Technetium exists in valence states ranging from +7 to -1, but in natural environments, the most stable valence states are +7 and +4 under oxidizing and reducing conditions, respectively. Technetium forms a reduced species [predominantly Tc(IV)] at redox potential (Eh) values below about 220 mV with respect to standard hydrogen electrode (SHE) in neutral pH conditions (Figure 1) 9. At higher Eh, it occurs as Tc(VII)O₄^-. Due to its weak interaction with mineral surfaces, TcO₄^- is considered as one of the most mobile radionuclides in the environment. In contrast, lower-valence Tc(IV) species [such as TcO₂·nH₂O with n = 1–2; equivalent to TcO(OH)₂° in Figure 1 when n = 1] are expected to be strongly retarded due to their strong sorption and/or precipitation; the solubility of TcO₂·nH₂O(s) in carbonate-containing groundwater was reported to be about 10^-8 M 1011.

The presence of reductants in the host rock (e.g., Fe(II) in pyrite FeS₂) can contribute to the reduction of Tc(VII) to Tc(IV) 12. For example, reducing groundwater was observed in boreholes on or near Yucca Mountain, in the western part of the NTS, several of which are known to contain pyrite 13. Recent work of Hu et al. 14 indicated that the groundwater in the deep NTS aquifer exhibited variable redox conditions. Under reducing conditions, Cui and Eriksen 1516 reported that TcO₄^- was reduced to TcO₂·nH₂O(s) by Fe(II)-bearing fracture-filling minerals on which Tc(IV)_aq was rapidly sorbed. Reduction of Tc(VII) to Tc(IV) occurred with Fe(II)-containing solid phases but not by aqueous Fe(II) species 11171819. Peretyazhko et al. 19 recently demonstrated that mineral-associated Fe(II) can be an effective heterogeneous reductant of Tc(VII) under anoxic conditions, yielding insoluble Tc(IV) precipitates, coprecipitates, and/or surface complexes that may significantly retard Tc migration. Their experimental results suggested the following affinity series for heterogeneous Tc(VII) reduction by Fe(II): Fe(II) adsorbed on Fe(III) oxides >> structural Fe(II) in phyllosilicates >> adsorbed Fe(II) in phyllosilicates [ion-exchangeable and some edge-complexed Fe(II)] ~aqueous Fe(II).

Lieser and Bauscher 9 observed wide variations in ⁹⁹Tc distribution coefficients for sediment-water experiments performed under aerobic and anaerobic conditions. By varying the redox potential, they observed a change in the K_d by about 3 orders of magnitude within a small range of Eh at 190 ± 30 mV for a pH of 7 ± 0.5. Chemical equilibrium modeling using EQ3/6 software has also shown the enhanced retardation of ⁹⁹Tc under reducing conditions in the saturated zone at Yucca Mountain 20. To summarize, ⁹⁹Tc can behave as either a non-sorbing species (like chloride) or a strongly sorbing species (like Am) – simply because of a modest change in redox conditions. The assumption that ⁹⁹Tc will always behave as a mobile species oversimplifies its geochemical behavior and may overestimate its transport rates.

The fate and transport of ¹²⁹I in groundwater is also dictated by its chemical speciation. Aqueous iodine usually occurs as the highly mobile iodide anion (I^-). Under more oxidizing conditions, iodine will be present as the iodate anion (IO₃^-), which is more reactive than iodide and could be sorbed onto positively-charged sites locally existing in clays and organic matter 2122. Unlike other redox-sensitive radionuclides (such as ⁹⁹Tc), iodine sorption may decrease under reducing conditions when I^- is the predominant species [cf., 23]. However, coexistence of several iodine species (iodide, iodate, and organic iodine species) has been reported in various aqueous systems, which will tend to make the sorption behavior of iodine more difficult to predict.

A large volume of literature exists on the geochemical behavior of actinides in the environment; topical review papers include Kim 24, Dozol and Hagemann 25, Silva and Nitsche 26, and Kersting and Reimus 27. In general, the mobility of actinides in aqueous systems is dependent on (1) their thermodynamic properties, which determine solubility and speciation as a function of pH and redox potential, (2) the availability of inorganic and organic ligands to form soluble complexes, and (3) the composition and abundance of minerals and mineral colloids present in the system. The valence state of redox sensitive radionuclides (including U, Np and Pu) plays a major role in defining the geochemical reactions and migration behavior of these elements. Solubility-limited concentrations, complexation reactions, sorption onto minerals, and colloid formation all differ considerably as a function of oxidation state 28.

