Stability of ¹²⁵I and ¹⁴C Labelled Boom Clay Organic Matter

The candidate host formation for the disposal of radioactive waste in Belgium is Boom Clay which may contain up to 4% organic matter (OM) [1]. A limited fraction (less than 0.05%) of this OM is mobile. OM can complex radionuclides and so influence their migration. The migration behaviour of the OM itself has been extensively studied but to date such studies have used absorbancy measurements to quantify the OM. Unfortunately various problems accompany the use of absorbancy measurements. The particular problems may be overcome by using radiolabelled OM. Accordingly as a precursor to planned in situ migration experiments in Boom Clay (BC) using radiolabelled OM, stability studies on ¹²⁵I and ¹⁴C labelled materials have been conducted. The ¹²⁵I containing solutions were analysed using Gel Permeation Chromatography (GPC) and the ¹⁴C solutions using High Performance Size Exclusion Chromatography (HPSEC). Dissappointingly at the relevant pH of 8.5, even in the absence of the clay, the ¹²⁵I label was found to be unstable. However the ¹⁴C labelled OM (¹⁴C-BC-OM) was stable under the mild conditions employed in the test, so its stability was investigated in the presence of Boom Clay. The results were compared with that of ¹⁴C labelled humic acids (¹⁴C-HA), treated similarly. Unexpectedly the ¹⁴C labelled material was found to be partially unstable in the presence of Boom Clay. However the instability has not hampered the laboratory column experiments and should not hamper the proposed in situ experiments with this material.