Environ. Sci. Technol. 1996, 30, 2263-2269
Reduction of Pertechnetate in Solution by Heterogeneous Electron Transfer from Fe(II)-Containing Geological Material DAQING CUI* AND TRYGVE E. ERIKSEN Department of Chemistry, Nuclear Chemistry, Royal Institute of Technology, 100 44 Stockholm, Sweden
We have studied the surface-mediated reduction of pertechnetate (TcO4- ) in solution by Fe(II)-bearing fracture filling material from a natural fracture in granite, hornblende, and magnetite. The disappearance of technetium from solution was found to follow pseudo first order kinetics, the rate constant being dependent on the specific surface area and Fe(II) content of the solid. Comparison of the rate constants obtained in the experiments with fracture filling material containing chlorite as Fe(II)-bearing mineral, hornblende, and magnetite indicates a strong influence of the binding manner of Fe(II) in the solid phase. Magnetite, with a low band gap (0.1 eV) between the valence and conduction bands was found to be the most efficient reductant, and based on the ionic strength and pH dependence of the rate of TcO4reduction, it is concluded that sorption of TcO4- on the magnetite surface by a ligand-exchange mechanism is the rate-determining reaction step: >SOH + TcO4- a >SOTcO3 + OH-. Oxidative desorption of sorbed/precipitated TcO2(s) into air-saturated groundwater was found to be very slow, most probably due to competing reactions between oxygen and the surface of the Fe(II)-bearing solid.
Introduction Migration of radionuclides through water-carrying fractures in bedrock is controlled by their solubility in groundwater and their sorption on the fracture filling material of watercarrying fractures. The oxidation state may have a dramatic effect on the speciation, solubility, and sorption behavior of multivalent radionuclides. The fission product 99Tc constitutes a long-term potential hazard due to its long half-life and abundance in nuclear wastes. In oxic groundwaters, highly soluble Tc(VII) is present as the TcO4- anion with very slight sorptive interaction with geological material. Under the reducing conditions expected to prevail in a deep bedrock repository for high-level nuclear wastes, the solubility, which is controlled by precipitation of the tetravalent hydrous oxide TcO2‚nH2O(s) ()Tc(OH)4(s)), is of the order 10-8 mol dm-3 (1).
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1996 American Chemical Society
The redox potentials in deep groundwater systems have been found to be consistent with redox reactions involving Fe(II) in solution and hydrous iron(III) oxide phases (2). Several research groups have reported the reduction of TcO4- by Fe(II) in solution (3-6), but the interpretation of the experimental data is far from straightforward due to the presence of added or unintentionally formed Fe(II)containing solid phases in several of the systems studied. In a recent study, using reaction vessels with hydrophobic inner surfaces to suppress Fe(II) sorption on the vessel walls, we found the reduction of TcO4- by Fe(II) in neutral to slightly alkaline solution to proceed very slow, if at all (7). Sorption of technetium from solutions initially containing TcO4- onto granite under reducing conditions or in the absence of oxygen has been reported by several researchers (8-12). According to Vandergraaf et al. (12) technetium is removed from anoxic solutions , and in some cases in the presence of air by iron oxides but not by minerals containing ferrous iron as an integral part of their crystal lattice. Haines et al. (13) demonstrated in a Fourier transform infrared (FTIR) study that the reaction between TcO4- and magnetite occurs via surface-mediated reduction to Tc(IV) and precipitation of TcO2‚nH2O on the Fe3O4 surface. The focus of this paper is on the reduction of TcO4- in solution by heterogeneous electron transfer from redox active Fe(II)-bearing fracture fillings and minerals.
