An official website of the United States government
Abstract During aerobic oxidation of methane (CH4) in seawater, a process which mitigates atmospheric emissions, the12C‐isotopologue reacts with a slightly greater rate constant than the13C‐isotopologue, leaving the residual CH4isotopically fractionated. Prior studies have attempted to exploit this systematic isotopic fractionation from methane oxidation to quantify the extent that a CH4pool has been oxidized in seawater. However, cultivation‐based studies have suggested that isotopic fractionation fundamentally changes as a microbial population blooms in response to an influx of reactive substrates. Using a systematic mesocosm incubation study with recently collected seawater, here we investigate the fundamental isotopic kinetics of aerobic CH4oxidation during a microbial bloom. As detailed in a companion paper, seawater samples were collected from seep fields in Hudson Canyon, U.S. Atlantic Margin, and atop Woolsey Mound (also known as Sleeping Dragon) which is part of lease block MC118 in the northern Gulf of Mexico, and used in these investigations. The results from both Hudson Canyon and MC118 show that in these natural environments isotopic fraction for CH4oxidation follows a first‐order kinetic process. The results also show that the isotopic fractionation factor remains constant during this methanotrophic bloom once rapid CH4oxidation begins and that the magnitude of the fractionation factor appears correlated with the first‐order reaction rate constant. These findings greatly simplify the use of natural stable isotope changes in CH4to assess the extent that CH4is oxidized in seawater following seafloor release.
more »
« less
Investigations of Aerobic Methane Oxidation in Two Marine Seep Environments: Part 1—Chemical Kinetics
Abstract Microbial aerobic oxidation is known to be a significant sink of marine methane (CH4), contributing to the relatively minor atmospheric release of this greenhouse gas over vast stretches of the ocean. However, the chemical kinetics of aerobic CH4oxidation are not well established, making it difficult to predict and assess the extent that CH4is oxidized in seawater following seafloor release. Here we investigate the kinetics of aerobic CH4oxidation using mesocosm incubations of fresh seawater samples collected from seep fields in Hudson Canyon, U.S. Atlantic Margin and MC118, Gulf of Mexico to gain a fundamental chemical understanding of this CH4sink. The goals of this investigation were to determine the response or lag time following CH4release until more rapid oxidation begins, the reaction order, and the stoichiometry of reactants utilized (i.e., CH4, oxygen, nitrate, phosphate, trace metals) during CH4oxidation. The results for both Hudson Canyon and MC118 environments show that CH4oxidation rates sharply increased within less than one month following the CH4inoculation of seawater. However, the exact temporal characteristics of this more rapid CH4oxidation varied based on location, possibly dependent on the local circulation and biogeochemical conditions at the point of seawater collection. The data further suggest that methane oxidation behaves as a first‐order kinetic process and that the reaction rate constant remains constant once rapid CH4oxidation begins.
more »
« less
- Award ID(s):
- 1756947
- PAR ID:
- 10456388
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 124
- Issue:
- 12
- ISSN:
- 2169-9275
- Page Range / eLocation ID:
- p. 8852-8868
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Microbial aerobic methane oxidation is an important sink for aquatic methane worldwide. Despite its importance to global methane fluxes, few aerobic methane oxidation rates have been obtained in freshwater or marine environments without imposing changes to the microbial community through use of ex situ methods. A novel in situ incubation method for continuous time‐series measurements was used in Jordan Lake, North Carolina, during 2020–2021, to determine reaction kinetics for aerobic methane oxidation rates across a wide range of naturally varying methane (55–1833 nM) and dissolved oxygen (DO; 28–366 μM) concentrations and temperatures (17–30°C). Methane oxidation began immediately at the start of each of 21 incubations and methane oxidation rates were 1storder with respect to methane. The data density allowed for accurate calculation of 1st‐order rate constants,k, that ranged from 0.018 to 0.462 h−1(R2 > 0.967). Addition of ammonium (20–45 μM) to natural concentrations ranging from 0.057 to 2.4 μM did not change aerobic methane oxidation rate kinetics, suggesting that the natural population of aerobic methane oxidizers in this eutrophic lake was not nitrogen limited. Values ofkinversely correlated most strongly with initial DO concentrations (R2 = 0.82) rather than temperature. Values forkincreased with Julian day throughout our sampling period, suggesting seasonal influences on methane oxidation via responses to geochemical changes or shifts in microbial community abundance and composition. These experiments demonstrate a high variability in the enzymatic capacity for 1st‐order methane oxidation rates in this eutrophic lake that is tightly and inversely coupled to oxygen concentrations. Measurements of in situ aerobic methane oxidation rate constants allow for the direct quantification and modeling of the microbial community's capacity for methane oxidation over a wide range of natural methane concentrations.more » « less
-
Abstract The complex structure of the catalytic active phase, and surface‐gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na2WO4/SiO2catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H2‐TPR, TAP and steady‐state kinetics) experiments, that the long speculated crystalline Na2WO4active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na‐WOxsites. Kinetic analysis via temporal analysis of products (TAP) and steady‐state OCM reaction studies demonstrate that (i) surface Na‐WOxsites are responsible for selectively activating CH4to C2Hxand over‐oxidizing CHyto CO and (ii) molten Na2WO4phase is mainly responsible for over‐oxidation of CH4to CO2and also assists in oxidative dehydrogenation of C2H6to C2H4. These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na2WO4/SiO2catalysts.more » « less
-
Electrocatalytic Methane Functionalization with d 0 Early Transition Metals Under Ambient ConditionsAbstract The undesirable loss of methane (CH4) at remote locations welcomes approaches that ambiently functionalize CH4on‐site without intense infrastructure investment. Recently, we found that electrochemical oxidation of vanadium(V)‐oxo with bisulfate ligand leads to CH4activation at ambient conditions. The key question is whether such an observation is a one‐off coincidence or a general strategy for electrocatalyst design. Here, a general scheme of electrocatalytic CH4activation with d0early transition metals is established. The pre‐catalysts’ molecular structure, electrocatalytic kinetics, and mechanism were detailed for titanium (IV), vanadium (V), and chromium (VI) species as model systems. After a turnover‐limiting one‐electron electrochemical oxidation, the yielded ligand‐centered cation radicals activate CH4with low activation energy and high selectivity. The reactivities are universal among early transition metals from Period 4 to 6, and the reactivities trend for different early transition metals correlate with their d orbital energies across periodic table. Our results offer new chemical insights towards developing advanced ambient electrocatalysts of natural gas.more » « less
-
Abstract Although most class (b) transition metals have been studied in regard to CH4activation, divalent silver (AgII), possibly owing to its reactive nature, is the only class (b) high‐valent transition metal center that is not yet reported to exhibit reactivities towards CH4activation. We now report that electrochemically generated AgIImetalloradical readily functionalizes CH4into methyl bisulfate (CH3OSO3H) at ambient conditions in 98 % H2SO4. Mechanistic investigation experimentally unveils a low activation energy of 13.1 kcal mol−1, a high pseudo‐first‐order rate constant of CH4activation up to 2.8×103 h−1at room temperature and a CH4pressure of 85 psi, and two competing reaction pathways preferable towards CH4activation over solvent oxidation. Reaction kinetic data suggest a Faradaic efficiency exceeding 99 % beyond 180 psi CH4at room temperature for potential chemical production from widely distributed natural gas resources with minimal infrastructure reliance.more » « less
