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Organo–clay complexes could be adsorbed and subsequently occluded into soil mineral matrices under local supersaturated solution conditions, leading to inaccessibility of microorganisms and their extracellular enzymes, which plays an important contribution to stabilization of soil organic matter (SOM) and affects their biogeochemical cycle. However, the underlying molecular mechanisms remain poorly understood. Here we apply Raman spectroscopy to analyze the LAPONITE®-sugar (monosaccharide glucose (Glu) and 5/20 kDa dextran (Dex-5/20) polysaccharides) interactions and use in situatomic force microscopy (AFM) to observe their occlusion processes within calcite. As shown by Raman spectra, the LAPONITE®-sugar complexes form with the mix of sugars and LAPONITE®, and the degree of elution is mediated by the molecular weight of sugars and more Glu would be eluted compared with Dex-5 and Dex-20. Then the LAPONITE®-sugar complexes could be occluded within calcite observed by AFM, and the occlusion of the LAPONITE®-sugar complexes within calcite hillocks are influenced by molecular weight with the trend of Dex-20 > Dex-5 > Glu. The binding force between sugars and calcite (1014) surfaces are measured by AFM-based dynamic force spectroscopy (DFS) to record the molecular-scale interactions, and high-molecular weight sugar such as Dex-20 exhibits strongest binds with calcite surfaces as shown by DFS data. These in situnanoscale observations and single-molecule determinations in a model system may provide insights into the clay-SOM-calcite fixation mechanisms by sugar in alkaline soils, with potential implications for global carbon cycling.

One beneficial approach to phosphorus recovery from wastewater is through struvite (MgNH4PO4·6H2O) crystallization, which could potentially be used as a slow-release fertilizer. However, it is often ignored that the reactivity and fate can be effectively influenced by naturally abundant metal ions, such as Ca2+in soil solutions, which results in the formation of sparingly soluble calcium phosphate precipitates on dissolved struvite crystal surfaces. Here, we use in situ atomic force microscopy coupled with a fluid reaction cell to observe interfacial dissolution–reprecipitation reactions of Ca2+-bearing solutions with distinct struvite surfaces. Our results show the formation of acidic amorphous calcium phosphate and its subsequent transformation to monetite (CaHPO4) crystals on the (011) face of struvite at a wider pH range; by contrast, the occurrence of basic amorphous calcium phosphate and its subsequent transformation to whitlockite (Ca29Mg(HPO4)3(PO4)18) and β-TCP (β-Ca3(PO4)2) is observed on the (001) face of struvite under acidic and alkaline conditions, respectively. Owing to Mg2+cations possessing a single oxygen deficit relative to the saturation coordination on the (001) surface, the {011} faceted surfaces most likely control an invariant Ca/P atomic ratio (at 1.0) in amorphous precursor phases through the formation of phosphate-bridged ternary complexes (≡Mg–P–Ca), which produce CaHPO40precipitates rather than whitlockite and β-TCP. Surface-specific dissolution of struvite is thus linked to the simultaneous growth of different calcium phosphate phases through the formation of precursors with distinct Ca/P atomic ratios, which are likely to be key factors in controlling the interfacial reactivity and fate of struvite under varied environmental solution conditions.

Both clinical observations and in vitrostudies have confirmed that hyperoxaluria or hypercalciuria is one of the main factors that determine the phase formation of pathological calcium oxalate (CaOx) stones, including the monohydrate (COM) at a low Ca/Ox molar ratio and a dihydrate (COD) at a high Ca/Ox molar ratio. However, in situnanoscale crystallization kinetics and molecular mechanisms remain unclear. Here we show, by time-resolved imaging and phase detection using atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM), two amorphous calcium oxalate (ACO) phases that are completely different in size can be formed in solutions with different Ca/Ox molar ratios and subsequently aggregate and crystallize into two different hydrate phases via their own different nonclassical pathways. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses demonstrate that the water content in the two amorphous precursor phases are consistent with that in their corresponding crystalline phases. Moreover, we also observe that COD, as a metastable phase, can spontaneously transform to COM in solutions with a high Ca/Ox molar ratio. Our findings provide direct evidence for Ca to Ox molar ratios in controlling phase selections and crystallization pathways during pathological CaOx stone formation.

