Journal of Nuclear Materials 429 (2012) 201–209

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Journal of Nuclear Materials

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Iron oxide waste form for stabilizing 99Tc

Wooyong Um a,⇑, Hyunshik Chang a,1, Jonathan P. Icenhower b, Wayne W. Lukens b, R. Jeffrey Serne a,Nik Qafoku a, Ravi K. Kukkadapu a, Joseph H. Westsik Jr. a

a Pacific Northwest National Laboratory, USAb Lawrence Berkeley National Laboratory, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 23 February 2012Accepted 1 June 2012Available online 3 May 2012

0022-3115/$ - see front matter Published by Elsevierhttp://dx.doi.org/10.1016/j.jnucmat.2012.06.004

⇑ Corresponding author. Address: Pacific Northwest999, P7-54, 902, Battelle Boulevard, Richland, WA 96227; fax: +1 509 371 6919.

E-mail address: wooyong.um@pnnl.gov (W. Um).1 Present address: Savannah River Ecology Laborat

29802, USA.

Crystals of goethite were synthesized with reduced technetium [99Tc(IV)] incorporated within the solidlattice. The presence of 99Tc(IV) as a substituting cation in the matrix and ‘‘armoring’’ by an additionallayer of precipitated goethite isolated the reduced 99Tc(IV) from oxidizing agents. These products wereused to make monolithic pellets to quantify an effective diffusion coefficient for 99Tc from goethite wasteform contacted with a synthetic Hanford IDF (Integrated Disposal Facility) pore water solution (pH = 7.2and I = 0.05 M) at room temperature for up to 120 days in static reactors. XANES analysis of the goethitesolids recovered post-run demonstrated that the 99Tc in the goethite crystals remains in the reduced99Tc(IV) state. The slow release of pertechnetate concentration with time in the static experiments withthe monolith followed a square root of time dependence, consistent with diffusion control for 99Tc release.An apparent diffusion coefficient of 6.15 � 10�11 cm2/s was calculated for the 99Tc–goethite pellet sampleand the corresponding leaching index (LI) was 10.2. The results of this study indicate that technetium canbe immobilized in a stable, low-cost Fe oxide matrix that is easy to fabricate and these findings can be use-ful in designing long-term solutions for nuclear waste disposal.

Published by Elsevier B.V.

1. Introduction

Neutron-induced fission of 235U-enriched nuclear fuel yieldstechnetium isotopes in relatively large amounts, the most impor-tant of which is technetium-99 (99Tc) [1–3]. Approximately 1 kgof 99Tc with a long half-life (2.13 � 105 years) is produced for everyton of nuclear fuel ‘‘burned’’ in a typical reactor [4]. The radiologicaland chemical properties of 99Tc present some unique problems fornuclear waste disposal. In virtually all near surface conditions,99Tc exists in the highly soluble and mobile pertechnetate form½TcðVIIÞO�4 � [5,6]. Environmental concerns have been raised becauseof the long half-life and high mobility of 99Tc in oxidizing subsurfaceenvironments [7]. The highly soluble pertechnetate oxyanion,99TcðVIIÞO�4 , does not sorb onto most terrestrial sediments [8], so99TcO�4 migrates at nearly the same velocity as groundwater [9]under common subsurface conditions (i.e., pH close to neutral orslightly alkaline and suboxic conditions). In the natural environ-ment, there may be cases in which pertechnetate may react inlocally reducing conditions, caused by microbial activity or withreduced inorganic metal phases, primarily magnetite and sulfides,

B.V.

National Laboratory, PO Box9354, USA. Tel.: +1 509 372

ory, P.O. Drawer E, Aiken, SC

that will induce reduction and retard the mobility of 99Tc.Under reducing conditions, pertechnetate can precipitate as99Tc(IV)O2�2H2O [10,11], sorb to mineral phases [7], and be retainedin different natural environments [12–14]. However, even whenreduced to more insoluble forms, the reoxidation of 99Tc(IV) bychanging redox conditions, such as contact with oxygen, can resultin release of pertechnetate back into the environment [14], leadingto the prediction of high 99Tc release rates in many performanceassessments [15–17]. For example, Lee and Bondietti [10] foundthat oxidation of an FeS/Tc(IV) precipitate allowed 70% of the 99Tcto return to solution over a period of 8 months, which could bedue to reoxidation back to 99TcO�4 , or to solubilization of 99Tc(IV)species at the low pH associated with oxidation of the sulfide [18].Further, the long half-life of 99Tc provides ample opportunity for99Tc(IV) to reoxidize and frustrates efforts to fashion a long-termstable immobilization matrix.

To prevent 99Tc release into the environment, plans are beingdrawn to sequester technetium into a stable waste form that willbe inert to dissolution and oxidation reactions. Numerous wasteforms have been proposed, including cement/grout [16,19], glass[20], hydroceramics [21,22], phosphate-bonded ceramics [21,23],and metal alloys [24]. Evaluating the efficacy of these proposedwaste forms is difficult, because data that bear on the long-termimmobilization of 99Tc are sparse. Short-term laboratory testshave, however, underscored a potential pitfall for many of thesematerials: 99Tc reduced to the more immobile 99Tc(IV) form is vul-nerable to the infiltration of oxidizing agents, especially oxygen.

