Las tensiones

Soil quality assessment is one of the key factors for evaluation the extent of contamination in the environment has increasingly emphasised the need to consider metals fractions reactivity in soils around anthropogenic activities (Lock and Janssen, 2001; Masto et al., 2015). Based on various studies, organic ligands, trace metals, inorganic fractions and microbial proportions have been used as soil quality indicators (de Haan and Visser-Reyneveld, 1996; Li et al., 2005; Ivezić et al., 2015). Trace metals in soils can be originated from a natural source or varying consequences of economic activities. Thus, aquatic soils constitute the most important pool or sink of different chemical forms of metals and other contaminants governed by lithology of the soils and rock in contact with them (Liang et al., 2014). Soils scientists have long emphasised that trace metals fractions and their toxicity are essentially absorbed and uptaken by biota from the soil solutions (Zhang and Young, 2005; Wuana and Okieimen, 2011). The behavior of trace metals in soils can be influenced by their sorptions and desorptions reactions with varying soil components and matrix. The bioavailability of trace metals in soil solutions can be influenced by various processes including metals dissociation and release from chemical contaminants (e.g., crude oil, agricultural practices (pesticides and fertilizer), paints and economic ports activities)

(Zhang and Young, 2005; Rocha et al., 2011). Furthermore, the chemical forms of metals in soils; water-soluble fractions, organic/or inorganic ligands, exchangeable and Fe/or Mn oxides bound, are kinetically determine the metals lability and mobility (Iwegbue, 2011). Soil contamination with crude oil has become a global environmental concern especially regions surrounding oil exploration, heavy industries and oil refineries, which are more threatened. Crude oil contains different concentrations of saturated, aromatic, and resins constituents at varying molecular weight (MW) range, depending oil type on degradation and microbial activities as described above (Xiao et al., 2010; Iwegbue, 2011). Within hydrocarbons in soils or sediments derived from spilled or discharged oil, organic complexes matrix, recalcitrant hydrocarbons as poly aromatic/substituted aromatic molecules (Wang and Stout, 2010; Omaka et al., 2011), including resins and hydrocarbons containing different functional groups (like carboxylic acids, ethers, other organic acids etc.), are the most dominant fractions, and represent more than 45% of the released oil components in the contaminated area (Tissot and Welte, 1984; Abha and Singh, 2012; Osam et al., 2013).

In most circumstances, free metals ions, inorganic and organic fractions released from crude oil spill are the key species that influence the trace metals speciation in the soil (sediments) (Guéguen et al., 2011; Majolagbe et al., 2012; Adesina and Adelasoye, 2014; Fu et al., 2014).

However, there is a lack of consistency may be partially interpreted by the lack of a standardised experiments to generate and ensure reliable, reproducible and comparable obtained data. Very little fieldwork has been reported in the literature regarding the effect of crude oil contamination on trace metals loads in soils applying conventional techniques and extraction methods. However, still a lack of important

standardised laboratory protocols to better understand the influence of crude oil constituents on the trace metals speciation in soils impacted with crude oil using reliable technique. It was found (Omaka et al., 2011; Shukry et al., 2013; Fu et al., 2014) that crude oil complexes in soils can either enhance or decrease the availability of trace metals depending on the oil composition. The decreased concentrations were probably due to the complexation with strong organic and inorganic ligands of high molecular weight from oil.

Considerable research has been conducted in Nigeria and the Dammam zone in the east of Saudi Arabia on the effect of crude oil contaminated soil on the availability of nutrient elements and the properties of soil (Agbogidi, 2013; Shukry et al., 2013). They treated the soils with different levels of oil (w/w). The results in both areas showed that an increase of trace metals Cu, Mn, Cd and Pb, Fe and Pb in soils and plant tissues with increasing crude oil concentration oil. Iwegbue (2011) and Omaka et al. (2011) investigated the effect of organic and inorganic fractions on the trace metals mobility and bioavailability in soils contaminated with crude oil in the Niger Delta using conventional extraction methods and 0.2-µm nucleopure polycarbonate membrane filters before being analysed for metals by atomic absorption spectrophotometry (Perkin Elmer, A3100). Cu and Cd were found at low concentrations in the soils affected with oil due to the formation of organometallic complexes with organic matter from oil. Ni and Cr were governed by residual fractions and found at higher concentrations maybe due to their release from oil contamination. Fu et al. (2014) pointed out that oil industries in the field contribute to the trace metal loads in the soil located nearby oil refineries and extraction processes using acid conventional extraction and high pressure microwave digestion before analysis by ICP-MS, 7700i, algilent, USA. The results demonstrated that all targeted

metals were increased and by fractionation analysis the mobility of them in soil contaminated with crude oil decreased in the order Cd > Mn >Zn > Ni > Pb > Cu > Cr > V. All studies revealed that some trace metals (Ni, Cu, Cd and Cr) in contaminated soils and sediments were found at a wide range of varieties, maybe due to the oil composition and level of pollutants entering the soil.

Most studies on the effect of crude oil constituents on trace metals in soils focused on measuring the total concentrations rather than speciation and investigating the influence of the matrix in biogeochemical processes in the environment. All studies revealed that some trace metals (Ni, Cu, Cd and Cr) in contaminated soils and sediments were found at a wide range of varieties, may be due to the oil composition and level of pollutants entering the soil.

Relatively limited or no sufficient information is available regarding the effect of crude oil constituents on trace metals availability in soils. For the three decades the sultanate of Oman relied on crude oil and petroleum products as a main source for economic support. Moreover, the industrial estates and economic port activities were increased. On the other hand, this rapid growth leads to serious environmental problems on the environment in terms of its increasing risks of contamination of coastal soils and communities. Sohar industrial region (SIR) in Sultanate of Oman has crude oil refinery and large number of industries operating in a confined area closer to agriculture farms, which cultivates area as fodder for their livestock (Jamrah et al., 2007; Abdul-Wahab and Jupp, 2009; Al-Rashdi and Sulaiman, 2013). (SIR) has indicated an interest in evaluating the potential influence of industrial and crude oil ligands on the trace metals availability in costal soils around industrial area. However, there is still limited studies of heavy metals in economic ports in Sultanate of Oman; in particular, published work is scarce for the main economic ports and industrial

regions (Al-Husaini et al., 2014). Moreover, there was lack information about trace metals speciation in these regions. Therefore, it is very difficult to interpret the contaminants inputs due to unavailable of natural concentrations of dissolved trace metals. These ambiguous findings probably can be explained by lack of standardised laboratory protocols, by treating different soils of varying properties with different levels of crude oil (%w/w) followed by field assessment to see the contribution of oil ligands to trace metals fractionation (Omaka et al., 2011; Kleindienst et al., 2015). The in situ speciation technique, DGT, has alternatively been used to determine the amount of metals that kinetically resupplied from the solid phase to the soil solutions (Zhang et al., 2004; Zhang and Young, 2005). The reduction in the resupply rate of metals fractions from solid phase into soil solution is typically attributed to the presence of high levels of large organic ligands and colloids from contamination. This could increase the amounts of binding sites for retaining cations (Kovaříková et al., 2007). If the R = CDGT/Csoln is 0.1 < R < 0.9, it indicates the soil solid phase able to resupply

metals fractions into soil solution. When greater than (0.9), the metal is present in soil solution kinetically labile fractions and mobile due to high capacity of solid phase to resupply soil solution with metals species. If the R value is very low and less than 0.1, it indicates that capacity of solid phase to resupply soil solutions by metals species is very low or no metals can be resupplied and kinetically limited (Zhang et al., 1998; Senila et al., 2012).

2.12. Assessment of trace metals pollution in sediments