Heavy Element Photophysics and Photochemistry
Photocatalytic
Destruction of Organic Complexants
We have shown that europium, a fission product element that is often
present in nuclear waste, can act photocatalytically to destroy a strongly
oxidation resistant complexant in aqueous solution. The term photocatalysis
denotes a process in which light and a chemical material termed a catalyst
interact in such a way that the desired chemical change occurs with no
net change in the chemical state of the catalyst which in this case is
the europium 3+ ion (Eu3+). In addition, we have shown that
laser-induced fluorescence is a sensitive monitor for the progress of
europium-based photocatalytic destruction of complexants.
 The
image shows an example in which europium played such a dual role by acting
both as a photocatalyst and a means for assessing the degree of complexant
destruction achieved. The figure shows the observed fluorescence decay
(note the logarithmic intensity scale) of Eu3+ions following
pulsed laser excitation (a) in the presence of a complexant, (b) in the
same solutionfollowing brief ultraviolet photolysis, and (c) with no complexant
present (click on the image for more information).
The fluorescence decay of the 5D0state of Eu3+
is a sensitive reflection of the ligands that immediately surround the
ion (often called the coordination environment of the ion). In this case,
the complexant suppresses processes by which electronically excited Eu3+ions
lose energy, thereby slowing the observed fluorescence decay rate ( bluesymbols
). Brief ultraviolet photolysis of a solution of this complexant and Eu3+
ions photocatalytically converted the complexant into products that are
relatively ineffective metal ion complexants, thereby increasing the observed
fluorescence decay rate ( redsymbols ). Conversion of the complexant into
water and carbon dioxide (complete destruction of it) would result in
an observed fluorescence decay rate that is comparable to that shown for
Eu3+surrounded only by water molecules (green symbols).
Many U.S. Department of Energy radioactive wastes are categorized as
"mixed" waste due to the presence of organic complexants and radionuclides.
Destruction of the complexants in such waste would reduce the hazard category
of the waste and the cost of disposing of it.
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