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Classical crystallization models successfully depict the dependence of growth kinetics on thermodynamic driving force but cannot predict the roles of solution chemistry. Yet, it has become increasingly clear that crystal growth rate at fixed supersaturations depends on pH, ionic strength, and the relative abundance of cations and anions in the parent solutions. We conducted experiments to isolate the effect of individual solution-chemistry parameters on calcite step growth kinetics. Our results revealed a distinct correlation between step velocity and pH in acute and obtuse directions and a simultaneous trend change at pH ∼9.5. Step speeds varied with solution stoichiometry, and each direction reached its maximal rate at a different [ Ca 2 + ] / [ CO 3 2 - ] value. In addition, the solution cation/anion at which maximal step speed occurred appeared to be pH dependent as well. Limited effect of ionic strength on step growth was observed in the obtuse direction at low background electrolyte concentrations, but no obvious change was found for growth at the acute side. We proposed a growth model based upon independent incorporation of cations and anions and interpreted these observations as: (1) solute constituents enter kinks through direct solution diffusion at obtuse steps but surface diffusion at acute steps; (2) growth is limited by Ca 2+ dehydration at the obtuse side, but by surface adsorption and reorientation and rearrangement of CO 3 2− in the acute direction; and (3) background electrolyte (NaCl in our study) competes with solutes for surface sites at more easily accessible kinks. [ABSTRACT FROM AUTHOR]

Abstract: The behaviour of gypsum {010} cleavage faces, both (010) and (01¯0), in undersaturated conditions is studied at real time to probe the mineral dissolution process. The observations in surface topographic change and step motion made by in situ fluid cell atomic force microscopy show that: (i) the only type of etch pits on the cleavage faces is that embraced by the [100] and [001] steps, disagreeing with previous reports that suggested the involvement of the [101] direction; (ii) compared to calcite, the anisotropy in step velocity is much more pronounced, presumably due to the more significant difference in atomic structure between the [100] and [001] steps on gypsum relative to that between the <4¯41>+ and <4¯41>− on calcite; (iii) the step kinetic behavior follows the prediction of the transition state theory; (iv) the dissolution process on gypsum {010} faces is not characterized by the formation of deep etch pits, even at conditions far from equilibrium. The lack of deep pits most likely results from the observed faster dissolution of unstable steps in which the increase in pit depth is out-paced by that in the lateral dimension. [Copyright &y& Elsevier]

Dissolution experiments were conducted on {101¯4} cleavage faces of calcite at various under-saturations to determine how the saturation state controls etch pit formation. Experimental observations were made by using in situ fluid cell Atomic Force Microscopy. Three dissolution modes were observed. When the saturation index Ω > 0.541, no etch pit formation was seen and dissolution primarily occurred at existing steps. When Ω decreased to Ωc = 0.541–0.410, the first visible pits appeared and continuous reduction in saturation state slowly increased the pit density on terraces while dissolution simultaneously proceeded at step edges. Finally, when the saturation state fell below Ωmax = ∼0.007, a precipitous increase in pit density took place that sharply contrasted to the ordered fashion of pit formation observed at saturation conditions above this level. These observations are interpreted to be two-dimensional and unassisted pit formation at Ω < ∼0.007, defect- and step-assisted dissolution in between Ω = 0.541 and 0.007, and existing step-induced dissolution for Ω > 0.541.The values of Ωc are in good agreement with the dislocation theory's predicted critical under-saturations for pit formation at line dislocations. The occurrence of Ωmax is not directly predicted but is a logical consequence of dissolution thermodynamics. These findings suggest that (1) dissolution near and far from equilibrium (i.e., Ω > Ωc, Ω < Ωmax) is not controlled by dislocations, therefore (2) dislocation density should significantly impact dissolution rate only in the saturation range of Ωmax < Ω < Ωc; (3) dissolution kinetics and chemical affinity of dissolution reactions should have a non-linear relationship: at sufficiently close to equilibrium, when dislocations cannot open up to form etch pits, the dissolution kinetics will be limited by the number of existing steps; at far from equilibrium, when pits are able to form in defect-free regions, the dissolution rate will be capped by the maximum number of achievable steps.These findings may provide explanations for several well-observed geochemical relationships, including the weak dependence of dissolution rate upon dislocation density in distilled water and the ‘plateau’ behavior of dissolution kinetics both near and far from equilibrium. The explosive occurrence of unassisted pit nucleation at Ω ∼ Ωmax is not predicted by the current dissolution rate equations. This suggests that an accurate ‘general’ rate law describing universal dissolution processes has yet to be developed. [Copyright &y& Elsevier]

Investigates the thermodynamic controls on carbonate growth by using atomic force microscopy. Complexity of biomineralized structures; Effect of aspartic acid on growth morphology and on the magnitude of the surface energy.

Presents a study which examined the interactions of acidic amino acids aspartate (Asp) with calcite {1014} in relation to its implications for biomineralization. Indepth look at the involvement of the matrix macromolecules which regulated the biological growth of the calcite; Methodology used to conduct the study; Results of the study.

The article presents a study regarding the involvement of calcite-aspartic acid (ASP) functional groups and the geometry of the surface binding. An effective strategy to gain a deeper insight into ASP-calcite interfacial reaction is to monitor that changes of microtopography. The study uses succinic acid to react with calcite. After further observation the study revealed that the changes in the surface microtopography resulted primarily from the appearance of newly form etch pits.

Carbonic anhydrase (CA) has been shown to promote calcite dissolution (Liu, 2001, https://doi.org/10.1111/j.1755-6724.2001.tb00531.x; Subhas et al., 2017, https://doi.org/10.1073/pnas.1703604114), and understanding the catalytic mechanism will facilitate our understanding of the oceanic alkalinity cycle. We use atomic force microscopy (AFM) to directly observe calcite dissolution in CA‐bearing solution. CA is found to etch the calcite surface only when in extreme proximity (~1 nm) to the mineral. Subsequently, the CA‐induced etch pits create step edges that serve as active dissolution sites. The possible catalytic mechanism is through the adsorption of CA on the calcite surface, followed by proton transfer from the CA catalytic center to the calcite surface during CO2 hydration. This study shows that the accessibility of CA to particulate inorganic carbon (PIC) in the ocean is critical in properly estimating oceanic CaCO3 and alkalinity cycles. Plain Language Summary: Calcite dissolution in aqueous solution is one of the most biogeochemically important reaction and has a significant impact on atmospheric CO2 regulation and climate change. Carbonic anhydrase (CA) is an enzyme that regulates pH and CO2 balance inside or surrounding organisms in aqueous and terrestrial environments and is shown to promote calcite dissolution. In this study, we investigate why this ubiquitous enzyme enhances calcite dissolution rates in order to facilitate the proper estimate of dissolution fluxes in natural environments. When observed at atomic scale, we find that the enzyme etches the mineral only in extreme proximity. This observation provides important clue for the catalysis mechanism and reveals that the accessibility of CA to CaCO3 in the ocean is critical in properly estimating the natural fluxes. Key Points: Carbonic anhydrase induces etch pit formation when in contact or in extreme proximity (~1 nm) with calciteThe catalysis is likely via the adsorption of carbonic anhydrase on the mineral, followed by proton transfer to the calcite surfaceThe accessibility of the enzyme to particulate inorganic carbon is vital in properly estimating oceanic CaCO3 and alkalinity cycles [ABSTRACT FROM AUTHOR]

