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The most stable Cm3+ sorption species at the water-gibbsite interface are predicted to be a tridentate surface complex, [t 7 -S(OH) 3 –Cm(H 2 O) 5 ]3+, in weakly acidic/neutral solution condition and a bidentate one, [b 6 -S(OH) 2 –Cm(OH)(H 2 O) 4 ]2+, in the alkaline solution condition by combining the AIMD simulations and static DFT geometry optimization calculations. Based on the high-accuracy SO-CAS(7e,7o)SCF/NEVPT2 calculations, the simulated emission spectra of the Cm3+ aquo ion and the two surface complexes show a gradual decrease trend in the emission energy in good agreement with TRLFS experimental observation of a red shift of peak maximum with pH increasing from 5 to 11. [Display omitted] The information of structure and stability of actinide species is key to understand the sorption mechanism of actinides at mineral–water interface. Such information is approximately derived from experimental spectroscopic measurements and needs to be accurately obtained by a direct atomic-scale modelling. Herein, systematic first-principles calculations and ab initio molecular dynamics (AIMD) simulations are carried out to study the coordination structures and absorption energies of Cm(III) surface complexes at gibbsite-water interface. Eleven representative complexing sites are investigated. The most stable Cm3+ sorption species are predicted to be a tridentate surface complex in weakly acidic/neutral solution condition and a bidentate one in the alkaline solution condition. Moreover, luminescence spectra of the Cm3+ aqua ion and the two surface complexes are predicted based on the high-accuracy ab initio wave function theory (WFT). The results give a gradually decreasing emission energy in good agreement with experimental observation of a red shift of peak maximum with pH increasing from 5 to 11. This work is a comprehensive computational study involving AIMD and ab initio WFT methods to gain the coordination structures, stabilities, and electronic spectra of actinide sorption species at the mineral–water interface, thus providing important theoretical support for geological disposal of actinide waste. [ABSTRACT FROM AUTHOR]

Based on static DFT calculations and AIMD simulations, the stable outer- and inner-sphere adsorption complexes are respectively formed through hydrogen bonding and coordinate bonding with aluminol groups of the kaolinite(0 0 1) surface. The most stable inner-sphere complexes are predicted to be tridentate surface complexes with an eight-folded coordination structure in the first coordination sphere of Am3+. [Display omitted] • Adsorption of Am3+ and Am(OH)2+ aqua ions at the kaolinite(0 0 1)-water interface is investigated by static DFT and AIMD simulations. • Stable outer- and inner-sphere Am(III) adsorption complexes are identified, highlighting the role of hydrogen bonding and coordinate bonding with surface aluminol groups. • A tridentate, eight-folded coordination structure for the most stable Am(III) inner-sphere complexes is predicted across a pH range of 5.5–7. • The efficiency and accuracy of combining static DFT calculations for initial screening and AIMD simulations for comprehensive dynamic analysis are demonstrated. Understanding the structures and stabilities of actinide species is crucial for comprehending the sorption mechanisms at mineral–water interfaces. However, the relevant molecular-level fundamental aspects of actinide coordination chemistry at the mineral–water interface remain unclear. We herein conduct a systematic theoretical study of coordination structures and adsorption energies of Am3+ and Am(OH)2+ sorption complexes at the kaolinite(0 0 1)-water interface by integrating static density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. Computational results show that stable outer- and inner-sphere adsorption complexes are respectively formed through hydrogen bonding and coordinate bonding with surface aluminol groups. The most stable inner-sphere species are predicted to be tridentate surface complexes with an eight-folded coordination structure in the first coordination sphere of Am(III). This work deepens the understanding of Am(III) adsorption and complexation behaviors at the mineral–water interface, providing essential insights for actinide migration in natural environments and nuclear waste disposal. The synergy between static DFT calculations for initial screening and AIMD simulations for comprehensive dynamic analysis in this work enhances the efficiency and accuracy of theoretical predictions for actinide environmental chemistry and can be helpful for the broader research fields of geochemistry and environmental science. [ABSTRACT FROM AUTHOR]