202 W. Um et al. / Journal of Nuclear Materials 429 (2012) 201–209

Upon contact with oxygen, reduced 99Tc(IV) has been found toreoxidize rapidly to pertechnetate [16,18]. Only the Savannah RiverSite ‘‘saltstone’’ [17] has been shown to hinder reoxidation of99Tc(IV), but mainly because of slow diffusion of oxygen into theimmense size of waste form slab, and not because of any inherentreoxidation resistant qualities of the material. Thus, formulating awaste form that isolates 99Tc(IV) from contact with O2 is key to thesuccess of any proposed immobilization strategy.

Several previous studies have shown that when 99Tc(IV) issubstituted into a mineral lattice, technetium is effective sealedfrom oxygen. For example, investigations into the interaction ofpertechnetate with Fe (oxy) hydroxide and sulfide minerals haveshown that once 99Tc(VII) is reduced, the association of 99Tc(IV)with iron is strong and limited reoxidation of 99Tc(IV) is found be-cause direct substitution of 99Tc(IV) for Fe in the Fe oxide structureis possible during precipitation and crystal growth reactions [25–28]. With these studies as a guide, we recently reported successfulexperiments in which pertechnetate was reduced through interac-tion of Fe2+ with the oxy-hydroxide phase, goethite [a-FeO(III)OH].We demonstrated that 99Tc(IV) was incorporated within the goe-thite mineral lattice during mineral growth and following addi-tional goethite overgrowth or ‘‘armoring’’ limited reoxidation of99Tc(IV), despite exposure of 99Tc(IV)-bearing goethite was to solu-tions in ambient oxidizing conditions [28].

There are additional reasons why goethite-based waste formsshould be considered for immobilizing technetium. As an end-stage weathering product in temperate environments, goethite isan extremely stable mineral phase at or near the Earth’s surface[29]. Previous studies have shown that hydrogen and oxygenatoms in the goethite structure are resistant to exchange withthe environment for up to 50 Myr [30], attesting to the thermody-namic and kinetic stability of goethite. In addition, goethite can bereadily synthesized in the laboratory with particle sizes and sur-face areas that can be manipulated readily [31], processes thatcan be easily and cheaply up-scaled to industrial levels. Further-more, iron in goethite is in the stable trivalent (III) state so thatin the oxidizing environments that will likely prevail in the dis-posal repository setting, there will be minimal chemical potentialfor redox reactions that may affect greater mobility of 99Tc(IV).Therefore, a waste form based on goethite appears to have the po-tential to be a simple, durable, cost-efficient, and effective candi-date for disposing of 99Tc from radioactive wastes.

Despite the arguments marshaled above and from our previousstudy on a goethite-based waste form, we still lack quantitative dataon the release of technetium to the environment. Our previous workdemonstrated that the release of 99Tc to the environment due to dis-solution of goethite is very slow under a range of geochemical con-ditions, and we postulated that the mechanism of release istransport limited in circum-neutral solutions. Diffusion of 99Tc(VII)through the goethite lattice must be preceded by the incursion ofoxygen. Isolation of 99Tc(IV) from oxidizing agents appears to bekey to its immobilization, as has been argued by previous investiga-tors [32]. Accordingly, the experiments described herein investi-gated 99Tc diffusion coefficient from the 99Tc–goethite waste formin batch experiments, and evaluated mineralogical changes andreoxidation of 99Tc(IV) using macroscopic and spectroscopicanalyses.

2. Material and methods

2.1. Synthesis of iron oxides

Goethite was synthesized based on a scaled-down procedure ofSchwertmann and Cornell [31]. To summarize, ferric nitrate [11.4 gof Fe(NO3)3�9H2O] was dissolved into NANOpure� water (100 mL)

and reacted with 2-M sodium hydroxide (NaOH) (150 mL). Theslurry was heated in an oven (80 �C) for 7 days. The solid productwas filtered from solution by vacuum filtration, washed with freshNANOpure� water two times, and air dried overnight. The air-driedsolid was then gently crushed to a powder form. In addition togoethite, two-line ferrihydrite (Fe5HO8�4H2O) was also synthesizedusing a modified method of Schwertmann and Cornell [31]. Thesynthesis method for ferrihydrite was similar to that of goethitewithout the heating step. Briefly, ferric nitrate [8.0 g ofFe(NO3)3�9H2O] was dissolved into NANOpure� water (100 mL) ina polyethylene bottle (250 mL); 1 M NaOH was added dropwisewhile stirring the slurry, until a pH of 6 was obtained. To obtain apH of 7–8, additional low-concentration NaOH (i.e., 0.01 M) wasadded while continuously stirring. The final solid product wasfiltered from solution by vacuum filtration, washed with freshNANOpure� water four times, and air dried overnight before use.

2.2. Technetium removal by Fe(II)-iron oxides

Batch experiments to determine the extent of Fe(II) adsorptiononto either goethite or ferrihydrite and the subsequent ability ofthe iron oxides to sequester 99Tc were conducted at different pHlevels. These experiments were conducted to study the effect ofpH on dissolved Fe(II) concentration and subsequent 99Tc(VII) re-moval by the Fe(II)–goethite or –ferrihydrite solids. Some of thepH variation tests were performed with only Fe(II) in solution(i.e., no goethite or ferrihydrite solid present) to determine empir-ically the solubility of Fe(II/III) oxyhydroxides. Solubility diagramsfor Fe(OH)3(s) and Fe(OH)2(s) were also constructed based on thestability constants of Fe(II)/Fe(III) hydrolysis species [33] and com-pared with measured Fe(II) concentrations in the effluent solutionsfrom the various batch tests. At each sub-sampling step, a smallaliquot of filtered solution was used to measure the pH and theconcentrations of 99Tc(VII), Fe(II) and total Fe. Concentrations of99Tc(VII) and total Fe in the supernatants were determined usingICP-MS and ICP-OES, respectively. The dissolved ferrous Fe(II) con-centration was determined using the ferrozine colorimetric meth-od [34] with a HACH DR/890 colorimeter.

