Environ. Sci. Technol. 2005, 39, 7084-7088
Effect of pH and Aging Time on the Kinetic Dissociation of 243Am(III) from Humic Acid-Coated γ-Al2O3: A Chelating Resin Exchange Study X I A N G K E W A N G , * ,† C H A N G L U N C H E N , † JINZHOU DU,‡ XIAOLI TAN,† DI XU,† AND SHAOMING YU§ Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, 230031, People’s Republic of China, State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, 20062, People’s Republic of China, and College of Chemical Engineering, Hefei University of Technology, 230009, Hefei, People’s Republic of China
The chelating resin was studied to assess its influence on metal availability and mobility in the environment. The association of organic-inorganic colloid-borne trace elements was investigated in this work. The radionuclide 243Am(III) was chosen as the representative and chemical homologue for trivalent lanthanide and actinide ions present in radioactive nuclear waste. The kinetic dissociation behavior of 243Am(III) from humic acid-coated γ-Al2O3 was studied at pH values of 4.0 ( 0.1, 5.0 ( 0.2, and 6.0 ( 0.2 with a contact time of 2 days after the addition of a chelating cation exchanger resin. The concentrations of the components were: 243Am(III) 3.0 × 10-7 mol/L, γ-Al2O3 0.5 g/L, HA 10 mg/L (pH 4.0 ( 0.1, 5.0 ( 0.2, and 6.0 ( 0.2) and 50 mg/L (pH 6.0 ( 0.2), respectively. The kinetics of dissociation of 243Am(III) after different equilibration time with humic acid-coated γ-Al2O3 was also investigated at pH 5.0 ( 0.2. The experiments were carried out in air and at ambient temperature. The results suggest that the fraction of irreversible bonding of radionuclides to HAcoated Al2O3 increases with increasing pH and is independent of aging time. The assumption of two different 243Am(III)HA-Al2O3 species, with “fast” and “slow” dissociation kinetics, is required to explain the experimental results. 243Am(III) species present on HA-Al O colloids moves from 2 3 the “fast” to the “slow” dissociating sites with the increase of aging time.
Introduction Immobilization of long-lived radionuclides, especially lanthanide and actinide ions, is an important goal of radioactive nuclear waste disposal. In this respect, humic acids-coated oxide minerals are of importance for the transport and sorption of lanthanides and actinides. Natural groundwater colloids can be characterized as organic, for example, humic acid (HA) and fulvic acid (FA), inorganic colloids, for example, clay minerals, silica or metal oxide colloids, or organicinorganic colloids, for example, humate-coated clay minerals * Corresponding author phone (fax): +86-551-5592788; e-mail:
[email protected]. † Chinese Academy of Sciences. ‡ East China Normal University. § Hefei University of Technology. 7084
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or metal oxide colloids (1). Because aquatic colloids have high specific surface area and chemical reactive, polyvalent actinides can be strongly sorbed onto the surface or into the structure of organic-inorganic colloids. The humic substances (HS) concentration in groundwater may increase to >100 mg per liter in geological formations with a lignite intercalation. Multivalent actinides are found to have a strong interaction with humic colloids (2-4). The migration of actinides in aqueous systems is therefore strongly influenced by the interaction of actinides with humic acid and mineral oxide colloids. The earlier batch experiments suggested that humic substance enhances the sorption of actinides at low pH and decreases at high pH values (5-8). The increase of sorption is explained by the bonding of HS onto the mineral surface followed by the interaction of actinides with surface sorbed HS, and the reduction of sorption due to the formation of soluble HS-An complexes, which stabilizes the actinides in aqueous solution. Several studies of metal complexation with humic substances in natural water systems have been done using equilibration models, and the assumptions of local equilibrium have been the dominant tool for explaining the metal speciation in freshwaters (3, 4). One problem with the equilibration assumption is that freshwater environments are often nonequilibrium systems, requiring kinetic approaches to determine the chemical speciation of metals more realistically (9-12). Kinetic dissociation study cannot only differentiate chemical species according to the kinetic parameters, but can also give information on the distribution in the environment during chemical processes, which they may undergo. The question is whether the interaction of actinides with organic-inorganic colloids undergoes reversible/irreversible sorption of actinides on colloids, and under what circumstances reversible or irreversible reaction becomes prevalent. The questions on the kinetic aspects of actinide-colloid interactions, that is, how fast is the desorption reaction of colloid-borne actinides under natural aquatic conditions, have been explained by several authors. The different rates of dissociation observed in various investigations (13-15) for actinide ions or REE humate complexes were attributed to the “strong” actinide binding within the coiled humic acid structure or “weak” binding to peripheral sites showing either slow or fast dissociation kinetics (16, 17). The strong binding sites are assumed to arise from folding of the humic acid molecule, thus stabilizing the metal-humic acid-mineral particle interaction. The application of the chelating resins for environmental samples has received considerable attention due to its influence on metal availability and mobility in the environment (17-19). Strong complexation between the chelating resin and the metal ions may be with respect to the metal bioavailability and remobilization from sediments and aquifers.
