Remediation of Petroleum Contaminated Soils in Urban Area Using Thermal Desorption

Introduction
Growing use of hydrocarbon constituents in various industries have generated significant quantities of hazardous wastes which lead to environmental pollution. In recent years, the strong global demands for fossil fuels and industrial chemicals have resulted in the release of some of these compounds into soil, water and air.Hydrocarbon-contaminated soils are among the important environmental issues which could be present around refineries, fuel stations, pipelines and storage tanks. Selection of an effective solution for cleanup of contaminated sites depends on several factors including soil conditions and pollutant concentrations, thus requiring one or more treatment techniques for cleanup of such site. In general, contaminated sites remediation methods can be classified as in situ or ex situ types. Several technologies, such as chemical oxidation, biostimulation and bioagumentation, or electrokinetic oxidation, can be used for the treatment of these specific polluted sites.  Therefore, many different technologies have been developed for remediation of contaminated soils: biological treatment, soil washing with surfactants, air stripping, thermal desorption, incineration, etc. Among the various processes, thermal desorption is a very attractive one because it can promptly treat the contaminated soils with high efficiency, irrespective of their chemical species.They are the most popular and versatile due to their removal efficiency, cost and required time. According to their treatment temperatures, treatments can be classified as low-temperature (100–350 ◦C) and high-temperature (350–600 ◦C) thermal desorption, involving the physical separation of contaminants from the soil, and thermal destruction (600–1000 ◦C), involving the chemical modification of contaminants.
 
In this study, soil samples were collected from Baghershahr contaminated area in south of Tehran, adjacent to Tehran oil refinery.The soil in the area is polluted with different hazardous hydrocarbons due to the leaching of the refinery. LTTD remediation method were evaluated for decontamination of this area soil. To assess the temperature and residence time on removal efficiency of gasoline from contaminated soil, LTTD were conducted on temperature of 90, 110, and 150 ° C and residence time of 10, 20 and 25 minutes.
 
Materials and Methods
Soil sampling was carried out at the vicinity of Tehran oil refinery area. Samples were collected from 10-30 cm depths of the ground surface and placed in glass jars. After transferring the samples to the laboratory, tests were performed to determine the moisture content and geotechnical properties of the samples such as soil texture, Atterberg’s limits, soil classification and the amount of organic matter. The EPA and ASTM methodswere applied to test the samples.
 
Due to low concentrations of hydrocarbons in the soil, the samples were spiked with gasoline at two concentrations of 4000 and 10000 ppm. The spiking of the samples was carried out to acquire more clear results regarding the performance of LTTD experiments. Low temperature thermal desorption experiments were performed in a reaction glass.Moreover, two thermometers were used for measuring temperatures inside the chamber and the exhaust gas. A condenser was also connected to reaction glass to liquidity the generated vapors during the test.
 
In order to investigate the effects of temperature and residence times on the removals of contaminants, the experiments were performed at different temperatures of 90, 110, 150 ° C and at three residence times of 10, 20 and 25 minutes.
A gas chromatography with Flame Ionization Detector(GC-FID) instrumentwas used for analyzing the gasoline concentration.
 
Results
 
Moisture content
The solids processing capacity of a thermal desorption system is inversely proportional to the moisture content of the feed material. The presence of moisture in the excavated soils to be treated in the LTTD unit will determine the residence time required and heating requirements for effective removal of contaminants. In order for desorption of petroleum constituents to occur, most of the soil moisture must be evaporated in the desorber. This process can require significant additional thermal input to the desorber and excessive residence time for the soil in the desorber. In many LTTD studies which were conducted on the petroleum contaminated soils, the optimum ranges of moisture contents were between 10% to 20%(W/W). In this study, however the moisture content of the soil was measured to be 15%.
Effects of operating parameters were investigated for the thermal treatment of petroleum contaminated soils in a desorber. Batch operation result shows that achieving to the significant efficiency depends on temperature and residence time.
 
Removal efficiency
The minimum removal efficiency achieved for the soils with the 10000 ppm contaminants was 70.1% at the residence time of 10 minutes and temperature of 90 ° C. Nevertheless, regarding the 10000 ppm samples, the maximum removal efficiency of 87% was achieved for the samples at residence time of 25 minute and temperature of 150°C.Likewise, for 4000 ppm gasoline concentration soil samples, Maximum and minimum removal efficiencies were 80% and 94.45%, respectively. The overall results indicate that LTTD method is an appropriate remediation method for hydrocarbon contaminated sites.
 
Temperature
In this study, temperature effect on the low temperature thermal desorbtion were evaluated. Treatment temperature is a key parameter affecting the degree of treatment of organic components. The required treatment temperature depends upon the specific types of petroleum contamination in the soil. The recommended treatment temperatures for various petroleum products and the operating temperature is in the range of boiling point of the chemicals. The actual temperature achieved by an LTTD system is a function of the moisture content and heat capacity of the soil, soil particle size, and the heat transfer and mixing characteristics of the thermal desorber. The results indicate that more significant removal efficiencies of removing gasoline from contaminated soil samples will achieve as temperature increased. For instance, as temperature increased from 110 to 150 °C in 4000 ppm gasoline contaminated sample and resident time of 25 minutes, the removal efficiencies increase from 90.3% to 93.4%.
 
Residence Time
To demonstrate the effects of residence time on removal of gasoil from soil, experiments were conducted under 10, 20, and 25 residence time, which is a key parameter affecting the degree to which decontamination is achievable. Residence time depends upon the design and operation of the system, characteristics of the contaminants and the soil, and the degree of treatment required.Moreover, the removals of contaminants from the samples increased by increasing heating residence times. The maximum LTTD removal efficiency of 94.5% occurred in a time period between 20 to 25 minutes (i.e. 23 minutes) for the samples with 4000 ppm concentration, which is considered as optimum time for this concentration. Nevertheless, greater residence time is essential for decontamination of contaminated soil whit concentration of 10000 ppm of gasoline.
 
Discussion and conclusions
For sites with petroleum contaminated soils, the primary concern is to reduce the residual concentration of the organic constituents to or below regulatory levels. This criterion applies to both the soil surrounding the excavation and the soil that was excavated and thermally treated. The results indicate that LTTD remediation method is capable of gasoline contaminated soil removal with efficiency of greater than 90%.
The resultsof LTTD tests also indicated that the increase in temperature from 90 to 110 and 150 °C increases the gasoline removal efficiencies.  Furthermore, as residence time of process increased, the removal efficiencies of gasoline from soil increased. For instance, removal efficiency increased from 71% to 82.5% for 4000 ppm contaminated samples as the residence times increased from 10 minutes to 25 minutes.The data obtained from the low temperature thermal desorption experiments indicate that the higher removal efficiencies of volatile gasoline hydrocarbons in soils with concentrations of 10000 ppm will be achieved at residence times and temperature of above 25 minutes and 150 °C.