Development and validation of a general-purpose ReaxFF reactive force field for earth material modeling.

ReaxFF reactive force field bridges the gap between nonreactive molecular simulations and quantum mechanical calculations and has been widely applied during the past two decades. However, its application to earth materials, especially those under high T-P conditions relevant to Earth's interior, is still limited due to the lack of available parameters. Here, we present the development and validation of a ReaxFF force field containing several of the most common elements in Earth's crust, i.e., Si/Al/O/H/Na/K. The force field was trained against a large data set obtained from density functional theory (DFT) calculations, including charges, bond/angle distortion curves, equation of states, ion migration energy profiles, and condensation reaction energies. Different coordination environments were considered in the training set. The fitting results showed that the current force field can well reproduce the DFT data (the Pearson correlation coefficient, Rp, is 0.95). We validated the force field on mineral-water interfaces, hydrous melts/supercritical geofluids, and bulk crystals. It was found that the current force field performed excellently in predicting the structural, thermodynamic, and transport properties of various systems (Rp = 0.95). Moreover, possible applications and future development have been discussed. The results obtained in this study suggest that the current force field holds good promise to model a wide range of processes and thus open opportunities to advance the application of ReaxFF in earth material modeling.

Diversity of Fungal Community and Its Constraints in the Yifeng Lithium Mines, Eastern China.

It is well accepted that biodiversity and ecosystem functions are strongly shaped by environmental conditions; however, relatively little is known about how they depend on the mineralogical assemblage of local environments, especially in mines. This study aims to reveal the diversity characteristics of the fungal community in the surface of granite lithium ores and their weathering products sampled from the Yifeng lithium mines in Jiangxi Province, eastern China. According to the analysis of internal transcribed spacer1 (ITS1) high-throughput sequencing, significant differences in fungal community diversity on the surface of lithium ores and their weathering products have been revealed. The operational taxonomic unit (OTU) of the ore surface and its weathering products ranged from 280 to 624, which may depend on the mineral composition as well as the degree of weathering. The community composition of each sample was significantly different at the phylum level, especially between the weathering products in Ascomycota and Basidiomycota. Although Ascomycota and Basidiomycota were the dominant fungal communities in all samples, each sample has its own distinctive fungi. The trophic modes of the fungi were more complex than that of the bacteria. 10 different fungal trophic modes and 25 dominant functional fungal groups were disclosed, and the saprophytic community was found to be the dominant group. These fungi could accelerate the decomposition of environmental organic matter in the environment by producing hydrolases and oxidases. Chytridiomycota with the function of producing and regulating secondary metabolites were the representative fungi in all samples. Our findings would provide theoretical basis and research clues for understanding the relationship between weathering of granite lithium and fungal communities.

Molecular Insights into the Salinity Effects on Movability of Oil-Brine in Shale Nanopore-Throat Systems.

Low-salinity flooding has been well recognized as a promising strategy to increase shale oil recovery, but the underlying mechanism remains unclarified, especially for complex nanopore networks filled with oil-brine fluids. In this study, the pressure-driven flow of an oil-brine fluid with varying salinities in shale nanopore-throat channels was first investigated based on molecular dynamics simulations. The critical pressure driving oil to intrude into a nanothroat filled with brine of varying salinities was determined. Simulation results indicate that the salinity of brine exhibits great effects on the movability of oil, and low salinity favors the increase of oil movability. Further analysis of the interactions between fluid and pore walls as well as the displacement pressures reveals dual effects of brine salinity on oil transportation in a nanopore-throat. On the one hand, hydrated cations anchoring onto throat walls enlarge the effective flow width in the throat before the hydration complexes reach the maximum. On the other hand, the interfacial tension between oil and brine increases with the brine salinity, which increases the capillary resistance and leads to a higher displacement pressure. These findings highlight the effects of brine salinity on oil movability in a nanopore-throat, which will promote the understanding of oil accumulation and dissipation in petroleum systems, as well as help to develop enhanced oil recovery.

