Dégradation photochimique d'herbicides chlorophenoxyalcanoïques en milieux aqueux

La dégradation photochimique de cinq herbicides appartenant à la famille des chlorophénoxyalcanoïques a été étudiée en solution aqueuse par irradiation à 254 nm selon trois systèmes : UV seul, UV/H2O2 et UV/H2O2/ FeIII (photo-Fenton). Le procédé photochimique semble constituer une alternative prometteuse aux méthodes existantes de traitement chimique des eaux polluées; en effet il permet de détruire photochimiquement l'herbicide initial et d'obtenir, dans les conditions opératoires initiales, sa minéralisation complète en CO2 et H2O. Il s'agit d'un procédé d'oxydation avancé, utilisant comme agent oxydant, des radicaux OH. produits in situ photochimiquement. L'évolution de la composition chimique des solutions d'herbicides étudiés a été suivie par chromatographie liquide à haute performance (CLHP). La minéralisation a été évaluée par mesure de la demande chimique en oxygène (DCO) et par le dosage des ions chlorures libérés. La cinétique de photodégradation, la nature et l'évolution des produits formés ainsi que le rendement du procédé ont été déterminés.Contamination of surface and ground waters by persistent organic pollutants constitutes a serious environmental problem. A number of physical and biological methods have been proposed to remove these pollutants from industrial wastewater. However, many organic contaminants are not destroyed by these techniques. Various chemical treatment methods for polluted waters have been proposed. These methods are based on catalytic, electrochemical and photochemical reactions, known as advanced oxidation processes (AOPs). However, despite these treatments, there is presently no universal technique available. Because of their widespread agricultural use, chlorophenoxyacid herbicides contaminate waterways and ground waters in France and many other European countries.In the present study, we have investigated the photochemical degradation of several chlorophenoxyacids in aqueous solution at room temperature, by ultraviolet (UV) irradiation at 254 nm. We compared the efficiency of three different systems: UV alone; assisted photochemistry (UV/H2O2); and photo-Fenton reaction (UV/H2O2/FeIII). The latter photochemical system was found to represent a promising alternative approach, relative to existing methods of polluted water chemical treatment. The method photochemically destroyed the initial herbicides into CO2 and H2O. Also, it constitutes an AOP based on the in situ photochemical formation of OH· radicals. The evolution of the chemical composition of the herbicide samples studied was monitored by high performance liquid chromatography (HPLC). The mineralization of the initial herbicides was evaluated by the measurement of the chemical oxygen demand (COD) and the determination of chloride ions. The photodegradation kinetics, the nature and evolution of the photoproducts as well as the process yield were studied for the three photochemical systems for five different chlorophenoxyacid herbicides, including 2-(2-methyl-4-chlorophenoxy)propionic acid (MCPP), 4-chloro-2-methylphenoxyacetic acid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4-D), 2-(2,4-dichlorophenoxy)propionic acid (2,4-DP) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T).Photodegradation by UV alone was relatively slow and partial, taking place in 1 h for MCPA, 1.5 h for MCPP and 3 h for 2,4-D. This process did not completely destroy the photoproducts. The mineralization yields for the organic matter were 64 %, for MCPA, 69 % for MCPP, 42 % for 2,4-DP and 35 % for 2,4-D, following an irradiation time of 2 h. The technique based on the photolysis of hydrogen peroxide (UV/H2O2) produced a more rapid photodecomposition, occurring within about 30 min for MCPA and MCPP and more than 60 min for 2,4-D and 2,4,5-T. The corresponding mineralization yields for the organic matter were 79 % for MCPP and 2,4-DP and 56 % for 2,4-D. The absolute rate constants for the reaction with hydroxyl radicals were found to be1.5×109, 1.6×109, 3.2×109 and 3.6×109 M-1 s-1 respectively for 2,4,5-T, 2,4-DP, MCPP and MCPA using this technique.The photo-Fenton system significantly improved the kinetic performance and mineralization yield. The photodegradation times were 7 min for MCPA, 10 min for MCPP, 40 min for 2,4-D and 60 min for 2,4,5-T, whereas the mineralization yields reach respective values of 96 % for MCPP, 95 % for MCPA, 80 % for 2,4-D, 94 % for 2,4-DP and 89 % for 2,4,5-T

Remediation of PAH-Contaminated Soils: Experimental Analysis and Modeling of Hydrodynamics and Mass Transfer in a Soil-Slurry Bioreactor

Extended Abstract Polycyclic Aromatic Hydrocarbon (PAHs)-contaminated soils are a great environmental and public health concern nowadays. Their remediation is an important field of research as several remediation techniques have been developed with the purpose of removing PAHs from soil. However, further researches are necessary to develop environmental friendly biotechnologies that allows public and private sectors to implement efficient and adaptable treatments for contaminated soils. Aerobic soil-slurry bioreactor technology has emerged as one of these technologies with high potential as an effective and feasible treatment technic for PAH-contaminated soils. For this treatment, soil is excavated, conditioned and loaded into an aerated aqueous bioreactor. Then, mechanical and/or pneumatic mixing maintains aerobic conditions and homogeneity. Furthermore, air supply and mixing represent the most energy intensive units Although, extensive research has been done on this topic, mechanisms involved in the removal of PAHs from soil are still not completely understood. In addition to the biological processes involved, important mass transfer mechanisms need to be considered (oxygen gas-liquid mass transfer, adsorption-desorption, volatilization of PAH, etc.). In general, even for volatile PAHs, volatilization is not considered in the studies whereas, in some conditions (high aeration rate), it can be a major mechanism of "PAH removal". The soil composition and concentration in the reactor should influence strongly the fluid viscosity, which is a key parameter governing the hydrodynamics and thus the mass transfer phenomena. Therefore, the aeration and mixing optimization requires a fine understanding of how different operational parameters influence the mixing and mass transfer mechanisms involved in the removal of PAHs from soil In this study, the influence of soil content (composition and concentrations) and operating conditions (air superficial velocity, stirring rate, etc.) on the mixing (rheology, etc.) and mass transfer phenomena (gas-liquid, adsorption-desorption) is addressed. Experiments are performed in a glass standard bioreactor designed to control hydrodynamic conditions and temperature. Air is injected from the bottom through a porous glass sparger. Mechanical agitation is performed by a marine propeller connected to a motor. Hydrodynamic parameters are monitored in order to study their influence on the process and, particularly on the oxygen and PAH transfer phenomena. Rheological behavior of soil/water matrix has been measured with a capillary rheometer The oxygen transfer tests showed that for a given air superficial velocity and stirring rate, the oxygen transfer coefficient in soil/water matrix is reduced in comparison with clean water results. This decrease depends on the soil composition and was more pronounced with an increase in the soil content. Moreover, the soil/water matrix could be assimilated to a non-Newtonian fluid with shear-thinning behavior (mainly pronounced for high soil content). The impact

Metomyl degradation by electro-Fenton and electro-Fenton like processes: A kinetics study of the effect of the nature and concentration of some transition metal ions as catalyst.

International audienc

Mineralisation of acephate by electro Fenton process (Minéralisation de l'acéphate par procédé electro-Fenton).

International audienc

Cold incineration of chlorophenols in aqueous solution by electro-Fenton process. Effect of number and position of chlorine atoms on the degradation kinetics

International audienc