CONCLUSION

1. Nuclear Waste Disposal and Safety Safety Research for Nuclear Waste Disposal Long-lived Radionuclides in Final Disposal Systems Interaction of selenite with iron sulphide minerals: FeS versus FeS2 Breynaert E., Dom D., Wangermez W., Scheinost A., Kirschhock C.E.A., Maes A. • Introduction • Selenium: essential micronutrient, toxic at higher concentrations • Longlived 79Se radioisotope (t½ = 295 ky) present in high-level RadWaste • Se release from mining waste (e.g. Western Phosphate Resource Area) • Se mobility decreases in reducing conditions (low soluble Se0 and FeSex minerals) [ GAO-12-505 Phosphate Mining on Federal Land - http://www.gao.gov/products/GAO-12-505 ] Se(IV) interaction with pyrite (FeS2) and mackinawite, troilite (FeS) • EXAFS • XAS wavelet analysis • Se(IV) + FeS2 (pyrite) ⇨ Se0↓ • Se(IV) + FeS (mackinawite)⇨ FeSe↓ • Se(IV) + FeS (troilite) ⇨ FeSex↓ Se Fe Bruggemanet al. (2005). Env. Poll. 137: 209 – 221 Breynaert et al. (2008). ES&T, 42(10), 3595-3601 Scheinost, et al. (2008) J. Contam. Hydrol., 102(3-4), 228-45 Breynaert et al. (2010). ES&T 47: 6649 - 6655 Why do FeS2 and FeS react geochemically different ? FeS2 FeS • Surface S2-2 oxidised to S2O32- • Se(IV) + FeS2 (pyrite) ⇨ selenopolythionate • 4 S2O32- + H2SeO3 + 4 H+⇨ S4O62- + Se(S2O3)22- + 3 H2O • Se(S2O3)22-⇨ Se0 • In presence of excess sulphur donors (e.g.) S2O32-, SO32- • Se(IV) homogeneously reduced S-2 • Se(IV) ⇨ Se0(am) • Se0(am) reductive dissolution • Se0(am) ⇨ HSe-, Se2-,Se22- • Se(s) + H+ + 2e− = HSe− • log K0 = -7.5 (Eh = -200 mV) • Fe2+ from FeS solubility • Fe2+ + Se2- = FeSe • Fe2+ + 2Se-= FeSe2 Druschel & Borda (2006). Geochim. Cosmochim. Acta, 70(20), 5246-5250. REFERENCES SAMPLES 77Se NMR Se(S2O3)22- SeO32- • Wavelets assist to map maximum amplitudes on k-space to peaks in r-space • thiosulphate pathway • Se(IV) + FeS2 • ⇩ Se(S2O3)22- + Se0↓ • polysulphide pathway • Se(IV) + FeS ⇩ FeSex ↓ CONCLUSION What is the impact of seleno- polythionate on Se mobility in pyrite-containing anoxic sediments? [ ppm ] Dr. Eric Breynaert | KULeuven | Center for Surface Chemistry and Catalysis | eric.breynaert@biw.kuleuven.be | www.biw.kuleuven.be/m2s/cok/ This work is performed in cooperation with, and with the financial support of ONDRAF/NIRAS, the Belgian Agency for Radioactive Waste and Fissile Materials, as part of the programme on geological disposal of high-level/long-lived radioactive waste that is carried out by ONDRAF/NIRAS