Arsenopyrite dissolution is an important source of arsenic's environmental occurrence and is well-studied under oxidative conditions. However, little is known of anoxic leaching of arsenopyrite by water, though it is probably one of the dominant abiotic processes controlling arsenic mobility in oxygen-limited environments such as groundwaters and paddy fields. The present study attempts to probe the mechanism of arsenopyrite-water interaction in anoxic environment through examining the pH dependence of dissolution kinetics and stoichiometry, as well as speciation changes both on the surface and in solution. Batch and flow-through experiments were first carried out to measure dissolution rates and the results showed alkaline pH mildly encourages As release. Furthermore, the dissolution was found to be near stoichiometric with respect to As and S but not with Fe/As or Fe/S, and the non-stoichiometry exhibited a negative dependence on pH. Combining speciation analyses that revealed the presence of H 2 O 2 , SO 4 2− and S 2 O 3 2− in solutions and a positive correlation of surface species Fe(III)-O, S(-II), and As(I), As(III), As(V), and As(-III) to solution pH, we interpreted the anoxic dissolution behavior of arsenopyrite through a mechanism that combines surface complexation and interfacial redox reactions. The kinetic measurements came to be concerning as data indicates arsenopyrite dissolution in anoxic environment can lead to water having arsenic concentrations above the 10 ppb WHO guideline value in one day at the experimental conditions. The observed H 2 O 2 production indicated the dissolution is associated with or initiated by electron transfer despite the anoxic condition, further suggesting that a more comprehensive theory other than surface complexation model may be needed to better understand the geochemical behavior of As and possibly S in the context of sulfide mineral–water interactions. [ABSTRACT FROM AUTHOR]

The ability of fungi to weather a wide range of minerals influences plant nutrition and enhances global biogeochemical cycles of life-essential elements. The fungus-mineral interface plays a key role in weathering, but the specific mechanisms underlying these processes remain poorly understood. Here, we examined fungal-mineral weathering using hematite and Trichoderma guizhouense. We showed that hematite dissolution increased over cultivation time, with the formation of secondary minerals up to ∼3000 µm−2 at the interfaces after 66 h cultivation. Of the hematite associated with hyphae, approximately 15% was converted to the secondary mineral ferrihydrite. Importantly, superoxide radicals were detected at the hyphal tips and along the whole hyphae. During cultivation, a high concentration (∼1000 nM) of hydroxyl radical was also detected. Synchrotron radiation based spectromicroscopies at fungus-mineral interfaces suggest that fungus hyphae alter the local redox state of iron and thus are redox-active. These findings indicate that fungus-initiated catalytic reactions occur at hyphal-mineral interfaces, in view of the fact that superoxide does not diffuse far from the site of formation. Furthermore, these results also suggest that the catalytic reactions may serve as a new strategy for microbial iron uptake. Together, these findings constitute a significant step forward in understanding the ways that fungi make minerals available to biological systems. [ABSTRACT FROM AUTHOR]

The solubilization and mobilization of elements from silicate minerals during fungal weathering are predominantly promoted by acidification and complexation reactions. However, stark differences exist in the release rates of different elements driven by fungal-derived low molecular weight organic compounds (LMWOCs) when acidity maintains constant, raising the question of whether the release of individual element during dissolution is ligand-specific. In this work, we investigate this question by characterizing the release of Mg, Si, Fe, and Ni from lizardite [(Mg, Fe, Ni) 3 Si 2 O 5 (OH) 4 ]. Miniaturized batch reactors (microplate wells of 250 µL) were used in dissolution experiments in the presence and absence of the indigenous fungus Talaromyces flavus. Abiotic chemical weathering experiments with metabolically relevant organics and HCl were also carried out to isolate the vital effects of elemental releases. The initial release rates of Mg, Si, Fe, and Ni were obtained from determining the dissolved elemental concentrations. The results show that T. flavus enhances the release of Mg and Si by a factor of ∼2, but that of Fe and Ni by a factor of >10, relative to the rates measured in the control experiment. Additionally, the measurements show an overexcretion of siderophores and oxalic acid, as well as acidification of bulk solution, during bioweathering. Abiotic chemical dissolution of lizardite confirms the release of Fe and Ni proceeds mainly via a ligand-promoted pathway. In addition, the results indicate that siderophores and oxalic acid are responsible for the solubilization of Fe and Ni, respectively. These findings provide direct evidence that the rate and mechanism of elemental release from silicate during bioweathering are ligand-specific, and both synergistic and inhibitory effects may be involved. Given that siderophore- and oxalic acid-producing fungi are highly active in soils, these results may have the potential to advance our understanding of the critical roles of fungi in rhizosphere geochemistry and ecology. [ABSTRACT FROM AUTHOR]

Background: Plants show developmental plasticity with variations in environmental nutrients. Considering low-cost rock dust has been identified as a potential alternative to artificial fertilizers for more sustainable agriculture, the growth responses of Arabidopsis seedlings on three rock meals (basalt, granite, and marlstone) were examined for the different foraging behavior, biomass accumulation, and root architecture. Results: Compared to ½ MS medium, basalt and granite meal increased primary root length by 13% and 38%, respectively, but marlstone caused a 66% decrease, and they all drastically reduced initiation and elongation of lateral roots but lengthened root hairs. Simultaneous supply of organic nutrients and trace elements increased fresh weight due to the increased length of primary roots and root hairs. When nitrogen (N), phosphorus (P), and potassium (K) were supplied individually, N proved most effective in improving fresh weight of seedlings growing on basalt and granite, whereas K, followed by P, was most effective for those growing on marlstone. Unexpectedly, the addition of N to marlstone negatively affected seedling growth, which was associated with repressed auxin biosynthesis in roots. Conclusions: Our data indicate that plants can recognize and adapt to complex mineral deficiency by adjusting hormonal homeostasis to achieve environmental sensitivity and developmental plasticity, which provide a basis for ecologically sound and sustainable strategies to maximize the use of natural resources and reduce the production of artificial fertilizers. [ABSTRACT FROM AUTHOR]

Terrestrial reactive oxygen species (ROS) may have played a central role in the formation of oxic environments and evolution of early life. The abiotic origin of ROS on the Archean Earth has been heavily studied, and ROS are conventionally thought to have originated from H2O/CO2 dissociation. Here, we report experiments that lead to a mineral-based source of oxygen, rather than water alone. The mechanism involves ROS generation at abraded mineral–water interfaces in various geodynamic processes (e.g., water currents and earthquakes) which are active where free electrons are created via open-shell electrons and point defects, high pressure, water/ice interactions, and combinations of these processes. The experiments reported here show that quartz or silicate minerals may produce reactive oxygen-containing sites (≡SiO•, ≡SiOO•) that initially emerge in cleaving Si–O bonds in silicates and generate ROS during contact with water. Experimental isotope-labeling experiments show that the hydroxylation of the peroxy radical (≡SiOO•) is the predominant pathway for H2O2 generation. This heterogeneous ROS production chemistry allows the transfer of oxygen atoms between water and rocks and alters their isotopic compositions. This process may be pervasive in the natural environment, and mineral-based production of H2O2 and accompanying O2 could occur on Earth and potentially on other terrestrial planets, providing initial oxidants and free oxygen, and be a component in the evolution of life and planetary habitability. [ABSTRACT FROM AUTHOR]

