Rodolfo J. Brandi

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.

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.

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.

Modeling of radiation absorption in a flat plate photocatalytic reactor

The radiation field inside a flat-plate photocatalytic reactor has been modeled. The solid-liquid heterogeneous reactor is irradiated by two tubular lamps and two parabolic reflectors. The model accounts for absorption and scattering effects by the solid suspension. From the lamp and reflector characteristics, the catalyst properties and concentration, and the reactor dimensions, the model provides a detailed description of the spatial and angular direction distributions of radiation intensities inside the reactor. These results permit to calculate at each point of the reactor the local value of the volumetric rate of radiation energy absorption (LVREA), a property that is always required to formulate the initiation rate of any photocatalytic process.

A methodology for modeling photocatalytic reactors for indoor pollution control using previously estimated kinetic parameters.

A methodology for modeling photocatalytic reactors for their application in indoor air pollution control is carried out. The methodology implies, firstly, the determination of intrinsic reaction kinetics for the removal of formaldehyde. This is achieved by means of a simple geometry, continuous reactor operating under kinetic control regime and steady state. The kinetic parameters were estimated from experimental data by means of a nonlinear optimization algorithm. The second step was the application of the obtained kinetic parameters to a very different photoreactor configuration. In this case, the reactor is a corrugated wall type using nanosize TiO(2) as catalyst irradiated by UV lamps that provided a spatially uniform radiation field. The radiative transfer within the reactor was modeled through a superficial emission model for the lamps, the ray tracing method and the computation of view factors. The velocity and concentration fields were evaluated by means of a commercial CFD tool (Fluent 12) where the radiation model was introduced externally. The results of the model were compared experimentally in a corrugated wall, bench scale reactor constructed in the laboratory. The overall pollutant conversion showed good agreement between model predictions and experiments, with a root mean square error less than 4%.

Scaling up of a photoreactor for formic acid degradation employing hydrogen peroxide and UV radiation

A model for scaling up a homogeneous photoreactor was developed and experimentally verified in a pilot-plant-size apparatus. The procedure is exemplified by the oxidation of dilute aqueous HCOOH solutions with UV radiation (254 nm) and H2O2. First, the kinetic model and the kinetic parameters of the HCOOH degradation were obtained in a well-stirred, small, batch flat-plate photoreactor (volume=70 ml). The method employed in the analysis of the experimental results yielded reaction-rate expressions for HCOOH and H2O2 that were independent of the reactor configuration. These kinetic equations and the corresponding kinetic constants were then used in a mathematical, fully deterministic model of a continuous-flow, 2-m-long, annular reactor (0.0065 m2 of cross section for flow) operating in a laminar-flow regime to predict exit concentrations of HCOOH. Irradiation was provided in both cases by two different types of germicidal lamps. No additional experiments were made to adjust the reactor-model parameters. Theoretical predictions from the representation of the reactor performance obtained were compared with experimental data furnished by experiments in the much-larger-size, cylindrical-flow reactor. Results showed good agreement for the range of variables explored; they corresponded to expected operating conditions in water streams polluted with low concentrations of organic compounds.

Water disinfection with UVC radiation and H2O2. A comparative study

A generalized kinetic model resulting from several modifications of the one originally known as the Series Event Model has been applied to describe three different disinfection processes and compare their efficiencies. The work was performed in a well-defined, versatile batch reactor employing Escherichia coli as a subrogate bacteria. The following systems were studied: (i) UVC radiation alone, (ii) hydrogen peroxide alone and (iii) UVC radiation combined with hydrogen peroxide. The kinetic parameters of the three models were determined. Within the range of studied operating conditions, the use of UVC alone has shown to produce the best results.

A novel approach to explain the inactivation mechanism of Escherichia coli employing a commercially available peracetic acid

The chemical inactivation of Escherichia coli employing a commercial mixture of peracetic acid (PAA) was studied. For this purpose, experiments were carried out using dilutions of the unmodified mixture, and also the same mixture but altered with hydrogen peroxide (HP) previously inhibited. Also, these results were compared to those obtained before employing HP alone. It was found that the mixture is much more efficient than HP and PAA acting separately. Furthermore, it was found that PAA without HP is much more efficient than HP alone. A plausible explanation is presented. The homolysis of PAA would give rise to a chain reaction that generates a significant number of highly oxidizing radicals. An attacking scheme to bacteria in two stages is proposed, where the initial step, mainly caused by PAA, is very fast and eliminates some specific components of the bacteria that would otherwise inhibit the parallel action of HP. Thereafter, the emergence of a potentiating synergetic action of the second oxidant seems to be immediately unveiled.

Arsenic sorption onto titanium dioxide, granular ferric hydroxide and activated alumina: Batch and dynamic studies

The aim of this work was to evaluate and compare the efficiencies of three different adsorbents for arsenic (As) removal from water: titanium dioxide (TiO2), granular ferric hydroxide (GFH) and activated alumina (AA). Equilibrium experiments for dissolved arsenite and arsenate were carried out through batch tests. Freundlich and Langmuir isotherm models were adopted and their parameters were estimated by non-linear regressions. In addition, dynamic experiments were performed in mini fixed bed columns and breakthrough curves were obtained for each combination of sorbate/adsorbent. Experimental results obtained by column assays were compared with predictions of well-known breakthrough models (Bohart–Adams and Clark). Results indicate that As(V) is more easily adsorbed than As(III) for AA and GFH, while TiO2 has a similar behavior for both species. The titanium-based material is the most efficient adsorbent to carry out the process, followed by the GFH.