Considerations on the X-ray exposure mechanism Although it is outside the scope of this paper to identify the full molecular scale mechanism that leads to the release of Nd ions from either DTPA or DHDP under X-ray exposure, several possibilities were explored. One possibility is that X-ray exposure reduces Nd 3+ to Nd 2+ , thereby weakening its binding. The oxidation state of 3+ for lanthanides, including Nd, is known to be stable under the solution conditions of these experiments, however, it has been demonstrated that exposure to an ultra-high pressure mercury lamp can reduce the oxidation state to 2+. The authors were successful in synthesizing insoluble Nd(II), Sm(II), and Eu(II) sulfates but were unable to produce Gd(II), citing its lower reduction potential. Nevertheless, if reduction to 2+ was the root cause of these X-ray induced results, the same transition is not expected to occur readily for Gd(III) due to its very stable half-filled 4f-shell. Therefore, samples containing GdCl3 substituted for NdCl3 were used to test the role of lanthanide reduction in the observed X-ray effects. Measurements on pH 4.5 samples containing Gd 3+ demonstrated a similar cyclical process as those with Nd 3+ (Fig. ), indicating that reduction of the lanthanide ion is not required for the X-ray exposure-induced adsorption cycle demonstrated by the data in Fig. .