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Pyrite oxidation in the excavation disturbed zone of the second shaft

Pyrite oxidation in the excavation disturbed zone of the second shaft. Exchange Meeting N° 3, EIG Euridice – SCK•CEN – ONDRAF/NIRAS Mol, November 09, 2001 M. Van Geet , M. De Craen, N. Maes, and P. De Cannière. Content: two parts. Pierre De Cannière Motivation Background Maarten Van Geet

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Pyrite oxidation in the excavation disturbed zone of the second shaft

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  1. Pyrite oxidation in the excavation disturbed zone of the second shaft Exchange Meeting N° 3, EIG Euridice – SCK•CEN – ONDRAF/NIRAS Mol, November 09, 2001 M. Van Geet, M. De Craen, N. Maes, and P. De Cannière

  2. Content: two parts • Pierre De Cannière • Motivation • Background • Maarten Van Geet • Experimental observations • Mineralogy: SEM, EDX • Chemistry: leaching, SO42– extraction + Questions, Discussions • Futures perspectives • Recommendations: concept, architecture, PA

  3. Pyrite oxidation: why a renewal of interest for an old problem ? • Excavation Disturbed Zone (EDZ) around the second shaft • oxidation by air along open fractures • SAFIR-2: concerns on oxidation • FEP’s catalog and future prospective: • selection of relevant phenomena, and • rejection of irrelevant processes • Recent experimental concerns in the lab

  4. No oxygen can subsist at great depth in saturated porous media • Reducing conditions in marine sediments • Pyrite: FeS2 • Siderite: FeCO3 • Organic matter: OM + Fe2+ • No more oxygen below ~ 20 m under the water table • diffusion controlled • consumed by reductants (reactive transport) • Lab work: anaerobic conditions needed: gloveboxes: O2 < 1 ppm

  5. Boom Clay (B) Below the water table Marine sediment (30 Ma) Strongly reducing clay Solubility O2 in water: ~ 8 mg dm-3 O2 completely consumed in depth Backfill foreseen:no air in closed galleries Yucca Mountain (Nevada) Above the water table (300 m) Volcanic tuff (10 Ma) Oxidized silicates (SiO2) O2 in air: 21  % O2 present in fissures and matrix pores No backfill in galleries: air present in open galleries Difference Boom Clay >< Yucca

  6. Boom Clay is slightly alkaline • in situ pH: 8.2 (without loss of CO2) • in situ PCO2: 3 x 10-3 atm • pH range: 8.5 —> 9.5 (with loss of CO2) • NaHCO3: 2 x 10-2 M • Calcite: CaCO3 ± 1 % • Siderite: FeCO3 ± 1 % • presence of large Septaria nodules

  7. Boom Clay is rich in Organics • Organic material (OM): 2 – 3 % dry wt. • DOC: 200 mg / L • immobile: 99.95 % (humic-clay complex) • mobile: 0.05 % (low MW) • Humic acids (HA) ~ 70 % • Fulvic acids (FA) ~ 30 % • more than 50 % mobile OM is < 1 000 MW

  8. Boom Clay is strongly reducing • Eh < -250 mV (SHE) • slurry: -265 mV • water: -400 mV • calculations: -280 to -300 mV • Pyrite: FeS2 ± 1 % (pool of Fe2+ and S22-) • Siderite: FeCO3 • Glauconite (green color from Fe2+) • Sorbed Fe2+

  9. Pyrite could be the Eh controlling phase Eh = -0.0687 pH + 0.2613 Pyrite oxidation: 4 FeS2 + 15 O2 + 10 H2O —> 4 FeO(OH) + 16 H+ + 8 SO42-

  10. Ubiquitous presence of micro-dispersed Pyrite: Janus God (Custodian of the Universe in Roman Religion) • Peace Face: decreases the solubility, and increases the sorption of many radionuclides, e.g.uranium:“ Pyrite has protected and will protect the World ”(H. Ohmoto, Migration’97, Sendai) • War Face: pyrite oxidation is very detrimental for metallic canisters: the release of H2SO4 and S2O32- induces strong pitting corrosion !!!

