Anal. Chem. 2005, 77, 5048-5054
Column Precipitation Chromatography: An Approach to Quantitative Analysis of Eigencolloids E. Breynaert and A. Maes*
Department of Interphase Chemistry, K. U. Leuven, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium
A new column precipitation chromatography (CPC) technique, capable of quantitatively measuring technetium eigencolloids in aqueous solutions, is presented. The CPC technique is based on the destabilization and precipitation of eigencolloids by polycations in a confined matrix. Tc(IV) colloids can be quantitatively determined from their precipitation onto the CPC column (separation step) and their subsequent elution upon oxidation to pertechnetate by peroxide (elution step). A clean-bed particle removal model was used to explain the experimental results. The chemistry of reduced technetium species has been subject to investigations initiated from both the medical community and the nuclear industry. Studies concerned with medical applications of Tc (more specifically 99mTc, t1/2 ) 6.2 h, γ emitter) are often focused on the extensive complexation chemistry of Tc because technetium is one of the principal radiolabeling agents used in medical nuclear imaging applications (planar scintigraphy, single photon emission tomography, etc.).2 Research initiated by the nuclear industry is mainly targeted to two different issues: (a) the separation of 99Tc (t1/2 ) 2.14 × 105 years, β- emitter) from high-level nuclear waste streams,3 a potential application for the recycling of nuclear waste, and (b) the geochemical behavior of 99Tc, an important issue with regard to the safety of the geological disposal facilities for long-term storage of nuclear waste.4,5 99Tc has been identified as one of the critical radionuclides that could impair the long-term safety of these facilities. Both medical and environmental studies concerned with the solubility and the complexation chemistry of technetium have encountered colloidal Tc(IV)-forms. Although the existence of the Tc colloids has been proven by various techniques,6-11 their quantitative determination still remains an issue. *
[email protected]. (1) Mota, M.; Teixeira, J. A.; Yelshin, A.; Bowen, W. R. Extended abstracts of the International conference Solid-Liquid Separation 2002, Falmouth, England, June 18-20, 2002. (2) Klaus, S. Technetium: Chemistry and Radiopharmaceutical Applications; Wiley-WCH Verlag GmbH: Weinheim, 2000. (3) Madic, C.; Lecomte, M.; Baron, P.; Boullis, B. C. R. Phys. 2002, 3, 797811. (4) NIRAS/ONDRAF. Technisch overzicht van het SAFIR 2-rapport (NIROND 2001-05 N), December 2001. (5) Artinger, R.; Buckau, G.; Zeh, P.; Geraedts, K.; Vancluysen, J.; Maes, A.; Kim, J. I. Radiochim. Acta 2003, 91, 743-750. (6) Grossmann, B.; Muenze, R. Int. J. Appl. Radiat. Isot. 1982, 33, 189-192. (7) Mang′era, K.; Vanbilloen, H.; Cleynhens, B.; de Groot, T.; Bormans, G.; Verbruggen, A.; Verbeke, K. Nucl. Med. Biol. 2000, 27, 781-789. (8) Sekine, T.; Narushima, H.; Kino, Y.; Kudo, H.; Lin, M. Z.; Katsumura, Y. Radiochim. Acta 2002, 90, 611-616.
