An Improved Gravimetric Method to Determine Total Petroleum Hydrocarbons in Contaminated Soils
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A gravimetric method to determine heavy fractions of total petroleum hydrocarbons (TPH) in soils is reported. The method was adapted and calibrated by modifying previous standard methods published, incorporating energy and cost savings where possible. Artificially contaminated soils with different organic matter content, and aged in stationary mode for a period of 8 months were used for calibration. Insufficient solvent evaporation was identified as the most prevalent and largest positive interference in the gravimetric detection. To overcome this, while minimizing the need for heating, a combination of three 10-min rotary evaporator steps and 30 min of vacuum in a desiccator were applied, for a total solvent volume of 60 ml. Hexane was chosen as the extraction solvent and a 40–60 min treatment in an ultrasound bath of 260 W was found suitable to extract 80–95% of TPH extracted by the Soxhlet method. Finally, the use of silica gel for cleanup of co-extracted natural organic matter was found unnecessary, because of the low amounts co-extracted for soils with up to 5% organic carbon, and because the chemical nature of the co-extracted organic matter prevents its selective adsorption to silica.
KeywordsContaminated soils Gravimetric method Silica gel Solvent evaporation Total petroleum hydrocarbons Ultrasound extraction
The authors are grateful to: Dr. Arturo Aguirre Gómez from the Department of Chemistry at FES-Cuautitlán, UNAM for supplying the soils of study, for the pH and electric conductivity data for these, and for his assistance in the organic carbon determinations; Dr. Rutilio Ortiz, Geography Institute, UNAM, for supplying the high organic matter content soils, and for the pH and organic carbon data for these; Rosaura Paez, Geography Institute, UNAM, Morelia Unit, for help in acquiring the gas chromatograms; Ricardo Alfaro, Chemistry Institute, UNAM, for his assistance in the initial methods development; and two anonymous reviewers who helped improve considerably the legibility of the manuscript.
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