Distribution, composition and risk assessment of hydrocarbon residue in surficial sediments of El-Dakhla, El-Kharga and El-Farafra oases, Egypt

This work examined the polycyclic aromatic hydrocarbons (PAHs) and n-alkanes quantities, sources, and hazards in sediments collected from the Egyptian Western Desert Oases namely: Dakhla, Kharga and Farafra oases. The n-alkane (C9–C20) residue concentrations have ranged from 0.66 to 2417.91 µg/g recorded for the three Oases. On the other hand, the total n-alkane ranged from 448.54 µg/g to 8442.60 µg/g. Higher carbon preference index (CPI) values (> 1.0) proposed that the natural sources could be the main contributor to n-alkanes in the Oases sediment. GC-MS/MS (selected reaction monitoring (SRM) method) was used for the determination of the ΣPAHs concentrations in the studied sediments. The ΣPAHs concentrations (ng/g, dry weight) in the studied three Oases varied from 10.18 to 790.14, 10.55 to 667.72, and from 38.27 to 362.77 for the Kharga, Dakhla and Farafra Oases, respectively. The higher molecular weight PAHs were the most abundant compounds in the collected samples. Assessing potential ecological and human health issues highlighted serious dangers for living things and people. All the investigated PAHs had cancer risk values between 1.43 × 10–4 and 1.64 × 10–1, this finding suggests that PAHs in the samples under study pose a moderate risk of cancer. The main sources of PAHs in this study are biomass, natural gas, and gasoline/diesel burning emissions.


Sample collection
The Western Desert of Egypt contains seven oases and Dakhla Oasis is one of them.It is located between the Farafra and Kharga Oasis in the New Valley Governorate, 350 kilometres from the Nile.It is around 80 km from east to west and 25 km from north to south 5 .Along with several sub-oases, the Dakhla Oasis is home to several villages.Along with a few other minor villages, Mut, El-Masara, and Al-Qasr are the main communities.In all, 40 surface sediment samples were taken from three Oases in Egypt's New Valley Governorate (Fig. 1) by the mean of the van Veen grab sampler.The areas were chosen while considering the anticipated pollution brought on by human activity (Table S1 in the Supplementary Material).A global positioning system (GPS) was applied to get the coordinates of the sampling sites.The sediments were gathered using a stainless steel grab.Each site received six grabs, and the top 3 cm were scooped into clean, wide-mouth glass bottles, transported frozen, and stored at 20 °C until analysis.Each sample was gathered and weighed at a weight of around 5 g in an aluminium dish.The C-9 to C-20 n-Alkanes and 13 EPA PAHs were analysed in the sediment samples.

Sample pre-treatments
Following a dichloromethane-hexane extraction, sediment heterogeneity in terms of the PAHs distribution was first investigated to look into variations in PAH contents between replicate sediment samples.The findings revealed that the pollutants were evenly distributed throughout the soils, with no variation in total PAH amounts across replicate soil samples (data not shown).The sample was extracted with the use of ultrasound.
Individual samples were taken out of the refrigerator, defrosted at ambient temperature for about five hours, and then dried at 50 °C for a whole night before being subjected to chemical treatment.Then, 5 g of anhydrous sodium sulphate was well mixed with 5.0 g of each sample.Each sediment sample was used to make duplicates.Next, the sediment sample was sonicated in an ultrasonic bath with 2 × 100 mL n-Hexane for 30 min each, and a third extraction with CH 2 Cl 2 (100 mL) was performed.The three extracts were then mixed, desulfurized using Cu metal-activated powder, and concentrated to a few millilitres at around 35 °C in a rotary evaporator.Finally, they were concentrated using a N 2 gas down to approximately 1 mL.Cleaning and fractionation were accomplished by putting the concentrated extract through a column made of silica and Al 2 O 3 .The column was made ready by slurry packing 20 mL (10 g) of silica, 10 mL (10 g) of Al 2 O 3 , and then 1.0 g of anhydrous Na 2 SO 4 .For the saturated aliphatic fraction (F1), the extract (1 mL) was successively eluted from the column with 25 mL of n-Hexane.Then 70 mL of dichloromethane and n-hexane (80:20) were added for the PAH fraction (F2).To prepare F1 and F2 for further chromatographic examination, a gentle stream of pure N2 was used to concentrate them.Since hexane is a nonpolar solvent, it was utilised as the injection solvent to enhance fraction separation.

