Showing papers by "F. Javier Benitez published in 2007"

Kinetics of phenylurea herbicides oxidation by Fenton and photo‐Fenton processes

Abstract: The chemical oxidation of four selected phenylurea herbicides (linuron, chlortoluron, diuron, and isoproturon) was studied by means of the Fenton system. The influence of the initial concentrations of hydrogen peroxide and ferrous ions, the pH and the type of buffer (perchloric acid/perchlorate, acetic acid/acetate, or phosphoric acid/phosphate) was established according to the degradation levels obtained. In the kinetic study, the general decomposition reaction was divided into two stages with different reaction rates, which was justified by considering the whole reaction mechanism for this system. In this kinetic study, a competition kinetics model, which used p-chlorobenzoic acid as a reference compound, was applied for the evaluation of the rate constants for each reaction between the herbicides and the hydroxyl radical. The proposed values for these rate constants are: 7.5 × 109 L mol−1 s−1 for chlortoluron, 5.6 × 109 L mol−1 s−1 for linuron, 7.1 × 109 L mol−1 s−1 for diuron and 5.7 × 109 L mol−1 s−1 for isoproturon. Finally, some experiments with the photo-Fenton system reveal increases in the decomposition levels of the herbicides, due to additional generation reactions of hydroxyl radicals. Copyright © 2007 Society of Chemical Industry

Removal of diazinon by various advanced oxidation processes

Abstract: Diazinon is a widely used organophosphorus insecticide that is an important pollutant in aquatic environments. The chemical removal of diazinon has been studied using UV radiation, ozone, Fenton's reagent, UV radiation plus hydrogen peroxide, ozone plus hydrogen peroxide and photo-Fenton as oxidation processes. In the photodegradation process the observed quantum yields had values ranging between 2.42 × 10−2 and 6.36 × 10−2 mol E−1. Similarly, the ozonation reaction gave values for the rate constant ranging between 0.100 and 0.193 min−1. In the combined systems UV/H2O2 and O3/H2O2 the partial contributions to the global oxidation reaction of the direct and radical pathways were deduced. In the Fenton's reagent and photo-Fenton systems, the mechanism of reaction has been partially discussed, and the predominant role of the radical pathway pointed out. Additionally, the rate constant for the reaction between diazinon and the hydroxyl radicals was determined, with the value 8.4 × 109 L mol−1 s−1 obtained. A comparison of the different oxidation systems tested under the same operating conditions revealed that UV radiation alone had a moderate oxidation efficiency, which is enhanced in the case of ozone, while the most efficient oxidant is the photo-Fenton system. Copyright © 2007 Society of Chemical Industry

Kinetics of reactions between chlorine or bromine and the herbicides diuron and isoproturon

Abstract: The chemical oxidation of two herbicide derivatives of the phenylurea group—diuron and isoproturon—has been carried out by means of chlorine, in the absence and in the presence of bromide ion. Apparent second-order rate constants for the reactions between chlorine and the herbicides were determined to be below 0.45 L mol−1 s−1. Hypobromous acid reacts faster with the investigated herbicides, especially with isoproturon (kapp = 24.8 L mol−1 s−1 at pH 7). While pH exerts a negative effect on the bromination rate, the maximum chlorination rate was found to be at circumneutral pH. In a second stage, the oxidation of each compound was conducted in different natural waters, in order to simulate the processes which take place in water purification plants. Again, chlorine was used as an oxidant, and bromide ion was added in some experiments with the aim of producing the more reactive HOBr oxidant. The herbicide oxidation rate was inversely proportional to the organic matter content of the natural water. However, the formation of trihalomethanes (THMs) was directly proportional to the organic matter content and constitutes a limitation for the application of chlorine during drinking water treatment. Finally, the evolution of herbicide concentration was modeled and predicted by applying a kinetics approach based on the rate constants for the reactions between the herbicides and the active oxidants. Copyright © 2007 Society of Chemical Industry

Elimination of benzene and chlorobenzenes by photodegradation and ozonation processes

Abstract: Benzene (B) and two representative chlorobenzenes (1,4-dichlorobenzene (DCB) and 1,2,3-trichlorobenzene (TCB)) were oxidized by means of UV irradiation alone, ozone alone, and the combinations UV/H2O2 and O3/H2O2. In the single photolytic process, the influence on the photodegradation of the pH, temperature, and type of radiation source used was established. A kinetic study was performed by evaluating the first-order rate constants and the quantum yields. The effect of the additional presence of hydrogen peroxide was pointed out in the combined process UV/H2O2,with the determination of the specific contribution of the radical pathway to the overall photodegradation system. In the oxidation by ozone based systems (ozone alone and the combination O3/H2O2), the rate constants at 20°C for the reaction of each compound with ozone and hydroxyl radicals were determined.