Recent insights into xerogel and aerogel mineral composites for CO 2 mineral sequestration

Supercritically dried composites have already been analysed and proposed as carbon dioxide sequesters. However, the economical and energetic costs of the supercritical drying process had to be re-evaluated, and were eventually found not to enhance the feasibility of the proposed route for CO2 mineral sequestration. Different composites series were synthesised with the only difference being the drying method. The structures of the porous matrix were characterised as well as their ability to capture CO2. The first results showed that the xerogel matrix is as good a host as the aerogel one, and also avoids expensive procedures such as supercritical drying for sample preparation without losing CO2 capture capacity and enhancing the efficiency of the whole carbon sequestration process. In this case, the sample preparation was simplified as much as possible, with the aim of reducing energetic and economic costs. Although good carbonation efficiencies were obtained with these cheap samples, the first results showed that previous high carbonation efficiencies could not be repeated.Junta de Andalucía TEP115Ministerio de Ciencia e Innovación PIA42008-3

Artificial weathering pools of calcium-rich industrial waste for CO2 sequestration

Processes of carbonation of calcium-rich aqueous industrial wastes from acetylene production were performed mimicking rock weathering, using the atmospheric carbon dioxide as reactant. This residue was carbonated exposing it to the air in artificial pools with controlled solid-to-liquid and surface-to-volume ratios, and the efficiency of this simple mineral carbonation process was maximized. Considering realistic values of just one acetylene production plant, the intelligent handling of the calcium-rich waste would make it possible to counteract the emission of around 800t of carbon dioxide per year, so the CO2 emissions of the acetylene production could be completely compensated and its carbon footprint significantly reduced.X-ray diffraction patterns and thermogravimetric analyses reported the conversion, up to 88%, of the calcium hydroxide into calcium carbonate under atmospheric conditions. So, considering a realistic industrial scale-up, 476kg of CO2 could be captured with 1t of dry waste. The morphology of the grains is resolved by electron microscopy, and can be described as needles 15nm wide and 200nm long arranged in grains smaller than 1 micron. We exploit these nanometric textural parameters (nanometric pores and particles having a specific surface area ∼50m2/g) to design an efficient carbon fixation procedure. The aim of this work is to propose this simple carbonation technology, based on aqueous alkaline industrial waste, as a contribution to reducing global CO2 emissions.Junta de Andalucía TEP115Ministerio de Ciencia e Innovación PIA42008-3

Removal of basic yellow cationic dye by an aqueous dispersion of Moroccan stevensite

The aim of this study was to investigate the adsorption of basic yellow, a cationic dye, from aqueous solution by natural stevensite, with 104m2/g of specific surface area. The kinetics and the effects of several experimental parameters such as the pH of the solution, adsorbent dose and initial dye concentration were researched using a batch adsorption technique. The results showed that an alkaline pH favoured basic yellow adsorption and the adsorption reached equilibrium in about 20min. It was concluded that the adsorption process was governed by the electrostatic interaction. The isothermal data were fitted by means of Langmuir and Freundlich equations, and a monolayer adsorption capacity of 454.54mg/g was calculated. Finally, a good agreement was found between the pseudo-second order model and the experimental data. A high maximum adsorption capacity was obtained (526mg/g) and a maximum surface density of ~9 dye molecules/nm2 was estimated, involving a columnar arrangement of the adsorbed molecules.Agencia Española de Cooperación Internacional A/018025/08Junta de Andalucía TEP11

Calcium silicates synthesised from industrial residues with the ability for CO2 sequestration

This work explored several synthesis routes to obtain calcium silicates from different calcium-rich and silica-rich industrial residues. Larnite, wollastonite and calcium silicate chloride were successfully synthesised with moderate heat treatments below standard temperatures. These procedures help to not only conserve natural resources, but also to reduce the energy requirements and CO2 emissions. In addition, these silicates have been successfully tested as carbon dioxide sequesters, to enhance the viability of CO2 mineral sequestration technologies using calcium-rich industrial by-products as sequestration agents. Two different carbon sequestration experiments were performed under ambient conditions. Static experiments revealed carbonation efficiencies close to 100% and real-time resolved experiments characterised the dynamic behaviour and ability of these samples to reduce the CO2 concentration within a mixture of gases. The CO2 concentration was reduced up to 70%, with a carbon fixation dynamic ratio of 3.2 mg CO2 per g of sequestration agent and minute. Our results confirm the suitability of the proposed synthesis routes to synthesise different calcium silicates recycling industrial residues, being therefore energetically more efficient and environmentally friendly procedures for the cement industry.Ministerio de Ciencia e Innovación CIT-440000-09-0

