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Selective YF3 Nanoparticle Formation in Polyelectrolyte Capsules as Microcontainers for Yttrium Recovery from Aqueous Solutions Dmitry G. Shchukin† and Gleb B. Sukhorukov*,‡ Institute for Physico-Chemical Problems, 220050 Minsk, Belarus, and Max Planck Institute of Colloids and Interfaces, D14424 Potsdam, Germany Received November 19, 2002. In Final Form: March 4, 2003 The usage of hollow poly(styrenesulfonate)/poly(allylamine hydrochloride) polyelectrolyte capsules for the selective precipitation of YF3 from water solution was described. Weakly crystallized YF3 particles of 6-9 nm diameter were synthesized exclusively inside the restricted volume of the polyelectrolyte capsule and characterized by transmission electron microscopy, scanning electron microscopy, and wide-angle X-ray scattering methods. Fluoride loaded poly(styrenesulfonate)/poly(allylamine hydrochloride) capsules were shown to be an effective tool for recovery of Y3+ traces from water solutions with up to 99% yield of recovery. The influence of the initial concentration of Y3+ as well as of the concentration of attendant metals (Na+, K+, Sn2+, ZrO2+, Fe3+) on the recovery yield was also investigated.
* To whom correspondence should be addressed. E-mail: gleb@ mpikg-golm.mpg.de. Phone: +49 (0)331-567-9429. Fax: +49 (0)331-567-9222. † Institute for Physico-Chemical Problems. ‡ Institute of Colloids and Interfaces.
adsorption on solid adsorbents including ion-exchange columns and membranes.12,13 Besides extraction and adsorption methods, selective precipitation,14,15 dried vapor transport of volatile halide complexes,16 and electrolysis on cation-exchange polymeric resin17 have also been reported. The explored methods have different selectivities and recovery yields (60-100%) depending on the rare earth element concentration. However, the main drawback of the precipitation methods is the small (several nanometers) size of the particles, that complicates their further separation from solution and results in inevitable losses. Extraction and adsorption methods suffer from rather low selectivity of recovering micromolar quantities of rare earth elements from their mixtures with other metal salts of higher concentrations. Hollow polyelectrolyte capsules, owing to their confined micron-sized volume, can be both microreactors for synthesis of insoluble precipitates and microcontainers during the separation. They were first documented in refs 18-20 as an extension of a flat polyelectrolyte multilayer assembly on the surface of colloid particles.21,22 Polyelectrolyte capsules are assembled by the LbL adsorption of cationic and anionic polyelectrolytes on the surface of a colloidal template core of 0.1-10 µm diameter.23 Different materials can be taken as the template core: organic and
(1) Antonietti, M. Curr. Opin. Colloid Interface Sci. 2001, 6, 244248. (2) Caruso, R. A.; Antonietti, M. Adv. Funct. Mater. 2002, 12, 307312. (3) Heuer, A. H.; Fink, D. J.; Laraia, V. J.; Arias, J. L.; Calvert, P. D.; Kendall, K.; Messing, G. L.; Blackwell, J.; Rieke, P. C.; Thompson, D. H.; Wheeler, A. P.; Veis, A.; Caplan, A. I. Science 1992, 255, 10981105. (4) Addadi, L.; Weiner, S. Angew. Chem., Int. Ed. Engl. 1992, 31, 153-169. (5) Mann, S.; Hannington, J. P.; Williams, R. J. P. Nature 1986, 324, 565-567. (6) Tricot, Y.-M.; Fendler, J. H. J. Phys. Chem. 1986, 90, 3369-3374. (7) Bhandarker, S.; Bose, A. J. Colloid Interface Sci. 1990, 135, 531536. (8) Meldrum, F. C.; Wade, V. J.; Nimmo, D. L.; Heywood, B. R.; Mann, S. Nature 1991, 349, 684-687. (9) Hubicki, Z.; Olszak, M. Adsorpt. Sci. Technol. 1998, 16, 817-836. (10) Karavaiko, G. I.; Kareva, A. S.; Avakian, Z. A.; Zakharova, V. I.; Korenevsky, A. A. Biotechnol. Lett. 1996, 18, 1291-1296. (11) Benedetto, J. D. S.; Grewal, I.; Dreisinger, D. B. Sep. Sci. Technol. 1995, 17, 3339-3349.
