• Controls On Antimony And Arsenic Speciation Via Sorption And Redox Chemistry At The Clay Mineral - Water Interface In Natural And Laboratory Settings

      Ilgen, Anastasia Gennadyevna; Trainor, Thomas P. (2010)
      Adsorption and redox transformations of contaminants in soil and aqueous environments are often controlled by the available mineral substrates. Aluminosilicates and aluminum oxides are ubiquitous and can influence the speciation and, therefore, the transport and bio-availability of toxic elements such as arsenic (As) and antimony (Sb). It is important to understand the partitioning and redox reactions promoted by these substrates in order to understand and model the transport of As and Sb in soils and surface waters. This study provides a detailed investigation of the sorption and redox behavior of As and Sb in clay-rich natural and laboratory systems. Since Fe 3+ is often found substituting for Al3+ in clay mineral structures, we also investigated the role of structural Fe in redox transformations of As and Sb adsorbed at the clay surface in controlled laboratory experiments. In a natural system affected by the release of spent geothermal fluids (Mutnovsky geothermal fields, Kamchatka, Russia), As concentrations are elevated above background levels in the Falshivaia River water and sediments (< 65 microm size fraction). Arsenic from the geothermal source fluids is in the reduced As3+ form, and is oxidized to As5+ after mixing with river water. Both As3+ and As5+ are found in aqueous and adsorbed forms. Analysis of the extended x-ray absorption fine structure (EXAFS) spectra shows that sediment-phase arsenic is associated with both Al- and Fe-rich phases with a bi-dentate corner-sharing local geometry. A series of laboratory experiments were performed in order to investigate Sb adsorption by Al-rich mineral substrates at a macroscopic and molecular level. The EXAFS analysis of the experimental samples concluded that both Sb3+ and Sb5+ form inner-sphere sorption complexes on the surfaces of hydrous aluminum oxide (HAO), and the clay minerals kaolinite (KGa-1b) and nontronite (NAu-1). Primarily, bi-dentate corner-sharing with a minor amount of mono-dentate corner-sharing complexes were formed. The oxidation state of the clay structural Fe affects the adsorption capacity of nontronite; if the clay is partially reduced, the Sb5+ uptake is increased significantly. The long term dynamics of the As aqueous speciation in clay suspensions where reduced arsenic (As3+) was added initially is complex. A fast disappearance of As3+ due to oxidation to As 5+ was followed by a slow increase of aqueous As3+. This behavior is explained by two simultaneous reactions: fast oxidation of As3+ by structural Fe3+ (anaerobic) or Fe 3+ and dissolved O2 (aerobic) and the slow reduction of As5+ by dissolved Fe2+. The ability of the structural Fe in nontronite clay to promote oxidation of As3+/Sb 3+ was greatly affected by its oxidation state: if all structural Fe was in an oxidized Fe3+ form, no oxidation was observed; however, when ~ 20 % of structural Fe was reduced to Fe2+, the clay promoted the most extensive oxidation under both aerobic and anaerobic conditions. The structural Fe2+ is not able to reduce As 5+/Sb5+, but reduction was seen when aqueous Fe 2+ was present in the systems. These research findings indicate that As and Sb can be effectively immobilized by Al-rich phases, while the substrate nature and oxidation state of structural Fe, along with the presence of dissolved Fe2+, can greatly affect the fate and transport of As and Sb. The increase in Sb5+ uptake in response to reducing structural Fe, possible increase or decrease in uptake of As due to As5+ reduction by aqueous Fe 2+, or oxidation of As3+ by clay structural Fe 3+, is likely to take place in a natural clay-rich soil or aquifer environment in moderate to slightly reducing conditions.
    • The Role Of Ice Surfaces In Affecting Nighttime Removal Of Nitrogen Oxides In High Latitude Plumes

      Huff, Deanna M.; Simpson, William (2010)
      Nitrogen oxides play an important role in the atmosphere by affecting ozone-mediated oxidation pathways. Nitrogen oxide removal from the atmosphere occurs via nitric acid formation. This nitric acid deposits to Earth's surface, leading to acidification and nitrogen fertilization. Under dark and cold conditions that commonly exist in the winter at high latitudes, nighttime reactions oxidize NO2 to the nitrate radical, NO3, and these molecules react to form N2O5. The heterogeneous hydrolysis of N2O5, which is catalyzed by surfaces, forms nitric acid. Modeling studies indicate that a majority of the Nx removal at high latitudes results from nighttime N2O5 chemistry. The N2O5 intermediate molecules may react on snowpack surfaces or on atmospheric particles. Past field studies demonstrated that aerosol surfaces are not solely responsible for the removal of N2O 5 near Earth's surface at high latitudes. In this work, we have used aerodynamic gradient micrometeorological methods to measure the deposition velocity of N2O5 to snowpack. This measurement is the first time that snowpack deposition has been quantified directly. We have found that snowpack deposition near Earth's surface at high latitudes is a significant chemical loss process for N2O5. Further studies demonstrated higher mixing ratios and longer lifetimes of N2 O5 aloft. Increasing N2O5 abundance and longevity with altitude implicates different loss mechanisms contribute at various altitudes in the atmosphere. Near Earth's surface, N2O 5 is very reactive, while aloft it acts more as a reservoir species that can transport further. Understanding the controlling mechanisms for N x removal under high latitude conditions will lead to better characterization of the NOx transport in pollution plumes and nitric acid deposition patterns.