Abinash Agrawal (Advisor), Songlin Cheng (Committee Member), Christina Powell (Committee Member)
Master of Science (MS)
The biogeochemical cycling of nitrogen in terrestrial systems is a major source of nitrous oxide (N2O), which is among key greenhouse gases (GHG). While biotic processes are commonly accepted as the major drivers of N2O production, the role of abiotic processes is less understood, and their importance may be underestimated. This study investigates abiotic reactions in the formation and breakdown of N trace gases, including nitric oxide (NO) and nitrous oxide (N2O) by naturally occurring nanoscale magnetite in soil as denitrification (reduction of nitrite) intermediates. Nitrogen biogeochemical cycling has been studied extensively with respect to microbial processes, atmospheric pollution and greenhouse gases. In addition to being potent greenhouse gases, NO catalyzes ozone production in the troposphere and N2O contributes to the destruction of ozone in the stratosphere.
Iron oxyhydroxides such as lepidocrocite were reported to reduce nitrite and nitrate to NO and N2O. Likewise, Magnetite's power to reduce pollutants like carbon tetrachloride (CT) and nitrobenzenes was also demonstrated in past works, suggesting that magnetite may have the ability to reduce nitrite and nitrate. In this study, the potential of chemogenic magnetite nanoparticles to abiotically reduce nitrite and nitrate to other nitrogen species (i.e. NO, N2O, N2 and NH3) was investigated in bench-scale batch reactors by characterizing reaction kinetics and quantifying various product mole fractions. The study focused on mass-fractions of NO and N2O that may be produced from nitrite and nitrate with magnetite.
The results confirm that chemogenic magnetite was capable of rapidly degrading nitrite into N2O with some N2 exhibiting pseudo first order reaction kinetics. Results show that 1.16 g L-1 (5 mM) magnetite in batch experiments under anaerobic conditions at pH 7 with no Fe2+ degraded almost all of 0.025 millimoles of nitrite resulting in about 50% N2O-N in about 2 days. N2O-N production was reduced by increases in pH and the amount of magnetite used. Ammonia was produced under basic conditions and N2 gas yields increased under basic conditions and in the presence of aqueous Fe(II). The presence of aqueous Fe(II) also increased the rate of the reaction such that nearly all of 0.025 millimoles of nitrite were removed within two to three hours. NO became a major product when the initial rate constant of the reaction (kobs) was low or when the magnetite was insufficient to degrade the nitrite that was present. Fe(II) additions degraded the NO and the reaction continued until the NO concentration stabilized again, suggesting a relationship between NO concentration and denitrification reaction. Increasing magnetite concentration increased kobs. Although nitrate has been said to be reactive in some cases, the reaction of nitrate to magnetite was nearly negligible in this investigation. Any reaction that did take place appeared to have only N2 as a product.
Magnetite reactions toward nitrite and nitrate are newly reported and the implications of this redox system are not yet clearly indicated. However, it is suggested that in interface zones of Fe3+ reduction that may form magnetite, denitrification of nitrite may take place, especially in areas where farm practices include excessive fertilization.
Department or Program
Department of Earth and Environmental Sciences
Year Degree Awarded
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