(12) Wen, B.; Shan, X.-Q.; Xu, S.-G. Analyst 1999, 124, 621-626. (13) Zhu, W.; Kennedy, M.; Alaerts, G. Fresenius J. Anal. Chem. 1998, 360, 74-80. (14) Iwata, Y.; Imura, H.; Suzuki, N. J. Radioanal. Nucl. Chem. 1993, 172, 305-312. (15) Weterings, K.; Janssen, J. Hydrometallurgy 1985, 15, 173-190. (16) Jiang, J.; Ozaki, T.; Machida, K.; Adachi, G. J. Alloys Compd. 1997, 260, 222-235. (17) Pinto, D. V. B. S.; Martins, A. H. Hydrometallurgy 2001, 60, 99-104. (18) Sukhorukov, G. B.; Donath, E.; Davis, S.; Lichtenfeld, H.; Caruso, F.; Popov, V. I.; Mohwald, H. Polym. Adv. Technol. 1998, 9, 759-765. (19) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202-2205. (20) Sukhorukov, G. B. Designed Nano-engineered Polymer Films on Colloidal Particles and Capsules. In Novel Methods to Study Interfacial Layers; Mobius, D., Miller, R., Ed.; Elsevier Science B. V.: 2001; pp 384-414. (21) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831-835. (22) Decher, G. Science 1997, 277, 1232-1237.
Introduction Synthesis of new nanosized inorganic materials possessing well-defined characteristics such as size, crystal modification, optical properties, and so forth is one of the main objectives in modern inorganic chemistry in the past decade. A number of approaches were developed to regulate parameters of resulting materials and synthetic reaction. Among them, template synthesis;1,2 usage of organic additives (biopolymers, surfactants, polyelectrolytes, etc.) to control particle size, shape, and crystallinity;3,4 and synthesis in a restricted volume5-8 were well studied. Another important practical goal of inorganic and environmental chemistry is to develop methods for recovering traces of valuable chemical elements (e.g. rare earth metals) from water effluents containing other elements at concentrations several orders of magnitude higher than those to be recovered. The best known methods for rare earth metal separation and recovery are extraction in nonpolar solvents from aqueous solution9-11 and
10.1021/la0268707 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/04/2003
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inorganic nano- and microparticles,18,23-25 proteins,26,27 and so forth. Polyelectrolyte capsule walls are permeable toward ions and small organic molecules.23 By changing solvent, pH, or ionic strength of the solution, the permeability of the capsules can be tuned toward macromolecules and colloidal particles of submicron diameter.28,29 Differences in permeability allow filling the capsule interior with polymers and inorganic nanoparticles by either encapsulation as a template26,30,31 or incorporation in already formed polyelectrolyte capsules from solution.32,33 The presence of the polyelectrolytes inside the capsule establishes a pH-gradient over the capsule shell34 and opens possibilities to load the polyelectrolyte capsule with different cations (anions) as counterions for polyelectrolyte molecules. Entrapped ions can further participate