Structure, Stability, and Acidity of the Uranyl Arsenate Dimer in Aqueous Solution.

The migration of uranium (U) in the surficial environment has received considerable attention. Due to their high natural abundance and low solubility, autunite-group minerals play a key role in controlling the mobility of U. However, the formation mechanism for these minerals has yet to be understood. In this work, we took the uranyl arsenate dimer ([UO2(HAsO4)(H2AsO4)(H2O)]22-) as a model molecule and carried out a series of first-principles molecular dynamics (FPMD) simulations to explore the early stage of the formation of trögerite (UO2HAsO4·4H2O), a representative autunite-group mineral. By using the potential-of-mean-force (PMF) method and vertical energy gap method, the dissociation free energies and the acidity constants (pKa's) of the dimer were calculated. Our results show that the U in the dimer holds a 4-coordinate structure, which is consistent with the coordination environment observed in trögerite mineralogy, in contrast to the 5-coordinate U in the monomer. Furthermore, the dimerization is thermodynamically favorable in solution. The FPMD results also suggest that tetramerization and even polyreactions would occur at pH > 2, as observed experimentally. Additionally, it is found that trögerite and the dimer have very similar local structural parameters. These findings imply that the dimer could serve as an important link between the U-As complexes in solution and the autunite-type sheet of trögerite. Given the nearly identical physicochemical properties of arsenate and phosphate, our findings suggest that uranyl phosphate minerals with the autunite-type sheet may form in a similar manner. This study therefore fills a critical gap in atomic-scale knowledge of the formation of autunite-group minerals and provides a theoretical basis for regulating uranium mobilization in P/As-bearing tailing water.

Lithium extraction from typical lithium silicate ores by two bacteria with different metabolic characteristics: Experiments, mechanism and significance.

Microorganisms obtain inorganic nutrients or energy from specific minerals to selectively weather minerals, but few studies on the differences in metabolic components of different functional bacteria lead to different weathering effects. This study evaluated the leaching effects of two bacteria with distinct metabolic characteristics on lithium silicate minerals with different structures. We aimed to understand the microscopic mechanism of crystal destruction of lithium silicate minerals with different structures under the action of microorganisms. The results showed that the metabolites produced by an acid producing silicate strain Raoultella sp. Z107 (strain Z107) had a high content of organic acids, among which lactic acid was up to about 11 g/L. Bacillus mucilaginosus 21,699 (strain BM) secreted capsular polysaccharide with a high content of 14.84 mg/L. The metabolic activities of the two strains were significantly different. Through the analysis of the leaching residue, it was found that the lithium silicate minerals were acid etched, interlayer domains expanded, crystallinity decreased, and metal bonds were broken under the action of bacteria. The dissolution of lithium silicate minerals by bacteria is a combination of bacterial adsorption, organic acid corrosion, and complexation of small molecular organic acids and macromolecular polymers with metal ions. The acid erosion and complexation effects of organic acids are greater than the single complexation of capsular polysaccharides, and the layered lepidolite is more likely to be decomposed by the weathering of bacterial metabolites than the chain structure spodumene. These results indicate that the diversity of metabolic activity of bacteria from different sources and the sequence and decomposition mechanism of metal ions released from minerals after lattice destruction are also different. Microorganisms decompose minerals for energy and nutrients, and eventually become the main players in the transformation of elements in biogeology.

Superionic Silica-Water and Silica-Hydrogen Compounds in the Deep Interiors of Uranus and Neptune.

Silica, water, and hydrogen are known to be the major components of celestial bodies, and have significant influence on the formation and evolution of giant planets, such as Uranus and Neptune. Thus, it is of fundamental importance to investigate their states and possible reactions under the planetary conditions. Here, using advanced crystal structure searches and first-principles calculations in the Si-O-H system, we find that a silica-water compound (SiO_{2})_{2}(H_{2}O) and a silica-hydrogen compound SiO_{2}H_{2} can exist under high pressures above 450 and 650 GPa, respectively. Further simulations reveal that, at high pressure and high temperature conditions corresponding to the interiors of Uranus and Neptune, these compounds exhibit superionic behavior, in which protons diffuse freely like liquid while the silicon and oxygen framework is fixed as solid. Therefore, these superionic silica-water and silica-hydrogen compounds could be regarded as important components of the deep mantle or core of giants, which also provides an alternative origin for their anomalous magnetic fields. These unexpected physical and chemical properties of the most common natural materials at high pressure offer key clues to understand some abstruse issues including demixing and erosion of the core in giant planets, and shed light on building reliable models for solar giants and exoplanets.

