Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 3, pp. 197-204, 2005

Current Status of the Thermodynamic Data for Technetium and Its Compounds andAqueous Species
Joseph A. Rard*
Energy and Environment Directorate, Lawrence Livermore National Laboratory, University of California,
Livermore, California 94550, U.S.A.
Received: July 31, 2005; In Final Form: September 30, 2005
The technetium isotope 99Tc is a major fission product from nuclear reactors. Because 99Tc has few applications
outside of scientific research, most of this technetium will ultimately be disposed of as nuclear waste. Geochemical
modeling of the dissolution of nuclear waste, and of the solubility and speciation of the dissolved radionuclides ingroundwaters, is an important part of the Performance Assessment of the safety of a nuclear waste repository that
relies on the availability of a critically-assessed thermodynamic database. Such a database for technetium was
published in the book Chemical Thermodynamics of Technetium, covering the published literature through 1998.
This database is described here, along with a discussion of more recent relevant studies. Gaps in the knowledge ofthe chemical and thermodynamic properties of technetium are identified here, and recommendations are made formeasurements that are required to eliminate these gaps.

1. Introduction
Technetium is produced in major amounts by the thermal
neutron fission of 235U and 239Pu reactor fuels. According to
Lieser,1 the burn up of 1 ton of 3% enriched uranium in a nuclear
reactor produces about 1 kg of 99Tc (6.13% fission yield), which
is about 10 mass-% of the fission products. Popova et al.2 in2003 cited an estimate that 60 tonnes of 99Tc have been accumulated from spent reactor fuel. El-Weare et al.3 cited an estimate that 10% of the 99Tc produced by nuclear reactors has beenreleased into the environment, along with an order of magnitude smaller amount of 99Tc as a relic from the atmospherictesting of nuclear weapons. The metastable isotope 99mTc (produced by neutron irradiation of 98Mo) is used extensively for
diagnostic purposes in nuclear medicine; 4 99mTc undergoes
gamma decay to the ground state 99Tc, with t1/2 = 6.0 hours,
which then occurs in the medical waste. 99Tc is also producedby the spontaneous fission of the 238U present in uranium ores,
but the amount formed is so small as to be nearly undetectable.5
Unlike 99mTc, 99Tc and other technetium isotopes have relatively few uses outside of scientific research. Depending on thetype of fuel used in the nuclear reactor, the extent of burn up,
and other reactor conditions, intermetallic (Mo,Tc,Ru,Rh,Pd)particles may form. These particles are quite difficult to dissolve and usually become part of the highly active waste.1, 5 The
remaining waste is generally dissolved in nitric acid as part of
the PUREX process or its modifications, and the dissolved 99Tc
is co-extracted with uranium and plutonium as mixed anion
complexes such as UO2(NO3)(TcO4)(TBP)2 solvated with tributlyphosphate (TBP).1, 2, 6 It is possible to destroy 99Tc by transmuting it into stable ruthenium isotopes using neutron irradiation,2
but this process is not currently used on a wide scale, and thus
much of the surplus 99Tc will eventually need to be disposed ofas part of the low level waste.
Under typical oxidizing environmental conditions, technetium
forms the pertechnetate ion TcO4−, whose salts are soluble andquite mobile in groundwater. TcO−4 is only slightly absorbed onmost rocks and minerals under oxic conditions,3, 7 probably by
surface ion exchange,7 and it is excluded from sorption in some*Corresponding author. E-mail: rard1@llnl.gov. FAX: +925
424 3255.

negatively charged sediments by repulsion within the charged
double layer thereby leading to an enhanced geochemical
mobility.8 However, TcO−4 can be reduced by, and absorbed on,minerals containing Fe(II) or sulfide (e.g. FeS, FeS2 , PbS,
Sb2S3, FeAs2),9, 10 and by common bacteria such as Clostridium
sphenoides.11 Reduced technetium can bioaccumulate in microorganisms and plants11−13 and in some invertebrates.12
TcO 4− (aq) can be reduced to the sparingly soluble (~10 −8
mol·kg−1) phase TcO2·xH2O(s) under strongly reducing conditions.5 Complex formation with humic acids increases the solubility of the reduced technetium species by one to two orders ofmagnitude, and depending on the reducing conditions and pH,
the technetium can be present as either Tc(III) or Tc(IV),14, 15
possibly as small hydrolyzed polymers.16 Chelating agents suchat EDTA, NTA, and oxalate are used in some waste separation
processes, and some of their complexes with reduced technetium may persist for long periods in alkaline tank waste.17
Understanding the dissolution and potential migration oftechnetium from a waste repository is critical for Performance
Assessment evaluations, and involves the use of geochemical
modeling calculations. The reliability of these geochemical
modeling calculations clearly depends on the availability of acomplete and accurate thermochemical database for the pertinent species, compounds, and complexes. The Thermochemical
Database Project (TDB) of the OECD Nuclear Energy Agency(NEA) published a series of books giving critically assessed
thermodynamic data for selected elements pertinent to radioactive waste management. Of these, Chemical Thermodynamics
of Technetium by Rard et al.5 was published in 1999, covering
the literature through 1998. A subsequent update volume18 forvarious radioactive elements including technetium, with literature coverage through 2002, did not produce changes in thetechnetium thermochemical database. In the present report,
the database from Chemical Thermodynamics of Technetium issummarized, some newer studies elucidating the chemical andthermodynamic properties of technetium are described, andrecommendations are made for experimental measurementsthat are needed to fill existing gaps in the available data.
2. The Chemistry and Thermodynamics of Technetium
Fundamental thermodynamic quantities of substances that© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2005

