Electrochemical generation of the Fenton's reagent: application to atrazine degradation.

The degradation of refractory chemicals in water requires chemical oxidation by hydroxyl radicals. Among the systems that may be used to generate OH(o), the Fenton's reagent consists of the mixing of ferrous iron and hydrogen peroxide. Even though this system is very simple, the oxidation of an organic compound is difficult to control and the ferrous iron regeneration is limited. Very recently, electrochemical systems have merged that allow the electrochemical production of ferrous iron and/or hydrogen peroxide, thereby allowing the generation of OH(o). So a simple electro-Fenton system has been used and tested for its efficiency in producing hydroxyl radicals. Atrazine was chosen as a model organic compound as its reaction with OH(o) has been extensively studied. Comparison with the classical Fenton system gives advantage to the electrochemical system, due to a more thorough oxidation of atrazine.

Determination of trihalomethanes in chlorinated sea water samples using a purge-and-trap system coupled to gas chromatography.

In power stations, the cooling effluents are chlorinated to avoid excessive biofouling. Yet, this disinfecting treatment leads to the formation of halogenated by-products, mainly trihalomethanes. So, there is a need for precise and accurate methods that allow trace levels determination of these compounds. A system that combines purge-and-trap and gas chromatography (with an electron capture detector) was used in this study. After careful choice of the experimental conditions, the performance of the system were evaluated. Precise and accurate determinations were obtained, allowing the determination of trihalomethanes in sea water samples chlorinated on site in three French coastal power stations. Bromoform was the predominant component formed, while traces of dibromochloromethane, chloroform and bromodichloromethane were also detected.

Determination of dihaloacetonitriles and halophenols in chlorinated sea water.

The cooling effluents in power stations are chlorinated to avoid excessive biofouling. However, this disinfecting treatment leads to the formation of halogenated by-products, mainly trihalomethanes, haloacetonitriles and halophenols. There is therefore a need for precise and accurate methods that allow trace level determinations of these compounds. Experimental procedures combining liquid-liquid extraction and gas chromatography coupled to mass spectrometry were developed in this study to determine dihaloacetonitriles and halophenols. The extraction step was improved and the performance of both methods evaluated. Precise and accurate determinations were obtained, allowing the measurement of the investigated compounds in chlorinated sea water samples from three French nuclear power stations.

Determination of traces of pesticides in water by solid-phase extraction and liquid chromatography-ionspray mass spectrometry.

A multi-residue analytical method for six pesticides (atrazine, hydroxyatrazine, carbofuran, promecarb, linuron and monolinuron) in drinking water has been developed. The method combines liquid chromatography and mass spectrometry using an ionspray interface. The linearity domain, as well as the limits of detection and quantification, were determined for each compound. Although satisfactory performance could be achieved, present drinking water regulations (0.1 microgram l-1 for single pesticide) requires a pre-concentration step. This was performed using solid-phase extraction with octadecyl-bonded silica cartridges. The analytical procedure was tested on water samples spiked at the 0.04 and 0.08 microgram l-1 levels, and allowed the determination of the investigated pesticides (except hydroxyatrazine) at these trace concentrations.

Biodegradation of p-nitrophenol in an aqueous waste stream by immobilized bacteria.

Microbiological analyses of activated sludge reactors after repeated exposure to 100 mg of p-nitrophenol (PNP) per liter resulted in the isolation of three Pseudomonas species able to utilize PNP as a sole source of carbon and energy. Cell suspensions of the three Pseudomonas sp., designated PNP1, PNP2, and PNP3, mineralized 70, 60, and 45% of a 70-mg/liter dose of PNP in 24, 48, and 96 h, respectively. Mass-balance analyses of PNP residues for all three cultures showed that undegraded PNP was less than 1% (less than 50 micrograms); volatile metabolites, less than 1%; cell residues, 8.4 to 14.9%; and water-soluble metabolites, 1.2 to 6.7%. A mixed culture of all three PNP-degrading Pseudomonas sp. was immobilized by adsorption onto diatomaceous earth biocarrier in a 1.75-liter Plexiglas column. The column was aerated and exposed to a synthetic waste stream containing 629 to 2,513 mg of PNP per liter at flow rates of 2 to 15 ml/min. Chemical loading studies showed that the threshold concentration for acute toxicity of PNP to the immobilized bacteria was 2,100 to 2,500 mg/liter. Further studies at PNP concentrations of 1,200 to 1,800 mg/liter showed that greater than 99 and 91 to 99% removal of PNP was achieved by immobilized bacteria at flow rates of 10 and 12 ml/min, respectively. These values represent hydraulic retention times of 48 to 58 min and PNP removal rates of 0.99 to 1.1 mg/h per g of biocarrier at 25 degrees C under optimal conditions. This study shows the successful use of immobilized bacteria technology to remove high concentrations of PNP from aqueous waste streams.

Estadística Médica / Medical Statistics

Este texto abarca dos materias bien diferenciadas: La Metodología Estadística y la Estadística Aplicada a la Salud Pública. La primera interesa al investigador en cualquier rama del saber, al trabajador de las más variadas actividades abstractas o concretas y no solamente al que se ocupa de salud individual o colectiva. Por su claridad, su coherencia y sencillez, llegará facilmente al destinatario. La segunda parte del texto está hecha para guiar la enseñanza de la estadística aplicada a la salud pública.