Aerosol-based synthesis of pure and stable amorphous calcium carbonate

A facile aerosol-based method for the synthesis of pure and stable amorphous calcium carbonate (ACC) is presented. The method relies on the instantaneous carbonation of calcium hydroxide aerosols with carbon dioxide followed by rapid drying of the freshly formed ACC. The ACC display extended stability against humidity induced crystallization.ISSN:1359-7345ISSN:1364-548

Droplet-based in situ X-ray absorption spectroscopy cell for studying crystallization processes at the tender X-ray energy range

The understanding of nucleation and crystallization is fundamental in science and technology. In solution, these processes are complex involving multiple transformations from ions and ion pairs through amorphous intermediates to multiple crystalline phases. X-ray absorption spectroscopy (XAS), which is sensitive to liquid, amorphous and crystalline phases offers prospects of demystifying these processes. However, for low Z elements the use of in situ X-ray absorption spectroscopy requires the tender X-ray range, which is often limited by vacuum requirements thereby complicating these measurements. To overcome these challenges, we developed a versatile and user-friendly droplet-based in situ X-ray absorption spectroscopy cell for studying crystallization processes. Time-resolved in situ experiments under ambient conditions are carried out in the cell whilst the cell is mounted in the vacuum chamber of a tender X-ray beamline. By following changes in the Ca K-edge X-ray absorption near edge structure (XANES), we captured in situ the intermediate phases involved during calcium carbonate crystallization from aqueous solutions. In addition, through linear combination fitting it was possible to qualitatively observe the evolution of each phase during the reaction demonstrating the potential of the cell in studying complex multiphase chemical processes.ISSN:2046-206

Tuning the Incorporation of Magnesium into Calcite during Its Crystallization from Additive-Free Aqueous Solution

Under ambient conditions, marine organisms are able to synthesize a variety of functional materials, ranging from eye lenses to protective shells through the meticulous control over magnesium incorporation into calcite during its crystallization. The mechanistic understanding of how they achieve such exquisite control, at a constant magnesium-to-calcium ratio and at ambient conditions, is important in the development of bioinspired functional materials. However, the replication of these processes in the laboratory is still challenging. Herein, we present a systematic study on how to tune magnesium incorporation into calcite and polymorph selection in the Ca-Mg-CO3 system through the precise control of the inorganic solutions chemistry at ambient conditions of temperature and pressure, and at a magnesium-to-calcium ratio of 5:1, which is analogous to the ratio found in most seas. By varying the pH, cation-to-anion ratio, and solution concentration, the controlled synthesis of magnesium calcites with 10-45% magnesium was achieved at room temperature. The mechanism of formation is consistent with that observed during biomineralization, during which an intermediate magnesium-rich amorphous calcium carbonate (Mg-ACC) phase forms first and later transforms into high magnesium calcite. Once crystallization occurs, the magnesium calcites that form are stable in solution and exhibit slow growth through Ostwald ripening. Our findings suggest that the precise control of saturation levels is key in driving nucleation and crystallization

Photochemical degradation of iron(III) citrate/citric acid aerosol quantified with the combination of three complementary experimental techniques and a kinetic process model

