Basic Research Needs for Geosciences - Energetics Meetings and ...

CROSSCUTTING ISSUE:HIGHLY REACTIVE SUBSURFACE MATERIALS AND ENVIRONMENTSpartitioning of elements and isotopes between coexisting species and phases relevant togeological sequestration.Corrosion at metal, metal alloy, and waste form surfaces, deliquescent brine formation andevaporation on reactive surfaces, and condensation of volatiles into liquid films can result inhighly concentrated and/or complex aqueous solutions and complex solids, whose chemical andphysical properties are unknown. Much of the existing geochemical database is best employed tocalculate equilibrium solubility of pure mineral phases in dilute aqueous solutions. Activitycoefficient effects in liquid, crystalline, and amorphous phases are lacking for many solid andsolution compositions critical to chemical migration issues. Little is known about the volatility ofacids in complex brines. Competitive adsorption effects are ill-defined, particularly for tracelevels of exotic species in complex brines.The efficacy of subsurface sequestration depends on our fundamental understanding of the extentand rate of CO 2 -mineral interactions and the associated effects on the fate of the injected carbonand rock characteristics. CO 2 -mediated dissolution reactions produce solutes that buffer pH,thereby controlling the extent to which formation waters become acidified. These fluid-mineralreactions also have the potential to alter the porosity and flow permeability regimes. Thesechanges may impact the injection process, but more importantly they would change the longtermflow characteristics and storage capacity of the aquifer.Studies of mineral-solution reaction kinetics have been focused on steady-state dissolution ratesand mechanisms in highly undersaturated aqueous solutions. Few investigations of nearequilibriumreactions have been undertaken and even fewer of mineral precipitation kinetics.Systematic investigations of solution composition effects are rare. Attempts to derive empiricalor theoretical linkages between multiple regions of solution saturation state are rare (Nagy andLasaga 1992; Dove et al. 2005). No theory has yet been formulated to apply to all minerals in allsolutions. While a multitude of experimental systems could be investigated, the first majorchallenge is to identify key fluid and solid compositions that will lead to an integrated empiricaland/or theoretical framework for predicting mineral-dissolution and precipitation rates in themulti-compositional environments of interest. The second major challenge is to develop fastmethods for quantifying rates as a function of composition and saturation state (Bénézeth et al.2007). Finally, reaction mechanisms must involve the structure, dynamics and electrostatics ofmineral/water interfaces, but there have been few attempts to link heterogeneous reaction rateswith the atomic-scale properties of the interface (Oelkers et al. 1994; Ludwig and Casey 1996;Dove 1999; Brantley 2004; Bickmore et al. 2006).Superposed or fluctuating environmental perturbations could cause feedbacks that areunpredictable based on models using empirical reaction rate laws from simple systems. Shortlivedcoupled processes, such as simultaneous adsorption and solid dissolution reactions, canresult in surface precipitation of new phases that rapidly and irreversibly immobilizecontaminants, which would have a positive impact on performance assessment. Alternatively,cumulative release of an element by non-steady-state reactions can be orders of magnitudehigher than those estimated with steady-state rates (Samson and Eggleston 2000), effectivelyincreasing risk. Trace components derived from reactions triggered by perturbations candramatically change the rate of subsequent reactions, for example by initiating autocatalysis orpoisoning sorption at specific structural surface sites. The challenge is to be able to identify and160 Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems