GC-FID methods for the analysis of Petroleum hydrocarbons were developed and optimised. Contamination of soil from the Niger Delta was investigated about 40 days after crude oil spillage from the Shell Petroleum marginal well head. Soil samples and controls were collected at depths of 0 - 15 cm, 15 - 30 cm and 30 - 60 cm. Samples were analysed using gas chromatography fitted with a flame ionisation detector. Penetration and migration of C 10-C 26 and C 26-C 34 hydrocarbons through the soil layers were assessed by cluster analysis to determine the spatial distribution, penetration and similarity of these compounds over the contaminated area. The results also indicated elevated levels of total hydrocarbon contents when compared with the reference sites. Recommendations are made to carefully monitor and remediate the environment.
Environmental pollution due to the increasing release of hazardous and toxic substances into the soil, water, sediment and air in Niger Delta, Nigeria has been a widespread problem in recent years. Indeed soil contamination by oil exploration activities has quickly become a considerable environmental issue [
Gas chromatography is one of the most powerful, popular, unique and readily versatile analytical techniques used for the separation, identification, and quantitative assay of compounds in the vapour state. It still remains the most important single technique for oil spill identification partly because the equipment is relatively available, and easy to operate with small amount of operator time and considerable amount of information can be gathered on using a high resolution (capillary) column. This study sought to establish the concentration of the contaminants, and apply cluster observation analysis to characterize chemically similar hydrocarbons over the contaminated area. This analytical information would be usefully employed in each petroleum spill site to access the level of contamination and to efficiently and safely remove the spilled petroleum products from the soil with the aim of returning the soil back to a useable form. Attempts for complete removal may not be practically more attainable than to remediate the soils to the concentration levels that will be harmless to plants, fauna, human health and the entire ecosystem [
Sampling is considered a vital and one of the most crucial steps in the procedure of analysis of organic contaminants in soils and sediments of our environment [
Auger boring often provides the simplest method [
Sample location | Sample identity | Sample depth each (cm) | Total number of samples |
---|---|---|---|
A | A1 to A10 | 0 - 15 15 - 30 30 - 60 | 30 |
B | A11 to A14 | 0 - 15 15 - 30 30 - 60 | 12 |
C | A15 to A17 | 0 - 15 15 - 30 30 - 60 | 5 |
47 |
Hole | Sample ID | Description | No. of Samples | Depth (cm) | Position coordinates north east |
---|---|---|---|---|---|
1 | A1 | 5 meters (m) from the Well head―the Reference Point (RP) | 3 | 0 - 15 15 - 30 30 - 60 | 4˚41'49.4'' & 7˚41'09.8'' |
2 | A2 | 10 m from Well head RP | 3 | 0 - 15 15 - 30 30 - 60 | 4˚41'50.5'' & 7˚41'09.12'' |
3 | A3 | 20 m from Well head RP | 3 | 0 - 15 15 - 30 30 - 60 | 4˚41'49.3'' & 7˚41'11.4'' |
4 | A4 | 50 m from RP | 3 | 0 - 15 15 - 30 30 - 60 | 4˚41'48.6'' & 7˚41'10.1'' |
5 | A5 | 100 m from RP | 3 | 0 - 15 15 - 30 30 - 60 | 4˚41'47.2'' & 7˚41'09.9'' |
6 | A6 | 150 m from RP | 3 | 0 - 15 15 - 30 30 - 60 | 4˚41'48.8'' & 7˚41'09.1'' |
7 | A7 | 100 m from RP | 3 | 0 - 15 15 - 30 30 - 60 | 4˚41'49.7'' & 7˚41'09.3'' |
8 | A8 | 150 m from RP | 3 | 0 - 15 15 - 30 30 - 60 | 4˚41'47.7'' & 7˚41'07.6'' |
9 | A9 | 10 m from RP | 3 | 0 - 15 15 - 30 30 - 60 | 4˚41'50.3'' & 7˚41'10.6'' |
10 | A10 | Well Head, (WH)-(Spillage and Reference Point. RP) | 3 | 0 - 15 15 - 30 30 - 60 | 4˚41'49.8'' & 7˚41'10.4'' |
11 | A11 | 50 m from RP | 3 | 0 - 15 15 - 30 30 - 60 | 4˚41'48.4'' & 7˚41'09.7'' |
12 | A12 | 150 m from RP | 3 | 0 - 15 15 - 30 30 - 60 | 4˚41'49'' & 7˚41'09.5'' |
---|---|---|---|---|---|
13 | A13 | NA | 3 | Same as above | - |
14 | A14 | NA | 3 | Same as above | - |
15 | A15 | NA | 3 | Same as above | - |
16 - 17 | A16 A17 | NA | 3 | Same as above | - |
(sub-surface) and bottom layer of 30 to 60 cm. The auger was cleaned with deionised water and rinsed with methanol after each sampling point. Three sub-samples were collected at each sampling point of the designated depths. Representative soil samples from the auger were taken into fresh polythene bags with seal and further placed in a pre-cleaned glass bottle with a clean Teflon-lined lid. Approximately 500 g of soil was collected at each sample depth. Four control site samples (duplicate sample blanks) were taken from the same geographically uncontaminated soil to determine the background levels of petroleum hydrocarbons for comparison with the contaminated soil.
