Gen-Tao Zhou

An experimental study of oxygen isotope fractionation between inorganically precipitated aragonite and water at low temperatures

Abstract To determine oxygen isotope fractionation between aragonite and water, aragonite was slowly precipitated from Ca(HCO3)2 solution at 0 to 50°C in the presence of Mg2+ or SO42−. The phase compositions and morphologies of synthetic minerals were detected by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The effects of aragonite precipitation rate and excess dissolved CO2 gas in the initial Ca(HCO3)2 solution on oxygen isotope fractionation between aragonite and water were investigated. For the CaCO3 minerals slowly precipitated by the CaCO3 or NaHCO3 dissolution method at 0 to 50°C, the XRD and SEM analyses show that the rate of aragonite precipitation increased with temperature. Correspondingly, oxygen isotope fractionations between aragonite and water deviated progressively farther from equilibrium. Additionally, an excess of dissolved CO2 gas in the initial Ca(HCO3)2 solution results in an increase in apparent oxygen isotope fractionations. As a consequence, the experimentally determined oxygen isotope fractionations at 50°C indicate disequilibrium, whereas the relatively lower fractionation values obtained at 0 and 25°C from the solution with less dissolved CO2 gas and low precipitation rates indicate a closer approach to equilibrium. Combining the lower values at 0 and 25°C with previous data derived from a two-step overgrowth technique at 50 and 70°C, a fractionation equation for the aragonite-water system at 0 to 70°C is obtained as follows: 10 3 ln α=20.44×10 3 /T−41.48. This equation represents the first experimental calibration of oxygen isotope fractionation between inorganically precipitated aragonite and water at low temperatures. By considering the kinetic mechanism of oxygen isotope disequilibrium, we argue that this equation is a close proxy for thermodynamic equilibrium fractionation in the low-temperature mineral. Therefore, the discrepancies in CaCO3-H2O fractionation factors between different synthesis experiments may imply that some of the studies reflect steady-state fractionations during aragonite precipitation and subsequent polymorphic transition to calcite at different run conditions.

Sonochemical synthesis of aragonite-type calcium carbonate with different morphologies

Aragonite, a high-pressure polymorph of CaCO3, was synthesized for the first time from simple Ca(HCO3)2 solutions by using high-power ultrasound irradiation. XRD, FT-IR and SEM techniques were used to characterize the phase composition and morphology of the products. The intensity of the ultrasound irradiation was found to have a remarkable effect on the morphology of the as-synthesized aragonite. The morphology evolved from rod-shaped to spindle-shaped when the acoustic amplitude is increased from 50% to 70% of the full amplitude. A possible mechanism for the formation of this controllable aragonite formation is proposed. The dissolved CO2 plays a crucial role in the homogeneous nucleation and growth of aragonite under sonication. Vaterite with flower- or dendrite-like structures could also be obtained by using 75% or 80% of the full acoustic amplitude, respectively. A nonequilibrium kinetics process controls the formation of unstable vaterite with unusual structures.

Formation of aragonite mesocrystals and implication for biomineralization

Abstract Highly oriented aragonite tablets have been found in the nacre layers of molluscan shell (or mother of pearl). In this article, we show that highly organized aragonite rods can be prepared over a broad range of pH values (1.5 to 6.9) and in the absence of any bio- or organic macromolecules. The organized rods were characterized by XRD, FTIR, FESEM, TEM, SAED, and EDX techniques. FESEM results reveal that the mesoscale aragonite rods are not only assembled with aragonite microrods end-to-end, and side-to-side, but are also partially fused to one another, forming flat, faceted surfaces, i.e., mesocrystal structure. TEM and SAED analyses confirm that the organized rods have the same crystallographic symmetry as single-crystal aragonite, and thus the self-assembly process is energetically favorable. Similar assembly processes also occur for the mineral strontianite of the aragonite group, revealing the occurrence of a general self-assembly process. The driving force controlling the self-assembly process may originate from the inherent anisotropic dipole-dipole interactions between the assembled units. Such dipole interaction may generally occur in biomineralization of nacre layers in molluscan shell, and orchestrate aragonite nanocrystals in an aragonite tablet to coherently orient and array. Furthermore, the dipole-dipole interactions may also contribute to the co-orientation of the aragonite tablets in the same nacreous column. Therefore, our experimental results may provide insight into biomineralization mechanisms. It appears that biological genetic and crystallochemical factors may synergistically operate in biomineralization.

