Petroleum Hydrocarbons in Saudi Desert Sands: Survey, Ecotoxicity and Remediation

Abstract

Petroleum, also known as “crude oil”, may enter the environment in large and small volumes due to accidental spillages such as acts of war, or terrorism or through natural processes. Crude oil is comprised of a complex mixture of compounds of which the main constituents are aliphatic hydrocarbons, with smaller proportions of aromatic compounds. The aims of this research are is to assess the background concentrations of total petroleum hydrocarbons (TPHs) in the Saudi Arabian desert, assess their ecotoxicology and to develop simple means of remediation. In this project, physio-chemical properties for Saudi Arabian desert sands (n=16) were determined. The pH ranged from 8.06 to 9.64. The sand moisture ranged from 0.04% to 0.68%. The organic matter content for sand samples ranged from 0.22% to 0.91%. The percentage of Carbon in sand ranged from 0.05% to 3.30%, while Nitrogen ranged from 0.01% to 0.03%. Elemental analysis was applied to all sand samples and Calcium (ranging from 1310 mg/kg to 14823 mg/kg) was found to be the most abundant element with an average of 5543 mg/kg, followed by Iron (ranging from 134 mg/kg to 6839 mg/kg) with an average of 2590 mg/kg. In addition, Aluminium ranged from 1065 mg/kg to 4575 mg/kg with an average of 1963 mg/kg. Magnesium ranged from 166 mg/kg to 2007 mg/kg with an average of 987 mg/kg. Hydrocarbon fractionation of crude oil and diesel into aliphatic and aromatic fractions was successfully done using column chromatography. For diesel, around 80% to 90% of the TPH were recovered using fractionation. For crude oil, approximately 30% of TPH were recovered. Therefore, around 20% of diesel and 70% of crude oil was lost because some compounds were volatile or stuck to the silica gel. Deionized water was also used to extract hydrocarbons from five of the sixteen samples. The water extractable hydrocarbons ranged from C21 to C36 with a TPH concentration of 586 mg/kg in sample Riyadh and 6651 mg/kg in Hufuf. Individual hydrocarbon concentrations also differed. The concentration of C36 in sample Hufuf was 3.48 mg/kg and C29 concentration was 1260 mg/kg. Dichloromethane (DCM) was similarly used to extract hydrocarbons from 13 samples. The concentration of TPHs extracted ranged from 234 mg/kg in sample Khobar to 34708 mg/kg in Karj. Hydrocarbons ranged from C20 to C31 with the same distribution in all samples. The C24 and C25 had the highest concentrations, while C31 was mostly the lowest. The concentrations in sample Hufuf were 137.93 mg/kg for C21 and 3142.43 mg/kg for C24. vii

To assess the ecotoxicology of the background hydrocarbons in crude oil contaminated sand (0.5% up to 10%) and control sand (0 % of TPHs), the germination of seven different types of plant seeds (ryegrass, wheat, coriander, lettuce, radish, cabbage red drumhead and cabbage greyhound) was studied noting germination rate, root and shoot length. Variation was observed in respect to the concentration of crude oil and the type of seeds. Germination experiments were conducted in both the United Kingdom (UK) and the Kingdom of Saudi Arabia (KSA) in order to determine the effect of temperature on plant growth and percentage seed germination. For some plants, seed germination was better in the UK than in KSA. Conversely, some plants germinated better in KSA than in the UK. In the UK, wheat roots and shoots grew better in contaminated sand at all contamination levels than in the control sand (there was a significant difference between roots and shoots compared to control sand as p value < 0.05, apart from shoot at 0.5 % and 5 %, as there was no significant difference as p value > 0.05). In KSA, wheat root and shoot growth decreased in contaminated sand at all contamination levels, except for a 0.5% contamination. In the UK, coriander root and shoot growth decreased at all contamination levels, while the root and shoot g