======================================================================
= Technetium =
======================================================================
Introduction
======================================================================
Technetium is a chemical element; it has symbol Tc and atomic number
43. It is the lightest element whose isotopes are all radioactive.
Technetium and promethium are the only radioactive elements whose
neighbours in the sense of atomic number are both stable. All
available technetium is produced as a synthetic element. Naturally
occurring technetium is a spontaneous fission product in uranium ore
and thorium ore (the most common source), or the product of neutron
capture in molybdenum ores. This silvery gray, crystalline transition
metal lies between manganese and rhenium in group 7 of the periodic
table, and its chemical properties are intermediate between those of
both adjacent elements. The most common naturally occurring isotope is
99Tc, in traces only.
Many of technetium's properties had been predicted by Dmitri Mendeleev
before it was discovered; Mendeleev noted a gap in his periodic table
and gave the undiscovered element the provisional name 'ekamanganese'
('Em'). In 1937, technetium became the first predominantly artificial
element to be produced, hence its name (from the Greek ',
'artificial', +
One short-lived gamma ray-emitting nuclear isomer, technetium-99m, is
used in nuclear medicine for a wide variety of tests, such as bone
cancer diagnoses. The ground state of the nuclide technetium-99 is
used as a gamma ray-free source of beta particles. Long-lived
technetium isotopes produced commercially are byproducts of the
fission of uranium-235 in nuclear reactors and are extracted from
nuclear fuel rods. Because even the longest-lived isotope of
technetium has a relatively short half-life (4.21 million years), the
1952 detection of technetium in red giants helped to prove that stars
can produce heavier elements.
Early assumptions
===================
From the 1860s through 1871, early forms of the periodic table
proposed by Dmitri Mendeleev contained a gap between molybdenum
(element 42) and ruthenium (element 44). In 1871, Mendeleev predicted
this missing element would occupy the empty place below manganese and
have similar chemical properties. Mendeleev gave it the provisional
name 'eka-manganese' (from 'eka', the Sanskrit word for 'one') because
it was one place down from the known element manganese.
Early misidentifications
==========================
Many early researchers, both before and after the periodic table was
published, were eager to be the first to discover and name the missing
element. Its location in the table suggested that it should be easier
to find than other undiscovered elements. This turned out not to be
the case, due to technetium's radioactivity.
Year Claimant Suggested name Actual material
|1828 |Gottfried Osann |Polinium |Iridium
|1845 |Heinrich Rose |Pelopium |Niobium-tantalum alloy
|1847 |R. Hermann |Ilmenium |Niobium-tantalum alloy
|1877 |Serge Kern |Davyum |Iridium-rhodium-iron alloy
|1896 |Prosper Barrière |Lucium |Yttrium
|1908 |Masataka Ogawa |Nipponium |Rhenium, which was the unknown
dvi-manganese
Irreproducible results
========================
German chemists Walter Noddack, Otto Berg, and Ida Tacke reported the
discovery of element 75 and element 43 in 1925, and named element 43
'masurium' (after Masuria in eastern Prussia, now in Poland, the
region where Walter Noddack's family originated). This name caused
significant resentment in the scientific community, because it was
interpreted as referring to a series of victories of the German army
over the Russian army in the Masuria region during World War I; as the
Noddacks remained in their academic positions while the Nazis were in
power, suspicions and hostility against their claim for discovering
element 43 continued. The group bombarded columbite with a beam of
electrons and deduced element 43 was present by examining X-ray
emission spectrograms. The wavelength of the X-rays produced is
related to the atomic number by a formula derived by Henry Moseley in
1913. The team claimed to detect a faint X-ray signal at a wavelength
produced by element 43. Later experimenters could not replicate the
discovery, and it was dismissed as an error. Still, in 1933, a series
of articles on the discovery of elements quoted the name 'masurium'
for element 43. Some more recent attempts have been made to
rehabilitate the Noddacks' claims, but they are disproved by Paul
Kuroda's study on the amount of technetium that could have been
present in the ores they studied: it could not have exceeded of ore,
and thus would have been undetectable by the Noddacks' methods.