in chemical reactions inside the capsules. This phenomenon was used for carrying out selective physicochemical processes (dye precipitation32 and inorganic carbonate35 and oxide36 synthesis) inside polyelectrolyte capsules. Here we investigate the formation of water-insoluble yttrium fluoride exclusively inside polyelectrolyte capsules containing an excess of poly(allylamine). Before precipitating YF3, the capsules were enriched with F- ions as counterions for positively charged amine groups of poly(allylamine) molecules. The resulting YF3 deposit was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and wide-angle X-ray scattering (WAXS) methods. The possible use of F- -loaded polyelectrolyte capsules for recovery of yttrium traces (∼10-5 M) from water solutions containing another attendant metal cation at higher concentration (∼10-1 M) was also studied. Experimental Methods Materials. Sodium poly(styrenesulfonate) (PSS, MW ∼ 70 000), poly(allylamine hydrochloride) (PAH, MW ∼ 50 000), fluorescein isothiocyanate, citric acid, acetone, Y(NO3)3‚6H2O, HF, ZrO(NO3)2‚6H2O, Fe(NO3)3‚9H2O, SnCl2, NaCl, KCl, MnSO4, and NH4HCO3 were purchased from Aldrich. All chemicals were used as received. The water used in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18 MΩ‚cm. Polyelectrolyte Capsule Formation. PAH/PSS capsules containing 0.1 monoM PAH monomers inside were fabricated (23) Sukhorukov, G. B.; Donath, E.; Lichtenfeld, H.; Knippel, M.; Budde, A.; Mo¨hwald, H. Colloid Surf., A 1998, 137, 253-266. (24) Voigt, A.; Lichtenfeld, H.; Sukhorukov, G. B.; Zastrow, H.; Donath, E.; Baumler, H.; Mohwald, H. Ind. Eng. Chem. Res. 1999, 38, 4037-4043. (25) Radtchenko, I. L.; Sukhorukov, G. B.; Leporatti, S.; Khomutov, G. B.; Donath, E.; Mo¨hwald, H. J. Colloid Interface Sci. 2000, 230, 272-280. (26) Balabushevitch, N. G.; Sukhorukov, G. B.; Moroz, N. A.; Volodkin, D.; Larionov, N. I.; Donath, E.; Mo¨hwald, H. Biotechnol. Bioeng. 2001, 76, 207-213. (27) Tiourina, O. P.; Antipov, A. A.; Sukhorukov, G. B.; Lvov, Y.; Mo¨hwald, H. Macromol. Biosci. 2001, 1, 209-214. (28) Lvov, Y.; Antipov, A. A.; Mamedov, A.; Mo¨hwald, H.; Sukhorukov, G. B. Nanoletters 2001, 1, 125-128. (29) Sukhorukov, G. B.; Antipov, A. A.; Voigt, A.; Donath, E.; Mo¨hwald, H. Macromol. Rapid Commun. 2001, 22, 44-46. (30) Shi, X.; Caruso, F. Langmuir 2001, 17, 2036-2042. (31) Antipov, A. A.; Sukhorukov, G. B.; Donath, E.; Mo¨hwald, H. J. Phys. Chem. B 2001, 105, 2281-2284. (32) Sukhorukov, G.; Da¨hne, L.; Hartmann, J.; Donath, E.; Mo¨hwald, H. Adv. Mater. 2000, 12, 112-115. (33) Da¨hne, L.; Leporatti, S.; Donath, E.; Mo¨hwald, H. J. Am. Chem. Soc. 2001, 123, 5431-5436. (34) Sukhorukov, G. B.; Brumen, M.; Donath, E.; Mo¨hwald, H. J. Phys. Chem. B 1999, 103, 6434-6440. (35) Sukhorukov, G. B.; Susha, A. S.; Davis, S.; Leporatti, S.; Donath, E.; Hartmann, J.; Mo¨hwald, H. J. Colloid Interface Sci. 2002, 247, 251254. (36) Radtchenko, I. L.; Giersig, M.; Sukhorukov, G. B. Langmuir 2002, 18, 8204-8208.