Ferric ions can bind strongly with dissolved organic matter (DOM), including humic acids (HA), fulvic acids (FA), and protein-like substances, whereas isolation of Fe-DOM precipitates (Fe-DOMP) and their biochemical characteristics remain unclear. In this work FeCl3 was used to isolate DOM components from various sources, including river, lake, soil, cow dung, and standard tryptophan and tyrosine, through precipitation at pH 7.5–8.5. The Fe-DOMP contribute to total DOM by approximately 38.6–93.8% of FA, 76.2% of HA and 25.0–30.4% of tryptophan and tyrosine, whilst fluorescence spectra allowed to monitor/discriminate the various DOM fractions in the samples. The relative intensity of the main infrared peaks such as 3406‒3383 cm−1 (aromatic OH), 1689‒1635 cm−1 (‒COOH), 1523–1504 cm−1 (amide) and 1176–1033 cm−1 (‒S=O) show either to decline or disappear in Fe‒DOMP. These results suggest the occurrence of Fe bonds with various functional groups of DOM, indicating the formation of π–d electron bonding systems of different strengths in Fe‒DOMP. The novel method used for isolation of Fe-DOMP shows promising in opening a new frontier both at laboratory and industrial purposes. Furthermore, results obtained may provide a better understanding of metal–organic complexes involved in the regulation of the long-term stabilization/sequestration of DOM in soils and waters. [ABSTRACT FROM AUTHOR]

While noble metals often occur as minor components in host minerals in various ore deposits, little theoretical assessment exists to predict the occurrence of these metals. Here, we probe the fundamental controls responsible for the occurrence of trace elements in host minerals through first-principles calculations. We apply the theoretical model to understanding the debated issues concerning the occurrence of gold (Au) in pyrite, in which the valence of Au is ascribed to either positive or negative values. Our results indicate that (1) both positive and negative valent Au may occur in pyrite and (2) higher sulfur fugacity and lower temperature lead to more Au+ occupying Fe sites in pyrite. These findings suggest that chemical states and speciation of the Au in host pyrite are ultimately controlled by temperature and sulfur fugacity, providing insight into the formation conditions of ore deposits and facilitating strategy design for beneficiation. [ABSTRACT FROM AUTHOR]

Pharmacolite (CaHAsO 4 ·2H 2 O) mineralization is an important process controlling arsenate mobility in carbonate areas but is poorly understood in terms of crystallization mechanism and kinetics. The present study intends to investigate pharmacolite formation initiated by heterogenous nucleation through in situ observations of the mineral–water interface. Experiments were performed by exposing gypsum substrate to solutions supersaturated with respect to pharmacolite followed by atomic force microscopy imaging to record changes in surface morphology and topography. The data are used to determine growth mode, step speed, and the rate of step birth. Experimental results show the crystallization is achieved by direction-specific mono-molecular layer growth via 2D surface nucleation or spiral hillock development in accord with the classical crystallization model. Subsequent theoretical analyses allow to determine step energies and kinetic coefficients in major growth directions on (0 1 0) and indicate that [0 0 1] steps are energetically the most stable and morphologically the most prominent while [1 0 0] and [1 0 1] steps are controlled either by thermodynamic or kinetics. Gypsum plays an important role in aiding pharmacolite formation through epitaxy as the two minerals share a range of structural commonalities. The notably reduced supersaturation needed in substrate-assisted pharmacolite crystallization relative to bulk solution nucleation suggests gypsum may significantly reduce the energy barrier for the mineralization reactions and hence may find applications in As remediation practice. [ABSTRACT FROM AUTHOR]

Soil plays an important role in controlling the biogeochemical cycling of nutrients and contaminants in the environment. Reliable analytical techniques are critical to better understand their transformation in soils. Diffusive Gradients in Thin-films technique (DGT) is one of such techniques, which has become increasingly popular in soil research since 1998. Undeniably, DGT attracts increasing attentions from multidisciplinary researchers worldwide. This is mainly due to its inherent merits, including ease of use, in situ measurement, and biomimetic applications. Further, four important aspects of DGT applications in soil research were summarized. They include: 1) chemical speciation, 2) bioavailability measurement, 3) desorption kinetic modeling, and 4) interfacial process mapping of nutrients and contaminants, with main focus on the progresses achieved during the past five years. In short, DGT is effective in studying the biochemical behaviors of nutrients (e.g., phosphate and nitrate), trace elements (e.g., Cd, Ni, Zn, Cu, Pb, and As), and polar organic compounds (e.g., antibiotics, pesticides, and bisphenols) in soils. Toward the end, we proposed several aspects of applying DGT as a versatile tool in soil research. [ABSTRACT FROM AUTHOR]

The major goal of this study was to evaluate the potential of fungal species indigenous to mine tailing soils in accelerating Mg release from lizardite (a polymorph of serpentine) at ambient T/P conditions. We characterized the culturable fungal isolates at three sampling sites representative of different degrees of mineral weathering by isolating the genomic subunits and internal transcribed spacer (ITS) rRNA genes using PCR and sequencing of cloned fragments. We chose the specific strain primarily identified asTalaromyces sp.for the further experiments with lizardite because of this strain's remarkable tolerance to high [Mg2+] (1 mol·L−1) and [Ni2+] (10 mM·L−1) levels in the screening test and its ubiquity in the most severely weathered samples. Results of dissolution experiments revealed that both magnesium-release rate and efficiency were significantly increased (e.g., by a factor of up to 15) in the presence of fungal cells than those in the abiotic controls. The enhanced dissolution of lizardite was mainly attributed to the fungal production of organic acids including oxalic acid, gluconic acid, formic acid, and fumaric acid added to the solution. The proton-promoted dissolution, however, was indicated not to be the only mechanism for fungus-lizardite interactions as much lesser Mg (in wt.%) was recovered in the abiotic system where the solution pH was constantly adjusted to match that of the fungal system. We also explored the dependence of fungal dissolution (of lizardite) on temperature and mineral particle sizes. In particular, we found that up to ∼ 50 wt.% of Mg was released from mineral particles of ∼ 50 μm within 30 days at 38°C, ∼ 26% and 8% higher than that at 18°C and 28°C, respectively. At the same temperature of 28°C, the Mg-release efficiency increased from 12.2 wt% for particles of ∼ 270 μm to 38.4 wt% for those of ∼100 μm although no apparent difference was recognized when the particle size decreased below 100 μm. The nonlinear correlation of dissolution rates with particle surface areas suggested that the dissolution process was controlled by mineral surface-structural modification along with Mg release and by fungal cells’ interaction with these surface structures. An amorphous layer of Mg-depleted silica was detected at the reacted mineral surface by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Formation of glushinskite (MgC2O4·2H2O) was also observed when oxalate was accumulated to certain concentrations in the solution. Overall, this study showed that the isolatedTalaromyces sp.was a promising bioagent to improve the efficacy of cation release from serpentine minerals for the purpose of carbon sequestration and resource recovery. [ABSTRACT FROM AUTHOR]