198 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005
are included in thermochemical databases are the standard
molar heat capacity at constant pressure Cp̊,m , the standard
molar entropy Sm̊ , the standard molar enthalpy of formation∆f Hm̊, the standard molar Gibbs free energy of formation ∆f Gm̊,and the equilibrium constant K. Because some of these thermodynamic quantities contain redundant information, not allof them will appear in a particular database. The standard
entropy of a pure element is required for the calculation of ∆f Sm̊
and, in combination with ∆f Hm̊, of the values of ∆f Gm̊.
2.1. Technetium metal. Technetium metal occurs as a hcp
crystal (P63/mmc space group) up to the melting point of (2430± 30) K. 5 Heat capacities available at the time of the NEA
review were limited to those of Trainor and Brodsky19 from 3 to
15 K, which yield the Debye temperature and electronic heat
capacity coefficient, and of Spitsyn et al.20 from about 950 to1600 K from thermal diffusion measurements. Unfortunately,
the heat capacities of Spitsyn et al. were only presented graphically and not numerically. Fernández Guillermet and Grimvall21
calculated the heat capacities and entropies of Tc(cr) with a theoretically-motivated model, using the concept of a temperaturedependent entropy Debye temperature θ S, which is based on alogarithmic average of phonon frequencies, and an empirical
relation between θ S for the corresponding 4d and 5d elements.
For Tc(cr) and Re(cr) their empirical relations yield θ S (Tc, cr,
T) = (1.198 ± 0.016) θ S (Re, cr, T), and values of θ S (Tc, cr, T)
thus calculated were used to calculate the vibrational contribution to the heat capacity and entropy. The heat capacities andentropies of Tc(cr) recommended for the NEA review5 from7.86 K (superconducting transition temperature) to 2430 K(melting temperature) were based on this approach, but wererevised using a more recent critical evaluation for Re(cr) toredetermine θ S (Re, cr, T). Heat capacity differences between
the source model21 and the revised model5 are very small nearroom temperature, but above about 500 K the differences
become significant and increase with temperature, reaching a
difference of about 8% at 1600 K.
Shirasu and Minato22 subsequently determined the heat capacities of Tc(cr) from 323 to 1073 K using differential scanning calorimetry. Their results essentially confirm the calculated results
reported in Reference 5 in this temperature region. Figure 1 compares these Cp̊,mvalues with those from two other recent determinations23, 24 and # the model calculations. The experimental Cp̊,mvalues of van der Laan and Konings23 are somewhat higher thanthose of Reference 22, whereas those of Boucharat24 are very low.
The measurements of van der Laan and Konings23 were madeusing a Tc0.85Ru0.15 alloy that was obtained by transmutation ofpart of the 99Tc to 100Ru, and it was non-uniform in composition.
Their calculation of Cp̊,m for Tc(cr) required two approximations: i) that the heat capacities measured for their non-uniform
sample are equal to those of a uniform sample having the averagecomposition, and ii) the heat capacity change from forming thealloy is zero. Considering the uncertainty introduced by theseapproximations, the agreement of their results23 with the recommended values5 is satisfactory. The minimum and maximum
exhibited by the heat capacities of Boucharat24 are quite improbable for a metal that has the same crystal structure over the studied temperature interval, and probably result from significant
chemical contamination of Boucharat’s Tc(cr) sample or a serious calibration error for the differential scanning calorimeter.
The excellent agreement of the experimental heat capacitiesof Shirasu and Minato22 with those recommended in Reference
5 gives us considerable confidence in the recommended values. However, the calculation of Sm̊ requires the evaluation of∫T{(Cp̊,m /T)dT}, and there are no experimental Cp̊,m data between

T=0

15 and 323 K to compare with the calculated values.5 The theoretical-based model using the entropy Debye temperature methodology begins to loose its validity below about 100 K, resulting

Rard

Figure 1. Comparison of experimental heat capacities of technetium
metal to those recommended in Reference 5. “Present data” refers to
the differential scanning calorimetric measurements of Shirasu andMinato.22 Reprinted from the Journal of Alloys and Compounds, volume 337, Y. Shirasu and K. Minato, Heat Capacities of Technetium
Metal and Technetium-Ruthenium Alloy, pages 243–247, copyright
2002, with permission from Elsevier.

in some uncertainty in the integral at low temperatures, andconsequently in the calculated Sm̊ values. Heat capacity measurements for high purity Tc(cr) in this temperature region are
definitely needed.
Assessed thermodynamic values for Tc(g) are available froma combination of statistical thermodynamic calculations combined with enthalpies of sublimation from three independentstudies.5 Although the latter measurements have appreciable
uncertainty, the recommended results should be adequate formost applications.
2.2. Technetium oxides and hydrous oxides. Two solidoxides in the technetium-oxygen system have been chemicallyand structurally characterized:5 TcO2 (cr) which occurs in the
monoclinic space group P21/c, and Tc2O7(cr) which occurs in
the orthorhombic space group Pbca. Tc2O7(cr) melts at (392.7 ±0.1) K and extrapolation of the vapor pressures of Tc2O7 (l)yields an estimated normal boiling temperature of 584 K.
Equations for the vapor pressures of Tc2O7(cr) and Tc2O7(l) as
functions of temperature are given in Table 1. Several other
TABLE 1: Parameters for the Vapor Pressure Equationln(p/bar) = a + b(K/T) + cln(T/K) for Tc(cr) and Various
Inorganic Technetium Compoundsa
Phase

c

Temperature
Range / Kab

Tc(cr)

17.342–80978≈1900–2350

Tc2O7(s)35.469–16590362.2–391.2

Tc2O7(l)14.101–8222.5

393.3–529.4

TcF6 (cr, ortho.)