Nanostructured sonogels

Acoustic cavitation effects in sol-gel liquid processing permits to obtain nanostructured materials, with size-dependent properties. The so-called "hot spots" produce very high temperatures and pressures which act as nanoreactors. Ultrasounds force the dissolution and the reaction stars. The products (alcohol, water and silanol) help to continue the dissolution, being catalyst content, temperature bath and alkyl group length dependent. Popular choices used in the preparation of silica-based gels are tetramethoxysilane (TMOS), Si(OCH3)4 and tetraethoxysilane (TEOS), Si(OC 2H5)4. The resultant "sonogels" are denser gels with finer and homogeneous porosity than those of classic ones. They have a high surface/volume ratio and are built by small particles (1 nm radius) and a high cross-linked network with low -OH surface coverage radicals. In this way a cluster model is presented based on randomly-packed spheres in several hierarchical levels that represent the real sonoaerogel. Organic modified silicates (ORMOSIL) were obtained by supercritical drying in ethanol of the corresponding alcogel producing a hybrid organic/inorganic aerogel. The new material takes the advantages of the organic polymers as flexibility, low density, toughness and formability whereas the inorganic part contributes with surface hardness, modulus strength, transparency and high refractive index. The sonocatalytic method has proven to be adequate to prepare silica matrices for fine and uniform dispersion of CdS and PbS quantum dots (QDs), which show exciton quantum confinement. We present results of characterization of these materials, such as nitrogen physisorption, small angle X-ray/neutrons scattering, electron microscopy, uniaxial compression and nanoindentation. Finally these materials find application as biomaterials for tissue engineering and for CO2 sequestration by means the carbonation reaction.Ministerio de Ciencia y Tecnología MAT2005-158

Larnite powders and larnite/silica aerogel composites as effective agents for CO2 sequestration by carbonation

This paper presents the results of the carbonation reaction of two sample types: larnite (Ca2SiO4) powders and larnite/silica aerogel composites, the larnite acting as an active phase in a process of direct mineral carbonation. First, larnite powders were synthesized by the reaction of colloidal silica and calcium nitrate in the presence of ethylene glycol. Then, to synthesize the composites, the surface of the larnite powders was chemically modified with 3-aminopropyltriethoxysilane (APTES), and later this mixture was added to a silica sol previously prepared from tetraethylorthosilicate (TEOS). The resulting humid gel was dried in an autoclave under supercritical conditions for the ethanol. The textures and chemical compositions of the powders and composites were characterized.The carbonation reaction of both types of samples was evaluated by means of X-ray diffraction and thermogravimetric analysis. Both techniques confirm the high efficiency of the reaction at room temperature and atmospheric pressure. A complete transformation of the silicate into carbonate resulted after submitting the samples to a flow of pure CO2 for 15 min. This indicates that for this reaction time, 1 t of larnite could eliminate about 550 kg of CO2. The grain size, porosity, and specific surface area are the factors controlling the reaction.Ministerio de Medio Ambiente A266/2007/3-11.1Ministerio de Educación y Ciencia MAT2005-0158

Intragranular carbon nanotubes in alumina-based composites for reinforced ceramics

The traditional methods for the synthesis of reinforced alumina-based matrix composites with carbon nanotubes (CNTs) have presented serious difficulties for obtaining well-dispersed and homogeneously distributed CNTs within the matrix. Besides this, the CNTs are typically found in the grain boundaries of the matrix. These features involve a non-optimal reinforcement role of the CNTs. With the aim of maximizing the efficiency of the reinforcement of the CNT, this work reconsiders a sol-gel-based procedure for ceramic composite fabrication with a two fold objective: to achieve a good dispersion of the CNTs and to promote the intragranular location of the CNTs. The mixing of precursors and CNTs has been developed under the presence of high-power ultrasounds, followed by a rapid in-situ gelation that “froze” the nanotubes inside the gel. The chemical and physical relationships between the ceramic matrix and the embedded reinforcing phase have been researched. First results confirm the success of the synthesis procedure for the preparation of alumina-based composite powders starting from a commercial boehmite sol and multiwalled carbon nanotubes. X-ray diffraction and Raman analyses confirmed the formation of the α-Al 2 O 3 and the persistence of the non-damaged nanotube structure. N 2 physisorption and electron microscopy were used to check the evolution of the nanostructure and to confirm the presence of intragranular carbon nanotube within the polycrystalline powder. Therefore, the alumina-based composite powder prepared by this new procedure is a good candidate for the preparation of reinforced ceramic matrix composites.Junta de Andalucía P12-FQM-107

Procedure to use phosphogypsum industrial waste for mineral CO 2 sequestration

Industrial wet phosphoric acid production in Huelva (SW Spain) has led to the controversial stockpiling of waste phosphogypsum by-products, resulting in the release of significant quantities of toxic impurities in salt marshes in the Tinto river estuary. In the framework of the fight against global climate change and the effort to reduce carbon dioxide emissions, a simple and efficient procedure for CO 2 mineral sequestration is presented in this work, using phosphogypsum waste as a calcium source. Our results demonstrate the high efficiency of portlandite precipitation by phosphogypsum dissolution using an alkaline soda solution. Carbonation experiments performed at ambient pressure and temperature resulted in total conversion of the portlandite into carbonate. The fate of trace elements present in the phosphogypsum waste was also investigated, and trace impurities were found to be completely transferred to the final calcite. We believe that the procedure proposed here should be considered not only as a solution for reducing old stockpiles of phosphogypsum wastes, but also for future phosphoric acid and other gypsum-producing industrial processes, resulting in more sustainable production.Junta de Andalucía TEP115Ministerio de Ciencia e Innovación PIA42008-3