The Use of Bitumen in the Stabilization of Lead Contaminated Iraqi Soil

Iraq is one of the biggest countries in oil production and its processes. Bitumen is a byproduct material that remains after the process of crude oil. This study is designed to investigate the role of bitumen in the stabilization of lead contaminated Iraqi soil; as measured by a newly developed “mini” Jet Device and the dispersion ratio method (DR,%). The results revealed that both bitumen amount and its method of mixing with soil, as a direct or as emulsion mixing, can significantly improve the stability of lead contaminated soil. Hence, only 3% bitumen as emulsion mixing was required to get the optimum soil stabilization, compared with 9% in direct mixing. A 3% bitumen emulsion has reduced both the scouring depth (SD, from 24.5 to1.6 mm; R= -0.90) and the erodibility coefficient (Kd, from 1.09 to 0.14; R= -0.86) as a result of improving soil engineering properties related to soil stabilization. However, the 3% of bitumen emulsion has also improved the soil chemical properties that have an important role in soil aggregation and lead mobility; such as pH, EC and SAR. Therefore, 3% of bitumen emulsion has successfully decreased both the dispersion ratio (DR, % from 7.03 to 4.48; R= -0.90) and the lead solubility (Pb from 48.8 to 4.9ppm; R= -0.96) in the solutions of lead contaminated soil.


No.22-(3) 2019 Journal of Petroleum Research & Studies (JPR&S)
E67 polymeric framework around the waste. [3] The S/S is achieved by mixing the contaminated material with appropriate amounts of binder/ stabilizer and water. The mixture sets and cures to form a solidified matrix and contain the waste.

Bitumen Properties
Bitumen is a by-product that remains after the processing of crude petroleum. Bitumen emulsions are complex fluids, their stability depend on the intermolecular forces, a result of a balance of repulsive and attractive forces. An emulsion is a dispersion of small droplets of one liquid in another one. Emulsions can be formed by any two immiscible liquids, but in mostly one of the phases is water. In multiple emulsions, the disperse phase contains another phase which may not have the same composition as the continuous phase [4]. Bitumen emulsion can be divided into anionic, cationic and nonionic types depending on the charge of their head groups in water; although this may be a pH-dependent.
However, the charge on the emulsion head group mostly determines the net charge on the bitumen surfaces [5].
The basic process involved in bitumen stabilization of fine-grained soils is a waterproofing phenomenon. Soil particles or aggregates are coated with bitumen that can stop or minimize the penetration of water which could normally result in a decrease in soil strength. In addition, it can improve soil stability by making the soil resistant to the detrimental effects of water. In soil stabilization with bitumen, two basic mechanisms are active: Waterproofing and adhesion. The aggregate particles adhere to the bitumen and the bitumen becomes a as binder and that increases both the cohesion and the shear strength [6].

Soil Samples
In this study, a Silt Clay Loam soil (Lean Clay, Table (1)) were used as a sample to carry out the experiments, acquired from Al-Taji region, North of Baghdad, from depth (0 to 90) cm. The soil samples were tested and analyzed according to the ASTM standards

Bitumen Material
The bitumen used in this study is a product under the name " Prakcoat" made by the

Experimental Work:
Most physical and engineering properties were determined at the Hydraulic laboratory of the Environmental Engineering Department, Engineering College, Al-Mustansiriya University according to the ASTM standards, as described by the same author. [7] However, scouring depth (SD), erodibility coefficient (Kd) and critical shear stress (τ c ) of soils were determined by a "mini" Jet Erosion device developed by Al-Madhhachi et al., [8]. Digital Shore-D Durometer to measure the degree of hardness was also used at the Materials Department laboratory, Engineering College, Al-Mustansiriya University.

Preparing Soil Samples for Chemical Tests:
x All the soil samples were air-dried, broken into small sizes and sieved through a 4.75 mm sieve according to ASTM standard. The sieving was performed to ensure that the soil was of uniform grade.
x The artificial Pb-contaminated soil samples were prepared by mixing lead nitrate, as the source of lead (Pb), to produce a soil of 4000 mg/kg lead concentration from the natural soil. As a reminder, 300ppm in soil is the maximum acceptable Pb concentration in EU countries [9].

No.22-(3) 2019 Journal of Petroleum Research & Studies (JPR&S)
E70 x Different percentages of bitumen (0%, 1%, 2%, 3%, 6% and 9%) by soil weight were added to the soil and mix by hand until the mixture seems to be homogeneous and then left to air dry. As explained in Section (2.2), two mixing methods were used to prepare soil samples; as Direct Mixing and as Emulsion.
x 25 g of soil for each bitumen percent placed in a conical flask and 50 ml of distilled water was added (1:2 soil water ratio) to the soil sample and shaken by hand for 1 minute then left for 2 hours to settle down. The soil solution remaining after sedimentation was subjected to the tests of the dispersed solids (DR %) and then filtered to measure Pb, PH, EC, and SAR of soil solutions.

Soil Solution Testing Methods
All tests on soil sample solutions were carried out at the sanitary laboratory of the Environment Department, Engineering College, Al-Mustansiriya University.