5048 Analytical Chemistry, Vol. 77, No. 15, August 1, 2005
In 2002, technetium(IV) oxide colloids, radiolytically formed by γ-irradiation of aqueous TcO4- solutions, were visualized for the first time by transmission electron spectroscopy.8 This experiment confirmed the existence of nanosized (2-130-nm o.d.) Tc(IV) colloids in reducing aqueous technetium solutions. Recently X-ray absorption fine structure spectroscopy also provided evidence for the existence of colloidal or polymeric Tc(IV) species in mixed chloride/sulfate media11,12 and laboratory-scale natural systems containing humic substances.9,13 In addition, it has also been shown that colloidal technetium can be stabilized in solution through an association with natural organic matter. This interaction has been described for various natural systems and could be quantified by a KHS value.9,14 The model used by Maes et al.14 quantitatively describes the interaction of dissolved organic matter with neutral Tc(IV) species, following KHS
Tc(neutral) + HS y\z Tc(neutral)-HS
(1)
It can be predicted from the KHS that the concentration of unassociated neutral Tc(IV) species will increase with the total Tc concentration in the system. Maes et al. have indeed shown that this concentration can largely exceed the expected solubility for crystalline and amorphous TcO2‚xH2O precipitates, previously determined to be in the range from 1 × 10-9 to 2 × 10-9 M Tc.15 Colloidal technetium species are also encountered and accounted for in medical applications of 99mTc.16 Radiopharmacists have developed various thin-layer chromatography- (TLC) or paper chromatography-based protocols that enable the assessment of the colloidal content of 99mTc solutions used for medical imaging. Quantification is done based on the γ-activity of the 99m nuclide. This approach can only be used successfully for the determination of 99mTc colloids, because the weak β-emission of 99Tc is difficult (9) Maes, A.; Geraedts, K.; Bruggeman, C.; Vancluysen, J.; Rossberg, A.; Hennig, H. Environ. Sci. Technol. 2004, 38, 2044-2051. (10) Ben Said, K.; Seimbille, Y.; Fattahi, M.; Houee-Levin, C.; Abbe, J. C. Appl. Radiat. Isot. 2001, 54, 45-51. (11) Vichot, L.; Ouvrard, G.; Montavon, G.; Fattahi, M.; Musikas, C.; Grambow, B. Radiochim. Acta 2002, 90, 575-579. (12) Vichot, L.; Fattahi, M.; Musikas, C.; Grambow, B. Radiochim. Acta 2003, 91, 263-271. (13) Geraedts, K.; Bruggeman, C.; Maes, A.; Van Loon, L. R.; Rossberg, A.; Reich, T. Radiochim. Acta 2002, 90, 879-884. (14) Maes, A.; Bruggeman, C.; Geraedts, K.; Vancluysen, J. Environ. Sci. Technol. 2003, 37, 747-753. (15) Rard, J. A.; Rand, M. H.; Anderegg, G.; Wanner, H. Chemical Thermodynamics of Technetium; Elsevier Science B. V.: Amsterdam, 1999. (16) Hung, J. C.; Budde, P. A.; Wilson, M. E. Am. J. Health-Syst. Pharm. 1995, 52, 310-313. 10.1021/ac050546+ CCC: $30.25
© 2005 American Chemical Society Published on Web 06/10/2005
to measure quantitatively on TLC plates. More recently highperformance liquid chromatography and size exclusion chromatography have also been successfully applied for the determination of the radiochemical purity and for the identification of the different species coexisting in the 99mTc-labeled solutions.17 In these LC techniques, colloidal technetium species often show some unwanted side effects; i.e., they partially adsorb to the column matrix, which results in an incomplete recovery. This behavior is consistent with experimental data from environmental studies that show a high affinity of reduced technetium species for a wide range of solid surfaces.5,18,19 Next to the aforementioned literature focused on Tc, other authors also have acknowledged that colloidal transition metal species (Pu, Zr, ...) can have a pronounced influence on the behavior of these elements in both artificial and natural environments.20-24 INSTRUMENTAL SETUP The separations described in this paper were made on a Pharmacia Biotech size exclusion chromatography system in which the classical polymer gel-based column was replaced with a newly developed precipitation column. The system was further equipped with a fraction collector, a gradient elution