『Abstract
Biotite dissolution under conditions of high pH and high aluminum,
sodium, and nitrate concentrations analogous to those found in
tank wastes at the Hanford Site was investigated using continuously
stirred flow-through reactions at 22 to 25℃. Experiments were
designed to simulate tank leaks into the Hanford vadose zone where
Fe(II) from biotite is the dominant reducing agent available to
immobilize certain contaminants. Both non-steady-state and steady-state
dissolution kinetics were quantified; interest in non-steady-state
kinetics derives from the inherently transitory nature of tank
leaks. Biotite was conditioned in pH 8 solutions to simulate the
alkaline environment of the Hanford sediment, and then reacted
in pH 10-14 solutions, some including 0.055 M Al(NO3)3 and/or 2 M or 6 M NaNO3.
Initial dissolution transients (intervals of rapid release rates
that decay to slower steady-state rates) showed fast preferential
release of K followed by near-stoichiometric release of Si, Al,
and Mg, and slower release of Fe. Each increase in pH resulted
in a second transient with the greatest amounts of Si, Al, and
K released at pH 14, followed by pHs 13, 12, 11, and 10. Fe release
also was highest at pH 14, but unchanging at pHs 10-13 within
experimental error. Transient releases at high pH are attributed
to dissolution of amphoteric secondary phases such as ferrihydrite
that are inferred from saturation calculations and solid analyses
to form during the conditioning interval. Transient release of
Si was inhibited by the presence of 0.055 M Al(NO3)3; the effects of Al(NO3)3 and NaNO3 on the release
rates of Al, Fe, Mg, and K were variable and generally outweighed
by the effect of pH. Quasi-steady-state release rates were slowest
at pH 11-12 (10-12.2 mol biotite m-1 s-1
for Si) and increased i either direction in pH away from this
minimum (to 10-11.5 at pHs 8 and 14 for Si). Fe release
rates at high pH were sufficient to account for observed Cr(VI)
reduction at Hanford. The net release rates of the major framework
cations, from which the biotite dissolution rate is inferred,
may reflect the precipitation of secondary phases or the alteration
of biotite to vermiculite. The most extensive solid-phase alterations
were observed in Na-enriched solutions.』
1. Introduction
2. Experimental
2.1. Pretreatment and characterization of solid
2.2. Experimental procedure
2.3. Calculation of release rates
2.4. Analyses
3. Results
3.1. Elemental release
3.2. Solids characterization
4. Discussion
4.1. Solid phase alteration
4.2. Dissolution transients following changes in pH
4.2.1. Non-stoichiometry and sources of transient release
4.2.2. Magnitude of transient releases
4.3. Steady-state dissolution rates
4.4. Implications for the Hanford Site and weathering
5. Conclusions
Acknowledgments
References
Appendix