Official discovery and later history
======================================
The discovery of element 43 was finally confirmed in a 1937 experiment
at the University of Palermo in Sicily by Carlo Perrier and Emilio
Segrè. In mid-1936, Segrè visited the United States, first Columbia
University in New York and then the Lawrence Berkeley National
Laboratory in California. He persuaded cyclotron inventor Ernest
Lawrence to let him take back some discarded cyclotron parts that had
become radioactive. Lawrence mailed him a molybdenum foil that had
been part of the deflector in the cyclotron.
Segrè enlisted his colleague Perrier to attempt to prove, through
comparative chemistry, that the molybdenum activity was indeed from an
element with the atomic number 43, which they did. University of
Palermo officials wanted them to name their discovery , after the
Latin name for Palermo, '. In 1947, element 43 was named after the
Greek word (), meaning 'artificial', since it was the first element
to be artificially produced.
Segrè returned to Berkeley and met Glenn T. Seaborg. They isolated the
metastable isotope technetium-99m, which is now used in some ten
million medical diagnostic procedures annually.
In 1952, the astronomer Paul W. Merrill detected the spectral
signature of technetium (specifically wavelengths of 403.1 nm, 423.8
nm, 426.2 nm, and 429.7 nm) in light from S-type red giants. The stars
were near the end of their lives but were rich in the short-lived
element, which indicated that it was being produced in the stars by
nuclear reactions. That evidence bolstered the hypothesis that heavier
elements are the product of nucleosynthesis in stars. More recently,
such observations provided evidence that elements are formed by
neutron capture in the s-process.
Since that discovery, there have been many searches in terrestrial
materials for natural sources of technetium. In 1962, technetium-99
was isolated and identified in pitchblende from the Belgian Congo in
very small quantities (about 0.2 ng/kg), where it originates as a
spontaneous fission product of uranium-238. The natural nuclear
fission reactor in Oklo contains evidence that significant amounts of
technetium-99 were produced and have since decayed into ruthenium-99.
Physical properties
=====================
Technetium is a silvery-gray radioactive metal with an appearance
similar to platinum, commonly obtained as a gray powder. The crystal
structure of the bulk pure metal is hexagonal close-packed. Atomic
technetium has characteristic emission lines at wavelengths of 363.3
nm, 403.1 nm, 426.2 nm, 429.7 nm, and 485.3 nm. The unit cell
parameters of the orthorhombic Tc metal were reported when Tc is
contaminated with carbon ( = 0.2805(4), = 0.4958(8), = 0.4474(5)·nm
for Tc-C with 1.38 wt% C and = 0.2815(4), = 0.4963(8), =
0.4482(5)·nm for Tc-C with 1.96 wt% C ). The metal form is slightly
paramagnetic, meaning its magnetic dipoles align with external
magnetic fields, but will assume random orientations once the field is
removed. Pure, metallic, single-crystal technetium becomes a type-II
superconductor at temperatures below 7.46 K.
Below this temperature, technetium has a very high magnetic
penetration depth, greater than any other element except niobium.
Chemical properties
=====================
Technetium is located in group 7 of the periodic table, between
rhenium and manganese. As predicted by the periodic law, its chemical
properties are between those two elements. Of the two, technetium more
closely resembles rhenium, particularly in its chemical inertness and
tendency to form covalent bonds. This is consistent with the tendency
of period 5 elements to resemble their counterparts in period 6 more
than period 4 due to the lanthanide contraction. Unlike manganese,
technetium does not readily form cations (ions with net positive
charge). Technetium exhibits nine oxidation states from −1 to +7, with
+4, +5, and +7 being the most common. Technetium dissolves in aqua
regia, nitric acid, and concentrated sulfuric acid, but 'not' in
hydrochloric acid of any concentration.