system and a UV detector. The precipitation column was prepared manually by dry packing a polyacrylic tube (length, 30 cm; inner diameter, 10 mm) with 6 g of Kieselguhr G (5-40 µm, Merck 8129, 15% CaSO4), which is a commercial macroporous, low specific surface (1.6 m2 g-1) product, with a high porosity (0.8), commonly used in TLC applications. After packing, the column was slowly wetted ((3 mm/min), from the bottom up to avoid air inclusions, with a saturated CaSO4‚2H2O solution. Two CaSO4‚2H2O (Merck, pro analyze) saturated eluents (R and O) were used; eluent R was a reducing solution consisting of ultrapure (MilliQ) water with hydrazine (8 × 10-4 mol/L added, Acros hydrazine monohydrate 99%) and was continuously flushed with N2 to remove dissolved oxygen. Eluent O, was a CaSO4‚2H2O saturated, oxidizing, H2O2/ tungstate solution (respectively, 3.5 wt % and 3.5 × 10-3 mol/L). The pH and Eh values for eluent R were respectively 7-8 and about -200 mV versus Ag/AgCl. Typical pH and redox conditions for eluent O were a pH value of 6.5-7 and an Eh value of +330 mV versus Ag/AgCl. After the initial packing procedure, the column was first eluted with the saturated CaSO4‚2H2O MQ solution (eluent R) for 48 h to stabilize the column packing and to remove colloidal silicon particles. This procedure was followed (17) Vanderghinste, D.; Van Eeckhoudt, M.; Terwinghe, C.; Mortelmans, L.; Bormans, G. M.; Verbruggen, A. M.; Vanbilloen, H. P. J. Pharm. Biomed. Anal. 2003, 32, 679-685. (18) Vandergraaf, T. T.; Ticknor, K. V.; George, I. M. ACS Symp. Ser. 1984, 246, 25-43. (19) Kumata, M.; Vandergraaf, T. T. Radioact. Waste Manage. Environmental Restor. 1993, 17, 107-117. (20) Honeyman, B. D. Nature 1999, 397, 23-24. (21) Artinger, R.; Rabung, T.; Kim, J. I.; Sachs, S.; Schmeide, K.; Heise, K. H.; Bernhard, G.; Nitsche, H. J. Contam. Hydrol. 2002, 58, 1-12. (22) Kersting, A. B.; Efurd, D. W.; Finnegan, D. L.; Rokop, D. J.; Smith, D. K.; Thompson, J. L. Nature 1999, 397, 56-59. (23) Mori, A.; Alexander, W. R.; Geckeis, H.; Hauser, W.; Schafer, T.; Eikenberg, J.; Fierz, T.; Degueldre, C.; Missana, T. Colloids Surf., A 2003, 217, 3347. (24) Walther, C.; Bitea, C.; Kim, J. I.; Kratz, J.-V.; Sherbaum, F. J. Institut fu ¨r Kernchemie, 2001.
by an elution during 6 h with eluent O to remove all oxidizable contaminants from the packing material. Throughout the packing procedure and the experiments, the column elution rate was always 1 mL/min. This can further be used as a column precipitation chromatography (CPC) column for eigencolloid determination. EXPERIMENTAL SECTION The void volume of the precipitation column was determined by monitoring the absorption (280 nm, Pharmacia monitor UV-1) of the UV-sensitive H2O2 in the eluate after switching to the peroxide eluent (eluent O). Before injection of a sample, the column was eluted for at least 30 min with eluent R to remove oxygen from the system. After injection of the sample (1.11-mL sample loop), the following elution procedure was applied: first the column was eluted with 12 mL of eluent R, followed by 9 mL of eluent O, and then again 15 mL of eluent R to completely remove all peroxide from the column. During the experiments, the eluate was continuously monitored (UV absorption) and was fractionated into 1.5-mL fractions in 5-mL polyethylene liquid scintillation vials. All β- activity (99Tc Emax ) 300 keV) was determined twice by liquid scintillation counting in both a Packard Tricarb 1600CA and a Packard Tricarb 1500 liquid scintillation analyzer, using Ultima Gold XR scintillation cocktail in 2:1 cocktail-sample proportions. Counting times were 30 min/sample for the Tricarb 1600CA and 45 min/sample for the Tricarb 1500. Three types of solutions, reported by test solution type, were tested in this study: (1) pure TcO4- solutions, (2) unfiltered reduced technetium solutions containing Tc(IV) colloids, and (3) filtered (0.2 µm, PTFE, Chromafil) reactive reduction mixtures sampled in the course of a TcO4- reduction and thus containing TcO4- and reduced Tc(IV) species. The pertechnetate solutions (test solution type 1) were prepared by diluting a TcO4- stock solution (5 × 10-2 