Progress 08/01/07 to 11/30/08

Outputs
OUTPUTS: 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 TiO2 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. Soils with higher content of both titanium and calcium exhibit higher photoactivities. 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. Nitrate was produced over the temperature range of 50-200 degrees 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 degrees C for 2-50 hours or at 70 degrees 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. 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. Nitrate was also detected on soils heated at 70 degrees C suggesting that this process is likely occurring naturally on desert soils by the influence of sunlight heating. Consideration of these processes will likely help to explain the elevated nitrate levels observed in desert soils and groundwater which have been largely attributed to long-term atmospheric nitrate precipitation. PARTICIPANTS: We cooperated with Dr. CJ Bohlke at the USGS laboratory in Reston, Virginia. His lab graciously provided the isotope studies that supported the project. TARGET AUDIENCES: Not relevant to this project. PROJECT MODIFICATIONS: Not relevant to this project.

Impacts
This project identified a new mode for nitrogen fixation in the environment. Working with USGS scientists in Reston, Virginia, we were able to establish strong evidence for this novel process using nitrogen isotopes. We have applied for a patent for this process, working with the UNR patent attorneys. Thermal fixation of nitrogen can potentially be a very "green" method for producing nitrate fertilizers, since it does not require any natural gas.

Publications

• Al-taani, Ahmed, and Glenn C. Miller, "Thermal formation of nitrate on titanium dioxide and desert soils surfaces" A presentation at the National American Chemical Society Meetings in Philadelphia, PA, August, 2008.

Progress 01/01/07 to 12/31/07

Outputs
OUTPUTS: A novel process for non-biological fixation of nitrogen to form nitrate has been identified, based on thermal treatment of titanium dioxide (TiO2), anatase and rutile, in the presence of air or pure nitrogen gas. This finding was observed while preparing thin films of TiO2 by thermal evaporation of an aqueous suspension in Petri dishes. 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 degrees C following 2 hours of heating and gave yields that were linear with increases in temperature. Under the conditions employed, the yield of nitrate was a function of the area of the TiO2 on the Petri dish, and the thinnest films produced over 40 mg/kg following 2 hours of thermal treatment at 200 degrees C. At temperatures greater than 400 degrees C, formation of nitrate was also observed, although the rate of thermal degradation of nitrate tended to reduce the observed yields. Formation of minor amounts of nitrite was also observed. Successive heat treatment and aqueous extraction of the same TiO2 sample over 14 cycles resulted in effectively identical yields for each cycle, suggesting that this process is not a result of oxidation or release of nitrate that may have been contained in the TiO2. The pH of the final extracted TiO2 suspensions was lowered to approximately 3-5, depending on the amount of nitrate produced, consistent with the formation of nitric acid. Additions of stoichiometric amounts of sodium, potassium or calcium hydroxide increased the amount of nitrate observed. The mechanistic pathway by which NO3 is formed is unknown, though it is presumably occurred via catalytic oxidation of elemental N2 on heated TiO2 surface. Formation of nitrate was also observed on certain arid lands soils heated to 70 degrees C for 2-4 hours, although the rate of production was generally lower. This observation suggests that thermal fixation/formation of nitrate on arid lands soils may contribute to the elevated nitrate levels found in soils and groundwater in the western United States. PARTICIPANTS: Ahmed Al-taani, Glenn Miller, Viktoriya Lepak Weirach TARGET AUDIENCES: Agricultural and soil scientists regulatory agencies in the Western United States

Impacts
This project has identified a new, naturally occuring process for fixing atmospheric nitrogen using arid lands soils or titanium dioxide. This process is novel, and explains, in part, how high concentrations of nitrate are generated in desert soils. This has wide spread implications for understanding the enviornmental generation and fate of an important nutrient and contaminant in the western United States. While it still needs to be confirmed using esotope methods, it may well require a reassessment of nitrogen cycling in these arid systems.

Publications

• No publications reported this period