Dispersion Ratio
The term "dispersed ratio" refers to matter suspended in water or in soil solution, and is related to the percentage of clay (<0.002mm) in soil sample. Dispersed solids includes both total suspended solids; the portion of total solids retained by a filter, and total dissolved solids; the portion that passes through a filter [10].
Dispersed solids can be measured by evaporating a soil-water mixture in a weighed dish, and then drying the residue in an oven at 103 to 105° C. The increase in weight of the dish represents the dispersed solids and hence soil erodibility. The dispersion ratio in this study was measured according to Middleton principles and by a method modified by Mutter [7,11] In this study, 25g of lead contaminated soil were mixed in 250ml conical flask with 50ml water to get a 1:2 soil water mixture. The mixture was shaken by hand for 1min. and

Chemistry of Soil Solution
The soil solution chemical properties, namely pH, EC and SAR, were measured in the filtered 1:2 soil solution prepared in the previous suction. pH was measured by a pH-meter, EC by an Electrical Bridge, soluble Na, Ca and Mg (in meq/l) and Pb (as ppm) were measured at Ib Al-Haitam Education College, University of Baghdad. The sodium adsorption ratio was calculated by the following formula [13]:

SAR =
; where sodium, calcium, and magnesium are in meq/liter.
Sodium adsorption ratio (SAR) is a measure of the suitability of water for use in irrigation to prevent soils from the sodium hazard. In general, the higher the sodium adsorption ratio, the less suitable the water is for irrigation. It is also an index of the soil sodicity and can be determined from the chemical analysis of water extracted from the soil.

4.Results And Discussion
Due to results obtained in this work and the findings of another related detailed study published by the same author [14], Bitumen has markedly improved soil both engineering and chemical properties related to soil stabilization. It should be noted, however, that the 9% direct mixing of bitumen has given better results than the 3% as emulsion. But in terms of feasibility and costs analysis, the 3% emulsion has achieved the optimum results.

Bitumen Emulsion
Table (4) reveals that the addition of only 3% bitumen as an emulsion has improved the engineering properties related to soil stabilization and the Pb solubility and movement reduction. However, shear stress (τ c , pa), maximum dry density (M.D.D), degree of hardness (DH), the Atterberge limits and optimum moisture content have markedly affected by the bitumen. Hence, both the scouring depth (SD) and the erodibilty coefficient (Kd) were reduced (from 24.5 to 1.6 and 1.09 to 0.14, respectively).

No.22-(3) 2019 Journal of Petroleum Research & Studies (JPR&S)
E73 Chemically and because the use of 3% bitumen emulsion has yielded the optimum soil stabilization, a detailed coverage focusing on the impact of the 3% bitumen emulsion on the soil chemical properties are discussed in Figures (1 to 5). Coalescence of bitumen droplets also takes place and the trapped water diffuses out.
Droplets in contact with clay minerals spread on the surface eventually displace the water film on the aggregate surface [15,16].

No.22-(3) 2019 Journal of Petroleum Research & Studies (JPR&S)
E75 precipitation may also occur in the soil and water systems as Pb carbonates [17]. As reviewed by [18], lead in sulphide solutions may be reduced to precipitate as Pb sulphides.
This occurs usually by the action of sulphate-reducing bacteria or through an encounter with organic matter, which may be bitumen in this case. In alkaline conditions and in the presence of carbonate, sulphides and silicates; as in the case of the current studied soil   known as cationic bitumen emulsion [19]. From the figure it could be noticed that the pH increase with increasing bitumen percentage and that may be because bitumen has acted as

No.22-(3) 2019 Journal of Petroleum Research & Studies (JPR&S)
E76 positively charged cation and may actually neutralize the acid in the cationic emulsion causing the pH to rise [20].   is contaminated with lead ions and bitumen percentages (0%, 3%, 6%, and 9%). From the figure it could be noticed that electrical conductivity decreased (from 3.5 to 2.34) with increasing bitumen percentage. According to [21], Soil pH is negatively related with soil electrical conductivity. Low soil pH value should have high soluble salt content and therefore high electrical conductivity. In high pH values, the cation exchange capacity of soil increased and hence more cations are adsorbed on soil particles and this may reduce the soluble cations in soil solution and hence EC [18].

No.22-(3) 2019 Journal of Petroleum Research & Studies (JPR&S)
E77 Bitumen, %   percentage. The SAR depends largely on the Na + concentration in soil solution. These results are in agreement with [22] who studied the role of Na + in enhancing the clay dispersion in Sodic soils. When sodium ions are adsorbed by soil particles as exchangeable cations, soil becomes sodic and the soil structure is degraded by means of clay swelling and dispersion. In the current study, the lead that existing in the contaminated soil (4000 ppm) may act like Ca and Mg as a divalent cation and can reduce the dispersive action of Na in soil solution [13]. In addition, bitumen may also behave as a positively charged cation [20] and this may enhance the flocculation and aggregation of soil particles.

Conclusion:
Bitumen appears to be as a good Soil stabilizer. Bitumen can significantly improve both the engineering and the chemical properties of soils and that make it more stable and less erodible. In this study, the method of bitumen mixing with soil is also appeared to be of a prime importance. In the direct mixing method, 9% of bitumen was required to completely improve soil properties against erosion. While in emulsion mixing method, only 3% bitumen was successful in minimizing the soil erodiblity; by improving shear stress, degree of hardness, maximum dry density, optimum moisture content, Atterberg limits and hence reducing both the scouring depth and erodibility coefficient of lead contaminated clayey soil. In addition, the chemical impact of bitumen has also played a major part in improving soil stability. Therefore, the dispersion ratio, which is considered as a simple soil aggregate stability test, decreased with higher additions of bitumen emulsion. The bitumen has increased pH, but reduced both EC and SAR, in soil solution and this has markedly improved the soil stability and reduced the mobility of Pb in soil solution.