M NH4TcO4, Amersham). The reduced technetium solutions (test solution type 2) were prepared in a glovebox under a reducing atmosphere (N2/H2 95/5%) by contacting a dry, aged (6 months) TcO2‚xH2O precipitate with 50 mL of MQ water containing 1 × 10-5 mol/L hydrazine. The system was vigorously shaken, and after one week of gravitational precipitation, the supernatant was sampled for injection onto the CPC column. The dry TcO2‚xH2O precipitate was prepared by reduction of TcO4- with excess hydrazine in the glovebox. The reactive reduction mixtures (test solution type 3) were prepared in ambient air by addition of hydrazine (2.5 × 10-1 mol/L, Acros hydrazine monohydrate 99%) to pertechnetate solutions (5 × 10-7 mol/L) in polyethylene vials on scheduled times before injection onto the column. TcO4- was gradually reduced into Tc(IV) in this process. Three experiments were performed (Table 1); in a first experiment, three type 1 test solutions containing different pertechnetate concentrations (2.05 × 10-7, 6.05 × 10-7, and 1.02 × 10-5 mol/L) were injected to evaluate the interaction between pertechnetate and the column packing. In a second experiment, two types of test solutions were injected on the column, namely, TcO4- (2.05 × 10-7 mol/L) (type 1) and a reduced technetium solution (3.54 × 10-6 mol/L) (type 2). In a third experiment, five sample solutions were injected on the column, namely, TcO4- (5 × 10-7 mol/L) (test solution type 1) and four reactive reducing mixtures (test solution type 3) sampled and filtered (0.2 µm, PTFE, Chromafil) at, respectively, 1-4 h after mixing (T1-T4). Analytical Chemistry, Vol. 77, No. 15, August 1, 2005
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Table 1. Overview of the Experiments and Samples sample
type
conc (M Tc)
A B C
Experiment 1 1 1 1
2.05 × 10-7 6.05 × 10-7 1.02 × 10-5
A B
Experiment 2 1 2
2.05 × 10-7 3.54 × 10-6
T0 T1 T2 T3 T4
Experiment 3 1 3 3 3 3
5.00 × 10-7 4.35 × 10-7 4.32 × 10-7 4.26 × 10-7 4.20 × 10-7
Figure 1. N2 adsorption isotherm for the Kieselguhr G column packing.
Characterization of the packing material was done with N2 adsorption (Coulter Omnisorp 100). RESULTS The N2 adsorption isotherm for the Kieselguhr G column packing shown in Figure 1 is typical for a solid with a relatively low specific surface and a small contribution of micro- and mesopores additional to the macroporous channels that are typical for diatomaceous earth. This was confirmed by a BET and t-plot surface area analysis, which resulted in a surface area of, respectively, 1.595 (C ) 9.073) and 2.692 m2/g. The high porosity (0.8) due to the macroporous channels of the packing material makes this an excellent material for use in aqueous systems where a low-pressure drop is advantageous. The breakthrough curve for an injection with eluent O is shown as absorbance in Figure 2. The absorbance of eluent O (280 nm) equalled 0.6 taking eluent A as the blank. Using the first inflection point of the UV-monitored elution profile of eluent O (Figure 2), a void volume of 10 ( 0.2 mL was observed. Throughout the complete series of experiments, the column stability and the void volume were monitored by checking the UV elution profile of eluent O after the eluent switch from eluent R to eluent O. In a first series of experiments, the interaction of pertechnetate (TcO4-) with the column matrix was evaluated. The elution profiles of three samples (A-C) containing pure TcO4- solutions (type 1) with concentrations 2.05 × 10-7, 6.05 × 10-7, and 1.02 × 10-5 mol/L are shown in Figure 2. Their recoveries were respectively 98.9, 99.4, and 96.8%. The maximum of the elution profile for TcO4- was observed at 9.75 ( 0.75 mL and corresponds 5050 Analytical Chemistry, Vol. 77, No. 15, August 1, 2005
Figure 2. Elution profiles for three different TcO4- samples: (A) (2.05 × 10-7 M), (B) (6.05 × 10-7 M), and (C) (1.02 × 10-5 M). In the figure, counts per minute (cpm) for all samples are shown versus the primary (A, B) and secondary (C) Y-axis. Additionally, the breakthrough curve for the peroxide containing eluent O is shown as UV absorption (280 nm) versus the primary Y-axis (0-1400 cpm ) 0-0.6 abs).
