A review on ex situ mineral carbonation.

The increased CO2 quantities in the environment have led to many harmful effects. Therefore, it is very important to decrease the CO2 levels in the environment. CO2 capture along with safe and permanent storage using mineral CO2 sequestration method can play an important role to reduce carbon emissions into the environment. Mineral sequestration is a stable storage method that provides long-term storage and an appropriate substitute for the more popular geological storage method. The process is most suited for places where there is a lack of underground cavities for underground geological storage. Minerals rich in Ca and Mg are used predominantly in carbonation reactions. In addition, those alkaline wastes that are rich in Mg and Ca such as cement waste, steel slag and many process ashes can also be employed in CO2 sequestration. Mineral carbonation could be used for the sequestration of billions of tonnes of CO2 every year. However, various drawbacks related to mineral carbonation still need to be addressed, such as resolving the slow rate of reactions, necessity of large amounts of feedstock, decreasing the high overall cost of CO2 sequestration and reducing the huge energy requirements to accelerate the carbonation reaction. This study explores a number of carbonation methods, parameters that control the process and future potential applications of carbonated products.

Dunite carbonation in batch-tubular reactor.

For geological carbon sequestration, the reaction of aqueous CO2 with silicate rock permits carbonate formation, achieving permanent carbon sequestration. The fractures available in silicate rock provide significant surface area for the precipitation of carbonates. The experiments were performed in a batch tubular reactor under diffusion-limited condition, with a special arrangement of a narrow tube filled with a 2800 g/L dunite slurry. The tube was kept open from the top, standing vertically filled with a CO2-rich bulk solution under 1 barg CO2 and temperatures ranging from 25 to 75 oC for 7-30 days. After 7 days of the experiment, magnesite precipitation was seen inside the tube and the precipitation was continued for up to 30 days. The magnesite precipitation was identified by micro-Raman spectroscopy, X-ray diffraction, and scanning electron microscopy. Additionally, SiO2 formation was seen in relative close vicinity to the magnesite precipitation. The precipitation on the surface of silicate rock might cover the fractures and pore spaces available, which may over time reduce the dissolution rate of dunite. Graphical Abstract.

Dissolution of steel slags in aqueous media.

Steel slag is a major industrial waste in steel industries, and its dissolution behavior in water needs to be characterized in the larger context of its potential use as an agent for sequestering CO2. For this purpose, a small closed system batch reactor was used to conduct the dissolution of steel slags in an aqueous medium under various dissolution conditions. In this study, two different types of steel slags were procured from steel plants in India, having diverse structural features, mineralogical compositions, and particle sizes. The experiment was performed at different temperatures for 240 h of dissolution at atmospheric pressure. The dissolution rates of major and minor slag elements were quantified through liquid-phase elemental analysis using an inductively coupled plasma atomic emission spectroscopy at different time intervals. Advanced analytical techniques such as field emission gun-scanning electron microscope, energy-dispersive X-ray, BET, and XRD were also used to analyze mineralogical and structural changes in the slag particles. High dissolution of slags was observed irrespective of the particle size distribution, which suggests high carbonation potential. Concentrations of toxic heavy metals in the leachate were far below maximum acceptable limits. Thus, the present study investigates the dissolution behavior of different mineral ions of steel slag in aqueous media in light of its potential application in CO2 sequestration.

Experimental study of dissolution of minerals and CO2 sequestration in steel slag.

This study strives to achieve a substantial amount of steel slag carbonation without using any harmful chemicals. For this purpose, experiments were performed in an aqueous medium, in a semi-batch reactor, to investigate the effect of varying reaction conditions during the steel slag CO2 sequestration process. Further, studying the effect of dissolution on carbonation reactions and the mineralogical changes that subsequently occur within the slag helps provide insight into the parameters that ultimately have an impact on the carbonation rate as well the magnitude of the impact.

Formation and shape-control of hierarchical cobalt nanostructures using quaternary ammonium salts in aqueous media.

Aggregation and self-assembly are influenced by molecular interactions. With precise control of molecular interactions, in this study, a wide range of nanostructures ranging from zero-dimensional nanospheres to hierarchical nanoplates and spindles have been successfully synthesized at ambient temperature in aqueous solution. The nanostructures reported here are formed by aggregation of spherical seed particles (monomers) in presence of quaternary ammonium salts. Hydroxide ions and a magnetic moment of the monomers are essential to induce shape anisotropy in the nanostructures. The cobalt nanoplates are studied in detail, and a growth mechanism based on collision, aggregation, and crystal consolidation is proposed based on a electron microscopy studies. The growth mechanism is generalized for rods, spindles, and nearly spherical nanostructures, obtained by varying the cation group in the quaternary ammonium hydroxides. Electron diffraction shows different predominant lattice planes on the edge and on the surface of a nanoplate. The study explains, hereto unaddressed, the temporal evolution of complex magnetic nanostructures. These ferromagnetic nanostructures represent an interesting combination of shape anisotropy and magnetic characteristics.

