Evidence of Tunable On−Off Sorption Behaviors of Metal Oxide

Ind. Eng. Chem. Res. 2006, 45, 7737-7742

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Evidence of Tunable On-Off Sorption Behaviors of Metal Oxide Nanoparticles: Role of Ion Exchanger Support Pavan Puttamraju and Arup K. SenGupta* EnVironmental Engineering Program, 13 E. Packer AVenue, Lehigh UniVersity, Bethlehem, PennsylVania 18015

Metal oxide particlessnamely, oxides of Fe(III), Zr(IV), Ti(IV), and Al(III)sare environmentally benign and exhibit amphoteric sorption behaviors around neutral pH; i.e., they can selectively bind both transitionmetal cations (e.g., Cu2+) and anionic ligands (e.g., arsenate or HAsO42-). Because sorption sites reside predominantly on the surface, the metal oxides offer very high sorption capacity at nanoscale sizes on a mass basis, because of the high surface area-to-volume ratio. However, these nanoparticles are almost impermeable in fixed-bed columnar configuration or any flow-through system. The primary objective of this short communication is to present convincing experimental evidence to demonstrate that, by appropriately dispersing hydrated iron oxide (HFO) nanoparticles within a polymeric cation or anion exchanger, its amphoteric sorption capacity can be tailored to remove either metal cations or anionic ligands. Such hybrid cation and anion exchangers are also amenable to efficient regeneration. Thus, toxic metals and ligands can be separated and recovered quantitatively from the same solution using HFO nanosorbent but with different ion exchanger support. The on-off sorption behaviors of HFO and zirconium oxide nanoparticles within an ion-exchanger host are explained using the Donnan membrane principle. The selective binding sites always reside with inorganic metal oxide nanoparticles, whereas the polymeric ion exchanger support exerts the Donnan effect. Introduction It is well-established that hydrated Fe(III) oxides (HFOs) exhibit amphoteric sorption behaviors; i.e., they can selectively bind Lewis acids or transition-metal cations (e.g., Cu2+) as well as Lewis bases or anionic ligands (e.g., arsenates and phosphates) through the formation of inner sphere complexes.1-6 Thus, Cu2+, which is a Lewis acid and an environmentally regulated heavy metal, is sorbed in preference to other competing but innocuous alkaline and alkaline-earth metal cations, namely, Na+, Ca2+, and Mg2+. Similarly, the sorption of arsenate, which is an anionic ligand, is preferred over commonly encountered anions, namely, sulfate, chloride, and bicarbonate. The point of zero charge (PZC) of submicrometer crystalline or amorphous iron oxide particles in an inert electrolyte (e.g., sodium nitrate, sodium perchlorate, or equivalent) resides within a pH range of 7.0-8.5.5,6 At circum-neutral pH, iron oxide nanoparticles sorb both Cu2+ and H2AsO4- simultaneously and selectively in the presence of commonly occurring competing ions, such as Na+, Ca2+, Cl-, SO42-, and HCO3-, that can form only outer-sphere complexes through Coulombic interaction. Oxides of zirconium(IV), titanium(IV), and aluminum(III) also exhibit similar favorable sorption behaviors toward Lewis acids and Lewis bases.7-11 Because sorption or binding sites reside only on the surface, nanoscale metal oxide particles with a very high surface-to-volume ratio offer significantly enhanced sorption capacity. However, such metal oxide nanoparticles are unable to separate transition-metal cations from anionic ligands. Nanoparticles cause unusually high-pressure drops in fixed-bed columns or any flow-through system; therefore, attempts were made to dope activated carbon, alginate, chitoson, cellulose, or polymeric sorbents with metal oxide nanoparticles.11-16 These host materials improved permeability in flow-through systems but were unable to alter or influence the sorption behaviors of metal oxides, including the separation of Lewis acids from Lewis bases. * To whom correspondence should be addressed. E-mail address: id: [email protected].