closely to the void volume of the column. This behavior is consistent with literature data, indicating that there is no interaction between pertechnetate and a large number of solid phases.25 The rather strong uncertainty in the reported maximum for the breakthrough curve of TcO4- is due to the large volume of the recovered fractions (1.5 mL/fraction). In a second series of experiments, two samples were injected. In this experiment, the column was eluted with 40.5 mL (instead of 12.5 mL) of eluent R to investigate the occurrence of column bleeding. Figure 3 shows the elution profile of a type 1 sample [TcO4- (2.05 × 10-7 mol/L) and a type 2, reduced technetium solution (3.54 × 10-6 mol/L)). The elution profile of the type 2 sample (B) presents two distinct Tc peak positions. Initially a small peak elutes at the position expected for TcO4-, as confirmed by the Tc peak observed for the type 1 sample (A). This small peak corresponds to only 4.2% of the injected activity, indicating that the sample was completely reduced. After the eluent switch to the oxidizing eluent O, a second, large, peak containing 94.9% of the total sample activity elutes. The observed elution profile for this latter peak is not delayed when calculated with respect to the eluent switch. The comeasured UV absorbance (Figure 3) shows that the elution times of H2O2 and the reoxidized Tc(IV) species are identical. Therefore, the initially retained colloidal Tc activity is eluted as TcO4-, after oxidation by eluent O. The UV absorption measurements of elutions with type 1 samples (TcO4only) have shown that the presence of TcO4- in concentrations lower than 1 × 10-5 M (33 200 cpm mL-1) does not add significantly to the UV absorption reading. The recoveries of the type 1 (A) and type 2 (B) samples were respectively 98.9 and 99.0%. A detailed inspection of the first peak, shown magnified in the inset in Figure 3, reveals that the profile for this peak is nearly identical to that of a TcO4- sample with a similar concentration. This indicates that the species eluting at this position should be either TcO4- or a species with similar behavior that does not interact with the column packing. In the third series of experiments, two types of sample solutions were injected on the column: a type 1 TcO4- sample (T0) and four type 2 filtered (0.2 µm, PTFE, Chromafil) reactive (25) Lieser, K. H.; Bauscher, C. Radiochim. Acta 1988, 44-5, 125-128.
Figure 3. Elution profiles for sample A (type 1) containing pure TcO4- (2.05 × 10-7 M) and sample B (type 2) containing a reduced technetium (3.54 × 10-6 M) solution. Counts per minute for all samples are shown versus the elution volume. Additionally, the breakthrough for a peroxide containing eluent O is shown as UV absorption (280 nm) (0-4000 cpm ) 0-0.6 abs). The inset shows a magnification in the range 0-450 cpm.
Figure 4. Elution profiles of TcO4- (T0) and samples T1-T4, sampled from reactive reductive mixtures at, respectively, 1-4 h after mixing. The elution volume is shown versus cpm on the primary Y-axis. The averaged elution profile for all samples is shown versus the UV absorption (280 nm) on the secondary Y-axis.
reducing mixtures, sampled between 1 and 4 h after mixing TcO4and hydrazine (T1-T4) in ambient air. The total activities remaining in the sample solutions after filtration of the samples T1-T4 were respectively 86.9, 86.4, 85.2, and 84.1% of the initial TcO4- concentration. The portion of the total activity that was filtered off (about 10-15%) was considered as particulate Tc(IV). The elution profiles of the Tc(IV)-containing reactive mixtures (T1-T4) are shown in Figure 4 and demonstrates that the peak eluting at the position expected for the TcO4- fraction decreases as the reduction proceeds, while the respective second peak in the profile enlarges. This second peak containing the activity associated with the colloid content of the samples contains
respectively 0, 27.5, 33.2, 33.9, and 36.8% of the total activity present in samples T0-T4. The total recoveries were respectively 97.2, 102.1, 100.9, 98.8, and 97.4%. The results from this third series of experiments (Figure 4) confirm the processes identified in the second series of experiments (Figure 3). Indeed, as more Tc(IV) is formed in the course of the reduction, an increasing amount of Tc is retained onto the column and is eluted later as TcO4- after reoxidation by eluent O. Since the total amount of Tc(IV) initially retained onto the column largely exceeded the solubility of TcO2‚xH2O precipitates, the Tc(IV) species separated by this procedure are colloidal Tc(IV) species and will further be designated as Tc(IV) eigencolloids. Analytical Chemistry, Vol. 77, No. 15, August 1, 2005