Experimental Section Chemicals. The hydrous γ-Al2O3 particles (Degussa, Aluminum Oxide C), also used by several authors (20-22), were first washed with 0.1 mol/L HNO3, then with 0.1 mol/L NaOH up to pH 10, and finally rinsing with Milli-Q water until the conductivity of the washing solution reached a stable value around 1.4-1.6 µS. The purified alumina was then stored as a suspension (60 g/L) in Milli-Q water. The N2-BET method gave the specific surface area to be 105 m2/g. The particle size (∼100 nm) and the point of zero charge (pH ≈ 8.9) are measured using photon correlation spectroscopy (PCS) (20, 10.1021/es0506307 CCC: $30.25
2005 American Chemical Society Published on Web 08/09/2005
21). Humic acid was extracted from a soil sample from the Da-Ya bay of Guangdong province, China. The purification and characterization of the humic acid was as described earlier (23, 24). Methods. The solution of 243Am(III)-humic acid-Al2O3 was prepared under aerobic conditions at ionic strength 0.1 mol/L NaClO4, pH ) 4.0, 5.0, and 6.0, respectively. The concentration of Al2O3 was 0.5 g/L, HA was 10 mg/L, and 243 Am(III) was 3.0 × 10-7 mol/L. The volume of the solution was 200 mL, and the weight of the chelating resin was 0.5 g. At pH 6.0, HA concentration was increased to 50 mg/L to investigate the effect of humic acid concentration. After humic acid was contacted with Al2O3 for 2 days, the 243Am(III) solution was added, and after various contact times of 2 h, 2 days, 1 month, and 5 months, the purified chelating cation exchanger (chelating resin, 3M Empore, Extraction Disks, Varian, Switzerland) was added. From the batch experiments, 2 days is enough to achieve the equilibration of HA sorption on alumina. About 97-99% HA is sorbed on alumina by analysis of HA in supernatant using a UV-vis spectrophotometer. pH was adjusted to 4.0, 5.0, and 6.0, respectively (by addition of 0.01 mol/L NaOH or 0.01 mol/L HCl solutions) and monitored throughout the experiments. Manual mixing of the suspension was done occasionally every 1 or 2 days. All experiments were carried out at room temperature. After different periods of time, the 243Am(III) remaining in suspension was analyzed by liquid scintillation counting (Packard 3100 TR/AB Liquid Scintillation analyzer, PerkinElmer) with ULTIMA GOLD AB (Packard) scintillation cocktail. Purification of chelating resin and conversion into a mixed H+/Na+ form was done as described elsewhere (17, 18).
