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.