Physico-chemical characterization of metal-doped bone chars and their adsorption behavior for water defluoridation

New bone chars for fluoride adsorption from drinking water have been synthetized via metallic doping using aluminum and iron salts. A detailed statistical analysis of the metal doping process using the signal-to-noise ratios from Taguchi's experimental designs and its impact on the fluoride adsorption properties of modified bone chars have been performed. The best conditions, including the proper metallic salt, for metal doping were identified to improve the fluoride uptakes of modified bone chars. Results showed that the fluoride adsorption properties of bone chars can be enhanced up to 600% using aluminum sulfate for the surface modification. This aluminum-based adsorbent showed an adsorption capacity of 31 mg/g, which outperformed the fluoride uptakes reported for several adsorbents. Surface interactions involved in the defluoridation process were established using FTIR, DRX and XPS analysis. Defluoridation using the metal-doped bone chars occurred via an ion exchange process between fluoride ions and the hydroxyl groups on the adsorbent surface, whereas the Al(OH)xFy, FexFy, and CaF2 interactions could play also an important role in the removal process. These metal-doped adsorbents anticipate a promising behavior in water treatment, especially in developing countries where the efficiency – cost tradeoff is crucial for implementing new defluoridation technologies.

Assessment of CO2 adsorption capacity on activated carbons by a combination of batch and dynamic tests

In this work, batch and dynamic adsorption tests are coupled for an accurate evaluation of CO2 adsorption performance for three different activated carbons obtained from olives stones by chemical activation followed by physical activation with CO2 at varying times, i.e. 20, 40 and 60 h. Kinetic and thermodynamic CO2 adsorption tests from simulated flue-gas at different temperature and CO2 pressure are carried out both in batch (a manometric equipment operating with pure CO2) and dynamic (a lab-scale fixed-bed column operating with CO2/N2 mixture) conditions. The textural characterization of the activated carbon samples shows a direct dependence of both micropore and ultramicropore volume on the activation time, hence AC60 has the higher contribution. The adsorption tests conducted at 273 and 293 K showed that, when CO2 pressure is lower than 0.3 bar, the lower the activation time the higher CO2 adsorption capacity and a ranking ωeq(AC20)>ωeq(AC40)>ωeq(AC60) can be exactly defined when T= 293 K. This result can be likely ascribed to a narrower pore size distribution of the AC20 sample, whose smaller pores are more effective for CO2 capture at higher temperature and lower CO2 pressure, the latter representing operating conditions of major interest for decarbonation of a flue-gas effluent. Moreover, the experimental results obtained from dynamic tests confirm the results derived from the batch tests in terms of CO2 adsorption capacity. It is important to highlight that the adsorption of N2 on the synthesized AC samples can be considered negligible. Finally, the importance of a proper analysis of characterization data and adsorption experimental results is highlighted for a correct assessment of CO2 removal performances of activated carbons at different CO2 pressure and operating temperature.

Use of Eutectic Mixtures for Preparation of Monolithic Carbons with CO2-Adsorption and Gas-Separation Capabilities

With global warming becoming one of the main problems our society is facing nowadays, there is an urgent demand to develop materials suitable for CO2 storage as well as for gas separation. Within this context, hierarchical porous structures are of great interest for in-flow applications because of the desirable combination of an extensive internal reactive surface along narrow nanopores with facile molecular transport through broad “highways” leading to and from these pores. Deep eutectic solvents (DESs) have been recently used in the synthesis of carbon monoliths exhibiting a bicontinuous porous structure composed of continuous macroporous channels and a continuous carbon network that contains a certain microporosity and provides considerable surface area. In this work, we have prepared two DESs for the preparation of two hierarchical carbon monoliths with different compositions (e.g., either nitrogen-doped or not) and structure. It is worth noting that DESs played a capital role in the synthesis of hierarchical carbon monoliths not only promoting the spinodal decomposition that governs the formation of the bicontinuous porous structure but also providing the precursors required to tailor the composition and the molecular sieve structure of the resulting carbons. We have studied the performance of these two carbons for CO2, N2, and CH4 adsorption in both monolithic and powdered form. We have also studied the selective adsorption of CO2 versus CH4 in equilibrium and dynamic conditions. We found that these materials combined a high CO2-sorption capacity besides an excellent CO2/N2 and CO2/CH4 selectivity and, interestingly, this performance was preserved when processed in both monolithic and powdered form.

