Oxygen transfer in a gas-liquid system : kinetic influence of water salinity

Oxygen gas is widely used as oxidant in a variety of industrial processes, such as hydrometallurgy, biochemical industry, organic syntheses, and wastewater treatment [1]. However, the gas–liquid mass transfer of oxygen usually becomes a bottleneck of the whole process due to its sparing solubility in aqueous solutions. It is therefore a research subject to enhance oxygen mass transfer. This study is dedicated to an accurate evaluation of thermodynamic and kinetics aspects in the water oxygenation process. Oxygenation can be analyzed by means of kinetic study of oxygen dissolution from the oxygen mass transfer coefficient (KLa) and oxygen transfer rate (SOTR) [2]. A stirred, submerged aerated 4-liters system have been designed and the operational conditions has been optimized by studying the influence of hydraulic head, air flow and salinity of water using an optical oxygen sensor. Concerning the thermodynamic phase equilibria, experimental and modelling results are obtained from different binary systems (water/air) and ternary systems (water/air/salts). This information is necessary to predict the composition of the gas phase during the process and it is also important for an implementation in a process simulation. The oxygen mass transfer coefficients were firstly measured, monitoring in the time the oxygen concentration in various synthetic liquid phases containing either salts (NaCl, KCl, LiCl and MgCl). When compared to clean water, noticeable increase of KLa were observed; the variation of KLa and SOTR with the solution salinity was modelled and found dependent on the nature of cation in the salt added. For all cases, an increase of KLa with salinity increasing was observed. The present study clearly confirmed the importance to define the experimental conditions before to describe and to model appropriately the gas–liquid mass transfer phenomena

Equilibrium and Kinetic Aspects in the Sensitization of Monolayer Transparent TiO2 Thin Films with Porphyrin Dyes for DSSC Applications

Free base, Cu(II) and Zn(II) complexes of the 2,7,12,17-tetrapropionic acid of 3,8,13,18-tetramethyl-21H,23H porphyrin (CPI) in solution and bounded to transparent monolayer TiO2nanoparticle films were studied to determine their adsorption on TiO2surface, to measure the adsorption kinetics and isotherms, and to use the results obtained to optimize the preparation of DSSC photovoltaic cells. Adsorption studies were carried out on monolayer transparent TiO2films of a known thickness. Langmuir and Frendlich adsorption constants of CPI-dyes on TiO2monolayer surface have been calculated as a function of the equilibrium concentrations in the solutions. The amount of these adsorbed dyes showed the accordance with Langmuir isotherm. Kinetic data on the adsorption of dyes showed significantly better fits to pseudo-first-order model and the evaluated rate constants linearly increased with the grow of initial dye concentrations. The stoichiometry of the adsorption of CPI-dyes into TiO2and the influence of presence of coadsorbent (chenodeoxycholic acid) have been established. The DSSC obtained in the similar conditions showed that the best efficiency can be obtained in the absence of coadsorbent with short and established immersion times

Kinetic Model for Simultaneous Adsorption/Photodegradation Process of Alizarin Red S in Water Solution by Nano-TiO2 under Visible Light.

The simultaneous adsorption and visible light photodegradation of Alizarin Red S in water solutions were studied in real time mode by using nano-TiO2, such as Anatase and Aeroxide P-25, supported on polypropylene strips. Kinetic results of the overall process were compared with those obtained from separated steps of adsorption and photodegradation previously studied; kinetic advantages were evidenced with the simultaneous approach. From the study of different dye concentrations, a kinetic model has been proposed which describes the overall process. This model considered two consecutive processes: The adsorption of dye on TiO2 surface and its photodegradation. The obtained results were in good agreement with experimental data and can predict the profiles of free dye, dye adsorbed on TiO2 and photoproduct concentrations during the total process

Recent Advances in Graphene Based TiO2 Nanocomposites (GTiO2Ns) for Photocatalytic Degradation of Synthetic Dyes

