Formation des ions bromate dans une colonne à bulles: Effets du peroxyde d'hydrogène lors de l'ozonation

L'utilisation de l'ozone, aujourd'hui très répandue dans les filières de potabilisation, n'est pas sans effet secondaire. De nombreux sous-produits peuvent se former comme notamment les ions bromates, sous produits finaux d'oxydation des bromures contenus dans les eaux. Malheureusement, le mécanisme de production de cette espèce est complexe et dépend de nombreux paramètres difficiles à appréhender.Sur une installation pilote de type colonne à bulles fonctionnant à contre-courant, nous avons étudié l'influence de différents paramètres, comme le pH, le temps de contact, la dose d'ozone et la dose de peroxyde d'hydrogène, sur la formation des bromates et la dégradation des pesticides, représentée par l'atrazine.Les résultats de la littérature ont été confirmés lors de l'emploi unique de l'ozone. La formation des ions bromate est influencée par la présence du peroxyde d'hydrogène. Cet oxydant intervient de manière non négligeable sur la consommation des entités intermédiaires. Le couple HOBr/OBr- peut être oxydé par l'ozone moléculaire et le radical OH° mais peut également être réduit par l'ozone et par le peroxyde sous sa forme acide ou sa base conjuguée. En ce qui concerne la dégradation des pesticides, l'utilisation de peroxyde d'hydrogène couplé à l'ozone favorise l'oxydation de la molécule d'atrazine grâce à la présence plus importante de radicaux hydroxyles.Une pollution accidentelle en pesticides pourra être traitée par l'ajout ponctuel de peroxyde d'hydrogène avec une augmentation de pH, la formation des bromates sera, dans ce cas, faible. La désinfection sera alors assurée par l'étape de chloration.In drinking water treatment plants, ozonation is often used to disinfect, to remove micropollutants and to improve water taste and odour. Ozonation increases organic matter biodegradability before filtration through granular active carbon and reduces the concentration of haloform precursors that react in the final chlorination step. However, by-products that could be detrimental to human health could be formed. For example, bromates, which are classified as carcinogenic compounds by the I.A.R.C, are produced during the ozonation of bromide-containing water. The mechanism of bromate formation is complex, due to the participation of molecular ozone and radical (hydroxyl and carbonate) reactions. The optimisation of the process should allow for a good disinfection and a reduction in the levels of micropollutants, together with low by-product formation.Using a pilot-scale counter-current bubble column, we have measured the bromate concentration in relation to pesticide removal. Water spiked with bromide and atrazine was stored in a completely stirred-tank (2 m3) before being pumped to the top of the column. The inlet gaseous ozone was measured by an analyser using UV detection, the outlet gaseous ozone was monitored by the potassium iodide method, and the dissolved ozone concentration was determined by the indigo trisulfonate method. Bromides and bromates were quantified by ion chromatography with a conductimetric detector, with a sodium carbonate solution as the eluant. Samples for bromate analysis were pretreated by OnGuard-Ag and OnGuard-H cartridges in series before injection. Atrazine degradation was measured by high performance liquid chromatography with a diode array detector, with a CH3CN/H2O mixture as the eluant. The linearisation of atrazine removal allowed us to calculate the hydroxyl radical concentration in a series of a completely-stirred tank reactors and in a plug-flow reactor.We have studied the influence of several parameters on bromate formation, including pH, bromide concentration and hydrogen peroxide concentration. As bromate production is a function of bromide concentration, we have chosen to calculate the ratio between the real bromate concentration and the theoretical bromate concentration if all bromide were oxidised to bromate. The pH affects bromate formation: an increase in pH in the absence of hydrogen peroxide increases bromate production, but when this oxidant is applied bromate production decreases when the pH increases. If reaction progress is represented as a function of [O3]*TC, we note that the presence of hydrogen peroxide increases bromate formation because of the increase in hydroxyl radical concentration, which favours radical formation. Nevertheless, if we represent reaction progress as a function of [OH∘]*TC, hydrogen peroxide seems to be an initiator and a scavenger in the mechanism of bromate formation. If we calculate the rates of all the oxidation and reduction reactions for HOBr/OBr- species, the contribution to the reduction of HOBr/OBr- species by peroxide is very important in comparison to the oxidation reactions, which inhibits bromate production. Without the hydrogen peroxide, the contribution of oxidation is equal to that of the reduction reaction, and in this case bromate formation is effective. When, under the same initial operational conditions, we apply hydrogen peroxide with an increase in pH, we observe a decrease in bromate formation with a decrease of the dissolved ozone concentration, which hinders the desired disinfection. The main contribution to atrazine oxidation is from the free-radical reactions, which explains why removal is better when we apply hydrogen peroxide than when we use ozone alone. However, if we want to respect a low bromate level in drinking water, atrazine degradation should not be greater than 90% for the operational conditions on our pilot-scale.If an accidental high pesticide concentration is observed, an addition of hydrogen peroxide with a concurrent increase of pH, could treat the pollution. In this case, a subsequent chlorination step would then have to be used to assure the disinfection alone