The entire sampling exercise was carried out in one day. The average ambient temperature was 28˚C. All the samples were placed in icebox and transported to the laboratory. Storage was done at 4˚C until analysis was completed in two weeks.
Extraction of petroleum contaminants from the soil took place prior to analytical determination. Soxhlet extraction with modified Brinkmann Büchi automated apparatus was used. Soxhlet extraction is a U. S. EPA [
This procedure involves extraction of the petroleum contaminants from about 10 g weight of dry oil-spilled soil with a suitable solvent. Soxhlet extraction really ensures intimate contact of the sample matrix with the extraction solvent and a reasonably large amount of 3 - 20 g could be used to allow quantitative extraction. The Soxhlet technique is the usually adopted reference method. It has been proposed by many agencies [
The choice of this extraction method amidst other recent extraction techniques includes its peculiar application, availability in the laboratory and the ease of setting up with minimal or no cost. Its drawbacks include delayed extraction period and use of appreciable volume of solvent. Other methods of extraction may not be labour intensive but due to their cost may not be accessible to every laboratory.
Optimum extraction time of about 2.5 hours was established using dichloromethane (DCM) after comparing with other solvents such as methanol, hexane, acetone, toluene and ethyl benzene. DCM proved to be the most suitable solvent over hexane, acetone, toluene and ethyl benzene for this extraction due to its consistency, efficiency and ability of not interfering with BTEX retention time window (RTW)―C5-C9. This was indicative that Tropical soil in south-south Niger.
Delta favoured efficient extraction with DCM as solvent.
In this work, dichloromethane (99.8%) used as the extracting solvent was supplied by Sigma Aldrich. Reference standards used were BTEX (benzene, toluene, ethylbenzene and xylene) Mix, catalog No. 47993 supplied by Supelco analytical, Bellefonte, PA, USA; Kit for the chromatographic determination of hydrocarbon content in soil according to DIN ISO 16703 and in waste according to EN 14039 comprising of the following: Standard solution for the determination of the retention time window (RTW), cat. No. 67583; Mineral Oil standard mixture type A and B for DIN EN 14039 and ISO 16703 (cat. No. 69246); Alkane standard mixture (cat. No. 68281) for the assay of the system efficiency of GC’s (C10-C40); Heptane, Puriss. p.a (cat. No. 51745) and Dual layer Florisil®/Na2SO4 SPE Tube, 2 g/2 g/6 mL (Cat. No 40080-1ea-F) all supplied by Fluka Analytical, Sigma Aldrich.
Diesel Range Organics (DRO) Mix (Tennessee/Mississippi), catalog No. 31214, Lot No. AO62141 was supplied by Restek , PA , USA . Hydrocarbon internal standards C14, C15, verification standards (C10, C11, C14, C15 and TCD) prepared in the laboratory were all HPLC and of analytical reference grades.
About 500 g of labelled petroleum contaminated soil samples were collected into zipped plastic bags and put into a glass jar sealed with Teflon lined cap. Nickel plated carbon steel hand auger was used in scooping the soil at three different depths of 0 - 15, 15 - 30, 30 - 60 cm.
The samples were preserved in the field using methanol and stored at 4˚C until analysis. Extraction was done using Brinkmann Buchi automated apparatus. Sample clean up was undertaken with a 6 mL dual layer glass Florisil®/Na2SO4 SPE Tube 2 g/2 g. The clean-up procedure effectively removed polar hydrocarbons of natural origin, moisture, impurities, colour interferences and did not have any significant effect on the amount of petroleum hydrocarbons present.