The dissolution of minerals in contact with water plays a crucial role in geochemistry. However, obtaining molecular insight into interfacial chemistry is challenging. Dissolution typically involves the release of ions from the surface, giving rise to a charged mineral surface. This charge affects the interfacial water arrangement, which can be investigated by surface‐specific vibrational Sum Frequency Generation (v‐SFG) spectroscopy. For the fluorite‐water interface, recent spectroscopic studies concluded that fluoride adsorption/desorption determines the surface charge, which contrasts zeta potential measurements assigning this role to the calcium ion. By combining v‐SFG spectroscopy and flow experiments with systematically suppressed dissolution, we uncover the interplay of dominant fluoride and weak calcium adsorption/desorption, resolving the controversy in the literature. We infer the calcium contribution to be orders of magnitude smaller, emphasizing the sensitivity of our approach. [ABSTRACT FROM AUTHOR]

Barite (Ba[SO4]) is one of the promising candidates for sequestration of radioactive waste. Barite can incorporate radium (Ra) and form ideal solid solutions, i.e., (Ba,Ra)[SO4]. Together with isostructural celestite (Sr[SO4]), ternary solid solutions, (Ba,Sr,Ra)[SO4], may exist in natural conditions. Our fundamental understanding of the dissolution kinetics of isostructural sulfates is critically important for a better risk assessment of nuclear waste repositories utilizing this mineral for sequestration. So far, the barite-water interface has been studied with experimental methods and atomistic computer simulations. The direct connection between the molecular scale details of the interface structure and experimental observations at the microscopic scale is not yet well understood. Here, we began to investigate this connection by using a kinetic Monte Carlo approach to simulate the barite dissolution process. We constructed a microkinetic model for the dissolution process and identified the reactive sites. Identification of these sites is important for an improved understanding of the dissolution, adsorption, and crystal growth mechanisms at the barite–water interface. We parameterized the molecular detachment rates by using the experimentally observed etch pit morphologies and atomic step velocities. Our parameterization attempts demonstrated that local lattice coordination is not sufficient to differentiate between the kinetically important sites and estimate their detachment rates. We suggest that the water structure and dynamics at identified sites should substantially influence the detachment rates. However, it will require more work to improve the parameterization of the model by means of Molecular Dynamics and ab initio calculations. [ABSTRACT FROM AUTHOR]

Na+, K+, Cs+ and Ca2+ are common exogenous cations that could be introduced into soils either via long‐term tillage with inorganic fertilizers or the leakage of nuclear waste. However, the manner in which they co‐adsorb and compete with the intrinsic potassium ions in illite, the response of intrinsic potassium (K′+), as well as the underlying mechanisms of these interactions, remain unclear. These aspects were explored using molecular dynamics simulations based on a series of concentrations. Because of the competition between exogenous cations and K′+, the number of adsorbed exogenous cations such as Na+ increased by 11.2, whereas the corresponding number of adsorbed potassium decreased by 14.2, with the increase in concentration from 0.1 to 1.0 mol·L−1, indicating that long‐term application of inorganic fertilizers tends to promote the release of intrinsic potassium. This was further validated from the increased adsorption free energy of IS (inner‐sphere) K′+, which increased from −54.3 kJ·mol−1 (0.1 mol·L−1) to −42.9 kJ·mol−1 (1.0 mol·L−1). The stability of both exogenous cations and intrinsic potassium decreased with the increase in concentration. This was mainly caused by the reduction of illite–cation interactions, as evidenced by the radial distribution function analysis. Nonetheless, intrinsic potassium always dominated the competition, as its abundance was lower than that of exogenous cations only beyond 0.7 mol·L−1. Different exogenous cations exerted diverse effects on the intrinsic potassium and may vary with the concentration, as reflected by the distribution, adsorption number, coordination environment and adsorption stability. Generally, the effect of exogenous cations on intrinsic potassium decreased as Ca2+ > Na+ > Cs+, which may have been caused by the different adsorption sites. These results provide new insight into the environmental effect of the excessive use of inorganic fertilizers and the leakage of radioactive waste in soils. Highlights: Increase in exogenous cations tended to promote the release of intrinsic potassium in illite.Intrinsic potassium generally dominated the competition with exogenous cations.Stability of adsorbed mixed cations was reduced with increase of concentration.Exogenous cations exerted a diverse effect on the intrinsic potassium. [ABSTRACT FROM AUTHOR]