Nanoplastics are widely distributed in crop soils and can interact with other exposed organic contaminants such as pesticides, leading to enhanced toxicity to plants and soil-beneficial microorganisms. These combined organic pollutants can also interact physiochemically with mineral matrices, becoming selectively preserved and occluded. Inclusion organics within growing minerals and the pore spaces of mineral aggregates are potentially inaccessible to plant root cells and soil microorganisms due to the limitation of their movement, but the microscopic mechanisms that control occlusion processes in the presence of nanoplastics mixed with pesticides remain poorly understood. Here, we use time-resolved atomic force microscopy (AFM) to observe how model soil minerals interact in situwith different functional groups of polystyrene (PSFG) mixed with glyphosate (PMG). Our results show that the PSFG–PMG complexes are occluded within calcite and iron hydroxide particles through hillock growth and aggregation, respectively. The free energies of binding between the functional groups of polystyrene and calcite surfaces measured using AFM-based dynamic force spectroscopy in the presence of different concentrations of PMG account for the molecular interactions involved in the occlusion process and the effects of the PMG concentration. These in situnanoscale observations and molecular-scale energetic measurements in a simple model system may provide insights into the immobilization of both nanoplastics and pesticides by soil minerals, with potential implications relating to multiple pollutant sequestration.

Chemical reactions at the mineral–solution interface control important interfacial processes, such as geochemical element cycling, nutrient recovery from eutrophicated waters, sequestration of toxic contaminants, and geological carbon storage by mineral carbonation. By time-resolved in situ imaging of nanoscale mineral interfacial reactions, it is possible to clarify the mechanisms governing mineral–fluid reactions.

Amelotin (AMTN) as a matrix protein exerts a direct effect on biomineralization by modulating apatite (HAP) formation during the dental enamel maturation stage through the specific interaction of a potentially phosphorylated Ser–Ser–Glu–Glu–Leu (SSEEL) peptide fragment with calcium phosphate (Ca–P) surfaces. However, the roles of (non)phosphorylation of this evolutionarily conserved subdomain within AMTN remain poorly understood. Here, we show, by time-resolved atomic force microscopy (AFM) imaging of in situ HAP crystallization via the HPO42–-rich amorphous calcium phosphate (acidic ACP), the on/off switching of the phase transformation process through a nonphosphorylation-to-phosphorylation transition of the SSEEL motif. Using high-resolution transmission electron microscopy (HRTEM), we observed that the acidic ACP phase is stabilized by the phosphorylated SSEEL motif, delaying its transformation to HAP, whereas the nonphosphorylated counterpart promotes HAP formation by accelerating the dissolution–recrystallization of the acidic ACP substrate. Dynamic force spectroscopy measurements demonstrate greater binding energies of nonphosphorylated SSEEL to the acidic ACP substrate by the formation of molecular peptide–ACP bonding, explaining the enhanced dissolution of the acidic ACP substrate by stronger complexion with surface Ca2+ions. Our findings demonstrate direct evidence for the switching role of (non)phosphorylation of an evolutionarily conserved subdomain within AMTN in controlling the phase transition of growing enamel and designing tissue regeneration biomaterials.

Antimony, which has damaging effects on the human body and the ecosystem, can be released into soils, ground-, and surface waters either from ore minerals that weather in near surface environments, or due to anthropogenic releases from waste rich in antimony, a component used in batteries, electronics, ammunitions, plastics, and many other industrial applications. Here, we show that dissolved Sb can interact with calcite, a widespread carbonate mineral, through a coupled dissolution–precipitation mechanism. The process is imaged in situ, at room temperature, at the nanometer scale by using an atomic force microscope equipped with a flow-through cell. Time-resolved imaging allowed following the coupled process of calcite dissolution, nucleation of precipitates at the calcite surface and growth of these precipitates. Sb(V) forms a precipitate, whereas Sb(III) needs to be oxidized to Sb(V) before being incorporated in the new phase. Scanning-electron microscopy and Raman spectroscopy allowed identification of the precipitates as two different calcium–antimony phases (Ca2Sb2O7). This coupled dissolution–precipitation process that occurs in a boundary layer at the calcite surface can sequester Sb as a solid phase on calcite, which has environmental implications as it may reduce the mobility of this hazardous compound in soils and groundwaters.