Synthetic dyes are widely used in textile, paper, food, cosmetic, and pharmaceutical industries. During industrial processes, some of these dyes are released into the wastewater and their successive release into rivers and lakes produces serious environmental problems. TiO2 is one of the most widely studied and used photocatalysts for environmental remediation. However, it is mainly active under UV-light irradiation due to its band gap of 3.2 eV, while it shows low efficiency under the visible light spectrum. Regarding the exploration of TiO2 activation in the visible light region of the total solar spectrum, the incorporation of carbon nanomaterials, such as graphene, in order to form carbon-TiO2 composites is a promising area. Graphene, in fact, has a large surface area which makes it a good adsorbent for organic pollutants removal through the combination of electrostatic attraction and π-π interaction. Furthermore, it has a high electron mobility and therefore it reduces the electron-hole pair recombination, improving the photocatalytic activity of the semiconductor. In recent years, there was an increasing interest in the preparation of graphene-based TiO2 photocatalysts. The present short review describes the recent advances in TiO2 photocatalyst coupling with graphene materials with the aim of extending the light absorption of TiO2 from UV wavelengths into the visible region, focusing on recent progress in the design and applications in the photocatalytic degradation of synthetic dyes

Selective Colorimetric Detection of Pb(II) Ions by Using Green Synthesized Gold Nanoparticles with Orange Peel Extract

Gold nanoparticles (AuNPs) were prepared by using a green approach that employed orange (citrus sinensis) peel water extract (OPE) as a reducing agent. In this case, the organic compounds present in orange peel were able to reduce Au(III) to Au(0) and, at the same time, to act as a capping agent, functionalizing the surface of the AuNPs, stabilizing them in a water solution. This “green” approach valorizes orange peel waste as a resourceful material and makes the synthetic process of AuNPs more environmentally sustainable, safe, and economically feasible than the traditional methods. The obtained gold nanoparticles (AuNPs@OPE) were characterized by FT–IR, DLS, SEM analysis, and UV–Vis spectroscopy; the latter showed a characteristic surface plasmon resonance (SPR) band at 530 nm, typical of spherical gold nanoparticles. The AuNPs@OPE were then tested as colorimetric sensors for heavy metals in water, showing an affinity and selectivity toward Pb2+. In fact, in the presence of Pb2+, the added cation favors the aggregation process, and, in this case, nanoparticles form clusters due to the interactions between Pb2+ and the carboxyl/hydroxyl groups on the surface of the AuNPs@OPE, increasing the size of the nanostructure. This process is accompanied by a change in color of the AuNPs@OPE from pink to violet, with a formation of a second, new SPR band, at a higher wavelength, relative to the aggregate formation. The colorimetric assay was tested at different times with the addition of Pb2+ ions showing different LOD values of 13.31 μM and 0.05 μM after 15 min and 90 min, respectively. The proposed colorimetric assay was also tested for analyzing Pb2+ in drinking water samples demonstrating the reliability to use AuNPs@OPE with real samples

Optimization of Photocathode for Tandem-Dye Solar Cell

Tandem dye sensitized solar cells(DSSCs) is a modification of n-type DSSCs, and could be a new device for increasing the efficiency of solar cells by converting more of the solar spectrum than can be obtained by one photoelectrode alone. The solar device is composed by two electrodes which are sensitized with two different and complimentary dyes that collect lower energy photons on one electrode and higher energy photons on the other [1]. NiO oxide is used as p-type semiconductor, and the sensitizers is anchoring on it; under irradiation, the sensitizer is excited and decays by hole injection into the VB of the NiO, forming a charge separated state. A redox couple, in most cases an iodine/triiodide couple, reacts with the charge sensitizer to regenerate the fundamental state and transports the electron to the counter electrode. The Open-Circuit Photovoltage(Voc) is the difference between the potential of the redox couple and the NiO Fermi Level. The efficiency of tandem solar cells is limited by the p-type photocathode and the higher efficiency reached until now is 1.3% [2]. The most restriction in this case is the recombination process between the hole in the NiO to reduced dye, that limits photocurrent, and the recombination to electrolyte. So, in order to have an efficient device, the dye regeneration and the charge injection into NiO must be able to compete with recombination. In this research we test new sensitizers: one based on boron-dipyrromethene and a cationic acceptor dye for application in tandem DSSCs [3]; in particular we focused the attention on the optimization of the NiO p-DSSC. We also study the influence of co-adsorbents in order to limit the aggregation and the recombination process

Graphene/TiO2 Nanocomposite for Efficient Visible-Light Photocatalysis: Synthesis, Characterization and Photocatalytic Applications.