Results and Discussion The dominant 243Am(III) species is Am3+ at pH < 7 (25). Geckeis et al. (17) studied the lanthanides and actinides species in humic acid solutions in the absence and presence of the chelating resin and found that humic acid forms strong complexation with actinide ions. A calculation considering the presence of the chelating resin suggests that most of the actinides are sorbed onto the resin (17). A relative fast reaction is found for the sorption of 243Am(III) to the chelating resin in the absence of humic acid and alumina (Figure 1A). The 243Am(III) concentration decreases rapidly to below 1% of the original concentration, and after 2 days the activity of 243Am(III) in solution is close to background. The sorption reaction of 243Am(III) to the chelating resin in the absence of humic acid and alumina can be expressed as
Am3+ + 3Na/H - Chelex f Am - Chelex + 3Na+/H+ (1) which is rather fast. The sorption of 243Am(III) to the chelating resin is delayed in the presence of humic acid and alumina depending on the pH values. Hence, the dissociation reaction of 243Am(III) from the humic acid-Al2O3 colloids is responsible for the shapes of the curves observed in the experiment (Figure 1B). The dissociation reaction of 243Am(III) from the humic acidAl2O3 colloids and sorption reaction onto the chelating resin can be expressed as step 1
≡ (Al - O) - HA - Am3+ 98 step 2
Am3+ ‚‚‚ +3Na/H - Chelex 98 Am - Chelex + 3Na+/H+ (2) The dissociation reaction of 243Am(III) from humic acidAl2O3 colloids (step 1) is very slow as compared to the sorption reaction of 243Am(III) onto the chelating resin (step 2). The
FIGURE 1. Fractions of 243Am(III) species in suspension as a function of the contact time with the chelating resin. The equilibration time of 243Am(III)-HA-Al2O3 was 2 days. C[243Am(III)] ) 3.0 × 10-7 mol/L, C[Al2O3] ) 0.5 g/L, C[HA] ) 10 mg/L. (A) Only chelating resin and 243Am(III); (B) presence of HA and Al O . 2 3 rate of sorption reaction of 243Am(III) onto the chelating resin is thus dependent on the dissociation reaction of 243Am(III) from humic acid-Al2O3 colloids. Pseudo-first-order kinetics was assumed to investigate the dissociation of metal ions from the humic acid-coated oxide colloids by several authors (16-18). In these studies, two types of metal ion/humic acid surface complexes, “fast” dissociating species and “slow” dissociating species, were needed to fit the experimental data. The contributions of the fast and slow species are indicated in the following:
( )
( )
Cfinal Csusp(t) -t -t ) + A1‚exp + A2‚exp Cinitial Cinitial τ1 τ2
(3)
where Csusp(t) is the concentration of 243Am(III) in suspension at time t (h); Cinitial is the initial total concentration of 243Am(III) in suspension (t ) 0) before the addition of the chelating resin; Cfinal is the final concentration of 243Am(III) in suspension after equilibration with the chelating resin; and A1(%) and A2 (%) are the fractions of 243Am(III) humic acid-Al2O3 colloids dissociating with a time constant τ1 (h) and τ2 (h), respectively. Considering that in the absence of the chelating agent the concentration of 243Am(III) remaining in solution is extremely low, one should state that Cfinal/Cinitial describes the fraction of 243Am(III) that is not being dissociated from the colloids. The sum of A1 and A2 can be considered as the fraction of reversible sorption sites, and Cfinal/Cinitial is the fraction of irreversible sorption sites. The results obtained VOL. 39, NO. 18, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Fractions of 243Am(III) species in suspension as a function of the contact time with the chelating resin and pH values. The equilibration time of 243Am(III)-HA-Al2O3 was 2 days. C[243Am(III)] ) 3.0 × 10-7 mol/L, C[Al2O3] ) 0.5 g/L.
FIGURE 3. Fractions of 243Am(III) in suspension as a function of contact time with the chelating resin. The different curves give the results for 243Am-HA-Al2O3 colloids, which have been equilibrated for the times given in the figure. pH ) 5.0 ( 0.2, C[243Am(III)] ) 3.0 × 10-7 mol/L, C[Al2O3] ) 0.5 g/L, C[HA] ) 10 mg/L. by the simulation of experimental data (Figures 1-3) using the kinetic model (eq 3) are listed in Table 1. The solid lines in Figures 1-3 are the simulation of the experimental data. Effect of pH. The results for the dissociation of 243Am(III) from humic acid-Al2O3 colloids at pH 4.0 ( 0.1, 5.0 ( 0.2, and 6.0 ( 0.2 are shown in Figure 2. The concentration of 243Am(III) decreases rapidly at pH 4.0, and the 243Am(III) dissociation rate slows down with increasing pH. As compared to the sorption of 243Am(III) onto the chelating resin in the absence of humic acid and alumina (Figure 1A), the sorption of 243Am(III) to the chelating resin is delayed in the presence of humic acid and alumina depending on the solution pH. The dissociation of 243Am(III) is initially fast and then slows down with increasing contact time with the chelating resin. Figure 1B shows the fast reaction of 243Am(III) sorption onto the chelating resin at the first contact time with the chelating resin, and the dissociation rate of 243Am(III) from humic acid-Al2O3 colloids decreases with increasing pH values. From the parameters listed in Table 1, it is clear that the “fast” and “slow” dissociating fractions (A1 and A2) diminish from pH 4.0 to 6.0, and the fractions of final concentration in suspension (Cfinal/Cinitial) (i.e., fraction of irreversible sorption sites) go up with increasing pH. This is 7086
