Volume 11, Issue 2 - Special Issue to Honor Professor Alberto E. Cassano, Guest Editors: Orlando Alfano and Eric Bandala

This article reports the development of a novel drum photocatalytic reactor for treating dye effluent streams. The parameters for operation including drum rotation speed, light source distance, catalyst loading and H 2 O 2 doping have been investigated using methylene blue as a model pollutant. Effluent can be generated by a number of domestic and industrial sources, including pharmaceutical, oil and gas, agricultural, food and chemical sectors. The work reported here proposes the application of semiconductor photocatalysis as a final polishing step for the removal of hydrocarbons from effluents sources, initial studies have proved effective in removing residual hydrocarbons from the effluent.

This study was focused on the synthesis of titania-based photocatalytic coatings with high photocatalytic activity, attrition resistance, and stability. Five different photocatalytic coatings were synthesized using the sol–gel technique. Three coatings were prepared using aqueous sols of TiO 2 nanoparticles with different amounts of titanium tetraisopropoxide and different quantities and types of acids. The other two photocatalysts were composite sol–gel coatings which were prepared by incorporating commercial Degussa P25 into the TiO 2 synthesized through sol–gel technique. The physical and optical properties of the immobilized photocatalysts were characterized with UV–vis spectroscopy, X-ray diffraction, scanning electron microscopy, and light scattering. The photocatalytic activity of each coating was determined using a lab-scale differential photoreactor by measuring the degradation rate of a model micropollutant, the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). The conversions of 2,4-D obtained with the TiO 2 coatings without Degussa P25 were in the order of 7–23%, whereas the two composite coatings provided conversions in the range of 66–69%. In addition, one of the composite coatings showed a more homogeneous morphology and less cracking, and for this reason, it was more durable and showed lower attrition.

This study reports the kinetic parameter estimation in the photocatalytic degradation of phenol over different TiO 2 catalysts by using the Genetic Algorithm (GA) and nonlinear regression. Reaction networks are based on a previously reported unified kinetic model (UKM) of the Langmuir–Hinshelwood type. Nonlinear least-squares fitting and GA are used to find the values for the kinetic constants. The computed parameters were found to predict experimental data for phenol photodegradation at different levels of concentrations. It is shown that both methods render close values for the kinetic constants. This suggests that UKM approach gives the global minimum and as a result, this method provides good and objective parameter estimates with low to moderate cross-correlation among kinetic constants and acceptable 95% Confidence Intervals (CIs). Global optimization by using GA requires extensive computer times of up to 5 minutes. Least square fitting provides the same results with computer times of seconds only. It is then concluded that the UKM approach effectively avoids overparameterization by finding the global optimum when optimizing the kinetic constants.

A theoretical/computational approach has been developed to satisfy mass transfer rates and molar fraction constraints simultaneously when modeling mass transfer from a finite source towards an infinite surrounding media. The procedure is based on using a mass balance of individual molecular species coupled with individual mass transfer rates. Since the surrounding media is assumed insoluble in the finite media, the total number of moles transferred across the interface is computed on the basis of modified molar fractions of the transferring species. Two simple examples are shown: the first is an evaporating droplet in air and the second example is a droplet being dissolved in water. These examples have very large differences in time scales and illustrate the versatility and accuracy of the computational procedure.

Computational Fluid Dynamics is a quite well established tool for carrying out realistic simulations of process apparatuses. However, as a difference from single phase systems, for multiphase systems the development of CFD models is still in progress. Among the two-phase systems, gas–liquid systems are characterised by an additional complexity level, related to the fact that bubble sizes are not known in advance, being rather the result of formation and breakage-coalescence dynamics and therefore of complex phenomena related to flow dynamics and interfacial effects. In the present work, Euler–Euler Reynolds-averaged flow simulations of an air-lift reactor are reported. All bubbles are assumed to share the same size, and a simplified approach is adopted for modelling inter-phase momentum exchange, that involves bubble terminal velocity as the sole parameter needed. Good agreement between simulation results and literature experimental data is found for all the gas flow rates simulated. This result implies that, despite the many simplifications that have to be adopted in order to make them viable, fully predictive CFD simulations of gas–liquid systems can give rise to reasonably accurate predictions of reactor fluid dynamics.