Potential pathways for inhibiting crystal growth are via either disrupting local microenvironments surrounding crystal–solution interfaces or physically blocking solute molecule attachment. However, the actual mode of inhibition may be more complicated due to the characteristic time scale for the inhibitor adsorption and relaxation to a well-bound state at crystal surfaces. Here we demonstrate the role of citrate (CA) and hydroxycitrate (HCA) in brushite (DCPD, CaHPO4·2H2O) crystallization over a broad range of both inhibitor concentrations and supersaturations by in situatomic force microscopy (AFM). We observed that both inhibitors exhibit two distinct actions: control of surface crystallization by the decrease of step density at high supersaturations and the decrease of the [1̅00]Ccstep velocity at high inhibitor concentration and low supersaturation. The switching of the two distinct modes depends on the terrace lifetime, and the slow kinetics along the [1̅00]Ccstep direction provides specific sites for the newly formed dislocations. Molecular modeling shows the strong HCA–crystal interaction by molecular recognition, explaining the AFM observations for the formation of new steps and surface dissolution along the [101]Ccdirection due to the introduction of strong localized strain in the crystal lattice. These direct observations highlight the importance of the inhibitor coverage on mineral surfaces, as well as the solution supersaturation in predicting the inhibition efficacy, and reveal an improved understanding of inhibition of calcium phosphate biomineralization, with clinical implications for the full therapeutic potential of small-molecule inhibitors for kidney stone disease.

The coprecipitation of iron (Fe) and phosphorus (P) in natural environments limits their bioavailability. Plant root-secreted organic acids can dissolve Fe–P precipitates, but the molecular mechanism underlying mobilizing biogenic elements from highly insoluble inorganic minerals remains poorly understood. Here, we investigated vivianite (Fe3(PO4)2·8H2O) dissolution by organic acids (oxalic acid (OA), citric acid (CA), and 2′-dehydroxymugineic acid (DMA)) at three different pH values (4.0, 6.0, and 8.0). With increasing pH, the vivianite dissolution efficiency by OA and CA was decreased while that by DMA was increased, indicating various dissolution mechanisms of different organic acids. Under acidic conditions, weak ligand OA (HC2O4–> C2O42–at pH 4.0 and C2O42–at pH 6.0) dissolved vivianite through the H+effect to form irregular pits, but under alkaline condition (pH 8.0), the completely deprotonated OA was insufficient to dissolve vivianite. At pH 4.0, CA (H2Cit–> HCit2–> H3Cit) dissolved vivianite to form irregular pits through a proton-promoted mechanism, while at pH 6.0 (HCit2–> Cit3–) and pH 8.0 (Cit3–), CA dissolved vivianite to form near-rhombohedral pits through a ligand-promoted mechanism. At three pH values ((H0)DMA3–> (H1)DMA2–at pH 4.0, (H0)DMA3–at pH 6.0, and (H0)DMA3–and one deprotonated imino at pH 8.0), strong ligand DMA dissolved vivianite to form near-rhombohedral pits via ligand-promoted mechanisms. Raman spectroscopy showed that the deprotonated carboxyl groups (COO–) and imino groups were bound to Fe on the vivianite (010) face. The surface free energy of vivianite coated with OA decreased from 29.32 mJ m–2to 24.23 mJ m–2and then to 13.47 mJ m–2with increasing pH, and that coated with CA resulted in a similar pH-dependent vivianite surface free-energy decrease while that coated with DMA increased the vivianite surface free energy from 31.92 mJ m–2to 39.26 mJ m–2and then to 49.93 mJ m–2. Density functional theory (DFT)-based calculations confirmed these findings. Our findings provide insight into the mechanism by which organic acids dissolved vivianite through proton and ligand effects.