88.0741–8208.3 –10.787

256.83–267.68

TcF6 (cr, cubic)

28.6883–5015.0–2.295268.32–310.03

TcF6 (l)50.5040–5537.5–5.8036

311.11–324.82

TcO3F(s)22.042–7459.0

TcO3F(l)12.761–4753.9

291.43–324.97

TcOF4 (solid I)

31.718–12793298.15–357.7

264.37–291.43

TcOF4 (solid II)

21.702–9210

357.7–406

TcOF4 (l)16.682–5844

406–423a

The information in this table was taken from Reference 5, whichshould be consulted for references and estimated uncertainties. The
parameters for Tc(cr) were calculated from an equation for the Gibbs
free energy of sublimation given in their Appendix B.2.

J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 199

Current Status of the Thermodynamic Datasolid oxides have been suggested in the literature, in particular
TcO3(s), but their existence has not been definitely established.
Tc2O7(cr) sublimates congruently to yield Tc2O7(g). Many othergaseous oxides that have been identified at very high temperature by mass spectrometry are Tc2O6 (g), Tc2O5 (g), Tc2O 4 (g),TcO3 (g), TcO2 (g), and TcO(g). Out of all of these anhydrousoxides, only TcO2 (cr), Tc2O7(cr), Tc2O7(l), and Tc2O7(g) arelikely to be pertinent to radioactive waste management.
The enthalpy of formation of Tc2O7(cr) has been determinedboth by oxygen combustion calorimetry and by solution calorimetry, and that of TcO2 (cr) by oxidative dissolution using
aqueous Ce(IV). Because of experimental difficulties,25 thevalue of ∆ f Hm̊ (TcO2, cr, 298.15 K) has a large uncertainty. A
redetermination of ∆f Hm̊ (TcO2, cr) using an alternative reaction
medium is desirable.
The calculation of ∆f Gm̊ from ∆f Hm̊ also requires the entropies
of TcO2(cr) and Tc2O7(cr), but no experimental values are available. The entropy of TcO2 (cr) at 298.15 K was estimated fromknown values for other transition metal dioxides, and values for
Tc2O7(cr) as a function of temperature were obtained from statistical thermodynamic calculations for Tc2O7(g) combined with theentropy of sublimation of Tc2O7(cr).5 Unfortunately, the vapor
pressures for Tc2O7(cr), see Figure 2, are available only over anarrow temperature range and are of low precision. A more precise determination of the vapor pressures of Tc2O7(cr) over a
wider temperature range, and/or its heat capacities from low toroom temperature, are needed to improve the accuracy of thevalues of ∆f Gm̊ for Tc2O7(cr) and Tc2O7(g).
Evaporation of water from HTcO4(aq) yields a reddish-blacksolid having the empirical composition Tc2 O 7·H 2 O(cr) or
HTcO4 (cr). The crystal structure of this compound (triclinic−
space group P1) is complicated and is not fully established, but
based on studies by Russian scientists, the compound is probably
ionic and polymeric (private communication from K. E. German
to J. A. Rard, cited in Reference 5). Values of ∆f Hm̊, ∆f Gm̊, and5Sm̊ at 298.15 K were derived from decomposition vapor pressure measurements, assuming that the empirical formula is
Tc2O7·H2O(cr):
Tc2O7·H2O(cr) →← Tc2O7(cr) + H2O(g)(1)

Tc2O7·H2O(cr) is extremely hygroscopic and deliquesces when
exposed to atmospheric air.

The most commonly encountered hydrous oxide is that ofTc(IV), and it is the most important one under reducing conditions. Tc4O5 ·xH2O(s) and Tc2O5 ·xH2O(s) have been reported,2, 5
but such mixed valence hydrated oxides are not likely to be foundoutside of a laboratory setting. There are several reports of aTc(III) hydrous oxide being produced during the electrolytic
reduction of TcO−4, but Tc(III) is unstable with regard to disproportion to other valence states at pHs above 3 or 4, and is very
sensitive to oxidation by even trace amounts of oxygen.5 Tc(II)and Tc(I) hydrous oxides have also been claimed to form undervery reducing conditions,5, 26 but they are very likely thermodynamically unstable.
The solid hydrous oxide of Tc(IV) has variously been described
as being amorphous or poorly crystalline. It has been formulated variously as Tc(OH) 4, TcO(OH) 2, TcO2, and TcO2·xH2O,
where x = 0.44 to 4.22.5 Some of this variation in the measured
water content and crystallinity may arise from different preparation conditions, i.e. electrolytic or chemical reduction of TcO4−
versus hydrolysis of Tc(IV) compounds, or sample age, but
determining the water content accurately is difficult owing to
the small sample sizes typically used. In Reference 5 the formulation TcO2·1.6H2O(am) was accepted as a reasonable average of reported compositions for this hydrous oxide.
Recent extended X-ray absorption fine structure (EXAFS)
studies now provide definite information about the nature of
the Tc(IV) hydrous oxides.16, 27, 28 TcO 2 ·xH 2O(s) has a onedimensional chain structure with two µ-oxo bridges between
neighboring technetium atoms (four bridges per technetium).
The Tc-Tc distance is (2.5–2.6) × 10−10 m and the bridging Tc-O
distance is (2.0–2.1) × 10−10 m. The remaining two coordinatedsites of each Tc(IV) are occupied by trans water molecules.
The recommended value of ∆ f Gm̊ (TcO2·1.6H2O, am, 298.15
K) was calculated using the emf data from two concordantstudies for the reversible electrochemical cell reaction5→ TcO ·1.6H O(am) + 0.4H O(l) (2)
TcO−4 (aq) + 4H+(aq) + 3e− ←2
2
2The available emfs are restricted to 298.15 K, and additional
measurements at other temperatures are needed to determine∆f Gm̊ of this important solid phase as a function of temperature.
2.3. Mixed oxides of technetium and pertechnetate salts.
Numerous ternary oxides containing Tc(VII), Tc(VI), Tc(V),and Tc(IV) have been synthesized, as have several quaternaryoxides containing Tc(IV). Their compositions and crystal
structures are listed in Table V.40 of Reference 5. Some of these
Tc(IV) or Tc(VII) mixed oxides may form during calcining of
low level nuclear waste, but none have been characterized thermodynamically.
TABLE 2: Solubilities of Pertechnetate Salts in Watera
Solid Phase