Instrumental analysis
Because of the excellent specificity and sensitivity of this analytical method for contaminated soil samples, individual PAHs were identified and quantified using GC-MS/MS.A DB5 ms ultra-inert capillary column, with an internal diameter of 0.25 mm, length of 30 m, and thickness of 0.25 m, was used to separate the samples.Splitless mode and a 1 mL injection volume were used for the analysis.Detection of the analytes was performed by employing a Thermo TRACE™ 1300 gas chromatography equipped with PTV mode splitless injector (Temperature 80 °C, Splitless time 1 min, Purge flow 5 mL/min, Carrier gas saver flow 20 mL/min for 5 min, Transfer temperature delay 1 min, Injection pressure 70 kPa for 0.1 min, Transfer pressure 210 kPa, Transfer rate 10 °C/ sec, Transfer temperature 300 °C, Transfer time 3 min, Cleaning rate 10 °C/sec, Cleaning temperature 320 °C, and Cleaning time 10 min) coupled to a TSQ 8000 Evo mass spectrometer (Thermo, USA) operating in selected reaction monitoring (SRM) mode and the Ar gas collision cell was set at 1.5 mL/min.Thermo TriPlus RSH Autosampler was used [19][20][21][22] .
The transfer line was set to 300 °C, the ion source temperature was set to 270 °C, the total scan duration was set to 0.1 sec, the predicted chromatographic peak width was set to 2.0 sec, and the timed scan type was set to SRM for the study of PAHs.The integrated peak area ratio of the target ion to the external standard was used to quantify the analytes.Target ions and retention time order allowed for the identification of the PAH analytes.Table S2 (Supplementary Material) displays the SRM and EI energies applied to PAHs.The GC oven temperature program used for PAH analysis starts at 60 °C and holds that temperature for 1 minute before increasing to 140 °C at a rate of 20.0 °C/min and holding that temperature for 0.0 min before increasing to 300 °C at a rate of 5.0 °C/min and holding that temperature for 4 mins.Thermo Scientific Xcalibur was used for data capture, reprocessing, and report production.
The transfer line was set to 300 °C, the ion source temperature was set to 270 °C, the total scan duration was set to 0.1 sec, the predicted chromatographic peak width was set to 2.0 sec, and the timed scan type was set to SRM for the study of n-alkanes.The integrated peak area ratio of the target ion to the external standard was used to quantify the n-alkanes.Target ions and retention time order were used to identify the n-alkanes.For n-alkanes, the SRM and EI energies are displayed in Table S3 (Supplementary material).The GC oven temperature program used for n-alkanes analysis starts at 65 °C and holds that temperature for 2 minutes before increasing to 250 °C at a rate of 15.0 °C/min and holding that temperature for 0.0 mins before increasing to 300 °C at a rate of 8.0 °C/ min and holding that temperature for 2 mins.Thermo Scientific Xcalibur was used for data capture, reprocessing, and report production.
Quality control (QC) and quality assurance (QA) techniques were used to guarantee the correctness and precision of the analytical results.Blank analysis, duplicate extract analysis, and reanalysis of samples with relative percent differences greater than 20% were performed 23,24 .Additionally, standard curves were calibrated daily using reference standards, and calibration levels were checked after every ten extract analyses.The analyte amounts in the blank samples were insignificant or below the limit of detection (LOD).The TSQ instrument's regression coefficient for the calibration curves of analytes varied from 0.975 to 0.997.Each method's results were extremely accurate, with a relative error range of 0.1 to 5.0%, and the findings of each approach were quite accurate.The recovery rates of n-alkanes and PAH residues from water and sediment samples ranged from 92.4 to 107.8%.

Physiochemical parameters
Table S1 (in Supplementary Material) displays the results of physicochemical parameters for the water body of the sampling sites of the collected surficial sediment.The water's average Temp., pH, EC and TDS were 19.6 °C, 8.12, 41364.9μs/cm and 26490.5 ppm, respectively.