Phosphorus (P) availability is widely assumed to be limited by the formation of metal (Ca, Fe, or Al) phosphate precipitates that are modulated by soil organic matter (SOM), but the SOM–precipitate interactions remain uncertain because of their environmental complexities. Here, we present a model system by quantifying the in situ nanoscale nucleation kinetics of calcium phosphates (Ca-Ps) on mica in environmentally relevant aqueous solutions by liquid-cell atomic force microscopy. We find that Ca-P precipitate formation is slower when humic acid (HA) concentration is higher. High-resolution transmission electron microscopy observations demonstrate that HA strongly stabilizes amorphous calcium phosphate (ACP), delaying its subsequent transformation to thermodynamically more stable phases. Consistent with the formation of molecular organo–mineral bonding, dynamic force spectroscopy measurements display larger binding energies of organic ligands with certain chemical functionalities on HA to the initially formed ACP than to mica that are responsible for stabilization of ACP through stronger HA–ACP interactions. Our results provide direct evidence for the proposed importance of SOM in inhibiting Ca-P precipitation/transformation. We suggest that similar studies of binding strength in SOM–Fe/Al–P may reveal how both organic matter and metal ions control P availability and fate, and thus the eventual P management for agronomical and environmental sustainability.

Cadmium (Cd2+) and Arsenate (As5+) are the main toxic elements in soil environments and are easily taken up by plants. Unraveling the kinetics of the adsorption and subsequent precipitation/immobilization on mineral surfaces is of considerable importance for predicting the fate of these dissolved species in soils. Here we used in situ atomic force microscopy (AFM) to image the dissolution on the (010) face of brushite (dicalcium phosphate dihydrate, CaHPO4·2H2O) in CdCl2- or Na2HAsO4-bearing solutions over a broad pH and concentration range. During the initial dissolution processes, we observed that Cd or As adsorbed on step edges to modify the morphology of etch pits from the normal triangular shape to a four-sided trapezium. Following extended reaction times, the respective precipitates were formed on brushite through a coupled dissolution–precipitation mechanism. In the presence of both CdCl2and Na2HAsO4in reaction solutions at pH 8.0, high-resolution transmission electron microscopy (HRTEM) showed a coexistence of both amorphous and crystalline phases, i.e., a mixed precipitate of amorphous and crystalline Cd(5–x)Cax(AsO4)(3–y)(PO4)yOH phases was detected. These direct dynamic observations of the transformation of adsorbed species to surface precipitates may improve the mechanistic understanding of the calcium phosphate mineral interface-induced simultaneous immobilization of both Cd and As and subsequent sequestration in diverse soils.

Mineral–soil organic matter (SOM including DNA, proteins, and polysaccharides) associations formed through various interactions, play a key role in regulating long-term SOM preservation. The mechanisms underlying DNA–mineral and DNA–protein/polysaccharide interactions at nanometer and molecular scales in environmentally relevant solutions remain uncertain. Here, we present a model mineral–SOM system consisting of mineral (mica)–nucleic acid (environmental DNA, eDNA)/protein (bovine serum albumin)/polysaccharide (alginate), and combine atomic force microscopy (AFM)-based dynamic force spectroscopy and PeakForce quantitative nanomechanical mapping using DNA-decorated tips. Single-molecule binding and adhesion force of eDNA to mineral and to mineral adsorbed by protein/polysaccharide reveal the noncovalent bonds and that systematically changing ion compositions, ionic strength, and pH result in significant differences in organic–organic and organic–mineral binding energies. Consistent with the bond-strength measurements, protein, rather than polysaccharide, promotes mineral-bound DNA molecules by ex situ AFM deposition observations in relatively high concentrations of divalent cation-containing acidic solutions. These molecular-scale determinations and nanoscale observations should substantially improve our understanding of how environmental factors influence the organic–mineral interfacial interactions through the synergy of collective noncovalent and/or covalent bonds in mineral–organic associations.