Solubility at
298.15K / mol·kg–1

Solubility
Product at
298.15 K, KS

NaTcO 4 ·4H 2Ob

11.30

6.17

KTcO4

0.105

5.15×10 –3

CsTcO 4AgTcO4

RbTcO4

Figure 2. Logarithmic plot of vapor pressures of Tc2O7(s, l) against
the inverse of temperature. This is Figure V.4, Vapour pressure of
Tc2O7, from Chemical Thermodynamics of Technetium, 5 ©OECD
1999, with permission from the OECD 2005.

Other
Temperatures
Studied273.15 K
293.150.017

2.4 ×10–4288.15–313.15 K

0.027

5.4 ×10 –4

288.15–313.15 K

TlTcO40.0025

4.8 ×10 –6

293.15–313.15 K

NH4TcO4

0.59

0.123

(CH3) 4NTcO4

0.14 (293.15 K)

Ph4AsTcO4

2.95×10 –5

Ba(TcO4) 2≈0.2 (interpolated)a293.15, 308.15 K

The information in this table was taken from Reference 5, whichshould be consulted for references and estimated uncertainties.
b
The degree of hydration of this salt is uncertain, but it is probably
the tetrahydrate.

200 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005−
4

Dissolved technetium occurs as TcO in aqueous solution underoxic conditions, and many pertechnetate salts have been prepared.5
Table 2 summarizes the solubilities of these pertechnetate salts at
298.15 K along with their solubility products (activity products).
Under normal environmental conditions, these pertechnetate salts
are too soluble to become solubility limiting phases.
KTcO4 (cr) is the most studied pertechnetate salt, with heat
capacities (and thus entropies) being available from 9.00 to
308.56 K, enthalpies of solution at 298.15 K, and accurate solubilities at 273.15 and 298.15 K.5 These data were used to calculate the entropy of formation of TcO−4, and when combined withthe enthalpy of formation of this ion obtained from Tc2O7(cr)data, yielded ∆f Gm̊(TcO−4, aq, 298.15 K). ∆f Gm̊(TcO−4, aq, 298.15
K) was then used for the calculation of ∆ f Gm̊ (TcO2·1.6H 2O, s,
298.15 K) using eq 2, and of ∆ f G˚m for the various Tc(IV)hydrolyzed aqueous species.5
2.4. Binary technetium halides and oxyhalides. The following binary halides have been reported: TcF 6 (cubic andorthorhombic cr, l, g), TcF5(cr, l), TcCl6 (cr), TcCl4 (s), TcBr4 (s),and TcBr3 (s). 5 TcCl6 (cr) is unstable with regard to chlorine
loss, and TcCl4 (s) and TcBr4 (s) are hydrolytically unstable in
the presence of water vapor or liquid water. The most important of these compounds is TcF6. Technetium is co-extractedwith uranium and plutonium during the reprocessing of spent
nuclear fuel by the PUREX process, and any technetium notsubsequently removed remains with the uranium or plutonium.
Conversion of this uranium to UF6 for isotopic enrichment byvapor diffusion of UF6 (g) yields volatile TcF6 as a byproduct,
and TcF6 (g) has been a source of technetium loss from reprocessing plants.
The following technetium oxyhalides have been reported:
TcOF4 (two crystalline forms, l, g), (TcOF4) 3 (cr), TcO2F 3 (cr),TcO3F(s, l, g), Tc2O5F4(s), TcOCl3(s, g), TcOBr3(s, g), TcOCl4(s),and TcO3Cl(s, l, g).5 Most, if not all, of these compounds arehydrolytically unstable. Equations for the vapor pressures of
TcF6, TcO3F, and TcOF4 are given in Table 1. Experimental heat
capacities (and thus entropies) are available from 5.809 to
347.937 K for TcF6 (both solid forms and liquid), and for TcF6(g)from statistical thermodynamic calculations. A determinationof the standard enthalpy of formation for TcF6 (cubic) or TcF6 (g)is needed to complete the calculation of ∆ f Gm̊ of one of thesephases, which in turn can be combined with the vapor pressure
data to yield ∆f Gm̊ of the other phase.
2.5. Ternary technetium halides, hydroxyhalides, and oxyhalides. A very large number of ternary of Tc(IV) halides havebeen prepared with formulae of M(I)2TcF6 (cr), M(I)2TcCl6 (cr),M(I)2TcBr6(cr), M(I)2TcI6(cr), M(II)TcCl6(cr), and M(II)TcBr6(cr),where M(I) is a monovalent cation and M(II) a divalent cation.5
The tendency for the Tc(IV) hexahalo salts to undergo aquationand subsequent hydrolysis in aqueous solutions falls in the order
2−
2−
2−
TcI2−
6 >> TcBr6 > TcCl6 >> TcF6 , with the fluoride salts being
quite resistant to hydrolysis whereas the iodide salts are readilyhydrolyzed. The TcBr62− and TcCl62− ions are stabilized by very
high concentrations of the corresponding halide ions and highlevels of acidity, and are resistant to hydrolysis when stored in
the dark to preclude photochemical aquation. Solubilities ofseveral of these chloride and bromide salts are available at298.15 K in their corresponding concentrated hydrohalic acids.5
Numerous dinuclear and polynuclear technetium halides
have been prepared with average technetium valences of 4 orless. 5, 29 Because most of these polymeric technetium compounds are formed under very reducing conditions, they will
not form or survive under normal environmental conditions.
Reduction of TcO 4− with concentrated aqueous HCl in thepresence of the iodide ion, or by HI in the presence of chloride
or bromide ions, yields salts containing the TcCl 5 (OH) 2− or
TcBr5(OH)2− anions.5 There is a lack of thermodynamic data forthese salts, but they are probably unstable with respect to hydrolysis by water.

