Technetium

group 7 · period 5 · d-block · transition metal

full Most contaminants are slowed on their way through an aquifer because something holds them back: a charge, a surface, a mineral that will take them in. Technetium-99 is one of the most mobile radionuclides in water precisely because almost nothing does. In oxic water it is the pertechnetate anion TcO₄⁻, large, singly charged and weakly hydrated, and so barely sorbed at the pH of ordinary groundwater that it is carried at very nearly the speed of the water itself; decay is no help either, because the half life is far longer than the time any plume takes to move. That is why the isotope defines the groundwater plumes under Hanford and the other fuel cycle sites. Anything done about it has to change either the charge or the oxidation state: reduce it with Fe(II) or sulfide to a Tc(IV) oxide of very low solubility, remembering that the return of oxygen can undo that, or catch the anion on an exchanger against the nitrate, sulfate and chloride competing for the same site. Technetium has no stable isotope and no chemical limit, so what stands in place of one is radiological: a WHO guidance level of 100 Bq/L and a US derived concentration of 900 pCi/L.

Typical wastewaters

nuclear fuel reprocessing and uranium enrichment plant discharges
technetium-99 as pertechnetate TcO₄⁻, weakly sorbed and mobile
nuclear site groundwater pump and treat (Hanford tank leaks and disposal cribs)
pertechnetate TcO₄⁻ in oxic groundwater above the 900 pCi/L standard in plume areas
alkaline nuclear tank waste streams (Hanford, Savannah River)
TcO₄⁻ in 1 M sodium hydroxide with nitrate, sulfate and chloride competing
removal work uses perrhenate as the surrogate
hospital effluent and sewage (radiopharmaceuticals)
technetium-99m from medical imaging, decaying with a 6 hour half-life; species not given by the source

1 · Identity

Symbol, number: Tc, 43
Oxidation states in water: +7 as pertechnetate TcO₄⁻ in oxic water; +4 as TcO₂ hydrate (and Tc(IV) sulfides) under reducing conditions
Note: The element chapter covers the isotopes and the pertechnetate chemistry. Tc-99 is a pure beta emitter with a 210,000 year half-life; Tc-99m (6 h) from medical imaging reaches hospital effluent and sewage but decays away.

2 · Occurrence in water

Natural sources: Negligible: traces from spontaneous fission of uranium (element chapter); the EPA fact sheet notes very small natural amounts in the crust and very low concentrations in air, seawater, soils and biota from fallout.
Anthropogenic sources: Fission product of uranium and plutonium in reactors; atmospheric weapons tests; fuel reprocessing and uranium enrichment plant discharges; groundwater beneath uranium processing facilities at US federal sites; Tc-99m in hospital and laboratory effluent. At Hanford the isotope is present in the vadose zone and groundwater from tank leaks and disposal cribs.

matrix	typical range	note
contaminated groundwater, Hanford	above the 900 pCi/L standard in plume areas pCi/Lone site; maxima not quoted	Washington Ecology lists Tc-99 among the eight main Hanford groundwater contaminants, water soluble and highly mobile, with limited methods to capture it in the environment
contaminated vadose zone sediment, Hanford	34 to 3800 pCi/gsediment, not water	range of the sediments used in the PNNL gas-phase treatment tests

3 · Speciation

In oxic water technetium is pertechnetate, TcO₄⁻, a large weakly hydrated anion that is weakly sorbed at neutral and alkaline pH and moves with the water. Under anoxic conditions Fe(II) and sulfide reduce Tc(VII) to Tc(IV), which precipitates as a hydrous TcO₂ phase (and sulfides where sulfide is present) of very low solubility; re-oxidation on return of oxygen remobilises it, so immobilisation may not be permanent. Organic rich soils retain it.

condition	dominant species	note
oxic groundwater and surface water, pH 6 to 9	TcO₄⁻	highly mobile; radioactive decay does not attenuate a plume over any practical time
anoxic, Fe(II) or sulfide bearing sediment	TcO₂.nH₂O (s), Tc(IV) sulfides	immobilised; the basis of reductive treatment
organic rich soil	retained Tc, not characterised in the fact sheet	EPA: retained and not highly mobile

Solubility: Pertechnetate salts are freely soluble; TcO₂ hydrate is very insoluble.
Hydrolysis: None for TcO₄⁻; Tc(IV) is fully hydrolysed.
Complexation: No complexation with natural ligands is described here; on an exchanger the anion competes with nitrate, sulfate and chloride.
Precipitates: TcO₂.nH₂O, Tc(IV) sulfide.