GC-FID method for the determination of petroleum hydrocarbons in soil was developed based on modifications of the reported methods [
The system consisted of a Varian BV , CP-3800 gas chromatograph coupled to the FID detector and equipped with an automatic sampler CTC Analytics CombiPAL and the 1177 split/splitless injector. The GC capillary (WCOT) column was a non polar, CP-Sil 8 CB Low Bleed/MS polymer; 30 m long, 0.25 mm inside diameter (id), 0.39 mm outside diameter (0 d), 0.25 µm film thickness, coating of 95% methyl, 5% phenylpolysiloxane. The initial column temperature programmed at 30˚C for 3 min. to identify and separate the volatile components. The temperature was increased to 320˚C at the rate of 8˚C min−1 and held for extra 15 min. The carrier gas was helium (99.99% pure) at velocity of 26 cm∙sec−1. Sample injection volume of 1 µL, 1:25 split ratio and column flow rate of 1.0 mL∙min−1 were applied. Detector (FID) and injector temperature were 320˚C and 300˚C respectively.
Identification and quantification of the samples was based on the comparison of the chromatographic data with the reference standards 1) Alkane Mix, C10-C40, 2) DRO, C10-C25 3) BTEX (benzene, toluene, ethylbenzene and total xylene) (iv) Retention Time window―RTW, C10 & C40 (v) hydrocarbons C10, C11, C14, C15, TCD (Thermal conductivity detector) (Figures 1-4). Quantitative analysis was computed based on [
Samples were analysed and chromatograms overlaid to confirm the identity and retention times with the reference standards. The average peak values of all the samples were recorded and their standard deviation and % RSD calculated at 95% confidence level.
The analysis of the solvent blank (DCM), thimble extract and control samples taken from similar geographical non-spilled areas, randomly collected and analyzed as the standards along side with other samples had no indication of trace of hydrocarbons or other contaminants since appropriate controls were carried out.
The Diesel Range Organics (DRO) reference standard was analysed along with the prepared laboratory standard to validate the eluted peaks (
The peaks eluted from all the five standards C10, C11, C14, C15 and the reference standard, DRO (C10-C25) were analyzed concurrently in order to ascertain the reproducibility of the chromatographic optimization process.
The three reference standards―DRO, alkane mix and BTEX were analysed with the sample at the same time using the optimised method (
All the petroleum contaminants in the soil that were qualitatively identified and quantitatively validated fell within C10 to C34 (
Sample number 3 (top soil-A3.1) was used to represent the other samples because it showed the widest spread of the contaminants in the optimized retention time window method with the last compound eluting at 45.73 min. RTWs were established to compensate for the minor shifts in absolute retention times as a result of sample loadings and normal chromatographic variability.
The overall concentration of total petroleum hydrocarbon (TPH) of each sample depth at the site were established and presented in
In
The greatest depths measured (60 cm) in most of the samples recorded had significantly low value of TPH though concentrations of total petroleum hydrocarbons did not decrease generally with depth.
The concentration of TPH at the mid/sub-soil (15 - 30 cm) depth was higher than the concentration range reported by [
High concentration levels of hydrocarbons present in contaminated sites could pose a health risk to humans, plants and animal lives. Assessment of the penetration capability of the hydrocarbon contaminants were carried out. It became imperative to assess the type of hydrocarbons and the extent of depth
No. | Sample Mass (g) | *Average Peak Area mV−sec) | Hydrocarbon Content (mg∙kg−1) or (ppm) | |||||||
---|---|---|---|---|---|---|---|---|---|---|
TS | MS | BL | TS | MS | BL | TS | MS | BL | ||
SITE A | ||||||||||
1 | 3 | 10.16 | 10.00 | 10.06 | 546 | 889 | 439 | 94 ± 25 | 156 ± 26 | 76 ± 39 |
2 | 2 | 10.49 | 10.25 | ** | 797 | 388 | ** | 133 ± 38 | 66 ± 19 | ** |
3 | 3 | 10.00 | 10.10 | 10.00 | 1343 | 1697 | 933 | 235 ± 5 | 294 ± 16 | 163 ± 15 |
4 | 3 | 10.01 | 10.11 | 10.02 | 443 | 482 | 482 | 77 ± 44 | 84 ± 18 | 84 ± 14 |
