3.1 Analysis of weathered Oil Using Gas Chromatography-Mass
Spectrometry (GC-MS)
Any extraction procedure used in analytical chemistry is primarily
intended to detach the analyte from its matrix quantitatively, speedily
and utilising as little solvent as possible [9]. A procedure known
as accelerated solvent extraction (ASE) has been employed for the
quantitative extraction of a number of semi-volatiles comprising
Polycyclic Aromatic Hydrocarbons (PAHs), phenols, polychlorinated
biphenyls (PCBs) as well as total petroleum hydrocarbons
[8;10].
An instrumental method known as GC-MS consisting of a gas chromatograph
(GC) coupled to a mass spectrometer (MS) in an effort to separate,
identify and quantify complex mixture of chemicals. This proves to be
the most appropriate method to analyse hundreds of compounds with
comparatively LMW. The compound itself has to be thermally stable and
satisfactorily volatile so that it can be analysed using GC-MS
technique. However, this study was conducted using GS-MS to analysed
weathered oil in soil. Preliminary study was conducted for the Kuwait
oil sand, the initial results from the GC-MS analyses indicated that the
concentrations of TPH varied from 183,432 to 212,993 mg/kg. No traces of
PAH compounds were found in the sample by this analysis. The GC profile
for TPH showed that after 15 minutes the TPH level started to increase,
indicating that the sample contains long chain hydrocarbon compounds,
Figure (1).
Figure 1: Here
Before carrying out GC analysis, chemical modification of the sample is
essential to eliminate interfering compounds that may affect the quality
of the data obtained. Hence, a high content of asphaltenes was removed
to get a correct reading for aliphatic and, aromatic compounds and to
protect the GC. However, the main portion of the diesel oil is not
characterized as most of the constituents cannot be resolved. In the
chromatogram, these constituents look like a ‘hump’, which is termed
unresolved complex mixture’ (UCM), containing branched, cyclicalkanes
and polar transformation products [11]. The resolved hydrocarbons
are named ‘total resolved hydrocarbons (TRH). The TRH and UCM combine to
become TPH. In the GC-MS, the TRH are shown as peaks, which are
non-degraded hydrocarbons, as shown in Figure (2).
Figure 2: Here
However, this method faced some challenges during the measurement, one
of the main difficulties caused by GC column which was blocked after
each run. This necessitated the development of another alternative
method for the analysis of TPH from soil samples, as given in the next
section.
Analysis of Weathered Crude Oil Using Gravimetric
Method
Currently, there is no standard method for determination of TPH in
contaminated soil with petroleum hydrocarbon products [12]. A number
of researchers [13; 14] have developed and utilised various methods
for this purpose. Most of the techniques for separating oil from the
sand in the laboratory involves ASE, soxhlet extraction or
ultrasonication. Ultrasonication with aid of an appropriate solvent has
been selected to extract oil from the sand, since it is easily available
with high extraction efficiency and economical. The amount of weathered
crude oil can then be calculated by gravimetric
technique.
Different sample
weight
starting with the most polluted sample, the 5 g and 1 g weights of the
oil-contaminated samples were selected depending on contamination level.
The 1g of contaminated soil was cleaned in three cycles whilst the 5g of
contaminated soil required six cycles. After the washing process is
completed, the weight of the oil-contaminated soil sample after
extraction and the weight of the crude oil were calculated using the
equation 1, data is presented in Table (1).
Table 1: Here
One-gram sample was cleaned in 30 min with 15 ml of solvent, in
comparison with the 5 g sample, which needs 60 min with 150 ml of
solvent to complete cleaning. However, the interaction between the
extraction solvent and oil contaminants was unlikely to be changed by
increasing the weight of the soil
sample. Therefore, 1 g was
recommended in this study.
Type of
solvent
This technique involved determining the suitable solvents for the
extraction of crude oil from sand in a short period with high extraction
efficiency and a less harmful product. There is a wide range of solvents
which can be used, such as hexane, dichloromethane (DCM) and chloroform.
The selected solvents for this experiment are DCM, DCM: hexane (1:1 mix)
and acetone: hexane (1:1mix), these solvents were found to be the most
effective solvent compared to other solvents in isolation of organic
compounds from soil [8; 15]. The selected solvents were tested using
the same procedures carried out in three trials by applying the basic
method described in Section 2.4. The ultrasonication process cleaned 1 g
of contaminated soil, in three washes all with different solvents
trialed. After the completion of the solvent recovery process, the
weight of sand for all three samples and recovered oil were calculated,
as exhibited in Table (2).
Table 2: Here
Based on Table (2), the highest crude oil concentration on average was
341,000 mg/kg for the solvent combination of acetone: hexane (1:1 v/v).
Chloroform is not recommended in this study since it affects the central
nervous system, such that people who are exposed to chloroform with a
certain amount can become unconscious. The mixture of acetone: hexane
has a high extraction efficiency due to its dependency on the solvent
mixture polarity for eliminating oil contaminants. Moreover, different
authors have used them in the extraction of petroleum products from soil
[16; 17; 18; 19]. This experiment has shown that the best solvent
system of choice for the extraction of oil from sand was mixture of
acetone: hexane (1:1 v/v).
Different ultrasonication
cycles
The main aim of trying a different ultrasonication cycle is to
investigate whether a reduction of cycle improved the extraction
process. The previous experiments (Section 3.2.2) required 5 ml of
acetone: hexane (1:1 v/v) for 10 minutes per cycle for three cycles and
these results were then compared with an alternative experiment, which
used 5 ml of acetone: hexane (1:1 v/v) solvent mixture to clean 1g of
soil sample for four cycles with a shorter cycle time of 5 minutes for
each cycle. This test demonstrates that the four cycles were required if
the sample washed for 5 minutes per cycle, therefore a total volume of
20 ml of solvent was required to complete the extraction process.
However, samples washed within 10 minutes per cycle required three
cycles, therefore, 30 ml of of solvent was required for the whole
process and the outcomes are presented in Figure
(3).
Figure 3: Here
Based on Figure (3), there are no-significant differences in
concentration of weathered crude oil was observed irrespective of the
washing cycle time. The outcome showed that the mean values for 5 and 10
minutes were 344,000 and 331,000, respectively (Post-hoc Scheffe
test, P < 0.05). The previous experiment (Section
3.2.2) required three cycles, 15 ml of solvent and 10 minutes per cycle
to wash two samples in one hour. The alternative experiment used more
solvent with less time, so that three samples could be washed in one
hour. Therefore, this outcome suggested the choice of 5 minute cycle
over the 10 minute cycle.
Validation of gravimetric method
One-gram of clean sand was mixed with 5 ml of fresh crude oil. The
crude oil was obtained from Kuwait Oil Company (KOC). The mixture of
sand and crude oil were washed in 20 minutes with 20 ml of solvents in
four wash cycles (Section 2.4). After the completion of the solvent
recovery process, the weight of sand and recovered oil were
calculated, as exhibited in Table (3).
Table 3:
The outcome of Table (3) indicated that the previous experiment (Section
2.4) was able to recover about 97.20% of the crude oil. This study
suggested that the selection process in recovery of the crude oil is
influenced by four principal characteristics: (1) minimise the cost of
the extraction solvent by reducing the quantity of solvent (2) the
solvent must dissolve the crude oil in the shortest time (3) high
extraction efficiency and (4) select the solvent that has the least side
effect. In this study, the temperature of the extraction was not
examined since the volatility of the selected solvents was already high.