Details of the synthesis procedures and the strategy for incorpo-rating reduced 99Tc(IV) into goethite can be found in the previouspaper (similar to Sample 2–5 preparation) [28]. Even though ourprevious study worked for waste form development using bothoff-gas secondary waste stream [28] and simple de-ionized (DI)water conditions, this study focused more for 99Tc diffusive leach-ability from Tc–goethite waste form prepared in simple DI solution.Succinctly, synthesized goethite powder (2.75 g) was re-suspendedin de-aerated and DI water (250 mL) and the pH was lowered to62.0 by adding 2 M nitric acid. Powdered FeCl2�4H2O (3.48 g) wasdirectly added to the goethite slurry as the Fe(II) source and reactedfor 1 day in an anoxic chamber (Coy Laboratory Products) equippedwith a H2/O2 gas analyzer and palladium-coated alumina catalyst. Amixture of N2 (97%) and H2 (3%) was used as the anaerobic gas in thechamber. Following this, 0.25 mL of 99Tc(VII) from a NaTcO4 stan-dard solution (2.2 � 10�2 M) was prepared in Pacific Northwest Na-tional Laboratory (PNNL) using NaTcO4 salt and added to make atotal 2.2 � 10�5 M of 99Tc in the Fe(II)–goethite slurry (250 mL)and homogenized for 1–2 days on a platform shaker. After mixing,sodium hydroxide (150 mL of 2 M NaOH) was added to facilitate99Tc–goethite precipitates. The 99Tc–goethite solids were furthermodified to armor the 99Tc–goethite solids with additional goethiteprecipitates using separately prepared Fe(NO3)3�9H2O (11.4 g/100 mL) solution. After 1–2 days of reaction with added ferric ni-trate and sodium hydroxide solutions, the bottle containing the finalslurry was removed from the anaerobic chamber and placed insidean oven at 80 �C for 7 days. The final 99Tc–goethite solid products

W. Um et al. / Journal of Nuclear Materials 429 (2012) 201–209 203

were separated by filtration, dried in air, and used for additionalanalysis.

2.3. Solid phase characterization

The initial goethite and ferrihydrite substrates and the final so-lid product before and after diffusion test were first characterizedusing a Scintag XRD unit equipped with a Cu K-alpha radiation(40 kV, 35 mA) source. The bulk 99Tc–goethite samples werehomogenized by grinding in an agate mortar and pestle, andmounted into a small circular sample holder before scanning from2 to 75 degrees 2-theta. Data reduction and phase identificationwere done by JADE software with PDF X-ray diffraction (XRD) data-base. Acid extraction, using 8 M HNO3 with heat at 90 �C was usedto determine the total 99Tc in the final 99Tc–goethite solid.

Transmission electron microscopy (TEM) samples were pre-pared by dispersing a small amount of the 99Tc-laden Fe(II)–goe-thite slurry in methanol and depositing this onto a lacey carbonTEM copper (Cu) grid. TEM characterization was performed usinga FEI Tecnai T30 operated at 300 keV equipped with a Gatan ORIUSdigital camera. Analysis was also performed by identifying themineralogy with selected area electron diffraction (SAED).

The Mössbauer sample was prepared by mixing a dried 99Tc–goethite sample with Vaseline in a Cu holder sealed at one end withclear tape. After mixing the sample in the holder, the opened end ofthe holder was sealed with clear tape. The Mössbauer spectra werecollected at room temperature (RT; data not shown) and liquidnitrogen (77 K) using a 50-mCi (initial strength) 57Co/Rh single-linethin source. The velocity transducer, MVT-1000 (WissEL) was oper-ated in a constant acceleration mode (23 Hz, 5± or ±12 mm/s). AnAr–Kr proportional counter was used to detect the radiation trans-mitted through the holder, and the counts were stored in a multi-channel scalar as a function of energy (transducer velocity) usinga 1024-channel analyzer. Data were folded to 512 channels to givea flat background and a zero-velocity position corresponding to thecenter shift (CSd) of a metal Fe foil at room temperature (RT). Cali-bration spectra were obtained with a 25-lm-thick metal Fe foil(Amersham, England) placed in the same position as the samplesto minimize any errors due to changes in geometry. A closed-cyclecryostat (ARS, Allentown, Pennsylvania) was used for 77 K measure-ment. The Mössbauer data were modeled with the Recoil software(University of Ottawa, Canada) using a Voigt based structural fittingroutine. The coefficient of variation of the spectral areas of the indi-vidual sites generally ranged between 1% and 2% of the fitted values.

Table 1Chemical composition of the solution and the list of ingredients used to prepare thesimulated IDF pore water from the Hanford 200-East Area.

