Radical hydroxyle | EPFL Graph Search

Le radical hydroxyle est le radical libre de formule chimique HO. Utilisation Il apparaît le plus souvent de façon transitoire dans les réactions radicalaires, souvent amorcées par des amorceurs radicalaires, par exemple en polymérisation. Il peut être stabilisé dans certains solvants aprotiques. Production Il est produit par exemple lors de réactions de dissociation de peroxydes, en présence d'ions de métaux de transition (Fe(II), Cu(I), réaction de Fenton), de chauffage ou de rayonnement. Dans l'atmosphère Ce radical hydroxyle (HO) apparaît spontanément dans la vapeur d'eau atmosphérique sous l'effet du rayonnement solaire en présence de certains polluants tels que l'ozone et le protoxyde d'azote. Mais le radical HO est si réactif qu'il disparait presque instantanément. Ce caractère éphémère rend très difficile la mesure de l'HO. On le cherche donc via des proxies, des produits chimiques interagissant avec l'hydroxyle ;

Iron-oxides and iron-citrate as new photocatalysts in solar inactivation of Escherichia coli in water

Cristina del Socorro Lonfat

This study addresses the bacterial inactivation mechanism by photo-Fenton process at near-neutral pH, focusing on iron-oxides and iron-citrate as photocatalysts for solar water disinfection and using E. coli as a bacteria model. Cell envelope damage during bacterial inactivation by photo-Fenton and TiO2 photocatalysis were investing providing evidence for lipid peroxidation and cell permeability. TiO2 photocatalysis induced significant cell membrane damage, in contrast to the photo-Fenton process, but the inactivation kinetics for both disinfection processes was similar. A higher efficiency of photo-generation of reactive oxygen species (ROS) in the presence of TiO2 photocatalyst compared with the photo-Fenton system was observed. The bactericidal effect of Fe3+/hv seems possible due to the adsorption of Fe3+ ions on the bacterial cell wall followed by photosensitization of iron-bacteria exciplexes oxidizing the cell membrane. In contrast, the effect of Fe2+/hv was associated with diffusion into the cell giving raise to intracellular dark Fenton¿s reactions. We suggest that cell envelope damage might not necessarily be a unique pathway in bacterial inactivation by photo-Fenton treatment. In particular, the enhancement of an internal (photo)-Fenton process by the synergistic action of UVA and the external Fenton's reactants appears to be an important contribution to bacterial inactivation. Bacterial inactivation by the heterogeneous photo-Fenton process was carried out via iron (hydr)oxide particles, i.e. hematite, goethite, wüstite and magnetite. We found that, the iron (hydr)oxides act as photocatalytic semiconductors and catalysts in the heterogeneous photo-Fenton process with the exception of magnetite, which needs H2O2 as electron acceptors. The Hydroxyl radical and superoxide radical were the principal ROS produced by iron (hydr)oxide particles under light in the absence or presence of H2O2. Natural organic matter (NOM) and inorganic substances did not interfere with the photocatalytic semiconducting action of hematite during bacterial inactivation, but enhanced bacterial inactivation mediated by hematite used as the photo-Fenton reagent. Our results demonstrated, for the first time, that low concentration of iron (hydr)oxides (0.6 mg/L) under sunlight, acting both as semiconductors or catalysts of the heterogeneous photo-Fenton process, may serve as a disinfection method for waterborne bacterial pathogens. Bacterial inactivation by the homogeneous photo-Fenton process was carried out using Fe¿citrate complex as a source of iron. The efficiency of the homogeneous photo-Fenton process using Fe-citrate complex strongly improved bacterial inactivation as compared with the FeSO4 and goethite as sources of iron. The bacterial inactivation rate increased in the order of goethite < FeSO4 < Fe-citrate, which agreed with the ¿OH radicals detected by ESR. Encouraging results were also obtained while applying this treatment for bacterial inactivation in natural water samples at pH 8.5. No bacterial reactivation and/or growth were observed showing that Fe-citrate-based photo-Fenton process efficiently inactivate bacteria using a low iron concentration of Fe-citrate, while avoiding precipitation of ferric hydroxides. The application of the photo-Fenton process at near-neutral pH is a promising technique for bacterial inactivation, due to its simplicity, the use of the sun, the low concentration of reagents and does not produce toxic waste.