The influence of surfactant loading level in a montmorillonite on the thermal, mechanical and rheological properties of EVA nanocomposites

In this work, montmorillonite (Mt) has been organically modified with ethyl hexadecyl dimethyl ammonium (EHDDMA) in 20, 50, 80 and 100% of the nominal exchange capacity (CEC) of the Mt. A full characterization of the organo-montmorillonite (OMt) obtained has been made, including thermal analysis, X-Ray Diffraction, elemental analysis CHN and nitrogen adsorption. According to the results, 12% in mass of the surfactant added is strongly retained by the Mt. When the mass percentage of EHDDMA exchanged in the OMt is increased up to this level, the interactions OMt–EHDDMA are steeply reduced depending on the EHDDMA content. Clay polymer nanocomposites (CPN) were prepared by melt mixing of EVA and different loads of OMt. The CPN were compress molded to obtain 1 mm thick sheets, which have been characterized according to their mechanical, thermal and rheological behaviors. The major changes in the structure of the OMt are obtained for low contents of EHDDMA. Nevertheless, the CPN containing OMt exchanged at 20 and 50% of the CEC show relatively low effect of the EHDDMA while the mechanical response and rheological behavior of CPN with OMt modified at 80 and 100% of the CEC are much more pronounced.

Beyond the H2/CO2 upper bound: one-step crystallization and separation of nano-sized ZIF-11 by centrifugation and its application in mixed matrix membranes

The synthesis of nano-sized ZIF-11 with an average size of 36 ± 6 nm is reported. This material has been named nano-zeolitic imidazolate framework-11 (nZIF-11). It has the same chemical composition and thermal stability and analogous H2 and CO2 adsorption properties to the conventional microcrystalline ZIF-11 (i.e. 1.9 ± 0.9 μm). nZIF-11 has been obtained following the centrifugation route, typically used for solid separation, as a fast new technique (pioneering for MOFs) for obtaining nanomaterials where the temperature, time and rotation speed can easily be controlled. Compared to the traditional synthesis consisting of stirring + separation, the reaction time was lowered from several hours to a few minutes when using this centrifugation synthesis technique. Employing the same reaction time (2, 5 or 10 min), micro-sized ZIF-11 was obtained using the traditional synthesis while nano-scale ZIF-11 was achieved only by using centrifugation synthesis. The small particle size obtained for nZIF-11 allowed the use of the wet MOF sample as a colloidal suspension stable in chloroform. This helped to prepare mixed matrix membranes (MMMs) by direct addition of the membrane polymer (polyimide Matrimid®) to the colloidal suspension, avoiding particle agglomeration resulting from drying. The MMMs were tested for H2/CO2 separation, improving the pure polymer membrane performance, with permeation values of 95.9 Barrer of H2 and a H2/CO2 separation selectivity of 4.4 at 35 °C. When measured at 200 °C, these values increased to 535 Barrer and 9.1.

Activated Carbons Impregnated with Na2S and H2SO4: Texture, Surface Chemistry and Application to Mercury Removal from Aqueous Solutions

