Relative proportions of organic carbon functional groups in biochars as influenced by spectral data collection and processing.

Solid-state 13C Nuclear Magnetic Resonance (NMR) and synchrotron-based X-ray Absorption Near-Edge Structure (XANES) have applications for determining the relative proportions of organic C functional groups in materials. Spectral data obtained by NMR is typically processed using integration (INTEG) whereas XANES spectral data is typically processed using deconvolution (DECONV). The objective of this study was to examine the impact of spectral data collection and processing on the estimated relative proportions of organic C functional groups in biochars. Biochars showed large variations in aromatic C (45-97%), alkyl C (0-23%), O-alkyl C (1-41%), phenolic C (0-20%) and carboxylic C (0-20%). NMR had a better ability than XANES to differentiate % aromatic C across biochars, and the mean % aromatic C was always greater for NMR-INTEG and NMR-DECONV than for XANES-INTEG or XANES-DECONV. NMR-INTEG showed significant associations with NMR-DECONV and XANES-INTEG for % aromatic C and alkyl C, but there were no significant associations between NMR and XANES for % O-alkyl C, phenolic C and carboxylic C. As well, there was no association between NMR-INTEG and XANES-DECONV for any organic C functional group, and in some cases, spectral data collection and processing influenced the quantification of organic C functional groups in a given biochar to the extent that the differences observed were as large as differences observed between biochars when analyzed using the same spectral data collection and processing technique. We conclude that great caution must be taken when comparing studies that determined organic C functional groups in materials using NMR-INTEG versus XANES-DECONV.

Nitrogen speciation and transformations in fire-derived organic matter.

Vegetation fires are known to have broad geochemical effects on carbon (C) cycles in the Earth system, yet limited information is available for nitrogen (N). In this study, we evaluated how charring organic matter (OM) to pyrogenic OM (PyOM) altered the N molecular structure and affected subsequent C and N mineralization. Nitrogen near-edge X-ray absorption fine structure (NEXAFS) of uncharred OM, PyOM, PyOM toluene extract, and PyOM after toluene extraction were used to predict PyOM-C and -N mineralization potentials. PyOM was produced from three different plants (e.g. Maize-Zea mays L.; Ryegrass-Lollium perenne L.; and Willow-Salix viminalix L.) each with varying initial N contents at three pyrolysis temperatures (350, 500 and 700 °C). Mineralization of C and N was measured from incubations of uncharred OM and PyOM in a sand matrix for 256 days at 30 °C. As pyrolysis temperature increased from 350 to 700 °C, aromatic C[bond, double bond]N in 6-membered rings (putative) increased threefold. Aromatic C[bond, double bond]N in 6-membered oxygenated ring increased sevenfold, and quaternary aromatic N doubled. Initial uncharred OM-N content was positively correlated with the proportion of heterocyclic aromatic N in PyOM (R2 = 0.44; P < 0.0001; n = 42). A 55% increase of aromatic N heterocycles at high OM-N content, when compared to low OM-N content, suggests that higher concentrations of N favor the incorporation of N atoms into aromatic structures by overcoming the energy barrier associated with the electronic and atomic configuration of the C structure. A ten-fold increase of aromatic C[bond, double bond]N in 6-membered rings (putative) in PyOM (as proportion of all PyOM-N) decreased C mineralization by 87%, whereas total N contents and C:N ratios of PyOM had no effects on C mineralization of PyOM-C for both pyrolysis temperatures (for PyOM-350 °C, R2 = 0.15; P < 0.27; for PyOM-700 °C, R2 = 0.22; P < 0.21). Oxidized aromatic N in PyOM toluene extracts correlated with higher C mineralization, whereas aromatic N in 6-membered heterocycles correlated with reduced C mineralization (R2 = 0.56; P = 0.001; n = 100). Similarly, aromatic N in 6-membered heterocycles in PyOM remaining after toluene extraction reduced PyOM-C mineralization (R2 = 0.49; P = 0.0006; n = 100). PyOM-C mineralization increased when N atoms were located at the edge of the C network in the form of oxidized N functionalities or when more N was found in PyOM toluene extracts and was more accessible to microbial oxidation. These results confirm the hypothesis that C persistence of fire-derived OM is significantly affected by its molecular N structure and the presented quantitative structure-activity relationship can be utilized for predictive modeling purposes.

Fire-derived organic matter retains ammonia through covalent bond formation.