Chemicals Concentration (M)

CaSO4 1.2 � 10�2

NaNO3 3.4 � 10�3

NaHCO3 3.0 � 10�4

NaCl 2.1 � 10�3

MgSO4 2.6 � 10�3

MgCl2 2.4 � 10�3

KCl 7.0 � 10�4

Ionic strength 0.05 MpH 7.2Alkalinity 29 mg/L

2.4. X-ray Absorption Near Edge Structure (XANES) spectroscopy

Solid standards of KTcO4, NaTcO4, TcO�4 adsorbed on Reillex-HPQ resin and TcO2�2H2O, and final 99Tc–goethite sample beforeand after diffusion tests were analyzed to determine the 99Tc oxi-dation state change. The XANES spectra were collected on beam-line 4-1 at the Stanford Synchrotron Radiation Laboratory (SSRL).The solid samples were mounted on Teflon sample holders andsealed with Kapton tape. A Si(220) double-flat crystal monochro-mator was used and the energy was calibrated by using the firstinflection point of the 99Tc K edge spectrum of the 99Tc(VII) stan-dard, defined as 21.044 keV. The 99Tc-standards and 99Tc–goethitespectra were collected in transmission and fluorescence mode,respectively, at room temperature using a 13-element germaniumdetector.

Data reduction and analysis were performed using the softwareIFEFFIT [35] and ATHENA/ARTEMIS [36] after correction for detec-tor dead-time. The XANES spectra for the 99Tc–goethite sampleswere fit using a linear combination of the XANES spectra of 99Tc(IV)and 99Tc(VII) standards collected by Lukens et al. [11].

2.5. Effective diffusion coefficient measurement for 99Tc

The ground-powder 99Tc–goethite sample was compacted intocylindrical monolithic pellets by mixing with dissolved polyethyl-ene glycol organic binder at about 3 wt%, and was prepared for dif-fusion leach testing. The 99Tc–goethite–organic binder slurry wasdried overnight in a heated bath at 80 �C and then pressed withup to 2500 kg of load force in a 1.3-cm-diameter die with a CarverPress, giving a resultant pressure of �150 MPa. A minor amount ofoleic acid was used on the final monolithic pellet surfaces to aid inpreserving the pellet integrity. The resultant monolithic pelletscontaining the 99Tc goethite solids were used to determine theeffective diffusion coefficient for 99Tc leached from the monolithimmersed in Integrated Disposal Facility (IDF) pore water solution(500 mL) in the Hanford Site 200 East Area at room temperature[37]. The IDF pore water solution is germane to the expected aque-ous solution compositions coursing through the solidified radioac-tive wastes disposal environment. The chemicals used to preparethe IDF pore water leaching solution are listed in Table 1. Photo-graphs of the monolithic pellets with 99Tc–goethite with a diame-ter of 1.3 cm and a height of 0.32 cm before and after the diffusionleach testing are shown in Fig. 1.

For the monolith diffusion tests, a subsample (1 mL) of leachatewas periodically collected from 30 min after the test commencedto 120 days through the leaching period using a 0.45-lm Nalgenesyringe filter and submitted for analyses of dissolved Fe(tot) and99Tc. The pH was directly measured in the slurry solution wheneach of the leachate samples was collected. Analysis methods forFe(tot) and 99Tc were the same as described above. After the diffu-sion tests were completed, the monolithic 99Tc–goethite samplewas collected and prepared for characterization of the solids.

Because goethite has a low solubility at neutral pH values sim-ilar to that of the IDF pore water (see the leaching results below),the contaminant-specific, effective diffusion coefficient is com-monly used to quantify the migration rate of 99Tc out of the mono-lithic waste form. The observed effective diffusivity for 99Tc out ofthe monolithic waste form was calculated using the analyticalsolution, Eq. (1), for simple radial diffusion from a cylindricalmonolithic 99Tc–goethite pellet as presented by Crank [38]:

Di ¼ pMti

2qCoðffiffiffiffitip�

ffiffiffiffiffiffiffiffiti�1p

Þ

� �2

ð1Þ

where Di is the observed diffusivity of 99Tc for the leaching interval,i [m2/s]; Mti

the Tc mass released during the leaching interval i [mg/m2]; ti the cumulative contact time after the leaching interval, i [s];ti�1 the cumulative contact time after the leaching interval, i � 1[s]; Co the initial leachable 99Tc content [mg/Kg] in a monolithic pel-let; and q is the pellet density [Kg-dry/m3]. The mean observed 99Tcdiffusivity was determined by taking the average of each of theinterval-observed diffusivities.

Fig. 1. Cylindrical monolithic pellets of 99Tc–goethite samples. (Left) pellet sample before diffusion leach testing by IDF pore water; (right) pellet sample after diffusion leachtesting for 120 days by IDF pore water. The difference in color is from the wetted pellet after leaching. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

204 W. Um et al. / Journal of Nuclear Materials 429 (2012) 201–209

The leachability index (LI), which is a parameter derived di-rectly from diffusion test results, is also used to evaluate diffu-sion-controlled 99Tc release with respect to time. The LI of 99Tc iscalculated with:

LIn ¼ �logDn

cm2s

� �ð2Þ

where LI is the leachability index and Dn is the effective diffusivityfor 99Tc (cm2/s) during the leach interval n. Because the LI is thenegative logarithm of the measured effective diffusivity of 99Tc, alarger LI value indicates that less 99Tc has leached during the diffu-sion test.