Metallic technetium slowly tarnishes in moist air and, in powder form,
burns in oxygen. When reacting with hydrogen at high pressure, it
forms the non-stoichiometric hydride TcH and while reacting with
carbon it forms TcC, with cell parameter 0.398 nm.
Technetium can catalyse the destruction of hydrazine by nitric acid,
and this property is due to its multiplicity of valencies. This caused
a problem in the separation of plutonium from uranium in nuclear fuel
processing, where hydrazine is used as a protective reductant to keep
plutonium in the trivalent rather than the more stable tetravalent
state. The problem was exacerbated by the mutually enhanced solvent
extraction of technetium and zirconium at the previous stage, and
required a process modification.
Pertechnetate and other derivatives
=====================================
The most prevalent form of technetium that is easily accessible is
sodium pertechnetate, Na[TcO4]. The majority of this material is
produced by radioactive decay from [99MoO4]2−:
Pertechnetate () is only weakly hydrated in aqueous solutions, and it
behaves analogously to perchlorate anion, both of which are
tetrahedral. Unlike permanganate (), it is only a weak oxidizing
agent.
Related to pertechnetate is technetium heptoxide. This pale-yellow,
volatile solid is produced by oxidation of Tc metal and related
precursors:
It is a molecular metal oxide, analogous to manganese heptoxide. It
adopts a centrosymmetric structure with two types of Tc−O bonds with
167 and 184 pm bond lengths.
Technetium heptoxide hydrolyzes to pertechnetate and pertechnetic
acid, depending on the pH:
HTcO4 is a strong acid. In concentrated sulfuric acid, [TcO4]−
converts to the octahedral form TcO3(OH)(H2O)2, the conjugate base of
the hypothetical triaquo complex [TcO3(H2O)3]+.
Other chalcogenide derivatives
================================
Technetium forms a dioxide, disulfide, diselenide, and ditelluride. An
ill-defined Tc2S7 forms upon treating pertechnate with hydrogen
sulfide. It thermally decomposes into disulfide and elemental sulfur.
Similarly the dioxide can be produced by reduction of the Tc2O7.
Unlike the case for rhenium, a trioxide has not been isolated for
technetium. However, TcO3 has been identified in the gas phase using
mass spectrometry.
Simple hydride and halide complexes
=====================================
Technetium forms the complex . The potassium salt is isostructural
with Potassium nonahydridorhenate. At high pressure formation of
TcH1.3 from elements was also reported.
The following binary (containing only two elements) technetium halides
are known: TcF6, TcF5, TcCl4, TcBr4, TcBr3, α-TcCl3, β-TcCl3, TcI3,
α-TcCl2, and β-TcCl2. The oxidation states range from Tc(VI) to
Tc(II). Technetium halides exhibit different structure types, such as
molecular octahedral complexes, extended chains, layered sheets, and
metal clusters arranged in a three-dimensional network. These
compounds are produced by combining the metal and halogen or by less
direct reactions.
TcCl4 is obtained by chlorination of Tc metal or Tc2O7. Upon heating,
TcCl4 gives the corresponding Tc(III) and Tc(II) chlorides.
The structure of TcCl4 is composed of infinite zigzag chains of
edge-sharing TcCl6 octahedra. It is isomorphous to transition metal
tetrachlorides of zirconium, hafnium, and platinum.
Two polymorphs of technetium trichloride exist, α- and β-TcCl3. The α
polymorph is also denoted as Tc3Cl9. It adopts a confacial
bioctahedral structure. It is prepared by treating the chloro-acetate
Tc2(O2CCH3)4Cl2 with HCl. Like Re3Cl9, the structure of the
α-polymorph consists of triangles with short M-M distances. β-TcCl3
features octahedral Tc centers, which are organized in pairs, as seen
also for molybdenum trichloride. TcBr3 does not adopt the structure of
either trichloride phase. Instead it has the structure of molybdenum
tribromide, consisting of chains of confacial octahedra with
alternating short and long Tc--Tc contacts. TcI3 has the same
structure as the high temperature phase of TiI3, featuring chains of
confacial octahedra with equal Tc--Tc contacts.