In earlier investigations, we successfully dispersed HFO nanoparticles within the gel phase of a polymeric cation exchanger using a chemical-thermal technique.16,17 The cation exchanger had negatively charged sulfonic acid functional groups covalently attached to the polymer matrix. However, similar methodology was not applicable to anion exchangers. Dispersing HFO nanoparticles within the gel phase of a strongly ionized anion exchanger is scientifically challenging because both ferric ion (Fe3+) and quaternary ammonium group (R4N+) in an anion-exchange resin are positively charged. Only recently have we been successful in dispersing HFO nanoparticles within polymeric anion exchangers through use of an anionic oxidizing agent, namely, permanganate (MnO4-) and hypochlorite (OCl-).18 It was conceptualized that if HFO or other metal oxide nanoparticles were dispersed within a cation or anion exchanger, the anions or cations will be rejected by the respective ion exchanger, in accordance with the Donnan co-ion exclusion effect.19,20 Consequently, amphoteric HFO nanoparticles will selectively bind either transition-metal cations (e.g., Cu2+) or anionic ligands (HAsO42-), as illustrated in Figure 1A-C. The sole objective of this short communication is to present sufficient experimental evidence to validate how the choice of the ionexchanger host material for HFO nanoparticles can completely reject the transition-metal cations while allowing enhanced selective sorption of anionic ligands and vice versa. Thus, in principle, an amphoteric metal oxide nanoparticle can be tailored to behave either as a strictly metal-selective sorbent or as a ligand-selective exchanger. It is likely that such tunability for complete rejection or selective uptake of target ions may also be easily incorporated in ion exchange membranes. Additional experimental evidence in support of zirconium oxide nanoparticles dispersed in ion exchanger media is also presented. Experiments Materials Preparation, Characterization, and Laboratory Procedure. A gel-type cation exchanger with sulfonic acid functional groups (Purolite C-100, Purolite Co., Philadelphia,

10.1021/ie060803g CCC: $33.50 © 2006 American Chemical Society Published on Web 09/28/2006

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Figure 1. Illustration of (A) distribution of surface functional groups of hydrated iron oxide (HFO) and their predominant interactions with different solutes, (B) inaccessibility of anions to HFO nanoparticles dispersed within a cation exchanger, and (C) inaccessibility of cations to HFO nanoparticles dispersed within an anion exchanger.

PA) and a gel-type anion exchanger with quaternary ammonium functional groups (Purolite A-400) were used for the preparation of HFO-dispersed ion exchangers. Commercially available granulated ferric hydroxide (or GFH) was obtained from U.S. Filter Co., Newark, NJ. Cation exchangers dispersed with HFO nanoparticles, referred to as hybrid cation exchanger or HCIX-Fe, were prepared in accordance with the technique developed earlier by DeMarco, SenGupta, and Greenleaf.16 As already explained in the previous section, the same procedure is not applicable to prepare hybrid

Figure 2. Scanning electron microscopy (SEM) microphotographs of (A) of freshly prepared HFO particles (20 000×), (B) spherical HAIX-Fe beads (10×), and (C) a sliced HCIX-Fe bead (40 000×).

anion exchanger or HAIX-Fe because both Fe3+ and anion exchanger’s quaternary ammonium functional groups (R4N+) are positively charged. A recently developed technique using an intermediate oxidizing agent (MnO4-) was used to disperse HFO nanoparticles within the anion exchanger gel phase.18 A zirconium oxide-dispersed cation exchanger (HCIX-Zr) was prepared, following the procedure described by Suzuki et al.21 Fixed-bed column runs were conducted using glass columns (11 mm in diameter), constant-flow stainless steel pumps, and

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Figure 4. Concentration profiles of (A) Cu(II) and (B) As(V) during regeneration of HCIX-Fe and HAIX-Fe (abbreviations: SLV ) superficial liquid-phase velocity and EBCT ) empty bed contact time). Copper recovery was 90%, whereas As(V) recovery was 96%.

Figure 3. Concentration profiles of Cu(II) and As(V) during the column runs for (A) granulated ferric hydroxide (or GFH), (B) HCIX-Fe, and (C) HAIX-Fe. (Abbreviations: SLV ) superficial liquid-phase velocity and EBCT ) empty bed contact time.)

