HR: 1340h
AN: GC13A-1210    [Abstracts]
TI: Mineral Chemistry and Precipitation Kinetics of Dawsonite in the Geological Sequestration of CO2
AU: * Duan, R
EM: rduan@lanl.gov
AF: Earth and Environmental Sciences and Chemistry Divisions Los Alamos National Laboratory, MS D469, Los Alamos, NM 87545 United States
AU: Carey, J W
EM: bcarey@lanl.gov
AF: Earth and Environmental Sciences and Chemistry Divisions Los Alamos National Laboratory, MS D469, Los Alamos, NM 87545 United States
AU: Kaszuba, J P
EM: jkaszuba@lanl.gov
AF: Earth and Environmental Sciences and Chemistry Divisions Los Alamos National Laboratory, MS D469, Los Alamos, NM 87545 United States
AB: Dawsonite [NaAl(OH)₂CO₃] is a relatively rare mineral that may play an important role in the geological sequestration of CO₂: thermodynamic and reactive transport calculations predict that dawsonite should precipitate under a variety of CO₂ injection scenarios by dissolution of Al-bearing minerals in Na-bearing brine. The precipitation of dawsonite is potentially beneficial as a means of fixing the CO₂ in solid form but could also modify porosity and permeability. The relative rarity of natural dawsonite occurrences, however, indicates that either appropriate CO₂-rich environments are rare in nature or that dawsonite formation is inhibited or more complex than the simplified geochemical systems used in the model calculations. We have conducted synthesis experiments and dissolution studies to help understand precipitation dynamics, Na-K substitution, and solubility. Both Na- and K-dawsonite were readily synthesized from mixtures of (Na,K)HCO₃ and gibbsite [Al(OH)₃] at 150°C for 24 hours. [Syntheses were made in unstirred, closed Parr^TM reaction vessels and consisted generally of 15 ml of H₂O with 6 g (Na,K)HCO₃ and sufficient Al-bearing mineral to achieve a (Na,K)/Al ratio of 8.] At lower temperatures, a 41% yield of dawsonite was achieved at 120°C; 1% at 94°C; and no reaction was observed at ≤75°C. In contrast, other Al-bearing minerals including albite, analcime and pyrophyllite at 150°C (28 days) and albite, pyrophyllite, and clinoptilolite at 200°C (66 days) showed no evidence of dawsonite formation but did yield analcime. Kaolinite, however, did produce 1% dawsonite at 75°C (40 days), 37% at 150°C (90 days), and some dawsonite in addition to a complex mixture of albite, cancrisilite, and other phases at 200°C (66 days). Thermodynamic calculations suggest that silica activity restricts the range of dawsonite stability and synthesis experiments with gibbsite-quartz and gibbsite-opal-CT were significantly different than gibbsite alone. At 150°C, the yield of dawsonite was smaller and partially replaced by analcime with the higher silica activity of opal-CT having a stronger effect than quartz. Long-term experiments with gibbsite and quartz or opal-CT (72 days at 150°C or 7 days at 200°C) showed that an assemblage of analcime and mordenite was more stable than dawsonite. X-ray diffraction and chemical analyses of dawsonite synthesized from mixed Na-K bicarbonate and gibbsite at 150°C demonstrated that there is no solid solution between the Na- and K-endmembers of dawsonite. In fact, K-dawsonite has a distinct crystal structure and the two endmembers co-precipitate at 0.68 ≤ X_KHCO3 ≤ 0.80. The solubilities of Na- and K-dawsonite were determined in DI water and in 0.025 M bicarbonate solutions. Dawsonite dissolves incongruently in DI water to yield a mixture of aluminum hydroxides (gibbsite, bayerite, doyleite, and nordstrandite) and dissolves congruently in 0.025 M bicarbonate solution. The DI results were in close agreement with published thermodynamic data for Na-dawsonite but the bicarbonate-bearing solutions increased the apparent solubility of dawsonite as evident in higher (10X) total Al concentrations.
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1699 General or miscellaneous
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3630 Experimental mineralogy and petrology
SC: Global Climate Change [GC]
MN: Fall Meeting 2005