3. Results and discussion

3.1. 99Tc(VII) and Fe(II) removal in solutions containing no Fe oxidesolid

Changes in dissolved Fe(II) and 99Tc concentrations in the finalsupernatant solution were investigated as a function of pH withoutthe presence of Fe-oxide minerals. Dissolved Fe(II) and Fe(tot) con-centrations in solution without 99Tc(VII) addition were similar toeach other and stable (remained at the initial concentrations)when pH was below 4.5, thereby indicating that most of the Fein solution existed as ferrous iron, Fe(II) (Fig. 2, left). However,the dissolved Fe(II) concentration decreased with increasing pH(>5.0) and approached zero in solutions at pH higher than 8.0. Total

Fig. 2. Changes in Fe or 99Tc concentrations in experiments over pH. (Left) Fe(II) andsolution without the presence of Fe(III) oxide solids.

dissolved Fe concentrations also showed the same decreasingtrend with increasing pH and no detectable Fe(II) and Fe(tot) con-centrations were found in solution containing no 99Tc(VII) at pHhigher than 8.0. There were minor differences between total Feand Fe(II) concentrations in solutions containing no 99Tc(VII) whenthe pH was below 4.5, suggesting that minor amounts of oxidizedFe(III) could be present even in these more acidic solutions. All theexperiments were conducted inside an anaerobic chamber thatwas monitored for oxygen concentrations. Despite these precau-tions, small amounts of oxygen could be present in the chamberduring the experiments; thus, one cannot eliminate the possibilitythat small amounts of Fe(II) were oxidized to Fe(III). Nearly com-plete removal (�100%) of dissolved Fe(II) at pH values higher than7.0 was attributed to precipitation of Fe(II) hydroxide as Fe(OH)2(s)(white rust) and magnetite. Independently prepared Fe(II) precipi-tate at pH = 7.5 from a similar solution composition showed anidentical XRD pattern to crystalline Fe(OH)2(s) [28].

Dissolved Fe(II) concentrations were slightly different in the ab-sence or presence of 99Tc(VII) in solution. Lower concentrations ofdissolved Fe(II) than Fe(tot) were found even at low pH values (3.2and 4.1) when 99Tc(VII) was present (Fig. 2, left), in comparison tothe same acidic solutions containing Fe(II) only below pH 5.0.When 99Tc(VII) was not present in solution, dissolved Fe(II) con-centrations at these low pH levels were stable and similar to boththe initial Fe(II) and total Fe concentrations. Because no 99Tc(VII)had been added to the Fe(II)-only solutions, no significant redoxreactions were expected to occur in which Fe(II) would be oxidizedto Fe(III) (Fig. 2, left). As mentioned, there could have been some

Fe(total) for experiments without Fe(III) oxide solids; (right) 99Tc in Fe(II)-bearing

-10

-9

-8

-7

-6-5

-4

-3

-2

-1

0

1 2 3 4 5 6 7 8 9 10 11 12

pH

Log

Fe(to

tal)

Con

cent

ratio

n (M

)

Fe(II)-sol only

Fe(II)-sol+Tc

Fe(II)-sol+G

Fe(II)-sol+F

Initial Fe(II)

FeOH3(s)

FeOH2(s)

Fig. 3. Solubility curves for Fe(OH)3(s) and Fe(OH)2(s) along with measured Fe(II)solution concentrations at different pHs and with different Fe oxides such asgoethite (G) and ferrihydrite (F). The brown line is the calculated solutionconcentration of Fe(total) controlled by the solubility of Fe(OH)3(s) and the blueline is the Fe(total) controlled by Fe(OH)2(s).

W. Um et al. / Journal of Nuclear Materials 429 (2012) 201–209 205

minor oxidation of Fe(II) from oxygen absorption in the solution,even if no 99Tc(VII) was added. However, when 99Tc(VII) and oxy-gen contamination were present together in solution, there was astrong driver for oxidation of Fe(II) as shown by the measurabledifference between measured Fe(II) and Fe(tot) concentrationseven at low pH (<5.0). Because the Fe(tot) concentration inFe(II) + Tc(VII) test solution at low pH values (<5.0) showed thesame concentration as the initial Fe(II) concentration, the concen-trations of dissolved ferric iron, Fe(III), could be determined by thedifference between measured Fe(II) and Fe(tot) concentrations inFe(II) + Tc(VII) test solutions. The calculated Fe(III) concentrationin the Fe(II) + Tc(VII) test solutions was about 50% and 40% of initialFe(II) concentration added to the test tubes at pH = 3.2 and 4.1,respectively. The mass of 99Tc(VII) added to the solutions was lessthan 0.1% of the mass of starting Fe(II) so that even if all the99Tc(VII) was reduced to 99Tc(IV), the electrons that each mole of99Tc(VII) requires to be reduced would be supplied easily by theoxidation of minor amounts of Fe(II). Complete reduction of allthe 99Tc(VII) present in the solution would not require nearly asmuch Fe(II) to be oxidized to Fe(III) as suggested by the apparentingrowth of Fe(III) calculated from the difference in the measuredtotal Fe and measured Fe(II) remaining in these solutions. There-fore, transformation of the Fe(II) to Fe(III) could also result from re-dox reactions between Fe(II) and the oxygen contamination withinthe 99Tc(VII)-spiked solution. Negligible 99Tc removal from solu-tion at these low pH values (Fig. 2, right) also suggests that mostof the formation of Fe(III) was caused by oxygen contaminationintroduced with 99Tc addition.

Independent saturation index (SI) calculations indicate that atthe concentrations of 99Tc in these tests, precipitation of TcO2�xH2Owould not occur until pH values exceeded 11. In addition, theslight decrease in Fe(III) concentration at pH (�4.1) compared tosolution at pH 3.2 (Fig. 2, left) indicates that minor amounts ofFe(OH)3(s) or magnetite containing both Fe(II) and Fe(III) may haveformed, even though no visible precipitate was observed in theseexperiments. In contrast, dark green magnetic precipitates wereobserved in solutions above pH 7 containing both Fe(II) and99Tc(VII). These neophases exhibited magnetic properties and wereattracted to the magnetic stirring bar used to agitate the slurry[28]. The dark green precipitate was likely a mixture of green rust(layered hydrous oxides containing Fe(II)/Fe(III) with interlayer an-ions) and magnetite that resulted from mineral transformation ofinitial precursor, white rust, Fe(OH)2(s), after reacting with dis-solved Fe(III) at higher pH values. The green precipitate (greenrust) also can be formed from solution containing both dissolvedFe(II) and Fe(III), which transform quickly to more crystalline ferricoxides dominated by magnetite with minor contents of goethite, aswere found in the SEM/TEM photomicrographs for similar samplecondition [28].