Rard
−
4

Reduction of TcO by aqueous HCl or HBr yields salts containing the TcOCl52−, TcOBr52−, TcOCl 4−, and TcOBr 4− ions withtechnetium in the unstable Tc(V) valence state.5 Lower reaction
temperatures of 265 to 273 K favor the formation of these Tc(V)salts, whereas Tc(IV) salts are produced at higher reaction temperatures. These Tc(V) oxyhalide salts should be unstable withrespect to hydrolysis and disproportionation to TcO−4 and Tc(IV).
A mixed valence oxychloride has been reported and formulated
as K3Tc2O2Cl8(s), but its characterization is incomplete and thus
its stoichiometry is uncertain.
2.6. Binary technetium sulfides. Reaction of TcO 4− (aq)with H2S yields Tc2S7(s), and heating Tc2S7(s) in the absence ofoxygen produces TcS2 (s).5 Because of its very low solubility inwater, under reducing conditions Tc2S7(s) has the potential forbeing the solubility-limiting phase for technetium in groundwater. However, two attempts to determine the solubility product of Tc2S7(s) have been unsuccessful,3, 30 because Tc2S7 readily
forms colloids in aqueous solutions30, 31 and the smaller colloids
could not be removed by filtration.30 Kunze et al.30 studied theprecipitation of Tc2S7(s) from Na2S solutions at pH = 8 to 10 insimulated brines and in the presence of iron or zircalloy. The
observed dissolved technetium concentrations were < 10 −8
mol·L −1 after 150 days of equilibration, which is consistent with
TcO2·xH2O(s) having replaced Tc2S7(s) as the solubility-limiting phase.
2.7. Technetium redox chemistry in aqueous solutions in
the absence of a complexing ligand. The valence state of dissolved technetium in aqueous solutions varies between +7 in
TcO−4 to +3. A stability field for Tc(II) appears in many potential-pH diagrams, even though this species has not been experimentally observed. As discussed in Reference 5, this resultsfrom the uncritical use of a Tc2+ (aq)/Tc(s) redox potential of
unknown origin given by Latimer.32
The electrolytic reduction of TcO 4− (aq) occurs through aseries of complicated and irreversible processes, involving multiple (mostly irreversible) steps and alternate pathways that dependon the pH of the solution and kinetic factors. However, by using
very rapid scanning techniques such as pulse polarography or
fast cyclic voltammetry, a reversible reduction of TcO 4− (aq)involving one electron was observed in alkaline solutions:5→ TcO2− (aq)TcO−4 (aq) + e− ←4
→ TcO−4 (aq) + Tc(V) → further reactions(3)

The TcO42− (aq) ion is very unstable and rapidly disproportionates irreversibly to form TcO4− (aq) and Tc(V), and the unstable
Tc(V) further disproportionates to Tc(IV) and a higher valence
state of technetium. Using conventional (slower) electrochemical techniques, TcO4− (aq) is reduced by a two-electron process
to Tc(V) which similarly disproportionates. In acidic solutions,TcO−4 (aq) can be reduced to Tc(III) or Tc(IV) depending on thepH, which involves converting some of the oxygen atoms into
water molecules. These other redox reactions are irreversible
and do not yield thermodynamic data. The Tc(VI) and Tc(V)species are thermodynamically unstable with regard to disproportionation.
A quasi-reversible potential for a Tc(IV)/Tc(III) redox couple
has been reported, but does not yield unambiguous results
because of uncertainties about the hydrolysis state of Tc(III),and the reduction of Tc(III) to Tc(s) in acidic solutions is irreversible.5 However, the Tc(IV)/Tc(III) redox boundary is probably very close to the boundary for reduction of H 2O, andTc(III) will not be a significant aqueous species under normal
environmental conditions.
2.8. Aqueous Tc(IV) species and carbonate complexes. The
variation of the solubility of TcO2·1.6H2O(am) with pH was analyzed to yield the solubility product of TcO2·1.6H2O(am), the hydrolysis constants relating TcO2+(aq), TcO(OH)+(aq), TcO(OH)2(aq),and TcO(OH)−3 (aq), and, in carbonate solutions, the stability con-J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 201