Crystal truncation rod (CTR) X-ray diffraction is an invaluable tool for measuring mineral surface and adsorbate structures, and has been applied to several environmentally and geochemically important systems. Traditionally, the method has been restricted to single crystals with lateral dimensions >3 mm. Minerals that meet this size criterion represent a minute fraction of those that are relevant to interfacial geochemistry questions, however. Crystal screening, data collection, and CTR measurement methods have been developed for crystals of <0.3 mm in lateral size using the manganese oxide mineral chalcophanite (ZnMn3O7·3H2O) as a case study. This work demonstrates the feasibility of applying the CTR technique to previously inaccessible surfaces, opening up a large suite of candidate substrates for future study. [ABSTRACT FROM AUTHOR]

Due to the supply-side reform and environmental protection in China, many small coal mines have been closed since 2015. However, acid mine drainage from these coal mines are continuously discharging into many rural creeks, which requires the systematical investigation on the variations of geochemical and environmental biological aspects in these water systems. In this study, from a classic acid mine drainage (AMD) from a closed coal mine of Hunan, China, various sediments and water samples in different sections were collected and analyzed. According to the corresponding Mineralogical and simple bacterial characteristics analysis (16S rRNA gene sequencing), the main findings were: 1) Secondary iron-containing minerals gradually transited from Gr(CO 3 2−) (green rust), Sh (schwertmannite) to Akg (Akaganeite) and more stable Gt (Goethite); 2) compared to the pristine sediment, these minerals decreased the acid-neutralizing capacity and cation exchange capacity (CEC) of sediments; 3) Proteobacteria and Firmicutes were the dominant phyla and the obvious variation of Firmicutes species was observed in the creek affected by AMD, which probably could been a biological index to diagnose the natural attenuation of AMD. These results could be greatly significant to understand typical variations of creek attenuation and bacterial community in the presence of high metal and sulfate concentration. Unlabelled Image • The secondary Fe-containing minerals gradually vary along AMD-affected Creek. • Firmicutes community significantly increases in the polluted sites. • The adsorption capacity and cation exchange capacity (CEC) of sediments decreases due to the instead of secondary minerals. [ABSTRACT FROM AUTHOR]

Reactions of elements at minerals-water interface are critical for their environmental fate in aquatic environment. Aiming at a mechanistic understanding of Se(IV) interaction with metal hydroxide, we carried out first-principles molecular dynamics (FPMD) simulations to quantitatively explore the microscopic structural characteristics and thermodynamic properties of Se(IV) complexation on different surfaces of gibbsite (Al(OH) 3). The complexing structures in different ways of HSeO 3 − adsorbed on basal and edge surfaces are characterized in detail. On basal surface, outer-sphere complexation of HSeO 3 − through hydrogen bonding and ternary inner-sphere complexation by cation bridging are formed. On edge surface, HSeO 3 − can be complexed as monodentate, mononuclear bidentate and binuclear bidentate inner-sphere ways onto different binding sites, and the complexing bond lengths agree well with available data by spectroscopic experiments. Desorption free energies indicate that inner-sphere edge complexes are favorable and spontaneous, and the binuclear bidentate complex on (010) edge is the most favored. The calculated acidity constants (p K a) of edge complexes indicate that these HSeO 3 − complexes can get deprotonated at ambient conditions. Accordingly, reaction pathways of the ligand exchange and thermodynamic stabilities of the different inner-sphere complexes are clarified. Consequently, the crystal facet- and pH-dependent complexation reaction mechanism of Se(IV) is disclosed based on the acquired surface reactive sites, geometric structures and coordination characteristics, electron transfer and bonding analysis, and thermodynamics, which is beneficial to inspiring an improved understanding of the aqueous environmental processes of inorganic contaminants. [Display omitted] • Complexations of Se(IV) on different surfaces of gibbsite were revealed by FPMD. • Outer- and inner-sphere complexes formed on basal surface via H-bonding and cation bridging, respectively. • On edge surfaces, Se(IV) formed different inner-sphere complexes. • Binuclear bidentate complexing is the most preferential species. • Se(IV)-cation-edge ternary complexation is probable at very high pH. [ABSTRACT FROM AUTHOR]