Global soil carbon cycling plays a key role in regulating and stabilizing the earth’s climate change because of soils with amounts of carbon at least three times greater than those of other ecological systems. Soil minerals have also been shown to underlie the persistence of soil organic matter (SOM) through both adsorption and occlusion, but the microscopic mechanisms that control the latter process are poorly understood. Here, using time-resolved in situ atomic force microscopy (AFM) to observe how calcite, a representative mineral in alkaline soils, interacts with humic substances, we show that following adsorption, humic substances are gradually occluded by the advancing steps of spirals on the calcite (1014) face grown in relatively high supersaturated solutions, through the embedment, compression, and closure of humic substance particles into cavities. This occlusion progress is inhibited by phytate at high concentrations (10–100 μM) due to the formation of phytate-Ca precipitates on step edges to prevent the step advancement, whereas phytate at relatively low concentrations (≤1 μM) and oxalate at high concentrations (100 μM) have little effect on this process. These in situ observations may provide new insights into the organo–mineral interaction, resulting in the incorporation of humic substances into minerals with a longer storage time to delay degradation in soils. This will improve our understanding of carbon cycling and immobilization in soil ecological systems.

The majority of human kidney stones are composed of multiple calcium oxalate crystals with variable amounts of brushite [dicalcium phosphate dihydrate (DCPD)] and hydroxyapatite (HAP) as a nucleus, in which fluid-mediated dissolution and reprecipitation may result in the phase transformation of DCPD to HAP. However, the underlying mechanisms of the phase transition and its modulation by natural inhibitors, such as osteopontin (OPN) proteins, remain poorly understood. Here, the in vitro formation of new phases on the DCPD (010) surface is observed in situ using atomic force microscopy in a simulated hypercalciuria milieu. We demonstrate the presence of an acidic amorphous calcium phosphate (ACP) phase with a characteristic Raman band of ν1HPO42–and the octacalcium phosphate (OCP)-like phase during the transformation process. High-resolution transmission electron microscopy analyses also confirm the existence of OCP and HAP within an amorphous matrix phase. In support of clinical observations, we further demonstrate the inhibitory effect of OPN peptide segments on the dissolution of DCPD and reprecipitation of acidic ACP. The definition of respective roles of DCPD and OPN thereby provides insights into the control of nucleus formation and subsequent inhibition of pathological mineralization.

Modulation of mineralization and demineralization of calcium phosphates (Ca-Ps) with organic macromolecules is a critical process which prevents human kidney stone disease. As a long unbranched polysaccharide of urinary glycosaminoglycans, chondroitin 4-sulfate (Ch4S) has been shown to play an essential role in inhibiting the formation of kidney stones. However, the mechanism of the role of Ch4S remains poorly understood. Here, we used in situ atomic force microscopy to observe the growth and dissolution of spirals on brushite (CaHPO4·2H2O) (010) surfaces. The results show that Ch4S preferentially inhibits the [101]Ccstep growth/dissolution by step pinning. This step–specific effect appears to be related to specific binding of Ch4S to Ca sites, as the observed inhibition is not seen in other crystallographic directions where there are fewer Ca terminations. Moreover, Ch4S promotes an increase in the terrace width of [101̅]Ccby the modification of the interfacial energies of the step edge. These in vitro direct observations of Ch4S modulating brushite mineralization and demineralization reveal a dual control of both step kinetics and interfacial energy.

Bioavailability and mobility of cadmium (Cd2+) and arsenate (As5+) in soils can be effectively lowered through the dissolution of brushite (dicalcium phosphate dihydrate, CaHPO4·2H2O) coupled with the precipitation of a more stable mineral phase containing both Cd and As. Due to the ubiquitous presence of humic acid (HA) in soil environments, it is more complex to predict the fate of dissolved Cd and As during such sequestration. Here, we used in situatomic force microscopy (AFM) to image the kinetics of simultaneous precipitation of Cd and As at the brushite–fluid interface in the presence of HA. Results show that HA inhibits the formation of both amorphous and crystalline Cd(5–x)Cax(PO4)(3–y)(AsO4)y(OH) on the (010) face of brushite. A combination of X-ray photoelectron spectroscopy (XPS) and real-time surface-enhanced Raman spectroscopy (SERS) reveals that part of As5+reduction into As3+with HA and [HA-Cd] complexation occurs, modulating the concentrations of free Cd2+and As5+ions to inhibit subsequent precipitation of a Cd(5–x)Cax(PO4)(3–y)(AsO4)y(OH) phase on the dissolving brushite surface. A combination of AFM imaging, SERS analyses, and PhreeqC simulations suggests that environmentally relevant humic substances can limit the precipitation of Cd and As at mineral surfaces through a mechanism of oxidation/reduction and aqueous/surface complexation. This may exacerbate the transportation of these contaminants into waters by subsurface fluid flow, and research attempts to weaken the negative effect of HA are needed.

