Dissolution of Clay Minerals and Redox Reactions between Clay Minerals and Uranium under High Pressure Co2 Conditions
Open Access
- Author:
- Liu, Yan
- Graduate Program:
- Environmental Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- February 25, 2013
- Committee Members:
- William D Burgos, Thesis Advisor/Co-Advisor
Fred Scott Cannon, Thesis Advisor/Co-Advisor
Christopher Gorski, Thesis Advisor/Co-Advisor - Keywords:
- clay
uranium
carbon sequestration - Abstract:
- Geological carbon sequestration is a promising technology to mitigate global warming. However, there are concerns that the acidic nature of the injected supercritical CO2 may dissolve minerals in the injection well seals or in other regions of the storage reservoir. Mineral dissolution could create pathways for CO2 to migrate to other geologic units. If mineral dissolution also mobilized contaminants (e.g., U, Pb, As), leakage of CO2 could pose risks to overlying drinking water reservoirs. In this study we examined the dissolution of specimen clay minerals in synthetic brine solutions under high pressure CO2 conditions (PT = 9.66 bar, PCO2 ≥ 8.66 bar CO2). Specimen clay minerals included nontronite NAu-2, montmorillonite SWy-2, and chlroite CCa-2. Unaltered and partially reduced NAu-2 (R-NAu-2) were used to measure the effect of clay-Fe(III) reduction on acid-promoted dissolution. Synthetic brines included 1.0 M NaCl, 0.33 M CaCl2, and 0.33 M Na2SO4. Uranium was used as a redox-active inorganic contaminant and was added either as soluble uranyl(VI) or sparingly soluble biogenic uraninite(IV). High pressure CO2 conditions were established by adding a measured mass of dry ice to a pressure tube containing clay mineral, brine, and U. Reaction kinetics were measured over a 15 day period by sacrificing pressure tubes, analyzing the supernatant for dissolved mineral components and U, and analyzing the speciation of Fe(II/III) in solution and in the clay mineral pellet. H3PO4-H2SO4 (1.40 M-0.50 M) was used to measure acid-promoted clay mineral dissolution in a stronger acid. Control reactors were maintained under ambient pressure conditions (PT = 1.0 bar, N2:H2 = 95:5%). For three types of unaltered clay minerals in three different brines, very little dissolution occurred, showing these clay minerals are stable under high pressure CO2 conditions. In 1.40 M H3PO4-0.50 M H2SO4 solution, increased acidity enhanced dissolution of unaltered NAu-2 and partially reduced NAu-2. R-NAu-2 was easier to be dissolved than unaltered NAu-2 under high pressure CO2 conditions and in 1.40 M H3PO4-0.50 M H2SO4. Fe(II) was less stable in clay mineral and acid dissolved Fe had higher Fe(II)/Fe ratios than clay mineral itself. No redox reactions between unaltered or partially reduced NAu-2 with U(VI) was observed. UO2 was oxidized by unaltered NAu-2, and reaction had a faster rate and greater extent under high pressure CO2 conditions than under ambient pressure conditions ((PT = 1.0 bar, N2:H2 = 95:5%)). Also, the redox reaction enhanced dissolution of unaltered NAu-2. Such CO2 enhanced dissolution of clay mineral and enhanced oxidation of UO2 to U(VI) (aq) indicates possible environmental risks related with carbon sequestration process.