Soil contamination is a serious problem and major environmental problem worldwide. Oil contamination of soils can occur as a result of oil drilling and exploration operations, leakage of storage tanks and wells, tanker accidents, spillage during transportation, etc. Research studies have shown that the presence of hydro carbons can affect the engineering properties of soils. The degree of alteration has been found to depend on the type of soil, the type of contaminant, and the concentration of the contaminant.
In an article published in the International Journal of Geo-Engineering from the Department of Civil Engineering, Federal University of Sao Carlos, Brazil, researchers demonstrated the capacity of lime treatment in improving the engineering properties of diesel contaminated soils. The research was geared towards demonstrating ways to reuse non-hazardous petroleum-contaminated soils in civil engineering applications, such as asphalt concrete, cold-mix asphalt, construction material, roadway sub-bases and alternative daily cover materials for landfills.
To carry out the study, the researchers collected an uncontaminated coarse grained soil sample from Sao Paulo, Brazil, and subjected it to laboratory tests such as specific gravity test, Atterberg limit test, particle size distribution test, compaction test, CBR, unconfined compression test, and pH test. Based on the Unified Soil Classification System, the coarse grain soil is classified as SC soil and mineralogical characteristics include quartz, feldspars, iron oxides and Kaolinite Hematite and Chlorite as secondary minerals. The diesel oil used in the study as the organic contaminant was a commercially available diesel oil named S500. Diesel properties include relative density of 0.834 at 20°C, kinematic viscosity of 2.0–5.0 cm2 s−1 at 40°C, pH of 6.0 and biodiesel up to 7.0% v/v. The diesel oil was directly mixed with dry soil to prepare oil-contaminated soil samples. The different ratios of diesel to dry soil were set to 4, 8, 12 and 16% to simulate different levels of contamination in the field.
From the study, it was observed that the presence of diesel affected consistency limits of natural soil, while pH values were not affected. The liquid limit and plastic limit of the soil increased with increase in diesel content, thereby leading to increase in the plasticity index. High liquid limit and plasticity index is an indication of weak soil. No significant changes were observed in ΔpH values, remaining negative for all diesel contents, thus not altering soil electric charges with diesel content increase.
From the compaction test result, it was observed that the compaction properties were altered due to the presence of diesel contamination. Significant decrease in maximum dry unit weight and an increase in optimum moisture contents with diesel addition were observed. In the unconfined compression test, about 70% reduction in strength was observed as the diesel content increased.
However, when the contaminated soil was treated with lime, the pH and ΔpH values increased, while the consistency limits reduced. After lime addition, calcite minerals were predominate in natural and oil-contaminated soils. The addition of lime in the diesel-contaminated soil resulted in a flocculated structure soil similar to that of lime-treated natural soil. Compaction parameters of diesel-contaminated soils were less altered by the presence of lime. Lime allowed 8% diesel-contaminated to sufficiently increase in 300% the UCS soil and recover some mechanical properties of natural soil, evidencing possible carbonation reactions in the mixture.
Reference
Correia N., Portelinha F.N.M, Mendes I.S., Batista da Silva J. W. (2020): Lime treatment of a diesel‑contaminated coarse‑grained soil for reuse in geotechnical applications. International Journal of Geo-Engineering (2020) 11(8). https://doi.org/10.1186/s40703-020-00115-2
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