[Display omitted] • Toxicity of the composite contaminants varies with exposure sequence. • Cd-An exposure leads to the strongest decline in biodiversity, network stability, and functional redundancy. • Simple mixture of Cd-An exhibits the mildest adverse effect. • Combined toxicity and sequence effect cannot be predicted from individual toxicity essay. Joint toxicity of organic-metal co-contamination can vary depending on organisms, toxicants, and even the sequence of exposure. This study examines how the combined toxicity of aniline (An) and cadmium (Cd) to soil bacteria in microcosms changes when the order of contaminant introduction is altered. Through analyzing biodiversity, molecular ecological network, functional redundancy, functional genes and pathways, we find the treatment of Cd followed by An brings about the strongest adverse impact to the bacterial consortium, followed by the reverse-ordered exposure and the simple mixture of the two chemicals. On the level of individual organisms, exposure sequence also affects the bacteria that are otherwise resistant to the standalone toxicity of both An and Cd. The dynamic behavior of aniline-cadmium composite is interpreted by considering the tolerance of organisms to individual chemicals, the interactions of the two toxicants, the recovery time, as well as the priority effect. The overall effect of the composite contamination is conceptualized by treating the chemicals as environmental filters screening the growth of the community. [ABSTRACT FROM AUTHOR]

Abstract: It has been observed that the metabolism of sulfate-reducing bacteria can overcome the energy barrier to Mg2+ incorporation into growing Ca–Mg carbonates and enhance dolomite precipitation, although the role of SRB in dolomite formation is still under debate. In this study, we presented in situ AFM observations of Ca–Mg carbonate {104} surface growing from supersaturated solutions. Our data showed that not only can Mg2+ modify the morphology of the polygonal growth hillocks and impede step growth, but it can also inhibit 1-D step nucleation, resulting in the inability for spirals to continue their vertical growth. However, in the presence of dissolved sulfide, both the 1-D step nucleation and step growth which had been retarded by Mg2+ ions were significantly enhanced. For example, in the presence as low as 0.13mM dissolved sulfide, the step velocity can be increased by more than 9 times compared to that in contact with solutions containing Mg2+ ions but no dissolved sulfide. The Ca–Mg carbonate growth hillock in contact with dissolved sulfide-bearing growth solutions eventually developed a micromosaic-like structure. Based on our observations, we propose that the overall catalytic effect of dissolved sulfide may be twofold, one to stabilize the critical nuclei during 1-D step nucleation by the adsorption of dissolved sulfide on Ca–Mg carbonate surfaces and two to facilitate the dehydration of surface Mg2+–water complexes during growth. We hypothesize that dissolved sulfide may adsorb on crystal faces through hydrogen bonding between the H in HS−/H2S and the O in calcite to weaken the rigid Mg2+ hydration shell, resulting in an elevated activation entropy for particle attachment and hence a larger kinetic coefficient for step growth. Together with previous studies on disorder dolomite precipitation induced by dissolved sulfide, we demonstrate the catalysis role of dissolved sulfide in sedimentary dolomite formation associate with SRB, which may shed new light on the long-standing “dolomite problem”. [Copyright &y& Elsevier]

Abstract: The dissolution of calcite in a full range of saturation conditions is investigated to explore the kinetic effect of Gibbs free energy, dislocation density, and the presence of atmospheric pCO2. Experiments are carried out in a mixed-flow reactor at room temperature (25°C) in both closed and open (to air) settings, and calcite samples are prepared by fragmentation and milling to generate different defect densities. Experimental observations show a highly nonlinear dependence of the dissolution rates on the Gibbs free energy; however, the kinetics does not seem to be affected by the samples'' dislocation density, nor the presence of atmospheric pCO2 at any saturation condition. Fitting the conventional transition state model (TST) to the observed rate - free energy relationship indicates that, though the TST rate equation is sufficient to describe the dissolution kinetics near and far from equilibrium, it clearly overestimates the dissolution rate when the system sits in between. These results suggest that: (i) the classic TST model may not be sufficient to depict the relation between dissolution rate and Gibbs free energy once solution saturation falls from its extrema. (ii) The steps associated with increased crystal defects may be overwhelmed by those regenerated at corners and edges of calcite particles through layer-by-layer dissolution along the cleavage directions. (iii) The presence of CO2 in ambient environments bears little importance to calcite dissolution possibly due to the slow response of aqueous HCO3 − to pCO2 change at low CO2 partial pressure conditions. [Copyright &y& Elsevier]

[Display omitted] • Muscovite granites may result from extremely fractionated granitic magmas. • Muscovite granites contain two stages of zircon grains. • Deep-seated magma chamber may have survived longer than 20 Myr. • The residual magma enriched in metals and generated the giant tungsten deposit. Granites generated in massive volume during an orogeny frequently concentrate abundant trace metals over the requirement of commercial mining. Tungsten mineralization usually develops in massive granite generated during orogenic activities. However, there is no general metallogenic model capable of explaining metal enrichment. In specific, the source of the metals and criteria to distinguish barren from mineralized granites have yet to be clearly identified. In this paper, we present that the muscovite alkali-feldspar granites, as the parental rocks to the tungsten mineralization in the Nanling Range (South China), may result from extremely fractionated granitic magma from the deep-seated magma chamber. Further, the geochemical and geochronological evidence on hydrothermal rims and magmatic cores of zircon grains from these granites reveals two stages of magmatic activities, i.e. the syn-orogenic biotite monzogranites at 155.0 ± 1.0 Ma and the post-orogenic muscovite alkali-feldspar granites at 133.4 ± 1.0 Ma respectively. Since the two granites with the similar zircon ε Hf (t) values and in the same location should develop from the same magma chamber during the Yanshanian movement, we interpreted that the deep-seated magma chamber survived longer than 20 Myr, within which a continuously fractional crystallization led to unusual concentration of incompatible elements (tungsten, volatile, and aqueous fluid) in the residual magma. When the regional tectonic environment entered the extensional regime at ~133 Ma, the highly fractionated residual magma ascended rapidly from the magma chamber, generated the muscovite alkali-feldspar granite, and developed the accompanying giant tungsten deposit. Our thermal modeling supports the possibility of an ultra-long magma residence time at deep-seated chamber. [ABSTRACT FROM AUTHOR]