The chemistry of uranium in the environment is dominated by the difference in behavior of the U(IV) and U(VI) ions. The tetravalent form generally has low solubility while the hexavalent form is relatively soluble as the uranyl (UO₂²⁺) ion and its complexes 29. As shown in Figure 2, the uranyl ion commonly forms soluble complexes with carbonate ligands at pH values typical of NTS groundwaters 3031. Even at relatively low oxidation potentials, uranyl species may dominate aqueous uranium speciation, although uraninite (UO₂) is the stable solid phase. Uranium is more readily sorbed onto minerals or organic matter when present as the positively charged uranyl species, and this step may precede reduction to less soluble U(IV) solids 32333435. However, the strong affinity of carbonate ligands for uranyl [e.g., 36] in solution effectively competes with sorption, thereby limiting the sorption of uranyl carbonate complexes 28.

An important feature of neptunium chemistry in aqueous systems is the large stability range for Np(V) 37. The pentavalent NpO₂⁺ species is dominant at pH values < 8 whereas Np(V) carbonate complexes tend to dominate at higher pH values 28 (Figure 3). Since Np(V) solid phases are relatively soluble and Np(V) aqueous species sorb weakly onto common minerals, Np(V) is relatively mobile in the environment. Under reducing conditions, Np(IV) is present as the low solubility Np(OH)₄ (aq) species at pH values > 5 28. Np(IV) shows a strong tendency for sorption to mineral surfaces 3738, which limits its mobility in aqueous systems.

The redox speciation of plutonium is affected by a number of competing variables, and Pu is observed to coexist in multiple valence states in natural waters 39 (Figure 4; note that the stability fields are shown only for the aqueous species). Pu(III) and Pu(IV) tend to be less stable than Pu(V) and Pu(VI) under oxidizing, near-neutral pH conditions, though Pu(IV) exhibits the strongest tendency to form ligand complexes 40. The aqueous chemistry of plutonium is further complicated by the fact that Pu(IV) disproportionates to Pu(III) and Pu(VI), Pu(V) disproportionates to Pu(IV) and Pu(VI), and Pu(VI) is easily reduced 4142. Kersting and Reimus 27 showed that Pu(V) reduction to Pu(IV) is an important mechanism for Pu sorption to mineral surfaces.

Figure 5 summarizes the valence states for several radionuclides as a function of redox potentials, and the figure also includes the expected equilibrium redox potentials associated with the common electron-acceptor couples encountered in groundwater 43. In the laboratory experiments conducted during this study, the redox conditions were controlled by modifying the oxygen concentration in air, and by spiking the solutions with Fe²⁺ and S^2-.

The extent of radionuclide sorption is dependent on the physical and chemical properties of the radionuclide-aquifer-groundwater system. The lithologic (aquifer) materials used in this study included alluvium, welded/devitrified tuff, zeolitic tuff, and carbonate rock. All lithologic materials were crushed and sieved to a 75–500 μm size range 5. The alluvium was collected from an exposed section of the U-1a.102 drift in the U-1a tunnel complex (at a depth of 295 m below the land surface) beneath Yucca Flat. The devitrified tuff sample was a Topopah Spring welded volcanic tuff collected from the Yucca Mountain tunnel (~300 m below ground surface). The original glassy matrix in this material has altered to fine-grained crystalline solids that include feldspar, quartz, cristobalite, and some smectite 44. The zeolitic tuff sample was from drill core UE-7az (496 m below ground surface) within the tuff confining unit (TCU) of Yucca Flat. Zeolitic tuff is formed by the alteration of glassy vitric tuff near and below the water table 45, and is typically comprised of clinoptilolite (> 60%), mordenite, opal, feldspar, and quartz. The carbonate rock was from drill core ER-6-1 (833 m below ground surface) within the lower carbonate aquifer (LCA) at Yucca Flat, and consists of massive dolomite with calcite veinlets.

A companion synthetic groundwater was used for each solid matrix. Synthetic LCA (Ca-Mg-HCO₃) and TCU (Na-K-HCO₃) waters were used in carbonate rock and zeolitic tuff sorption experiments, respectively. The solution compositions were based on measured concentrations of major ions in groundwaters sampled from the respective hydrostratigraphic units, except that sulfate, nitrate, and fluoride were omitted from the background solution because of their minimal role in radionuclide retardation processes. Synthetic J-13 water (Na-K-HCO₃ type) was used for the sorption of radionuclides onto alluvium and devitrified tuff. According to the thermodynamic database "thermo" in the Geochemist's Workbench, a solution composed of J-13 water should be supersaturated with respect to a number of phases, including calcite 46. To avoid potential mineral precipitation issues, synthetic J-13 water was prepared using a recipe from Viani 46 that is predicted to be stable at 25°C and atmospheric pCO₂. Measured concentrations of constituents in the prepared synthetic background waters are presented in Table 2.

