: New York,2006.

(4) Earley, J. E., Sr.; Tofan, D. C.; Amadei, G. A. Reduction ofperchlorate by titanous ions in ethanolic solution. In PerchlorateEnvironmental Occurrence, Interactions and Treatment. Gu, B.,Coates, J. D., Eds.; Springer: New York, 2006.

(5) Cao, J. S.; Elliott, D.; Zhang, W. Perchlorate reduction bynanoscale iron particles. J. Nanopart. Res. 2005, 7, 499-506.

(6) U.S. Environmental Protection Agency. Perchlorate TreatmentTechnology Update; EPA 542-R-05-015; U.S. EPA: Washington,DC, May 2005.

(7) Zhou, P.; Brown, G. M.; Gu, B. Membranes and other treatmenttechnologies-pros and cons. In Perchlorate EnvironmentalOccurrence, Interactions and Treatment; Gu, B., Coates, J. D.,Eds.; Springer: New York, 2006.

(8) Gu, B.; Ku, Y-K.; Brown, G. M. Sorption and desorptionperchlorate and U(VI) by strong-base anion-exchange resins.Environ. Sci. Technol. 2005, 39, 901-907.

(9) Gu, B.; Brown, G. M. Recent advances in ion exchange forperchlorate, treatment, recovery and destruction. In PerchlorateEnvironmental Occurrence, Interactions and Treatment. Gu, B.,Coates, J. D., Eds.; Springer: New York, 2006.

(10) Tripp, A. R.; Clifford, D. A. Ion exchange for the remediation ofperchlorate-contaminated drinking water. J. Am. Water WorksAssoc. 2006, 98, 105-114.

(11) Diallo, M. S.; Chritie, S.; Swaminathan, P.; Johnson, J. H., Jr.;Goddard, W. A., III Dendrimer enhanced ultrafiltration. 1.Recovery of Cu(II) from aqueous solutions using Gx-NH2

PAMAM dendrimers with ethylene diamine core. Environ. Sci.Technol. 2005, 39, 1366-1377.

(12) Diallo, M. S. Water treatment by dendrimer enhanced filtration.U.S. Patent Application; Publication No. US 1006/0021938 A1;Publication Date February 2, 2006.

(13) Savage, N.; Diallo, M. S. Nanomaterials and water purification.J. Nanopart. Res. 2005, 7, 331-342.

(14) Martell, A. E.; Hancock, R. D. Metal Complexes in AqueousSolutions; Plenum Press: New York, 1996.

(15) Beer, P. D.; Gale, P. A. Anion recognition and sensing: the stateof the art and future perspectives. Angew. Chem., Int. Ed. 2001,40, 486-516.

(16) Tomalia, D. A.; Naylor A. M.; Goddard, W. A., III Starburstdendrimers-molecular-level control of size, shape, surface-chemistry, topology and flexibility from atoms to macro-scopic matter. Angew. Chem., Int. Ed. Engl. 1990, 29, 138-175.

(17) Twyman, L. J.; Beezer, A. E.; Esfand, R.; Hardy, M. J.; Mitchell,J. C. The synthesis of water soluble dendrimers and theirapplication as possible drug delivery systems, Tetrahedron Lett.1999, 40, 1743-1746.

(18) Birnbaum, E. R.; Rau, K. C.; Sauer, N. N. Selective anion bindingfrom water using soluble polymer. Sep. Sci. Technol. 2003, 38,389-404.

(19) Daniel, M. C.; Ruiz, J.; Blais, J. C.; Daro, N.; Astruc, D. Synthesisof five generations of redox-stable pentamethylamidoferrocenyldendrimers and comparison of amidoferrocenyl- and pentam-ethylamidoferrocenyl dendrimers as electrochemical exore-ceptors for the selective recognition of H2PO4

-, HSO4- and

adenosine 5′-triphosphate (ATP) anions: stereoelectronic andhydrophobic roles of cyclopentadienyl permethylation. Chem.Eur. J. 2003, 9, 4371-4379.

(20) Stephan, H.; Spies, H.; Johansnsen, B.; Klein, L.; Vogtle, F.Lipophilic urea-functionalized dendrimers as efficient carriersfor oxyanions. Chem. Commun. 1999, 1875-1876.

(21) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. About dendrimers:Structure, physical properties, and applications. Chem. Rev.1999, 99, 1665-1688.

(22) Jansen, J. F. G. A.; Debrabandervandenberg, E. M. M.; Meijer,E. W. Encapsulation of Guest Molecules Into a Dendritic Box.Science 1994, 266, 1226-1229.

(23) Pistolis, G.; Malliaris, A. study of poly(propylene imine) den-drimers in water by exciplex formation. Langmuir 2002, 18,246-251.

(24) Kabanov, V. A.; Zezin, A. B.; Rogacheva, V. B.; Gulyaeva, V. B.;Zansochova, M. F.; Joosten, J. G. H.; Brackman, J. Polyelectrolytebehavior of Astramol poly(propyleneimine) dendrimers. Mac-romolecules 1998, 31, 5142-5144.

6526 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 18, 2007

(25) Diallo, M. S.; Chritie, S.; Swaminathan, P.; Balogh, L.; Shi, X.;Um, W.; Papelis, L.; Goddard, W. A., III; Johnson, J. H., Jr.Dendritic chelating agents 1. Cu(II) binding to ethylene diaminecore poly(amidoamine) dendrimers in aqueous solutions.Langmuir 2004, 20, 2640-2651.

(26) Cakara, D.; Kleiman, J.; Borkovec, M. Microscopic protonationequilibria of poly(amidoamine) dendrimers from macroscopictitrations. Macromolecules 2003, 36, 4201-4207.

(27) Niu, Y.; Sun, Li.; Crooks, R. M. Determination of the intrinsicproton binding constants for poly(amidoamine) dendrimersvia potentiometric pH titration. Macromolecules 2003, 36, 5725-5731.

(28) Scherrenberg, R.; Betty, C.; van Vliet, P.; Edouard, G., Brackman,J.; de Brabander, E.; Mortensen, K. The molecular characteristicsof poly(propyleneimine) dendrimers as studied by small angleneutron scattering, viscosimetry and molecular dynamics.Macromolecules 1998, 31, 456-461.

(29) Dvornic, P. R.; Uppuluri, S. Rheology and Solution Propertiesof Dendrimers. In Dendrimers and other Dendritic Polymers;Frechet, J. M. J., Tomalia, D. A., Eds.; Wiley and Sons: NewYork, 2001.

(30) Maiti, P. K.; Cagin, T.; Lin, S. T.; Goddard, W. A., III Effect ofsolvent and pH on the structure of PAMAM dendrimers.Macromolecules 2005, 38, 979-991.

(31) Wikipedia. Properties table of common solvents; Available athttp://en.wikipedia.org/wiki/Solvent.

(32) Barron, R. E.; Fritz, J. S. Effect of functional group structure andexchange capacity on the selectivity of anion exchangers fordivalent anions. J. Chromatogr. 1994, 316, 201-210.

(33) Diallo, M. S. Mixtures of dendrimers for selective recovery ofanions from aqueous solutions and electrolytes by dendrimerfiltration. U.S. Patent Application, Pending.

(34) Diallo, M. S. Anion-selective dendritic hosts: computer aidedmolecular design and applications to separations, sensing andcatalysis. U.S. Patent Application, Pending.

Received for review October 26, 2006. Revised manuscriptreceived May 10, 2007. Accepted May 29, 2007.

ES0710959

VOL. 41, NO. 18, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 6527