Several anionic technetium halides are known. The binary tetrahalides
can be converted to the hexahalides [TcX6]2− (X = F, Cl, Br, I), which
adopt octahedral molecular geometry. More reduced halides form anionic
clusters with Tc-Tc bonds. The situation is similar for the related
elements of Mo, W, Re. These clusters have the nuclearity Tc4, Tc6,
Tc8, and Tc13. The more stable Tc6 and Tc8 clusters have prism shapes
where vertical pairs of Tc atoms are connected by triple bonds and the
planar atoms by single bonds. Every technetium atom makes six bonds,
and the remaining valence electrons can be saturated by one axial and
two bridging ligand halogen atoms such as chlorine or bromine.
Coordination and organometallic complexes
===========================================
Technetium forms a variety of coordination complexes with organic
ligands. Many have been well-investigated because of their relevance
to nuclear medicine.
Technetium forms a variety of compounds with Tc-C bonds, i.e.
organotechnetium complexes. Prominent members of this class are
complexes with CO, arene, and cyclopentadienyl ligands. The binary
carbonyl Tc2(CO)10 is a white volatile solid. In this molecule, two
technetium atoms are bound to each other; each atom is surrounded by
octahedra of five carbonyl ligands. The bond length between technetium
atoms, 303 pm, is significantly larger than the distance between two
atoms in metallic technetium (272 pm). Similar carbonyls are formed by
technetium's congeners, manganese and rhenium. Interest in
organotechnetium compounds has also been motivated by applications in
nuclear medicine. Technetium also forms aquo-carbonyl complexes, one
prominent complex being [Tc(CO)3(H2O)3]+, which are unusual compared
to other metal carbonyls.
Isotopes
======================================================================
Technetium, with atomic number 'Z' = 43, is the lowest-numbered
element in the periodic table for which all isotopes are radioactive.
The second-lightest exclusively radioactive element, promethium, has
atomic number 61. Atomic nuclei with an odd number of protons are less
stable than those with even numbers, even when the total number of
nucleons (protons + neutrons) is even, and odd numbered elements have
fewer stable isotopes.
The most stable radioactive isotopes are technetium-97 with a
half-life of million years and technetium-98 with million years;
current measurements of their half-lives give overlapping confidence
intervals corresponding to one standard deviation and therefore do not
allow a definite assignment of technetium's most stable isotope. The
next most stable isotope is technetium-99, which has a half-life of
211,100 years. Thirty-four other radioisotopes have been characterized
with mass numbers ranging from 86 to 122. Most of these have
half-lives that are less than an hour, the exceptions being
technetium-93 (2.73 hours), technetium-94 (4.88 hours), technetium-95
(20 hours), and technetium-96 (4.3 days).
The primary decay mode for isotopes lighter than technetium-98 (98Tc)
is electron capture, producing molybdenum ('Z' = 42). For
technetium-98 and heavier isotopes, the primary mode is beta emission
(the emission of an electron or positron), producing ruthenium ('Z' =
44), with the exception that technetium-100 can decay both by beta
emission and electron capture.
Technetium also has numerous nuclear isomers, which are isotopes with
one or more excited nucleons. Technetium-97m (97mTc; "m" stands for
metastability) is the most stable, with a half-life of 91 days and
excitation energy 0.0965 MeV.
This is followed by technetium-95m (61 days, 0.03 MeV), and
technetium-99m (6.01 hours, 0.142 MeV).
Technetium-99 (99Tc) is a major product of the fission of uranium-235
(235U), making it the most common and most readily available isotope
of technetium. One gram of technetium-99 produces per second (in
other words, the specific activity of 99Tc is 0.62 GBq/g).
Occurrence and production
======================================================================
Technetium occurs naturally in the Earth's crust in minute
concentrations of about 0.003 parts per trillion. Technetium is so
rare because the half-lives of 97Tc and 98Tc are only More than a
thousand of such periods have passed since the formation of the Earth,
so the probability of survival of even one atom of primordial
technetium is effectively zero. However, small amounts exist as
spontaneous fission products in uranium ores. A kilogram of uranium
contains an estimated 1 nanogram , equivalent to ten trillion atoms,
of technetium.
