Alberto E. Cassano

Supported titanium oxide as photocatalyst in water decontamination: State of the art

Abstract Over the last two decades the interest on systems based on supported titanium oxide as a photocatalyst for water decontamination has grown significantly. A variety of supporting materials, coating methods, and reactor arrangements have been investigated from engineering as well as from more fundamental points of view. A thorough search of the published reports of these investigationes was carried out and they are analyzed in this paper. An overview of the state of the art in the subject is given.

Reaction engineering of suspended solid heterogeneous photocatalytic reactors

Abstract A revision of the authors’ work on suspended solid photocatalytic reactors is presented. Thus, heterogeneous reactions in aqueous media, involving the presence of both fine particles of titanium dioxide and UV radiation are analyzed. Under these conditions, light scattering is a complex phenomenon that must be included in the description of the system performance. Suspended solid photocatalytic reactors are just a family of the well-known slurry reactors for which considerable progress has been reported in the reaction engineering literature. However, ab initio design methods for these particular — light activated — reactors are not known. Previous developed methods should be readily applicable if: (1) the photocatalytic activation is properly described, (2) an intrinsic kinetic expression can be developed, and (3) the scalar field of radiation intensities inside reaction spaces of different geometries can also be known. With this purpose, in order to model the radiation field and also the initiation step, the radiative transfer equation (RTE) for heterogeneous media must be solved. Solution of the RTE in photocatalytic reactors implies the knowledge of different system properties that are not usually known. New methods for obtaining this information are described and typical results are presented. Scale up procedures, derived from the application of first principles, cannot be applied if intrinsic reaction kinetic data are not available. A key point in developing these reaction rate expressions is the knowledge of the radiation absorption by the catalytic particles. The required model is also presented. Furthermore, a simple reactor design and special operating conditions for appropriate laboratory experiments were developed that can be used to produce this type of data reducing to a minimum the difficulties associated with the analysis of composition vs. time information. In this review, the photocatalytic oxidation of trichloroethylene is used as a model reaction. Finally, the description of the radiation field distribution in a typical flat plate, photocatalytic solar reactor simulator illustrates the method that should be used for scaling-up purposes. With this objective the photon absorption rate was obtained as a function of position and the developed model was verified with careful experimental measurements.

Radiation field modelling in photoreactors. I: Homogeneous media

Abstract The present work reports an analysis of the existing models for the description of the radiation field inside photochemical reactors for homogeneous systems. With this purpose incidence and emission models are critically analysed and the contributions of the main research groups are presented. The review classifies the results known up to the present in a methodological way and, as a logical consequence, projects those areas where further work is needed.

LARGE SCALE STUDIES IN SOLAR CATALYTIC WASTEWATER TREATMENT

Abstract The practical applicability of the double-skin sheet reactor (DSSR) for solar catalytic wastewater treatment has been checked at laboratory scale, in Hannover, and at the outdoor test-field of the Plataforma Solar de Almeria (PSA), in Spain, using dichloro acetic acid (DCA) as the standard test pollutant. This type of photoreactor has also been successfully used for the treatment of biologically pretreated industrial wastewater on laboratory and pilot plant scale. To analyze the experimental results, a first-order rate law in concentration and radiation density flux (assumed constant for each run) has been derived: R V =k 3 (A R /V R ) q UV c , where k3 is a lump kinetic parameter, AR is the illuminated reactor area, VR is the volume of the photoreactor, q UV is the time averaged radiation density flux of UV energy, and c is the concentration of the pollutant. It has been observed that the integrated form of this rate law fits fairly well the experimental data obtained with a model compound and solar energy, and that an expression derived from this rate can be used for an approximate scaling-up of a solar catalytic wastewater treatment plant.

Degradation kinetics of 2,4-D in water employing hydrogen peroxide and UV radiation

This paper reports the photooxidation of 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous solution employing hydrogen peroxide and ultraviolet radiation. A kinetic model to represent the degradation of 2,4-D and that of equally toxic intermediate products, such as 2,4-dichlorophenol (DCP) and chlorohydroquinone (CHQ), is presented. The model includes the parallel, direct photolysis of 2,4-D and the most important reaction intermediates. The experimental work was performed in a batch, well-stirred tank reactor irradiated from its bottom using a low power, germicidal, tubular lamp placed at the focal axis of a cylindrical reflector of parabolic cross-section. Herbicide degradation initial rates twenty times faster than those obtained employing UV radiation alone were found. In order to reach more useful conclusions about the ability of the process to reduce the contamination to innocuous final products, simultaneous measurements of the total organic carbon (TOC) were performed. By application of the kinetic model to the whole set of concentration versus time experimental data, the values of the kinetic parameters were obtained. The model permits a good representation of the reaction evolution in a rather wide range of 2,4-D and H2O2 initial concentrations.

Absolute Quantum Yields In Photocatalytic Slurry Reactors

Photocatalysis in slurry reactors have the particular characteristic that the reacting medium not only absorbs but also scatters photons and both phenomena occurs simultaneously. When this is the case, typical methods used in classical photochemistry such as homogeneous actinometry are useless because they cannot take into account radiation scattering. Consequently present methods in use always underestimate quantum yields calculations. A method has been developed to produce absolute values of photocatalytic quantum yields in slurry reacting systems employing titanium dioxide and near UV radiation. The new procedure resorts to the rigorous solution of the radiative transfer equation inside the reaction space in order to obtain the true value of the photonic absorption rate. Employing monochromatic light, values for the photocatalytic decomposition of phenol and 1,4-dioxane are reported under precisely defined conditions. The method can be used to compare reactivities of different catalysts or, for a given catalyst, reactivities with different compounds regardless the reactor shape, size or configuration.

