Экология/6. Экологический мониторинг

Abilev M., PhD Alimzhanova M., PhD Kenessov B.

Al-Farabi Kazakh national university, Kazakhstan

Optimization of solid-phase microextraction conditions for semi-quantitative determination of total content of petroleum hydrocarbons in soil

Oil production and refining, which are maintained for decades, are accompanied by the accumulation of oil waste, sludge, spills of drilling fluids and formation waters that are hazardous to human habitat and wildlife in oil production and refining areas of the country.

The Caspian Sea region occupies a special place among the zones of ecological stress in Kazakhstan. The rapid development of oil and gas industry of Kazakhstan has created many environmental problems. The ecological situation in the region is influenced by both natural and anthropogenic factors - regression and transgression of the Caspian Sea, which are in close contact with the parts of soil and vegetation, wildlife and water balance, as well as large amounts and intensity of oil and related environmental pollution.

Analysis of available methodological basis for determining the total content of petroleum hydracarbons showed that these thechniques are based on the traditional methods of extracting of petroleum hydracarbons from oil-contaminated soil and sludge. These techniques are mostly suitable for the general characteristics of soil contamination by oil. But in many cases it is need to determine the individual compounds in the contaminated soils which is achieved by using the gas chromatography. Solid-phase microextraction is used to increase the efficiency of this technique [1].

Application of the method of solid-phase microextraction for determining the total content of petroleum hydrocarbons in soils allows to significantly reduce the time and cost required to perform an analysis, to fully automate the process of analysis and sample preparation, and GC/MS will provide the most possible information on toxic chemical compounds that are present in the sample [2].

The development of methods for determining the total content of petroleum hydrocarbons in soil samples included the following stages:

1) selection of the optimal extracting coating;

2) the choice of optimal extraction conditions - temperature and time;

3) obtaining calibration curves for different soil types;

4) obtaining calibration curves for different types of crude oil and petroleum products.

Experiments were performed on soil samples, sampled from areas of oil deposits of Koschagyl and Zhetybai (Kazakhstan), as well as oil pollution in the laboratory (10 mg/kg).

Sample weighing 2 g was placed in a vial of 20 mL for the preparation of model samples of soil. Then, crude oil in the volume which was calculated on the basis of the measured value of its density was added using the microsyringe of 50 mL (scale 1 mL), the mass of oil in the microsyringe was 20 mg. Since all of oil samples had high viscosity, their were pre-warmed to 50°C. To prevent the hardening of oil during the sampling, syringe was also pre-warmed to the temperature of 50°C. 

Selection of the optimal extracting coating

Following types of extraction coatings were tested during the experiments: 100 µm polydimethylsiloxane (PDMS); 7 µm polydimethylsiloxane; 65 µm polydimethylsiloxane/divinylbenzene (PDMS/DVB); 85 µm carboxen/polydimethylsiloxane (CAR/PDMS).

Following parameters were used for the experiment:

extraction temperature: 95°C,

extraction time: 1 min,

evaporator temperature: 250°C

column: HP-Innowax 30 m long, i. d. 0,25 mm and film thickness of 0,25 µm, 

chromatographing temperature: 40°C (holding for 10 min), heating rate of 5°C/min to 240°C (holding for 30 min)

detection: total ion stream in the range of mass numbers of m/z 34-600.

The results of experiment are shown in Figure 1. 

Figure 1 – Efficiency of extraction of petroleum hydrocarbons from soils contaminated with oil from the Koschagyl deposit

Based on these data it was concluded that the 100 µm coating of polydimethylsiloxane provides the greatest degree of extraction of petroleum hydrocarbons from contaminated soils. The high efficiency of this coating due to strong hydrophobic polydimethylsiloxane, which observed its high affinity to the oil hydrophobic hydrocarbon. In addition, the coating based on polydimethylsiloxane has a high chemical and thermal resistance for a long time.

Thus, the 100 µm PDMS coating was chosen as an optimal and was used in all subsequent experiments. 

Selection of optimal temperature of extraction

The following temperatures were tested for the extraction: 70, 95, 120 and 150°C.

It was shown during the experiments (Figure 2) that with increasing of temperature from 70 to 150°C peak area of hydrocarbons continuously increase and the temperature of 150°C provides the greatest analytical signal. It also should be noted that with increasing of temperature, the chromatogram obtained using SPME, increasingly corresponds to the actual chemical composition of oil in the soil. 

Figure 2 - Effect of extraction temperature on the peak area of petroleum hydrocarbons

However, it was found in subsequent testing of the given regime on real samples of different types of soil and moisture that at temperatures of 120 and 150°C the pressure in the extraction vessel significantly increases due to evaporation of water, the content of which in soil samples from areas of oil deposits varies from 5 to 30%. In this case increased pressure leads to a partial depressurization of the extraction vessel and the loss of analytes and can lead to complete destruction of the vials.

Sensitivity of the method based on solid-phase microextraction is very high, therefore it is expedient at the expense of increasing the signal to achieve the greatest reliability of the final methodology. Furthermore, it should be considered that at lower temperatures the lighter petroleum fractions selectively extracted, representing the greatest threat to the environment and man [3].

Therefore, the temperature of 95°C was chosen as optimal for the extraction of petroleum hydrocarbons from soils. Increasing the extraction temperature to 150°C is only necessary when analyzing very small concentrations of hydrocarbons, and is permitted only for samples with a moisture content of less than 10% of either pre-dried at 105°C. 