\ce{TcO4^- + 4 H+ + 3 e- -> TcO2 (s) + 2 H2O}

the Tc(VII) to Tc(IV) half reaction that every reductive treatment rests on; it runs once the redox potential falls into the iron reducing and sulfate reducing range of an anoxic aquifer, and reverses when air returns; written as the electron balance of the reduction the PNNL report describes, which prints no standard potential

\ce{TcO4^- + 3 Fe^2+ + 7 H2O -> TcO2 (s) + 3 Fe(OH)3 (s) + 5 H+}

reduction of pertechnetate by aqueous or mineral Fe(II) to a hydrous Tc(IV) oxide, the reaction described for anoxic Hanford sediments and cited by the PNNL report (Zachara et al. 2007); balanced statement of that redox pair, the hydration water of the oxide omitted

\ce{4 TcO2 (s) + 3 O2 + 4 OH^- -> 4 TcO4^- + 2 H2O}

re-oxidation of the reduced solid when oxygen returns, the reason reductive immobilisation may not be permanent; alkaline to circumneutral water; electron balance of the re-oxidation the PNNL report names, not printed there

4 · Role in treatment

as a problem

pertechnetate plumes in groundwater

weak sorption of the anion, no decay, so the plume travels at groundwater velocity

Hanford 200 Area groundwater is pumped and treated; the vadose zone is the long term source

5 · Removal and control

in situ reductive immobilisation with hydrogen sulfide and ammonia gases (vadose zone)

hydrogen sulfide reduces Tc(VII) to Tc(IV); ammonia vapour dissolves sediment aluminosilicates so that precipitates coat and enclose the reduced technetium

laboratory tests on Hanford sediments at 34 to 3800 pCi/g, 1 to 8 percent moisture, gas delivery of minutes to hours, sequential or parallel gases

Efficiency: a large fraction of the treated Tc-99 resisted leaching in saturated column tests
Interferences: re-oxidation on return of air; alkaline co-contaminants

anion exchange and selective anion sorbents (pump and treat, waste streams)

TcO₄⁻ exchanges onto quaternary ammonium, imidazolium or other cationic frameworks; perrhenate is the non-radioactive surrogate in development work

\ce{RCl + TcO4^- -> RTcO4 + Cl^-}

a cationic polymer sorbent reached 1052 mg/g perrhenate after 24 h in 1 M sodium hydroxide and removed TcO4^- rapidly from simulated Hanford and Savannah River waste; a zinc oxide biochar composite took 25,916 mg/kg perrhenate with selectivity over iodide, nitrite, nitrate, sulfate and phosphate; the exchange is written in the usual resin notation (R a quaternary ammonium site) for a chloride form bed, the sources describe the sorbents and capacities rather than the exchange step

Efficiency: as quoted for the surrogate; no plant performance figure read
Interferences: nitrate, sulfate and chloride compete; radiolysis and high alkalinity degrade organic resins

zero valent iron and Fe(II) reduction in reactive barriers

reduction to Tc(IV) oxide and sulfide on iron surfaces

\ce{3 Fe (s) + 2 TcO4^- + 8 H+ -> 3 Fe^2+ + 2 TcO2 (s) + 4 H2O}

anoxic; sulfidised iron holds the reduced technetium longer; the stoichiometry is the electron balance for the reduction to Tc(IV) oxide that the PNNL report describes, not printed there

Efficiency: not recorded
Interferences: oxygen re-mobilises Tc(IV)

6 · Analytics

method	standard	detection limit	note
liquid scintillation counting after separation	ISO 22125-1:2019	about 5 to 20 Bq/kg with 30 min counting of 14 to 40 mL	all water types: drinking, rain, surface, ground, cooling, industrial and domestic wastewater
ICP-MS after separation	ISO 22125-2:2019	not recorded	catalogue entry only; the separation chemistry and the handling of interferences are not described here