Current Status of the Thermodynamic Data−
3

5

stants for Tc(CO3)(OH)2(aq) and Tc(CO3)(OH) (aq). The recommended equilibrium constants were derived from thesolubility studies of Meyer et al.33 and Eriksen et al.34 whose
results are compared in Figure 3. Nguyen Trung et al.35 measured solubilities that are probably consistent, but the underlying
experimental data have not yet been published. Hess et al.36
recently reported very extensive solubility experiments for
TcO2 ·1.6H 2O(am) in the presence of hydrazine (to maintainreducing conditions) with different concentrations of added
NaCl. They were able to represent their solubilities with thesolubility product and hydrolysis constants from Reference 5,
but adjusting in addition the hydrolysis constant for the reaction:→ TcO(OH) (aq) + H+(aq)TcO(OH) +(aq) + H2O(l) ←2(4)

As can be seen by comparing the left- and right-hand plots in
Figure 4, using the adjusted value of log10 K for reaction (4) gives
a better representation of the solubility data of Hess et al.36 for 4
and 11 day equilibrations, whereas the results of their 29 and 65
day equilibrations are better represented by the hydrolysis constant recommended in Reference 5. These plots indicate thataging effects of unknown origin were present in their saturated
solutions when pH < 4. Table 3 gives a summary of the reac-

TABLE 3: Solubility Product for TcO 2 ·1.6H 2 O(s) andEquilibrium Constants for the Hydrolysis of Tc(IV) andFormation of Complexes with Inorganic Ions at ≈ 298 K
Reaction

Referencea

log10 K

→ TcO(OH) (aq) + 0.6H O(l)TcO ·1.6H O(s) ←–8.4 ± 0.5 Rard et al.5

→ TcO(OH) (aq) + 2H + (aq)
TcO2+ (aq) + 2H 2O(l) ←2> –4.0→ TcO(OH) (aq) + H (aq)
TcO(OH) (aq) + H 2O(l) ←2–2.5 ± 0.3 Rard et al.5

2

2

2

2

+

+

Rard et al.5

–4.0 ± 0.4 Hess et al.36→ TcO(OH)− (aq) + H + (aq)
TcO(OH) 2 (aq) + H 2O(l) ←3–10.9 ± 0.4 Rard et al.5

→ Tc(CO )(OH) (aq)
TcO(OH) 2 (aq) + CO2 (g) ←32

1.1 ± 0.3 Rard et al.5

→ Tc(CO )(OH) − (aq)
TcO(OH)2 (aq) + CO2 (g) + H2O(l) ←3
3–7.2 ± 0.6 Rard et al.5
+ H + (aq)
→ TcCl (aq) +TcO2 ·1.6H 2O(am) + 4Cl– (aq) + 4H + ←4
3.6H 2O(l)–2.8 ± 0.2b Hess et al.36a

The source solubility data used for the evaluation in Reference 5 were fromthe studies of Meyer et al.33 and Eriksen et al.34 b This value should only
be used in combination with the adjusted hydrolysis constant (reported
above) from that study.

tions involving Tc(IV) and the recommended log10 K values.
It is very difficult to distinguish between oxy- and hydroxycoordination in sparingly soluble Tc(IV) species. The formulation of the hydrolyzed Tc(IV) species as TcO(OH)2−nn (aq) rather
than as Tc(OH) m4−m (aq) was mainly due to the recognition thatthe charge of the Tc(IV) species never increased above +2 evenat pH = 0. The recent extended EXAFS studies16, 27, 28 indicatethat TcO2·xH2O(s) contains µ-oxo rather than hydroxo bridges,
suggesting that formulating the monomeric core solution species as TcO2+(aq) was a reasonable choice.5 Because of the strong
tendency of Tc(IV) to form hexacoordinated complexes, any
unassigned coordination sites on the Tc(IV) species are presumed to be occupied by water molecules.
A reported dimerization constant for the reaction→ {TcO(OH) } (aq)
2 TcO(OH)2 (aq) ←2 2

Figure 3. Plot of solubility of TcO2 ·xH2O(s) against the negative logarithm of the hydrogen ion concentration. This is Figure V.3, Solubility
measurements of hydrous Tc(IV) oxide at 25 ˚C, from Chemical
Thermodynamics of Technetium,5 ©OECD 1999, with permission fromthe OECD 2005.
-2(5)

was not accepted in the NEA review5 because of the probablepresence of significant concentrations of TcO−4 (aq), and possible
colloid formation from the apparently oversaturated solutions.
The chemical reduction of TcO−4 (aq) to form oversaturated solutions of Tc(IV) initially yields small hydrolyzed polymers28 andvery fine colloids that could be removed by ultrafiltration but
-2

4-day
11-day29-day65-dayRard et al. 1999

-3
-4

4-day
11-day29-day65-dayModel-3-4
TcCl 40

-5

-5

-6

-6
TcO(OH)+

TcCl 62--7-7

TcO 2+-8-8
TcO(OH)-90
2

TcO(OH)20-9

TcO2 • 1.6H 2O(am)T cO2+

-10
-1

0

1

2

3

pH

4

5

6

7

TcO(OH)+

TcCl4(am)

