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Accueil du site → Doctorat → États-Unis → 2008 → Non-biological fixation of atmospheric nitrogen to nitrate on titanium dioxide and desert soil surfaces

University of Nevada, Reno (2008)

Non-biological fixation of atmospheric nitrogen to nitrate on titanium dioxide and desert soil surfaces

Al-Taani, Ahmed A.

Titre : Non-biological fixation of atmospheric nitrogen to nitrate on titanium dioxide and desert soil surfaces

Auteur : Al-Taani, Ahmed A.

Université de soutenance : University of Nevada, Reno

Grade : Doctor of Philosophy (PhD) 2008

Résumé
Elevated nitrate levels have frequently been observed in soils and the associated groundwater in arid regions of the U.S, many of which are distant from anthropogenic sources. Although these elevated nitrate concentrations have generally been linked to atmospheric precipitation, the current study indicates that at least a portion of these nitrates may have been formed through photochemical and thermal transformation reactions on soil surfaces. Photochemical nitrogen fixation to nitrate was observed on pure TiO 2 (both anatase and rutile) and desert soil surfaces when exposed to sunlight from 2 to 80 days. The yields of nitrate were generally proportional to irradiation time and increased substantially when sodium hydroxide was added. Larger surface films of soils or TiO2 generated higher yields of nitrate. Soils with higher content of both titanium and calcium exhibit higher photoactivities, and the production rate varied slightly with particle size. Traces of nitrite and ammonia detected on irradiated TiO2 surface were similar to background levels, and are probably not intermediates in the formation of nitrate. TiO2 and soils obtained from Atacama Desert in northern Chile and Pyramid Lake, NV were irradiated with sunlight for 32 days in either 15N labeled or unlabeled nitrogen and produced nitrates enriched in 15N and that nearly all isotopic values were higher than that of atmospheric 15N. Nitrate produced photochemically on Atacama Desert soils have isotopic values that are similar to those of the subsoil nitrates of the Atacama Desert. During our experimental investigation and while preparing thin films of TiO2 by thermal evaporation of an aqueous suspension in Petri dishes, we consistently observed an increase in nitrate concentrations in all samples (even the controls) whenever TiO2 slurries came in contact with heat and air. An expanded series of experiments was carried out in a conventional oven in the absence of light ; photocatalytic reactions are not involved. Nitrate was produced over the temperature range of 50-200°C following 2 hours of heating and gave yields that were linear with increases in temperature. Nitrate formation was also observed on certain arid land soils thermally treated in the normal atmosphere at 200°C for 2-50 hours or at 70°C for 15 hours or one week, although the rate of nitrate formation varied with different soils. Under the conditions employed, the yield of nitrate was a function of the area of the TiO2 or soils on the Petri dish. Formation of minor amounts of nitrite was also observed. Nitrate yields were produced in approximately equal amounts following a series of successive cycles of heating and extraction of the same soil fractions or TiO2 material indicating that the measured nitrate concentrations are not a result of soil nitrate release. Soils from Atacama Desert and Pyramid Lake have shown higher thermal activities and produced larger yields of nitrate than that measured for other soils tested. Additions of stoichiometric amounts of sodium, potassium or calcium hydroxide increased the amount of nitrate observed on both TiO 2 and on soils. Nitrates generated thermally on TiO2 or on soils from Pyramid Lake and from Atacama Desert have been enriched in 15N when heated in 5ml of 15N labeled nitrogen. The majority of d15N values of nitrate produced on TiO2 or on soil surfaces heated in air have d 15N ratios larger than that of atmospheric N. The isotopic composition of nitrate formed on heated soil surface has values similar to those observed for desert subsoil nitrates and linked to atmospheric processes. Nitrate was also detected on soils heated at 70°C suggesting that this process is likely occurring naturally on desert soils by the influence of sunlight heating. Consideration of these processes will likely raise the question on the origin of subsoil nitrate in arid and semiarid land and potentially help to explain the elevated nitrate levels observed in desert soils and groundwater which have been largely attributed to long-term atmospheric nitrate precipitation.

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