We present the first examination of calcite dissolution in seawater using Atomic Force Microscopy (AFM). We quantify step retreat velocity and etch pit density to compare dissolution in seawater to low ionic strength water, and also to compare calcite dissolution under AFM conditions to those conducted in bulk solution experiments (e.g. Subhas et al., 2015, Dong et al., 2018). Bulk dissolution rates and step retreat velocities are slower at high and mid-saturation state (Ω) values and become comparable to low ionic strength water rates at low Ω. The onset of defect-assisted etch pit formation in seawater is at Ω ∼ 0.85 (defined as Ω critical), higher than in low ionic strength water (Ω ∼ 0.54). There is an abrupt increase in etch pit density (from ∼106 cm−2 to ∼108 cm−2) occurring when Ω falls below 0.7 in seawater, compared to Ω ∼ 0.1 in low ionic strength water, suggesting a transition from defect-assisted dissolution to homogeneous dissolution much closer to equilibrium in seawater. The step retreat velocity (v) does not scale linearly with undersaturation (1-Ω) across an Ω range of 0.4 to 0.9 in seawater, potentially indicating a high order correlation between kink rate and Ω for non-Kossel crystals such as calcite, or surface complexation processes during calcite dissolution in seawater. [ABSTRACT FROM AUTHOR]

Rhizosphere is a soil volume of high spatio-temporal heterogeneity and intensive plant-soil-microbial interactions, for which visualization and process quantification is of highest scientific and applied relevance, but still very challenging. A novel methodology for quick assessment of two-dimensional distribution of available phosphorus (P) in rhizosphere was suggested, tested, and development up to the application platform. Available P was firstly trapped by an in-situ diffusive gradients in thin-films (DGT) sampler with precipitated zirconia as the binding gel, and subsequently, the loaded gel was analyzed with an optimized colorimetric imaging densitometry (CID). The imaging platform was established linking: i) DGT, ii) planar optode, and iii) soil zymography techniques to simultaneously determine available P, oxygen, and acid phosphatase in rhizosphere at sub-millimeter spatial scales. The DGT identified available P level in rice rhizosphere were spatially overlapping to the localized redox hotspots and phosphatase activity. The spatial relationship between available P and acid phosphatase activity was dependent on root development. The root radial oxygen loss (ROL) remained active during the experimental observations (2–3 days), while a flux of available P of 10 pg cm−2 s−1 was visualized within 2–3 mm of roots, confirming the correlative response of rice roots to oxygen secretion and P uptake. Summarizing, the established imaging platform is suitable to capture spatial heterogeneity and temporal dynamics of root activities, nutrient bioavailability, ROL and enzyme activities in rhizosphere. [Display omitted] • Quick establishment of the two-dimensional distribution of available P was realized. • A versatile platform was established for imaging multiple analytes in rhizosphere. • Rhizosphere P dynamics were linked to localized redox conditions and root activity. • Relationship between P and acid phosphatase activities varied as root developed. [ABSTRACT FROM AUTHOR]

Scorodite (FeAsO 4 ·2H 2 O)–water interaction is critical to As distribution and storage in surface environment but is inadequately understood due to ambiguities in the mineral's stability and weathering rate at atmospheric conditions. In the present study we attempted to experimentally determine the thermodynamic and kinetic parameters needed to compute solubility and dissolution rate of scorodite at 25 °C. Experiments were carried out in low pH (1.15) solutions using specially synthesized large scorodite crystals (with Mean Diameter of 27.9 μm). Such experimental conditions ensured the results were not subject to the influence of secondary Fe-bearing phase precipitation and grain size effect. Measured equilibrium concentrations of Fe and As, along with newly published Fe-As complexes association constants, were first used to determine the solubility products K sp and dissolution rates r n at 50–90 °C. The obtained K sp ∼ T and r n ∼ T dependence was then used to derive the Gibbs free energy, enthalpy, entropy, heat capacity, and activation energy for the dissolution reaction. Finally, we extrapolated the measurements to 25 °C and obtained room temperature solubility and dissolution rate, scrutinized the pH effect on dissolution, and analyzed the Δ G ∼ r n relation of the dissolution reaction. Our results show that literature data are likely overestimated scorodite solubility at pH > 4 ∼ 4.5 due to neglecting the effect of ferric iron hydrolysis. Estimated ambient condition dissolution rate is an order magnitude lower than the earlier report, implicating the importance of size effect, but is one to two orders of magnitude higher than that of common rock-forming minerals, cautioning the proposed use of scorodite for As fixation and storage. The determined r n ∼ ΔG relation cannot be fully fitted by the transition state model, particularly at near equilibrium, suggesting dissolution in this study may be controlled by defect-assisted surface reactions. [ABSTRACT FROM AUTHOR]

The occurrence of the {2 1 0} shape of pyrite crystals has been used as an empirical indicator for gold enrichment in ore deposits. Such indicating, also called 'typomorphism' in ore mineralogy, was conventionally attributed to environmental factors such as sulfur fugacity and temperature and pressure conditions in the ore-forming hydrothermal fluid. Here we show that the crystal-face specific redox reactivity of pyrite is also an important governing potency that drives gold enrichment in the pyrite crystals with particular shapes. This conclusion was drawn from data compiled from chemical analysis of field samples, laboratory testing of gold deposition on natural pyrite crystals, as well as theoretical calculations. Kinetic measurements of gold reductive deposition first revealed the highest precipitation rate on the {2 1 0} faces, particularly when pyrite crystals are enclosed by complex forms containing {2 1 0}. Corroborating with the experimental observations, density functional theory (DFT) calculations further indicated that the {2 1 0} possesses a high adsorption energy for gold, a high energy state of the highest occupied molecular orbital and consequently a low Eh. These results afford evidence supporting the view that the face specific reactivity, an intrinsic property of crystal, plays an important role in the gold concentration in pyrite and the occurrence of specific pyrite shapes. Our findings not only provide a surface chemical view for understanding the empirical prospecting method but also exemplified the significance of crystal face specific-reactivity in understanding crystallographic orientation related geochemical processes. The crystal face specific reactivity may be a critical factor controlling geochemical reactions at mineral-water interface, such as prebiotic synthesis on pyrite surfaces and mineral-water interface controlled kinetic isotope fractionation. [ABSTRACT FROM AUTHOR]

Abstract The generation of reactive oxygen species H 2 O 2 and OH from pyrite in anaerobic environments plays an important role in the evolution of early Earth. What remains debatable is the underlying mechanism leading to the OH generation reactions. Using a comprehensive approach combining X-ray photoelectron spectroscopy and ab initio calculations, we investigated binding energies and valence band structures of defective pyrite surfaces in an attempt to interrogate pyrite-mediated H 2 O dissociation in the presence and absence of crystal defects. The results show that, while energetically inhibited on perfect crystal faces, H 2 O dissociation is thermodynamically favored at defective sites. Furthermore, the formation of surface defects can lead to an energy shift in valence bands and thereby forming two defect states. Simultaneously, interaction between both defect states and water molecules makes the hydroxylation energetically favored on the pyrite surface. The hydroxylation occurs through proton transfer from water to a defective S monomer, resulting in an Fe O(H)···H S structure. These findings provide new insight into pyrite-assisted OH formation processes in anaerobic conditions and may be important for understanding prebiotic chemistry and the evolution of early Earth. [ABSTRACT FROM AUTHOR]