Biominerals are crucial materials that play a vital role in many forms of life. Understanding the various steps through which ions in aqueous environment associate to form increasingly structured particles that eventually transform into the final crystalline or amorphous poly(a)morph in the presence of biologically active molecules is therefore of great significance. In this context, computer modeling is now able to provide an accurate atomistic picture of the dynamics and thermodynamics of possible association events in solution, as well as to make predictions as to particle stability and possible alternative nucleation pathways, as a complement to experiment. This review provides a general overview of the most significant computational methods and of their achievements in this field, with a focus on calcium carbonate as the most abundant biomineral. [ABSTRACT FROM AUTHOR]

Reactive iron (Fe) mineral coatings found in subsurface reduction-oxidation transition zones (RTZs) contribute to the attenuation of contaminants. An 18.3-m anoxic core was collected from the site, where constituents of concern (COCs) in groundwater included chlorinated solvents. Reactive Fe mineral coatings were found to be abundant in the RTZs. This research focused on evaluating reaction kinetics with anoxic sediments bearing ferrous mineral nano-coatings spiked with either tetrachloroethylene (PCE), trichloroethylene (TCE), or 1,4-dichlorobenzene (1,4-DCB). Reaction kinetics with RTZ sediments followed pseudo-first-order reactions for the three contaminants with 90% degradation achieved in less than 39 days. The second-order rate constants for the three COCs ranged from 6.20 × 10-4 to 1.73 × 10-3 Lg-1h-1 with pyrite (FeS 2), 4.97 × 10-5 to 1.24 × 10-3 Lg-1h-1with mackinawite (FeS), 1.25 × 10-4 to 1.89 × 10-4 Lg-1h-1 with siderite (FeCO 3), and 1.79 × 10-4 to 1.10 × 10-3 Lg-1h-1 with magnetite (Fe 3 O 4). For these three chlorinated solvents, the trend for the rate constants followed: Fe(II) sulfide minerals > magnetite > siderite. The high reactivity of Fe mineral coatings is hypothesized to be due to the large surface areas of the nano-mineral coatings. As a result, these surfaces are expected to play an important role in the attenuation of chlorinated solvents in contaminated subsurface environments. [Display omitted] • Reactive Fe mineral coatings play an important role in contaminant dehalogenation. • Dechlorination for 1,4-DCB was observed with reactive Fe minerals coatings. • 90% degradation of 1,4-DCB, TCE, and PCE with RTZ sediments less than 39 days. • In degradation, the following trend was found: Fe(II) sulfides>magnetite>siderite. [ABSTRACT FROM AUTHOR]

Reactions occurring at mineral–fluid interfaces are important in all geochemical processes and essential for the cycling of elements within the Earth. Understanding the mechanism of the transformation of one solid phase to another and the role of fluids is fundamental to many natural and industrial processes. Problems such as the interaction of minerals with CO 2 -saturated water, the durability of nuclear waste materials, the remediation of polluted water, and mineral reactions that can destroy our stone-based cultural heritage, are related by the common feature that a mineral assemblage in contact with a fluid may be replaced by a more stable assemblage. [ABSTRACT FROM AUTHOR]

Abstract: Nanogeochemistry—a newly emerging research field—attempts to understand geochemical reactions and mass transfers at nanometer scales, especially with regards to the formation of nanostructures in geochemical systems, emergent properties of these structures, and their controls on geochemical processes. The research also includes use of nanotechnology to design new materials and engineering approaches for effective natural resource extraction and environmental management. At the core of this new research field is the concept that, as the size of a material is reduced to nanometers, novel physical or chemical properties of the material may emerge that can be drastically different from those of the corresponding bulk phase and the material properties then become size-dependent. Nanostructures, which frequently occur in geologic materials, may directly control mineral phase stability, mineral–water interface chemistry, geochemical reaction kinetics, geo-fluid migration and transport, and even global biogeochemical cycles as a whole. This paper aims to provide a comprehensive review of recent progress in nanogeochemical research. The review is focused on two general types of nanostructures—nano solid phases and nanopores (nanofluids)—with an emphasis on the occurrence of each nanostructure in natural environments, the associated emergent properties, and the potential geochemical implications. Stemming from an increasing interest in shale gas research, a special discussion is provided on gas/oil disposition and migration in unconventional low-permeability reservoirs, wherein shale is treated as a nanocomposite material. Nanogeochemistry is a relatively young research field, and much remains to be explored. There is an urgent need for systematically characterizing specific nanostructures over the whole nanometer-size range and developing a general theoretical framework for data analysis and synthesis. There is also a need for developing experimental and modeling techniques to extrapolate the knowledge obtained from simple model systems to complex natural systems. [Copyright &y& Elsevier]

