ACS Earth and Space Chemistry最新文献

The reduction of Hg^II to Hg^I or Hg⁰ can lead to significant changes in Hg toxicity and mobility in the environment. Photochemical reduction is the primary process for the reduction of Hg^II to Hg⁰ in sunlit environments; however, dark reduction of Hg^II can occur via microbial metabolic processes and/or reduction by reduced natural organic matter, Fe^II mineral phases, Fe^II sorbed to minerals, or aqueous Fe^II. Here, we demonstrate a novel Hg^II reduction pathway involving another environmentally relevant reductant, Mn^II. Abiotic reduction of Hg^IIO by Mn^II was studied as a function of pH and anion environment (perchlorate, sulfate, chloride) using X-ray absorption spectroscopy to characterize the solid-phase Hg and Mn species. At circumneutral pH of 7.5, about 70% of Hg^II was reduced to elemental Hg⁰ within 2 h. In contrast, 12 h were needed to achieve the same extent of reduction at pH 6.9. In the presence of sulfate and chloride, Hg^I species were formed. Hg^II reduction was initially rapid and coupled with the oxidation of soluble Mn^II-oxides to insoluble Mn^IV-oxides, followed by a significantly slower reduction of Hg^II during the Mn^II-catalyzed transformation of the Mn^IV-oxides to hydroxide and oxyhydroxide minerals. The observed reduction of Hg^II by Mn^II at circumneutral pH could be an important transformation pathway for environmental Hg, affecting its bioavailability and mobility under mildly reducing conditions.

Investigating the habitability of ocean worlds is a priority of current and future NASA missions. The Europa Clipper mission will conduct approximately 50 flybys of Jupiter’s moon Europa, returning a detailed portrait of its interior from the synthesis of data from its instrument suite. The magnetometer on board has the capability of decoupling Europa’s induced magnetic field to high precision, and when these data are inverted, the electrical conductivity profile from the electrically conducting subsurface salty ocean may be constrained. To optimize the interpretation of magnetic induction data near ocean worlds and constrain salinity from electrical conductivity, accurate laboratory electrical conductivity data are needed under the conditions expected in their subsurface oceans. At the high-pressure, low-temperature (HPLT) conditions of icy worlds, comprehensive conductivity data sets are sparse or absent from either laboratory data or simulations. We conducted molecular dynamics simulations of candidate ocean compositions of aqueous NaCl under HPLT conditions at multiple concentrations. Our results predict electrical conductivity as a function of temperature, pressure, and composition, showing a decrease in conductivity as the pressure increases deeper into the interior of an icy moon. These data can guide laboratory experiments at conditions relevant to icy moons and can be used in tandem to forward-model the magnetic induction signals at ocean worlds and compare with future spacecraft data. We discuss implications for the Europa Clipper mission.

Hydrogen cyanide (HCN)-derived molecules and polymers are featured in several hypotheses on the origin of life. Over half a century of investigations into HCN self-reactions have led to many suggestions regarding the structural nature of the products and an even greater number of proposed polymerization pathways. A comprehensive overview of possible reactions and structures is missing. In this work, we use quantum chemical calculations to map the relative Gibbs free energy of most HCN-derived molecules and polymers that have been discussed in the literature. Our computed free energies indicate that several previously considered polymerization pathways are not spontaneous and should be discarded from future consideration. Among the most thermodynamically favored products are polyaminoimidazole and adenine.

Reactive transport modeling is a critical tool for elucidating the coupling among geochemical reactions, transport processes, and fracture permeability. This study explores the implications of preferential clay-rich regions near fracture surfaces of a Mancos shale sample on the reactive evolution of the fracture by using reactive transport simulations. These simulations utilized heterogeneous mineralogy data obtained from an in-depth analysis of a clay-rich region near the fracture surface, sourced from a mechanically induced fracture. We compared these simulations with counterparts assuming homogeneous mineral distributions based on bulk X-ray diffraction (XRD) data and prior imaging of the sample matrix. The results consistently show increased reactivity in cells near the inlet and fracture surface across all scenarios. The most significant changes in the porosity, mineral composition, and ion concentration occur in cells adjacent to the fracture at the system inlet. Comparative analysis reveals variations in mineral and porosity evolutions among the three systems. Over longer simulation periods, dissolution and porosity increase occur more rapidly in simulations, reflecting mineral heterogeneity, particularly within the clay-rich region near the fracture. A sensitivity analysis of mineral surface area (SA) values shows consistent trends using both low and high Brunauer–Emmett–Teller (BET) SA values over short time scales (days) but substantial disparities over longer time scales (years). These findings hold promise for improving our ability to predict the evolution of reactive fractures, with implications for subsurface CO₂ sequestration and oil recovery. In summary, this study advances our understanding of reactive transport in fractured systems, offering new avenues for predictive modeling.