-10
-1

0

1

2

3

4

5

6

7

pH

Figure 4. Plots of log10 (CTc(IV)/mol·kg−1) against the pH from the study of Hess et al.36 where CTc(IV) is the total concentration of dissolved Tc(IV) and
0.02 mol·L−1 hydrazine was present to maintain reducing conditions. Solid curves: comparison of these experimental measurements against model
predictions made using the assessed ∆f Gm̊ values from (left plot) Reference 5, and from (right plot) the parameterized model of Hess et al. with inclusion of chloride complexes. This is Figure 8 from Journal of Solution Chemistry, volume 33, pages 199–226, 2004, N. J. Hess, Y. Xia, D. Rai, and S.
D. Conradson, Thermodynamic Model for the Solubility of TcO2·xH2O(am) in the Aqueous Tc(IV)–Na+ –Cl− –H+ –OH− –H2O System, ©2004 Plenum
Publishing Corporation, with kind permission from Springer Science and Business Media.

202 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005

Rard

37

not by sedimentation. At higher degrees of Tc(IV) oversaturation, colloid growth was observed, and the addition of increasing
amounts of NaCl caused coagulation and then precipitation.37
2.9. Aqueous Tc(IV) and Tc(III) chloride and bromide complexes. The following Tc(IV) halide complexes have been pro2−−
posed in the literature: TcF2−
6 (aq), TcCl6 (aq), TcCl5 (aq), TcCl4(aq),+2−−
TcCl3(aq), TcBr 6 (aq), TcBr 5 (aq), and TcBr4(aq), as have several
Tc(III) and Tc(IV) monohydroxy chloride and bromide complexes.5 Equilibrium is reached very slowly in the Tc(IV) halidesystems in the absence of chemical or photochemical catalysis.
Equilibrium constants are available for the equilibria
−
−
→
TcX2−
6 (aq) ← TcX5 (aq) + X (aq)(6)

where X = Cl or Br, but the reported equilibrium constants arelimited to concentrated acidic chloride and bromide solutions.5, 38
A recent study by Ben Said et al.39 indicates that aged solutionsof Tc(IV) in 1 mol·L −1 HCl contain TcCl−5 (aq) and TcCl4 (aq).
However, a recent thesis38 reports strong evidence for formation4−of the Tc(IV) complex Tc2OCl10
at pH = 0 and 1 in aqueous solu−1−
tions containing 3 mol·L Cl , by the dimerization reaction:→ Tc OCl 4− (aq) + 2H+(aq) + H O(l) (7)2TcCl5(OH2) − (aq) ←2
102

Evidence was also obtained for the partially dissociated formof this ion, Tc2OCl82− (aq), and possibly for the µ-oxo bridged
x−6
Tc2O2Cl10−x(aq) complexes.
The results from Tc(IV) solubility measurements by Hess etal.36 in 0.8, 2.5, and 5.0 mol·L −1 NaCl are shown in Figure 5.
Based on the change in slope at pH ≈ 0.5, they concluded that
although the solid phase was TcO2·xH2O(am) at higher pHs, a
new phase, possibly TcCl4(am), occurred at lower pHs and highchloride concentrations. Because the identification of this second phase is speculative, and more solution species may havebeen present than actually assumed, their data at low pHs may-2

TcO2·1.6H2O(am) + 4Cl− (aq) + 4H+(8)→ TcCl (aq) + 3.6H O(l)←42An equilibrium constant, calculated from information given in
their paper, is reported in Table 3.
2.10. Thermodynamic modeling at finite ionic strengths.
Tables 4 and 5 summarize the ∆f Gm̊, ∆f Hm̊, Sm̊, and Cp̊,m values
that are available for technetium compounds and aqueous ions.
The many gaps in these tables indicate areas where additional
thermodynamic measurements are needed. These standard
state values are for pure phases and ideal aqueous solutions. At
finite solution concentrations, the NEA recommends the use of
the Specific Ion Interaction Theory (SIT), and they havereported the available SIT parameter values.5, 18 At high ionicstrengths, Pitzer’s ion interaction model40 is widely used andhas been incorporated in many geochemical modeling codes.
Ion-interaction parameters are available for several single aqueous pertechnetates and some mixtures at 298.15 K,41−44 andthese values are given in Tables 6 and 7. Pitzer’s review40 andother reports by Pitzer should be consulted for the significanceof these parameters.
Acknowledgments. This work was performed under the
auspices of the U. S. Department of Energy by University ofCalifornia, Lawrence Livermore National Laboratory undercontract No. W-7405-ENG-48. The author is grateful to theorganizing committee of IST-2005, especially Professors Tsutomu
Sekine and Yasuhiko Fujii, Dr. Nancy J. Hess for providing
electronic copies of Figures 4 and 5, Prof. Bernd Grambow for a
copy of Reference 38, and Dr. Ananda M. Wijesinghe for helpful suggestions.