Experimental Section All experiments were carried out at ambient temperature (22 ( 2 °C) in a controlled atmosphere box flushed with argon to minimize the intrusion of oxygen into the reaction vessel. Materials. The material used as sorbents was collected from drillcores containing natural fractures taken 360 m below ground level in the Stripa Mine, Sweden. The color of the fracture filling materials from different drillcores varied in color from black to light green. Hornblende and magnetite were used as pure reference minerals. The solids were crushed in argon atmosphere using an Agate mortar and pestle, sieved to 60-90-, 90-125-,125250, and 250-500-µm size fractions and washed with acetone in a ultrasonic bath. The 125-250-µm size fraction of light green colored fracture filling material was separated into nine fractions with respect to iron content using a Frantz Model L-1 isodynamic separator. The mineral composition of the various crushed fractions was characterized by qualitative or semiquantitative X-ray diffraction and the BET surface areas measured using a Micromeritics Flow Sorb II 2300 apparatus with N2 as sorbing gas. The mineral composition of the solids used are given in Tables 1 and 2. All chemicals were of analytical grade and were used as received. The water used to prepare perchlorate and groundwater solutions was deionized, triple distilled in quartz, and purged with argon (AGA 5.7 quality) containing less than 0.5 ppm oxygen. The composition of the synthetic groundwater is given in Table 3. 99Tc was purchased from Amersham as TcO4- in 0.1 mol dm-3 NH4OH aqueous solution. Analyses. The Fe(II) content of the fracture filling material was determined as FeO by the method described
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TABLE 1
Qualitative Mineral Analysis by X-ray Diffraction granite fracture filling (black) fracture filling (dark green)
quartz quartz quartz
microcline epidote muskovite
albite calcite chlorite
muscovite chlorite pyroxeme
chlorite microcline albite
calcite muscovite
TABLE 2
Fracture Filling, Mineral Composition, and Proerties of Magnetically Separated Fractions of Light Green Fraction Filling fraction no.
wt (%) surface (m2/g) CEC (µquiv/g) Fe(II) (%) (titrat.) Fe total % (INAA) quartz % microcline % albite (%) plagioclase (%) iron chlorite (%) muscovite mica (%)
orig
1
2
3
4
5
6
7
8
9
100 1.02 22.7 0.96 1.25 22 23 37 15 0.8 1.7
0.82 1.8 61.3 16.45 17.34 17 17 20
2.67 3.93 55.9 6.01 6.81 23 22 42
6.68 4.29 53.2 2.58 3.62 19 23 38 18 2 7
18.9 2.44 30.0 1.37 1.93 18 22 41 17 0.7 4
7.92 0.97 21.4 0.86 1.14 19 21 38 16 0.5 2
7.83 0.65 17.6 0.63 0.77 19 26 40 16 0.7
5.56 0.47 13.4 0.43 0.59 19 24 38 15 0.5 0.1
18.45 0.46 9.1 0.28 0.38 22 25 35 13
31.18 0.3 3.6 0.11 0.14 26 23
quartz microcline albite
37 9
5 8
Ideal Composition intermediate plagioclase muscovite iron chlorite
SiO2 KAlSi3O8 NaAlSi3O8
16
0.1
Na0.5Ca0.5Al1.5Si2.5O8 KAl2(Si3Al)O10(OH)2 Fe3(Si,Al)4O10(OH)2.Fe3(OH)6
TABLE 3
Composition of Synthetic Groundwater HCO3123
species concn (mg dm-3)
SO429.6
Cl70
SiO2 12
Ca2+ 18
Mg2+ 4.3
K+ 3.9
Na+ 65
TABLE 4
Experimental Conditionsa exp 1 2 3 (1) 3 (2) 3 (3) 3 (4) 3 (5) 4 (1) 4 (2) 4 (3)
solution SGW* SGW SGW SGW SGW SGW SGW, H2O2 NaClO4, pH 9 0.1-10-4 M NaClO4, 10-3 M pH 7.8-9.5 SGW
CTc 10--6(M) 2.0 1.6 1.5
1.2 1.2 2.0
atm
sorbent
Ar Ar/H2 Ar/1% H2 Ar Ar + air air air Ar/1% H2 Ar/1% H2 Ar + 1% H2
FCGM none MSFF MSFF MSFF MSFF MSFF magnetite magnetite magnetite
m/V (g/cm3) 1/10.5 1/7.5 1/3.75 1/3.75 1/3.75 1/3.75 1/1 1/1 1/7
comments S.-R. R. S.-R. D.-O. D.-O. D.-O. D.-O. S.-R. different ionic strength S.-R. different pH S.-R.
