Search

It is a challenge to make thorough but efficient experimental designs for the coupled mineral dissolution and precipitation studies in a multi-mineral system, because it is difficult to speculate the best experimental duration, optimal sampling schedule, effects of different experimental conditions, and how to maximize the experimental outputs prior to the actual experiments. Geochemical modeling is an efficient and effective tool to assist the experimental design by virtually running all scenarios of interest for the studied system and predicting the experimental outcomes. Here we demonstrated an example of geochemical modeling assisted experimental design of coupled labradorite dissolution and calcite and clayey mineral precipitation using multiple isotope tracers. In this study, labradorite (plagioclase) was chosen as the reactant because it is both a major component and one of the most reactive minerals in basalt. Following our isotope doping studies of single minerals in the last ten years, initial solutions in the simulations were doped with multiple isotopes (e.g., Ca and Si). Geochemical modeling results show that the use of isotope tracers gives us orders of magnitude more sensitivity than the conventional method based on concentrations and allows us to decouple dissolution and precipitation reactions at near-equilibrium condition. The simulations suggest that the precise unidirectional dissolution rates can inform us which rate laws plagioclase dissolution has followed. Calcite precipitation occurred at near-equilibrium and the multiple isotope tracer experiments would provide near-equilibrium precipitation rates, which was a challenge for the conventional concentration-based experiments. In addition, whether the precipitation of clayey phases is the rate-limiting step in some multi-mineral systems will be revealed. Overall, the modeling results of multi-mineral reaction kinetics will improve the understanding of the coupled dissolution–precipitation in the multi-mineral systems and the quality of geochemical modeling prediction of CO2 removal and storage efficacy in the basalt systems. [ABSTRACT FROM AUTHOR]

Kyanite is an important and slow-dissolving mineral. Earlier work has measured its dissolution rate at high temperature and acidic pH, but experimental measurements at low temperature and near neutral pH were lacking. The rate equation by Palandri and Kharaka (A compilation of rate parameters of water–mineral interaction kinetics for application to geochemical modeling. US Geological Survey, Open File Report 2004-1068, 2004) indicates that the rate of kyanite dissolution at room temperature and near neutral pH is on the order of 10−17 mol m−2 s−1, orders of magnitudes slower than most common silicate minerals such as albite and quartz. This study used an externally-stirred mixed-flow reactor, which allows high solid:solution ratios, to measure the dissolution rate of kyanite at 0–22 °C and pH of 3.5–7.5. The measured dissolution rate of kyanite is 4.6–7.6 × 10−13 mol m−2 s−1 at 22 °C, and the apparent activation energy is 73.5 kJ mol−1. This dissolution rate is close to the rate of quartz dissolution and four orders of magnitude faster than the prediction by rate equation of Palandri and Kharaka (2004). Based on our new experimental data, we recommend the following rate equation for modeling the dissolution of kyanite at ambient temperatures. r = k e - E a R 1 T - 1 298.15 where k = 5.08 × 10−13 mol m−2 s−1, and Ea = 73.5 kJ mol−1. Review of literature data (Carroll in The dissolution behavior of corundum, kaolinite, and andalusite: a surface complex reaction model for the dissolution of aluminosilicate minerals in diagenetic and weathering environs. Dissertation, Northwestern University, 1989) led to a recommended rate equation for andalusite as for T = 25 °C and pH = 2–10: r = k 1 a H + n 1 + k 2 + k 3 a H + n 3 where k1 = 4.04 × 10−10 mol m−2 s−1, k2 = 7.95 × 10−10 mol m−2 s−1, k3 = 1.01 × 10−17 mol m−2 s−1, n1 = 1.2 and n3 = − 0.6. [ABSTRACT FROM AUTHOR]

This paper explores how dissolution and precipitation reactions are coupled in batch reactor experimental systems at elevated temperatures. This is the fifth paper in our series of 'Coupled Alkali Feldspar Dissolution and Secondary Mineral Precipitation in Batch Systems.' In the previous four papers we presented batch experiments of alkali-feldspar hydrolysis and explored the coupling of dissolution and precipitation reactions (Fu et al. in Chem Geol 91:955-964, ; Zhu and Lu in Geochim Cosmochim Acta 73:3171-3200, ; Zhu et al.in Geochim Cosmochim Acta 74:3963-3983, ; Lu et al. in Appl Geochem 30:75-90, ). Here, we present the results of additional K-rich feldspar hydrolysis experiments at 150 °C. Our solution chemistry measurements have constrained feldspar dissolution rates, and our high resolution transmission electron microscopy work has identified boehmite precipitation. Reaction path modeling of K-feldspar dissolution and boehmite precipitation simulated the coupled reactions, but only with forced changes of boehmite rate law in the middle of experimental duration. The results which are reported in this article lend further 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. in Water-rock interaction, ). [ABSTRACT FROM AUTHOR]

Reactions and reaction rates within aquifers are fundamental components of critical hydrological processes. However, reactions simulated in laboratory experiments typically demonstrate rates that are much faster than those observed in the field. Therefore, it is necessary to conduct more reaction rate analyses in natural settings. This study of geochemical reactions in the regional aquifer in the Pajarito Plateau near Los Alamos, New Mexico combines modeling with petrographic assessment to further knowledge and understanding of complex natural hydrologic systems. Groundwater geochemistry shows marked evolution along assumed flow paths. The flow path chosen for this study was evaluated using inverse mass balance modeling to calculate the mass transfer. X-ray diffraction and field emission gun scanning electron microscopy were used to identify possible reactants and products. Considering the mineralogy of the aquifer and saturation indices for the regional water refined initial interpretations. Calculations yielded dissolution rates for plagioclase on the order of 10−15 mol s−1 m−2 and for K-feldspar on the order of 10−17 mol s−1 m−2, orders of magnitude slower than laboratory rates. While these rates agree with other aquifer studies, they must be considered in the light of the uncertainty associated with geometric surface area estimates, 14C ages, and aquifer properties. [ABSTRACT FROM AUTHOR]