-2

-3

-3

0.8 M NaCl
4-day
14-day29-day
Model

-4
-5

2.5 M NaCl
3-day
10-day61-day
Model

-4
-5

-6

-6

TcO2+

TcO 2+

-7

-7

-8

-8
TcO(OH)20-9

TcO(OH)+0

TcCl4(am)

-10

1

2

3

4

TcO(OH)+TcO 2 • 1.6H 2 O(am)TcCl4 (am)-1

TcO(OH)20-9

TcO 2 • 1.6H 2 O(am)-10require reinterpretation in the future. However, they were able
to evaluate the Gibbs free energy change for the reaction

5

6

7

-1

0

1

2

3

4

5

6

7

pCH

pCH
-2
-3

5.0 M NaCl
3-day
11-day-4

TcCl 40

Figure 5. Plots of log10 (CTc(IV)/mol·kg−1) against the pH at different
concentrations of added NaCl from the study of Hess et al.36 whereCTc(IV) is the total concentration of dissolved Tc(IV) and 0.02 mol·L −1
hydrazine was present to maintain reducing conditions. Solid curve:
comparison of these experimental measurements against model predictions made using the model of Hess et al. with inclusion of chloride complexes. This is part of Figure 9 from Journal of Solution
Chemistry, volume 33, pages 199–226, 2004, N. J. Hess, Y. Xia, D.
Rai, and S. D. Conradson, Thermodynamic Model for the Solubilityof TcO2·xH 2O(am) in the Aqueous Tc(IV)–Na + –Cl− –H + –OH− –H2O
System, ©2004 Plenum Publishing Corporation, with kind permission from Springer Science and Business Media.

29-dayModel

TcCl 62-

-5
-6

TcO2+

-7
-8

TcO(OH) 20-9

TcO(OH) +TcO2 • 1.6H 2O(am)TcCl 4(am)

-10
-1

0

1

2

3

pCH

4

5

6

7

J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 203

Current Status of the Thermodynamic Data

TABLE 4: Solid and Gaseous Technetium Compounds andPertechnetate Salts with Assessed Standard Thermodynamic
Quantities at 298.15 K and 0.1 MPa (1 bar) a
Substance∆fGm̊

∆f Hm̊

Sm̊

Cp̊,mTc(cr)Tc(g)
TcO(g)
TcO2 (cr)
Tc2O7(s)Tc2O7(g)Tc2O7·H2O(s) b
TcO2 ·1.6H 2O(am)NaTcO4 ·4H 2O(s) c
KTcO4 (cr)CsTcO4 (cr)TlTcO4 (cr)AgTcO4 (cr)
NH4TcO4 (cr)TcF6 (cr, cubic)
TcF6 (g)TcO3F(g)TcO3Cl(g)TcC(g)TcS(g)

X
X
X
X
X
X
X
X
X
X
X
X
X
X

X
X
X
X
X
X
X

X
X
X
X
X
X
X

X
X
X

X

X

X
X

X

X
X

X
X

X
X
X
X
X

TABLE 5: Aqueous Technetium Species with AssessedStandard Thermodynamic Quantities at 298.15 K and 0.1
MPa (1 bar) a
Species∆fGm̊

∆f Hm̊

Sm̊

Cp̊,m

TcO4−
TcO42−TcO2+
TcO(OH) +TcO(OH) 2
TcO(OH)3−
Tc(CO3)(OH) 2
Tc(CO3)(OH)3−
TcCl4

X
X
Xb
X
X
X
X
X
X

X

X

X

a

Most of the information in this table was taken from Reference 5,
which should be consulted for references and numerical values. An
alternative value of ∆f Gm̊ for TcO(OH) + is given by Hess et al.36 who
also reported the value for TcCl4 (aq). An X denotes that an evaluated value is available, whereas a blank denotes that no reliable value
is available. b Only a lower bound is available for ∆f Gm̊ of this ion.

TABLE 6: Experimentally Based Ion-interaction Model(Pitzer’s Model) Parameters for Aqueous Pertechnetate
Electrolytes at 298.15 K

HTcO 4 (aq)Cφ/mol–2 ·kg2

β (0) /
mol–1·kg

β (1) /
mol–1·kg

0.078540.256850.01839 Rard and Miller41

Reference

0.011110.159540.00236 Könnecke et al.42c–0.0578

0.006

0Neck et al.43, 44

CsTcO 4 (aq) c–0.1884–0.15880Könnecke et al.42

Mg(TcO 4) 2 (aq) a

0.31381.840

0.0114

Neck et al.43, 44c0.2964

1.661

0Neck et al.44

NaTcO 4 (aq)KTcO 4 (aq)a,bCa(TcO 4) 2 (aq)a

θ Cl,TcO4 / θ SO4,TcO4 / ψ M,Cl,TcO4 / ψ M,SO4,TcO4 /mol–1·kg mol–1·kg mol–2 ·kg2 mol–2 ·kg2

TcO 4−

0.067

Reference
Könnecke et al.42
Neck et al.43

0.179

Na +, TcO 4−–0.0085–0.003Könnecke et al.,42
Neck et al.43

K+, TcO 4−–0.011

0.002

Neck et al.43

Cs +,TcO 4−–0.0011

0.024

Könnecke et al.,42
Neck et al.43

Mg2+, TcO 4−–0.0115–0.030

Neck et al.43

Ca 2+, TcO 4−–0.033Neck et al.44

The value ψ Na,Mg,TcO4 = –0.020 mol–2·kg2 is also given in Reference 43.

References
X
X
X
X
X
X

The information in this table was taken from Reference 5, whichshould be consulted for references and numerical values. An X
denotes that an evaluated value is available, whereas a blank denotesthat no reliable value is available. b The simplest empirical formulaof this substance is Tc2O7·H2O(s), but it is probably polymeric. c The
degree of hydration of this solid phase is uncertain, but it is probably
the tetrahydrate.

a

ions

a

a

SoluteTABLE 7: Experimentally Based Ion-interaction Model(Pitzer’s Model) Parameters for Aqueous Mixtures
Containing the Pertechnetate Ion at 298.15 Ka

Based on isopiestic experiments. b Also see Rard and Miller.41 c Basedon solubility measurements.

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