Phosphorus (P) recovery from wastewater is essential for sustainable P management. A biomimetic hexapeptide (SGAGKT) has been demonstrated to bind inorganic P in P-rich environments, however the dynamics and molecular mechanisms of P-binding to the hexapeptide still remain largely unknown. We used dynamic force spectroscopy (DFS) to directly distinguish the P-unbound and P-bound SGAGKT adsorbed to a mica (001) surface by measuring the single-molecule binding free energy (ΔGb). Using atomic force microscopy (AFM) to determine real-time step retreat velocities of triangular etch pits formed at the nanoscale on the dissolving (010) face of brushite (CaHPO4·2H2O) in the presence of SGAGKT, we observed that SGAGKT peptides promoted in situ dissolution through an enhanced P-binding driven by hydrogen bonds in a P-loop being capable of discriminating phosphate over arsenate, concomitantly forming a thermodynamically favored SGAGKT–HPO42–complexation at pH 8.0 and relatively low ionic strength, consistent with the DFS and isothermal titration calorimetry (ITC) determinations. The findings reveal the thermodynamic and kinetic basis for binding of phosphate to SGAGKT and provide direct evidence for phosphate discrimination in phosphate/arsenate-rich environments.

Understanding the mechanism and kinetics of the replacement of carbonates by fluorite has applications in Earth sciences and engineering. Samples of Carrara marble were reacted with an ammonium fluoride (NH4F) solution for different reaction times and temperatures. The microstructure of the product phase (fluorite) was analyzed using SEM. The kinetics of replacement was monitored using Rietveld refinements of X-ray powder diffraction patterns of the products. After reaction, all samples preserved their size and external morphology (a pseudomorphic replacement). The grain boundaries of the original marble were preserved although each calcite grain was replaced by multiple fine crystals of fluorite creating inter-crystal porosity. The empirical activation energy Ea(kJ/mol) of the replacement reaction was determined by both model-fitting and model-free methods. The isoconversional method yielded an empirical activation energy of 41 kJ/mol, and a statistical approach applied to the model-fitting method revealed that the replacement of Carrara marble by fluorite is better fitted to a diffusion-controlled process. These results suggest that the replacement reaction depends on the ion diffusion rate in the fluid phase through the newly formed porosity.

The epitaxial overgrowth of brushite (CaHPO4·2H2O) by the interaction of phosphate-bearing, slightly acidic, aqueous solutions with gypsum (CaSO4·2H2O) was investigated in situ using atomic force microscopy (AFM). Brushite growth nuclei were not observed to form on the {010} gypsum cleavage surface, but instead formed in areas of high dissolution, laterally attached to gypsum [101] step edges. During the brushite overgrowth the structural relationships between brushite (Aa) and gypsum (A2/a) result in several phenomena, including the development of induced twofold twining, habit polarity, and topographic effects due to coalescence of like-oriented crystals. The observed brushite growth is markedly anisotropic, with the growth rate along the main periodic bond chains (PBCs) in the brushite structure increasing in the order [101] > [101] > [010], leading to tabular forms elongated on [101]. Such a growth habit may result from the stabilization of the polar [101] direction of brushite due to changes in hydration of calcium ions induced by the presence of sulfate in solution, which is consistent with the stabilization of the gypsum [101] steps during dissolution in the presence of HPO2-4ions. The coupling between growth and dissolution was found to result in growth rate fluctuations controlled by the changes in the solution composition.

