The aim of this paper is to study the effects of oil production and industrial activity of the (North Oil Company) on the soil pollution by heavy metals in the Kirkuk area, using sequential extraction method.

All reagents were analytical reagent grade unless otherwise. Hydrogen peroxide, acetic acid, ammonium acetate, hydroxyl ammonium chloride, HCl and HNO₃ were of suprapure quality (Aldrich Chemie, Germany) All plastic and glassware were cleaned up by soaking dilute (1 + 9) and were rinsed with distilled water prior to use. Standard metals solution (1000 mg/l) were purchased from (BDH, Poole, UK) or prepared in lab from pure metals. All the soil samples were carried to laboratory and prevesed in a refrigerator at 3˚C - 4˚C.

Determination of cadmium, copper, zinc, and lead. The soil samples were digested using concentrated analytical grade HNO₃ and HClO₄. Approximate 0.5 g of dry soil sample was put into a glass vessel and 5 mL of concentrated HNO₃ was added. In succession, the glass vessel was heated on an electrothermal board at 150˚C until the mixture was nearly dried. Another 5 mL of concentrated HNO₃ and 3 mL of concentrated HClO₄ were added into the glass vessel and maintained at 150˚C for 3 hrs. During the digestion process, a tundish was used to cover the mouth of glass vessel for preventing acid splashing. After cooling, the digested sample was decanted into a glass tube and diluted to 25 mL with 2% HNO₃. All AAS measurements were carried out using a Shimadzu AA-640 (graphite) AAS (Shimadzu Scientific Instrumental, Kyoto, Japan).

The sequential extraction procedure used was similar to that of Tessier et al. [14] the procedure a pertionally groups heavy metals into the following five fractions:

F.1 = Soluble and exchangeable

The soluble and exchangeable metals from soil were extracted with 20 mL of a 1.0 M MgCl₂ solution adjusted to a pH of 7.0 by mechanical shaking for 1 h at 20˚C.

F.2 = Bound carbonates

The carbonates in the residue from the previous step were extracted with 20 mL of a 1.0 M NaAc solution adjusted to a pH of 5.0 with HAc by continuously shaking for 4 h at room temperature.

F.3 = Bound to iron and manganese oxides

Metals bound to iron and manganese oxides were extracted from the residue of the second extraction by shaking with 50 mL of a 0.04 M NH₂OH∙HCL/25% HAc solution. The extraction was performed at 96 = 3˚C for 5.5 h.

F.4 = Bound to organic matter

Metals bound to organic matter were extracted by pouring 7.5 mL of a 0.02 M HNO₃ solution and 12.5 mL of a 30% H₂O₂ solution adjusted to a pH 2.0 onto the residue from F3, then providing continuous agitation for 2 h at the temperature of 85˚C. An additional volume of 7.5 mL of the 30% H₂O₂ solution adjusted to a pH of 2.0 is then added, while maintaining continuous and a temperature of 85˚C for another 3 h. The solution was then cooled to room temperature. An aliquot of 12.5 mL of a 3.2 M NH₄Ac/20% NHO₃ solution was added and shaken for 30 min.

F.5 = Bounded to the soil (residual fraction)

The residue from F4 was quantitatively transferred into a digestion vessel and metals were dissolved in aqua regia using 7 mL of 10 M HCl and 2.3 mL of 15.8 M HNO₃. The temperature of the reaction mixture was slowly risen until reflux condition and maintained for >2 h. Three parallel extractions were carried out, in each case using 2.5 g soil. All solid/liquid separation was performed by centrifuging, at 5000 rpm for 15 min. The supernatant was removed and analyzed for metals. The residue was washed with 20 mL of de-ionized water. After centrifugation for 15 min the supernatant was discarded.

The exchangeable and bond to carbonates species, corresponding to the first two fractions, are generally called “Bioavailable”, as they exhibit a mobility relative to the environment and are potentially available for plants. Results show that the amounts of Zn, Cu, Cd and Pb in Bio-available from are very low, the abundance of metals in exchangeable fraction is (%1.35 Zn), (%3.75 Cu), (%10.96 Cd) and (%5.22 Pb) this for the soil inside the city, either with regard to percentages of the soil outside the city were ranged (%1.52 Zn), (%1.75 Cu), (%1.82 Cd) and (%5.29 Pb). Heavy metals in the exchangeable held by electrostatic adsorption represent the most mo-

bile and readily available for biological uptake in the environment thus this fraction can be regard as a pollution indicator [18] . Whereas, the percentage of carbonate fraction for the soil within the city were (%12.35 Zn), (%10.97 Cu), (%12.69 Cd), and (%14.02 Pb), and the value of the metals from outside the city were (%7.82 Zn), (%5.825 Cu), (%10.044 Cd), and (%10.145 Pb).

In this fraction the results of sequential extraction shows that the percentages were (%68.05 Zn), (%12.34 Cu), (%26.74 Cd) and (%8.62 Pb) this for the inside the city, the outside of the city is (%51.9 Zn), (%11.84 Cu), (%23.85 Cd) and (%21.27 Pb). Zinc was highly abound to Fe-Mn oxides [19] , in both samples more than the other elements, aboundace of Zn in other fractions is low and this refer to that Zn in this environment is more mobile than the metals that are mostly abundant in the residual fraction [20] . The residual fraction represents metal largely embedded in the crystal lattice of the soil fraction and should not be available for remobilization except under very harsh consideration [21] . Zinc strongly bound in the Fe-Mn oxides fraction and it has stability constants high enough to be concentrated in this fraction [22] [23] . Zn in high levels in the Fe/Mn oxide phase agree with (Fagbote. et al., 2010) and (Jian-Min et al., 2007) [23] [24] . According to Jian-Min results under reducing conditions metals present in the Fe-Mn oxide phase are unstable and easily released through dissolution releasing soluble metals. This implies that the potential risks of metal pollution will increase with time [25] . Also another regard that this fraction in comparison with carbonate minerals, Fe-Mn oxide minerals have relatively large area and surface site density. The Fe-Mn oxide, the reducible phase of the soil under oxidizing conditions is significant sink for the heavy metals [26] .

Cu was mainly bound to this fraction in both sites of the study. The percentages of two sites were (%5.23 Zn), (%43.1 Cu), (%2.41 Cd), and (%29.43 Pb) this is for inside the city, but the percentages of the outside the city is (%5.95 Zn), (%46.93 Cu), (%1.56 Cd) and (%20.53 Pb). The results of sequential extraction in this study revealed that copper is mostly associated with organic matters fraction. Copper can be easily complex with organic matters because of high formation of organic-compounds [27] [28] . Heavy metals with high abundance in the phase bound to organic matter are more available than heavy metals in the Residual Fraction. The organic phase is relatively stable in nature but can be mobilized under strong oxidizing conditions due to degradation of organic matter [14] [27] . Heavy metals with high abundance in the phase bound to organics are more available than heavy metals in the residual fraction.

The results shows that Cadmium and lead were observed to be highest in the residual fraction but the percentages of Cu and Zn were lowest in both sites the results shows inside the city as follows (%12.99 Zn), (%29.81 Cu), (%45.83 Cd) and (%44.01 Pb) but outside the city the results were (%32.77 Zn), (%33.64 Cu), (%62.71 Cd) and (%42.41 Pb). Lead and Cadmium levels were mostly associated with the residual phase. This agrees with Ramirez et al., 2005 who reported Cd and Pb were mostly abundant with the residual phase [29] . Metals present in the residual fraction are a measure of the degree of environmental pollution. The higher the metals present in this fraction, the lower degree of pollution and verse versa [30] .