Pyrite oxidation not only is environmentally significant in the formation of acid mine (or acid rock) drainage and oxidative acidification of lacustrine sediment but also is a critical stage in geochemical sulfur evolution. The oxidation process is always controlled by the reactivity of pyrite, which in turn is controlled by its surface structure. In this study, the oxidation behavior of naturally existing {1 0 0}, {1 1 1}, and {2 1 0} facets of pyrite was investigated using a comprehensive approach combining X-ray photoelectron spectroscopy, diffuse reflectance Fourier transform infrared spectroscopy, and time-of-flight secondary-ion mass spectrometry with periodic density functional theoretical (DFT) calculations. The experimental results show that (i) the initial oxidation rates of both pyrite {1 1 1} and {2 1 0} are much greater than that of pyrite {1 0 0}; (ii) the initial oxidation rate of pyrite {2 1 0} is greater than that of pyrite {1 1 1} in low relative humidity, which is reversed in high relative humidity; and (iii) inner sphere oxygen-bearing sulfur species are originally generated from surface reactions and then converted to outer sphere species. The facet dependent rate law can be expressed as: r { hkl } = k { hkl } h a P 0.5 ( t + 1 ) - 0.5 , where r { hkl } is the orientation dependent reaction rate, k { hkl } is the orientation dependent rate constant, h is the relative humidity, P is the oxygen partial pressure, and t is the oxidation time in seconds. {1 1 1} is the most sensitive facet for pyrite oxidation. Combined with DFT theoretical investigations, water catalyzed electron transfer is speculated as the rate-limiting step. These findings disclose the structure–reactivity dependence of pyrite, which not only presents new insight into the mechanism of pyrite oxidation but also provides fundamental data to evaluate sulfur speciation evolution, suggesting that the surface structure sensitivity should be considered to estimate the reactivity at the mineral–water interface. [ABSTRACT FROM AUTHOR]

The substrate effect of mica and hematite on the nucleation and crystallization of calcite was investigated using scanning electron microscope (SEM), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD) methods. On mica, we found, in the absence of Mg2+, the substrates' (001) surfaces with hexagonal and pseudo-hexagonal two-dimensional (2-D) structure can affect the orientation of calcite nucleation with calcite (001) ~// mica (001) and calcite (010) ~// mica (010) to be the major interfacial relationship. On hematite, we did not observe frequent twinning relationship between adjacent calcite gains, but often saw preferentially nucleation of calcite at surface steps on hematite substrate. We suggest that calcite crystals initially nucleate from the Ca2+ layers adsorbed on the surfaces. The pseudo-hexagonal symmetry on mica (001) surface also leads to the observed calcite (001) twinning. A second and less common orientation between calcite {104} and mica (001) was detected but could be due to local structure damage of the mica surface. Results in the presence of Mg2+ show that the substrate surfaces can weaken Mg toxicity to calcite nucleation and lead to a higher level of Mg incorporation into calcite lattice. [ABSTRACT FROM AUTHOR]

Correction to: I Scientific Reports i https://doi.org/10.1038/s41598-023-31831-1, published online 20 March 2023 The original version of this Article contained errors in the Results and discussion, under the subheading "Mechanisms for Fe-DOMP formation and isolation", "2 I FeCl i SB 3 sb + 6 I NaOH i + 6 I DOM i - I COOH i = 2 I Fe i SP 3 sp + 6 I DOM i - I COO i SP - sp + 2 I NaCl i SB 3 sb + 6 I H i SB 2 sb I O i ." now reads: "2 I FeCl i SB 3 sb + 6 I NaOH i + 6 I DOM i - I COOH i = 2 I Fe i SP 3 sp + 6 I DOM i - I COO i SP - sp + 6 I NaCl i + 6 I H i SB 2 sb I O i ." The original article can be found online at https://doi.org/10.1038/s41598-023-31831-1. [Extracted from the article]

Despite the availability of various reactors designed to study gas-liquid reactions, no appropriate devices are available to accurately investigate triple-phased mineral carbonation reactions involving CO2 gas, aqueous solutions (containing divalent cations), and carbonate minerals. This report presents a composite reactor that combines a modified conventional wetted-wall column, a pH control module, and an attachment to monitor precipitation reactions. Our test and calibration experiments show that the absorption column behaved largely in agreement with theoretical predictions and previous observations. Experimental confirmation of CO2 absorption in NaOH and ethanolamine supported the effectiveness of the column for gas-liquid interaction. A test run in the CO2-NH3-MgCl2 system carried out for real time investigation of the relevant carbonation reactions shows that the reactor's performance closely followed the expected reaction path reflected in pH change, the occurrence of precipitation, and the rate of NH3 addition, indicating the appropriateness of the composite device in studying triple-phase carbonation process. [ABSTRACT FROM AUTHOR]

We investigated the effect of Mg2+ on calcite hillock growth over a broad range of solution conditions in terms of supersaturation (Ωcalcite) and Mg/Ca ratios using atomic force microscopy and secondary ion mass spectrometry. We found that both the incorporation pattern/incorporated Mg2+ quantity in the hillock structure and the Mg2+-induced morphological change of the hillock surface showed strong dependence of the growth conditions. Specifically, when Mg/Ca was high (i.e., >5) and Ωcalcite was low (i.e., ~0.45), Mg2+ was predominantly incorporated into the negative sectors of the hillock structure, resulting in gradual loss of step structure and morphological amorphism on these vicinal surfaces. When Mg/Ca and Ωcalcite were in intermediate ranges (i.e., Mg/Ca < 5, and 0.45 < Ωcalcite < 1), the originally straight edges of the hillock steps exhibited curvatures of varying degrees and formed "tear-drop" morphologies. It is noted that such "tear-drop" morphology was stable within the duration of the experiments and did not evolve into other surface patterns. By contrast, when both Mg/Ca and Ωcalcite were high (i.e., Mg/Ca > 5, and Ωcalcite > 1.1), the growing hillocks experienced two phases of morphological changes, initiated with the formation of "tear-drops" followed by the development of linear ruptures along [481] and [441] directions. And the occurrence of these ruptures segmented the hillock surface effectively into multiple isolated plateaus. Significantly, we revealed the underlying mechanisms for these condition-specific effects of Mg2+ on calcite growth, which mainly resulted from the interplay among three major factors: (1) the size-mismatch between Mg2+ and Ca2+ that causes structural strains in magnesian calcite and leads to morphological amorphism in high-Mg2+ carbonate; (2) the asymmetry of the calcite crystal structure that sets a physical limitation for Mg2+ incorporation patterns in the hillock structure; and (3) the step advancing rate (i.e., the calcite growth kinetics) that affects both Mg2+ incorporation and the accommodation of Mg2+-induced structural strains in the hillock structure. Detailed discussions were given for each growth scenario. The results of our study provide a theoretical base to decipher the roles of Mg2+ in CaCO3 mineralization, and thus, have important implication for a range of processes that involve the growth of Mg-Ca-CO3 systems, such as biomineralization, carbon capture and storage, and scale controls in industrial settings. [ABSTRACT FROM AUTHOR]