EPFL2015

Comparison of methylisoborneol and geosmin abatement in surface water by conventional ozonation and an electro-peroxone process

Chao Chen, Urs von Gunten, Gang Yu

In this study methylisoborneol (MIB) and geosmin abatement in a surface water by conventional ozonation and the electro-peroxone (E-peroxone) process was compared. Batch tests with addition of ozone (03) stock solutions and semi-batch tests with continuous O-2/O-3 gas sparging (simulating real ozone contactors) were conducted to investigate O-3 decomposition, center dot OH production, MIB and geosmin abatement, and bromate formation during the two processes. Results show that with specific ozone doses typically used in routine drinking water treatment (0.5-1.0 mg O-3/mg dissolved organic carbon (DOC)), conventional ozonation could not adequately abate MIB and geosmin in a surface water. While increasing the specific ozone doses (1.0-2.5 mg O-3/mg DOC) could enhance MIB and geosmin abatement by conventional ozonation, this approach resulted in significant bromate formation. By installing a carbon-based cathode to electrochemically produce H2O2 from cathodic oxygen reduction, conventional ozonation can be conveniently upgraded to an E-peroxone process. The electro-generated H2O2 considerably enhanced the kinetics and to a lesser extent the yields of hydroxyl radical (center dot OH) from O-3 decomposition. Consequently, during the E-peroxone process, abatement of MIB and geosmin occurred at much higher rates than during conventional ozonation. In addition, for a given specific ozone dose, the MIB and geosmin abatement efficiencies increased moderately in the E-peroxone (by similar to 8-9% and similar to 10 25% in the batch and semi-batch tests, respectively) with significantly lower bromate formation compared to conventional ozonation. These results suggest that the E-peroxone process may serve as an attractive backup of conventional ozonation processes during accidental spills or seasonal events such as algal blooms when high ozone doses are required to enhance MIB and geosmin abatement. (C) 2016 Elsevier Ltd. All rights reserved.

Elsevier2017

Pretreatment options for micropollutant abatement before aquifer recharge in drinking water production: Assessment of UV/H2O2 and dense membranes

Robin Wünsch

Micropollutants (MP) such as residues of pharmaceuticals, industrial chemicals or pesticides can be detected in almost all water resources. Various processes can be used to abate them in drinking water treatment, e.g., ozonation, advanced oxidation processes (AOP), adsorption on activated carbon or membrane filtration. The selection of a suitable process combination is a complex task for water suppliers.This thesis compares two process chains in the context of a multi-barrier system including managed aquifer recharge (MAR). To protect soils and aquifers from MP in the future, an additional barrier against MP upstream of the MAR was investigated: (1) a full-stream treatment using the AOP UV/H2O2, and (2) a side-stream treatment with low-pressure reverse osmosis (LPRO) or nanofiltration (NF) and ozonation of the retentate to treat the concentrated MPs.Pilot-scale experiments were conducted to determine the relative abatements of MP by UV/H2O2 treatment and by a subsequent soil column treatment. Compared to a soil column fed with water without UV/H2O2 pretreatment, the performance of the combined process (UV/H2O2 + soil column) was more efficient. However, this could be explained by an additive effect of the individual processes.Relative abatements in the UV/H2O2 process are mainly based on the introduced UV fluence and hydroxyl radical exposure. These parameters could be calculated with a model based on the measured relative abatements of two MPs (probe compounds) and their kinetic data (second-order rate constants for the reactions with hydroxyl radical, absorbance and quantum yield).With this information, the relative abatements of other MPs could be predicted with an accuracy of ±20%. A sensitivity analysis of the model showed that the relative abatements of the selected probe compounds should be >50%. In this case, the accuracy of the calculated parameters depends mainly on the precision of the kinetic data.In the case of membrane-based treatment of water by LPRO or NF, a concentrate is produced. Ozonation to treat MP in the concentrate is accompanied by a formation of the possibly carcinogenic bromate by oxidation of bromide. To achieve the highest possible MP abatement with simultaneously low bromate formation, both membrane selection and subsequent concentrate ozonation were investigated. In laboratory tests with standardized concentrates, neither the water source nor the membrane type caused a change in bromate yield for similar relative MP abatements. However, NF membranes have lower relative retentions of bromide and MP than LPRO membranes, making NF concentrates more suitable for ozonation with limited bromate formation.A comparison of UV/H2O2 with membrane treatment showed that full-stream treatment with UV/H2O2 has about four times lower costs and an about five times lower environmental impact. In addition it achieves significantly higher disinfection. In contrast, side-stream treatment with LPRO causes up to two times lower impacts for the environment when the electrical energy source is wind or hydropower. This study thus contributes technical, economic, and environmental aspects to a holistic evaluation of the two process options.

EPFL2022