Shchukin and Sukhorukov
Figure 1. SEM image of MnCO3 template particles. employing the layer-by-layer assembling21,22 on the surface of monodisperse MnCO3 particles of ∼2.8 µm diameter. MnCO3 particles were prepared by a simple mixing method described in ref 37: Acidic manganese sulfate solution (9 × 10-3 M, pH ) 4.2 adjusted by sulfuric acid) was added at a one-to-one volume ratio to 2.25 × 10-3 M NH4HCO3. Then the stirred mixture was aged at 50 °C for 16 h. The resulting MnCO3 template particles had a round shape with an average diameter of 2.8 µm (Figure 1). To deposit a polyelectrolyte shell, 0.4 g of MnCO3 was added to 10 mL of 5 × 10-3 M citric acid solution and then mixed with 15 mL of a 1 mg/mL PAH solution; the amount of the latter is enough to form a relatively thick PAH/citrate shell (∼150 nm).34 Afterward, alternating adsorption of PAH/PSS monolayers starting from a PSS monolayer was performed using 2 mg/mL PSS or 1 mg/mL PAH solutions in distilled water (first three layers) and 2 mg/mL PSS solution or 1 mg/mL PAH solution in 0.5 M NaCl (next four layers). Washing out nonadsorbed polymer molecules followed each adsorption step. After formation of the PAH/PSS shell, the MnCO3 core was dissolved in 0.1 M HCl under Ar atmosphere to prevent oxidation of Mn(II), and monodisperse hollow capsules of ∼3.2 µm diameter composed of an inner PAH/citrate shell and an outer PAH/PSS one were obtained. Characterization. For SEM analysis a drop of each sample solution was applied to a glass wafer with sequential drying at room temperature overnight. Then the samples were sputtered with gold and measurements were conducted using a Gemini Leo 1550 instrument operated at an acceleration voltage of 3 keV. Confocal microscopy images of polyelectrolyte capsules in solution were obtained on a Leica TCS SP scanning system equipped with a 100× oil immersion objective. For confocal fluorescence microscopy PAH molecules were labeled with fluorescein isothiocyanate.23 To view the interior structure, dried polyelectrolyte capsules were embedded in gelatine holders filled with polymerized MMA (methyl methacrylate) and then cut into ultrathin sections (from 30 to 100 nm in thickness) using a Leica ultracut UCT ultramicrotome. Copper grids coated with carbon film were used to support the thin sections, and a Zeiss EM 912 Omega transmission electron microscope operating at 300 kV was employed for analysis. The crystallinity of YF3 (both encapsulated and deposited from water solution for comparison) was determined from wide-angle X-ray scattering (WAXS), EnrafNonius PDS-120. The concentration of Y3+ in solution was measured volumetrically employing xylenol orange as an indicator.38 The Y, Sn, Zr, Fe, Na, and K contents in polyelectrolyte capsules after yttrium recovery were determined with a Perkin-Elmer Analyst 700 atomic absorbance spectrometer. (37) Hamada, S.; Kudo, Y.; Okada, J.; Kano, H. J. Colloid Interface Sci. 1987, 118, 356-365. (38) Kolthoff, I. M., Elving, P. J., Sandell, E. B., Eds. Treatise on Analytical Chemistry; Wiley: New York, 1963.
YF3 Nanoparticles as Microcontainers for Yt Recovery
Langmuir, Vol. 19, No. 10, 2003 4429 Table 1. Efficiency of Yttrium Recovery from Solutions of Different Y3+ Concentration in the Presence of 0.1 M Attendant Metal Saltsa efficiency, % [Y3+], M 10-6
5 × 10-6 10-5 10-4 5 × 10-4 10-3
92.3 98.6 99.2 99.4 98.8 94.1
Fe3+
ZrO2+
Sn2+
Na+
K+
78.4 80.0 81.3 83.4 86.8 90.0
82.4 83.8 85.2 86.7 90.3 91.8
87.5 88.0 91.1 92.6 93.0 93.1
91.8 98.7 99.1 99.2 98.7 94.2
92.0 98.9 99.0 99.5 98.6 94.2
a Experimental conditions: 25 °C; 15% v/v of F--loaded PAH/ PSS capsules; duration of extraction, 18 h; pH ) 4.5.
Table 2. Y3+/Men+ Ratio in Polyelectrolyte Capsules after YF3 Precipitationa metal cation Fe3+
Figure 2. Schematic overview of the preparation of F--loaded PAH/PSS capsules followed by the precipitation of YF3.