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DISCUSSION The series of experiments shown above have clearly demonstrated that the proposed CPC is well suited to separate eigencolloids from other species in a solution. Second, the CPC technique showed that the eigencolloid content can be quantitatively determined. However, the detailed interpretation of the elution profiles for the different samples can possibly disclose much more information about the samples than only their eigencolloid content. To make these further interpretations possible, it is imperative to have a thorough understanding of the working principles of the column. It has been shown that polyvalent cations such as Ca2+ can induce flocculation and facilitated deposition in a colloidal system. Davis et al. 26 have successfully demonstrated the positive effect of Ca2+ on the deposition of sulfate latex particles (o.d. 0.31 µm) onto a 10-cm sand-packed column. In most applications where flocculation/precipitation is applied, centrifugal forces are used complementary to gravitational forces to accelerate precipitation of the flocculated species. In some applications, gravitational or centrifugal precipitation can, however, induce unwanted effects. This is, for example, the case when the flocculated species have hydrophobic properties and relatively small dimensions. It has been shown27 that even charged colloidal particles can suddenly pop up at the air-water interface due to surface charge screening by salts. This can lead to small colloids being strongly trapped at the interface of air bubbles in water by forces resulting from the surface tension. Indeed, since such small air bubbles are present in most aquatic environments that have been thoroughly mixed, centrifugation of such a system may lead to a partial flotation of the colloids that were intended to precipitate. In the case of solutions containing Tc(IV) colloids, the combined action of added Ca2+ ions and centrifugation in fact made it impossible to sample the supernatant solution of the systems under investigation and led to inconsistent results (deviations up to 800%, between four samples from the same supernatant), due to such a flotation mechanism resulting in a Tc “film” at the solution surface. The results of batch precipitation experiments were therefore not reported in detail in the current work. To eliminate these and similar problems, the CPC system utilizes the deposition of colloids onto the particles of a packed bed instead of centrifugation. In fact, this deposition should be interpreted mechanistically as a combination of (hetero-)coagulation between colloids and heteroadagulation of the colloids onto the packed bed in a flowing stream. In packed beds similar to the CPC column matrix, three predominant mechanisms are responsible for the transport of particles from the bulk fluid, passing through the bed, to the surface of the bed constituents: interception, gravity (sedimentation), and Brownian diffusion. Since the colloid loading of the discussed samples is very low in comparison to the surface of the column, the CPC column can be approached using a clean-bed removal model in which the packed bed is modeled as an assemblage of unit collectors with a certain known geometry. In this approach, the overall single (26) Davis, C. J.; Eschenazi, E.; Papadopoulos, K. D. Colloid Polym. Sci. 2002, 280, 52-58. (27) Mbamala, E. C.; von Grunberg, H. H. J. Phys.: Condens. Matter 2002, 14, 4881-4900.
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Analytical Chemistry, Vol. 77, No. 15, August 1, 2005
collector removal efficiency η can be obtained from
η ) Rη0
(2)
In this expression, R is an empirical collision efficiency factor and η0 the single collector efficiency determined from physical considerations and according to Yao et al.28 R can be described as the sum of three transport mechanisms:
η0 ) ηD + ηI + ηG
(3)
ηD is the transport efficiency by diffusion, ηI is the transport efficiency by interception, and ηG is the transport efficiency by gravity. The respective formulas for these transport efficiencies for a spherical collector in a porous system are given by eqs 4-6:
( ) ()
/3
ηD ) 4.0AS1
D∞ Udc
3 dp ηI ) AS 2 dc ηG )
2/3
(4)
2
(5)
(Fp - F) 2 gdp 18µU
(6)
Combined with the correlating equations formulated by Rajagopalan et al.29,30 η0 was evaluated as
( )
η0 ) 4.0AS1/3
D∞ Udc
2/3
+ ASNLO1/8R15/8 + 3.38 × 10-3 ASNG1.2R-0.4 (7)
where AS ) 2(1 - p5)/(2 - 3p + 3p5 - 2p6),31 p ) (1 - f)1/3, D8 is the bulk particle diffusion coefficient, U is the superficial velocity, dc is the collector diameter, dp is the particle diameter, NLO ) 4A/(9πµdp2U), A is a Hamaker constant, R ) dp/dc, and NG ) (Fp - F)gdp2/(18 µU). For a packed bed composed of uniform spheres, the overall bed removal efficiency of the particles was expressed as
(
)
3 (1 - f)ηL 1 - C/C0 ) 1 - exp 4 ac
(8)
where L is the depth of the bed, ac is the collector radius, η is the overall single collector removal efficiency, f is the packed-bed porosity, and C0 and C, respectively, are the influent and effluent particle concentrations. Using eqs 7 and 8, the overall bed particle removal efficiency for the CPC column can be calculated as a function of the colloid size. The single collector efficiency η0 in these equations is mainly an expression for the frequency of collisions between colloids and the column matrix and is therefore (28) Yao, K.-M.; Habibian, M. T.; O’Melia, C. R. Environ. Sci. Technol. 1971, 5, 1105-1112. (29) Rajagopalan, R.; Tien, C. Am. Inst. Chem. Eng. J. 1976, 22, 523-533. (30) Rajagopalan, R.; Tien, C.; Pfeffer, R. Am. Inst. Chem. Eng. J. 1982, 28, 871872. (31) Happel, J. Am. Inst. Chem. Eng. J. 1958, 4, 197-201.
