Reply #116: #2: The aqueous chemistry of Technetium is dominated by an anion.

[NNadir]

Most metals, as people who have taken a course in chemistry know, have cationic chemistry, meaning that they are generally characterized by positively charged ions, many of which have simple salts, several of which are generally water insoluble.

This is not the case with technetium. The most common oxidation state for technetium is +7, but it does not exist as an ion with such a high charge (which would be extraordinary) but instead is bonded to four oxygens (each of which have an oxidation state of -2). This means that the charge on the ion that represents the most common oxidation state for technetium is represented by the Tc04- or pertechnate ion (similar to the familiar purple permanganate ion for those who have taken an introductory college chemistry course). The pertechnate ion forms many soluble salts and can be quite mobile. (Most interestingly to an inorganic chemist, the cesium and potassium salts of pertechnate are at best sparingly soluble.) Pertechnate is in fact leaching out of the Hanford waste tanks filled during the 1950's by the folks who were building nuclear weapons in those days. The pertechnate so released is moving in the water table under the Hanford reservation in this form. Eventually some of it is expected to reach the Columbia River.

Because the common form of technetium is anionic as opposed to cationic, this complicates the removal of this species through agencies like ion exchange resins. The necessity of adding two types of resin, both cationic and anionic, as opposed to just a cationic resin adds expense. In addition, although one can capture technetium on strong anion exchange resins having quaternary ammonium salts, the behavior of these resins with respect to pertechnate is very sensitive to the chemical conditions of the waste water, the pH and types of other anions present. Therefore their management is problematic and difficult.

The pertechnate ion's chemistry has also been a problem at least one commercial nuclear fuel reprocessing plant. Technetium that was discharged from the Sellafield nuclear fuel reprocessing plant in the UK is detectable throughout the North Sea. Measures have been taken to ameliorate this unfortunate state of affairs that was both unnecessary and wasteful, but frankly, it's been the source of considerable public anxiety. This has had an untoward effect in a time where the crisis at hand almost demands public acceptance of nuclear energy.

The geochemistry of technetium is well understood from the naturally occurring Oklo nuclear reactors that operated almost two billion years ago.

http://www.curtin.edu.au/curtin/centre/waisrc/OKLO/Why/Why.html

Here is the Oklo periodic table which shows the long term fate of the radionuclides under experimentally verifiable geological conditions:



Note that the technetium moved. It didn't go far, but it definitely moved.