Results and Discussion Synthesis of YF3 inside Polyelectrolyte Capsules. Initial polyeletrolyte capsules were hollow spheres of ∼3.2 µm diameter composed of inner PAH/citrate layers, which are slightly soluble in NaOH or NaCl solutions, and outer PAH/PSS layers, which are rather stable against medium alkali, acidic, or salt treatment.20,23-25 Substitution of citrate anions inside polyelectrolyte capsules with fluoride anions was performed in two stages. At the first stage, citrate-loaded polyelectrolyte capsules were immersed into deaerated 0.01 M NaOH solution for 12 h, replacing citrate anions by OH- (for a schematic overview of the whole procedure, see Figure 2). Then the excess of NaOH was removed and the capsules were put into deaerated 0.5 M HF for 12 h to replace OH- anions with F-. Employing NaOH solution at the first stage was necessary to perform the neutralization reaction for complete replacement. The F--loaded PAH/PSS capsules were washed in acetone to prevent reverse substitution of F- by OH-. As shown in Figure 3, PAH molecules inside F--loaded capsules mostly do not dissolve after NaOH and HF treatment and form an inner PAH/F- shell in which F- anions are at the highest concentration. However, slight fluorescence from the capsule interior was observed, confirming the presence of a PAH/F- complex also in the capsule volume. Adding F--loaded capsules to the Y3+ water solution (10-3 to 10-6 M) leads to the formation of insoluble YF3 only inside the capsules without any traces of precipitate in the solution. The absence of YF3 outside the capsule resulted from high stability of the PAH/F- complex, prohibiting the diffusion of fluoride from the capsule volume (PAH molecules are captured inside polyelectrolyte capsules and cannot diffuse through the capsule shell because of their large size34). Introduction of Y3+ in the capsule volume leads to the decomposition of PAH/F- due to the low solubility product constant of YF3. The presence of yttrium inside was confirmed by atomic absorbance spectroscopy, and the YF3 deposit was identified using WAXS. As shown in Figure 4, the yttrium compound precipitated inside the capsules is weakly crystallized YF3 with traces of Y(OH)3 as a minor component. Formation of the latter can be explained by hydrolysis of yttrium salt in the presence of PAH molecules, which increase pH inside the capsule volume.34 The broadening of X-ray peaks
ZrO2+ Sn2+ Na+ K+
[Y3+]/[Men+] 5.2 6.7 11.8 27.5 61.2
a Experimental conditions: 25 °C; 15% v/v of F--loaded PAH/ PSS capsules; 10-4 M [Y3+]; 0.1 M [Men+]; duration of extraction, 18 h; pH ) 4.5.
corresponding to the YF3 indicates considerably lower particle (crystallite) size and ordering compared to the case of the YF3 precipitated from water solution at the same conditions in the absence of PAH/PSS capsules (Figure 4). The crystallite size for YF3 inside the capsules, derived from X-ray data, is around 7 nm, which agrees with the particle size estimated by TEM (see below). Figure 5 shows typical SEM images of the polyelectrolyte capsules either without YF3 (Figure 5a) or filled with yttrium fluoride (Figure 5b). The most interesting feature of these images is the spherical-like morphology of capsules containing YF3, which is similar to the original form of the polyelectrolyte capsule in solution. The size of the particles is rather monodisperse and does not exceed the initial size of the capsules in solution. Folds and creases spreading from one YF3-loaded capsule to another are observed, demonstrating partial shrinkage of the capsule shell. Moreover, part of the polyelectrolyte shell was exfoliated upon drying YF3-loaded capsules, exhibiting an inner YF3 layer. For comparison, in the absence of inorganic precipitate inside, the capsules collapse upon drying, forming a flat structure with a variety of folds. Figure 6 shows TEM micrographs of polyelectrolyte capsules with YF3 deposited from water solutions containing Y3+ cations in different concentrations, on which the accumulation of YF3 nanocrystals is seen as dark spots because of their higher contrast. TEM analysis of YF3loaded polyelectrolyte capsules established that the deposition of YF3 from Y3+ solution presumably occurs on the inner side of the PAH/PSS wall (Figure 6), where, as described above, F- anions have the highest concentration. YF3 nanoparticles were also found in the capsule interior. The thickness and particle size of the YF3 layer greatly depend on the concentration of Y3+ in solution. For high concentrations (10-3 to 5 × 10-5 M [Y3+]), a continuous 50-100 nm layer made up of 7-10 nm particles is observed while separate agglomerates or individual particles attached to the inner capsule wall are formed at [Y3+] < 10-6 M. Recovery of Y3+ from Water Solutions. Deposition of insoluble rare earth fluorides inside polyelectrolyte capsules permits us to use these capsules as microcontainers for rare earth element separation and recovery
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Figure 3. Fluorescence confocal microscopy image and fluorescence profile of F--loaded PAH/PSS polyelectrolyte capsules. Encapsulated PAH molecules were labeled with fluorescein isothiocyanate.