an ISCO fraction collector, as described in previous works.22-24 Trace amounts of Cu(II) and As(V) anions, namely, H2AsO4and HAsO42-, were present in the feed solutions along with other commonly occurring electrolytes. Both dissolved arsenic and copper were analyzed using an atomic absorption spectrophotometer with graphite furnace accessory (Perkin-Elmer Model 6500). The iron content of the hybrid material was determined after digestion with 10% sulfuric acid for 24 h and subsequent analyses of iron in the liquid phase, followed by a mass balance. Figure 2A shows nanoscale HFO particles that were prepared via controlled precipitation from the aqueous phase. Figure 2B

shows photographs of HAIX-Fe particles synthesized in our laboratory; note that the spherical geometry is retained after processing and the hybrid polymeric particles develop a deep brown color, because of the presence of HFO. Figure 2C shows a scanning electron microscopy (SEM) photograph of a sliced HCIX-Fe. Results Evidence of Tunability. In our laboratory, we performed three separate fixed-bed column runs, using (i) commercially available GFH from U.S. Filter Co. without any ion exchanger support material; (ii) HFO dispersed in a cation exchanger, which was referenced as a hybrid cation exchanger (HCIX-Fe); and (iii) HFO dispersed in an anion exchanger, which was referenced as a hybrid anion exchanger (HAIX-Fe). The feed composition was identical in all three cases, and trace concentrations of both anionic As(V) or H2AsO4- and Cu2+ were present as target solutes with other electrolytes. The hydrodynamic conditionssnamely, superficial liquid velocity (SLV) and empty bed contact time (EBCT)swere also identical for all three column runs. Figure 3A-C shows the column-run results; note

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Figure 5. Aqueous phase arsenic and copper concentration profiles during a batch sorption test using HCIX-Zr.

that (i) GFH removed both As(V) anions and Cu2+ quite significantly; (ii) HCIX-Fe removed only Cu2+ very selectively for well over 2000 bed volumes but rejected As(V) anions completely; and iii) HAIX-Fe showed extraordinary As(V) sorption with a minimum breakthrough for nearly 5000 bed volumes while Cu2+ broke through immediately. Note that the parent cation exchanger with sulfonic acid functional groups can sorb cations only through electrostatic interaction and, thus, offers no specific selectivity toward Cu2+ in the presence of greater concentrations of other competing cations, namely, calcium and sodium. Similarly, anion exchangers with quaternary ammonium functional groups exhibit no specific selectivity toward anionic arsenate in the presence of competing sulfate anions and the same has already been demonstrated in a previous study.17 Figure 4A demonstrates that the copper that is sorbed could be efficiently desorbed at slightly acidic pH from HCIXFe, whereas HAIX-Fe with arsenate was amenable to efficient regeneration with an alkaline solution (see Figure 4B). Thus, Cu2+ and HAsO42- could be separated and recovered quantitatively from the same solution using HFO nanosorbents through appropriate choice of ion-exchanger materials as supports. There was no measurable loss of iron from HCIX-Fe or HAIX-Fe following regeneration. To further confirm that the sorption behaviors of other amphoteric metal oxide nanoparticlessnamely, zirconium oxide (ZrO2)smay also be tuned in a similar way, we dispersed zirconium nanoparticles within a cation exchanger, using a procedure described earlier in the literature.21 The resulting hybrid cation exchanger (HCIX-Zr) was used in a batch sorption study where both copper and arsenate were present in trace concentrations. Figure 5 shows that while copper concentration decreased to almost zero within an hour, arsenic(V) remained essentially unchanged. Discussion Role of the Donnan Membrane Effect. Experimental results of Figures 3A-C and 5 clearly demonstrate that amphoteric sorption behaviors of metal oxide nanoparticles can be judiciously tuned by dispersing them within the gel phases of ion exchangers. Thus, the HFO or zirconium oxide nanoparticles

can be tailored to be selective either toward transition-metal cations or anionic ligands while completely rejecting the other. Such tunability of the hybrid material results essentially from the Donnan membrane effect exerted by the ion exchanger support. The Donnan membrane equilibrium, which is also referenced as the Gibbs-Donnan equation, arises from the inability of ions to diffuse outward from one phase in a heterogeneous system. The gel phase of an ion exchanger can be viewed as a polyelectrolyte where the functional groups (quaternary ammonium groups for anion exchanger and sulfonic acid groups for cation exchanger) are covalently attached and, hence, nondiffusible. The manifestation of the Donnan effect in two types of ion exchangers can be explained as follows: (i) a high concentration of fixed, nondiffusible negatively charged sulfonic acid functional groups in a cation exchanger disallow permeation of anions, including arsenate into the gel phase and hence arsenate sorption by HFO nanoparticles is negligible; (ii) conversely, a high concentration of positively charged nondiffusible quaternary ammonium groups in an anion exchanger imbibe arsenate into the exchanger phase but reject Cu2+. It is noteworthy that the conditions leading to the Donnan membrane equilibrium do not arise from the physical existence of a semipermeable membrane or externally charged surfaces. To further quantitatively illustrate the Donnan membrane effect, let us consider a single HFO dispersed hybrid cationexchange resin bead that has been dropped in a large volume of solution containing 10 mM CaCl2. The solution also contains 10-3 mM Cu2+ and HAsO42-, i.e., dissolved copper and arsenate concentrations are almost negligible, compared to calcium and chloride. The cation exchanger bead with covalently attached sulfonic acid functional groups was originally in calcium form and the exchange capacity is 2.0 M. Considering electroneutrality in the exchanger phase,

