Coupled alkali feldspar dissolution and secondary mineral precipitation in batch systems: 4. Numerical modeling of kinetic reaction paths

Abstract

This paper explores how dissolution and precipitation reactions are coupled in batch reactor experimental systems at elevated temperatures. This is the fourth paper in our series of "Coupled Alkali Feldspar Dissolution and Secondary Mineral Precipitation in Batch Systems". In our third paper, we demonstrated via speciation-solubility modeling that partial equilibrium between secondary minerals and aqueous solutions was not attained in feldspar hydrolysis batch reactors at 90-300°C and that a strong coupling between dissolution and precipitation reactions follows as a consequence of the slower precipitation of secondary minerals ). Here, we develop this concept further by using numerical reaction path models to elucidate how the dissolution and precipitation reactions are coupled. Modeling results show that a quasi-steady state was reached. At the quasi-steady state, dissolution reactions proceeded at rates that are orders of magnitude slower than the rates measured at far from equilibrium. The quasi-steady state is determined by the relative rate constants, and strongly influenced by the function of Gibbs free energy of reaction (DG r ) in the rate laws.To explore the potential effects of fluid flow rates on the coupling of reactions, we extrapolate a batch system (Ganor et al., 2007) to open systems and simulated one-dimensional reactive mass transport for oligoclase dissolution and kaolinite precipitation in homogeneous porous media. Different steady states were achieved at different locations along the one-dimensional domain. The time-space distribution and saturation indices (SI) at the steady states were a function of flow rates for a given kinetic model. Regardless of the differences in SI, the ratio between oligoclase dissolution rates and kaolinite precipitation rates remained 1.626, as in the batch system case (Ganor et al., 2007). Therefore, our simulation results demonstrated coupling among dissolution, precipitation, and flow rates.Results reported in this communication lend support to our hypothesis that slow secondary mineral precipitation explains part of the well-known apparent discrepancy between lab measured and field estimated feldspar dissolution rates (Zhu et al., 2004). Here we show how the slow secondary mineral precipitation provides a regulator to explain why the systems are held close to equilibrium and show how the most often-quoted "near equilibrium" explanation for an apparent field-lab discrepancy can work quantitatively. The substantiated hypothesis now offers the promise of reconciling part of the apparent fieldlab discrepancy.

Effects of rate law formulation on predicting CO2sequestration in sandstone formations

“…BCF-type rate law has successfully simulated secondary mineralization in feldspar hydrolysis experiments [19].…”

The efficient long-term inhibition of forsterite dissolution by common soil bacteria and fungi at Earth surface conditions

“…Finally, long-term olivine dissolution rates can also be limited by the slow precipitation rates of secondary minerals, which could lead the fluid to approach equilibrium with the dissolving mineral (e.g. Zhu and Lu, 2009;Zhu et al, 2010;Saldi et al, 2012). This study was motivated in part to assess how these various processes might influence forsterite dissolution rates over time-scales larger than those typically considered in laboratory studies.…”

An experimental study of magnesite precipitation rates at neutral to alkaline conditions and 100–200°C as a function of pH, aqueous solution composition and chemical affinity

“…Such models have also been used to predict mineral precipitation rates in geochemical modeling studies (e.g. Steefel and MacQuarrie, 1996;Knauss et al, 2005;Steefel and Maher, 2009;Zhu, 2009;Goddéris et al, 2010;Zhu et al, 2010;Liu et al, 2011), yet the applicability of such models has not been verified in detail and may be inconsistent with what is known about mineral precipitation kinetics (e.g. Nielsen, 1964Nielsen, , 1984Nancollas and Reddy, 1971;Reddy and Nancollas, 1976;Reddy and Gaillard, 1981;Inskeep and Bloom, 1985;Mucci, 1986;Van Cappellen and Berner, 1991;Piana et al, 2006;Kowacz et al, 2007;Vekilov, 2007;Bénézeth et al, 2008;Kowacz and Putnis, 2008;Perdikouri et al, 2009;Vavouraki et al, 2010;RodriguezBlanco et al, 2011).…”