Selenium is usually known as the ‘double-edged sword element’ for its dual toxic and beneficial character to health. Since the pioneer works by Schwarz and Foltz on the relationships between selenium deficiency and liver, muscle and heart diseases, many efforts have been undertaken to better understand the role of selenium in health. At the same time, an increasing number of publications have appeared during these last years on the selenium physico–chemical interactions within the environment. Both types of research represent ongoing efforts to correctly estimate the bioavailability of selenium species for health and the environment. Redox reactions, diffusion, adsorption and precipitation processes or interactions with organic matter and biota govern the speciation and mobility of selenium in the environment. This review intends to emphasize and collect the important advances made during these last years in the mechanistic understanding of processes which govern selenium cycling and bioavailability, like adsorption at the mineral/water interface, precipitation of elemental selenium, or bioavailability of nanoscaled precipitates. The advent of powerful spectroscopic techniques, like X-ray absorption spectroscopy, has allowed the structural description of adsorption and substitution processes that selenium undergoes in a variety of minerals. These and other structural details about selenium precipitates are reviewed here, together with their relationships to the bioavailability of the element in the environment. [ABSTRACT FROM AUTHOR]

Abstract: Chemical reactions occurring at the mineral–water interface are controlled by an interfacial layer, nanometers thick, whose properties may deviate from those of the respective bulk mineral and water phases. The molecular-scale structure of this interfacial layer, however, is poorly constrained, and correlations between macroscopic phenomena and molecular-scale processes remain speculative. The application of high-resolution X-ray scattering techniques has begun to provide substantial new insights into the molecular-scale structure of the mineral–water interface. In this review, we describe the characteristics of synchrotron-based X-ray scattering techniques that make them uniquely powerful probes of mineral–water interfacial structures and discuss the new insights that have been derived from their application. In particular, we focus on efforts to understand the structure and distribution of interfacial water as well as their dependence on substrate properties for major mineral classes including oxides, carbonates, sulfates, phosphates, silicates, halides and chromates. We compare these X-ray scattering results with those from other structural and spectroscopic techniques and integrate these to provide a conceptual framework upon which to base an understanding of the systematic variation of mineral–water interfacial structures. [Copyright &y& Elsevier]

The molecular-scale adsorption mechanism of heavy metal ions in the interlayer and nanopore regions of montmorillonite (MMT) were investigated by molecular dynamics simulations. Three typical heavy metals (zinc, cadmium, and lead) were selected as the model ions, and two types of MMT (Arizona and Wyoming) were considered. The results showed that Cd2+ and Pb2+ can form both inner- and outer-sphere complexes on Wyoming MMT, while Zn2+ only formed outer-sphere complex due to the stronger hydration interaction of Zn2+ than Cd2+ and Pb2+. For Arizona MMT, all of the three ions only formed outer-sphere complexes on its interlayer and external basal surface in which the cations remained a fully hydrated state. The calculated diffusion coefficients of three cations in interlayer and nanopore indicated that their diffusion abilities were significantly impaired, implying that MMT adsorbents have a strong ability to fix and retard heavy metal ions. The derived results and mechanisms are instrumental to a profound understanding of the transport and retention of heavy metal elements in subsurface environments, and provide guidance for the management of heavy metal pollution. [Display omitted] • Zn2+ cannot form inner-sphere adsorption on Wyoming MMT surface. • The adsorption strength of Wyoming MMT was slightly stronger than that of Arizona. • The electrostatic force was the dominant energy for adsorption of cations. • The mobilities of cations in the interlayer and nanopore of MMT was retarded. [ABSTRACT FROM AUTHOR]