Batch and Column Extraction of Lead from Al-Doura Oil Refinery Soil

The aim of this research is to investigate the extraction of lead (Pb) from soil of Al-Doura oil refinery in Baghdad/Iraq. Ethylenediaminetetraacetic acid disodium salt (Na2EDTA) and Hydrochloric acid (HCl) solution were used as extractants. Soil washing method was practiced in two ways, batch extraction and column extraction experiments. A set of batch experiments were carried out at different conditions of extractant (Na2EDTA, HCl) concentration, contact time, pH and agitation speed. From the batch experiments, the maximum removal percentages of Pb that have been obtained were 70 % using Na2EDTA (0.1M) at pH 4 , agitation speed 200 and at equilibrium time 4 hours and 65 % using HCl (1M) at pH 1.08 , agitation speed 200 at equilibrium time 5 hours. Column experiments were conducted at different conditions of extractant (Na2EDTA, HCl) concentration, contact time and flow rate. The maximum removal percentages of Pb were 78% using Na2EDTA (0.1M) at pH 4 , flow rate 30 ml/hr and equilibrium time 8 hours and 75% using HCl (1M) at pH 1.08, flow rate 20 ml/hr and equilibrium time 10 hours. The column extraction proved that the extractant volume required to achieve high removal efficiency is less than that of the batch extraction, but requires a longer contact time. The experimental data of batch and column extraction were applied in four kinetic models; first order, parabolic diffusion, two constant and Elovich model to find best fit model for extraction system. For batch extraction, the parabolic diffusion and two-constant models


Introduction
Heavy metals are chemical elements with a specific gravity that is at least 5 times the specific gravity of water. They are often problematic environmental pollutants, with wellknown toxic effects on living systems [1]. They are introduced into the environment during mining, refining of ores, combustion of fossil fuels and industrial processes. They cannot be degraded or destroyed [2].
Among the physical and biological processes in the subsurface soils, geochemistry plays a major role in the distribution, speciation, as well as the remediation potential of heavy metals. Heavy metals constitute an ill-defined group of inorganic chemical hazards, and those most commonly found at contaminated sites are lead (Pb), chromium (Cr), arsenic (As), zinc (Zn), cadmium (Cd), copper (Cu), mercury (Hg), and nickel (Ni) [3].
Heavy metals represent major risks regarding contamination of natural waters after release by metal-bearing soil constituents and migration via the soil solution downward to the water table [4]. The impact of contamination on the environment should be of scientific concern, in order to minimize the threat of soil and groundwater contamination [5].
The adequate protection and restoration of soil ecosystems contaminated with heavy metals require their characterization and remediation [6]. Technologies available for remediating metal contaminated soil can be divided mainly into two namely, immobilization methods and separation/concentration methods. In the first type of remediation, contaminants are immobilized thereby preventing the leaching of contaminants into the groundwater, the second type of remediation deals with separating the contaminants from the soils or reducing the volume of contaminated soil [7]. E120 Soil washing is represented one of the most suitable in-situ (on-site)/ex-situ physical/ chemical treatment technologies. Soil washing has been successfully used for the treatment of soils contaminated with heavy metals, hydrocarbons and semi-volatile organic compounds (SVOCS) [8]. Soil washing usually employs different extractants such as acids, bases, chelating agents, electrolytes, oxidizing agents and surfactants [9][10][11][12][13].
The technique of soil washing is to separate the metal from soil by using chelating agents by organic acid , ethylenediamminotetraacetic acid disodium salt Na2EDTA, a representative chelating agent, can extract heavy metals from contaminated soils with high efficiency. Strong inorganic acid can be used for useful washing solutions in terms of reasonable cost and simple handling of the effluent solution. It is an effective solvent due to high its removal efficiency on heavy metal extraction, especially hydrochloric acid (HCl) [14].
Soil washing is a variable treatment alternative for metal contaminated sites chemical extraction to enhance the efficiency of heavy metals extraction. Process parameters in soil washing include the mode of extraction (batch or column), extractant type and concentration, pH, liquid-to-solid ratio (L/S), retention time .The soil related parameters are pH, particle size distribution and mineral type of metal to be extracted and their concentration, distribution and physicochemical forms in the soils [15][16][17].
In the present study, soil washing method was used to extract lead from Al-Doura oil refinery soil. The samples of soil were taken, exactly, from department of receiving and pumping of TEL (Tetraethyl lead, (C2H5)4Pb) in the refinery. TEL is an organic lead compound used as an anti-knock additive for petrol. It is highly toxic, causing nervous system and brain damage. It, when burned, forms inorganic lead oxides and carbonates of lower toxicity. However, the toxicity of these compounds requires a strict design for blending plant that holding these compounds to prevent the leakage of TEL.