Fire-derived organic matter, often referred to as pyrogenic organic matter (PyOM), is present in the Earth's soil, sediment, atmosphere, and water. We investigated interactions of PyOM with ammonia (NH3) gas, which makes up much of the Earth's reactive nitrogen (N) pool. Here we show that PyOM's NH3 retention capacity under ambient conditions can exceed 180 mg N g-1 PyOM-carbon, resulting in a material with a higher N content than any unprocessed plant material and most animal manures. As PyOM is weathered, NH3 retention increases sixfold, with more than half of the N retained through chemisorption rather than physisorption. Near-edge X-ray absorption fine structure and nuclear magnetic resonance spectroscopy reveal that a variety of covalent bonds form between NH3-N and PyOM, more than 10% of which contained heterocyclic structures. We estimate that through these mechanisms soil PyOM stocks could retain more than 600-fold annual NH3 emissions from agriculture, exerting an important control on global N cycling.

Validating the scalability of soft X-ray spectromicroscopy for quantitative soil ecology and biogeochemistry research.

Synchrotron-based soft-X-ray scanning transmission X-ray microscopy (STXM) has the potential to provide nanoscale resolution of the associations among biological and geological materials. However, standard methods for how samples should be prepared, measured, and analyzed to allow the results from these nanoscale imaging and spectroscopic tools to be scaled to field scale biogeochemical results are not well established. We utilized a simple sample preparation technique that allows one to assess detailed mineral, metal, and microbe spectroscopic information at the nano- and microscale in soil colloids. We then evaluated three common approaches to collect and process nano- and micronscale information by STXM and the correspondence of these approaches to millimeter scale soil measurements. Finally, we assessed the reproducibility and spatial autocorrelation of nano- and micronscale protein, Fe(II) and Fe(III) densities in a soil sample. We demonstrate that linear combination fitting of entire spectra provides slightly different Fe(II) mineral densities compared to image resonance difference mapping but that difference mapping results are highly reproducible between among sample replicates. Further, STXM results scale to the mm scale in complex systems with an approximate geospatial range of 3 µm in these samples.

Aqueous Cu(II)-organic complexation studied in situ using soft X-ray and vibrational spectroscopies.

In situ aqueous solutions containing copper-ligand mixtures were measured at the Cu L-edge using X-ray absorption near edge structure (XANES) and with attenuated total reflectance infrared (ATR-FTIR) spectroscopies. Copper complexation with environmentally relevant ligands such as EDTA, citrate, and malate provided a bridge between spectroscopic studies and general environmental behavior and will allow for future study of complex environmental samples. XANES results show that the lowest unoccupied molecular orbital (LUMO) energy is governed by the ligand field strength and is related to Lewis acid/base properties of the ligand functional groups. Complementary ATR-FTIR studies confirmed the importance of water molecules in the structure of these Cu-ligand complexes and provided in-depth structural analysis to support the XANES data. Copper-malate is shown to have a 5/6-O-ring structure, and Cu-ethylenediaminetetraacetate has pentadentate coordination. Cu L-edge XANES also revealed direct Cu-N coordination in these aqueous solutions with amide functional groups.

Oxygen reduction electrocatalyst based on strongly coupled cobalt oxide nanocrystals and carbon nanotubes.

Electrocatalyst for oxygen reduction reaction (ORR) is crucial for a variety of renewable energy applications and energy-intensive industries. The design and synthesis of highly active ORR catalysts with strong durability at low cost is extremely desirable but remains challenging. Here, we used a simple two-step method to synthesize cobalt oxide/carbon nanotube (CNT) strongly coupled hybrid as efficient ORR catalyst by directly growing nanocrystals on oxidized multiwalled CNTs. The mildly oxidized CNTs provided functional groups on the outer walls to nucleate and anchor nanocrystals, while retaining intact inner walls for highly conducting network. Cobalt oxide was in the form of CoO due to a gas-phase annealing step in NH(3). The resulting CoO/nitrogen-doped CNT (NCNT) hybrid showed high ORR current density that outperformed Co(3)O(4)/graphene hybrid and commercial Pt/C catalyst at medium overpotential, mainly through a 4e reduction pathway. The metal oxide/carbon nanotube hybrid was found to be advantageous over the graphene counterpart in terms of active sites and charge transport. Last, the CoO/NCNT hybrid showed high ORR activity and stability under a highly corrosive condition of 10 M NaOH at 80 °C, demonstrating the potential of strongly coupled inorganic/nanocarbon hybrid as a novel catalyst system in oxygen depolarized cathode for chlor-alkali electrolysis.

Calibration method at the N K-edge using interstitial nitrogen gas in solid-state nitrogen-containing inorganic compounds.

The standard method of soft X-ray beamline calibration at the N K-edge uses the nu = 0 peak transition of gas-phase N(2). Interstitial N(2) gas trapped or formed within widely available solid-state ammonium- and amine-containing salts can be used for this purpose, bypassing gas-phase measurements. Evidence from non-nitrogen-containing compounds (KH(2)PO(4)) and from He-purged ammonium salts suggest that production of N(2) gas is through beam-induced decomposition. Compounds with nitrate or nitrite as anions produce coincident features and are not suitable for this calibration method.