Composition, distribution, and concentration of PAHs
Tables S4a-c (Supplementary Material) show each PAH's residue levels in all tested samples.The ∑PAHs concentrations varied significantly, ranging from 10.18 to 790.14 ng/g with a mean value of 166.44 ng/g.The average concentrations of ∑PAHs were recorded in the collected sediment samples in the following descending order: Farafra > Dakhla > Kharga.In all of the analysed samples, PHN was the predominant congener.08 ng/g.The current study's order demonstrates that HMW PAHs predominated.This could be explained by HMW PAHs in bottom sediments having a lower solubility due to their adsorption on particulate matter 16 .
The ∑PAHs, %, and ratios are presented in Tables 1, 2 and 3.For ∑PAHs, ∑PAH CARC (BaA +BbF + BaP + DahA + IP) were well-thought-out carcinogens 25 .The highest concentration level of ∑PAH CARC was documented in Kharga (69.48 ng/g), followed by Dakhla (65.04 ng/g).∑PAH CARC ranged from 94-334.55 ng/g with 37.58% of total PAHs.Moreover, the average of ∑PAH COMB (PYR + CHR + BkF + BghiP) ranged from 61.14 to 63.85 ng/g, Table 1.Interrelationship indices of PAHs recorded in Kharga Oasis (ng/g, dry weight (dw)). 1 Carcinogenic PAHs, 2 Combustion PAHs, 3 fossil-fuel PAHs, 4 Low molecular weight PAHs, 5 High molecular weight PAHs.Petroleum products or crude oil are the principal sources of petrogenic pollutant-derived PAHs, with three-to four-ring structures.While pyrolytic pollutant-produced PAHs have 4-6 rings built into them 26 .Generally, PAHs containing > 4-rings are believed to be more toxic than PAHs, which have two or three aromatic rings 27 .On the other hand, the contribution of potential pollution sources was identified and quantified using the diagnostic ratios of the residue levels of different PAHs 28,29 .The PHN/ANT ratios in this study's samples were all less than 10 (Tables 1, 2 and 3), confirming pyrolytic origin.Notably, the BaA/CHR isomeric ratio was close to 1, which supports the comparable source.The literature uses the IP/(IP + BghiP), FLR/(FLR + PYR), and ANT/(ANT + PHN) for source clarification [30][31][32] .Tables 1, 2 and 3 present the ratios calculated for PAHs, BaA/CHR ratio ranged from 0.76 to 1.13, which is similar to that for coke and coal emission (ratio ranged 1.05-1.17) 33.The range of BaP/BghiP was 1.30 to 6.33, which indicates that traffic emissions are the most likely cause.Traffic emissions are frequently present when the ratio of BaP/BghiP is greater than 0.60 34 .Additionally, a recorded IP/BghiP ratio greater than 0.4 indicates a diesel engine source.According to Caricchia et al. 35 , the IP/BghiP ratio for petrol engines is around 0.40 and rises to 1.00 for diesel engines.A dominating source of combustion is often considered one with an ANT/(ANT + PHN) ratio > 0.1 30 .All samples had a FLU/(FLU + PYR) ratio below 0.5, indicating petrol emissions, whereas ratios over 0.5 indicated diesel emissions 36 .According to Yunker et al. 30 , the IP/(IP + BghiP) ratio values ranged from 0.27 to 0.33, showing inputs from liquid fossil fuels and/or sources of vehicle combustion, as well as those from the process of burning biomass and coal.Additionally, IP/(IP + BghiP) ratios greater than 0.5 denote coal combustion, whereas IP/(IP + BghiP) ratios of 0.62 denote wood combustion, reflecting the same sources of pollution 30,36 .

