Search

HypothesisCalcium carbonate nucleation is often a complex and multistep process that is difficult to follow in situ. The time-resolved electrochemical and electrophoretic methods can provide a new insight into the nucleation pathway.ExperimentsHere, we used a combination of speciation calculations with time-resolved electrophoretic and potentiometric methods to monitor calcium carbonate precipitation from a slightly supersaturated solution.FindingsAfter an initial mixing period of three minutes in which metastable CaCO3 phases may have nucleated and subsequently dissolved due to locally-high supersaturations, bulk solution pH and Ca2+ concentrations stabilize before decreasing in tandem with the precipitation of a CaCO3 phase. After an hour, the precipitate is dominated by calcite that grows at the expense of dissolving vaterite. The time-dependent electrokinetic potential shows analogous signatures of multistage nucleation process: initial rapid changes in ζ-potential are followed by much slower equilibration starting about one hour after reagents are mixed. The changes in ζ-potential, solution pH, saturation indexes, and particle morphology are consistent with vaterite to calcite transformation via dissolution of the former and recrystallization of the latter. These findings highlight the potential use of ζ-potential measurements for monitoring polymorphic transformations of carbonate phases in-situ.

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 dMg ) 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 dMg 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 growth experiments. Moreover, the equilibrium temperature dependence of K dMg agrees well with field-based paleotemperature calibrations determined for low-Mg benthic and planktonic foraminifera at temperatures

Deserts comprise a large portion of the Earth's land area, yet their role in the fluxes and cycles of greenhouse gases is poorly known and their likely response to climate change largely unexplored. We report a reconnaissance investigation of the concentrations and fluxes of CO2, CH4, and N2O along two elevation (climate) gradients in the southwestern United States. In-soil concentrations of CO2 increased with elevation (up to 5,000 ppm). Concentrations of CH4 declined with depth in all soils (to less than 1 ppm), but the rates of decrease with depth increased with elevation. In contrast, concentrations and depth trends of N2O varied erratically. Soils were net CO2 sources (0 to >1,500 kg CO2·ha−1·year−1), and net CH4 sinks (0.2 to >3 kg CH4·ha−1·year−1). The small and variable N2O fluxes were inconsistent with the trends in soil N δ15N values, which decreased by 5‰ to 6‰ over about 1,000 m of elevation. The high soil N δ15N values (up to nearly 17‰ at the lowest elevation) indicate that there is a soil N loss mechanism that is highly depleted in 15N, and gaseous losses—either NH3 or N2O/N2—are suspected of driving these values. In summary, there appears to be a strong climate control on both soil CO2 and CH4 concentrations and to a lesser degree on calculated fluxes. The soil N trace gas concentrations indicate that deserts can be either small sources or sinks of N2O and that there may be significant consumption of arid soil N2O.

Cesium-137 is a major byproduct of nuclear energy generation and is environmentally threatening due to its long half-life and affinity for naturally occurring micaceous clays. Recent experimental observations of illite and phlogopite mica indicate that Cs+ is capable of exchanging with K+ bound in the anhydrous interlayers of layered silicates, forming sharp exchange fronts, leading to interstratification of Cs- and K-illite. We present here a coarse-grained (CG) model of the anhydrous illite interlayer developed using iterative Boltzmann inversion that qualitatively and quantitatively reproduces features of a previously proposed feedback mechanism of ion exchange. The CG model represents a 70-fold speedup over all-atom models of clay systems and predicts interlayer expansion for K-illite near ion exchange fronts. Contrary to the longstanding theory that ion exchange in a neighboring layer increases the binding of K in lattice counterion sites leading to interstratification, we find that the presence of neighboring exchanged layers leads to short-range structural relaxations that increase basal spacing and decrease cohesion of the neighboring K-illite layers. We also provide evidence that the formation of alternating Cs- and K-illite interlayers (i.e., ordered interstratification) is both thermodynamically and mechanically favorable compared to exchange in adjacent interlayers.