The solubility curves for Fe(OH)3(s) and Fe(OH)2(s), as well asthe aqueous Fe(II) concentrations measured in solution, are shownin Fig. 3. The saturation index (SI) for the solutions was calculatedbased on the solubility product (Ksp) for Fe(OH)2(s) of 10�15. Thecalculated SI was supersaturated with respect to ferrous hydroxide[Fe(OH)2(s)] at pH higher than 7.0, suggesting that it precipitates atthese conditions. These results are in accord with the XRD patternsof centrifuged solids in tests prepared separately without goethitebeing present [28], and indicates the likelihood of white rust,Fe(OH)2(s) at pH = 7.5. Rapid precipitation of Fe(OH)2(s) from the0.07 M Fe(II) solution can occur when the solution pH is raised tohigher than 7.0, even without goethite being present. Becausewhite rust, Fe(OH)2(s), is very unstable towards oxidation, theprecipitate rapidly transforms into magnetite and/or goethite withincreasing pH. However, this mineral transformation sequenceamong various Fe oxides occurs so fast that it cannot bedistinguished in this batch test. For high pH solutions (i.e.,

pH > 7.5), dissolved Fe(II) and Fe(tot) concentrations inFe(II) + 99Tc(VII) solutions were identical and very low, suggestingthat essentially all the Fe was precipitated and removed from solu-tion as solid phases.

The concentration of 99Tc in solution followed a similar trend tothat of dissolved Fe(tot) concentrations with varying pH levels upto 9.0 (Fig. 2, right). There was no 99Tc removal in test solutionsat low pH values (�3.2 and 4.1). However, at higher pH (7.5–9.5),most of the 99Tc(VII) in solution was removed by the newly precip-itated Fe(II/III) oxides. An exception to these trends occurred at pH11.2. The lower percentage of 99Tc removed from solution after1 day of reaction (only 69% of 99Tc removal from solution; seeFig. 2, right) was most likely caused by 99Tc(IV) reoxidation fromminor oxygen contamination entering the test tube. Because wefound no detectable aqueous iron [Fe(II) or Fe(tot)] concentrations,dissolution of newly precipitated 99Tc(IV)-bearing Fe(III) oxides didnot contribute to these higher 99Tc(VII) concentrations. We specu-late that 99Tc(IV) was present as TcO2�xH2O(s) that became vulner-able to oxygen diffusion into the tubes at longer contact time.

3.2. 99Tc(VII) and Fe(II) removal from solution containing goethite orferrihydrite

Removal of Fe(II) and 99Tc(VII) from solution in the presence ofgoethite or ferrihydrite was investigated over a range of pH values.At low pH (�2.0), after 1 day of contact, Fe(II) concentrations re-mained unchanged, suggesting that Fe(II) adsorption onto eithergoethite or ferrihydrite was negligible (Fig. 4, left). Adsorption ofFe(II) onto goethite was insignificant up to pH = 6.5 and equivalentto results from ‘‘blank’’ tests lacking goethite. On the other hand, adecrease of Fe was observed at low pH when Fe(OH)2(s) precipita-tion occurred. Even above pH 6.5, where goethite becomes stable,concentrations of Fe [both as Fe(II) and Fe(tot)] decrease rapidlyin some experiments, whether or not goethite is present (Figs. 2and 4), consistent with rapid precipitation of Fe(OH)2(s) (Fig. 3).For cases in which ferrihydrite was present, a steeper decrease inFe concentrations with time occurred as the pH values approached5.7. In the presence of ferrihydrite, these tests showed about 90%Fe(II) removal at pH = 5.7 by adsorption processes (Fig. 4, left). Re-moval of Fe(II) in solution by adsorption on ferrihydrite was alsomuch higher at a pH of 6.5 than in the systems without Fe oxide(Fig. 2, left) and with goethite present in solution (Fig. 4, left). Theobserved enhanced drop in both Fe(tot) and Fe(II) concentrations

Fig. 4. Dissolved Fe(II) and Fe(tot) concentrations in experiments conducted at different pHs with Fe oxides present (G = goethite; F = ferrihydrite) (left) and 99Tc removal inFe(II) solutions with Fe oxides present at different pHs (right). The solid concentration in these batch experiments was 0.3 g/50 mL.

206 W. Um et al. / Journal of Nuclear Materials 429 (2012) 201–209

in the ferrihydrite slurries at pH values between pH �5.5 to 6.5,conditions in which precipitation of Fe(OH)2(s) also occurs, con-firms that the dominant mechanism of Fe(II) initial removal in theferrihydrite slurries was Fe(II) adsorption on ferrihydrite, not pre-cipitation of Fe(OH)2(s). In general, goethite precipitation occurredonly above pH 6.5 and was a significant cause of Fe loss from solu-tion under these conditions. Thus, Fe(OH)2(s) and ferrihydrite arethe main arbiters of Fe removal from solution at low pH, whereFe(II) is stable, and these solid phases are precursors to goethite for-mation as the pH of the system was increased in the experiments.

