(559j) Total Internal Reflection Fluorescence Microscopy: Dual Color Imaging on Oil Sands Tailing Ponds

Total Internal Reflection Fluorescence Microscopy: Dual Color Imaging
on Oil Sands Tailing Ponds

Abstract:

Understanding the nature of bitumen, clay and their interactions in
tailing ponds is essential to support oil industry to develop effective methods
for tailing remediation. Clay minerals such as kaolinite has been reported to
have negative effect on bitumen extraction and product quality from oil sands.
Therefore, understanding of bitumen-clay and clay-clay interaction is crucial
in waste water treatment processes such as particle aggregation and
sedimentation. In this study, due to natural fluorescent of bitumen, high
intensity contrast between clay and bitumen, and high signal-to-noise ratio of
Total Internal Reflection Fluorescence (TIRF) microscopy, distribution of both
organic bitumen and inorganic clay in water-based bitumen extraction was
investigated. Multicolor imaging represented residual hydrophobic bitumen
bridging and covering fine clay particles and hydrophilic clay particles in mature
fine tailings.

Introduction

The oil sands deposit in the northern Alberta, Canada consists of
bitumen (4–18 wt.%), sands (55–80 wt.%), fine solids (particles smaller than
<44 μm (5–34 wt.%)) and water (2–15 wt.%) [1].
The semi-solid nature of bitumen requires extraction and recovery technologies
different from the conventional processes. These recovery processes result in
accumulation of almost one billion cubic metres of a clay slurries known as
mature fine tailings (MFT) [2] at 30-35% solid content of fine clays which
remain for decades in a fluid state because of its very slow agglomeration rate
[2].

Clay component of the oil sands ore is different than many mining
industry slurries. Most of the clay fine particles (<2 μm),
commonly of aluminum and magnesium, in Athabasca deposits are water-wet with a thin
water film of about 2 µm covered the sand grains. Clays are made of layers
built from components with tetrahedrally and octahedrally coordinated cations [2].
Kaolinite (Al₂Si₂O₅(OH)₄) has an
important role because of its high abundance in nature, relatively pure
chemical composition, and well characterized crystal [3]. Clays are attached
with organic materials at a nano-scale in oil sand
deposits. Lack of contrast, multistep sample preparation, scale limitation and
temperature and pressure dependency limit the study of interaction using
conventional methods such as X-ray photoelectron
spectroscopy (XPS) [4] and transmission electron microscopy (TEM) [5]. However,
due to natural fluorescence of bitumen [6], fluorescence microscopy has shown
excellent imaging results over a wide range of spectrum. Fluorescent dye
has been extensively used in biomedical diagnosis and biological imaging. A dye with well separated excitation and emission
peaks compared with bitumen improves detection of bitumen in MFT.

In this work, spectrofluorometery
was first used to verify the favorable bitumen excitation and emission
wavelengths for fluorescence microscopy. Moreover, confocal fluorescence
microscope was also used to verify bitumen fluorescence in presence of water
and clay. According to the results and in agreement with literature, a 488 nm laser source and 500−550 nm emission filter are the
optimum values for bitumen fluorescence imaging. A water-soluble fluorescent
dye labeled water to make TIRF multicolor images of tailing samples. Successful
detection of bitumen in the MFT samples can be achieved with this technique
including fluorescence.

Experimental
Section

Kaolin, a model clay, and Athabasca bitumen were provided by
Institute of Oil Sands Innovation (IOSI), University of Alberta, and MFT was
received from Syncrude Canada, Ltd. tailings ponds. MFT samples contain
approximately 31 wt. % solids, 4 wt. % bitumen, and 65 wt. % water. Fluorescent
dye (CF 660R) was purchased from Biotium Inc. USA,
and spectroscopic grade dimethyl sulfoxide was supplied from SigmaAldrich. To observe a real
time and condition MFT sample, a bowl-shaped sample holder was prepared using a
10 mm diameter circular hole in the center of a glass slide attached to a cover
slip. The sample was placed into the bowl to study using excitation wavelength
of 488 nm. The labeling stock solution was diluted in Dimethyl sulfoxide (10
mM) and stored at -20 °C. Water was fluorescence labeled using diluted stock
solution (100 times) and mixed with MFT sample.