Selection of the optimal time of extraction

In this experiment soil samples with a total hydrocarbon concentration of 10 µg/kg were analyzed by SPME/GC/MS using the time of extraction of 10, 30, 60, 120, 180 and 300 s.

The resulting curve of the response of hydrocarbons from the time of extraction is shown in Figure 3. As it can be seen from the figure the response of hydrocarbons increases with the time of extraction and reaches a plateau at the age of 60 seconds, then the response does not change significantly. It was concluded based on these data that the extraction time of 60 s is optimal, as it provides a high signal at the minimum time required for extraction.

Figure 3 - Effect of extraction time on peak area of petroleum hydrocarbons

Obtaining the calibration curves

The following samples were prepared to obtain the calibration curves:

- soil samples of type 1, contaminated with oil from the Koschagyl deposit with concentrations of 1,0; 5,0 and 50 g/kg;

- soil samples of type 2, contaminated by oil from the Koschagyl deposit with concentrations of 1,0; 5,0 and 50 g/kg;

- soil samples of type 1, contaminated with oil from the Zhetybai deposit with concentrations of 1,0; 5,0 and 50 g/kg;

- soil samples of type 2, contaminated by oil from the Zhetybai deposit with concentrations of 1,0; 5,0 and 50 g/kg;

The obtained calibration curves are shown in Figure 4.

Figure 4 – Calibration curves obtained for different types of soils contaminated with oil from the Koschagyl (1 and 2) and Zhetybai (3 and 4) deposits

As it is shown in the Figure 4, all the calibration dependence possess good linearity and, consequently, can be used for the quantitative determination of petroleum hydrocarbons in contaminated soils.Calibration curves obtained for different soil types differ among themselves unessentially - the slope of these curves are virtually identical.

Calibration curves obtained with the use of oil from different deposits differ unessentially, but for the quantitative analysis is recommended to use the grading, obtained with the use of oil from a particular deposit, that does not cause any difficulty.

Thus, the optimal parameters of method for determining the total content of petroleum hydrocarbons using solid-phase microextraction coupled with gas chromatography-mass spectrometry were established: absorption coating is 100 µm of polydimethylsiloxane, extraction temperature - 95°C, extraction time - 60 sec. Calibration dependence obtained using the optimal parameters is linear and can be used for quantitative analysis. 

Analysis of soil samples from Zhetybai and Koschagyl deposits

All the collected samples were analyzed semiquantitatively using the method of gas chromatography with mass spectrometric detection coupled with solid-phase microextraction SPME/GC/MS with the following parameters: the adsorption coating 100 µm of polydimethylsiloxane, extraction temperature 95°C, extraction time 10 min, evaporator temperature 250°C, column HP-Innowax 30 m long, i.d. 0,25 mm and film thickness 0,25 µm, chromatographing temperature 40°C (holding for 10 min), heating rate of 5°C/min to 240°C (holding for 30 min), detection in the mode of the total ion stream in the range of mass numbers of m/z 34-600.

Results of the analysis of samples showed that the soil in areas of oil spills is heavily contaminated by oil hydrocarbons, content of which reaches 500 g/kg. Analysis of the deep samples showed that the highest concentrations of petroleum hydrocarbons are character for surface samples (0-50 cm). Further, with increasing of the depth the oil hydrocarbon content drops and reaches zero at the depth of 1,0-1,5 m.

CONCLUSIONS

The following results were obtained:

1. The optimum parameters of technique of rapid determination of total petroleum hydrocarbons in contaminated soils by solid-phase microextraction coupled with gas chromatography-mass spectrometry were established: absorption coating 100 µm of polydimethylsiloxane, extraction temperature 95°C, extraction time 60 sec. Linear calibration curves for different types of soils and oil from different fields were obtained.

2. Ecological-chemical investigations of deposits of Koschagyl and Zhetybai were carried put, during which the basic contaminated sites, the nature of pollution, as well as features of the distribution of petroleum hydrocarbons in soil layers were established. Sampling points was shown in satellite images using a variety of geographic information systems.

3. Analysis of deep soil of deposits of Koschagyl and Zhetybai for the content of petroleum hydrocarbons showed that the hydrocarbons penetrate deep into the soil horizon, while the largest concentration typical for the surface horizons (up to 60 cm). This feature is caused by the chemical composition of oils from deposits of Koschagyl and Zhetybai, which are dominated by heavy and very heavy fractions, the high viscosity of which does not allow them to migrate along the soil horizon. 

4. The research results can be used to organize monitoring of the environment in the oil-producing regions, in studying the behavior of petroleum hydrocarbons in the environment and to develop effective methods for remediation and restoration of contaminated sites.

REFERENCES

1. Zhang Zh., Jang M.J., Pawliszyn J. Solid-phase microextraction: a solvent free alternative for sample preparation // Analytical Chemistry. - 1994. – Vol. 66(17). - 844A-853A.

2. Llompart M., Li K., Fingas M. Headspace solid phase microextraction (HS SPME) for the determination of volatile and semivolatile pollutants in soils // Talanta. – 1999. - Vol. 48. – Vol. 451-459.

3. Jaraula C.M.B., Kenig F., Doran P.T., Priscu J.C., Welch K.A. SPME-GCMS study of the natural attenuation of aviation diesel spilled on the perennial ice cover of Lake Fryxell, Antarctica // Science of the Total Environment. - 2008. – Vol. 407. – P. 250-262.