Technetium

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 plume maxima not read
• 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 from the element entry; not read from a water 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 entry 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 entry); 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.

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.

Solubility

Pertechnetate salts are freely soluble; TcO₂ hydrate is very insoluble.

Hydrolysis

None for TcO₄⁻; Tc(IV) is fully hydrolysed.

Complexation

Not read for natural ligands; the anion competes with nitrate, sulfate and chloride on exchangers.

Precipitates

TcO₂.nH₂O, Tc(IV) sulfide.

4 · Role in treatment

5 · Removal and control

6 · Analytics

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.

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 (EPA fact sheet).

Ecotoxicity

Not read.

Flags

• 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.

Gaps

• 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.

Sources

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

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.