Spectrofluorometery and TIRF Imaging

Spectrofluorometer was used to measure excitation and
emission spectra
related to fluorescent dye. A Cary Eclipse fluorescence spectrophotometer was applied
to choose the best laser source and emission filter for fluorescent dye observation.
A very diluted solution of dye and water was prepared to avoid high excitation
light scattering and 4ml of the solution was taken into glass cuvette to
measure the excitation and emission spectra.

TIRF microscopy illuminates a thin layer near the
interface of two media with different refractive indexes through total internal
reflection in the lower index medium at a laser excitation angle greater than
critical angle to reflect the generated light. A
Nikon eclipse Ti TIRF equipped with 488 nm and 633 nm
lasers sources (Melles Griot) were used for
excitation and fluorescence in the range 500-550 nm (green) and 650-730 nm
(red) were collected. A QuantEM 512sc CCD camera coupled
with a 100X objective was used to capture the images.

Result and Discussion

According to figure 1, spectrofluorometer result represented that CF
660R dye has the most excitation and emission in the range of 635-675 nm and
660-700 nm, respectively. Therefore, 633 nm laser line and 650-725 nm filter were used based on
availability for the fluorescent microscopy. Bitumen has the most
excitation on 488 nm laser line and omits light mostly in 500-550 nm [7]. These
fluorescence absorption and emission spectra were set to get the best images of
bitumen and dye in MFT samples.

Figure 1. CF 660R
spectrofluorometer graph of fluorescence intensity for a range of excitation
and emission wavelengths

As discussed earlier, fluorescent dye was used to label
water to locate clay, bitumen, and water at the same time in MFT sample. Bitumen, model clay and dyed water multicolor imaging is shown in
figure 2. Figure 2a shows a bright-field image of the clay and bitumen
interface and the fluorescence image of the clay and bitumen interface with
excitation at 488 nm and emission filter in 500-550 nm is represented in figure
2b. Fluorescence image of the clay and bitumen with excitation at 633 nm and
emission filter in of 660-700 nm is also shown is figure 2c. Figure 2d is the
superimposed 488 nm excitation laser TIRF and 633 nm excitation laser TIRF
images using ImageJ^® software that shows clay and bitumen interface
is clearly visible. The dark black particles in these images are clay
particles, while the green dots are bitumen.

Figure 2. (a) bright-field images of clay-bitumen
interface, (b) fluorescence image of the clay-bitumen interface (excitation at 488
nm and emission filter of 500-550 nm), (c) fluorescence image of the clay-bitumen
interface (excitation at 633 nm and emission filter of 660-700 nm), (d)
superimposed TIRF images of (b) and (c).

A bright-field image of the clay and bitumen interface in a tailing
sample is represented in figure 3a. Fluorescence images of the clay and bitumen
interface with excitation at 488 nm and emission filter in 500-550 nm and
excitation at 633 nm and emission filter in of 660-700 nm are shown in figure
3b and 3c, respectively. Figure 3d is the superimposed 488 nm excitation laser
TIRF and 633 nm excitation laser TIRF images to show clear clay and bitumen
interface. The dark black particles in these images are clay particles, while
the green dots are bitumen.

Figure 2. (a) bright-field images of MFT sample, (b)
fluorescence image of the MFT sample (excitation at 488 nm and emission filter
of 500-550 nm), (c) fluorescence image of the MFT sample (excitation at 633 nm
and emission filter of 660-700 nm), (d) superimposed TIRF images of (b) and
(c).

Conclusion

The bright-field images of clay and bitumen show that
the lack of contrast makes it difficult to observe bitumen. While fluorescence
microscopy clearly shows bitumen as a result of its strong fluorescence, clay is
still not detectable because it lacks substantial fluorescence. With dye added
to this mixture fluorescent images confirm that bitumen and clay can be
distinguished using multicolor fluorescence techniques. This is critically
important to analyze the bitumen clay interaction to improve effective
processes to accelerate aggregation and sedimentation during the MFT treatment.
Results indicate that in MFT sample, bitumen is not suspended freely in water
and residual bitumen is bridging and coating the clay agglomerates. It also confirms
the presence of water-wet clays due to small percentage of clay aggregates
attached to bitumen.

References

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