Acidity and metal complexation of edge surface of birnessite-type MnO2: Insight from first principles simulations.

Birnessite-type MnO2 plays key roles in scavenging trace elements in numerous natural environments and has also been regarded as a promising energy storage material. The interfacial properties of birnessite are highly pH-dependent due to the presence of various amphoteric groups on its edges, and, therefore, the acidity constants (pKa) of these groups are vital to the understanding of its electrochemical and environmental performances. However, an accurate acidity dataset for birnessite is absent yet. In this study, we employed first-principles molecular dynamics simulations and the vertical energy gap method to calculate the pKas of groups on the birnessite (010) edge. The interfacial hydration structure was characterized with a focus on the hydrogen bonding network. The obtained pKas suggest that MnOH2 is active while Mn2OH remains inert in a common pH range. Based on these results, the incorporation of transition metals on the edge surface was investigated by taking Ni2+ and Zn2+ as the model cations. The energy changes associated with the incorporation process of Ni2+ from the outer-sphere state indicate that incorporation on the edge surface is more feasible than that on the basal surface presumed in previous studies. Overall, the results obtained provide an atomic-scale insight into the acid-base chemistry of birnessite and form a physical basis for understanding the interfacial processes of birnessite.

Mixed Coordination Silica at Megabar Pressure.

Silica (SiO_{2}), as a raw material of silicon, glass, ceramics, abrasive, and refractory substances, etc., is of significant importance in industrial applications and fundamental research such as electronics and planetary science. Here, using a crystal structure searching method and first-principles calculations, we predicted that a ground state crystalline phase of silica with R3[over ¯] symmetry is stable at around 645-890 GPa, which contains six-, eight-, and nine-coordinated silicon atoms and results in an average coordination number of eight. This mixed-coordination silica fills in the density, electronic band gap, and coordination number gaps between the previously known sixfold pyrite-type and ninefold Fe_{2}P-type phases, and may appear in the core or mantle of super-Earth exoplanets, or even the solar giant planets such as the Neptune. In addition, we also found that some silicon superoxides, Cmcm SiO_{3} and Ccce SiO_{6}, are stable in this pressure range and may appear in an oxygen-rich environment. Our finding enriches the high-pressure phase diagram of silicon oxides and improves understanding of the interior structure of giant planets in our solar system.

Selection of Planar Chiral Conformations between Pillar[5,6]arenes Induced by Amino Acid Derivatives in Aqueous Media.

Chiral α-amino acids play critical roles in the metabolic process in nearly all life forms. So far, chiral recognition of α-amino acids has mainly focused on the determination of l/d enantiomers. Herein, selection of planar chiral conformations between water-soluble pillar[5]arene WP5 and pillar[6]arene WP6 was observed due to α-side chain or ethyl ester moieties of l-α-amino acid ethyl ester hydrochlorides binding with WP5 and WP6, respectively. Therefore, α-side chain and ethyl ester moieties of l-α-amino acid ethyl ester hydrochlorides were recognized by observing the induced CD signal and its inversion. This is a rare example of being able to detect the chiral region around α-carbon of a chiral α-amino acid molecule.

Surface Acidity and As(V) Complexation of Iron Oxyhydroxides: Insights from First-Principles Molecular Dynamics Simulations.

