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

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

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

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

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

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

Band Gap Implications on Nano-TiO2 Surface Modification with Ascorbic Acid for Visible Light-Active Polypropylene Coated Photocatalyst

The effect of surface modification using ascorbic acid as a surface modifier of nano-TiO2 heterogeneous photocatalyst was studied. The preparation of supported photocatalyst was made by a specific paste containing ascorbic acid modified TiO2 nanoparticles used to cover Polypropylene as a support material. The obtained heterogeneous photocatalyst was thoroughly characterized (scanning electron microscope (SEM), RAMAN, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Diffuse Reflectance Spectra (DRS) and successfully applied in the visible light photodegradation of Alizarin Red S in water solutions. In particular, this new supported TiO2 photocatalyst showed a change in the adsorption mechanism of dye with respect to that of only TiO2 due to the surface properties. In addition, an improvement of photocatalytic performances in the visible light photodegration was obtained, showing a strict correlation between efficiency and energy band gap values, evidencing the favorable surface modification of TiO2 nanoparticles

From TiO2 and Graphite to Graphene doped TiO2 for visible light photocatalytic degradation of refractory dye.

Graphene production is an ongoing challenge for large-scale applications. Many processes are used to produce graphene 1. Top-down method such as the exfoliation of graphite powder in liquid phase by sonication is a promising route to create high quality graphene in great quantity due to its simplicity, its versatility and its low-cost 2. Graphene with the thickness of a single carbon atom owns unique physical and chemical properties like large surface area, highly flexible structure, high electrical and thermal conductivity and high chemical stability 3. With these properties, graphene is an attractive material in applications that require a fast electron transfer, such as photocatalysis. In fact, 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. Furthermore, electrons are easily transported from TiO2 to graphene nano-sheets and the electron-hole recombination is reduced; this is enhances the oxidative reactivity 5. In this work, graphene doped TiO2 nanocomposite was used as photocatalytic materials for the Alizarin Red S degradation in water solutions. Graphene dispersions were prepared by liquid-phase exfoliation of graphite in the presence of a non-ionic surfactant, Triton X-100. The obtained graphene dispersion was characterized by X-Ray Diffraction, Dynamic Light Scattering and UV-Visible spectroscopy and was subsequently used for the preparation of graphene doped-TiO2 photocatalyst. Graphene doped-TiO2 nanocomposites showed higher adsorption of Alizarin Red S on the catalyst surface and higher photocatalytic activity for its degradation under visible light irradiation, respect to those obtained with pure TiO2 6. References: 1) Dimiev, A. M.; Tour, J. M. ACS Nano, 2014, 8, 3060 - 3068. 2) Samorì, P. et al. Chemical Society Reviews, 2014, 43, 381 - 398. 3) Geim, A.K.; Novoselov, K. S. Nature Materials, 2007, 6, 183 - 191. 4) Khalid, N. R.; Hong, Z. et al. Current Applied Physics, 2013, 13, 659 - 663. 5) Li, F.; Cheng, H. M. et al. Advanced Functional Materials, 2011, 21, 1717 - 1722. 6) Giovannetti, R.; D’ Amato, C. A. et al. Scientific Reports, 2015, 5, 17801