The effects of treatment of an activated carbon with Sulphur precursors on its textural properties and on the ability of the complex synthesized for mercury removal in aqueous solutions are studied. To this end, a commercial activated carbon has been modified by treatments with aqueous solutions of Na2S and H2SO4 at two temperatures (25 and 140 °C) to introduce sulphur species on its surface. The prepared adsorbents have been characterized by N2 (-196 °C) and CO2 (0 °C) adsorption, thermogravimetric analysis, temperature-programmed decomposition and X-ray photoelectron spectroscopy, and their adsorption capacities to remove Hg(II) ions in aqueous solutions have been determined. It has been shown that the impregnation treatments slightly modified the textural properties of the samples, with a small increase in the textural parameters (BET surface area and mesopore volumes). By contrast, surface oxygen content was increased when impregnation was carried out with Na2S, but it decreased when H2SO4 was used. However, the main effect of the impregnation treatments was the formation of surface sulphur complexes of thiol type, which was only achieved when the impregnation treatments were carried out at low temperature (25 °C). The presence of surface sulphur enhances the adsorption behaviour of these samples in the removal of Hg(II) cations in aqueous solutions at pH 2. In fact, complete Hg(II) removal is only obtained with the sulphur-containing activated carbons.

Removal of 8-quinolinecarboxylic acid pesticide from aqueous solution by adsorption on activated montmorillonites

Sodium montmorillonite (Na-M), acidic montmorillonite (H-M), and organo-acidic montmorillonite (Org-H-M) were applied to remove the herbicide 8-quinolinecarboxylic acid (8-QCA). The montmorillonites containing adsorbed 8-QCA were investigated by transmission electron microscopy, FT-IR spectroscopy, X-ray diffraction analysis, X-ray fluorescence thermogravimetric analysis, and physical adsorption of gases. Experiments showed that the amount of adsorbed 8-QCA increased at lower pH, reaching a maximum at pH 2. The adsorption kinetics was found to follow the pseudo-second-order kinetic model. The Langmuir model provided the best correlation of experimental data for adsorption equilibria. The adsorption of 8-QCA decreased in the order Org-H-M > H-M > Na-M. Isotherms were also used to obtain the thermodynamic parameters. The negative values of ΔG indicated the spontaneous nature of the adsorption process.

Activated carbon fibre monoliths

Activated carbon fibre monoliths were prepared by physical activation of carbon fibre monoliths derived from two kinds of pitch-based carbon fibre (CF) (carbon fibres from a coal tar pitch and carbon fibres derived from a petroleum pitch). The monoliths were conformed using a coal tar pitch binder. The carbon fibre monoliths and the activated carbon fibre monoliths were studied by scanning electron microscopy (SEM) and gas adsorption (i.e. N2 at 77 K and CO2 at 273 K). The results obtained reveal that monoliths perform a good activation process that produce a quite high development of microporosity (BET surface areas around 2600 m2/g and N2 micropore volume of 1.23 cm3/g). On the other hand, it is remarkable that the activation process used allow to easily control the degree of activation and hence to select the adsorption capacities of the activated carbon fibre monoliths.

Gas Storage Scale-up at Room Temperature on High Density Carbon Materials

In relation to the current interest on gas storage demand for environmental applications (e.g., gas transportation, and carbon dioxide capture) and for energy purposes (e.g., methane and hydrogen), high pressure adsorption (physisorption) on highly porous sorbents has become an attractive option. Considering that for high pressure adsorption, the sorbent requires both, high porosity and high density, the present paper investigates gas storage enhancement on selected carbon adsorbents, both on a gravimetric and on a volumetric basis. Results on carbon dioxide, methane, and hydrogen adsorption at room temperature (i.e., supercritical and subcritical gases) are reported. From the obtained results, the importance of both parameters (porosity and density) of the adsorbents is confirmed. Hence, the densest of the different carbon materials used is selected to study a scale-up gas storage system, with a 2.5 l cylinder tank containing 2.64 kg of adsorbent. The scale-up results are in agreement with the laboratory scale ones and highlight the importance of the adsorbent density for volumetric storage performances, reaching, at 20 bar and at RT, 376 g l-1, 104 g l-1, and 2.4 g l-1 for CO2, CH4,and H2, respectively.