The radionuclides and surrogate species used in the batch-sorption experiments were chosen because they represent a significant fraction of the radiologic source term at the NTS, and are expected to show a wide range of radionuclide retardation behavior. Synthetic groundwaters were spiked with radionuclides and surrogate species at the appropriate amount to achieve the target concentration (Table 3). The target concentration was based on a combination of instrument detection limits, expected background concentrations, and solubility limits. Tables 3A, 3B, and 3C summarize the measured radionuclide compositions of spiked blank solutions. The radionuclide stock solutions (⁹⁹Tc, ²³⁷Np, and Pu) were stored in a HNO₃ matrix. When spiking samples, a small amount of NH₄OH was added to minimize pH changes to solutions.

U(VI) was assumed to be the dominant U species in the stock solution based on its stability under oxidizing conditions. The Pu stock solution was purified using a TEVA column. The oxidation state of the Pu stock solution was characterized using both solvent extraction with PMBP (4-benzoyl-3-methyl-1-phenyl-2-pyrozolln-5-one) as described in 47484950 and Pu co-precipitation by lanthanum fluoride 51. The results indicated that the Pu stock solution consisted of 80% Pu(IV), 15% Pu(III) and 5% colloidal Pu. The ²³⁷Np stock was purified in concentrated HCl with KI solid using an AG1 × 8 (100–200 mesh) resin column. The Np was eluted from the column using 0.1 M HNO₃. The Np solution was dried in HNO₃ on a hotplate before re-dissolving in 1 M HCl to make a Np(V) solution. Np(V) oxidation state of the stock solution was confirmed by UV/VIS spectrometry.

Chemical forms used in the initial solution were Sr²⁺, Cs⁺, I^-, ⁹⁹TcO₄^-, and ReO₄^-. Valence states for the actinides were U(VI), ²³⁷Np(V), and Pu(IV); the predominant chemical forms are expected to be carbonate and/or hydroxide complexes for U, Np, and Pu. These are the valence states and chemical forms expected to occur in the oxidizing groundwaters at the NTS. Cs and Sr are not subject to changes in oxidation state and were included for comparison. ReO₄^- is sometimes used as the surrogate for TcO₄^- based on their similar crystal chemistry, electronic configuration, and thermodynamic data 525354. However, there is a difference between Re and Tc in their respective oxidation potentials. A comparison of the E_h-pH diagrams for Re and Tc shows that ReO₄^- is in equilibrium with ReO₃ and Re₂O₃ over a wide range of conditions. In contrast, there are no equilibrium fields for TcO₃ or Tc₂O₃ 55. Furthermore, the reduction of Re from +7 to +4 is more difficult than for Tc 555657.

Radionuclides Pu and ²³⁷Np used in the batch sorption solutions were from existing stocks at Lawrence Livermore National Laboratory. Standard Reference Materials, SRM 4288A for ⁹⁹Tc, SRM 4341 for ²³⁷Np, and SRM 4334H for ²⁴²Pu, obtained from the National Institute of Standards and Technology, were used to prepare the standard solution to calibrate the ICP-MS instrument for measuring the radionuclide concentrations in the batch-sorption samples.

The first batch test (Test A in Table 3) was carried out under atmospheric conditions. The test was conducted in accordance with ASTM method D4646 58, except that a solution to solid ratio of 5:1 (one gram of air-dry solid to 5 mL of sorption solution) was employed instead of the 20:1 ratio specified in the ASTM method. All sorption treatments (solid and sorption solution) were run in triplicate. Blank (solid and background solution) and control (no solid; only sorption solution) treatments were run in duplicate.

A 0.7 m³ glove box was used to modify and control the atmospheric composition for the other batch sorption experiments (Tests B-E). Either high purity Ar or a mixture of 99% Ar and 1% CO₂ was used to control the atmosphere composition inside the glove box. Tests B and C were conducted under a gas composition of about 1 and 0.1% O₂, controlled by maintaining the flow rate (pressure) of the pure Ar gas. Tests D and E were all conducted at 0.1% O₂ level, with test tubes further spiked with a reductant (FeCl₂ for Test D or Na₂S for Test E) (Table 3). Two gas sensors were placed inside the glove box to monitor air composition. The oxygen sensor (Pro O₂ analyzer, Nuvair Gas Analyzers, Oxnard, CA) has a detection limit of 0.1% O₂. A CO₂/temperature sensor (Model 7001, Telaire, Goleta, CA) was used to monitor the concomitant decrease of CO₂ with O₂ and to confirm the decreased O₂ level inside the glove box. The CO₂ sensor (working range of ~400 to 1 ppm) is more sensitive than the O₂ sensor (working range of 21 to 0.1%). The experimental temperature was 22.9 ± 0.5°C.

