The rates of mineral dissolution reactions in porous media are difficult to predict, in part because of a lack of understanding of mineral reactive surface area in natural porous media. Common estimates of mineral reactive surface area used in reactive transport models for porous media are typically ad hoc and often based on average grain size, increased to account for surface roughness or decreased by several orders of magnitude to account for reduced surface reactivity of field as opposed to laboratory samples. In this study, accessible mineral surface areas are determined for a sample from the reservoir formation at the Nagaoka pilot CO2 injection site (Japan) using a multi-scale image analysis based on synchrotron Xray microCT, SEMQEMSCAN, XRD, SANS, and FIB-SEM. This analysis not only accounts for accessibility of mineral surfaces to macro-pores, but also accessibility through connected micro-pores in smectite, the most abundant clay mineral in this sample. While the imaging analysis reveals that most of the micro-and macro-pores are well connected, some pore regions are unconnected and thus inaccessible to fluid flow and diffusion. To evaluate whether mineral accessible surface area accurately reflects reactive surface area a flow-through core experiment is performed and modeled at the continuum scale. The core experiment is performed under conditions replicating the pilot site and the evolution of effluent solutes in the aqueous phase is tracked.Various reactive surface area models are evaluated for their ability to capture the observed effluent chemistry, beginning with parameter values determined as a best fit to a disaggregated sediment experiment (Beckingham et al., 2016) described previously.Simulations that assume that all mineral surfaces are accessible (as in the disaggregated sediment experiment) over-predict the observed mineral reaction rates, suggesting that a reduction of RSA by a factor of 10-20 is required to match the core flood experimental data. While the fit of the effluent chemistry (and inferred mineral dissolution rates) greatly improve when the pore-accessible mineral surface areas are used, it was also necessary to include highly reactive glass phases to match the experimental observations, in agreement with conclusions from the disaggregated sediment experiment. It is hypothesized here that the 10-20 reduction in reactive surface areas based on the limited pore accessibility of reactive phases in core flood experiment may be reasonable for poorly sorted and cemented sediments like those at the Nagaoka site, although this reflects pore rather than larger scale heterogeneity.

Our limited understanding of mineral reactive surface area contributes to significant uncertainties in quantitative simulations of reactive chemical transport in subsurface processes. Continuum formulations for reactive transport typically use a number of different approximations for reactive surface area, including geometric, specific, and effective surface area. In this study, reactive surface area estimates are developed and evaluated for their ability to predict dissolution rates in a well-stirred flow-through reactor experiment using disaggregated samples from the Nagaoka pilot CO 2 injection site (Japan). The disaggregated samples are reacted with CO 2 acidified synthetic brine under conditions approximating the field conditions and the evolution of solute concentrations in the reactor effluent is tracked over time. The experiments, carried out in fluid-dominated conditions at a pH of 3.2 for 650 hours, resulted in substantial dissolution of the sample and release of a disproportionately large fraction of the divalent cations. Traditional reactive surface area estimation methods, including an adjusted geometric surface area and a BET-based surface area, are compared to a newly developed image-based method. Continuum reactive transport modeling is used to determine which of the reactive surface area models provides the best match with the effluent chemistry from the well-stirred reactor. The modeling incorporates laboratory derived mineral dissolution rates reported in the literature and the initial modal mineralogy of the Nagaoka sediment was determined from scanning electron microscopy (SEM) characterization. The closest match with the observed steady-state effluent concentrations was obtained using specific surface area estimates from the image-based approach supplemented by literature-derived BET measurements. To capture the evolving effluent chemistry, particularly over the first 300 hours of the experiment, it was also necessary to account for the grain size distribution in the sediment and the presence of a highly reactive volcanic glass phase that shows preferential cation leaching.