Olivine dissolution from Indian dunite in saline water.

The rate and mechanism of olivine dissolution was studied using naturally weathered dunite FO98.21(Mg1.884Fe0.391SiO4) from an Indian source, that also contains serpentine mineral lizardite. A series of batch dissolution experiments were carried out to check the influence of temperature (30-75 ∘C), initial dunite concentration (0.5 and 20 g/L), and salinity (0-35 g/L NaCl) under fixed head space CO2 pressure (P[Formula: see text] = 1 barg) on dunite dissolution. Dissolved Mg, Si, and Fe concentrations were determined by inductive coupled plasma atomic emission spectroscopy. End-product solids were characterized by scanning electron microscopy and X-ray diffraction. Initially, rates of dissolution of Si and Mg were observed to be in stoichiometric proportion. After 8 h, the dissolution rate was observed to decline. At the end of the experiment (504 h), an amorphous silica-rich layer was observed over the dunite surface. This results in decay of the dissolution rate. The operating conditions (i.e., salinity, temperature, and mineral loading) affect the dissolution kinetics in a very complex manner because of which the observed experimental trends do not exhibit a direct trend.

Mechanism of Nanorod Formation by Wormlike Micelle-Assisted Assembly of Nanospheres.

Hierarchical self-assembly is an elegant and energy-efficient bottom-up method for the structuring of complex materials. We demonstrate the synthesis of maghemite nanorods via directed self-assembly, assisted by wormlike micelles, under controlled shear. The experimental data are analyzed by formulating a "slithering snake" mechanism and simulating it on a cubic lattice, using a coarse-grained Monte Carlo framework. The influence of shear rate, precursor concentration, and length of Kuhn segment on the morphology of the nanorods is examined. Experiments indicate that the shear is necessary for the formation of nanorods, although diameter and length of the nanorods are insensitive to the shear rate, within the range of shear rates investigated. The model adequately captures the features of directional aggregation of particles, and the computed length and diameter correspond to the typical dimensions of the nanorods obtained experimentally. The protocol has considerable potential for producing nanorods of several materials simply by changing the precursors.

Superposition of Quantum Confinement Energy (SQCE) model for estimating shell thickness in core-shell quantum dots: validation and comparison.

A novel theoretical model based on superposition of core and shell band-gaps, termed as SQCE model, is developed and reported here, which enables one to estimate the shell thickness in a core-shell quantum dot (QD), which is critically important in deciding its optical and electronic properties. We apply the model to two experimental core-shell QD systems, CdSe-CdS and CdSe-ZnS, which we synthesize by microemulsion method. We synthesize and study two series of samples, R and S to study the optical properties. The core size is varied in the R-series (by varying water-to-surfactant ratio, R) whereas the shell thickness is varied in the S-series (by varying the shell-to-core precursor molar ratio, S). The core and core-shell QDs from R-series and S-series are characterized for particle size, shape and crystallographic information. The shell thickness for all core-shell QD samples is estimated by SQCE model, and experimentally measured with TEM and SAXS. A close match is observed between experimental values and model predictions, thus validating the model. Further, the optimum shell thickness (corresponding to maximum quantum yield) values for CdS and ZnS over a 4.26 nm CdSe core have been estimated as 0.585 nm and 0.689 nm, respectively, from the SQCE model. The SQCE model developed in this work is applicable to other core-shell quantum dots also, such as CdTe-CdS, CdTe-CdSe and CdS-ZnS, and will serve as a useful complement to experimental measurement.

Pulmonary targeting of nanoparticle drug matrices.

BACKGROUND: Nanoparticle drug matrices using lipids or liposomes, with diameters of 40-300 nm, have recently been developed to encapsulate drugs like Insulin, Budesonide, and Rifampicin for pulmonary delivery raising interest in their regional lung deposition. METHODS: Lung deposition has so far been modeled using a one-dimensional transport equation, with or without moving airway boundaries, and a lumped deposition term for particle diffusion, sedimentation, and impaction. Here, a two-dimensional transport model has been developed with an explicit treatment of radial diffusion, the primary mechanism for nanoparticle deposition. Regional lung deposition was calculated using Weibel's whole lung model geometry during normal breathing and medical inhalation cycles. CONCLUSIONS: Model predictions agree well with measurements of total and pulmonary lung deposition for particles of 10 nm to 10 µm, with earlier models incorporating moving boundaries and aerosol dynamics, and with the reported regional lung deposition of inhaled dry powder insulin. To simulate medical inhalation, the model was run with inhalation times from 2-6 sec and breath hold from 0-10 sec. A high and relatively invariant pulmonary deposition fraction between 70 and 95% was predicted for a broad nanoparticle size range (50-200 nm) for inhalation cycles with breathing rate between 500 and 2000 cm(3) sec(-1) and breath hold of 5-10 sec. Thus, nanoparticles may be able to deliver consistent lung doses, over modest breath hold periods, even with intrapatient variability in breathing rate. A linearized nomogram was provided as a heuristic for design of nanoparticle drug matrices to target the pulmonary lung.

