ORIGIN AND SOURCES OF POLYCYCLIC AROMATIC HYDROCARBONS (PAHs) IN SEDIMENTS CORE FROM TIGRIS, EUPHRATES AND SHATT AL-ARAB RIVERS

Due to the important area of the Tigris, Euphrates and Shatt Al-Arab rivers in Iraq, and the effect of pollutant to theses rivers, the object of study is the origin and sources of PAHs compounds in sediment core samples which collected in 2021 from six important stations that are (Tigris1, Tigris2, Euphrates1, Euphrates2, Shatt Al-Arab1, and Shatt Al-Arab2). Polycyclic aromatic hydrocarbons (PAHs) were analyzes by using capillary gas chromatography. The results of PAHs shown in two pattern low and high molecular weight. The total PAHs ranged between 79.141 ng/g at station No. 6 to 3.830 ng/g at station No. 3. The rush to develop industries across the globe accelerates environmental damage brought on by many contaminants, including PAHs. Organic compounds in the PAHs class have two or more aromatic rings. PAHs can be pyrogenic, petrogenic, or biogenic depending on how they develop. Pyrogenic PAHs are produced when various fuels, oil and gas, waste, or other organic materials like fume from oil industries in the area. The investigation showed two patterns of sources petrogenic and pyrogenic with the petrogenic source predominating according to the ratios (low molecular weight/high molecular weight), anthracene/(anthracene+phenanthrene) and fluoranthene/(fluoranthene+pyrene). Additionally, findings indicated that sediment pollution is of a moderate pollution. By adhering to sedimentary particles, PAHs get into the sediments. Based on the physicochemical characteristics of each fraction and the surrounding environment, sediments also serve as a source for some contaminants that re-enter the water column. Lighter PAHs predominated in water samples, while heavier compounds predominated in sediment samples, according to several studies. In addition, it is difficult to remove the high concentrations of PAHs in riverine sediments brought on by industrial activity. While other research indicated significant PAHs pollution in a variety of global environments. Due to the fact that such research helps to lessen the obvious shortage of information regarding such pollutants in Iraqi rivers, this study gives as the baselines for coming research.


Introduction
One of the major issues that have attracted a lot of attention globally is the contamination of sediment and aquatic environments with hydrocarbons. These compounds, which are derived from crude oil and products like diesel, gasoline, lubricating oil, and others, have been shown to be highly toxic, genotoxic, and carcinogenic in nature [1,2]. One large class of chemical compounds, known as hydrocarbons, have carbon and hydrogen as its basic building blocks, along with a variety of heteroatoms like oxygen, nitrogen, chlorine, sulfur, and others. Aliphatic, alicyclic, and aromatic compounds are the three main classes that can be used to categorize the hydrocarbon molecules [3,4].
There are two types of polycyclic hydrocarbons: low molecular and high molecular. Low molecular polycyclic hydrocarbons have two to three fused rings and are soluble in water and volatile, making them susceptible to degradation processes. High molecular polycyclic hydrocarbons have more than four fused rings and are less soluble, less volatile, and more lipophilic than low molecular polycyclic hydrocarbons [5].
After the confluence of the Euphrates and the Tigris Rivers, the Shatt Al-Arab River is formed near the city of Al-Qurna in southern Iraq [6]. The territory flowing to the Shatt Al-Arab region downstream of Al-Qurna is shared by Iran and Iraq. The Shatt Al-Arab River, which runs for 192 kilometers, widens along the way, from 250-300 meters near the Euphrates-Tigris confluence to ISSN 2664-9969 about 700 meters around Basrah and more than 800 meters as it approaches the river mouth. A total of 145,190 km 2 of land flows directly into the Shatt Al-Arab region downstream of the Euphrates-Tigris confluence, excluding the Euphrates and Tigris Basins [7].
This rivers is the most important source of fresh water in Iraq, and influenced by freshwater discharges from agricultural runoff, industrial activities, and untreated domestic sewage.
Thus, the object of study is the origin and sources of PAHs (polycyclic aromatic hydrocarbons) compounds in sediment core samples which collected in 2021 from six important stations that are (Tigris1, Tigris2, Euphrates1, Euphrates2, Shatt Al-Arab1, and Shatt Al-Arab2).

Research methodology
Sediment cores pipe of (120 cm length and 5 cm diameter) were collected from six stations in 4 December 2021 which represent different sites of the Euphrates and Tigris Rivers and the Shatt Al-Arab River ( Fig. 1) for analyzing and estimating the concentration of polycyclic aromatic hydrocarbons (PAHs) in these sediment core.
The cores were inserted into the water-sediment interface and pushed to ensure that they reached maximum depth. The cores were slowly retrieved back, closed with its cover immediately and marked as to which is the upward direction.
The samples were dried in an air grinded in an electrical stainless steel mortar and sieved. Through 63 µm sieve, 25 gm of sieved sediments were placed in cellulose thimble and soxhlet. Extracted using soxhlet intermittent extraction [8] with mixed solvents (120 ml) methanol:benzene (1:1 v/v) for 24-36 hrs. at temperature doesn't exceed 40 °C. At the end of this period, the combined extracts were saponification for 2 hrs. by adding (15 ml) 4M MeOH (KOH) at the same temperature, then cooled to room temperature, using separator funnel to extracted the un saponification matter with (40 ml) n-hexane. The upper un saponification matter with hexane was taking and passed through chromatographic column provided with glass wool at the bottom then a layer of silica gel and a layer of alumina, in the top, a layer of anhydrous sodium sulfate was placed to collect the aliphatic fraction, then 40 ml of benzene added to collect the aromatic fraction, analysis were done by using capillary gas chromatography.

Research results and discussion
At six stations, the total concentration of polycyclic aromatic hydrocarbons in sediment ranged from 79.141 ng/g at station No. 6 to 3.830 ng/g at station No. 3.The total averages of aromatic compounds concentrations in the regions were calculated in Table 1 and Fig. 2 and Fig. 3.
In all stations, the source of PAHs was petrogenic, as shown by the first of them, Low Molecular Weight/High Molecular Weight -PAHs in the (      The study found that the sources of polycyclic aromatic hydrocarbons were both petrogenic and pyrogynic, with the predominance of Flouren and Phenanthrene in high concentration indicating a Petrogenic source and the pre-sence of Anthracene in most stations indicating a pyrogenic origin. This finding is consistent with those of [9][10][11]. If to compare received data with other studies in the area let's find that it is within the range (Table 4).  The PAHs concentration in the sediments was compared with US National Oceanic sediment quality guidelines (Table 5) [20]. The recommended effect range low (ERL) and effect range median (ERM) target values were used to determine toxic effects in the sampling locations. When PAH concentrations vary between ERL and ERM values, a mild toxic effect is expected. In addition, no negative effect is expected for PAH concentrations lower than ERL values. All PAH compounds were below ERL, ERM values in all sampling locations, indicating no mild toxic effect.

ISSN 2664-9969
According to [21] classification which depending on the concentration of total PAHs in the sediment is classified as non-contaminated with the total PAHs <200 ng·g -1 ·dw, weakly contaminated 200-600 ng·g -1 ·dw, moderately contaminated 600-1000 ng·g -1 · dw, and heavily contaminated >1000 ng·g -1 ·dw. According to this classification, most samples in the current study were weakly contaminated by PAHs including (oil areas, roads, petrol stations, power plants and electrical generators) samples.