Sampling pitfalls: Keep the sample oxic: pertechnetate is dissolved and passes filters, but a sample that goes anoxic in the bottle loses technetium to the walls as Tc(IV). Ruthenium and molybdenum isobars must be separated before ICP-MS.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

drinking water

body	limit	note
WHO GDWQ Table A₆.1	100 Bq/L	technetium-99 guidance level at 0.1 mSv per year; Tc-99m is not listed in the rows read
US EPA NPDWR	900 pCi/L	derived concentration of Tc-99 assumed to yield the 4 mrem per year beta and photon MCL when Tc-99 is the only such nuclide (33 Bq/L, the writer's conversion); with other beta emitters the summed dose applies
US EPA 40 CFR 141.66	4 mrem per year	MCL for beta particle and photon radioactivity from man-made radionuclides; the section read lists tritium 20,000 and strontium-90 8 pCi/L, not technetium
EU ¹	0.1 mSv per year	indicative dose parametric value; gross beta screening 1.0 Bq/L; Tc-99 is not among the derived concentrations listed in the annex read
EU DWD 2020/2184	not set	radioactivity is left to Directive 2013/51/Euratom

discharge

body	limit	note
EU CWW BREF BAT-AEL ², BAT 12	not set	radionuclides are outside the CWW BAT conclusions

8 · Health and environmental effects

Toxicity: Radiological, not chemical: Tc-99 is a weak beta emitter, so external exposure is minimal and ingestion of contaminated water and plants is the route; once absorbed it concentrates in the thyroid and gastrointestinal tract and is excreted steadily (half in about 60 hours, Washington Ecology). Risk is an increased chance of cancer.
Bioaccumulation: Under aerobic conditions technetium in soil transfers readily to plants; brown algae concentrate Tc-99 from seawater and it transfers to marine animals ³.

Read these with care

Hanford groundwater maxima were not read; only the standard and the qualitative plume statements are quoted.
The pertechnetate sorbent capacities are perrhenate surrogate figures from laboratory materials, not plant performance.
The Fe(II) reduction equation is the writer's balanced statement of the redox pair described in the PNNL report; the original paper was not read.
The 900 pCi/L to 33 Bq/L conversion is the writer's arithmetic.

What this chapter does not cover

Tc-99 concentrations in groundwater (pCi/L) at Hanford or any other site were not read from a primary source.
Kd values for pertechnetate were not read.
Tc-99m in hospital effluent and sewage was not read.
The Euratom Annex III list read did not include Tc-99; the full annex may.
No GCC standard was read.
The oxidation product of hydrogen sulfide in the gas phase reduction of pertechnetate is not stated by the PNNL report, so no equation is written for that step.
Complexation of pertechnetate with natural ligands, the separation chemistry behind the mass spectrometric method, and any ecotoxicity test for technetium were not read.
Ecotoxicity: No ecotoxicity data for technetium are quoted in this chapter.