A batch cylindrical photocatalytic reactor, externally irradiated by 1–6 UV fluorescent lamps and containing a stirred slurry of polycrystalline TiO 2 , was modeled by coupling a modified Langmuir–Hinshelwood kinetics together with a two-dimensional light intensity field. The radiation field has been determined on the main assumptions of diffuse radiation, isotropic scattering and negligible backward reflected photon flow. The model has been applied to the photocatalytic oxidation of organic substrates which do not undergo homogeneous photochemical degradation. The model is characterized by the following four parameters: the kinetic constants of substrate adsorption, desorption and degradation and the exponent of the power law expressing the kinetics dependence on the light intensity. The model constants may be determined by applying a simple least-squares best fitting procedure.

This contribution focuses on the analysis of the hydrodynamics taking place near an electrode of cylindrical geometry in electrokinetic applications. Both the temperature development conditions and Joule heating effect are included. A boundary layer approach has been used to model the hydrodynamic in the system. This is based on the heat transfer model, the continuity equation, and the Navier–Stokes equation. The resulting set of two partial differential equations mutually coupled is solved applying the Von Karman integral approximation. A numerical solution of the differential–integral model is used to illustrate the behavior of the systems under a variety of conditions.

In this work, the use of previously reported figures-of-merit is proposed for the comparison of solar-driven photocatalytic disinfection technologies using NF-codoped TiO 2 . These figures-of-merit are based on the solar collection area per order ( A CO ) through the understanding of the overall kinetic behavior of the disinfection process under the tested conditions: pH 7, four different catalyst concentrations (0.0, 0.10, 0.25 and 50 mgmL − 1 ) and two solar radiation types (UV+visible and visible radiation alone). The results provide a direct link to the accumulated energy efficiency (the lowest the value the highest the efficiency) of the inactivation process, allowing the comparison between the efficiencies of a broad range of processes evaluating different experimental conditions.

This work presents a study focused on the development of a simple useful tool to predict the generation of trihalomethanes in drinking water purification systems, using two precursors and trichloromethane as model compounds through a simple chlorine decay model. This work proposed a semiempirical model without adjustable power parameters where fast and slow stages and the effect of pH were included. Despite that the model is not based in a complete kinetic scheme, using the proposed equations it is possible to predict the simultaneous evolution of chlorine and TCM with a set of linear kinetics parameters which characterize the system and will be obtained using simple routine laboratory measurements. The results show that both TCM formation and chlorine decay are strongly dependent on the chemical nature of the model precursor. Although resorcinol and phenol have different reactivity with chlorine and represent different functional groups which are present in natural compounds, the TCM generation appears to be properly described in both cases by the total chlorine consumption. Considering that during the potabilization processes the pH changes, the study of the effects of this variable is very important to achieve the minimization of THMs generation. The pH has a significant effect on the time evolution of chlorine-substituted hydroxybenzene intermediates and therefore on the TCM formation, since the properties of the reacting species are directly affected by the reaction medium for their participation in the different reaction paths. The study of the distribution and selectivity of the intermediate species allowed explaining the results obtained for the kinetics of formation of TCM. The results suggest that in order to understand the effect of pH, the nature of oxidation of HOCl and ClO ‒ , should be considered simultaneously with the electronegative nature of the precursor compounds. Finally, in terms of minimizing the generation of THM it is important to consider the potential impact of pH changes within the water treatment process and supply and the stages where chlorination may be carried out.

The central problem for standardization of photocatalytic efficiency of whatever substrate on an illuminated catalyst is the rate evaluation. For gas/solid experiments different reactors, like batch or flow-through either continuous stirred-tank reactor (CSTR) or plug flow reactor (PFR), could be envisaged. The basic equations governing these reactors and the rate expression for them are presented here. Experiments show that a CSTR configuration presents a lot of advantages for practical use, as any volume, any shape of catalyst and any flow of gas into the reactor can possibly be used. A CSTR configuration is superior to the standardized PFR as the resistance to mass transfer can be reduced by inside forced ventilation. Consequently, it gives an evaluation of the photocatalytic rate more close to the actual surface one. The rate for CSTR at steady state must be calculated as r ( C o ) = C o F η /S(1− η ), where η is the conversion.

For a long time now, it is known that an infinite number of compounds can be obtained from microalgae due to their promising composition. This fact is used by several companies to produce at medium-scale different high value products such as carotenoids. Furthermore, this last decade research in this topic has significantly increased given the potential solution these microorganisms present to meet World environmental agreements regarding CO 2 emissions. However, the culture of large-scale microalgae in order to obtain products which are required in huge quantities (e.g., oil for manufacturing biodiesel) and at the same time removing pollutants appeared as a difficult task due to the great number of variables that must be taken into account. The photobioreactor design for cultivating the microalgae is perhaps the most important task, but there are other bottle-necks, such as the harvesting system, cleaning system, control system, the chosen microalgae, and the extraction of products from the microalgae. This review focuses on around 200 patents covering microalgae technology, from different photobioreactors designs to microalgae applications, in order to construct a perfect large-scale facility removing CO 2 in the process.