Some red giant stars with the spectral types S, M, and N display a
spectral absorption line indicating the presence of technetium. These
red giants are known informally as technetium stars.
Fission waste product
=======================
In contrast to the rare natural occurrence, bulk quantities of
technetium-99 are produced each year from spent nuclear fuel rods,
which contain various fission products. The fission of a gram of
uranium-235 in nuclear reactors yields 27 mg of technetium-99, giving
technetium a fission product yield of 6.1%. Other fissile isotopes
produce similar yields of technetium, such as 4.9% from uranium-233
and 6.21% from plutonium-239. An estimated 49,000 TBq (78 metric tons)
of technetium was produced in nuclear reactors between 1983 and 1994,
by far the dominant source of terrestrial technetium.
Only a fraction of the production is used commercially.{{efn|
, technetium-99 in the form of ammonium pertechnetate is available to
holders of an Oak Ridge National Laboratory permit.
}}
Technetium-99 is produced by the nuclear fission of both uranium-235
and plutonium-239. It is therefore present in radioactive waste and in
the nuclear fallout of fission bomb explosions. Its decay, measured in
becquerels per amount of spent fuel, is the dominant contributor to
nuclear waste radioactivity after about after the creation of the
nuclear waste. From 1945-1994, an estimated 160 TBq (about 250 kg) of
technetium-99 was released into the environment during atmospheric
nuclear tests.
The amount of technetium-99 from nuclear reactors released into the
environment up to 1986 is on the order of 1000 TBq (about 1600 kg),
primarily by nuclear fuel reprocessing; most of this was discharged
into the sea. Reprocessing methods have reduced emissions since then,
but as of 2005 the primary release of technetium-99 into the
environment is by the Sellafield plant, which released an estimated
550 TBq (about 900 kg) from 1995 to 1999 into the Irish Sea.
From 2000 onwards the amount has been limited by regulation to 90 TBq
(about 140 kg) per year.
Discharge of technetium into the sea resulted in contamination of some
seafood with minuscule quantities of this element. For example,
European lobster and fish from west Cumbria contain about 1 Bq/kg of
technetium.
Fission product for commercial use
====================================
The metastable isotope technetium-99m is continuously produced as a
fission product from the fission of uranium or plutonium in nuclear
reactors:
^{238}_{92}U ->[\ce{sf}] ^{137}_{53}I + ^{99}_{39}Y + 2^{1}_{0}n
^{99}_{39}Y ->[\beta^-][1.47\,\ce{s}] ^{99}_{40}Zr
->[\beta^-][2.1\,\ce{s}] ^{99}_{41}Nb ->[\beta^-][15.0\,\ce{s}]
^{99}_{42}Mo ->[\beta^-][65.94\,\ce{h}] ^{99}_{43}Tc
->[\beta^-][211,100\,\ce{y}] ^{99}_{44}Ru
Because used fuel is allowed to stand for several years before
reprocessing, all molybdenum-99 and technetium-99m is decayed by the
time that the fission products are separated from the major actinides
in conventional nuclear reprocessing. The liquid left after
plutonium-uranium extraction (PUREX) contains a high concentration of
technetium as but almost all of this is technetium-99, not
technetium-99m.
The vast majority of the technetium-99m used in medical work is
produced by irradiating dedicated highly enriched uranium targets in a
reactor, extracting molybdenum-99 from the targets in reprocessing
facilities, and recovering at the diagnostic center the technetium-99m
produced upon decay of molybdenum-99. Molybdenum-99 in the form of
molybdate is adsorbed onto acid alumina () in a shielded column
chromatograph inside a technetium-99m generator ("technetium cow",
also occasionally called a "molybdenum cow"). Molybdenum-99 has a
half-life of 67 hours, so short-lived technetium-99m (half-life: 6
hours), which results from its decay, is being constantly produced.