Leucite and analcime have open framework aluminosilicate structures, where ion exchange by cation substitution has been previously used to explain the replacement of one phase by another. Using 18O-enriched NaCl solutions in hydrothermal reactions and run-product analyses using scanning electron microscopy, infrared and Raman spectroscopy, and time-of-flight secondary ion mass spectrometry, we show that the replacement of leucite by analcime is not a solid-state reaction involving cation exchange by volume diffusion. Textural features such as nano-pores and clusters, as well as the detection of high amounts of 18O in the framework of analcime, suggest that the reaction proceeds by dissolution of leucite and reprecipitation of analcime, where structural O atoms of the leucite framework are exchanged and a new analcime structure forms at a moving interface through the leucite parent crystal. The characteristic high porosity (on a nano-scale) in the analcime product phase results from some of the parent phase being lost to the solution to give a volume deficit reaction. However, external dimensions are maintained during the process to result in the pseudomorphic replacement of an open framework aluminosilicate structure by a coupled dissolution-reprecipitation mechanism.

Solid-fluid interactions often involve the replacement of one phase by another while retaining the morphology and structural details of the parent phase, i.e, pseudomorphism. We present in situ observations of the evolution of both the solid and fluid compositions at the interface during such a replacement reaction in the model system KBr-KCl-H2O, in which a single crystal of KBr is replaced by a single crystal of KCl. The pseudomorphism is initiated by epitaxial growth at the fluid-mineral interface, when the dissolution of the parent phase results in an interfacial fluid layer that is supersaturated with respect to a different solid composition. The subsequent evolution of the coupled dissolution and growth can be related to local equilibrium defined by a Lippmann diagram. The reaction features, including the development of porosity in the new solid phase, share many characteristics of replacement reactions in nature as well as in technical applications

Ferrihydrite and a vernadite-like mineral, in samples collected from the riverbeds and floodplains of the river draining the largest mining-contaminated site in the United States (the Clark Fork River Superfund Complex), have been studied with transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis. These poorly crystalline minerals are environmentally important in this system because contaminant heavy metals (As, Cu, Pb, and/or Zn) are always associated with them. Both two- and six-line ferrihydrite have been identified with selected-area electron diffraction. For the vernadite-like mineral, the two d values observed are approximately between 0.1 and 0.2 Å larger than those reported for vernadite, the Mn hydrous oxide that is thought to have a birnessitelike structure, but which is disordered in the layer stacking direction. In several field specimens, the ferrihydrite and vernadite-like minerals are intimately mixed on the nanoscale, but they also occur separately. It is suggested that the vernadite-like mineral, found separately, is produced biogenically by Mn-oxidizing bacteria, whereas the same mineral associated with ferrihydrite is produced abiotically via the heterogeneous oxidation of Mn2+aqinitially on ferrihydrite surfaces. Evidence from this study demonstrates that the vernadite-like mineral sorbs considerably more toxic metals than does ferrihydrite, demonstrating that it may be a good candidate for application to heavy-metal sorption in permeable reactive barriers.

Intracellular membrane-bound vesicles play important roles in the formation of biominerals, such as calcium oxalate monohydrate (COM) crystals, through the interactions of the vesicles and different crystal faces. However, in situ kinetics and the mechanism of occlusion of diverse vesicles, which have similar compositions, into the (1̅01) and (010) faces of COM remain unknown. Here, using time-resolved in situ atomic force microscopy (AFM), we observe that negatively charged phosphatidylcholine vesicles are adsorbed preferentially and then occluded into the (1̅01) face, whereas positively charged (2,3-dioleoyloxy-propyl)-trimethylammonium vesicles are only occluded into the (010) face, and zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine vesicles are rarely incorporated into COM crystals. The free energies of binding between the lipid vesicles and COM crystal faces measured by AFM-based single-molecule dynamic force spectroscopy account for the vesicle-crystal face interaction through an electrostatic attraction. These in situ kinetics and energetic analyses may improve our understanding of the mechanisms of lipid occlusion.