During the sorption experiments conducted inside the glove box, 15-mL sized centrifuge tubes were uncovered to permit gas exchange and placed on a shaker (Maxi-Mix III type 65800, Thermolyne, Dubuque, IA) at a shaker speed of 1,000 rpm. Batch tests were started when the O₂ level inside the glove box reached steady-state levels, usually after flushing for 2–4 hours. Then after 48 hours, batch sorption tests were stopped and the sample was filtered with a 0.2 μm PTFE membrane syringe filter (Acrodisc CR 13 mm, Pall Life Sciences, East Hills, NY). The sorption kinetics of radionuclides and the limited mixing during sorption experiments was not evaluated, as this study was intended to compare the effects of redox conditions on sorption under otherwise similar experimental conditions. Furthermore, redox reactions can be kinetically limited, requiring timescales on the order of weeks to achieve steady state 59. Consequently, observations at 48 hours may not capture the full extent of the redox manipulation. Comparative effects between samples at 48 hours address the initial response of radionuclides to abrupt redox changes.

An aliquot of filtrate (0.5 mL) was pipetted into 3.5 mL 2% HNO₃ solution for subsequent ICP-MS (Thermo Electron Model X7, Thermo Fisher Scientific, Inc, Waltham, MA) analysis of all elements of interest; 5 ng/L of ⁶Li, ⁴⁵Sc, ¹¹⁵In, and ²⁰⁹Bi were included as internal standards. The redox potential, relative to the standard hydrogen electrode, was measured using platinum as a sensing electrode and silver-silver chloride as a reference electrode (Thermo Orion Eh combination electrode, model 9678BN, Thermo Fisher Scientific, Inc, Waltham, MA) filled with saturated potassium chloride. Standard Zobell's solution (a solution of potassium ferric-ferrocyanide of known Eh) was used to verify the working operation of the measurement system 60. Measurement of pH for the filtrate was conducted using an Oakton meter (Eutech Instruments, Singapore) and glass pH electrode system (Orion Ross combination pH electrode, model 81-02) calibrated to three pH buffer standards (pH 4, 7, and 10). Dissolved oxygen in the filtrate was evaluated with a SympHony SB50D (Thermo Orion) with a detection limit of 0.1 mg/L. All these measurements, except for ICP-MS analyses, were performed inside the glove box under the desired O₂ level.

The principal redox couples in the subsurface are O₂/H₂O, NO₃^-/N₂, Mn(IV, III)/Mn(II), Fe(III)/Fe(II), SO₄^2-/H₂S, and CO₂/CH₄, which form a redox ladder from the most oxidizing O₂/H₂O to the reducing CO₂/CH₄couples. The theoretical (thermodynamically calculated) Eh values at pH 7 are 816, -182, and -215 mV for the O₂/H₂O, Fe(III)/Fe(II), and SO₄^2-/H₂S redox couples employed in this study. However, various authors suggest caution in using Eh to quantify redox condition 6162. The difficulty in interpreting redox from Eh measurements results from using an equilibrium approach to describe a highly dynamic system 63. Eh is a simple measure, but it gives at best only qualitative assessment of water redox conditions because the Pt electrode may not respond to many important redox couples 64. Thus, a wide range of Eh has been observed for the same redox couple and, as a result, several redox reactions may be relevant within the same Eh range 64. The measured Eh usually reflects "non-equilibrium" potentials and can only be qualitatively interpreted. Supplementary measurements of redox couple concentrations should be made in addition to Eh measurements to better evaluate redox conditions.

In this work, Eh values measured with a platinum electrode did not appear to reflect the Eh difference between the SO₄^2-/H₂S and Fe(III)/Fe(II) redox couples. In addition to the Eh measurements, sulfate production in the sulfate/sulfide redox couple was monitored. In Test E where 24 mg/L (0.3 mM) Na₂S was spiked into the sorption solution, about 2 mg/L sulfate (SO₄^2-) was produced. According to the reaction:

S^2- + 4H₂O = SO₄^2- + 8H⁺ + 8 e^-

this amount was equivalent to 0.67 mg/L sulfide (S^2-) being oxidized and suggested that sufficient sulfide remained in solution to influence redox conditions.