Figure 4. XRD patterns of YF3 synthesized inside F--loaded PAH/PSS capsules (a) and in solution (b). Arrows indicate peaks corresponding to the Y(OH)3. YF3 was deposited from a 10-4 M Y(NO3)3 solution. Peak assignment was made using Max Planck Society Powder Diffraction Database (http://merkur.mpikggolm.mpg.de/pdf2/).
from water effluents. A study was carried out on the removal of traces of yttrium from both pure Y(NO3)3 aqueous solution and its mixtures with attendant metal salts. Adding 100 µL of 15% v/v F- -loaded capsules to 0.9 mL of Y3+ solution results in quantitative (>98%) extraction of yttrium ions to the capsule interior at initial yttrium concentrations varying from 10-5 to 10-4 M (Table 1). High
yield of yttrium removal from water solution can be explained by taking into consideration a rather large quantity of F- ions inside PAH/PSS capsules. Judging from the PAH monomer concentration in the inner PAH/ F- layer (0.1 monoM, see Experimental Section), a similar concentration of fluoride anions inside the capsules can be predicted. However, a slight decrease of the extraction yield was observed at either low (10-4 M) yttrium content. The indicated reduction of the recovery yield at low concentrations of Y3+ can be caused by very small, but definite, solubility of the YF3 deposit, resulting in a detectable extra quantity of Y3+ in solution, whereas at high yttrium content not all Y3+ cations react with loaded fluoride anions due to the insufficient amount of the latter in the capsules. This hindrance can be overcome by adding more F--loaded capsules to the treated solution. An excess amount of attendant metals in the yttriumcontaining solution decreases in general the yttrium recovery yield (Table 1). Na+ and K+ ions do not influence the extraction efficiency, and their content in polyelectrolyte capsules after extraction is more than 10 times lower compared to that of yttrium (Table 2). Higher Na+ content is caused by the use of sodium salts while assembling the capsule shell. On the contrary, metals capable of complexing with F- anions (Fe, Zr, Sn) reduce yttrium recovery by a factor up to 1.2, and their presence (see Table 2) was found in polyelectrolyte capsules upon completing YF3 formation. The most pronounced changes of the extraction efficiency appeared after the addition of ferric salt. The observed reduction of the recovery yield and selectivity can be explained by two reasons: (1)
Figure 5. SEM images of hollow F--loaded polyelectrolyte capsules (a) and polyelectrolyte capsules filled with YF3 (b). YF3 was deposited from a 10-4 M Y(NO3)3 solution.
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Figure 6. TEM images of hollow F--loaded polyelectrolyte capsules (a) and polyelectrolyte capsules filled with YF3 deposited from 5 × 10-6 M Y(NO3)3 (b), 10-4 M Y(NO3)3 (c), and 10-3 M Y(NO3)3 (d) solutions.
multicharged cations can be adsorbed in the PAH/PSS shell as counterions for anionic PSS molecules;25 (2) part of the F- ions can form stable metal-fluoride complexes which compensate the positive charge of the inner PAH wall. However, despite the negative effect of multicharged cations, the recovery yield remains high and filled polyelectrolyte capsules can be easily collected from solution by a simple sedimentation or centrifugation procedure. Thus, the slightly crystallized yttrium fluoride can be synthesized exclusively inside F- -loaded PAH/PSS polyelectrolyte capsules of micron size. Deposition of YF3 mainly occurs on the inner side of the polyelectrolyte shell. Depending on the initial concentration of yttrium cations in water solution, the thickness of the YF3 layer ranges between 50 and 100 nm; at low Y3+ concentration (