Modeling of formation of nanoparticles in reverse micellar systems: Ostwald ripening of silver halide particles.

There are many possible size enhancement processes that affect the formation of nanoparticles in reverse micelles, such as coagulation and Ostwald ripening, and different physical systems are likely to follow one or more of these mechanisms depending upon the properties of the system. It has been suggested that silver halide particles, prepared from a reverse micellar system of AgNO3 and KCl in NP-6/cyclohexane solution, increase in size due to Ostwald ripening (Kimijima, K.; Sugimoto, T. J. Phys. Chem. B 2004, 108, 3735), which occurs due to the dependence of the solubility of the particles on the particle size so that the larger particles grow at the expense of smaller particles. This study provides a modeling framework to quantitatively analyze the ripening process of nanoparticles produced in reverse micellar systems.

CaCO(3) nanoparticle synthesis by carbonation of lime solution in microemulsion systems.

Various aspects of nanoparticle precipitation in gas-reverse micellar systems have been studied. The experimental system chosen for investigation deals with the precipitation of CaCO(3) nanoparticles. The effect of operating variables, such as water-to-surfactant molar ratio, different continuous phases, surfactant concentration and stirring speed, have been investigated experimentally. The results indicate that using low concentrations of Ca(OH)(2) particles outside the micelles, low surfactant concentrations, low stirring speeds and water-to-surfactant molar ratios lead to the formation of smaller nanoparticles in gas-reverse micellar systems.

Modeling shell formation in core-shell nanocrystals in reverse micelle systems.

The mechanisms responsible for the formation of the shell in core-shell nanocrystals are ion-displacement and heterogeneous nucleation. In the ion-displacement mechanism, the shell is formed by the displacement reaction at the surface of the core nanoparticle whereas in heterogeneous nucleation the core particle induces the nucleation (or direct deposition) of shell material on its surface. The formation of core-shell nanocrystals via the post-core route has been examined in the current investigation. A purely probabilistic Monte Carlo scheme for the formation of the shell has been developed to predict the experimental results of Hota et al. (Hota, G.; Jain, S.; Khilar, K. C. Colloids Surf., A 2004, 232, 119) for the precipitation of Ag2S-coated CdS (Ag2S@CdS) nanoparticles. The simulation procedure involves two stages. In the first stage, shell formation takes place as a result of the consumption of supersaturation, ion displacement, and reaction between Ag+ and excess sulfide ions. The growth in the second stage is driven by the coagulation of nanoparticles. The results indicate that the fraction of shell deposited by the ion-displacement mechanism increases with increasing ion ratio and decreases with increasing water-to-surfactant molar ratio.

Coagulation of nanoparticles in reverse micellar systems: a Monte Carlo model.

The process of formation of nanoparticles obtained by mixing two micellized, aqueous solutions has been simulated using the Monte Carlo technique. The model includes the phenomena of finite nucleation, growth via intermicellar exchange, and coagulation of nanoparticles after their formation. Using the model, an exploratory study has been conducted to analyze whether the coagulation of nanoparticles is the reason for the formation of nanoparticles whose sizes are comparable to the size of the reverse micelles. The model explains the possible mechanism of coagulation of semiconductor nanoparticles formed within reverse micelles and its effect on the evolution of their size with time. The model is predictive in nature, and the simulation results compare well with those observed experimentally.

Monte Carlo models for nanoparticle formation in two microemulsion systems.

The process of formation of nanoparticles obtained by mixing two micellized, aqueous solutions has been simulated using the Monte Carlo technique. The model includes the phenomena of finite reaction, nucleation, and growth via intermicellar exchange. This exploratory study examines the characteristic particle size distributions (PSDs) that result from using combinations of different initial reactant distributions (Poissonian and geometric) and different types of intermicellar exchange protocols (random, cooperative, and binomial). It is observed that the PSDs obtained using an initial Poissonian distribution of reactants and random exchange rules are similar to reported experimental results for CdS nanoparticles. The effect of exchange efficiency and reaction rate has also been studied. It is seen that a high exchange efficiency leads to relatively larger particle sizes. Also, a slow reaction rate has been shown to lead to the formation of larger nanoparticles.

Mathematical analysis of solid-liquid mass transfer in a batch reactor for the enzymatic transformation of testosterone to 4AD.

A previous mathematical analysis of mass transfer in a two-phase (solid-liquid) batch reactor for enzymatic transformation of testosterone to 4AD (Pereira et al., 1987) is extended to incorporate the effect of convective mixing. The results of the analysis showed that for a given enzyme loading, the mass transfer resistance in the solid (a function of the bead size) and the intensity of convective mixing (as embodied in the mass transfer coefficient) are two parameters that can be varied such that the overall mass transfer rate from the solid to the liquid phase ensures optimal reactor performance.