Effect of Different Crude Oil Levels on A bioavailability of Some Heavy Metals in Two Different Texture Soils

Soil polluted with crude oil is one of the most serious environmental problems in the world, which is mostly caused by human activity. Crude oil-associated heavy metals in the polluted soils are still considered as one of the major issue around the globe. Two soils were polluted with crude oil at levels of 0.0, 1.25, 2.5, 5, 10, 15 and 30 ml.kg−, then concentrations of heavy elements were added, 30 ml.kg− and 140 ml.kg− for lead and nickel and 40 ml.kg− and 150 ml.kg− for copper and zinc. When adding levels of (0, 1.25, 2.5, 5, 10, 15 and 30) ml.kg− of crude oil and (30 and 140) ml.kg− of lead and nickel, and (40 and 150) ml.kg− of zinc and copper to sandy and clay soils, it turns out a superiority in available lead concentrations over available nickel concentrations in sandy soil on the one hand, and a superiority in available nickel concentrations over available lead concentrations in clay soil when adding concentrations of 30 and 140 ml.kg− of the two elements on the other hand. It was also observed that the available form of copper concentrations were superior to the available form of zinc concentrations in sandy soil, and the available form of zinc concentrations were superior to the available form of copper concentrations in clay soil, when adding low concentrations (40 ml.kg−) and relatively high concentrations (150 ml.kg−). Generally, the results of this study varied depending on the effect of the crude oil levels and heavy metals concentrations.


Introduction
Oil spills usually occur due to the uncontrolled release of crude oil into the environment, either as crude oil or refined products releases contaminants into the environment including heavy metals [1].Soil polluted with crude oil is one of the most serious environmental problems in the world [2,3], which is mostly caused by human activity and changes in soil properties.It affects biological, chemical composition [4], as well as it leads to changes in plants' phytotoxic properties and state, soil enzyme activity, nitrogen turnover in the soil and changes in soil microorganisms [5][6][7].Crude oilassociated heavy metal (HMs) in the polluted soils is still considered as one of the major issue around the globe.Previous studies have established that crude oil contains several inorganic and carcinogenic pollutants such as heavy metals (HMs) that cause heavy risks to the surrounding environment [8,9].Heavy metals are one of these contaminants accumulated in soils and increased the risk of causing a range of diseases (e.g.cancers and affected infants birth for inhabitants living around) [10], as well as some heavy metals (trace elements) play a significant role in metabolic processes in all living organisms.Some of them are essential for the normal growth and development of plants (copper, cobalt, nickel, zinc, chromium, and manganese) because they take part in many enzymatic reactions.However, among trace elements there are heavy metals such as cadmium, lead, and mercury, which may be toxic to cultivated plants even in low concentrations [11][12][13][14].Heavy metals persist in the soil for a long time due to their adherent quality and therefore the soil is considered the major sink for HMs [15].The HMs after entering the soil may percolate deep beneath the ground and may lead to groundwater contaminations [16,17].This is considered a serious issue as it leads to food chain contaminations thereby causing risks and hazards to the biotic environment [18].

Materials and Methods
The research aimed to study of the effect of crude oil pollution and heavy metals on available concentrations of lead, nickel, zinc and copper in sandy and clayey soils.The two soils were air-dried, ground and passed through a sieve with a diameter of 2 mm and mixed well.The two soils were polluted with crude oil at levels of 0.0, 1.25, 2.5, 5, 10, 15 and 30 ml.kg -then concentrations of heavy elements were added, 30 mg.kg -and 140 ml.kg-for lead and nickel and 40 mg.kg -and 150 ml.kg -for copper and zinc.The soil samples were incubated for 30 days after being polluted with crude oil and heavy metals.Soil analyses were performed on the homogenized soil in three replicates and the results are presented as mean values in Table 1.The soil pH and electrical conductivity (EC) were measured in a slurry of a 1:1 w/w (soil/water) ratio [19,20].The soil particle size distribution was determined using the Bouyoucos hydrometer method [21], while the percentage of organic matter (OM) in the soil samples was determined using Walkley-Black's method [22].Cation exchange capacity was determined according to Savant method [23].The concentrations of the available heavy metals were determined according to [24].After the soil samples were filtered using filter paper Whatman 42, the concentrations of the available heavy metals were determined using Atomic Absorption Spectrophotometer (AAS).

Results and Discussions
When adding levels of (0.0, 1.25, 2.5, 5, 10, 15 and 30) ml.kg -of crude oil and (30 and 140) ml.kg -of lead and nickel, and (40 and 150) ml.kg -of zinc and copper to sandy and clay soils (Fig. 1-4), it turns out, a superiority in available lead concentrations over available nickel concentrations in sandy soil on the one hand, and a superiority in available nickel concentrations over available lead concentrations in clay soil when adding concentrations of 30 and 140 ml.kg-of the two elements on the other hand.It is caused by the occurrence of a chemical distribution of the two elements by the availability of quantities of water at the field capacity in sandy and clay soil samples, depending on the concentration of the element and its forms on which it is present, and on soil characteristics such as the pH value, organic matter, electrical conductivity, and texture [25], and when the effect of these characteristics is equal, with the exception of soil texture, it is certain that there is a significant effect of soil texture on the distribution of elements in the soil.[26] explained that lead showed a positive correlation with fine particles in clay soil compared with sand particles.This provides an opportunity for its superiority in sandy soils in the free (dissolved) form, and with a lower degree and strength of bonding with fine particles of clay present in small quantities in sandy soils due to weak non-specific adsorption mechanisms as a result of the difference in electrical charges of the element and soil particles in the exchangeable form [27].It was also observed that the available form of copper concentrations were superior to the available form of zinc concentrations in sandy soil on one hand, and the available form of zinc concentrations were superior to the available form of copper concentrations in clay soil on the other hand, when adding low concentrations (40 ml.kg -) and relatively high concentrations (150 ml.kg -) of the two elements, it may be caused by the low adsorption capacity of copper in the sandy soil because it does not contain fine clay particles that have a high specific surface area that provides binding sites.This element has a high adsorption capacity with clay soil particles compared to the zinc element [28], which makes it highly concentrated in the clay part of the soil, or it may interfere within the crystalline lattice of the soil [29,30].The reason may be, according to what was mentioned by [31][32][33][34], that clay soils with a high content of lime work to control the ions of dissolved copper in soil solution and adsorbed on the surfaces of its fine particles are subject to a specific adsorption mechanism with strong covalent bonds, in addition to having a high affinity for the copper element towards the dissolved organic ligands available in the soil solution as a result of adding levels of crude oil to form an element-organic ligand complex, which increases its mobility and facilitates its availability in the sandy soil compared to clay soil [35].

Figure 1 .
Figure 1.Available concentrations of heavy metals at levels of crude oil(ml.kg-) and 30 mg.kg -of Pb and Ni and 40 ml.kg -of Zn and Cu in sandy soil.

Figure 2 .
Figure 2. Available concentrations of heavy metals at levels of crude oil(ml.kg-) and 30 mg.kg -of Pb and Ni and 40 mg.kg -of Zn and Cu in clayey soil.

Figure 3 .
Figure 3. Available concentrations of heavy metals at levels of crude oil(ml.kg-) and 140 mg.kg-of Pb and Ni and 150 mg.kg -of Zn and Cu in sandy soil.

Figure 4 .
Figure 4. Available concentrations of heavy metals at levels of crude oil(ml.kg-) and 140 mg.kg-of Pb and Ni and 150 mg.kg -of Zn and Cu in clayey soil.

Table 1 .
Physio-chemical properties of sandy and clayey soils.