Notes

Euratom Directive 2013/51
Decision 2016/902
EPA fact sheet

Sources

US EPA, EPA Facts About Technetium-99 4: sources, half-life 210,000 years, environmental behaviour, 4 mrem per year MCL and the 900 pCi/L derived concentration
Washington State Department of Ecology, Hanford cleanup: groundwater contaminants (technetium-99 entry: 900 pCi/L standard, mobility, half-life, health effects)
Truex, M. J., Szecsody, J. E., Zhong, L. and Qafoku, N., Gas-Phase Treatment of Technetium in the Vadose Zone at the Hanford Site Central Plateau, PNNL-23665 / RPT-DVZ-AFRI-023 (September 2014), summary and introduction
Creation of cationic polymeric nanotrap featuring high anion density and exceptional alkaline stability for highly efficient pertechnetate removal from nuclear waste streams, ACS Central Science (2024) (open access, PMC10906250); and Synthesis of ZnO nanoparticle-anchored biochar composites for the selective removal of perrhenate, a surrogate for pertechnetate, Journal of Hazardous Materials (2020) (abstract, PubMed 31761646)
ISO 22125-1:2019, Water quality. Technetium-99. Part 1: Test method using liquid scintillation counting (scope and detection limit from the catalogue abstract)
ISO 22125-2:2019, Water quality. Technetium-99. Part 2: Test method using inductively coupled plasma mass spectrometry (catalogue entry)
40 CFR 141.66 Maximum contaminant levels for radionuclides (beta particle and photon radioactivity 4 mrem/year; gross alpha 15 pCi/L; combined radium 5 pCi/L; uranium 30 µg/L)
WHO Guidelines for Drinking-water Quality, 4th ed. incorporating the first and second addenda (2022), Annex 6 Table A6.1 guidance levels for radionuclides in drinking-water (individual dose criterion 0.1 mSv per year, levels rounded to the nearest order of magnitude)
Council Directive 2013/51/Euratom on radioactive substances in water intended for human consumption, Annex I (radon 100 Bq/l, tritium 100 Bq/l, indicative dose 0.1 mSv), Annex III screening (gross alpha 0.1 Bq/l, gross beta 1.0 Bq/l) and derived concentrations (annexes read on legislation.gov.uk)
Directive (EU) 2020/2184 on the quality of water intended for human consumption (recast), Annex I Parts B, C and D (annexes read on legislation.gov.uk)
US EPA, National Primary Drinking Water Regulations (table of MCLs; inorganic chemicals and radionuclides; beta particle and photon emitters 4 millirem per year)
Commission Implementing Decision (EU) 2016/902 establishing BAT conclusions for common waste water and waste gas treatment/management systems in the chemical sector (CWW), BAT 12 Tables 1 to 3 (TOC, COD, TSS, TN, Ninorg, TP, AOX, Cr, Cu, Ni, Zn) (annex read on legislation.gov.uk)
The Element Book, element chapter and reference text for Tc (isotopes, pertechnetate chemistry, medical Tc-99m, corrosion inhibition)

Identity

Name and symbol: Technetium, Tc
Atomic number: 43 protons
Position: group 7 · period 5 · d-block · transition metal
CAS number: 7440-26-8

Atomic structure

Atomic mass: 98 u
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d⁵
[Kr] 5s²⁴d⁵
Electrons per shell: 2, 8, 18, 13, 2
Valence electrons: 7 ns and (n-1)d

nuclide	half-life	decay
⁹⁷Tc	4.21 My	ε=100%
⁹⁸Tc	4.2 My	β-=100%; β+=0%
⁹⁹Tc	211.1 ky	β-=100%
⁹⁷Tcm	91.1 d	IT=96.06±1.8%; ε=3.94±1.8%

no stable isotope; the longest-lived nuclides

Physical properties

State at room temperature: Solid
Melting point: 2,430 K (2,156.85 °C)
Boiling point: 4,538 K (4,264.85 °C)
Density: 11 g/cm3
Appearance: shiny gray metal
Thermal conductivity: 50.6 W/(m·K)
Electrical resistivity: 200 nΩ·m at 20 °C
Electrical conductivity: 5 MS/m
Crystal structure: hexagonal close packed
Molar heat capacity: 24.27 J/(mol·K)

Chemical properties

Oxidation states: +7, +6, +4
Electronegativity: 1.9 (Pauling Scale)
Ionisation energy: 7.28 eV
1st 702, 2nd 1,470, 3rd 2,850 kJ/mol
Electron affinity: 0.55 eV
Atomic radius: empirical 147, covalent 147, van der Waals 209 pm
Ionic radius: Tc⁴⁺ 65; Tc⁵⁺ 60; Tc⁷⁺ 56 pm

Reactivity: A radioactive group 7 metal whose chemistry resembles rhenium: it tarnishes slowly in moist air, is dissolved by oxidising acids but not hydrochloric acid, and under oxidising conditions exists as the pertechnetate ion TcO4-.
with water: Does not react with water; the pertechnetate ion is its stable form in aerated water.
with oxygen, air: Tarnishes slowly in moist air; the powder burns in oxygen to the volatile heptoxide: $\ce{4 Tc + 7 O2 -> 2 Tc2O7}$ .
with acids: Dissolves in nitric acid, aqua regia and concentrated sulfuric acid, but not in hydrochloric acid of any strength.
with halogens: Combines with the halogens on heating; fluorine gives the hexafluoride: $\ce{Tc + 3 F2 -> TcF6}$ , and chlorine the tetrachloride: $\ce{Tc + 2 Cl2 -> TcCl4}$ .