Iron hydroxides are ubiquitous in soils and aquifers and have been adopted as adsorbents for As(V) removal. However, the complexation mechanisms of As(V) have not been well understood due to the lack of information on the reactive sites and acidities of iron hydroxides. In this work, we first calculated the acidity constants (pKas) of surface groups on lepidocrocite (010), (001), and (100) surfaces by using the first-principles molecular dynamics (FPMD)-based vertical energy gap method. Then, the desorption free energies of As(V) on goethite (110) and lepidocrocite (001) surfaces were calculated by using constrained FPMD simulations. The point of zero charges and reactive sites of individual surfaces were obtained based on the calculated pKas. The structures, thermodynamics, and pH dependence for As(V) complexation were derived by integrating the pKas and desorption free energies. The pKa data sets obtained are fundamental parameters that control the charging and adsorption behavior of iron oxyhydroxides and will be very useful in investigating the adsorption processes on these minerals. The pH-dependent complexation mechanisms of As(V) derived in this study would be helpful for the development of effective adsorbent materials and the prediction of the long-term behavior of As(V) in natural environments.

Facet-Dependent Photodegradation of Methylene Blue by Hematite Nanoplates in Visible Light.

The expression of specific crystal facets in different nanostructures is known to play a vital role in determining the sensitivity toward the photodegradation of organics, which can generally be ascribed to differences in surface structure and energy. Herein, we report the synthesis of hematite nanoplates with controlled relative exposure of basal (001) and edge (012) facets, enabling us to establish direct correlation between the surface structure and the photocatalytic degradation efficiency of methylene blue (MB) in the presence of hydrogen peroxide. MB adsorption experiments showed that the capacity on (001) is about three times larger than on (012). Density functional theory calculations suggest the adsorption energy on the (001) surface is 6.28 kcal/mol lower than that on the (012) surface. However, the MB photodegradation rate on the (001) surface is around 14.5 times faster than on the (012) surface. We attribute this to a higher availability of the photoelectron accepting surface Fe3+ sites on the (001) facet. This facilitates more efficient iron valence cycling and the heterogeneous photo-Fenton reaction yielding MB-oxidizing hydroxyl radicals at the surface. Our findings help establish a rational basis for the design and optimization of hematite nanostructures as photocatalysts for environmental remediation.

Enhanced Fluoride Uptake by Layered Double Hydroxides under Alkaline Conditions: Solid-State NMR Evidence of the Role of Surface >MgOH Sites.

Layered double hydroxides (LDHs) are potential low-cost filter materials for use in fluoride removal from drinking water, but molecular-scale defluoridation mechanisms are lacking. In this research, we employed 19F solid-state NMR spectroscopy to identify fluoride sorption products on 2:1 MgAl LDH and to reveal the relationship between fluoride sorption and the LDH structure. A set of six 19F NMR peaks centered at -140, -148, -156, -163, -176, and -183 ppm was resolved. Combining quantum chemical calculations based on density function theory (DFT) and 19F{27Al} transfer of populations in double resonance (TRAPDOR) analysis, we could assign the peaks at -140, -148, -156, and -163 ppm to Al-F (F coordinated to surface Al) and those at -176 and -183 ppm to Mg-F (F coordinated to surface Mg only). Interestingly, the spectroscopic data reveal that the formation of Al-F is the predominant mode of F- sorption at low pH, whereas the formation of Mg-F is predominant at high pH (or a higher Mg/Al ratio). This finding supports the fact that the F- uptake of 2:1 MgAl LDH was nearly six times that of activated alumina at pH 9. Overall, we explicitly revealed the different roles of the surface >MgOH and >AlOH sites of LDHs in defluoridation, which explained why the use of classic activated alumina for defluoridation is limited at high pH. The findings from this research may also provide new insights into material screening for potential filters for F- removal under alkaline conditions.

Closest-Packing Water Monolayer Stably Intercalated in Phyllosilicate Minerals under High Pressure.

