Engineering topics

Schwarz, Kaitlynn C.

Publications and source records attributed to Schwarz, Kaitlynn C..

3 records

DOE OSTI2020

Soil microbial EPS resiliency is influenced by carbon source accessibility

The adaptability of soil microbial communities to prolonged periods of drought is influenced by their ability to produce extracellular polymeric substances (EPS) with sufficient water retention properties. Microbial EPSs as water reservoirs during drought have been extensively investigated, but it remains unknown how carbon substrate accessibility to soil microbial communities will affect the chemical properties of the EPS they generate, and whether this in turn will alter their water retention ability. In this work, we observed that the accessibility of carbon substrates influenced microbial community structure and, consequently, the chemical properties of EPS produced by the microbial communities. Further, our results demonstrated that an insoluble carbon substrate (i.e., chitin), stimulated microbial communities to produce EPS with better water retention properties in comparison to a soluble carbon substrate (i.e., N-acetylglucosamine; NAG). In all, this study demonstrates the importance of carbon substrate accessibility by soil microorganisms in regulating the community structure and consequently, the EPS carbon chemistry, which in turn can greatly influence the adaptability of soil microbial communities to drought.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2020

Probing Pore Size and Connectivity in Porous Silicas Using 13C MAS NMR Spectroscopy of Supercritical Methane

Measuring the pore size and pore-size distributions and exploring the fluid-exchange dynamics between different types of pores in porous materials remains a significant experimental challenge but is critical to understanding catalysis, chromatography, nutrient cycling, and a whole range of geochemical phenomena, including shale gas and tight gas extraction. Here, we present the results of 1D 13 C NMR and 2D exchange spectroscopy (EXSY) NMR investigations of a porous silica using supercritical methane (scCH 4 ) as a direct probe of pore size and fluid exchange between pore types. The results show that the 13 C chemical shift of scCH 4 adsorbed in nanometer-scale silica pores becomes more negative with increasing pore diameter, in agreement with trends reported for gas hydrates, zeolites, MOFs, and clays and other microporous (<10 nm pores) geochemical materials. These 13 C chemical shifts follow a natural log–linear trend with pore size in the vacuum-dried porous silicas studied here, allowing one to predict pore size based on the 13 C chemical shift in dry silica nanopores. The EXSY and 1D 13 C NMR results both show that CH 4 exchanges between pore and bulk fluid environments over rate scales from 0.01 to 1 kHz in vacuum-dried samples and that exchange dynamics must be considered when interpreting 13 C NMR of pore-adsorbed CH 4 . Introducing H 2 O to the system causes the 13 C chemical shift to become more negative with increasing H 2 O content as a result of H 2 O preferentially filling small pores, forcing CH 4 to occupy larger pores that are better connected to the bulk environment. Likewise, the 1D 13 C data show a decreased distribution of exchange rates between pore-adsorbed and bulk CH 4 in the presence of H 2 O.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Evolution of Radicals from the Photolysis of High Ionic Strength Alkaline Nitrite Solutions

Reactive nitrogen species (RNS), along with reactive oxygen species (ROS), are significant products from radiolysis in solution. While much research has been focused on biological systems, these species are also important products in the autoradiolysis that occurs in nuclear waste. In this work, we determine the correlation between solution constituents, particularly nitrite, and radical products in highly alkaline solutions relevant to liquid waste. Because these radicals tend to be very short-lived, we employ spin trapping in conjunction with electron paramagnetic resonance (EPR) to detect them and quantify their production. Most spin traps do not function in these conditions (>1 M NaOH); however, nitroalkanes such as nitromethane will act as spin traps in their aci form, which is dominant at high pH. To restrict the products to those originating from nitrite, we use 280–480 nm UV light to generate radicals, avoiding products from the photolysis of water. Under these circumstances, nitric oxide, nitrite radicals, and hydroxyl radicals are detected, and the trends with the concentration of the constituents of the solutions are tracked. These include nitrite, nitrate, hydroxide, and carbonate. We find that, while the equilibrium shifts with increasing pH from hydroxyl radicals to the more slowly reacting oxide radicals, the production of nitrite radicals does not decrease.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