The interaction of dissolved Boom Clay and Gorleben humic substances with selenium oxyanions (selenite and selenate)

Batch experiments were performed in which the interaction of selenite (Se+IVO32-) and selenate (Se+VIO42-) with dissolved Boom Clay and Gorleben humic substances (HS) was followed as a function of time. Particulate HS (>25 nm) was first removed by centrifugation. The supernatant solutions were analyzed by a combination of Ion Chromatography, Gel Permeation Chromatography and Ultrafiltration to determine the Se solution speciation. A first experiment in which 1 × 10−6 M 75SeO42- was in contact with 128 ppm Boom Clay HS or 160 ppm Gorleben HS in a synthetic clay H2O background electrolyte, showed that the total Se solution concentration and Se speciation remained unchanged, indicating that there was no interaction whatsoever between SeO42- and HS during the three months’ observation time. Upon contact of 1 × 10−6 M 75SeO42- with the same HS-containing solution however, a decrease of ∼95% in total Se solution concentration after centrifugation was noted and final Se solution concentrations of 4.4 × 10−8 M (Boom Clay HS) and 1.3 × 10−7 M (Gorleben HS) were measured after six months observation. Together with the decrease in total solution concentration, a change in the Se solution speciation was observed. After one month, the solution speciation in both systems was dominated by a colloidal Se species that was associated with HS. The chemical nature of this Se species could not be determined. The SeO32- concentration decreased steadily to a final concentration of 1.9 × 10−8 M (Boom Clay HS) and 1.5 × 10−8 M (Gorleben HS). The detailed understanding of the mechanisms at work in these systems will necessitate spectroscopic measurements of the Se species present in the pellet and of Se associated with the humic substance fraction in solution.