The directional hydrogen-bond (HB) network and nondirectional van der Waals (vdW) interactions make up the specificity of water. Directional HBs could construct an ice-like monolayer in hydrophobic confinement even in the ambient regime. Here, we report a water monolayer dominated by vdW interactions confined in a phyllosilicate interlayer under high pressure. Surprisingly, it was in a thermodynamically stable state coupled with bulk water at the same pressure (P) and temperature (T), as revealed by the thermodynamic integration approach on the basis of molecular dynamics (MD) simulations. Both classical and ab initio MD simulations showed water O atoms were stably trapped and exhibited an ordered hexagonal closest-packing arrangement, but OH bonds of water reoriented frequently and exhibited a specific two-stage reorientation relaxation. Strikingly, hydration in the interlayer under high pressure had no relevance with surface hydrophilicity rationalized by the HB forming ability, which, however, determines wetting in the ambient regime. Intercalated water molecules were trapped by vdW interactions, which shaped the closest-packing arrangement and made hydration energetically available. The high pressure-volume term largely drives hydration, as it compensates the entropy penalty which is restricted by a relatively lower temperature. This vdW water monolayer should be ubiquitous in the high pressure but low-temperature regime.

Molecular dynamics simulation of CO2-switchable surfactant regulated reversible emulsification/demulsification processes of a dodecane-saline system.

CO2-Switchable surfactants are of great potential in a wide range of industrial applications related to their ability to stabilize and destabilize emulsions upon command. Molecular dynamics simulations have been performed to reveal the fundamental mechanism of the reversible emulsification/demulsification processes of a dodecane-saline system by a CO2-switchable surfactant that switches between active (i.e., N'-dodecyl-N,N-dimethylacetamidinium (DMAAH+)) and inactive (i.e., N'-dodecyl-N,N-dimethylacetamidine (DMAA)) forms. The density profiles indicate that DMAAH+ could increase the oil-water interfacial thickness to a greater extent compared to DMAA. DMAAH+ could sharply reduce the interfacial tension of the dodecane-saline system, while DMAA only exhibits a limited decrease, which is in accordance with the experimental observation that DMAAH+/DMAA can reversibly emulsify/demulsify alkane-water systems. Our simulations showed that both the number and lifetime of hydrogen bonds (HBs) between DMAA and water are almost equal to those between DMAAH+ and water. In DMAA, the N atom connecting with the alkyl tail acted as a HB acceptor, while the N atom attached by a proton in DMAAH+ acted as a HB donor. Furthermore, the HBs between DMAAH+ and HCO3- at the interfaces are relatively limited. Hence, it is deduced that the HBs are insufficient to achieve the CO2-switchability of DMAA/DMAAH+. The Lennard Jones and coulombic potentials between DMAA/DMAAH+ and other species show that the coulombic potentials between DMAAH+ and water or anions (i.e., Cl- and HCO3-) sharply decrease with the increase of DMAAH+ and are much lower than those in models with DMAA. The enhanced coulombic interactions between DMAAH+ and anions lead to a remarkable reduction in interfacial tension and the emulsification of the alkane-saline system. Therefore, coulombic interactions are of crucial importance to the reversible emulsification/demulsification processes regulated by CO2-switchable surfactants, namely DMAAH+/DMAA.

Distribution and Mobility of Crude Oil-Brine in Clay Mesopores: Insights from Molecular Dynamics Simulations.

