Treatment of Soil Contaminated with Petroleum Compounds using Stabilization/Solidification Method (case study: Salafchegan Industrial Estate)

Remediation of Oil Compounds Contaminated Soil Using Solidification and Stabilization
(Case Study: Salafchegan Industrial estate)
Introduction
The soils which are contaminated with oil hydrocarbons and heavy metals are among the environmental issues. Therefore, remediations of these soils are necessary according health and environment standards and needs to be under consideration by decision makers. Solidification and stabilization of oil hydrocarbons and heavy metals is well known as an appropriate technology in prohibition of contamination movement to the base layers of soil and underground water resources. The contamination of underground water resources often happens because of subsurface leakage of pollutants from soil to the subsurface layers. Organic hydrocarbon especially those originated from oil are the most prevalent soil contaminants. Hydrocarbon contaminants are among the family of carcinogens and neurotoxic organic pollutants. Therefore, remediation of contaminated soils with organic contaminants are necessary. The process of solidification and stabilization changes of the oil hydrocarbons and heavy metals into more stable materials from which the leakage of contamination is minimum. This process takes place by methods such as encapsulation or adsorption of the contaminant. Organophillic clay shows a great capacity in oil hydrocarbon or heavy metal contamination stabilization because of high specific area and cation exchange capacity. The main goal of this paper is to study the process of interaction between cement-lime-organophillic clay and oil hydrocarbon or heavy metal contamination considering microstructure in solidification and stabilization process. The leakage of contaminant from the solidified matrix is also studied and the best mixing fraction of stabilizer and solidifier for the best result is calculated.
Materials and Methods
The main focus of this research is on solidification and stabilization process in removal of the contaminants from the used bleaching soils of engine oil secondary treatments unit which are contaminated with oil hydrocarbons. This process is done using cement and quicklime accompanied by organophillic clay.
Stabilization/solidification is an effective disposal method for hazardous contaminants such as heavy metals and toxic materials. It was firstly used for nuclear waste treatment in 1950s and got widespread as a treatment technology for hazardous wastes in 1970s. US EPA has identified S/S as the Best Demonstrated Available Technology for 57 hazardous wastes which are regulated. Figure 7 shows the frequency of S/S treatment method compared to other technologies in US.




Figure 7. frequency of S/S treatment method compared to other technologies in US
Using S/S treatment method, chemical fixation and encapsulation of contaminants will be possible and the rate of contaminant migration will be minimized. During S/S process, the surface area exposed to leaching is decreased by isolating the wastes in an impenetrable capsule. The combination of stabilization and solidification results in improvement of strength and reduction in leachability which is the main goal of the process and hydraulic conductivity of treated soil. The aim of stabilization process is to decrease the solubility of contaminants by changing the chemical state of wastes or capsulation while solidification attempts to convert the waste into an easily handled solid with reduced hazards from leaching and higher strength. S/S can be applied both in situ and ex situ. In situ process contains the injection of agents into the contaminated soil using mixing equipment or pressure injection while in ex situ process the soil will be excavated. In ex situ systems, the control of process conditions is improved in comparison to in situ systems but its need to soil excavation is the most important drawback in comparison to in situ process. The most important parameter in S/S applicability is its limitation to immobilization of inorganic contaminants. However, under certain circumstances it can be applied to organic contaminants. The choice of appropriate binder in S/S process highly depends on the nature of contaminant and environmental conditions. The binders include inorganic cementitous materials (such as Portland cement, pozzolans, hydraulic slags and lime), organic binders, and thermosetting organic polymers.
In the first step, the characteristics of the wastes of the engine oil treatment industry are determined. For this purpose, the sampling takes place from the waste disposal site in Salafchegan Industrial City based on the method proposed by EPA. The wastes are measured to determine the total amount of oil hydrocarbons, total aromatic hydrocarbons, heavy metals, pH and humidity. The tests for determination of density, particle size distribution, Atterberg limits, diffusivity and oil contaminant content is done as well. Studying the amount and the types of the organic compounds in the soil is a necessity which determines the remediation methods in next processes. In other words, if the soil has a noticeable amount of organic and contaminating compounds (such as polycyclic aromatic hydrocarbons), it must be clarified before being used in solidification and stabilization process.
In the second step, the characteristics of the leakage of contaminants from the solidified samples is studied under different circumstances which is done using TCLP and free pushing resistance tests and analyzing the morphologies of the samples using SEM electroscopy.
Portland cement, the contaminated bleaching soil, organophillic clay and quicklime are the main materials used in this research. Some solvent and chemicals are used as well which are introduced in the paper.
Results and Discussion
The results show that the humidity content and specific gravity of the soil are 12.26% and 1.22 respectively. The results of the tests for measuring Atterberg limits and plasticity index shows that the soil which is being studied in this research, is from the types of the soils with high concentration of organic compounds with low diffusivity. Its pH is about neutral and it is heavy soil overall. The tests of heavy metal have shown the traces of iron, barium, lead, chrome, cobalt, cadmium, manganese, copper, nickel and zinc in the soil samples but their amounts are under the standard limits except barium which is above the standard limit.
The tests of aromatic hydrocarbons shows the existence of acenaphthylene, acenaphthene, fluorine, fluoranthene and pyrene in the soil in amounts more than other aromatic compounds but still their amounts are under the standard limits. Therefore, the soil being studied in this research does not have the polycyclic aromatic contamination potential.
To analyze the effectiveness of the solidification and stabilization of the soil, the leakage in a control sample (without organophillic clay and quicklime as solidifiers) is measured and compared to the samples containing solidifiers. The comparison between the leaked contaminants concentrations from stabilized samples and control sample shows that in all of the solidified and stabilized samples, an appropriate percentage of oil hydrocarbon removal has happened. In other words, most of the organic compounds and heavy metals diagnosed in the soil are encapsulated in the matrix and are not present during TCLP test. It shows that using organophillic clay has a good effectiveness for solidification and stabilization of the used bleaching soil in waste disposal site of Salafchegan Industrial City. The tests of free pushing resistance show that the addition of clay will increase the resistance of the samples.
Conclusions
The results show that addition of the clay has reduced the leakage in the soil. In total hydrocarbon removal, the samples with 20% of cement or quicklime and 30% of organophillic clay has the highest effectiveness with 98% removal of TPH and 98% removal of barium and producing 368-398 kPa pushing resistance.
Moreover, SEM electroscopy and TCLP test results show that increase in organophillic clay content and decrease in porosity volume in matrix are the two main reasons which enhanced the effectiveness of the stabilization process. The first will enhance the capability of the adsorption of the contaminant and the second will reduce the leakage amount.

Keywords: leakage test, organophillic clay, polycyclic aromatic hydrocarbons, solidification and stabilization, oil-contaminated soils.