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in agreement with the sorption of actinides onto alumina in the absence or presence of humic acid analyzed by batch sorption experiments (5, 20). The sorption of actinides on HS-oxides increases with increasing pH, and irreversible fractions of sorption also increase. Effect of Humic Acid Concentration. The dissociation of 243Am(III) from humic acid-Al O colloid at pH 6.0 for two 2 3 different humic acid concentrations, 10 and 50 mg/L, respectively, is also shown in Figure 2. The dissociation rate of 243Am(III) from the humic acid-Al2O3 colloid at initial time is very similar for the two systems, and then becomes different with increasing contact time. After 5000 h of contacting with the chelating resin, the fraction of 243Am(III) remaining at humic acid-Al2O3 colloid (Cfinal/Cinitial) is ∼65% for 10 mg/L HA, and ∼42% for 50 mg/L HA. The results suggest that humic acid contributes significantly to the sorption/desorption of actinides onto oxide particles. Geckeis et al. (17) had studied the dissociation of Eu(III) from Eu-HA complexes at pH 8 and found that only about 13% Eu(III) remains as Eu-HA complexes in solution after long time contacting with the chelex resin (which is very similar to the chelating resin). The irreversible retention ability of the actinides at humic acid-Al2O3 colloids is attributed to the properties of oxide and humic acid. The affinity of oxide particles to HA and metal ions is different as the functional groups are dependent on oxides (26, 27). It is a little surprising that a high content of HA decreases Cfinal/Cinitial from 65% (CHA ) 10 mg/L) to 42% (CHA ) 50 mg/ L). Our results were similar to the earlier reports (27, 28) that the retention of Th(IV) was hindered when HA and hematite was equilibrated beforehand during 24 h; this effect was more evident when the ratio between HA and surface sites exceeded a critical value (i.e., at high HA concentration). Humic substances have a macromolecular structure, and it is presumed that only a small fraction of the “sorbed” carboxylic and phenolic groups directly interact with alumina surface sites, while the remaining “sorbed” groups are free to interact with americium (29). As described in the earlier report (17), only about 13% Eu(III) remained as Eu-HA complexes in humic acid solution. The irreversible sorption fraction of Eu(III) on HA is low. The 243Am(III) at the outer sphere of HA layer is mainly presented as Am-HA complexes and is easily desorbed from HA-alumina colloids and then is adsorbed on the chelating resin. Therefore, Cfinal/Cinitial at high HA concentration is lower than that at low HA concentration. It is necessary to investigate the kinetic dissociation of 243Am(III) from Al2O3 and HA separately in further studies. Effect of Equilibration Time (Aging Time). The dissociation of 243Am(III) from the humic acid-Al2O3 colloid at pH 5.0 ( 0.2 after different contact times of 243Am(III)-humic acid-Al2O3 complex (2 h, 2 days, 1 month, and 5 months) (Figure 3) shows that the sorption of 243Am(III) to the chelating resin depends on the equilibration time prior to the contact with the chelating resin. The dissociation rate of 243Am(III) slows down with increasing aging time of 243Am(III)-humic acid-Al2O3 complex. In all experiments, a similar 243Am(III) concentration is attained after long contact time with the chelating resin (ca. 5000 h). The decrease of 243Am(III) concentration is described by using a similar time constant (τ1) for the fast and (τ2) for the slow dissociating components irrespective of different aging times. From the parameters listed in Table 1, A1 decreases a little, whereas A2 increases with the increasing contact time. 243Am(III) moves from a fast (A1) to a slow (A2) dissociation species with an increase in the reaction time; that is, the availability of the colloidborne 243Am(III) toward sorption to chelating resin decreases with aging time (17). The dissociation rates are similar to those reported by King et al. (30) by using a similar experimental approach, and slower than those reported by Geckeis et al. (17). The difference may be attributed to the
TABLE 1. Kinetic Parameters Obtained for Different Systems by Fitting Experimental Data to the Kinetic Equation (Eq 3)
a
aging time
pH
A1 (%)
τ1 (h)
A2 (%)
τ2 (h)
Cfinal/Cinitial (%)
2 days 2 days 2 days 2 daysa 2h 2 days 1 month 5 months
4.0 5.0 6.0 6.0 5.0 5.0 5.0 5.0
22 ( 2 13.0 ( 0.8 8.8 ( 2.2 13.5 ( 1.9 11.5 ( 1.3 13.0 ( 0.8 11.5 ( 2.6 6.1 ( 1.2
2.6 ( 0.7 23.5 ( 4.3 5.8 ( 4.0 2.8 ( 1.0 6.0 ( 1.82 23.5 ( 4.3 166 ( 69 3.7 ( 1.3
72.2 ( 1.2 69.8 ( 0.7 25.7 ( 1.2 45.2 ( 0.7 69.1 ( 0.7 69.8 ( 0.7 71.9 ( 2.0 76.0 ( 2.0
525 ( 21 1570 ( 50 571 ( 64 1862 ( 81 966 ( 31 1570 ( 50 1994 ( 162 2042 ( 35
5(1 16.5 ( 0.7 65 ( 3 42 ( 4 18.7 ( 0.7 16.5 ( 0.7 16.0 ( 1.6 18.0 ( 2.0
The concentration of HA was 50 mg/L.