The value of crude oil accommodated in shale has been recognized and has attracted increasing attention from the academic and industrial society. The occurrence and mobility of crude oil in clay pores, therefore, become essential issues for evaluation and recovery of shale oil. The distribution, structure, and transport of the oil-brine mixture confined in a slit-shaped montmorillonite mesopore with different water amounts have been investigated using equilibrium molecular dynamics and nonequilibrium molecular dynamics (NEMD) simulations. A mimic model of crude oil, a mixture of 19 organic molecules, was employed, and thus the behavior of different organic molecules could be characterized in detail. A temperature of 410 K and a pressure of 300 atm corresponding to a buried depth of 3 km were employed. The simulations indicate that the water amount determines the distribution of crude oil. Water and metal ions prefer to cover on hydrophilic montmorillonite surfaces, while nonpolar hydrocarbons tend to be far away from clay surfaces. As the water amount is too low to completely cover the clay surfaces, some polar organic molecules will come into contact with the uncovered clay surface. Abundant organic acid molecules adsorb onto montmorillonite surfaces mainly through participating in the inner-sphere complexes of Na+ ions closely located at montmorillonite surfaces (i.e., Na+ cation bridge) and forming hydrogen bonds with water molecules in the vicinity. Carbazole molecules tend to aggregate together due to π-π stacking, while thioether molecules mix within alkane molecules and exhibit no characteristic distributions. The mobility of all oil components decreases with the decrease of the water amount, and the mobility of polar components (i.e., organic acid and carbazole) is relatively lower than that of nonpolar hydrocarbons. NEMD simulations clearly indicate that the transport velocity of crude oil markedly increases with the water amount under a specific pressure gradient. The brine covering on clay surfaces significantly weakens oil-clay interfacial interactions. Polar components, especially organic acid, exhibit relatively low transport velocity compared with nonpolar hydrocarbons. These findings highlight the understanding of physical-chemical behaviors of shale oil and provide atomistic information for technology development for enhancing oil recovery.

Synthesis of 2D Hexagonal Hematite Nanosheets and the Crystal Growth Mechanism.

Nanostructure engineering of hematite is a promising strategy to overcome its performance limitations as a photodegradation catalyst for organic dyes or toxic organic chemicals. Precise control of exposed facets plays a vital role as an optimization strategy. Although great progress has been made in the synthesis of different crystal morphologies, design principles remain ad hoc, awaiting systematic control of facet expression within a clean synthesis protocol yielding versatile results. Herein, we report a simple method that enables precise morphology control to synthesize 2D hexagonal hematite nanosheets from two-line ferrihydrite. Nanosheet sizes and facet proportions were successfully tuned by changing the pH, the ratio of solvent ethanol to water, and the concentration of FeIII ions. The growth mechanism appears to involve a combination of ferrihydrite solid-state transformation and assembly into hematite, followed by an ion-by-ion growth that perfects the surface terminations. The finding helps to establish a rational basis for the design and optimization of hematite nanostructures.

Understanding the Heterogeneous Nucleation of Heavy Metal Phyllosilicates on Clay Edges with First-Principles Molecular Dynamics.

The nucleation and precipitation of heavy metal phyllosilicates can occur in the course of sorption onto clay edges, which will provide a long-term stabilization of heavy metal pollutants. However, a quantitative understanding of their reaction mechanisms is still lacking. Taking Ni2+ as the model cation, we characterized the atomic scale structures and thermodynamics of the early stage of nucleation by carrying out systematic first-principles molecular dynamics (FPMD) simulations, and the microscopic nucleation mechanisms were revealed. Two possible nucleation pathways were examined: a stepwise pathway (denoted as Path1) and a synchronous pathway (denoted as Path2). In Path1, Ni(OH)2 forms first and then transforms to Ni phyllosilicate via silicification; in Path2, Ni phyllosilicate forms on clay edges directly. The computed free energies of complexation and condensation reactions indicate that Path2 is much more thermodynamically favorable than Path1, meaning that, given that the solution contains dissolved Si initially, heavy metal phyllosilicates will nucleate on clay edges through Path2. By comparing these free energies with their counterpart values of the reaction in bulk solution, the effect of the surface has been uncovered. These findings provide valuable insights for an improved understanding of the stabilization and transformation of heavy metal elements in nature. The derived results form a quantitative basis for future studies on the heterogenous nucleation and precipitation of heavy metal cations.

Influences of Cation Ratio, Anion Type, and Water Content on Polytypism of Layered Double Hydroxides.