Composition, distribution, and concentration of n-alkanes
The individual n-alkanes (C9 to C20) ranged from 0.66 at M12 station to 2417.91 µg/g recorded at M15 station (Table S5 in Supplementary Material).The total n-alkane ranged from 448.54 µg/g at M28 location sample to 8442.60 µg/g at M15 location sample, with an average of 1390.10 ±1392.73µg/g.C-20 recorded the highest average value, while C-10 recorded the lowest value among all the studied stations.
The carbon preference index (CPI) has been applied to n-alkanes to help determine their origin.According to Zdanaviciute et al. 37 , the CPI values for petroleum were reported to range from 0.93 to 1.07.On the other hand, according to Bi et al. 15 , the CPI values for plants ranged from 2.3 to 54.3.The greater contribution of n-alkanes from manufactured sources, such as burning biomass and petroleum pollution, is indicated by CPI values that are near unity; higher CPI values imply a bigger contribution from natural sources, such as terrestrial vegetation and biogenic 38 .According to Tareq et al. 38 , when the carbon preference index is near 1, it is linked to the emissions from vehicles and other activities; when it is greater than 1, it is linked to terrestrial vegetation.Furthermore, Simoneit 39 showed that a CPI < 5 has a greater proportion of odd-numbered n-alkanes, which are produced by cracking and dehydrating n-alkanes and n-alcohols, respectively.The CPI was computed using the subsequent Eq. (1) 40 : (1) CPI = �(C9 − C19) ODD /� (C10 − C20) EVEN , where (Ci-Cj) ODD and (Ci-Cj) EVEN are the residue of the n-alkanes with an odd carbon number and that with an even carbon number, respectively, over the range i-j.
The computed CPI values for sediment samples ranged between 0.57 and 3.02 (Table S5 in Supplementary Material).For the sediment, the greatest high CPI values are found in station M22 with CPI > 1.The CPI values above 1.0 indicate that the n-alkanes at these locations may have come from natural sources.CPI levels near 1 are connected to many activities and automobile emissions.In our investigation, the values of CPI larger than 1 demonstrate a preference for n-alkanes with odd rather than even carbon numbers.Neither even nor is odd numbered alkane preponderance, which is typical of petroleum and/or mixed sources, represented by CPI values equal to 1 41 .
The n-alkanes mean carbon number (MCN) of the sediment samples was computed according to the subsequent Eq. ( 2): where [Ci] and [T-Cs] are the residues of the n-alkanes with carbon number i and that of the T-Cs, respectively.
For sediment, the MCN value (Table S5 in Supplementary Material) ranged between 12.12 and 17.94.The high MCN value was at M22 site, while the lower was at M28, with an average MCN value of 14.65 ±1.48.The high relative abundance of trees in comparison to grasses and herbs during the low flow of organic carbon might be the cause of the low MCN values 38 .

Risk assessment of PAHs
The 10 ng/g tolerance level for BAP has been suggested by the Joint Food and Agriculture Organisation and World Health Organisation Expert Committee on Food Additives, which has examined BAP as a general indication of PAHs 42 .Of the PAHs, BaP is the most hazardous 43 .BAP recorded an average of 15.38-20.91ng/g above the tolerance limit.A maximum of BaP was recorded in Kharga 109.19 ng/g > Dakhla 102.8 ng/g > Farafra 52.14 ng/g.In addition, the ratios of BaP were above that recorded for BbF, BkF and BghiP in all samples (Tables 1, 2 and 3).The HQ method, which reflects a ratio of the exposure dosage for each PAH to an oral chronic reference dose 44 , was used to assess the risk associated with dietary exposure to non-carcinogenic PAHs.It should be noted that RFD is a threshold dosage or intake that was cautiously selected.Therefore, there is little chance of a negative impact on health if the predicted consumption is lower than the reference dosage (HQ 1).The risk level of 10 -6 is used by the USEPA 45 , as the threshold at which risk management choices can be made.Decisions on risk management are often based on the cancer risk range of 10 -6 to 10 -4 .All the investigated PAHs had cancer risk values between 1.43 × 10 -4 and 1.64 × 10 -1 , as indicated in Table 4.This finding suggests that PAHs in the samples under study pose a moderate risk of cancer.Figure 3 shows the residue amounts of PAH with 3-, 4-, 5-, and 6-rings in the investigated sediment.

Conclusion
In all of the investigated species, ACE was the predominant congener.The following species' average residue levels (ng/g) of PAHs were found in descending order: The following values are in order of importance: ACE 158.

Figure 1 .
Figure 1.Sampling locations throughout the studied region.

Figure 2 .Figure 3 .
Figure 2. (a) Distribution of COMB-PAHs in the three studied Oases.(b) Distribution of F-PAHs in the three studied Oases.(c) Distribution of CARC-PAHs in the three studied Oases.

Table 4 .
Factorial analysis for residual PAHs (ng/g, dry weight (dw)).NA not available, ND not determined, HQ hazard quotient, CR cancer risk index, ADD Calculated average daily dose, CSF cancer slope factor mg/kg/ day, HI hazard index, RFD reference dose mg/kg/day.

Table 5 .
PAHs concentration levels in surficial sediments in the Egyptian Mediterranean coast in the present study compared with other coastal studies in the Mediterranean Sea and other world regions.