When driven out of equilibrium, many diverse systems can form complex spatial and dynamical patterns, even in the absence of attractive interactions. Using kinetic Monte Carlo simulations, we investigate the phase behavior of a binary system of particles of dissimilar size confined between semiflexible planar surfaces, in which the nanoconfinement introduces a non-local coupling between particles, which we model as an activation energy barrier to diffusion that decreases with the local fraction of the larger particle. The system autonomously reaches a cyclical non-equilibrium state characterized by the formation and dissolution of metastable micelle-like clusters with the small particles in the core and the large ones in the surrounding corona. The power spectrum of the fluctuations in the aggregation number exhibits 1/f noise reminiscent of self-organized critical systems. We suggest that the dynamical metastability of the micellar structures arises from an inversion of the energy landscape, in which the relaxation dynamics of one of the species induces a metastable phase for the other species.

The mobility of radiocesium in the environment is largely mediated by cation exchange in micaceous clays, in particular Illite-a non-swelling clay mineral that naturally contains interlayer K+ and has high affinity for Cs+. Although exchange of interlayer K+ for Cs+ is nearly thermodynamically nonselective, recent experiments show that direct, anhydrous Cs+-K+ exchange is kinetically viable and leads to the formation of phase-separated interlayers through a mechanism that remains unclear. Here, using classical atomistic simulations and density functional theory calculations, we identify a molecular-scale positive feedback mechanism in which exchange of the larger Cs+ for the smaller K+ significantly lowers the migration barrier of neighboring K+, allowing exchange to propagate rapidly once initiated at the clay edge. Barrier lowering upon slight increase in layer spacing (∼0.7 Å) during Cs+ exchange is an example of "chemical-mechanical coupling" that likely explains the observed sharp exchange fronts leading to interstratification. Interestingly, we find that these features are thermodynamically favored even in the absence of a heterogeneous layer charge distribution.

The mobility of radiocesium in the environment is largely mediated by cation exchange in micaceous clays, in particular Illite-a non-swelling clay mineral that naturally contains interlayer K+ and has high affinity for Cs+. Although exchange of interlayer K+ for Cs+ is nearly thermodynamically nonselective, recent experiments show that direct, anhydrous Cs+-K+ exchange is kinetically viable and leads to the formation of phase-separated interlayers through a mechanism that remains unclear. Here, using classical atomistic simulations and density functional theory calculations, we identify a molecular-scale positive feedback mechanism in which exchange of the larger Cs+ for the smaller K+ significantly lowers the migration barrier of neighboring K+, allowing exchange to propagate rapidly once initiated at the clay edge. Barrier lowering upon slight increase in layer spacing (∼0.7 Å) during Cs+ exchange is an example of "chemical-mechanical coupling" that likely explains the observed sharp exchange fronts leading to interstratification. Interestingly, we find that these features are thermodynamically favored even in the absence of a heterogeneous layer charge distribution.

Hypothesis: Calcium carbonate nucleation is often a complex and multistep process that is difficult to follow in situ. The time-resolved electrochemical and electrophoretic methods can provide a new insight into the nucleation pathway., Experiments: Here, we used a combination of speciation calculations with time-resolved electrophoretic and potentiometric methods to monitor calcium carbonate precipitation from a slightly supersaturated solution., Findings: After an initial mixing period of three minutes in which metastable CaCO 3 phases may have nucleated and subsequently dissolved due to locally-high supersaturations, bulk solution pH and Ca 2+ concentrations stabilize before decreasing in tandem with the precipitation of a CaCO 3 phase. After an hour, the precipitate is dominated by calcite that grows at the expense of dissolving vaterite. The time-dependent electrokinetic potential shows analogous signatures of multistage nucleation process: initial rapid changes in ζ-potential are followed by much slower equilibration starting about one hour after reagents are mixed. The changes in ζ-potential, solution pH, saturation indexes, and particle morphology are consistent with vaterite to calcite transformation via dissolution of the former and recrystallization of the latter. These findings highlight the potential use of ζ-potential measurements for monitoring polymorphic transformations of carbonate phases in-situ., (Copyright © 2019 Elsevier Inc. All rights reserved.)