Figure 5. Calculated packed-bed particle removal efficiency based on column and flow parameters applicable to the CPC column. The removal efficiency (%) is shown versus the colloid diameter dp (µm) for three different collision efficiencies: R ) 1, R ) 0.1, and R ) 0.05. The following parameters were used in the calculations: U ) 0.212 mm s-1, L ) 10 cm, f ) 0.8 1, dc ) 0.02 mm, T ) 293 K, A ) 1 × 10-20 J, and Fp ) 4 g cm-3.
largely influenced by flow and column parameters such as collector diameter dc and superficial velocity U. The collision efficiency R is, however, largely controlled by the surface chemistry of the colloids and the matrix surfaces. Under optimal chemical treatment, for example, efficient surface charge compensation, the absence of a repulsive energy barrier between the colloids and matrix surfaces results in particle attachment at every collision (R ) 1). Figure 5 shows the overall bed removal efficiency for three different collision efficiencies: R ) 1, R ) 0.1, and R ) 0.05. The figure was constructed based on eq 8, using the specific parameters applicable to the CPC system. Although the collision efficiency R for the colloids in this work could not be determined theoretically, a value higher than 0.1 is expected for R because a theoretical analysis by Elimelech32 indicates a large dependence of R on the inverse Debye length and thus the thickness of the diffuse double layer. Thus, larger R values correspond to smaller double layers. Additional to the high ionic strength in the system, inducing a thin diffuse double layer, the results from Maes et al.9,14 indicate that hydrophobic interactions also have an important influence on the interactions of Tc(IV) colloids with surfaces. It can be seen in Figure 5 that for R > 0.1, and colloid diameters smaller than 350 nm or larger than 2 µm, the bed particle removal efficiency is larger than 99%. If a collision efficiency of 0.2 is considered, the overall bed particle removal efficiency would exceed 99.9% for all particle diameters. This result is consistent with the experimentally observed performance of the CPC column, shown above. Song and Elimelech have, however, shown33 that eq 7 is an overestimation for the single collector efficiency at low particle Peclet numbers. The particle Peclet number is defined in eq 9. 2
NPE )
12πµUap kT
(9)
observed that single collector efficiencies slightly higher than unity were found for colloids smaller than 2 nm. By adapting the model used as a basis for definition of eq 7, Song and Elimelech were able to prove that the overall single collector removal efficiency η, evaluated using parameters similar to these used in the current work, approaches unity at low particle Peclet numbers Figure 5 would therefore not be influenced strongly by the overestimation of η0 in eq 7. A trial was made to determine the colloidal dimensions from dynamic light scattering measurements. The colloid concentration was, however, too low to allow the determination. Size exclusion effects can have a pronounced influence on the behavior of colloids in a porous system. This mechanism has been successfully exploited in size exclusion chromatography and has also been helpful in explaining the observed behavior of colloids in natural environments,20-24 e.g., the migration of Pu containing colloids in the Nevada dessert, where these colloids moved faster than the groundwater current. Even for very small colloids (o.d. 50 nm) transported in broad macroporous channels (o.d. 0.5-5 µm), size exclusion effects have been observed.34 If this effect should play an important role in the current setup, colloids and polymeric species that are larger than TcO4- would exhibit a shorter elution time than pertechnetate. Evidently this is not consistent with the experimental results shown above, because no species have eluted faster from the column than pertechnetate. In contrast to the symmetric elution profile of TcO4- in Figures 2 and 3, the elution profiles for T1-T4 in Figure 4 show a highly asymmetric first peak. This asymmetry in peak shape of the firsteluting fraction indicates that this fraction cannot solely be attributed to only unreduced pertechnetate remaining in solution. The activity eluted by eluent R is therefore interpreted as a combination of pertechnetate and dissolved mono- and polymeric Tc(IV) species. The exact identity of these species could not be elucidated in the framework of this study; however, a hypothesis on the formation of eigencolloids can be formulated by the following reactions:
[Tcx(OH)4x-z(H2O)2x+z]z + [Tcy(OH)4y-w(H2O)2y+w]w h [Tcx+y(OH)4(x+y)-(w+z) (H2O)2(x+y+w+z)]w+z
with
x ) 1 f n, y ) 1 f n, z ) -1 f 1, w ) -1 f 1 (10)
which represent the condensation of mononuclear hydrolyzed Tc(IV) species into binuclear and polynuclear Tc(IV) species. Equation 10 indicates that equilibrium exists between those dissolved species, and therefore, a broad spectrum of primary and secondary species should always be present in a solution containing Tc(IV) eigencolloids. The proposed reaction mechanism on the formation of Tc(IV) eigencolloids is related to the reactions formulated by Altmaier et al.35 on the formation of Th(IV) colloids. This working hypothesis is highly supported by the spectra in Figure 4. Indeed, the tailing between the two peaks in the spectrum may be assigned to the interaction of primary and secondary species with the column.