Glyphosate degradation in water employing the H2O2/UVC process

Glyphosate is the organophosphate herbicide most widely used in the world. Any form of spill or discharge, even if unintentional, can be transferred to the water due to its high solubility. The combination of hydrogen peroxide and UV radiation could be a suitable option to decrease glyphosate concentration to acceptable limits. In this work, the effects of initial pH, hydrogen peroxide initial concentration, and incident radiation in glyphosate degradation were studied. The experimental device was a cylinder irradiated with two tubular, germicidal lamps. Conversion of glyphosate increases significantly from pH = 3-7. From this value on, the increase becomes much less noticeable. The reaction rate depends on the initial herbicide concentration and has an optimum plateau of a hydrogen peroxide to glyphosate molar concentration ratio between 7 and 19. The expected non linear dependence on the irradiation rate was observed. The identification of critical reaction intermediaries, and the quantification of the main end products were possible and it led to propose a plausible degradation path. The achieved quantification of the mineralization extent is a positive indicator for the possible application of a rather simple technology for an in situ solution for some of the problems derived from the intensive use of glyphosate.

Rigorous model and experimental verification of the radiation field in a flat-plate solar collector simulator employed for photocatalytic reactions

Abstract In any kinetically controlled photocatalytic process the catalyst activation is always a photochemical act that depends upon the local value of the volumetric rate of energy absorption (LVREA). In a heterogeneous photocatalytic reaction, a precise evaluation of the LVREA can be obtained when the spatial and directional distributions of radiation intensities are known. With this purpose, a mathematical model of the radiation field inside a flat-plate heterogeneous reactor (a solar simulator) has been developed. The solid–liquid reactor is irradiated by two tubular UV lamps with the aid of two parabolic reflectors. Since titanium dioxide suspensions absorb and scatter radiation the model accounts for both effects. Resorting to information about the lamp and reflector characteristics, the catalyst optical properties and concentration, as well as the reactor dimensions and wall reactor properties, the solution of the mathematical model (a two-dimensional–two-directional model) provides a detailed description of the spatial and angular directional distributions of radiation intensities inside the reactor. Using this information, it is possible to precisely calculate the rate of absorbed radiation energy at each point inside the reactor. This is one of the key variables for reactor design and/or scale-up purposes. The radiation distribution inside the reactor was verified by computing forwardly transmitted and backwardly scattered radiation fluxes coming out of the reaction space through the glass reactor walls. These radiation fluxes were compared with experimental measurements made with a UV radiometer and good agreement was obtained; when no fouling was present and considering low catalyst loading to obtain measurable radiation fluxes, the maximum observed error was 12%. The predicted inlet boundary condition was also verified with actinometry and the error was smaller than 14% For this reactor configuration, when the radiation absorption performance is the only factor under consideration, it was found that Aldrich titania is more efficient than the Degussa P 25 variety.

Modeling of light scattering in photochemical reactors

Abstract Heterogeneous photochemical reactions in fluid (water or air) environments have been gaining increasing interests in the past fifteen years, the main application being in pollution abatement by solid semiconductor photocatalyzed systems. One of the unsolved problems is the proper evaluation of the rate of absorbed radiation energy by the participating medium due to the existence of simultaneous absorption and scattering. This is a key property in quantifying kinetic models and designing photoreactors. A model derived from radiative transport fundamentals and chemical reactor engineering concepts is presented. It permits a rigorous quantification of the local volumetric rate of radiant energy absorption (LVREA) inside the reactor. The method was experimentally verified by analyzing the scattering effects (performance changes) produced by addition of transparent, inert particles to a homogeneous photochemical reacting system. With the validated model, and employing radiation absorbing particles (titanium dioxide), the proposed approach was compared with other methods currently in use to evaluate the LVREA. It was shown, in a quantitative way, that the proper application of the radiative transfer equation is a requisite for obtaining accurate results. Finally, the results of changing particle concentration and particle size on the magnitude of the scattered light were explored.

Photocatalytic reactors for treating water pollution with solar illumination. II: A simplified analysis for flow reactors

Abstract Usual applications of photocatalytic reactors for treating wastewater exhibit the difficulty of handling fluids having varying composition and/or concentrations; thus, a detailed kinetic representation may not be possible. When the catalyst activation is obtained employing solar illumination an additional complexity always coexists: solar fluxes are permanently changing with time. For comparing different reacting systems under similar operating conditions and to provide approximate estimations for scaling up purposes, simplified models may be useful. For these approximations the model parameters should be restricted as much as possible to initial physical and boundary conditions such as: initial concentrations (expressed as such or as TOC measurements), flow rate or reactor volume, irradiated reactor area, incident radiation fluxes and a fairly simple experimental observation such as the photonic efficiency. A combination of a new concept: the “actual observed photonic efficiency” with ideal reactor models and empirical kinetic rate expressions can be used to provide rather simple working equations that can be efficiently used to describe the performance of practical reactors. In this paper, the method has been developed for the case of a photocatalytic batch reactor.