The soluble pertechnetate can then be chemically extracted by elution
using a saline solution. A drawback of this process is that it
requires targets containing uranium-235, which are subject to the
security precautions of fissile materials.
Almost two-thirds of the world's supply comes from two reactors; the
National Research Universal Reactor at Chalk River Laboratories in
Ontario, Canada, and the High Flux Reactor at Nuclear Research and
Consultancy Group in Petten, Netherlands. All major reactors that
produce technetium-99m were built in the 1960s and are close to the
end of life. The two new Canadian Multipurpose Applied Physics Lattice
Experiment reactors planned and built to produce 200% of the demand of
technetium-99m relieved all other producers from building their own
reactors. With the cancellation of the already tested reactors in
2008, the future supply of technetium-99m became problematic.
Waste disposal
================
The long half-life of technetium-99 and its potential to form anionic
species creates a major concern for long-term disposal of radioactive
waste. Many of the processes designed to remove fission products in
reprocessing plants aim at cationic species such as caesium (e.g.,
caesium-137) and strontium (e.g., strontium-90). Hence the
pertechnetate escapes through those processes. Current disposal
options favor burial in continental, geologically stable rock. The
primary danger with such practice is the likelihood that the waste
will contact water, which could leach radioactive contamination into
the environment. The anionic pertechnetate and iodide tend not to
adsorb into the surfaces of minerals, and are likely to be washed
away. By comparison plutonium, uranium, and caesium tend to bind to
soil particles. Technetium could be immobilized by some environments,
such as microbial activity in lake bottom sediments, and the
environmental chemistry of technetium is an area of active research.
An alternative disposal method, transmutation, has been demonstrated
at CERN for technetium-99. In this process, the technetium
(technetium-99 as a metal target) is bombarded with neutrons to form
the short-lived technetium-100 (half-life = 16 seconds) which decays
by beta decay to stable ruthenium-100. If recovery of usable ruthenium
is a goal, an extremely pure technetium target is needed; if small
traces of the minor actinides such as americium and curium are present
in the target, they are likely to undergo fission and form more
fission products which increase the radioactivity of the irradiated
target. The formation of ruthenium-106 (half-life 374 days) from the
'fresh fission' is likely to increase the activity of the final
ruthenium metal, which will then require a longer cooling time after
irradiation before the ruthenium can be used.
The actual separation of technetium-99 from spent nuclear fuel is a
long process. During fuel reprocessing, it comes out as a component of
the highly radioactive waste liquid. After sitting for several years,
the radioactivity reduces to a level where extraction of the
long-lived isotopes, including technetium-99, becomes feasible. A
series of chemical processes yields technetium-99 metal of high
purity.
Neutron activation
====================
Molybdenum-99, which decays to form technetium-99m, can be formed by
the neutron activation of molybdenum-98. When needed, other technetium
isotopes are not produced in significant quantities by fission, but
are manufactured by neutron irradiation of parent isotopes (for
example, technetium-97 can be made by neutron irradiation of
ruthenium-96).
Particle accelerators
=======================
The feasibility of technetium-99m production with the 22-MeV-proton
bombardment of a molybdenum-100 target in medical cyclotrons following
the reaction 100Mo(p,2n)99mTc was demonstrated in 1971. The recent
shortages of medical technetium-99m reignited the interest in its
production by proton bombardment of isotopically enriched (>99.5%)
molybdenum-100 targets. Other techniques are being investigated for
obtaining molybdenum-99 from molybdenum-100 via (n,2n) or (γ,n)
reactions in particle accelerators.