[RSO3-] ) 2.0 ) 2[ Ca2+]R

(1)

where “R” refers to the resin phase. Of all the cations and anions present in the system, only RSO3- is nondiffusible, i.e., it cannot permeate outward from the resin phase, even with a thermodynamically favorable chemical potential gradient. All cations and anions excepting nondiffusible RSO3- will redistribute

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Figure 3C. In both cases, however, metal oxide nanoparticles (e.g., HFO or ZrO2) are the active sorbents but their sorption affinities toward specific solutes have been altered by the Donnan effect exerted by the supporting ion exchanger. From a separation process viewpoint, the foregoing hybrid materials offer opportunities to remove, separate, and recover target contaminants from the aqueous phase, using the same metal oxide nanosorbent but with different ion-exchanger supports. According to information in the open literature, any deliberate attempt to utilize the Donnan membrane effect to tailor or tune the sorption behaviors of HFO or other metal oxide nanoparticles has not been reported to date. Acknowledgment Figure 6. Donnan equilibrium effect for HFO dispersed cation-exchange resin (HCIX-Fe): (A) conditions before immersing the HCIX-Fe bead in the solution and (B) conditions at equilibrium.

between the ion exchanger and the aqueous phase, in accordance with the Donnan equilibrium. Assuming ideality, the following equilibrium relationship will be observed at equilibrium:

[Cu2+]R [Cu2+]W

)

[C1-]W2 [C1-]R2

)

[Ca2+]R [Ca2+]W

)

[HAsO42-]W [HAsO42-]R

Literature Cited

(2)

The exchanger- and aqueous-phase electroneutrality conditions are as follows:

2[Ca2+]R + 2[Cu2+]R ) [RSO3-] + 2[HAsO42-]R + [Cl-]R (3) 2[Ca2+]W + 2[Cu2+]W ) 2[HAsO42-]W + [Cl-]W (4) Now considering the experimental conditions that

[Ca2+]W . [Cu2+]W and

[Cl-]W . [HAsO42-]W it may be shown that

[Cu2 + ]R [Cu2+]W

)

[HAsO42-]W [HAsO42-]R

≈ 100

The authors are thankful to Sudipta Sarkar for his assistance in preparing the illustrations. We gratefully acknowledge that the project was financed in part by a grant from the Commonwealth of Pennsylvania, Department of Community and Economic Development, through the Pennsylvania Infrastructure Technology Alliance (PITA).

(5)

The derivation of eq 5 can be followed by consulting the recently published English translation of Donnan’s original paper.20 Thus, the copper concentration inside the cation exchanger gel is several orders of magnitude greater than arsenate at equilibrium, as illustrated in Figures 6A and 6B. Dispersed HFO nanoparticles within HCIX-Fe, therefore, show high copper removal capacity while arsenic removal is absent, as demonstrated in Figure 3B. Similarly, zirconium oxide supported within a cation exchanger also exhibits high copper removal capacity (see Figure 5). If the monovalent arsenate (H2AsO4-) is the predominant arsenic species, the derivation leading to eq 5 will be more cumbersome; however, the general conclusion that the Cu ion will predominate over arsenate inside the cation exchanger gel phase will remain unchanged. Conversely, the gel phase of the anion exchanger with positively charged quaternary ammonium groups allow enhanced permeation of arsenate coupled with near-complete rejection of copper, thus leading to high arsenic removal capacity, as evidenced from