Introduction of coal fly ash into aquatic systems poses a potential environmental hazard because of its heavy metal content. Here we investigate the relationship between solid phase transformations, fluid composition, and metal release and speciation during prolonged wet aging of a class C and class F coal fly ash. The class C ash causes rapid alkalinization of water that is neutralized over time by CO(2) uptake from air and calcite precipitation. The resulting aqueous metal concentrations are below regulatory limits with the exception of Cr; solubility constraints suggest this is released as chromate. Limited As release is accompanied by no change in solid-phase speciation, but up to 35% of the Zn in the ash dissolves and reprecipitates in secondary phases. Similar processes inhibit Ba and Cu release. In contrast, the class F ash causes rapid acidification of water and initially releases substantial quantities of As, Se, Cr, Cu, Zn, and Ba. Arsenic concentrations decline during aging because of adsorption to the iron oxide-rich ash; this is aided by As(III) oxidation. Precipitation processes lower Ba and Cr concentrations during aging. Se, Cu, and Zn concentrations remain elevated during wet aging and solid-phase Zn speciation is not affected by ash-water reactions. Total metal contents were poor predictors of metal release, which is predominantly controlled by metal speciation and the effects of ash-water reactions on fluid pH. While contact with atmospheric gases has little effect on class F ash, carbonation of class C ash inhibits metal release and neutralizes the alkalinity produced by the ash.

Carbon isotope (δ13C) variations measured in carbonates have been attributed to large-scale phenomena throughout Earth history, such as changes in atmospheric oxygen or global glaciations. These interpretations follow from a model wherein the δ13C of marine dissolved inorganic carbon (DIC) is controlled by the relative sedimentary burial rates of biogenic carbonate (BC) and organic carbon (OC). A new model proposes authigenic carbonate (AC) as a third major sedimentary C pool, implying that δ13C anomalies are not necessarily indicative of extreme changes in the global carbon cycle and/or atmospheric oxygen. Two conditions are required for AC formation to significantly alter bulk carbonate δ13C: the AC isotopic composition must be at least ∼3‰ different from that of BC and the AC/BC ratio must be >0.1. We use pore fluid Ca and Sr concentrations to estimate rates of AC formation in Late Cenozoic marine sediments, then calculate relative fractions of AC, OC, and BC. Today AC is not expected to constitute a significant fraction of total sedimentary carbon (AC+OC+BC) globally; however, there are modern sites where local conditions promote elevated AC/BC and anaerobic metabolisms can alter the δ13C of pore fluids. We investigate these sites to determine what conditions might enable AC to alter δ13C of marine DIC. We find there is very little net addition of AC relative to BC, but large quantities of AC form today across many settings via recrystallization. In settings where remineralization of organic matter causes recrystallized carbonate to form with modified δ13C, AC/BC is generally too low for this recrystallization to significantly shift the δ13C of the bulk carbonate. However, exceptions are found in sites with very low BC and extensive methane oxidation, suggesting that this environment type would need to be globally extensive in the past in order for AC formation to change the δ13C of marine DIC.

Magnesian calcite is perhaps the most well studied solid solution in the geosciences due to the widespread use of marine carbonates to reconstruct paleoenvironment. Despite decades of research, the low temperature thermodynamic properties of magnesian calcite in seawater are poorly constrained, largely because very slow reaction kinetics prevent the direct measurement of equilibrium distribution coefficients (K d Mg) for anhydrous Mg-bearing minerals. In this study, we use the Mg content of authigenic calcite formed in deep-sea marine carbonate sediments to determine the dependence of K d Mg on temperature and aqueous Mg/Ca between ~2 and 25°C. We find that the solid activity coefficient of magnesite in Mg-calcite is strongly temperature dependent in this range, leading to predicted exsolution of Mg at low temperatures. At the temperatures typical of ocean bottom water, equilibrium Mg distribution coefficients are at least an order of magnitude lower than values inferred from inorganic calcite 1 growth experiments. Moreover, the equilibrium temperature dependence of K d Mg agrees well with field-based paleotemperature calibrations determined for low-Mg benthic and planktonic foraminifera at temperatures < 20°C. Partitioning of Mg in foraminiferal tests is expected to be highly dependent on the Mg/Ca ratio in the calcifying fluid, so ignoring secular variations in seawater Mg/Ca can lead to significant underestimation of paleotemperatures.