The role of organic matter in the colloid mobility and solid-liquid distribution of naturally abundant lanthanides and actinides: Case of the Boom Clay formation

In Belgium, deep geological disposal of radioactive waste is envisaged in poorly indurated clay formations like the Boom Clay (BC). In the present work we applied the Asymmetrical Flow Field-Flow Fractionation coupled to a UV-Vis spectrophotometer (AsFlFFF-UV) as well as the Liquid Chromatography coupled to Organic Carbon Detection, UV and ICP-MS (LC-OCD-UV-ICP-MS) to study dissolved organic matter (DOM) mobility. Small sized DOM fractions (<7 nm) are detected in all BC porewaters (BCPW) where the maximum of the species size distribution is at a hydrodynamic diameter of ∼1.8-1.9 nm. Pore waters originating from or influenced by the double band (DB) structure exhibit a multimodal size distribution ranging to larger sized colloidal entities up to ∼30 nm. The findings support the outcome of earlier investigations stating that the inter layer mobility of DOM in BC apparently is restricted to species with a diameter of <7 nm. Trace elements (Fe, Mn, Ni, lanthanides, Th, U) reveal a variable and complex association with colloidal species in BCPW and BC leachates where FeOOH/DOM aggregates play a role. An attempt is made to derive so-called in-situ K d(OM) -values i.e. the distribution of naturally abundant lanthanides and actinides between dissolved OM in BCPW and OM being extractable from solid BC samples by a leaching step using 15 mM NaHCO 3 solution. Results are compared with laboratory K d -values described in the literature and point to the significant contribution of the immobile OM to lanthanide and actinide retention.