a atm: atmosphere in reaction vessels; SGW: synthetic groundwater; SGW*: syntetic groundwater equilibrated with Stripa granite; FCGM: Fe(II)-containing geological material: granite, fracture filling (black and dark green) and hornblende; MSFF: magnetically separated fracture filling (light green); S.-R. sorption-reduction process; D.-O.: desorption-oxidation process.
by Graff (14). The pH measurements were made by a Radiometer Model pHM84 pH meter and a GK 202C glass electrode, and the redox potentials were monitored by a Metrohm Pt electrode connected to a Metrohm 632 pH meter. A Yokogawa SR20/AR24 (Ag/AgCl) electrode was used as reference. The electrodes were standarized using saturated quinhydrone buffers at pH 4 and 7. Samples drawn from the solutions with a syringe were weighed on an electronic balance with 10-4 g accuracy and analyzed for 99Tc in a Beckman Model 5801 Liquid Scintillation System using Ready Safe as the scintillation cocktail. The counter was calibrated with standard solutions containing known 99Tc concentrations. The counting error (2σ) was SOH2+ a SOH + H+ a >SO- + 2H+
(1)
describing the surface speciation of magnetite are according to Regazzoni et al. (17) pHzpc ) 6.8, pKa1s(int) ) 5.63. The effect of pH on the surface speciation is shown in Figure 8. The reduction of an oxidant in solution by heterogeneous electron transfer from structurally bound Fe(II) in minerals
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FIGURE 8. Protonation status of magnetite surface as function of pH.
is a complex process involving sorption, electron transfer between the solid interface and the adsorbed oxidant, solidstate electrochemical reactions, and electron transfer. Magnetite has a very low band gap (0.1 eV) between the valence and conduction band and fast electron transfer between Fe(II) and Fe(III) atoms (18). The influence of ionic strength and pH on the rate of reduction of TcO4- in neutral to alkaline solution by magnetite indicates that sorption by a ligand exchange reaction (19) replacing surface OH- group by TcO4- may be the rate-determining step:
>SOH + TcO4- a >S-OTcO3 + OH-
FIGURE 9. Relative rate constants for the reduction of TcO4- by magnetite, given as kpH/kpH)7, plotted against the fractional concentration of the uncharged surface species >SOH.
(2)
If this assumption is true, the rate of TcO4- reduction and hence the calculated pseudo-order rate constant should be proportional to the surface area of the solid phase and the surface concentration, denoted by brackets, of the species >SOH:
k ∝ A[>SOH]
(3)
In Figure 9, the experimental data are displayed in a kpH/kpH)7 versus [>SOH]pH/[>SOH]pH)7 plot. The slope obtained by linear regression analysis (0.96 ( 0.06) strongly corroborates the assumption of TcO4- sorption by the ligand exchange mechanism being the rate determining step. Desorption. Desorption of the reduced/sorbed technetium into argon-purged and aerated solution was found to be a very slow process in agreement with the results from our previous experiments (11). During 3 weeks exposure to air-saturated groundwater, the technetium concentration, initially corresponding to the solubility of TcO2‚nH2O(s), increased slightly (Figure 10). On the addition of H2O2 to a concentration of 3 mol dm-3 in the leach solution, a sudden increase in the technetium concentration was observed. A plausible explanation is that oxidation of the reduced technetium, sorbed or precipitated on the solids, is suppressed by competing reactions between oxygen and the surface of the Fe(II)bearing solid. On the addition of H2O2 in high concentra-
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FIGURE 10. Desorption of technetium from magnetically separated fractions of light green colored fracture filling under different redox conditions. Initial technetium concentration in the argon purged solution corresponds to the solubility of TcO2‚nH2O(s).
tion, the redox active reductant in the solids was consumed, allowing the oxidation of TcO2.nH2O.
Conclusions The fracture filling matrial and the aqueous phase form two different compartments in transmissive fractures in the bedrock. The three-electron reduction of TcO4- to the sparingly soluble TcO2‚nH2O by Fe(II) in the aqueous phase, although thermodynamically possible, is kinetically hindered. Reduction at the interface between Fe(II)-bearing fracture fillings and the aqueous phase will therefore be the most important retardation process for technetium. The rate of reduction is strongly influenced by sorption of TcO4- and the binding manner of Fe(II) in the solid phase.