The calculated equilibrium solubility curve for Fe(OH)2(s) plot-ted in Fig. 3 matched very well with the measured Fe(II) concentra-tions in the sample with goethite present at pHs higher than 7.5(Fig. 3). However, the measured Fe(II) solution concentrations inthe presence of ferrihydrite fell to the left of the calculated equilib-rium solubility curve line in the region of undersaturation, sup-porting our hypothesis that Fe(II) removal in these solutionsresulted from adsorption of Fe(II) onto ferrihydrite and not precip-itation of Fe(OH)2(s).

Removal of 99Tc from solution was also measured in the batchtests at different pH values containing either goethite or ferrihy-drite. When 99Tc(VII) was spiked in Fe(II) solutions containing goe-thite or ferrihydrite solids at low pH (<3.0), the concentration of99Tc was constant and similar to the initial 99Tc concentration

Fig. 5. (A) TEM image and electron diffraction pattern (B) of goethite crystals from99Tc–goethite powder sample. The specific crystallographic planes on the goethitecrystallite have been identified on the figure (A). The electron diffraction patternshows the presence of some overlapping crystals (B).

(Fig. 4, right). Because Fe(II) adsorption onto goethite or ferrihy-drite was insignificant and the solution was undersaturated withrespect to Fe(OH)2(s) at these low pH values (<3.0), we infer thatthe key to 99Tc(VII) reduction is sorption of Fe(II) onto solid Fe(oxy)hydroxide phases, where the heterogeneous catalysis of tech-netium reduction occurs. The constant concentrations of 99Tc(VII)near those at the beginning of the experiment also indicates thatTcO2�xH2O(s) is not precipitating at this pH. However, 99Tc(VII) re-moval was fast and complete even after 1 day of reaction in solu-tion with ferrihydrite or goethite present at higher pH values(>5.0), most likely resulting from 99Tc-reduction during reactionwith either adsorbed Fe(II) on the ferrihydrite or the Fe(OH)2 pre-cipitates that also formed in both systems.

3.3. Solid phase characterization of 99Tc–goethite

The mineralogy of the initial goethite and ferrihydrite, 99Tc–goe-thite powder sample, and the final 99Tc–goethite pellets after diffu-sion leach test was determined and compared using XRD analysis.Initially synthesized solid substrate was identified as goethite or2-line ferrihydrite [39]. The measured XRD pattern of the 99Tc–goe-thite powder was also found to be solely goethite, which is the sameas Sample 2–5 in Um et al. [28] based on the exact match to the XRD-JADE program. Acicular goethite grains were observed by TEM in thetechnetium-bearing sample (Fig. 5). The SAED data for this samplealso clearly indicated that goethite was well crystallized (Fig. 5b).

Fig. 6. 57Fe transmission Mössbauer spectra of 99Tc–goethite sample at 77 Ktemperature.

Fig. 7. Calculated 99Tc effective diffusivities for 99Tc–goethite pellet as a function of cumulative leaching time (a) and the linear dependence between cumulative 99Tc releaseand cumulative leaching time, t½.

Fig. 8. Normalized XANES spectra for 99Tc(VII) and 99Tc(IV) standards as well as99Tc goethite samples before and after diffusion leach test. The rectangular blacksymbols and red lines indicate the measured data and a linear combination fit,respectively. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

W. Um et al. / Journal of Nuclear Materials 429 (2012) 201–209 207

The Mössbauer spectral features of room temperature (data notshown) and 77 K (Fig. 6) are similar to goethite [40]. More impor-tantly, Mössbauer spectral analysis showed the sample to be freeof adsorbed Fe(II) species that exhibits distinct peaks around 0 and2.5 mm/sec respectively [41], confirming that all the Fe present inthe final 99Tc–goethite product was Fe(III) in 99Tc–goethite sample.More solid characterization results for the 99Tc–goethite using SEMand TEM can be found in our previous paper [28]. The final 99Tc–goe-thite pellet sample after 120 day reaction with the simulated IDFpore water solution also showed identical XRD patterns to the initial99Tc–goethite sample [42]. The lack of mineralogical change evenafter 120 days contact with oxygenated aqueous solution indicatesthat goethite is very stable at circumneutral pH conditions, whichis germane to waste form storage at the Hanford Site.

3.4. Diffusion coefficient and leachability index for 99Tc

Monolithic and cylindrical pellets of the 99Tc goethite wasteform were used to determine an effective diffusion coefficient (orleachability index) for 99Tc immersed in IDF pore water. A smallaliquot of leachate was removed from the leaching container attime intervals from 30 min to 120 days throughout the leachingperiod. The small aliquot was analyzed for 99Tc concentrationand pH. The pH remained constant at a value close to the initialpH (�7.2) of the IDF pore water. The diffusivity of 99Tc from99Tc–goethite pellet sample was calculated with Eq. (1), and the re-sults are displayed in Fig. 7. The first value of 99Tc diffusivity after30 min of contact was low, likely because of effects imposed by theoleic acid used on the final monolithic pellet surfaces to keep thepellets intact. Thereafter, the data showed a low effective diffusionconstant for 99Tc over the course of the experiment (Fig. 7).