Étude du traitement et du recyclage des eaux issues des serres horticoles

La gestion de l'eau dans les systèmes de culture hors-sol fait apparaître deux problèmes distincts. D'une part, les ressources en eau doivent être de bonne qualité et ne pas contenir de pesticides ou de germes pathogènes. D'autre part, les rejets fortement " chargés " en nutriments (NO3-, PO43-) polluants pour l'environnement, doivent être limités par le biais de leur recyclage ce qui implique nécessairement la désinfection des effluents.La technique mise en œuvre pour obtenir cette maîtrise de la qualité tant chimique que microbiologique des solutions circulantes en culture hors-sol est celle d'une oxydation à l'ozone seul et couplé au peroxyde d'hydrogène dans des réacteurs constitués de mélangeurs statiques. Les conditions de traitement sont une dose d'oxydant de 10 g O3/m3 d'effluent à traiter, un rapport H2O2/O3 de 0,15 g/g pour un temps de contact dans le réacteur de l'ordre de la seconde. Etudié sur site dans le cadre du traitement de effluents de serre réels, le procédé s'est révélé tout à fait adapté pour abattre les pesticides (# 90 % pour l'atrazine), maîtriser la prolifération des micro-organismes (Flore aérobie mésophile, flore fongique) et en particulier des germes pathogènes (Clavibacter michiganensis, Fusarium, Pythium sp ).Le procédé novateur O3/H2O2 sur mélangeurs statiques constitue donc pour les serristes une réponse nouvelle dont l'un des intérêts est de combiner les effets " détoxiquant " et désinfectant.The management of water resources in soil-less cultures presents two difficulties. On one hand, the quality of these resources has to be good, that is to say without pesticides or pathogens. On the other hand, the effluents contain high concentrations of nutrients (NO3-, PO43-), damageable for the environment, and should be recycled. Thus, recycling has to include necessarily a disinfection step to satisfy the quality requirement. The main disinfection treatments used in soil-less cultures are slow sand filtration, ultraviolet treatment, heat treatment, nanofiltration, ozone or hydrogen peroxide oxidation, iodine or chlorine treatment.In order to control the chemical as well as the microbiological quality of the recycled nutrient solution, we suggest oxidation (O3) and advanced oxidation (O3/H2O2) processes, carried out in static mixers as chemical reactors instead of bubble columns. We have been studying this process in situ for the treatment of a 1-hectare greenhouse. The pilot plant unit can be configured under three setups (Figure 2) according to the aim to favor either the molecular action of ozone or the formation of very reactive radical species such as the hydroxyl radical. In this second case, the mechanism of ozone decomposition is given by Figure 1.The first step of the study was to measure the influence of the nutrient solution to be recycled on the efficiency of atrazine removal (Figures 3 and 4). In comparison with tap water, the percentage of pesticide removal is lower by about 10 to 20 %. Solutions with nutrients do not drastically change the process efficiency. The experiments were carried out with various ozone dosages and various ozone / hydrogen peroxide mass ratios, using the three configurations (Figures 5 and 6). With these results, the best operating conditions for micropollutant removal are a treatment rate of about 10 g O3 /m3 of treated solution, a H2O2/O3 ratio equal to 0.15 g/g and a contact time in the reactor in the range of 1 to 2 seconds. The influence of the configuration type is not really marked. The results show that, under these conditions, this technique leads to good pesticide removal efficiencies (about 90 % for atrazine).In a second step, experiments were carried out on real solutions containing microorganisms from the greenhouse, sometimes spiked with special bacteria (Clavibacter) or fungi (Fusarium). Some results are reported in Figures 7, 8 and 9. With the same oxidant dosage conditions, the role of the configuration is clearly demonstrated. The best results are obtained with a molecular action of ozone in the first static mixed reactor followed by a free-radical action within the second reactor. Thus, it is possible to prevent germ proliferation (aerobic mesophilic flora and fungi flora) and particularly pathogenic species. The abatement of Clavibacter michiganensis reaches 3.5 to 4 logarithmic units, 1 to 1.5 units for Pythium and 2 to 4 units for Fusarium. The treatment does not effect a complete sterilization, e.g., the beneficial bacterium Pseudomonas fluorescens survives. The global impact of the treatment on the nutritive quality of the treated solution is negligible. Nevertheless, we can note that the process induces a decrease of the ion concentrations of Fe (II) (- 5 to 30 %) and Mn (II) (-10 to 15 %) as a result of the oxidation of the EDTA chelate. In fact, this problem is observed with all oxidation and UV treatments. The residual oxidant (O3, H2O2) concentrations are low and do not induce obvious toxic effects on the cultures.Thus, the technique is consistent with a recycling of the treated effluents. The advantages of the process include very short contact times, compactness of the equipment, no need for pretreatment, reasonable investment and operating costs, an increase of the oxygen concentration in the treated effluent, and possible curative effects on the culture's germ contamination due to the residual concentration of hydrogen peroxide. The disinfection efficiency of this suggested process is similar to those obtained with more common techniques like UV irradiation. Moreover, the studied process can also reduce, for example, an eventual chemical pollution of the water resource. In conclusion, the O3, H2O2 process in static mixers appears to be a new solution for greenhouse farmers