  11. Previous Studies • Pyrite oxidation: Aging effects in Boom ClayBaeyens and Henrion (1981, 1984) • First detection of thiosulfate: Cerberus Project: 60Co gamma sources Noynaert, Beaucaire, et al. (1992)

  12. Potential oxidizers in an underground HLW repository • From air: • gaseous and dissolved O2 • Radiolysis: • peroxides + H2O2 , H-O–O•, H-O•, • Eurobitum waste: • NaNO3 • From pyrite oxidation itself: • Fe3+ • Micro-organisms • thio-oxidans, ferro-oxidans bacteria

  13. Pyrite oxidation: what it is • Initial chemical reaction • Calcite dissolution • Cation exchange • Water chemistry evolution • Eh-pH diagram

  14. Oxidation of pure pyrite only 4 FeS2 + 15 O2 + 10 H2O 4 FeO(OH) + 16 H+ + 8 SO42- via a metastable intermediate: S2O32– (thiosulfate) S2O32–

  15. Formation of thiosulfate (S2O32–) 4 FeS2 + 7 O2 + 6 H2O —> 4 FeO(OH) + 4 S2O32– + 8 H+ 4 S2O32– + 8 O2 + 4 H2O —> 8 SO42– + 8 H+ 4 FeS2 + 15 O2 + 10 H2O —> 4 FeO(OH) + 8 SO42– + 16 H+ S2O32– electrophoretic mobility higher than these of Cl–: Strong depolarizer: induce pitting corrosion of stainless steel ! Detected around the Cerberus experiment at 2 × 10-4 M

  16. Pyrite Oxidation in Boom Clay • Pyrite oxidation in itself 4 FeS2 + 15 O2 + 10 H2O —> 4 FeO(OH) + 8 SO42– + 16 H+ • Calcite dissolution by H+ 8 CaCO3 + 16 H+ —> 8 Ca2+ + 8 H2O + 8 CO2 • Cation exchange Ca2+ 2 Na+ 16 X —Na + 8 Ca2+ —> 8 X=Ca + 16 Na+

  17. Global reaction in Boom Clay 4 FeS2 + 15 O2 + 8 CaCO3 + 2 H2O + 16 X–Na4 FeO(OH)+ 8 Na2SO4 + 8 CO2 + 8 X=Ca • With the formation of alteration products: • Ferrihydrite (FeO(OH), hydrous ferric oxide) • Jarosite (KFe3(SO4)2(OH)6 ) (yellow) • Gypsum (CaSO4 · 2 H2O) • Thenardite (Na2SO4), mirabilite (Na2SO4 · 10 H2O),

  18. Formation of alteration products • Ferrihydrite (FeO(OH), hydrous ferric oxide, Fe oxy-hydroxide) • Jarosite (KFe3(SO4)2(OH)6 ) (yellow) • Gypsum (CaSO4 · 2 H2O) • Thenardite (Na2SO4) • Mirabilite (Na2SO4 · 10 H2O),

  19. Boom Clay Oxidation Na+ SO42– Undisturbed pH: (8.2 - 9.5) Eh: -250 to -400 mV(SHE) Slightly oxidized pH: (6 - 7) Eh: less negative thiosulphate/sulphate cation exchange Strongly oxidized pH: (3 - 4) Eh > 0 mV(SHE) calcite dissolution cation exchange Na+ Na+ HCO3– HCO3– SO42– S2O32–

  20. Strong chemical perturbations may arise from pyrite oxidation 4 FeS2 + 15 O2 + 10 H2O —> 4 FeOOH + 8 SO42– + 16 H+ • Strong acidification: clay may become positively charged • cations as Ca2+, Sr2+ are no longer sorbed • anions as I– are sorbed • Iron oxy-hydroxides: strong sorbent for oxy-anions as HPO42–, H3SiO4–, SeO32–, IO3–, Organic Matter (OM) • Sulphate: may induce the co-precipitation of Sr2+, Ra2+ • Redox-sensitive elements (Se, Tc, U, Np, Pu) are mobile