FIGURE 4. Fractions of fast (A1) and slow (A2) dissociating 243Am(III) as a function of aging time of 243Am(III)-HA-Al2O3. different sources of the humic substances. The results indicate that the contribution of different humic substances on sorption and complexation of metal ions is different. In our earlier report (21), time-resolved laser fluorescence spectroscopy (TRLFS) studies of the sorption of Cm(III) on humic acid-coated γ-Al2O3 had indicated that the equilibration can be achieved in several hours, and the microstructure/ species of Cm(III) on the colloids changes with increasing time. The sorption of Cm(III) on humic acid-coated γ-Al2O3 colloids is similar to that of Am(III), and the results of TRLFS study are consistent with the present results. Comparison of A1, A2, and Cfinal/Cinitial. The fraction of 243Am(III) bound in “fast” (A ) and “slow” (A ) dissociating 1 2 configurations is shown as a function of aging time of 243Am(III)-humic acid-Al O complex in Figure 4. The “fast” 2 3 dissociating fraction (A1) goes down, and the “slow” dissociation fraction (A2) goes up with increasing aging time. The sum of A1 and A2 for different aging times is in the range of 81-84%, which is in agreement with the fraction of 243Am(III) remaining on HA-Al O colloids in solution (i.e., 2 3 Cfinal/Cinitial ) 16-18%). Considering the uncertainties in the simulation and measurement, one can see the fraction of irreversible sorption is independent of aging time. Fractions of A1, A2, and Cfinal/Cinitial and the sum of A1, A2, and Cfinal/Cinitial at pH 4.0, 5.0, and 6.0 are shown in Figure 5. Cfinal/Cinitial increases with increasing pH, while A1 and A2 decrease with increasing pH. The sum of A1, A2, and Cfinal/ Cinitial is about 100% in the uncertain scope. From the kinetic dissociation study, one can conclude: (1) Sorption and desorption of 243Am(III) on humic substances-coated oxide particles is strongly influenced by pH. (2) The sorption of 243Am(III) on organic-inorganic colloids depends on aging time. The bond between 243Am(III) and colloids becomes strong, and the dissociating rate decreases with increasing aging time. The sorption of 243Am(III) moves
FIGURE 5. Fractions of A1, A2, and Cfinal/Cinitial of 243Am(III) as a function of pH. The open points were derived from the system of C[HA] ) 50 mg/L. from a “fast” to a “slow” dissociating species/sites with increasing aging time of 243Am(III) in humic acid-alumina complex. (3) The fraction of irreversible sorption sites in humic acid-alumina colloids is dependent on pH values, but independent of the equilibration contact time of metal ions with humic acid-alumina colloids.
Acknowledgments Centurial Project support from the Chinese Academy of Sciences in IPP is acknowledged. We also thank Dr. Geckeis and Dr. Rabung (INE, Forschungszentrum Karlsruhe, Germany) for providing us with samples of the chelating resin and alumina. Dr. D. A. Dzombak, Associate Editor, and three anonymous reviewers provided thoughtful recommendations for revision that improved the manuscript.
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Received for review April 2, 2005. Revised manuscript received July 8, 2005. Accepted July 11, 2005. ES0506307