Nuclear medicine and biology
==============================
Technetium-99m ("m" indicates that this is a metastable nuclear
isomer) is used in radioactive isotope medical tests. For example,
technetium-99m is a radioactive tracer that medical imaging equipment
tracks in the human body. It is well suited to the role because it
emits readily detectable 140 keV gamma rays, and its half-life is 6.01
hours (meaning that about 94% of it decays to technetium-99 in 24
hours). The chemistry of technetium allows it to be bound to a variety
of biochemical compounds, each of which determines how it is
metabolized and deposited in the body, and this single isotope can be
used for a multitude of diagnostic tests. More than 50 common
radiopharmaceuticals are based on technetium-99m for imaging and
functional studies of the brain, heart muscle, thyroid, lungs, liver,
gall bladder, kidneys, skeleton, blood, and tumors.
The longer-lived isotope, technetium-95m with a half-life of 61 days,
is used as a radioactive tracer to study the movement of technetium in
the environment and in plant and animal systems.
Industrial and chemical
=========================
Technetium-99 decays almost entirely by beta decay, emitting beta
particles with consistent low energies and no accompanying gamma rays.
Moreover, its long half-life means that this emission decreases very
slowly with time. It can also be extracted to a high chemical and
isotopic purity from radioactive waste. For these reasons, it is a
U.S. National Institute of Standards and Technology (NIST) standard
beta emitter, and is used for equipment calibration. Technetium-99 has
also been proposed for optoelectronic devices and nanoscale nuclear
batteries.
Like rhenium and palladium, technetium can serve as a catalyst. In
processes such as the dehydrogenation of isopropyl alcohol, it is a
far more effective catalyst than either rhenium or palladium. However,
its radioactivity is a major problem in safe catalytic applications.
When steel is immersed in water, adding a small concentration (55 ppm)
of potassium pertechnetate(VII) to the water protects the steel from
corrosion, even if the temperature is raised to 250 C. For this
reason, pertechnetate has been used as an anodic corrosion inhibitor
for steel, although technetium's radioactivity poses problems that
limit this application to self-contained systems. While (for example)
can also inhibit corrosion, it requires a concentration ten times as
high. In one experiment, a specimen of carbon steel was kept in an
aqueous solution of pertechnetate for 20 years and was still
uncorroded. The mechanism by which pertechnetate prevents corrosion is
not well understood, but seems to involve the reversible formation of
a thin surface layer (passivation). One theory holds that the
pertechnetate reacts with the steel surface to form a layer of
technetium dioxide which prevents further corrosion; the same effect
explains how iron powder can be used to remove pertechnetate from
water. The effect disappears rapidly if the concentration of
pertechnetate falls below the minimum concentration or if too high a
concentration of other ions is added.
As noted, the radioactive nature of technetium (3 MBq/L at the
concentrations required) makes this corrosion protection impractical
in almost all situations. Nevertheless, corrosion protection by
pertechnetate ions was proposed (but never adopted) for use in boiling
water reactors.
Precautions and biological effect
======================================================================
Technetium plays no natural biological role and is not normally found
in the human body. Technetium is produced in quantity by nuclear
fission, and spreads more readily than many radionuclides. It appears
to have low chemical toxicity. For example, no significant change in
blood formula, body and organ weights, and food consumption could be
detected for rats which ingested up to 15 μg of technetium-99 per gram
of food for several weeks. In the body, technetium quickly gets
converted to the stable ion, which is highly water-soluble and
quickly excreted. The radiological toxicity of technetium (per unit of
mass) is a function of compound, type of radiation for the isotope in
question, and the isotope's half-life.
All isotopes of technetium must be handled carefully. The most common
isotope, technetium-99, is a weak beta emitter; such radiation is
stopped by the walls of laboratory glassware. The primary hazard when
working with technetium is inhalation of dust; such radioactive
contamination in the lungs can pose a significant cancer risk. For
most work, careful handling in a fume hood is sufficient, and a glove
box is not needed.
Being close to noble metals, technetium is not very susceptible to
corrosion, and during biofouling, its ability to self-cleanse has been
recorded due to its radiotoxic effect on biota.
References
======================================================================
S. Garg and B. Maheshwari, et al., Atomic Data and Nuclear Data Tables
150, 101546 (2023)
https://doi.org/10.1016/j.adt.2022.101546
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Technetium