In the calculations made to construct Figure 5, we have indeed (32) Elimelech, M. Water Res. 1992, 26, 1-8. (33) Song, L.; Elimelech, M. J. Colloid Interface Sci. 1992, 153, 294-297.
(34) Sirivithayapakorn, S.; Keller, A. Water Resour. Res. 2003, 39. (35) Naito, S.; Sekine, T.; Kino, Y.; Kudo, H. Radiochim. Acta 1998, 82, 129132.
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CONCLUSION The elution profiles of the samples containing reduced technetium prove the existence of Tc(IV) eigencolloids. Moreover, the new column precipitation chromatography technique provides a means for the quantitative measurement of the colloidal Tc(IV) fraction, determined as the amount of Tc that is retained onto the column and thus eluted as the second peak. The proposed CPC system can be used in solubility studies to discriminate between the contribution of colloidal and dissolved mono- and polynuclear species to the measured concentration in solution. The results of solubility studies of solids containing transition metals capable of forming stable eigencolloids can be seriously distorted by the formation of stable eigencolloids, which exhibit a behavior similar to dissolved species and which are difficult to detect. Another, more practical, preparative, application would be the removal of eigencolloids from preparations containing technetium complexes used in medical imaging. The preparative removal of technetium eigencolloids from solutions containing technetium complexes can be fairly easily implemented using a disposable “solid-phase extraction” version of the CPC column. The main limit in this case is the solubility of the Ca form of the prepared anionic complex.
SAFETY CONSIDERATIONS Hydrazine hydrate. Hydrazine hydrate is toxic by inhalation, in contact with skin, and if swallowed. It causes burns and may cause sensitization by skin contact. It may cause cancer. It is very toxic to aquatic organisms and may cause long-term adverse effects in the aquatic environment. It is corrosive, air sensitive, and a cancer suspect agent. Waste containing hydrazine hydrate should be classified as special waste and disposed off as prescribed by the local regulations. 99Tc. 99Tc is a β-emitting radioactive nuclide and should be handled with caution. Limited use can minimize the risk of severe
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radioactive contamination and all wastes should be disposed of as radioactive waste. ACKNOWLEDGMENT The authors acknowledge a grant from Katholieke Universiteit Leuven and financial support from the KULeuven Geconcerteerde Onderzoeksacties (GOA2005/13) and NIRAS/ONDRAF (Contract CCHO 20004004862). GLOSSARY R
collision efficiency
A
Hamaker’s constant (1.00 × 10-20 J)
As
porosity dependent parameter
ac
collector radius
C
particle concentration
dc
collector diameter
dp
particle diameter
Dm
bulk particle diffusion coefficient
f
filter medium porosity
g
gravitational acceleration constant
η
overall single collector efficiency
η0
single collector efficiency
ηD
transport efficiency by diffusion
ηI
transport efficiency by interception
ηG
transport efficiency by gravity
k
Boltzman’s constant
L
filter bed depth
µ
dynamic viscosity (water: 0.001 002 Pa s)
F
density
R
dp/dc
T
absolute temperature
U
approach (superficial) velocity
Received for review March 31, 2005. Accepted May 11, 2005. AC050546+