Redox properties of the iron-sulfur clusters in activated Fe-hydrogenase from Desulfovibrio vulgaris (Hildenborough)

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65750/1/j.1432-1033.1992.tb17261.x.pd

Photocatalytic degradation of pesticides in pure water and a commercial agricultural solution on TiO2 coated media

International audienc

N-nitrosamines, emerging disinfection by-products of health concern: an overview of occurrence, mechanisms of formation, control and analysis in water

International audienceThe presence of N-nitrosamines in water bodies used for drinking water purposes may present a more serious risk for humans than regulated disinfection byproducts (DBPs) species. Hence, understanding and controlling the incidence of N-nitrosamines represents a contemporary challenge to the water industry. Although many of these mols. potentially formed as DBPs are detected in chlorinated natural waters, few studies have focused on the formation, occurrence, and anal. of N-nitrosamines. Until now, nine N-nitrosamines have been detected in water samples; N-nitrosodimethylamine is the most frequently reported nitrosamine in drinking water. Although there are currently no federal regulations for these mols. in drinking water, this family of N-DBPs is one of three potential groups of contaminants highlighted for possible regulatory action in the near future. This paper gives an overview of the current knowledge concerning the occurrence, precursors, and formation mechanisms of N-nitrosamines in water. In addn., the existing regulations are described and relevant anal. methods used for their quantification in water samples are also discussed

Adsorption of pesticides onto granular activated carbon: determination of surface diffusivities using simple batch experiments

International audienceThe Homogeneous Surface Diffusion Model (HSDM) has been successfully used to predict the adsorption kinetics for several chemicals inside batch adsorber vessels. In addition to the adsorption equilibrium, this model is based on external mass transfer and surface diffusion. This paper presents the determination of the surface diffusion coefficient (Ds) using a differential column batch reactor (DCBR). The adsorption kinetics for three pesticides onto granular activated carbon have been established experimentally. Their corresponding three diffusion coefficients were determined by fitting the computer simulations to the experimental concentration-time data. The results showthat this original apparatus increases by an order of magnitude the range of reachable diffusion coefficient compared to perfectly mixed contactors. Moreover the computed Ds values are more accurate because of the better assessment of the external mass transfer coefficient (kf) for fixed beds