The production of graphene in large quantities is an ongoing challenge for large-scale applications. A number of processes are used to produce graphene from graphene oxide but they need strong oxidizing and reducing agents [1]. However, graphene fabricated under these chemical conditions tends to have a certain number of structural defects, when compared to that produced from other techniques. For that purpose, top-down method such as the exfoliation of graphite powder in liquid phase by sonication is a very promising route due to its simplicity, its versatility and its low-cost [2]; besides, ultrasound treatment offers a suitable option to create high-quality graphene in great quantity. Graphene with the thickness of a single carbon atom owns unique physical and chemical properties including highly flexible structure, large surface area, high electrical and thermal conductivity and high chemical stability; also, in graphene, electrons have a linear relation between energy and momentum, so its band structure has no energy gap [3]. With these properties, graphene is an attractive material in applications that require a fast electron transfer, such as photocatalysis; it has been reported that graphene based semiconductor nanocomposites are considered as good photocatalyst for pollutant degradation [4]. Graphene is an ideal nanomaterial for doping TiO2 because the formation of Ti-O-C bonds extend the visible light absorption of TiO2. Moreover, electrons are easily transported from TiO2 to the graphene nano-sheets and the electron-hole recombination is significantly reduced; this is enhances the oxidative reactivity [5]. In this work, it was used an aqueous solution of a non-ionic surfactant, that acted like dispersing agent and as stabilizer to prevent layer stacking, for the direct exfoliation of graphite by sonication. The obtained graphene dispersion is characterized by X-Ray Diffraction (XRD), Dynamic Light Scattering (DLS) and UV-Visible spectroscopy, and it is used for the preparation of heterogeneous GR@TiO2 photocatalyst supported on polypropylene (PP). GR@TiO2 nanocomposites are used to treat water with environmental pollutants by photocatalytic

Temperature and salinity effects on the Raman OH-stretching vibration bands of water: starting point to know hydrate occupancy and unreacted water in the gas hydrates.

Properties of gas hydrates can be in situ detected by Raman spectroscopy in order to determine structure, evolution, composition, cage occupancies and vibrational spectra of the guest molecules. Temperature, pressure, dissolved ions and cooperative effects related to hydrogen bonds and solute solvation promote structural changes on water molecules. In order to know the effects of different parameters on water OH-stretching vibrations, we present an experimental Raman study on how the addition of sodium chloride to liquid water under different temperature conditions influences the water hydrogen bonds. To this purpose, Raman spectra of ultrapure water and of NaCl water solutions were acquired under different temperature conditions. Experiments were performed with an excitation wavelength of 532 nm, a total acquisition time of 300 seconds at 20 and -20 °C respectively. The obtained preliminary results demonstrated that salinity and temperature affected OH-stretching vibrations and that the results can be correlated with the NaCl concentrations. These obtained results represent the starting point in order to obtain data about hydrate occupancy and unreacted water in the gas hydrate mass in different experimental conditions

Kinetic Model for Photocatalytic Degradation of Alizarin Red-S by Polypropylene coated nano-TiO2

The aim of this study is optimize and clarify the total mechanism of adsorption/ visible-photodegradation of Alizarin Red S polypropylene coated nano-TiO2 Degussa P-25 and TiO2 Anatase as photocatalysts. The characterization of Alizarin Red S and its chemical interaction with TiO2 surface has been studied. The acid dissociation constants of Alizarin Red S are determined. Adsorption and photodegradation steps were simultaneously studied in order to propose a simple kinetic model which can describe the process in an adequate way. The results obtained from this kinetics model are in good agreement with experimental data

Constant Pressure CO2 Replacement of CH4 in Different Hydrate Environments: Structure and Morphology

Gas hydrates (GHs) are solid, ice-like compounds composed of water molecules forming a lattice structure that hosts gas molecules, produced under high pressure and low temperature. The structure of the hydrate structure is affected by the surrounding environment, and in this context, a structural characterization of GHs prepared in different environments, ultrapure (UP) water, seawater, synthetic sand, natural sand, and sodium dodecyl sulfate, has been proposed. In particular, the Raman spectroscopy has been used to investigate the structural changes in the water cages, the gas uptake in the hydrate structure, the CH4 cage occupancies, the hydration number, and the yield of carbon dioxide replacement at constant pressure. For this comparison, CH4-hydrates, CO2-hydrates, and CH4/CO2-hydrates (obtained from an implemented replacement process) were prepared in five different experimental conditions mentioned above and structurally characterized. From Raman investigation, pure CH4-hydrates displayed almost full (>95%) occupation in the large cage and a significant change in the small cage occupation related to the different tested media. The cage occupancy calculation of CO2/CH4-hydrates showed that a higher yield of replacement can be obtained in UP water and that CH4-hydrates in natural sand and in seawater, which are the most representing of natural environments, displayed a good replacement of CH4 with CO2. Additionally, the ex situ morphological characterization of the GHs by scanning electron microscopy (SEM) allowed the highlighting of morphological differences among the investigated samples