The influence of sonicating a TiO 2 suspension was studied and, in addition, the effect of the TiO 2 aggregate size. 2,4-Dichlorophenol (DCP) was degraded by photocatalysis with TiO 2 . In a first experimental series, the suspension was prepared just stirring the photocatalyst. Then, the experimental runs were repeated but stirring and sonicating the TiO 2 . The sonicated suspension showed to be much more effective than the non-sonicated, and the kinetic constants were around 70% higher for DCP removal and 40% higher for total organic carbon (TOC) mineralization. The absorption and scattering coefficients and the diffuse reflectance were also determined and presented for both types of suspensions. The values were calculated by transmittance measurements for different amounts of TiO 2 . The values obtained were fitted to the radiative transfer equation using the Schuster–Schwarzschild approximation. In agreement with the results of the degradation experiments, it was found that the absorption coefficient was higher for the sonicated suspension.

The heterogeneous photocatalysis (HP) procedure has been demonstrated to be an interesting method for disinfecting water. It is effective for inactivating Pseudomonas aeruginosa , but its effectiveness may be reduced due to the action of several factors which are able to affect bacterial radio-sensitivity. The results reported here show the influence of nutritional stress and pre-exposure to sub-lethal UVA doses on the efficiency of the inactivation of P. aeruginosa by HP. Both previous exposures to low UVA fluencies and nutrient deprivation induce bacterial resistance to this process, in concordance with previous observations about the lethal effect of direct UVA irradiation. Starvation plus pre-irradiation did not have synergistic or antagonist effects. Kinetic parameters are presented for all three cases. These factors should be taken into account in the design of a water treatment process.

CeO 2 nanoparticles (NPs) were tested to assess their toxicity on Escherichia coli strain in the presence of non-ionic surfactants. The NPs were dispersed in water by sonication at different pH values and times then mixed with three different surfactants (i.e., Triton X-100, Polyvinyl Pyrrolidone (PVP) and Tween 80) with a concentration of 0.001% v/v. It was found that sonication favored dispersion of the material and produced particles having 100 nm sizes in average. The material show toxicity to E. coli at pH 7 when growth using only minimal M9 media; no toxic response was observed for bacteria growth in rich media. The toxic effect in minimal media was enhanced by adding any of the non-ionic surfactants to the media. The use of CeO 2 plus surfactant decreased the minimal inhibitory concentration (MIC) value of E. coli . The highest effect was observed for addition of Tween 80, in this case MIC value was 0.150 mg mL – 1 compared to 3 mg mL – 1 of CeO 2 alone (almost 20 times improvement). These findings suggest the importance of different substances that can interact with NPs, like surfactants, usually present in wastewater systems that may lead to undesirable unexpected toxic characteristics in materials usually considered as innocuous.

The relation between the amount of incident photons, absorbed photons, oxidant species, reaction products and electrons in the external circuit should be analyzed individually to determine the step limiting the efficiency of the global photoelectrocatalytic processes. This work discusses the evaluation of three different titania electrodes for the oxidation of methanol in a photoelectrocatalytic reactor. The electrode prepared with three titania coating cycles shows a high efficiency in terms of photochemical (photons to product molecules), electrochemical (product molecules to electrons) and photoelectrochemical (photons to electrons) what explain its high activity for the photoelectrocatalytic oxidation of methanol.

The photocatalytic oxidation of seven typical indoor volatile organic compounds (VOCs) is experimentally investigated using novel nanocrystalline TiO 2 dip-coated catalysts. Not only the role of hydrophilicity of the reactants but also other physico-chemical properties and molecular descriptors are studied and related to kinetic and equilibrium constants. The main objective of this work consists in establishing simple relationships that will be useful to deepen the understanding of gas-phase heterogeneous photocatalytic mechanisms and for the prediction of degradation rates of these VOCs using an indoor air treatment process.

An analysis for boundary layer flows caused by natural convection due to heat generation caused by the Joule heating effect is presented. The integral approximation approach developed by Von Karman is used to model the boundary layer flow in the system. Effects of the heat generation on temperature and velocity profiles as well as on the boundary layer thickness are discussed, and their implication for possible convective mixing effects near the electrode region is highlighted. These are important pieces of information when designing applications in electrokinetic remediation and separation of biomolecules.