Extraction of PAHs from Soil Using Supercritical Fluids

Importance of the Topic

Efficient isolation of polycyclic aromatic hydrocarbons (PAHs) from environmental matrices such as soil is critical for accurate monitoring of pollution, risk assessment and regulatory compliance. Supercritical fluid extraction (SFE) using CO2 offers a greener alternative to traditional solvent-intensive techniques, but weathered or aged PAHs often remain bound to soil particles. Introducing minimal co-solvent improves analyte desorption and ensures reliable quantification in environmental and industrial laboratories.

Objectives and Study Overview

This study evaluated the impact of adding a fixed volume of methanol to soil samples prior to supercritical CO2 extraction. Using a soil standard reference material spiked with known PAH concentrations, the research compared SFE recoveries against established EPA liquid/solid extraction benchmarks, aiming to optimize extraction efficiency while minimizing solvent usage.

Methodology and Instrumentation

Samples of 0.5 g soil were placed in a 1 mL extraction vessel. Prior to SFE, each sample received 100 µL of pesticide-grade methanol as a co-solvent. Extractions employed supercritical CO2 at 7 000 psi and 80 °C with a flow rate of 2 L/min. A 10-minute static period was followed by 30 minutes of dynamic flow. Eluted analytes were captured on a 1 g, 6 mL C18 SPE cartridge and rinsed with 5 mL methanol containing internal standard.

Used Instrumentation

• Applied Separations Spe-ed SFE Supercritical Extraction System
• GC-MS for analyte quantification
• Supercritical CO2 grade gas and pesticide-grade methanol
• C18 SPE cartridges (1 g/6 mL)

Main Results and Discussion

Measured recoveries of key PAHs closely matched certified values:

• Naphthalene: 31 mg/kg (certified 32 mg/kg)
• Acenaphthene: 17 mg/kg (19 mg/kg)
• Phenanthrene: 1573 mg/kg (1618 mg/kg)
• Anthracene: 493 mg/kg (422 mg/kg)
• Benzo(bk)fluoranthene: 180 mg/kg (152 mg/kg)

These results demonstrate that 100 µL methanol suffices to displace PAHs from weathered soil without overwhelming the SPE trapping phase, delivering recovery rates comparable or superior to Soxhlet extraction while dramatically reducing extraction time and solvent consumption.

Benefits and Practical Applications

Integrating a small co-solvent volume into CO2-based SFE accelerates extraction, reduces organic solvent disposal, and maintains or improves analyte recovery. This approach supports high-throughput environmental testing, quality assurance in industries handling contaminated soils, and research laboratories seeking greener analytical workflows.

Future Trends and Opportunities

Advances may include online coupling of SFE with mass spectrometry for real-time analysis, exploration of alternative green co-solvents, method automation for field deployable units, and expansion to other hydrophobic pollutants such as chlorinated pesticides or PCBs. Further refinement of pressure-temperature protocols could optimize selectivity for target compounds.

Conclusion

The simple addition of 100 µL methanol to soil samples prior to supercritical CO2 extraction effectively releases both freshly spiked and weathered PAHs. This protocol offers a rapid, eco-friendly alternative to traditional liquid/solid extraction while delivering high recovery and reproducibility.

Reference

Applied Separations, Supercritical Extraction Application Note: PAHs in Soil (Allentown, PA).