Fungus-mineral interactions play unparalleled roles in shaping the planet Earth but are underappreciated relative to bacterial influences. Unique to fungus, but largely unknown, are the interfacial processes and extensiveness of hypha- versus spore-mineral interactions given the associated turgor pressure differences and the vast contact areas between mycelia and minerals in the critical zone. Here we examine lizardite [Mg3Si2O5(OH)4] dissolution by single cells of a native fungal strain using confocal laser scanning microscopy, atomic force microscopy, and transmission electron microscopy-energy dispersive X-ray spectroscopy to explore the mechanism, driving force, and magnitude of the interfacial reactions. Results from our inspection showed (1) significant pH reduction in the vicinity of cells upon mineral surface attachment, (2) exclusive Fe loss from the mineral at the cell-mineral interfaces, and (3) destruction of the mineral crystal structure below the area colonized by hyphae but not that by spores. Compared to the results from bulk experiments and at the mineral-water interface, these observations indicate that (1) only attached cells release siderophores and (2) biomechanical forces of hyphal growth are indispensable for fungal weathering and strong enough to breach the mineral lattice. Estimated mineral mass loss at the interface suggests that cellular dissolution can ultimately account for ~40%-50% of the overall bio-weathering, significantly larger than the previous estimate of ~1% contribution. [ABSTRACT FROM AUTHOR]

Abstract: Nesquehonite crystallization experiments were conducted in H2O and water/dimethylformamide (DMF) mixture to examine solvent effect on the nucleation kinetics. Results show a reduction of nucleation induction time in the presence of DMF and a positive correlation between nucleation rate and DMF concentration. Analysis in the context of the classical nucleation theory reveals an unexpected increase in the surface energy of nesquehonite upon the solvent change, implying that kinetic factors, rather than surface energetics, is the driving force behind the resultant enhancement in nucleation rate. Further analyses suggest that kinetic acceleration under higher surface energy conditions is possible if nucleation proceeds through a cluster aggregation mechanism instead of critical nucleus formation. Fitting the Smoluchowski’s coagulation theory to the experimental data seems to provide support to the operation of the non-classical nucleation mechanism as it shows that the cluster association rate is independent of the sizes of the precursors but positively correlated to the DMF concentration, consistent with the scenario of growing population of sub-critical nuclei and ensuing higher probability of cluster collision/aggregation leading to an accelerated nucleation. [Copyright &y& Elsevier]

Carbonation of natural brucite in H2O and diluted HCI is investigated at room temperature and moderate pCO2 to explore the products' mineralogy and reaction kinetics. Results show nesquehonite is by far the dominant carbonate species formed, despite its poorer thermodynamic stability relative to magnesite and possibly hydromagnesite. Time-dependent measurements reveal carbonate formation within 30 min, regardless of the original acidity of the slurry. However, while the fraction of reacted brucite in H2O increases gradually over time and approaches unity (∼98%) at 2.5 h, it rises rapidly in HCI within the first hour and levels off thereafter, leaving a significant amount of brucite unreacted. Such behavior suggests that the initial quantity of Mg2+ affects the reaction kinetics. Fitting a pseudo first-order rate law to the data yields a higher rate constant for the HCl experiments. These observations may imply that the carbonation does not proceed through heterogeneous reaction between gaseous CO2 and solid brucite. Solution chemistry analysis indicates that most CO2 stays in aqueous phase in both media; however, the concentration of HCO3- becomes high in H2O after about 2 h, agreeing with the observed inferior carbonation extent in HCI. [ABSTRACT FROM AUTHOR]

Abstract: Morphological characters and multi-element isotopic compositions of carbonates from the loess–paleosol sequences in Northwestern China are examined to explore the origin of the minerals. Samples are collected from various sections ranging from Holocene to 0.9Ma within the sequences and fractions with grain sizes >45 and <2μm are separated from the bulk soil and examined by SEM and TEM. The results show that the grains >45μm exhibit an almost perfect spherical shape while those <2μm are dominated by nano-rods having diameters of 30–50nm and lengths of 0.3–2μm, presumably indicating the detrital origin of the coarse fractions and the authigenic characters of the fine ones. Such implications are corroborated by the multi-elemental isotopic compositions of the carbonate minerals. A comparison of the δ13C and δ18O values between minerals and biologically originated samples indicates that the <2μm fractions have a similar composition to those of coexisting land snail shells. Additional differences between the two size fractions also manifest in the ratios of 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, and 87Sr/86Sr. These results suggest the utility of fine carbonate particles in the soil profile in reconstructing a potentially higher resolution δ13C and δ18O time series to elucidate the paleoclimatic fluctuation in the Chinese Loess Plateau during Pleistocene. The discovery of the nano-rod calcite in Chinese loess, together with previous findings of the similar mineral form in Asian dust, strongly suggests the possibility that these highly reactive CaCO3 form may alter the aerosol properties during transport. [Copyright &y& Elsevier]

Abstract: Microbiological flocculation in wastewater by a strain of Bacillus mucilaginosus was examined and the flocculation mechanism was investigated using Coomassie brilliant blue reaction, Anthrone reaction, thin-layer chromatography (TLC), scanning electronic microscope, and infrared spectroscopy. The GY03 strain, isolated from farmland soil, was identified as B. mucilaginosus by its morphology and 16S rDNA sequence. Cultivated in a nitrogen-free medium, the flocculation material (bacterium–mineral complex) was used to treat domestic, brewage, and pharmaceutical wastewater. On the basis of one-way experimental results, orthogonal experiments were carried out and the optimum processing conditions of the microbial flocculants were obtained. In the optimal conditions, the maximal removal ratio of COD in domestic, brewage, and pharmaceutical wastewater reached 74.6%, 70.5%, and 66.2%, respectively; the maximal removal ratio of BOD was 42.3%, 77.4%, and 41.7%, respectively; and the maximal removal of SS amounted 93.3%, 93.6%, and 88.4%, respectively. The treatment to kaolin suspended liquid by the GY03 strain may act as a model of studying microbial flocculation mechanisms in which bridging and charge neutralization hypothesis were proposed as the critical reasons based upon the experimental observations. [Copyright &y& Elsevier]

Abstract: A strain of thermophilic fungus Aspergillus fumigatus was cultured with K-bearing minerals to determine if microbe–mineral interactions enhance the release of mineralic potassium. Experiments were carried out in two settings, one with the mineral grains and the fungal cells in direct contact, and the other employing a membrane (pore size 0.22μm) to separate the two. Measurements over a period of 30 days showed that, irrespective of the experimental setup, the concentration of free K in the culture was drastically higher than those in any of the control experiments where no living organism was present. Moreover, the occurrence of mineral–cell physical contact enhanced potassium release by an additional factor of 3 to 4 in comparison to the separation experiments. For contact experiments, Electron Probe Microanalysis revealed the formation of mycelium–mineral aggregates, and Atomic Force Microscopy imaging further indicated the possible ingestion of mineral particles by the fungus cells. Contrasting to what was observed and expected in control experiments, the potassium solubilization rate showed a positive dependence upon pH when fungi and minerals were mixed directly, and exhibited no correlations with solution acidity if cell–rock contact was restrained. These results appear to suggest that A. fumigatus promoted potassium release by means of at least three likely routes, one through the complexation of soluble organic ligands, another appealing to the immobile biopolymers such as the insoluble components of secretion, and the third related to the mechanical forces in association with the direct physical contact between cells and mineral particles. [Copyright &y& Elsevier]