Study of dopamine reactivity on platinum single crystal electrode surfaces

Dopamine is the biological molecule responsible, among other functions, of the heart beat and blood pressure regulation. Its loss, in the human body, can result in serious diseases such as Parkinson's, schizophrenia or depression. Structurally, this molecule belongs to the group of catecholamines, together with epinephrine (adrenaline) and norepinephrine (noradrenaline). The hydroquinone moiety of the molecule can be easily oxidized to quinone, rendering the electrochemical methods a convenient approach for the development of dopamine biosensors. The reactivity of similar aromatic molecules, such as catechol and hydroquinone, at well-ordered platinum surfaces, has recently been investigated in our group. In this paper, we extend these studies to the structurally related molecule dopamine. The study has been performed in neutral pH, since this is closer to the natural conditions for these molecules in biological media. Cyclic voltammetry and in situ infra-red spectroscopy have been combined to extract information about the behavior of this molecule on well-defined platinum surfaces. Dopamine appears to be electrochemically active and reveals interesting adsorption phenomena at low potentials (0.15–0.25 V vs RHE), sensitive to the single crystal orientation. The adsorption of dopamine on these surfaces is very strong, taking place at much lower potentials than the electron transfer from solution species. Specifically, the voltammetry of Pt(1 1 1) and Pt(1 0 0) in dopamine solutions shows an oxidation peak at potentials close to the onset of hydrogen evolution, which is related to the desorption of hydrogen and the adsorption of dopamine. On the other hand, adsorption on Pt(1 1 0) is irreversible and the surface appears totally blocked. Spectroscopic results indicate that dopamine is adsorbed flat on the surface. At potentials higher than 0.6 V vs RHE the three basal planes show a common redox process. The initial formation of the quinone moiety is followed by a chemical step resulting in the formation of 5,6-dihydroxyindoline quinone as final product. This oxidation process has also been investigated by vibrational spectroscopy.

Microwave-assisted catalysis by iron oxide nanoparticles on MCM-41: Effect of the support morphology

Catalytically active heterogeneous catalysts have been prepared via microwave deposition of iron oxide nanoparticles (0.5–1.2 wt%) on MCM-41 type silica materials with different morphologies (particles, helical and spheres). This methodology leads to iron oxide nanoparticles composed by a mixture of FeO and Fe2O3 species, being the Fe(II)/Fe(III) peak ratio near to 1.11 by XPS. DRUV spectroscopy indicates the presence of tetrahedral coordinated Fe3+ in the silica framework of the three catalysts as well as some extraframework iron species in the catalysts with particle and sphere-like morphologies. The loading of the nanoparticles does neither affect the mesopore arrangement nor the textural properties of the silica supports, as indicated by SAXS and nitrogen adsorption/desorption isotherms. A detailed investigation of the morphology of the supports in various microwave-assisted catalyzed processes shows that helical mesostructures provide optimum catalytic activities and improved reusabilities in the microwave-assisted redox (selective oxidation of benzyl alcohol) catalyzed process probably due to a combination of lower particle size and higher acidity in comparison with the supports with particle and sphere morphology.

Novel silica membrane material for molecular sieve applications

Development of new silica membranes properties, e.g., molecular sieving properties, has been increasingly gaining importance in the last few years. A novel unsupported silica membrane, referred to as hydrophobic metal-doped silica, was developed by cobalt-doping within the organic templated silica matrix. The novel material was prepared by the acid-catalyzed hydrolysis and condensation process of tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES), which is the precursor for methyl ligand covalently bounded to the silica matrix. The synthesis and surface properties of the novel unsupported silica membrane as well as the unsupported blank silica and modified silica membranes were revealed by surface and microstructural techniques, such as water contact angle measurement, FTIR, X-ray, Solid-state 29Si MAS NMR, TGA and N2 and CO2 adsorption measurements. The results showed that the thermal stability of the organic templated silica matrix was enhanced by cobalt-doping process. A hydrophobic microporous silica membrane material with high thermal stability up to ∼560 °C in oxidizing atmosphere and a narrow pore size distribution centered at 1.1 nm was obtained. Therefore, a novel precursor material for molecular sieve silica membranes applications has been achieved and developed.