Typical compounds

KTcO₄ potassium pertechnetate
pertechnetate, the common Tc(VII) form; steel corrosion inhibitor
Tc₂O₇ technetium heptoxide
pale yellow volatile oxide, hydrolyses to pertechnetic acid
TcO₂ technetium dioxide
reduction product of the heptoxide
TcF₆ technetium hexafluoride
highest binary fluoride
TcCl₄ technetium tetrachloride
chain-structured chloride
Tc₂S₇ technetium heptasulfide
precursor to the disulfide

Occurrence, production and use

Crustal abundance: Not Applicable
Oceanic abundance: Not Applicable

Occurrence and sources

trace fission product in uranium ores
about 1 milligram per tonne of uranium
synthesis
separated from fission products of spent nuclear fuel in tonne quantities

Extraction, production: Separation from spent uranium fuel
no reaction stated by the source

Uses

Small amounts of technetium can retard the corrosion of steel, although this protection can only be applied to closed systems due to technetium's radioactivity. Technetium can also be used as a medical tracer and to calibrate particle detectors.

Medicine: technetium-99m radiopharmaceuticals for diagnostic imaging of many organs
Nuclear (closed systems): pertechnetate as a corrosion inhibitor for steel where radioactivity can be contained

Safety, toxicity: It is reported that mild carbon steels may be effectively protected by as little as 55 ppm of KTcO4 in aerated distilled water at temperatures up to 250°C. This corrosion protection is limited to closed systems, since technetium is radioative and must be confined. 98Tc has a specific activity of 6.2 x 108 Bq/g. Activity of this level must not be allowed to spread. 99Tc is a contamination hazard and should be handled in a glove box.

Discovery and name

Discovered by: Carlo Perrier and Emilio Segrè
Discovered: 1937
First isolated: not in sources
Named by: not in sources
Origin of the name: after Greek τεχνητός, 'artificial', for being the first artificially produced element

Technetium is a silvery-gray metal that tarnishes slowly in moist air. The common oxidation states of technetium are +7, +5, and +4. Under oxidizing conditions technetium (VII) will exist as the pertechnetate ion, TcO4-. The chemistry of technetium is said to be similar to that of rhenium. Technetium dissolves in nitric acid, aqua regia, and concentrated sulfuric acid, but is not soluble in hydrochloric acid of any strength. The element is a remarkable corrosion inhibitor for steel. The metal is an excellent superconductor at 11K and below.

Conventions

Valence electrons are counted by the usual convention: the outer shell for s- and p-block elements, ns and (n-1)d for the d-block, ns, (n-1)d and (n-2)f for the f-block. Lanthanides and actinides are placed in the f-block with no group number. Electrical conductivity is the reciprocal of the printed resistivity. Ionic radii are Shannon effective radii, six-coordinate unless noted. Where a field reads “not in sources” the value was not found; it is a gap, not a zero. Regulatory limits are the published values and change often, so check the standard in force at your site and the numbers written into your own permit before you design to them.

Data

Element records, isotopes, radii and the descriptive text come from PubChem (NCBI), the Los Alamos National Laboratory periodic table, IUPAC CIAAW and the IAEA Atomic Mass Data Center. Appearance, thermal conductivity, electrical resistivity, crystal structure, discovery and the origin of each name come from Wikipedia and Periodic-Table-JSON, used under CC BY-SA 4.0. Hazard classifications come from the ECHA C&L inventory via PubChem. Ionic radii follow R. D. Shannon (1976). The water chapters cite their own sources at the foot of each entry, and are written to the level of Snoeyink and Jenkins, Stumm and Morgan, MWH's Water Treatment and Metcalf and Eddy.