Iron(III) carboxylate photochemistry plays an important role in aerosol aging, especially in the lower troposphere. These complexes can absorb light over a broad wavelength range, inducing the reduction of iron(III) and the oxidation of carboxylate ligands. In the presence of O-2, the ensuing radical chemistry leads to further decarboxylation, and the production of center dot OH, HO2 center dot, peroxides, and oxygenated volatile organic compounds, contributing to particle mass loss. The center dot OH, H-2 center dot, and peroxides in turn reoxidize iron(II) back to iron(III), closing a photocatalytic cycle. This cycle is repeated, resulting in continual mass loss due to the release of CO2 and other volatile compounds. In a cold and/or dry atmosphere, organic aerosol particles tend to attain highly viscous states. While the impact of reduced mobility of aerosol constituents on dark chemical reactions has received substantial attention, studies on the effect of high viscosity on photochemical processes are scarce. Here, we choose iron(III) citrate (Fe-III (Cit)) as a model light-absorbing iron carboxylate complex that induces citric acid (CA) degradation to investigate how transport limitations influence photochemical processes. Three complementary experimental approaches were used to investigate kinetic transport limitations. The mass loss of single, levitated particles was measured with an elec- trodynamic balance, the oxidation state of deposited particles was measured with X-ray spectromicroscopy, and H-2 center dot radical production and release into the gas phase was observed in coated-wall flow-tube experiments. We observed significant photochemical degradation with up to 80 % mass loss within 24 h of light exposure. Interestingly, we also observed that mass loss always accelerated during irradiation, resulting in an increase of the mass loss rate by about a factor of 10. When we increased relative humidity (RH), the observed particle mass loss rate also increased. This is consistent with strong kinetic transport limitations for highly viscous particles. To quantitatively compare these experiments and determine important physical and chemical parameters, a numerical multilayered photochemical reaction and diffusion (PRAD) model was developed that treats chemical reactions and the transport of various species. The PRAD model was tuned to simultaneously reproduce all experimental results as closely as possible and captured the essential chemistry and transport during irradiation. In particular, the photolysis rate of Fe-III, the reoxidation rate of Fe-II, H-2 center dot production, and the diffusivity of O-2 in aqueous Fe-III (Cit)/CA system as function of RH and Fe-III (Cit)/CA molar ratio could be constrained. This led to satisfactory agreement within model uncertainty for most but not all experiments performed. Photochemical degradation under atmospheric conditions predicted by the PRAD model shows that release of CO2 and repartitioning of organic compounds to the gas phase may be very important when attempting to accurately predict organic aerosol aging processes.ISSN:1680-7375ISSN:1680-736

Photolytic radical persistence due to anoxia in viscous aerosol particles

In viscous, organic-rich aerosol particles containing iron, sunlight may induce anoxic conditions that stabilize reactive oxygen species (ROS) and carbon-centered radicals (CCRs). In laboratory experiments, we show mass loss, iron oxidation and radical formation and release from photoactive organic particles containing iron. Our results reveal a range of temperature and relative humidity, including ambient conditions, that control ROS build up and CCR persistence in photochemically active, viscous organic particles. We find that radicals can attain high concentrations, altering aerosol chemistry and exacerbating health hazards of aerosol exposure. Our physicochemical kinetic model confirmed these results, implying that oxygen does not penetrate such particles due to the combined effects of fast reaction and slow diffusion near the particle surface, allowing photochemically-produced radicals to be effectively trapped in an anoxic organic matrix.ISSN:2041-172

Supersaturated calcium carbonate solutions are classical.

Mechanisms of CaCO3 nucleation from solutions that depend on multistage pathways and the existence of species far more complex than simple ions or ion pairs have recently been proposed. Herein, we provide a tightly coupled theoretical and experimental study on the pathways that precede the initial stages of CaCO3 nucleation. Starting from molecular simulations, we succeed in correctly predicting bulk thermodynamic quantities and experimental data, including equilibrium constants, titration curves, and detailed x-ray absorption spectra taken from the supersaturated CaCO3 solutions. The picture that emerges is in complete agreement with classical views of cluster populations in which ions and ion pairs dominate, with the concomitant free energy landscapes following classical nucleation theory

Supersaturated calcium carbonate solutions are classical.

Mechanisms of CaCO3 nucleation from solutions that depend on multistage pathways and the existence of species far more complex than simple ions or ion pairs have recently been proposed. Herein, we provide a tightly coupled theoretical and experimental study on the pathways that precede the initial stages of CaCO3 nucleation. Starting from molecular simulations, we succeed in correctly predicting bulk thermodynamic quantities and experimental data, including equilibrium constants, titration curves, and detailed x-ray absorption spectra taken from the supersaturated CaCO3 solutions. The picture that emerges is in complete agreement with classical views of cluster populations in which ions and ion pairs dominate, with the concomitant free energy landscapes following classical nucleation theory