Soil Characterization
Al-Doura oil refinery soil was used in the experiments conducted in the present study.
After collecting samples from surface (0-20) cm, these were air dried at room temperature, sieved by using 2mm sieve to remove stones and large particles, and then mixed with hand to ensure uniformity.

Chemical Analysis
Contaminant in different soil sections were extracted by performing acid digestion in accordance with the Haswell (1991) [18]. Total concentration of lead was determined using this extraction procedure. For this procedure, the soil sample was crushed and approximately of 1 g of a representative sample was weighed accurately in a beaker (capacity of 250 ml) and then mixed with (15) ml of concentrated hydrochloric acid (HCl) and (5) ml of concentrated nitric acid (HNO3), the mixture was then heated in the heating sandy bath until the brown evaporation was disappeared and the sample arrived to dry state, this step takes about (45-60) minutes. Then cooling the beaker to laboratory temperature and then add 5 ml of concentrated hydrochloric acid (HCl) and then heated in the heating sandy bath, this step takes about (5)(6)(7)(8)(9)(10) minutes. Then cooling the beaker and add 5 ml of concentrated hydrochloric acid and 50 ml of heated distilled water to wash the sides of beaker from remains dissolved sample. After that heating the mixture to the boiling points to (2-3) minutes. Then filtrate the sample with filtration paper No. 42, and then keep it in volumetric flask capacity of 100 ml, then, washing the precipitate with distilled water and add the previous washed water to filtration and complete the size to 100 ml. Finally, the metal concentration was determined by atomic absorption spectrophotometer (AAS).

Extractants
Two types of the extractants (Na2EDTA and HCl ) were used for removing lead from contaminated soil.

Batch Extraction Experiments
Batch equilibrium tests are carried out to specify the best conditions of contact time, pH, Then, the solution is filtered using a whatman No.42 filter paper. The supernatant was analyzed for heavy metal using atomic absorption spectrometry (AAS). All tests were performed in triplicate and the results were presented as average of the duplicates extracts.
The removal of lead was calculated using the following equation [ 19,11,13 ] : Lead removal = Where CL and Cs are the concentration of lead in supernatant (in mg/L) and soil (in mg/kg), respectively. VL is the volume of supernatant (in L), and Ms is the dry mass of the soil (in kg). The flow rate (Q) of extractant (Na2EDTA , HCl ) solution was calculated using the following equation:

Column Extraction Experiments
where : A= Cross section area of the column (cm 2 ).

V= Darcy velocity (cm/hr).
Which is calculated from the following equation: Where: n = porosity of the soil.

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The calculated seepage velocities of extractant solution, using equations (2) and (3), for the three flow rates are 0.476, 0.714, 1.19 cm/hr. These velocities were adopted in the column extraction experiments.

Batch Experiments
A number of batch experiments were carried out for the contaminated soil. The effects of contact time and extractant (Na2EDTA, HCl) concentration were studied. The concentrations of lead in supernatant were obtained as shown in table (2), the value of lead concentration used in equation (1) to calculate the removal efficiency.

Kinetic Models
In order to examine the heavy metals extraction mechanism, kinetic data were fitted with four mathematical models: first order, parabolic diffusion, two constant and Elovich model.  (7), which were applied for experimental data in the present study.
Table (7): The mathematical models applying to fit experimental data.
From the application of various kinetic experiments, Elovich model get good correlation (coefficient of determination(R 2 )) in column mode; and in batch extraction, the parabolic diffusion and two-constant models obtained the best correlation (coefficient of determination(R 2 )).

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(a) Two-constant model in batch extraction.
(b) Elovich model in column extraction.    4-The parabolic diffusion and two-constant models gave the best correlation {coefficient of determination (R 2 )} with experimental data using HCl and Na2EDTA respectively.

Column Extraction
1-The column results indicated that several factors such as extraction or equilibrium time, solvent of extraction dosage and flow rate effect on the extraction process. However, the best values of these factors will achieve the maximum removal efficiency of heavy metals.