The calculated individual interval diffusivity values for the99Tc–goethite pellet showed a gradually decreasing trend as thecontact time increased, except one value that was determined atearly 30 min of contact (Fig. 7a). Based on the assumptions andboundary conditions for simple radial diffusion from a cylinderinto an infinite bath presented by Crank [38], the mass release of99Tc should follow a linear dependence on the square root of time,t½. Excluding the first data point collected at 30 min, the 99Tc–goe-thite pellet sample showed a mass release proportional to t½ witha linear correlation coefficient (R2 = 0.90) in Fig. 7b. The calculatedaverage 99Tc diffusivity for 99Tc–goethite pellet sample is6.15 � 10�11 cm2/s, and the leachability index (LI) is 10.2 accord-ingly. The low value for 99Tc diffusivity and the high LI value arecomparable to those found in several different Hanford grouts thatranged from 10�12 to 10�9 cm2/s with LIs from 9 to 12 [43]. The LI isused as a performance criterion for decision of use and disposal oftreated waste, and in most cases, treatment is considered effective

for solid waste forms when the LI value is equal to or greater than9.0 [44]. The value of LI for the 99Tc–goethite pellet is also similarto or slightly higher (better performance) than those of Cast Stone(LI = 9.4–10.3 for 99Tc) prepared with a low-activity waste simulant[19]. In addition, if 99Tc sequestered-goethite powders were usedas a part of ingredient material to produce the solidified wasteforms, the values of diffusivity and LI for 99Tc would be even lowerand higher, respectively than those values determined using 99Tc–goethite pellet here.

3.5. XANES analysis for 99Tc oxidation state on the 99Tc–goethite pellet

Because the XANES spectra for the 99Tc(VII) standards (NaTcO4,KTcO4, and TcO�4 adsorbed on ion exchange resin) are very similarand are characterized by a strong pre-edge feature due to the 1s to4d transition for the tetrahedral 99TcO�4 anion, only one XANESspectrum for 99TcO�4 collected from 99Tc-adsorbed resin is provided

208 W. Um et al. / Journal of Nuclear Materials 429 (2012) 201–209

in Fig. 8. The XANES spectrum of TcO2�2H2O standard for 99Tc(IV) isvery distinctive and characteristic for 99Tc(IV) coordinated by sixoxygen atoms in an octahedral geometry.

The oxidation states of 99Tc in the 99Tc–goethite powder sampleboth before and after contact with oxygenated aqueous solutionfor 120 days reaction were determined by fitting their collectedXANES spectra by a linear combination fit using the spectra forthe 99Tc(VII) and 99Tc(IV) standards. In all cases, the fitting resultsindicated that only 99Tc(IV) was present in both unleached and lea-ched 99Tc–goethite samples. The best fit results in Fig. 8 showedthat the fraction of 99Tc present as 99TcO�4 in the 99Tc–goethite pel-let sample even after 120 days diffusion leach test was less than1%, demonstrating that the reoxidation of the 99Tc(IV) initiallyincorporated within the Fe(II)-treated goethite mineral latticewas minimal and consequently diffusive transport of 99Tc(VII)was limited in the goethite waste form.

4. Conclusions

Based on the results of this study, the following conclusionswere drawn:

(1) The observed high-percentage 99Tc incorporation within theFe(II)-treated Fe-oxide mineral (mainly goethite) structureprovides a viable option for treating waste streams contain-ing 99Tc(VII) and forming stable 99Tc-bearing Fe oxide solidwaste forms.

(2) Reduced and incorporated 99Tc within the goethite is unli-kely to be released, even when the final 99Tc–goethite prod-uct is exposed to oxidizing conditions. In the circumneutralHanford pore water solution, the concentration and an effec-tive diffusion coefficient (6.15 � 10�11 cm2/s) of leached 99Tcfrom the goethite pellet waste form were very low, suggest-ing potential use of goethite as a waste form.

(3) A previous study of the evolution of 99Tc oxidation states incement waste forms showed that the fraction of 99Tc(VII)rapidly increased from about 10% to 40–50% after exposureto atmospheric oxygen within 4 months [16]. Even though99Tc(VII) can be initially reduced to form TcO2�2H2O(s) inthe solidified waste forms, reoxidation to 99Tc(IV) occursrapidly when it reacts with oxygen because 99Tc(IV) asTcO2�2H2O(s) is not chemically and structurally stable whencontacted with oxygen. However, reduced 99Tc(IV) that iscoprecipitated within goethite lattices and subsequentlyarmored with additional goethite layers is capable of pro-tecting 99Tc(IV) from future oxygen contact and reducesthe rate of 99Tc reoxidation from 99Tc(IV) to 99Tc(VII) dueto the chemical and structural stability of goethite.

Acknowledgments

Funding was provided by the DOE Environmental Management(EM-31) Program. PNNL is operated for the DOE by Battelle Memo-rial Institute under Contract DE-AC05-76RL0 1830. Part of this re-search was performed at Lawrence Berkeley National Laboratoryand was supported by the Director, Office of Science, Office of BasicEnergy Sciences, Chemical Sciences, Geosciences, and BiosciencesDivision, of the U.S. Department of Energy and by the Director, Of-fice of Science, of the U.S. Department of Energy under Contract No.DE-AC02-05CH11231. The XANES data collection was carried outat the SSRL, a national user facility operated by Standard Universityon behalf of the US DOE. A portion of funding was provided for thisresearch by WCU (World Class University) program at the Divisionof Advanced Nuclear Engineering (DANE) in POSTECH through the

National Research Foundation of Korea funded by the Ministry ofEducation, Science and Technology (R31-30005). A portion of solidcharacterization analyses was also performed using EMSL, a na-tional scientific user facility sponsored by the Department of En-ergy’s Office of Biological and Environmental Research andlocated at Pacific Northwest National Laboratory.

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