Kinetic mechanism of oxygen isotope disequilibrium in precipitated witherite and aragonite at low temperatures: an experimental study

Abstract To study what dictates oxygen isotope equilibrium fractionation between inorganic carbonate and water during carbonate precipitation from aqueous solutions, a direct precipitation approach was used to synthesize witherite, and an overgrowth technique was used to synthesize aragonite. The experiments were conducted at 50 and 70°C by one- and two-step approaches, respectively, with a difference in the time of oxygen isotope exchange between dissolved carbonate and water before carbonate precipitation. The two-step approach involved sufficient time to achieve oxygen isotope equilibrium between dissolved carbonate and water, whereas the one-step approach did not. The measured witherite-water fractionations are systematically lower than the aragonite-water fractionations regardless of exchange time between dissolved carbonate and water, pointing to cation effect on oxygen isotope partitioning between the barium and calcium carbonates when precipitating them from the solutions. The two-step approach experiments provide the equilibrium fractionations between the precipitated carbonates and water, whereas the one-step experiments do not. The present experiments show that approaching equilibrium oxygen isotope fractionation between precipitated carbonate and water proceeds via the following two processes: 1. Oxygen isotope exchange between [CO3]2− and H2O: (1) [ C 16 O 3 ] 2− +2 H 2 18 O =[ C 18 O 2 16 O ] 2− +2 H 2 16 O 2. A combination of divalent metal cation M2+ with the [CO3]2− to form carbonate: (2) M 2+ +[ C 18 O 2 16 O ] 2− + H 2 18 O = MC 18 O 3 + H 2 16 O Reaction 1 is the rate-limiting step for equilibrium oxygen isotope fractionation between carbonate and water, and reaction 2 bears the structural effect of carbonate crystallization on oxygen isotope fractionation during carbonate precipitation. The present results show that the time of [CO3]2−-H2O isotope exchange in the precipitation experiments is of critical importance in dictating the extent of isotopic equilibration between precipitated carbonate and water. If the time of oxygen isotope exchange between [CO3]2− and H2O is long enough to attain equilibrium before carbonate precipitation, the precipitated carbonate is able to achieve isotopic equilibration with water.

Controlled crystallization of unstable vaterite with distinct morphologies and their polymorphic transition to stable calcite

Vaterite with different morphologies, a thermodynamically unstable polymorph of calcium carbonate, was successfully synthesized by a simple injection-precipitation method. The phase composition and morphology of the products were characterized by the XRD, FT-IR, SEM, and TEM techniques. Experiments were performed at 37 and 25 °C using pH values of 1.5, 3.0, and 6.9. At 37 °C, precipitated vaterite has spindle-like morphology at the low pH 1.5 of the initial CaCl 2 solution, and shows coexistence of the spindle-, spheroid-, and cauliflower-like morphologies at the intermediate pH 3.0, whereas it is spheroidal at pH 6.9. SAED analyses revealed that the spindle-like vaterite superstructures were self-assembled by the oriented aggregation of vaterite micro-crystals along the crystallographic c direction. At 25 °C, however, the low pH (1.5) led to coexistence of cauliflower- and spheroid-shaped vaterite, whereas spherulite-like vaterite was always obtained at either pH 3.0 or 6.9. We show that simple inorganic precipitation processes lead to complex and unusual morphologies with hierarchical structure. Therefore, care must be taken when morphological criteria are claimed as proof for biogenic origin of minerals. Moreover, time studies of the polymorphic transition revealed that, although solution-mediated dissolution of precursor vaterite and reprecipitation of secondary calcite always occur, Ostwald ripening finally contributes to the formation of rhombohedral calcite. Therefore, the polymorphic transition sequentially proceeds from vaterite through irregular calcite aggregates to stable calcite rhombohedra.

Microwave-assisted controlled synthesis of monodisperse pyrite microspherolites

A microwave-assisted polyol method was reported to synthesize uniform and monodisperse pyrite (FeS2) microspherolites. The reaction processes for the synthesis involved the reduction of sulfur and reaction of the intermediate sulphur species with Fe2+. Polyvinyl pyrrolidone (PVP) added to the reaction system exerts an important role in the control of the phase composition and morphology of the products. The sample was characterized by powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and selected area electron diffraction (SAED) techniques. A series of TEM/HRTEM and SAED results reveal that the formation of pyrite FeS2 microspherolites is via a nanocrystal aggregation-based mechanism. The time-dependence experiments further demonstrate that primary FeS2 nanocrystals are first formed, and then aggregate into large spherolites, finally Ostwald ripening leads to the uniform and monodisperse microspherolites. The influence of microwave power on the size and morphology of the products and effect of microwave heating in the synthesis were also investigated.