5 | 3 | 10.10 | 10.30 | 10.00 | 605 | 515 | 560 | 105 ± 11 | 88 ± 4 | 98 ± 3 |
6 | 3 | 10.08 | 10.00 | 10.00 | 54 | 48 | 45 | 9 ± 7 | 8 ± 1 | 7 ± 1 |
7 | 3 | 10.00 | 10.02 | 10.01 | 45 | 51 | 46 | 10 ± 2 | 9 ± 1 | 8 ± 2 |
8 | 3 | 10.02 | 10.39 | 10.48 | 259 | 323 | 345 | 45 ± 11 | 54 ± 7 | 58 ± 13 |
SITE B | ||||||||||
1 | 3 | 10.13 | 10.10 | NA | 248 | 216 | NA | 43 ± 18 | 37 ± 6 | NA |
2 | 3 | 10.01 | 10.02 | NA | 269 | 202 | NA | 47 ± 10 | 34 ± 7 | NA |
3 | 3 | 10.08 | 10.02 | NA | 1668 | 1820 | NA | 289 ± 15 | 318 ± 4 | NA |
4 | 3 | 10.00 | 10.00 | 10.14 | 283 | 216 | 287 | 50 ± 13 | 38 ± 4 | 49 ± 3 |
Control samples | ||||||||||
1 | 3 | 10.11 | 10.02 | 10.10 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
2 | 3 | 10.01 | 10.11 | 10.00 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
3 | 3 | 10.20 | 10.14 | 10.11 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
4 | 3 | 10.00 | 10.12 | 10.13 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
TOTAL | 47 |
TS = Top soil; MS = Mid soil; BL = Bottom level. **Not sampled due to impervious rock obstruction.
penetration for the purpose of remediation and record. This section of investigation classified the hydrocarbons into groups based on their degree of penetration within the soil strata. Basically, three major groups of petroleum hydrocarbons are known, classified and adopted in this work. These are: 1) The Gasoline Range Organics (GRO), generally eluting in window C5-C9. 2) The Diesel Range Organics (DRO) elutes from C10-C26. 3) The Waste Oil Organics (WOO), eluting above C26. The presence and concentrations of C10-C26, C26 and above had been identified and quantified. The penetration, percentage distribution and migration of these groups of hydrocarbons in the samples were considered.
However, for the purpose of this assessment, the site samples were classified into three fundamental groups based on the observed pattern of the hydrocarbon penetration and distribution.
Group 1 pattern was exhibited by six (6) samples (A1, A3, A5, A8, B1 & B3), each comprising of three (3) sampling depths. This group had both DRO (C10-C26) and WOO (C26 and above) with competitive penetration capacity as represented in
Group 2 has three (3) samples (A6, A9 and B4) exhibiting similar spatial penetration and distribution pattern. In this group, 100% presence of DRO was found in all the soil depths with no contribution from WOO (
There are 3 samples (A2, A4 & A7) making group 3, in which DRO dominates the soil depths with little contribution from the Lubricating or waste oil hydrocarbon (WOO) range―C26 & above (
In the graphical representations in Figures 4-6, each sample shows three different depths of 15 cm (TS), 30 cm (MS) and 60 cm (BL). The chromatogram for each sample is represented with a bar graph side by side showing penetration and distribution of hydrocarbons in the range C10-C20 and C21-C34 through the measured soil levels.
Samples illustrated in
Soil sample A4 in
BTEX range of volatile organics were not detected in this analysis partly due to its reduced concentration resulting from evaporation after prolonged exposure of the spill before sampling was undertaken.
A method for the analysis of soils spilled with crude oil was developed and optimized. The results in this study revealed that the TPH concentration in the all the levels of soil strata measured ranged from 7 ± 1 to 318 ± 4 mg∙kg−1. The concentrations and penetration ranges for two groups of TPHs―C10-C26 (Diesel Range Organics) and C26-C34 (Waste Oil Organics) were recognised.
Despite limited information on the migration and depth penetration of hydrocarbons in soils, data from this study showed the types, distribution, migration and penetration capability of the petroleum hydrocarbon contaminants in the study area.
The spatial distribution and penetration pattern of petroleum contaminants at the investigated site were established as an informative guide to the Government and oil industries during remediation process.
The authors declare no conflicts of interest regarding the publication of this paper.
Okop, I. and Persaud, K. (2019) Qualitative and Quantitative Assessment of Petroleum Contaminants in Soils under Tropical Weather Conditions. American Journal of Analytical Chemistry, 10, 112-125. https://doi.org/10.4236/ajac.2019.103010