ABSTRACT
In this study, we observed natural methane (CH4) hydrate sediments, which are a type of unconventional natural gas resources, using x-ray computed tomography (CT). Because CH4 hydrates are formed by hydrogen bonding of water molecules with CH4, material decomposition becomes challenging when CH4 hydrates coexist with liquid or solid water in natural sediments. Tri-contrast (absorption, refraction, and scattering) imaging was performed via diffraction enhanced x-ray CT optics using monochromatic synchrotron x rays. The quantitative characterization of the contrast changes successfully enabled the decomposition of CH4 hydrates coexisting with frozen seawater (ice) in natural sediments obtained from the Okhotsk Sea. This study reveals complementary structural information about the microtexture and spatial relation among CH4 hydrates, ice, and pores by utilizing the distinct physical properties of x rays when passing through the materials. These results highlight the exceptional capabilities of high-resolution multicontrast x-ray tomography in materials science and geoscience applications.
ABSTRACT
Molecular and stable isotope compositions of hydrate-bound gases collected from 59 hydrate-bearing sites between 2005 to 2019 in the southern and central sub-basins of Lake Baikal are reported. The δ2H of the hydrate-bound methane is distributed between - 310 and - 270, approximately 120 lower than its value in the marine environment, due to the difference in δ2H between the lake water and seawater. Hydrate-bound gases originate from microbial (primary and secondary), thermogenic, and mixed gas sources. Gas hydrates with microbial ethane (δ13C: - 60, δ2H: between - 310 and - 250) were retrieved at approximately one-third of the total sites, and their stable isotope compositions were lower than those of thermogenic ethane (δ13C: - 25, δ2H: - 210). The low δ2H of ethane, which has rarely been reported, suggests for the first time that lake water with low hydrogen isotope ratios affects the formation process of microbial ethane as well as methane. Structure II hydrates containing enclathrated methane and ethane were collected from eight sites. In thermogenic gas, hydrocarbons heavier than ethane are biodegraded, resulting in a unique system of mixed methane-ethane gases. The decomposition and recrystallization of the hydrates that enclathrate methane and ethane resulted in the formation of structure II hydrates due to the enrichment of ethane.
ABSTRACT
We reported the characteristics of hydrate-bound hydrocarbons in lake-bottom sediments at the Kedr mud volcano in Lake Baikal. Twenty hydrate-bearing sediment cores were retrieved, and methane-stable isotopes of hydrate-bound gases (δ13C and δ2H of - 47.8 to - 44.0 V-PDB and - 280.5 to - 272.8 V-SMOW, respectively) indicated their thermogenic origin accompanied with secondary microbial methane. Powder X-ray diffraction patterns of the crystals and molecular composition of the hydrate-bound gases suggested that structure II crystals showed a high concentration of ethane (around 14% of hydrate-bound hydrocarbons), whereas structure I crystals showed a relatively low concentration of ethane (2-5% of hydrate-bound hydrocarbons). These different crystallographic structures comprised complicated layers in the sub-lacustrine sediment, suggesting that the gas hydrates partly dissociate, concentrate ethane and form structure II crystals. We concluded that a high concentration of thermogenic ethane primarily controls the crystallographic structure of gas hydrates and that propane, iso-butane (2-methylpropane) and neopentane (2,2-dimethylpropane) are encaged into crystals in the re-crystallisation process.
Subject(s)
Fossil Fuels , Methane/chemistry , Carbon Dioxide/chemistry , Ethane/chemistry , Models, Molecular , Particle Size , X-Ray DiffractionABSTRACT
A fast and accurate analytical method was established for the simultaneous direct determination of aluminum, calcium and iron in silicon carbide and silicon nitride powders by graphite furnace atomic absorption spectrometry using a slurry sampling technique and a Hitachi Model Z-9000 atomic absorption spectrometer. The slurry samples were prepared by the ultrasonication of silicon carbide or silicon nitride powders with 0.1 M nitric acid. Calibration curves were prepared by using a mixed standard solution containing aluminum, calcium, iron and 0.1 M nitric acid. The analytical results of the proposed method for aluminum, calcium and iron in silicon carbide and silicon nitride reference materials were in good agreement with the reference values. The detection limits for aluminum, calcium and iron were 0.6 microg/g, 0.15 microg/g and 2.5 microg/g, respectively, in solid samples, when 200 mg of powdered samples were suspended in 20 ml of 0.1 M nitric acid and a 10 microl portion of the slurry sample was then measured. The relative standard deviation of the determination of aluminum, calcium and iron was 5 - 33%.
ABSTRACT
Isotope dilution analysis of the sub-microg l(-1) levels of selenite and selenate in natural water samples by microwave-induced nitrogen plasma mass spectrometry (MIP-MS) was performed. An appropriate amount of a spike solution containing 78Se-selenite and 78Se-selenate was added to the natural water sample to be analyzed. Both analytes in the water were then concentrated simultaneously by passing the sample through a column that was filled with an anionic exchange resin. After the concentration process, all of the selenite and some of the selenate on the resin were eluted by 0.03 M nitric acid. The residual selenate was eluted by 0.13 M nitric acid. The eluted sample solutions were injected into MIP-MS, and isotope dilution analyses were carried out. Selenite and selenate concentrations as low as 0.01 microg l(-1) in the natural water sample were successfully determined by the proposed method.