A simple apparatus was designed to effectively isolate magnetotactic bacteria from soils or sediments based on their magnetotaxis. Through a series of processes including sample incubation, MTB harvesting, isolation, purification and identification, several strains of bacteria were isolated from the samples successfully. By Transmission Electron Microscopy (TEM) and Energy-Dispersive X-ray Analysis (EDXA), these bacteria were certificated to be magnetotactic bacteria. The phylogenetic relationship between the isolated magnetic strains and some known magnetotactic bacteria was inferred by the construction of phylogenetic tree based on 16SrDNA sequences. This apparatus has been proven to have the advantages of being inexpensive, simple to assemble, easy to perform and highly efficient to isolate novel magnetotactic bacteria. The research indicated that the combined approach of harvesting MTB by home-made apparatus and the method of plate colony isolation could purify and isolate magnetotactic bacteria effectively. [ABSTRACT FROM AUTHOR]

Abstract: Fluorosis, developed in the population who depends on water with high concentrations of fluoride for their daily drinking usage, is one of the most frequently occurring endemic diseases. To search for effective defluoridation agents, we studied F− adsorption by synthetic Mg/Al–CO3 hydrotalcite-like compounds (HT) and their calcined products (HTC). The adsorption experiments were carried out as a function of time, pH, and the adsorbate concentration. The affecting factors to fluoride sorption were found to be solution pH and the crystallinity of the HT. The results from our adsorption experiments indicated that F− uptake by HTC was much stronger than their precursors HT. It was further observed that the maximal adsorption takes place within the first 15 min. HTs and HTCs exhibit a reversed trend in F− uptake with regard to temperature of the hydrothermal treatment to HT. At any specific initial F− concentration, the removal efficiency of F− increases with the temperature for HTC, but decreases for HT. The removal efficiency is inversely related to the initial fluoride concentration for HTC obtained from HT synthesized at 130 °C (HTC130, the most efficient adsorbent seen in this study). The adsorption isotherm at pH 7 for HTC130 was linear and did not follow Langmuir equation; the K value (slope of the line) was calculated to be 0.46 L/g. HTC130 was shown to be able to bring F− concentration from 5 mg/L down to less than 1 mg/L in aqueous solution, suggesting that this material may be a possible candidate for F− removal. [Copyright &y& Elsevier]

Abstract: Biogenic carbonates spawned from microbial activities are common occurrences in soils. Here, we investigate the carbonate biomineralization mediated by the bacterium Bacillus megaterium, a dominant strain separated from a loess profile in China. Upon completing bacterial cultivation, the ensuring products are centrifuged, and the resultant supernatant and the concentrated bacterial sludge as well as the un-separated culture are added separately into a Ca-CO3 containing solution for crystallization experiments. Results of XRD and SEM analysis indicate that calcite is the dominant mineral phase formed when the bacteria are present. When the supernatant alone is used, however, a significant portion of vaterite is also precipitated. Experimental results further reveal that the bacteria have a strong tendency to colonize the center area of the calcite {10 4} faces. Observed crystal morphology suggests that the bacterial colony may promote the growth normal to each individual {10 4} face of calcite when the cell concentration is high, but may retard it or even cause dissolution of the immediate substrate surfaces when the concentration is low. SEM images taken at earlier stages of the crystallization experiments demonstrate the nucleation of calcite on the bacterial cell walls but do not show obvious morphological changes on the nanometer- to submicron-sized nuclei. δ13C measurements unveil that the crystals grown in the presence of bacteria are further enriched in the heavy carbon isotope, implying that the bacterial metabolism may not be the carbon sources for the mineralization. Based upon these findings, we propose a mechanism for the B. megaterium mediated calcite mineralization and conclude that the whole process involves epi- and inter-cellular growth in the local microenvironments whose conditions may be controlled by cell sequestration and proton pumping during bacterial respiration. [Copyright &y& Elsevier]

Abstract: Grain size- and crystallographic direction-dependence are among the fundamental characteristics of crystal solubility. However, such important material properties are routinely ignored and solubility is often conveniently approximated by a solubility product. In this study, we attempt to outline the relationship between solubility and solubility product using thermodynamic arguments, and to provide observations that demonstrate the occurrence of circumstances where the solubility product cannot properly approximate crystal solubility. Theoretical analysis shows that solubility is always greater than solubility product, but the difference is inversely related to the grain size. Furthermore, the difference can be crystallographic direction specific if the total surface energy change upon the attachment of an individual growth unit is nonequivalent for each symmetrically unrelated crystal faces. In situ AFM experiments conducted on the cleavage face of calcite demonstrate that the steps exhibit direction- and length-dependent behavior. Specifically, the measured critical step lengths are consistent with the predicted inverse relationship to saturation states. Moreover, step retreat at and advance at are observed simultaneously in a narrow range of saturation at near equilibrium conditions, indicating the existence of direction specific solubility. Whereas these findings justify the rationale for approximating solubility by solubility product in cases where large crystals are concerned, the results imply that the size and direction effect should not be ignored if nanocrystal growth/dissolution is the subject of interest. [Copyright &y& Elsevier]

• Goethite can transform tetracyclines under various environmental conditions. • Two different hydroxyl groups on goethite were reactive sites for TTC degradation. • N,N-dedimethylation was the major transformation pathway for TTC. • Antimicrobial properties of TTC were eliminated after transformation by goethite. • Dealkylation by goethite are expected for other drugs possess alkylamino group. Tetracycline antibiotics (TCs) are a group of the top selling and widely used antibiotics that have been frequently detected in various environments. The interaction between TCs and goethite (α-FeOOH), one of the most common crystalline oxide minerals in aqueous environment and soil, is unclear. Apart from adsorption, this study firstly demonstrated that transformation of tetracycline (TTC) occurred in the presence of goethite. The transformation kinetics and mechanism of TTC with goethite were investigated to gain a better understanding of the fate of TCs in the natural environment. The results showed that the transformation of TCs by goethite explicitly exhibited two-stage kinetics, wherein an initial period of fast transformation was followed by a continuous slow transformation. Hydroxyl groups on goethite were identified as major reactive sites, among which singly coordinated hydroxyls (FeOH) were more reactive than doubly coordinated hydroxyls (Fe 2 OH) towards the transformation of TTC. On the basis of transformation rates, speciation of TTC and functional groups on goethite surface, a kinetic model was established successfully describing the transformation of TTC by goethite under conditions of varying reactant concentration and pH. The transformation of TTC by goethite mainly resulted in a N,N-dedimethylation product that did not show antimicrobial properties towards Escherichia coli. This study indicates that Fe(III)-(hydro)oxides in soils and sediments may play an important role in the natural attenuation of tetracycline antibiotics and their bioactivity. [Display omitted] [ABSTRACT FROM AUTHOR]