Kinetic model of water disinfection using peracetic acid including synergistic effects.

The disinfection efficiencies of a commercial mixture of peracetic acid against Escherichia coli were studied in laboratory scale experiments. The joint and separate action of two disinfectant agents, hydrogen peroxide and peracetic acid, were evaluated in order to observe synergistic effects. A kinetic model for each component of the mixture and for the commercial mixture was proposed. Through simple mathematical equations, the model describes different stages of attack by disinfectants during the inactivation process. Based on the experiments and the kinetic parameters obtained, it could be established that the efficiency of hydrogen peroxide was much lower than that of peracetic acid alone. However, the contribution of hydrogen peroxide was very important in the commercial mixture. It should be noted that this improvement occurred only after peracetic acid had initiated the attack on the cell. This synergistic effect was successfully explained by the proposed scheme and was verified by experimental results. Besides providing a clearer mechanistic understanding of water disinfection, such models may improve our ability to design reactors.

Kinetics of the degradation of n-butyl benzyl phthalate using O3/UV, direct photolysis, direct ozonation and UV effects.

The aim of this work is to study the degradation kinetics of the endocrine disruptor benzyl butyl phthalate using ozone and UV radiation. The model comprises four parallel subsystems that are identified and isolated: (1) direct photolysis, (2) direct ozonation in the absence of hydroxyl radicals, (3) complete ozonation (direct + indirect oxidation), and (4) ozone + UV. To determine the nature of ozone attacks and the influence of ·OH radicals on O3 activity, two sets of experiments were performed: (i) conventional ozonation and (ii) the same ozonation experiments in the presence of tert-butanol as radical scavenger, where only the reactions involving molecular ozone are present. The explored variables were (i) ozone concentration, (ii) incident radiation rate at the reactor windows, (iii) reaction pH, and (iv) the presence of radical scavengers. Major intermediates of BBP degradation were identified. Degradation kinetics was correctly modeled by a pseudo-second-order kinetic model based on the sum of all the effects occurring during the treatment. The corresponding kinetic constants were obtained, and the relative contributions of each of the considered subsystems were evaluated.

Application of a montmorillonite clay modified with iron in photo-Fenton process. Comparison with goethite and nZVI.

Iron pillared clay (Fe-PILC) was prepared from a montmorillonite and was characterized by scanning electron microscopy and X-ray fluorescence. Fe-PILC catalytic activity was evaluated in photo-Fenton processes applied to the degradation of 2-clorophenol. Different catalyst loads were assayed. The Fe-PILC allowed almost complete degradation of the contaminant. An increase in the contaminant degradation rate was observed, following leaching of iron during catalytic assays, which suggest the existence of a homogeneous photo-Fenton mechanism. The catalytic performance of the Fe-PILC was compared with that for goethite and zero valent iron nanoparticles. Differences were found regarding the achieved degradation levels, the efficiency in oxidant consumption, and the extension of iron leaching.

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 (III) oxidation of water applying a combination of hydrogen peroxide and UVC radiation.

Arsenic is toxic to both plants and animals and inorganic arsenicals are proven carcinogens in humans. The oxidation of As(III) to As(v) is desirable for enhancing the immobilization of arsenic and is required for most arsenic removal technologies. The main objective of this research is to apply an Advanced Oxidation Process that combines ultraviolet radiation and hydrogen peroxide (UVC/H(2)O(2)) for oxidizing aqueous solutions of As(III). For that purpose, a discontinuous photochemical reactor (laboratory scale) was built with two 40 W tubular germicidal lamps (λ = 253.7 nm) operating inside a recycling system. The study was made beginning with a concentration of 200 µg L(-1) of As(III), changing the H(2)O(2) concentration and the spectral fluence rate on the reactor windows. Based on references in the literature on the photolysis of hydrogen peroxide, arsenic oxidation and our experimental results, a complete reaction scheme, apt for reaction kinetics mathematical modelling, is proposed. In addition, the effectiveness of arsenic oxidation was evaluated using a raw groundwater sample. It is concluded that the photochemical treatment of As(III) using H(2)O(2) and UVC radiation is a simple and feasible technique for the oxidation of As(III) to As(v).

Degradation of dichloroacetic acid in homogeneous aqueous media employing ozone and UVC radiation.

A tentative workable mechanism for dichloroacetic acid decomposition (DCA) in aqueous media employing ozone and UVC radiation has been developed. All experiments were made in a homogeneous medium under assured kinetic control regime. Under no circumstances did a headspace exist in the reactor volume. The starting point of the reaction with UVC radiation was always under the prerequisite of a confirmed state of initial equilibrium conditions for the mixture water-ozone-oxygen at 20 °C. The explored variables were: (i) DCA initial concentration, (ii) ozone concentration and (iii) fluence rate at the reactor window. The model comprises three parallel reactions: (1) direct photolysis, (2) direct ozonation and (3) ozone + UVC degradation. Complete DCA removal was achieved, and the mass balance, considering DCA disappearance and chloride ion formation, closed within very small error. The combination of ozone and UVC radiation produces a significant amount of hydrogen peroxide as an important reaction by-product. The direct photolysis can be well represented with a six step reaction sequence. The direct ozonation mechanism comprises 22 steps and, with the entire set of kinetic constants completed in this work, it is independent of the reaction pH in the range from 3 to 6.3. Lastly, the associated use of ozone and UVC radiation becomes necessary to consider the existence of radiation absorption by three species, namely DCA, ozone and hydrogen peroxide. The developed system, including the three parallel reactions, led to the proposal of a 37 step reaction mechanism. Finally the reaction kinetics, the mass balances and the radiation field corresponding to this complex system were rigorously modeled and the most significant features of the mathematical representation are briefly described. The simulation results rendered from this model agree very well with the measured experimental data. This outcome will be essential for deriving a complete reactor model that must be appropriate to describe, in the future, the more practical two-phase operating system.