Characterization of Carbon Molecular Sieve Membranes Supported on Ceramic Tubes

Carbon molecular sieve membranes have been analyzed in supported and unsupported configurations in this experimental study. The membranes were used to adsorb CO2, N2 and CH4, and their adsorption data were analyzed to establish differences in rate and capacity of adsorption between the two types of samples (supported and unsupported). Experimental results show an important effect of the support, which can be considered as an additional parameter to tailor pore size on these carbon membranes. Immersion calorimetry values were measured by immersing the membranes into liquids of different molecular dimensions (dichloromethane, benzene, n-hexane, 2,2-dimethylbutane). Similarities were found between adsorption and calorimetric analysis. The pore volume of the samples analyzed ranged from 0.016 to 0.263 cm3/g. The effect of the pyrolysis temperature, either 550 or 700 °C, under N2 atmosphere was also analyzed. Quantification of the pore-size distribution of the support was done by liquid-liquid displacement porosimetry. The composite membrane was used for CO2/CH4 separation before and after pore plugging was done. The ideal selectivity factors value (4.47) was over the Knudsen theoretical factor (0.60) for membrane pyrolyzed at 600 °C, which indicates the potential application of these membranes for the separation of low-molecular weight gases.

Applications for CO2-Activated Carbon Monoliths: I. Gas Storage

Carbon monoliths with high densities are studied as adsorbents for the storage of H2, CH4, and CO2 at ambient temperature and high pressures. The starting monolith A3 (produced by ATMI Co.) was activated under a CO2 flow at 1073 K, applying different activation times up to 48 h. Micropore volumes and apparent surface areas were deduced from N2 and CO2 adsorption isotherms at 77 K and 273 K, respectively. CO2 and CH4 isotherms were measured up to 3 MPa and H2 up to 20 MPa. The BET surface area of the starting monolith (941 m2/g) could be significantly increased up to 1586 m2/g, and the developed porosity is almost exclusively comprised of micropores <1 nm. Total storage amounts take into account the compressed gas in the void space of the material, in addition to the adsorbed gas. Remarkably, high total storage amounts are reached for CO2 (482 g/L), CH4 (123 g/L), and H2 (18 g/L). These values are much higher than for other sorbents with similar surface areas, due to the high density of the starting monolith and of the activated ones, for which the density decreases only slightly (from 1.0 g/cm3 to 0.8 g /cm3 upon CO2 activation). The findings reveal the suitability of high density activated carbon monoliths for gas storage application. Thus, the amounts of stored gas can be increased by more than a 70 % in the case of H2 at 20 MPa, almost 5.5 times in the case of CH4 at 3 MPa, and more than 7.5 times in the case of CO2 at 3 MPa when adsorbents are used for gas storage under the investigated conditions rather than simple compression. Furthermore, the obtained results have been recently confirmed by a scale-up study in which 2.64 kg of high density monolith adsorbent was filled a tank cylinder of 2.5 L (Carbon, 76, 2014, 123).

Assessment of naproxen adsorption on bone char in aqueous solutions using batch and fixed-bed processes

An integrated analysis of naproxen adsorption on bone char in batch and packed-bed column conditions has been performed. Kinetic, thermodynamic and breakthrough parameters have been calculated using adsorption models and artificial neural networks. Results show that naproxen removal using bone char in batch conditions is a feasible and effective process, which could involve electrostatic and non-electrostatic interactions depending mainly on pH conditions. However, the application of packed-bed column for naproxen adsorption on bone char is not effective for the treatment of diluted solutions due to the low degree of adsorbent utilization (below 4%) at tested operating conditions. The proposed mechanism for naproxen removal using bone char could include a complexation process via phosphate and naproxen, hydrogen bonding and the possibility of hydrophobic interactions via π–π electron. This study highlights the relevance of performing an integrated analysis of adsorbent effectiveness in batch and dynamic conditions to establish the best process configuration for the removal of emerging water pollutants such as pharmaceuticals.