Acknowledgments This work has been supported by the Swedish Nuclear Fuel and Waste Management Co. We are grateful to Prof. U. Håkansson for the gift of minerals and to Dr. P. Schweda for carrying out the X-ray analyses.
Literature Cited (1) Eriksen, T. E.; Ndalamba, P.; Cui, D.; Bruno, J.; Caceci, M.; Spahiu, K. Solubility of the redox-sensitive radionuclides 99Tc and 237Np under reducing conditions in neutral to alkaline solution. Effect of carbonate; SKB-TR 93-18; Swedish Nuclear Fuel and Waste Management Co.: Stockholm, 1993. (2) Grenthe, I.; Stumm, W.; Laaksuharju, M.; Nilsson, A.-C.; Wikberg, P. Chem. Geol. 1992, 98, 131-150. (3) Byegård, J.; Albinsson, Y.; Sharnemark, G.; Skålberg, M. Radiochim. Acta 1992, 58/59, 239-244 (4) Guppy, R. M.; Atkinson, A.; Valentine, T. M. Studies of the solubility of technetium under a range of redox conditions; Harwell ARE-R 13467; DOE/RW/89/102; 1989. (5) Lieser, K. H; Bauscher, C. H. Radiochim. Acta 1987, 42, 205-213 . (6) Hakanen, M.; Lindberg, A. Technetium, neptutium and uranium in simulated anaerobic groundwater conditions; Report YJT-9502; Nuclear Waste Commission of Finnish Power Companies: 1995. (7) Cui, D.; Eriksen, T. E. Environ. Sci. Technol. 1996, 30, 22592262. (8) Bonedetti, E. A.; Francis, C. W. Science 1979, 203, 1337-1340. (9) Allard, B. Actinide and technetium solubility limitations in groundwater of crystalline rock. In Scientific Basis for Nuclear Waste Management VII; McVay, G. L., Ed.; Elsevier: New York, 1984; pp 219-226. (10) Allard, B.; Kipatsi, H.; Torstenfelt, B. Radiochem. Radioanal. Lett. 1979, 37, 223-229.
(11) Eriksen, T. E.; Cui, D. On the interaction of granite with Tc(IV) and Tc(VII) in aqueous solution; SKB TR 91-47; Swedish Nuclear Fuel and Waste Management Co.: Stockholm, 1991. (12) Vandergraaf, T. T.; Ticknor, K. V.; George, I. M. In Geological Behaviour of Radioactive Waste; Barney, G. S., Ed.; American Chemical Society Symposium Series 246; American Chemical Society: Washington, DC, 1984; p 24. (13) Haines, R. I.; Owen, R. I.; Vandergraff, T. T. Nucl. J. Can. 1987, 1, 32-37. (14) Graff, P. R. Determination of FeO in Geological Materials. Nor. Geol. Unders. [Publ.] 1983, 388, 2-12. (15) Wikberg, P. The chemistry of deep groundwater in crystalline rock. Dissertation, The Royal Institute of Technology, Stockholm, 1987. (16) Eriksen, T. E. Radionuclide transport in a single fracture. A laboratory flow system for transport under reducing conditions; SKB TR 88-28, Swedish Nuclear Fuel and Waste Management Co.: Stockholm, 1988. (17) Regazzoni, A. E.; Blesa, M. A.; Maroto, A. J. G. J. Colloid Interface Sci. 1983, 91, 560-570. (18) White, A. F. Heterogeneous electrochemical reactions associated with oxidation of ferrous oxide and silicate surfaces. Hochella, M. F., White, A. F., Eds.; Reviews in Mineralogy Vol. 23; Mineralogical Society of America: Washington, DC, 1990; p 467. (19) Stumm, W.; Sulzberger, B. Geochim. Cosmochim. Acta 1992, 56, 3233-57.
Received for review August 22, 1995. Revised manuscript received December 21, 1995. Accepted February 26, 1996.X ES950627V X
Abstract published in Advance ACS Abstracts, May 1, 1996.
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