The heterogeneous photo-Fenton reaction using goethite as catalyst.

In the present work the degradation of 2-chlorophenol (2-CP) used as model compound, applying the Heterogeneous photo-Fenton reaction, was studied. Small particles of goethite or iron oxyhydroxide were used as a source of iron. The influence of catalyst loading, radiation intensity and the molar ratio between hydrogen peroxide and contaminant were examined. Improvement by illumination is highly significant. During the progress of 2-CP degradation, the reaction shows an unusual acceleration. This autocatalytic comportment, with stronger tendencies at higher temperatures, implies a completely different behaviour from the one typically expected. The autocatalytic performance is successfully explained by the joint action of two factors: (i) the evolution of the available iron in the homogeneous phase during the course of the reaction and (ii) the autocatalytic contribution of some of the reaction intermediates in the iron cycle. The small iron concentration leaching into the solution is produced by two typical liquid medium - solid goethite surface dissolution processes. A reaction mechanism has been proposed and, in a first stage, parameters have been obtained for the dark reaction. In a second step, the complete data for the irradiated operation were obtained.

Kinetic modelling of the photocatalytic inactivation of bacteria.

This work analyzes the kinetic modelling of the photocatalytic inactivation of E. coli in water using different types of kinetic models; from an empirical equation to an intrinsic kinetic model including explicit radiation absorption effects. Simple empirical equations lead to lower fitting errors, but require a total of 12 parameters to reproduce the results of four inactivation curves when the catalyst concentration was increased. Moreover, these parameters have no physical meaning and cannot be extrapolated to different experimental conditions. The use of a pseudo-mechanistic model based on a simplified reaction mechanism reduces the number of required kinetic parameters to 6, being the kinetic constant the only parameter that depends on the catalyst concentration. Finally, a simple modification of a kinetic model based on the intrinsic mechanism of photocatalytic reactions including explicit radiation absorption effects achieved the fitting of all the experiments with only three parameters. The main advantage of this approach is that the kinetic parameters estimated for the model become independent of the irradiation form, as well as the reactor size and its geometrical configuration, providing the necessary information for scaling-up and design of commercial-scale photoreactors for water disinfection.

The local and observed photochemical reaction rates revisited.

In a broad sense, photochemical reactions proceed through pathways involving several reaction steps. The initiation step is the absorption of energy both by the reactant or sensitizer molecules and in some cases, by the catalyst, leading to intermediate products that ultimately give rise to stable end products. Preferably, the reaction rate expression is derived from a proposed mechanism together with sound simplifying assumptions; otherwise, it may be adopted on an empirical basis. Under a kinetic control regime, the rate expression thus obtained depends on the local rate of photon absorption according to a power law whose exponent very often ranges from one half to unity. The kinetic expression should be valid at every point of the reactor volume. However, due to radiation attenuation in an absorbing and/or scattering medium, the value of the photon absorption rate is always a function of the spatial position. Therefore, the overall photochemical reaction rate will not be uniform throughout the entire reaction zone, and the distinction between local and volume average photochemical reaction rates becomes mandatory. Experimental values of reaction rates obtained from concentration measurements performed in well-mixed reaction cells are, necessarily, average values. Consequently, for validation purposes, experimental results from these cells must be compared with volume averages of the mechanistically or empirically derived local reaction rate expressions. In this work it is shown that unless the rate is first order with respect to the photon absorption rate or the attenuation in the absorbing and/or scattering medium is kept very low, when the averaging operation is not performed, significant errors may be expected.

Dichloroacetic acid degradation employing hydrogen peroxide and UV radiation.

The degradation reaction of dichloroacetic acid employing H(2)O(2) and UVC radiation (253.7nm) has been studied in a well mixed reactor operating inside a recycling system. It has been shown that in an aqueous solution no stable reaction intermediates are formed and, at every time during the reaction, two mols of hydrochloric acid are formed for every mol of dichloroacetic acid that is decomposed and, in the same way, there is a paired agreement between the calculated TOC concentration corresponding to the unaltered dichloroacetic acid and the experimental values measured in the solution. On this basis and classical references from the scientific literature for the H(2)O(2) photolysis, a complete reaction scheme, apt for reaction kinetics mathematical modeling and ulterior scale-up is proposed.

Rational design of photocatalytic reactors

Se presenta una breve descripción de diferentes reactores fotocatalíticos, así como los principales problemas asociados con esta tecnología. Finalmente, el trabajo se centra en modelos basados en los fundamentos de la ingeniería de las reacciones químicas aplicadas a reactores sólido-gas y sólido-líquido