1. [Se]in and [Se]out : initial and final supernatant Se concentration PT and NPT: samples pretreated and not pretreated with 1M HCl after crushing XAS analysis, Se K-edge (12.658 keV), fluorescence mod Oxidising Conditions Selenate (SeO 4 2- ) Weakly adsorbed on oxide surfaces Reduction kinetically hindered / poorly known No solubility limitation State-of-the-art regarding selenium geochemistry under Boom Clay conditions Bruggeman, C.; Breynaert E.; Maes, A Katholieke Universiteit Leuven, Laboratory for Colloid Chemistry, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium Se Redox Chemistry Se(VI) No significant reduction or sorption by Boom Clay components : Mildy Oxidising Conditions Selenite (SeO 3 2- ) Strongly adsorbed on oxide surfaces Reduction faster than SeO 4 -2, important process for storage No solubility limitation Reducing Conditions Se 0, Selenide (Se 2- ) Limited solubility Not well known pH-pe diagram [Se] tot = 10 -6 mol l -1 (NEA database project 2006) Se(IV) Se(0) Se(-II) Formation of FeSe by reduction of SeO 3 -2 by FeS, 18 days equilibrium 7.86 > 6.68  10 -2 mol/kg < 10 -6 6.07  10 -3 100 g·l -1 FeS < 100 µm PT 10 7.89> 6.10×10 -2 < 10 -6 6.08  10 -3 100 g·l -1 FeS < 100 µm NPT 9 pH[Se] S (mol/kg)[Se] out (M)[Se] in (M)Solid/LiquidDescriptionSample No association of Se 0 with purified Boom Clay Humic Acid (p-BCHA) Se 0 Se(IV) Se(IV) + p-BCHA + N 2 H 4 ; spn; pH 7.25 Se 0 + p-BCHA ; pellet Se 0 + p-BCHA + Fe(II); pellet Se(IV) + p-BCHA + N 2 H 4 ; spn; pH 5.25 Se(IV) + p-BCHA + N 2 H 4 ; pellet; pH 5.25 Sorption-Reduction by Fe(II) containing minerals (FeS 2, FeS, …) * Batch tests of SeO 3 2 ‑ with pure pyrite (FeS2) in synthetic Boom Clay water (no OM) Selenium on FeS2 solid phase (mol g ‑ 1 ) versus SeO 3 2 ‑ concentration (mol dm ‑ 3 ). Short contact times (3 and 7 days) : linear K d  sorption process Longer contact times : progressive convergeance to 3 × 10 ‑ 9 mol dm ‑ 3  reduction process. Sorption-reduction of SeO 3 2 ‑ on pyrite (FeS 2 ) TIME 7.425.25×10 -2 8.63×10 -4 6.10  10 -3 100 g·l -1 FeS 2 < 10 µm PT 4 7.806.11×10 -2 4.80  10 -5 6.10  10 -3 100 g·l -1 FeS 2 < 10 µm NPT 3 pH[Se] S (mol/kg)[Se] out (M)[Se] in (M)Solid/LiquidDescriptionSample Reduction of SeO 3 -2 to Se 0 by pyrite (FeS 2 ) [Se]in and [Se]out : initial and final supernatant Se concentration PT and NPT: samples pretreated and not pretreated with 1M HCl after crushing XAS analysis, Se K-edge (12.658 keV), fluorescence mode -Illite -Pyrite -HS Conclusions Under BK conditions -Se(VI)  unaltered -Se(IV)  initial sorption on oxidic surfaces  reduction by FeS 2 Knowledge Gaps ?  Se(VI) sorption on Boom Clay  Se(IV) sorption/reduction on CaCO 3 /FeCO 3 phases  Se speciation on Boom Clay solid phase Short term (1 day) batch sorption edges of Se(IV) on illite du Puy Typical anion adsorption edges TIME Association of Se with Boom Clay Organic Matter SeO 3 -2 sorption on Illite du Puy (10 g/l; 0.01 N NaCl) Sorption on oxidic surfaces (edges) of clay minerals -Association increases with time  speciation/valence ? Acknowledgements: ONDRAF/NIRAS – TRANCOM II – FUNMIG – FWO/NWO *Selenite reduction in Boom clay: Effect of FeS2, clay minerals and dissolved organic matter. Bruggeman C, Maes A, Vancluysen J and P. Vandemussele. ENVIRONMENTAL POLLUTION 137 (2): 209-221