In this study, fluorine and carbon compounds were added to the sample to
enhance the CO gas production during thermal decomposition. First,
following previous studies 11, 12,
polytetrafluoroethylene (PTFE) was used as the fluorine source. The
following inorganic fluorides were also tested: KF, NaF, LiF,
CaF2, AlF3 (FUJIFILM Wako Pure Chemical
Corporation, 99%, 99%, 98.0%, 99.9%, 31.0–34.0 (as Al) % purity,
respectively), and BaF2 (Kojundo Chemical Laboratory
Co., Ltd., 99% purity). Graphite powder (5–11 μm of particle size, AS
ONE Corporation) was used as a carbon source. Granular nickelized carbon
(50 wt% Ni, Elemental Microanalysis Co., Ltd.) was also tested instead
of graphite to enhance the reaction. This carbon has been nickelized and
is henceforth referred to as Ni/C. Ni/C is activated carbon doped with
metallic Ni nanoparticles; the doped Ni’s catalytic function is thought
to speed up the pace at which gas is produced as organic matter breaks
down 17.
2.2. Methods
2.2.1. Pre-treatment for clay
samples
Pre-treatment for montmorillonite samples were performed by the
following method: To remove impurities such as quartz and feldspar, the
clay fraction was extracted from the bentonite ores (TKN-01, -29, and
-31). The samples were dispersed in deionized water and placed in an
ultrasonic bath to aid in disintegration. After this, the samples were
separated into the < 2 μm size fraction by centrifugation
based on Stoke’s Law. Finally, the clay fractions were freeze-dried. The
appropriate size fraction was determined using powder X-ray diffraction,
and no impurities were observed in any fraction.
In order to obtain representative δ18O values for clay
minerals, removing adsorbed water molecules is necessary. The majority
of the adsorbed water linked to smectite is bound to the interlayer at
ambient temperature and humidity levels 18. In this
study, montmorillonite samples were ion-exchanged with
K+, which has a low hydration enthalpy. The smectite
saturated with K+ was the least hygroscopic compared
to other exchangeable cations (Ca2+,
Na+) and provided a more accurate H isotope ratio in
the structure 19. In this study, K+saturation for the montmorillonite samples was achieved by dispersion in
a 2M KCl solution, shaking for 15 min, and solid-liquid separation by
centrifugation. This procedure was repeated three times to produce
homoionic K+-saturated forms of each sample. After
saturation, the samples were dispersed in deionized water and shaken for
15 min to remove excess salt before solid-liquid separation. This
procedure was repeated three times. The samples were then freeze-dried
and gently ground into a fine powder using an agate mortar and pestle.
2.2.2. TC/EA configuration
The TC/EA-IRMS used in this study was in the standard configuration,
except for a gas trap between the reactor and GC column (Fig. 1). The
samples were inserted into a zero-blank autosampler (Costech), which
contained a maximum of 99 samples (Fig. 1(a)). The autosampler was
continually purged with He gas to remove atmospheric gases such as
N2, O2, and H2O. Thermal
decomposition was performed after the samples fell into the TC/EA
(Thermo Fischer Scientific) system. The central part of the TC/EA is a
high-temperature reactor composed of an outer ceramic tube and an inner
glassy carbon tube (Fig. 1(b)). The glassy carbon tube was filled with a
layer of silver wool at the bottom and a layer of glassy carbon granule
(several millimeters in size). The reaction residue was collected using
a graphite crucible that was held up by glassy carbon granules. The
crucible was positioned in the hottest zone of the reactor and was set
at 1450˚C in this study.
The thermal decomposition of quartz (SiO2) using this
method is represented by the following reaction:
SiO2 + 4NaF + 2*C → 2*CO +
SiF4 +4Na
Here, *C represents carbon derived from graphite or Ni/C powder. CO and
fluoride gases were generated. Although the chemical species of the
fluoride gases produced here have not been identified, it is assumed
that they are mainly SiF4.When PTFE is thermally
decomposed, CF4 gas is produced, which is converted into
more stable SiF4 in the presence of SiO220, 21. Similarly, the more stable
AlF3 (solid and gaseous) forms in the presence of
Al2O3 22. Silicate
minerals contain other cations such as Mg, Fe, as well as Si and Al in
their crystal structures, and it is thought that fluoride gases
containing these cations will form when they are decomposed in TC/EA.
The gases produced in the reactor were transferred to a gas trap. The
gas trap is 30 cm long which is filled with 20 cm of ascarite II®
adsorbent and 10 cm of Mg(ClO4)2granular (Fig. 1(c)). Mg(ClO4)2 was
employed as the moisture adsorbent, and Ascarite II was utilized to
remove fluoride gasses in order to safeguard the GC column. Unlike a
prior publication, which included a liquid N2-trap
between the reactor and the GC column to remove possible fluoride gas
remains 23, we did not use one. Although this point
should be examined in the future to prevent possible damage to the GC
column, no fluoride species were detected using the following mass
spectrometer. After passing through the gas trap, the CO gas flowed into
a GC column set at 90˚C and then to an IRMS via an open-slit interface
(Con Flo IV, Thermo Fischer Scientific) (Fig. 1(d)). The IRMS model used
here was the Delta V Advantage (Thermo Fischer Scientific). This
provided the CO peak areas for m /z 28, 29, and 30 (Fig.
1(e), (f)).