Document Type : Research Paper
The expanding production of fuels, drugs, fertilizers, chemicals, and hazardous materials has caused considerable environmental contaminations. The contamination of soil and groundwater with petroleum hydrocarbon-based fuels as a result of accidental spills or improper storage has been reported frequently. Iran is seriously facing soil contamination problem, due to owning 8.58% of the global oil fields; generating 35 million tons of petrochemical products; and having more than 20 000 km of pipelines. The extraction of 1 kg of crude oil usually generates 10–20 g of waste residues Petroleum refineries are burdened with the problem of handling large sludge quantities. It is estimated that more than 28,000 tons of petroleum oily sludge are being generated each year from each petroleum refinery. This oily sludge is recognized as hazardous waste under the Resource Conservation and Recovery Act (RCRA). Since the early 1970s leaks from evaporation ponds, storage tanks and under- ground pipelines at the Tehran Oil Refinery (TOR), which is located in the Shahre-Ray district, south of Tehran, Iran, were the major sources of soil and groundwater pollution in the area. For many years, wastes contaminated with chromium and mercury from the TOR site have contaminated the area, thus causing the pollution of soil, air and groundwater in the region. Heavy metals have toxic characteristics, and due to their non-degradability and persistency, they impose adverse effects on humans and ecosystems. The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) consider mercury and chromium among the 100 most dangerous toxic substances .Furthermore, the toxicity characteristic leaching procedure (TCLP) lists these metals as toxic metals, while their concentration in soil leachates should not exceed 0.2 and 5 ppm, respectively. Chromium is also the 21st most abundant element in the Earth’s crust with an average concentration of 100 ppm.
Chromium damages the kidneys the liver and blood cells through oxidation reactions When reaches the blood stream. Contact with products containing chromates can lead to allergic contact dermatitis and irritant dermatitis, resulting in ulceration of the skin. In addition to these sorts of effects, the carcinogenicity of chromate dust has been proved since 1980 when the first publication described the increasing cancer risk of workers in a chromate dye company.
Mercury (Hg) is a silvery liquid metal. The primary source of Hg is a sulfide ore called cinnabar (HgS). Although Hg usually obtained as the by-product of processing complex ores which contains mixed sulfides, chloride, oxides and minerals, it could occur as the principal ore product.
Mercury can be absorbed through the skin and mucous membranes. Mercury vapors can also be inhaled. Accordingly, containers of mercury are extremely sealed to avoid any spill and evaporation. In order to avoid exposure to mercury vapor, heating of mercury or decomposable compounds of it is always carried out with adequate ventilation.
Soil washing is a combination of using liquids (usually water, occasionally combined with solvents) and mechanical processes to scrub soils. “Solvents are selected on the basis of their ability to solubilize specific contaminants, and on their environmental and health effects.” The soil washing process separates fine soil (clay, silt, etc) from coarse soil (sand and gravel). Since hydrocarbon contaminants tend to bind to fine soil particles (mainly clay and silt), separating the smaller particles from the larger ones decreases the volume of contaminated soil. The smaller volume of soil, which contains the majority of clay and silt particles, can be further treated by other methods, such as incineration or bioremediation, or disposed in according to federal regulations. The clean, considerable volume of soil is seemed to be non-toxic and can be used as backfill. Generally, semi-volatile organic compounds (SVOCs), petroleum and fuel residuals, heavy metals, PCBs, PAHs, and pesticides are the target contaminant groups for soil washing. This technology lets the recovery of metals and it can purify a wide range of organic and inorganic contaminants from coarse-grained soils. because of reducing the quantity of material which would require further treatment, soil washing is cost-effective compare to other technologies.
Materials & Methods
In this paper, 3-mercaptopropionic acid reagent was used for soil washing of the samples. Brief description of this reagent is as follows:
• 3-Mercaptopropionic acid (MPA), HSCH 2CH 2COOH, is used in a variety of applications. The MPA itself is used as cocatalyst in the manufacture of Bisphenol A, which is a key raw material in Polycarbonate production. MPA enhances the process efficiency.
A 250-mL solution of 3-mercaptopropionic acid with the concentration of 1.22 kg/L was prepared in the laboratory for testing of the samples. This solution was used as the main washing reagent to evaluate the contaminants removal efficiency under various temperature and concentration conditions.
Twenty grams of contaminated soil were placed in a 600-mL beaker and 400 mL of reagents solution were added to the sample (1-20 is TCLP ratio). The soil was mixed with the designated washing solution using Jar test equipment for 4 hours at the rotational rate of 250 rpm.
To obtain the effects of temperature on contaminants removal efficiencies, three washing solutions were made at 25, 35 and 45°C (respectively named T1, T2, and T3). To maintain the desired testing temperatures, the samples were kept in different water baths during the washing procedure.
To evaluate effects of solutions concentration on the soil washing efficiency 4 different concentrations of 3-mercaptopropionic acid solutions (0.05, 0.1, 0.15 and 0.2 normal) were prepared.
U.S. Environmental Protection Agency (U.S. EPA) METHOD 3050B was applied to digest soil samples for ICM-MS tests. This method has been written to provide two separate digestion procedures, one for the preparation of sediments, sludge, and soil samples for analysis by flame atomic absorption spectrometry (FLAA) or inductively coupled plasma atomic emission spectrometry (ICP-AES) and one for the preparation of sediments, sludge, and soil samples for analysis of samples by Graphite Furnace AA (GFAA) or inductively coupled plasma mass spectrometry (ICP-MS).
The average values of 155.7 and 27.2 ppm, respectively were used as the concentrations of chromium and mercury in raw soil samples of Tehran oil refinery contaminated site throughout this paper.
Discussion of Results
Effects of solutions concentration on mercury removal efficiency
The removal efficiencies of the contaminants were 67.88, 73.39, 81.57, and 84.53% at 0.05, 0.15, 0.1 and 0.2 N concentrations of 3-mercaptopropionic acid solution, respectively.
Effects of solutions temperature on mercury removal efficiency
By using 3-mercaptopropionic acid in 4 different concentrations (0.05, 0.1, 0.15 and 0.2N) the removal efficiencies of mercury were measured to be 71.31, 79.12, 84.94, and 86.98% at 35°C and to be 75.15, 83.44, 86.79, and 87.90% at 45°C, respectively.
Effects of solutions concentration on chromium removal efficiency
The average amounts of chromium removals corresponding to 0.05 N and 0.2 M 3-mercaptopropionic acid solutions at 25°C were in the order of 51.37%, and 63.45%, respectively.
Effects of solutions temperature on chromium removal efficiency
By using 3-mercaptopropionic acid in 4 different concentrations (0.05, 0.1, 0.15 and 0.2N) the removal efficiencies of chromium were reported to be 52.35, 53.98, 57.89, and 67.63% at 35°C and to be 55.11, 57.89, 62.76, and 75.21% at 45°C, respectively.
The outcomes illustrate that the highest mercury and chromium removal efficiencies from the sludge samples achieved by using 0.2 N 3-mercaptopropionic acid solution at 45°C (87.90% and 75.21% respectively) Furthermore, by using 0.2 N 3-mercaptopropionic acid solution at 25 °C, %84.53 of Mercury and %63.45 of Chromium were extracted.