Origine du manganèse de la nappe alluviale de Beaucaire (Gard, France) Essai de démanganisation in situ (procédé Vyredox)

L'origine et le mécanisme de l'augmentation de la concentration en manganèse dans l'eau de la nappe phréatique de Beaucaire ont été recherchés afin d'orienter le choix d'un procédé de démanganisation adapté aux conditions du site.L'aquifère est caractérisé par son déficit en oxygène et par la présence de dépôts d'oxyde de manganèse (Mn(IV)) sur les sédiments. Dans ces conditions la microflore utilise les oxydes de manganèse conne accepteur final d'électrons (démontré dans les expériences in vitro) et le manganèse réduit passe alors en solution.Lors des essais d'application du procédé Vyredox le potentiel d'oxydoréduction de la nappe augmente et la concentration en manganèse dissous diminue. Rien n'indique une précipitation du manganèse et donc un colmatage à la périphérie de la partie oxygénée de la nappe.The origin and the mechanism of the increase of the dissolved manganese concentrations have been investigated to choose a system of demanganization well fitted b the site conditions.The aquifer characteristics are a depletion of the dissolved oxygen amount and deposits of manganese oxides (Mn(IV)) on the detriments. Under these conditions the microflora utilizes these manganese oxides as a final electron acceptor (as demonstrated by in vitro experiments) and the solubilization of manganese takes place.The field application of the Vyredox system increases the redox potential of the groundwater and the manganese concentration decreases. Nothing indicates that manganese precipitation and consequently warping of the water table takes place at the periphery of the oxygenated part of the aquifer

Oscillatory behavior of Pd-Au catalysts in toluene total oxidation

In this work, the activity of bimetallic Pd-Au doped hierarchically structured titania catalysts has been investigated in the total oxidation of toluene. In earlier works, doping titania with group Vb metal oxides ensured an increased catalytic performance in the elimination of VOC molecules. A synergy between gold and palladium loaded at the surface of titania supports provided better performances in VOC oxidation reactions. Therefore, the main focus in this work was to investigate the durability of the prepared catalysts under long time-on-stream periods. Vanadium-doped catalysts showed a stable activity throughout the whole 110 h test, whereas, surprisingly, niobium-doped catalysts presented a cycle-like activity while nevertheless maintaining a high performance in toluene elimination. Operando Diffuse Reflectance Infrared Fourrier Transform spectroscopy (DRIFT) experiments revealed that variations in the presence of OH radicals and the presence of carbonaceous compounds adsorbed at the surface of spent catalysts varies with the occurrence of oscillations. X-ray Photoelectron Spectroscopy (XPS) results show that interactions between the material and the active phase provided extra amounts of mobile oxygen species and participated in easing the reduction of palladium. An enhanced redox reaction scheme is thus obtained and allows the occurrence of the cyclic-like performance of the catalyst