AQUIFER KINETICS AT THE MOLECULAR SCALE: INSIGHTS FROM SYNCHROTRON RADIATION STUDIES AT MINERAL-WATER INTERFACES

Understanding reaction kinetics in aquifers is made difficult by their inaccessibility to in situ observation on the appropriate spatial and temporal scales. It is commonly observed that apparent steady-state reaction rates of silicate minerals in aquifers decrease with the duration of the reaction. This observation has been attributed to various factors, including: increasing total surface area, decreasing reactive surface area, accumulation of leached layers and surface precipitates and/or organic adsorbates, and approach of the system to thermodynamic equilibrium (decreasing reaction affinity). Many of these factors are now accessible to direct experimental observation at the molecular scale by using synchrotron x-ray scattering methods on single crystals with well defined surface areas and controlled reaction conditions (solution composition, temperature, flow rate). For example, x-ray scattering studies of orthoclase dissolution involving gem-quality single-crystal surfaces under acidic and basic, far-from-equilibrium conditions, have shown that layer-by-layer dissolution rates on different crystal faces can be measured precisely, dissolution is essentially congruent, and the commonly described “leached layer” consists of secondary Si- and/or Al-rich oxyhydroxide precipitates (Teng et al., GCA 65, 2001; Fenter et al., GCA 67, 2003). Growth rates, structures, and epitaxial relationships of secondary precipitates can also be measured precisely (e.g., Chiarello et al., GCA 61, 1997), as can those of adsorbed organic layers (e.g., Fenter and Sturchio, GCA 63, 1999). Recent results have begun to elucidate the structures of water and the electrical double-layer at the mineral-water interface (Fenter and Sturchio, Prog. Surf. Sci. 77, 2004; Zhang et al., Langmuir 20, 2004); these advances may lead to more accurate molecular-scale models of mineral-water interfacial reaction kinetics. Enhanced understanding of molecular-scale reaction mechanisms and kinetics at the mineral-water interface will ultimately be transferable to larger-scale (spatial and temporal) aquifer systems, reducing the prevailing uncertainties and enabling the formulation of more realistic aquifer models.