  21. Detrimental effects of oxidation on • Chemistry of interstitial water (pH, Eh, Na2SO4 type) • Mineralogy: oxidation products (FeO(OH), jarosite, mirabilite) • Migration of radionuclides • Solubility and sorption of redox-sensitive elements: Se, Tc, U, Np, Pu • Sorption of non redox-sensitive elements: Ra2+, Si(OH)4 • Altered sorption of simple cations (Ca2+, Sr2+) and anions (I–) • Metallic corrosion: localized corrosion, pitting (S2O32–) • Glass corrosion: • pH: no significant impact on the solubility of silica at low pH • FeO(OH): strong sorbent for dissolved silica • Spent fuel corrosion: could be increased

  22. Detrimental effects of oxidation on • Chemistry of interstitial water (pH, Eh, Na2SO4 type) • Mineralogy: oxidation products (FeO(OH), jarosite, gypsum, thenardite, mirabilite) • Migration of radionuclides • Solubility and sorption of redox-sensitive elements: Se, Tc, U, Np, Pu • Sorption of non redox-sensitive elements: Ra2+, Si(OH)4 • Altered sorption of simple species: cations (Ca2+, Sr2+) and anions (I–)

  23. Detrimental effects of oxidation on • Metallic corrosion: localized corrosion, pitting (S2O32–) • Glass corrosion: • No significant impact on the solubility of silica at low pH • FeO(OH): strong sorbent for dissolved silica • Spent fuel corrosion: could be increased

  24. Technical Problems • Long-term good preservation of clay samples: GLP: deficient PE-Al bags • Squeezing experiments (SO42–; + anion exclusion ?) • Drilling + closure of boreholes after Ar filling • 1  % pyrite —> SO42– => as high as160 g dm-3, but 500 mg dm-3 may already disturb trace elements whose chemical behaviour is following major elements • Oxygen is our Permanent Enemy • Extra Care Continuously Needed to Protect Boom Clay against Oxidation

  25. Two Transport Mechanisms • Diffusion of dissolved oxygen in the water = reactive transport: “burning candle”= very slow process, controlled by diffusion • Direct and fast ingress of gaseous oxygen from air through fractures in EDZ around shafts and galleries could easily go faster and farther !

  26. Oxygen penetration: How ? Gaseous, or dissolved O2 ? Clay pits: gaseous O2: Yes Infiltration of shallow aerated waters ! No dissolved O2 can subsist at great depth ! No ! Faults No ! Open galleries Gaseous O2 : Yes

  27. When is oxidation relevant ? • EDZ of Galleries: Chemically Disturbed Zone (CDZ) • Need to minimize the EDZ • EDZ of boreholes and piezometers • New down-hole camera: evidences of fracturation around boreholes • New piezo at ring 116: high SO42– content • Need to inert boreholes with Ar or N2

  28. —> Concept / Architecture • Limit the exposure of Boom Clay to O2 • excavate quickly the galleries, • dispose the waste immediately and, • backfill the galleries within 2 years • Do not leave the galleries open for 20 or 50 years ! • Accessibility for retrievibility would favour oxidation !

  29. When is oxidation not relevant ? FEP’s to be rejected ! • General oxidation of the deep formation from the surface; only possible if: • dramatic erosion • no more overlying formation (no more barrier !) [cataclysm: no alleviation measurements possible] • Direct injection of oxygen bearing water through an active and open fault • scenario in Sweden and Finland for granite under ice caps: not valid for faults in porous media !

  30. Sampling strategy at the bottom of the second shaft North Starting chamber (September 2001): Sampling of oxidized pyrite as function of the distance from: • the shaft wall • the fractures

  31. Gallery Section Sampling along gallery axis Less fissured Sampling as a distance from shaft

  32. More fissured Gallery Section Sampling perpendicular to a fracture Gaseous O2 ingress O2 diffusion in water Open fracture

  33. Credits to our colleagues Persons involved: M. De Craen, N. Maes, L. Wang, H. Moors, M. Van Gompel, L. Van Ravestyn, T. Maes, F. Vandervoort, T. Beauwens, J. Mertens, W. Bastiaens, F. BernierThe Hades Team and, Everyone we should have forgotten (sorry ;-)

  34. Non conservative parameters • Determination of non conservative parameters (PCO2, pH, Eh) • What is Eh in Boom Clay (Orpheus measurements) ?

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