(1) Gao, Y.; SenGupta, A. K.; Simpson, D. A new hybrid inorganic sorbent for heavy metals removal. Water Res. 1995, 29 (9), 2195-2205. (2) Dutta, P. K.; Ray, A. K.; Sharma, V. K.; Millero, F. J. Adsorption of arsenate and arsenite on titanium dioxide suspensions. J. Colloid Interface Sci. 2004, 278, 270-275. (3) Jang, J.-H. Surface chemistry of hydrous ferric oxide and hematite as based on their reactions with Fe(II) and U(VI), Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 2004. (4) Pierce, M. L.; Moore, C. B. Adsorption of As(III) and As(V) on amorphous iron hydroxide. Water Res. 1982, 6, 1247-1253. (5) Stum, W.; Morgan, J. J. Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters; Wiley-Interscience: New York, 1996. (6) Dzombak, D. A.; Morel, F. M. M. Surface Complexation Modeling. Hydrous Ferric Oxide; Wiley: New York, 1990. (7) Cotton, F. A.; Wilkinson, J. AdVanced Inorganic Chemistry; WileyInterscience: New York, 1998. (8) Ho, L. N.; Ishihara, T.; Ueshima, S.; Nishiguchi, H.; Takita, Y. Removal of fluoride from water through ion exchange by mesoporous Ti oxohydroxide. J. Colloid Interface Sci. 2004, 272, 399-403. (9) Ghosh, M. M.; Yuan, J. R. Adsorption of inorganic arsenic and organ arsenicals on hydrous oxides. EnViron. Prog. 1987, 3 (3), 150-157. (10) Leaser, K. H. In Ion Exchangers; Dorfner, K., Ed.; Walter de Gruyter: Berlin, 1991; p 519. (11) Yuchi, A.; Ogiso, A.; Muranaka, S.; Niwa, T. Preconcentration of phosphate and arsenate a sub-ng-ml-1 level with a chelating polymer-gel loaded with zirconium(IV). Anal. Chim. Acta 2003, 494, 81-86. (12) Guo, X.; Chen, F. Removal of arsenic by bead cellulose loaded with iron oxyhyrdoxide from groundwater. EnViron. Sci. Technol. 2005, 39, 6808-6818. (13) Min, J. M.; Hering, J. Arsenate sorption by Fe(III)-doped alginate gels. Water Res. 1998, 32, 1544-1552. (14) Cumbal, L.; Greenleaf, J.; Leun, D.; SenGupta, A. K. Polymer supported inorganic nanoparticles: characterization and environmental applications. React. Funct. Polym. 2003, 54, 167-180. (15) Zouboulis, A. I.; Katsoyiannis, I. A. Arsenic removal using iron oxide loaded alginate beads. Ind. Eng. Chem. Res. 2002, 41, 6149-6155. (16) DeMarco, M. J.; SenGupta, A. K.; Greenleaf, J. E. Arsenic removal using a polymer/hybrid inorganic hybrid sorbent. Water Res. 2003, 37, 164176. (17) Cumbal, L.; SenGupta, A. K. Arsenic removal using polymersupported hydrated Fe(III) oxide nanoparticles. EnViron. Sci. Technol. 2005, 39, 6508-6515. (18) SenGupta, A. K.; Cumbal, L. H. Manufacture and use of hybrid anion exchanger for selective removal of contaminating ligands from fluids. U. S. Patent Appl. Publ. U.S. 2005 156,136, July 21, 2005. (19) Donnan, F. G. Z. Electrochem. Angelwandte Phys. Chem. 1911, 17572-581. (In Ger.). (20) Donnan, F. G. Theory of membrane equlibria and membrane potentials in the presence of nondialysing electrolytes. A contribution to physical-chemical physiology. J. Membr. Sci. 1995, 100, 45-55. (Engl. transl.)

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(21) Suzuki, M. T.; Bomnai, J. O., Matsunga, H.; Yokoyama, T. Preparation of porous resin loaded with crystalline hydrous zirconium oxide and its application to removal of arsenic. React. Func. Polym. 2000, 43, 165-172. (22) Ramana, A.; SenGupta, A. K. Removing selenium(IV) and arsenic(V) oxyanions with tailored chelating polymers. J. EnViron. Eng. 1992, 118 (5), 755-775. (23) SenGupta, A. K.; Lim, L. Modeling chromate ion exchange processes. AIChE J. 1988, 34 (12), 2019-2029.

(24) Zhao, D.; SenGupta, A. K. Ultimate removal of phosphate using a new class of anion exchanger. Water Res. 1998, 32, 1613-1625.

ReceiVed for reView June 23, 2006 ReVised manuscript receiVed September 14, 2006 Accepted September 16, 2006 IE060803G