Layered double hydroxides (LDHs) are a significant sink of anions (CO32-, SO42-, NO3-, Cl-, etc.) and divalent transition-metal cations in soil. The anion exchange capacity gives rise to functional materials. The stability of LDHs is determined by the interaction between cation-bearing layers and intercalated water and anions, which is correlated with polytypism and coordination structure. A systematic investigation is performed to show the influence of cation ratio, anion type, and water content on polytypism, swelling behavior, and interlayer structure of Mg-Al-LDHs using molecular dynamics simulations. LDHs intercalated with NO3- ions exhibit a polytype transition from 3 R1 (three-layer rhombohedral polytype) to 1 T (one-layer trigonal polytype) with increasing water content. NO3- ions exhibit a D3 h point group symmetry at low water contents. The polytype transition coincides with the complete transformation into tilted NO3- ion with a C2 v point group symmetry. The transition appears at a lower water content when the Mg/Al ratio is lower. LDHs with SO42- ions exhibit a three-stage polytypism. The first and last stages are 3 R1. The intermediate stage could be 1 T or a mixture of different O(octahedra)-type interlayers, which depends on the cation ratio. The relative popularity of SO42- ions with a C s point group symmetry is characteristic for the intermediate stage, while mostly SO42- ions exhibit a C3 v symmetry. There is no clear relevance between cation ratio and water content at which a polytype transition happens. The configurational adjustments of NO3- and SO42- ions facilitate the swelling behavior of LDHs. LDHs with CO32- or Cl- ions always maintain a 3 R1 polytype irrespective of water content and hardly swell. The configurations of anions and water reflect local coordination structure due to hydrogen bonds. The layer-stacking way influences long-ranged Coulombic interactions. Hydrogen-bonding structure and long-ranged Coulombic interactions collectively determine polytypism and stability of LDHs.

Uranyl Arsenate Complexes in Aqueous Solution: Insights from First-Principles Molecular Dynamics Simulations.

In this study, the structures and acidity constants (p Ka's) of uranyl arsenate complexes in solutions have been revealed by using the first principle molecular dynamics technique. The results show that uranyl and arsenate form stable complexes with the U/As ratios of 1:1 and 1:2, and the bidentate complexation between U and As is highly favored. Speciation-pH distributions are derived based on free energy and p Ka calculations, which indicate that for the 1:1 species, UO2(H2AsO4)(H2O)3+ is the major species at pH < 7, while UO2(HAsO4)(H2O)30 and UO2(AsO4)(H2O)3- dominate in acid-to-alkaline and extreme alkaline pH ranges. For the 1:2 species, UO2(H2AsO4)2(H2O)0 is dominant under acid-to-neutral pH conditions, while UO2(HAsO4)(H2AsO4)(H2O)-, UO2(HAsO4)(HAsO4)(H2O)2-, and UO2(AsO4)(HAsO4)(H2O)3- become the major forms in the pH range of 7.2-10.7, 10.7-12.1, and >12.1, respectively.

Structural Incorporation of Manganese into Goethite and Its Enhancement of Pb(II) Adsorption.

Natural goethite (α-FeOOH) commonly accommodates various metal elements by substituting for Fe, which greatly alters the surface reactivity of goethite. This study discloses the enhancement of Mn-substitution for the Pb2+adsorption capacity of goethite. The incorporated Mn in the synthesized goethite presents as Mn(III) and causes a slight decrease in the a and c of the unit cell parameters and an observable increase in the b direction due to the Jahn-Teller effect of the Mn(III)O6 octahedra. With the Mn content increasing, the particle size decreases gradually, and the surface clearly becomes roughened. The Pb2+ adsorption capacity of goethite is observably enhanced by Mn substitution due to the modified surface complexes. And the increased surface-area-normalized adsorption capacity for Mn-substituted goethite indicated that the enhancement of Pb adsorption is not only attributed to the increase of surface area but also to the change of binding complexes. Extended X-ray absorption fine structure (EXAFS) analysis indicates that the binding structures of Pb2+ on goethite presents as edge-sharing complexes with a regular RPb-Fe = 3.31 Å. In the case of Mn-goethite, Pb2+ is also bound with the Mn surface site on the edge-sharing complex with a larger RPb-Mn = 3.47 Å. The mechanism for enhancing Pb2+ adsorption on Mn-goethite can be interpreted as the preferred Pb2+ binding on the Mn site of Mn-goethite surface. In a summary, the Mn-goethite has great potential for material development in environmental remediation.