Biomimetic synthesis of struvite with biogenic morphology and implication for pathological biomineralization

Recent studies have found that certain urinary proteins can efficiently inhibit stone formation. These discoveries are significant for developing effective therapies for stone disease, but the inhibition mechanism of crystallization remains elusive. In the present study, polyaspartic acid (PASP) was employed as a model peptide to investigate the effect of urinary proteins on the crystallization and morphological evolution of struvite. The results demonstrate that selective adsorption/binding of PASP onto the {010} and {101} faces of struvite crystals results in arrowhead-shaped morphology, which further evolves into X-shaped and unusual tabular structures with time. Noticeably, these morphologies are reminiscent of biogenic struvite morphology. Concentration-dependent experiments show that PASP can inhibit struvite growth and the inhibitory capacity increases with increasing PASP concentration, whereas aspartic acid monomers do not show a significant effect. Considering that PASP is a structural and functional analogue of the subdomains of aspartic acid-rich proteins, our results reveal that aspartic acid-rich proteins play a key role in regulating biogenic struvite morphology, and aspartic acid residues contribute to the inhibitory capacity of urinary proteins. The potential implications of PASP for developing therapeutic agents for urinary stone disease is also discussed.

Microwave-assisted synthesis of flower-like β-FeSe microstructures

Flower-like tetragonal iron selenide (β-FeSe) microstructures assembled from nanoplates have been successfully prepared using Se powder and FeCl3·6H2O as elemental precursors in the basic 1,2-propylene glycol (1,2-PG) solvent system by microwave irradiation for 1 h. The product was characterized by means of X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDX). The reaction mechanism and assembly process of the flower-like β-FeSe microstructures have been investigated and discussed, and a novel Se disproportionation-based reaction mechanism was proposed for the microwave-assisted synthesis of flower-like β-FeSe microstructures. In addition, ethylene glycol (EG) was also used as the reaction medium to investigate the influence of the different polyols on the size and structure of the product.

Preparation of Hollow Core/Shell Microspheres of Hematite and Its Adsorption Ability for Samarium

Hollow core/shell hematite microspheres with diameter of ca. 1-2 μm have been successfully achieved by calcining the precursor composite microspheres of pyrite and polyvinylpyrrolidone (PVP) in air. The synthesized products were characterized by a wide range of techniques including powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), and Brunauer-Emmett-Teller (BET) gas sorptometry. Temperature- and time-dependent experiments unveil that the precursor pyrite-PVP composite microspheres finally transform into hollow core/shell hematite microspheres in air through a multistep process including the oxidation and sulfation of pyrite, combustion of PVP occluded in the precursor, desulfation, aggregation, and fusion of nanosized hematite as well as mass transportation from the interior to the exterior of the microspheres. The formation of the hollow core/shell microspheres dominantly depends on the calcination temperature under current experimental conditions, and the aggregation of hematite nanocrystals and the core shrinking during the oxidation of pyrite are responsible for the formation of the hollow structures. Moreover, the adsorption ability of the hematite for Sm(III) was also tested. The results exhibit that the hematite microspheres have good adsorption activity for trivalent samarium, and that its adsorption capacity strongly depends on the pH of the solution, and the maximum adsorption capacity for Sm(III) is 14.48 mg/g at neutral pH. As samarium is a typical member of the lanthanide series, our results suggest that the hollow hematite microspheres have potential application in removal of rare earth elements (REEs) entering the water environment.

Biomineralization mediated by anaerobic methane-consuming cell consortia

Anaerobic methanotrophic archaea (ANME) play a significant role in global carbon cycles. These organisms consume more than 90% of ocean-derived methane and influence the landscape of the seafloor by stimulating the formation of carbonates. ANME frequently form cell consortia with sulfate-reducing bacteria (SRB) of the family Deltaproteobacteria. We investigated the mechanistic link between ANME and the natural consortium by examining anaerobic oxidation of methane (AOM) metabolism and the deposition of biogenetic minerals through high-resolution imaging analysis. All of the cell consortia found in a sample of marine sediment were encrusted by a thick siliceous envelope consisting of laminated and cementing substances, whereas carbonate minerals were not found attached to cells. Beside SRB cells, other bacteria (such as Betaproteobacteria) were found to link with the consortia by adhering to the siliceous crusts. Given the properties of siliceous minerals, we hypothesize that ANME cell consortia can interact with other microorganisms and their substrates via their siliceous envelope, and this mechanism of silicon accumulation may serve in clay mineral formation in marine sedimentary environments. A mechanism for biomineralization mediated by AOM consortia was suggested based on the above observations.