Extraction of Total Petroleum Hydrocarbons (TPH) from Soil Using Supercritical Fluids

Importance of Topic

The contamination of soil and groundwater by leaking underground storage tanks containing petroleum hydrocarbons represents a significant environmental challenge. Accurate, efficient, and reproducible measurement of total petroleum hydrocarbons (TPH) in soils is critical for regulatory compliance, risk assessment, and remediation efforts. Supercritical fluid extraction (SFE) with carbon dioxide offers a green alternative to traditional solvent-based methods, reducing solvent consumption and waste generation.

Study Overview and Objectives

This application note describes the use of supercritical fluid extraction to quantify TPH in soil samples. The primary objectives are to demonstrate the method's efficiency, compliance with EPA Method 3560, and versatility in sample types such as soils, sediments, and fly ash. The protocol outlines sample preparation, extraction conditions, and subsequent analysis via infrared detection or gas chromatography for target hydrocarbon fractions.

Methodology and Instrumentation

• Sample preparation: 3 g of soil placed in a pre-cleaned dish; addition of drying agent (e.g. diatomaceous earth or anhydrous magnesium sulfate) at a 1:1 ratio for samples with >20 % moisture.
• Loading: Transfer sample to a 10–24 mL extraction vessel packed with silanized glass wool plugs and Spe-ed Matrix support.
• Extraction conditions: 5000 psi pressure, 80 °C temperature, 1 L/min supercritical CO₂ flow, dynamic extraction for 30 minutes.
• Collection strategies: Either a C18 SPE cartridge (1 g/6 mL) or 3 mL of tetrachloroethylene solvent.
• Post-extraction analysis: Tetrachloroethylene analyzed by EPA IR Method 8440; C18 cartridge eluted with tetrachloroethylene (EPA IR Method 8840) or methylene chloride (EPA GC Method 8015) for non-halogenated volatile organics.

Instrumention Used

• Applied Separations Spe-ed™ SFE System
• Spe-ed Matrix support (Cat. #7950)
• Carbon dioxide (SFC/SFE grade)
• Tetrachloroethylene (spectrophotometric grade)
• C18 SPE cartridges (1 g/6 mL, Cat. #12007)
• Methylene chloride (pesticide grade)
• Diatomaceous earth

Main Results and Discussion

The method achieves efficient extraction of TPH from various soil matrices with high reproducibility. Supercritical CO₂ effectively solubilizes hydrocarbon fractions, while the chosen collection media ensure quantitative recovery. The defined extraction parameters provide an optimal balance between throughput and analyte yield, consistently meeting EPA performance criteria for TPH monitoring.

Benefits and Practical Applications

• Significantly reduced solvent consumption and hazardous waste compared to traditional Soxhlet extraction.
• Rapid extraction cycle of 30 minutes enhances sample throughput in analytical laboratories.
• Regulatory acceptance through EPA Method 3560 compliance.
• Adaptable detection options (infrared spectroscopy or gas chromatography) to suit laboratory infrastructure.

Future Trends and Potential Uses

Ongoing developments in supercritical extraction will aim to shorten extraction times further, integrate inline detection systems, and broaden the spectrum of extractable analytes. Emerging green co-solvents and novel sorbents may improve selectivity and environmental performance. Coupling SFE with high-resolution mass spectrometry holds promise for detailed molecular characterization of complex hydrocarbon mixtures.

Conclusion

Supercritical fluid extraction with CO₂ provides an efficient, environmentally friendly, and regulatory-compliant approach for quantifying total petroleum hydrocarbons in soils. Its speed, reproducibility, and reduced solvent usage support robust environmental monitoring and remediation programs.

Reference

• EPA Method 3560, Supercritical Fluid Extraction