Quantification of Volatile Organics in Soil Aging Experiments Using Fourier Transform Infrared Spectroscopy

Abstract

On-line Fourier transform infrared (FT-IR) spectroscopy was applied to monitor the concentration of halogenated volatile organic compounds in a sample-preparation process that simulates long-term, slow accumulation of contaminants in soils (i.e., aging). Artificial aging is conducted by circulating a supercritical fluid solution containing the contaminant(s) of interest through a packed soil column. Mid-infrared spectra of several volatile halocarbons were measured in supercritical Xe and CO₂ to evaluate possible interferences from the strong absorption of CO₂. Although some of the C–X bands were partially masked in supercritical CO₂, all of the compounds studied had distinct spectral features in the region 1400–700 cm⁻¹ and could be monitored in either solvent. Quantitative measurements of halogenated volatile organics in supercritical CO₂ were demonstrated with CCl₄. Excellent results were obtained over the range 7–280 mM. Representative artificial aging experiments were conducted on two test soils using CCl₄ as the contaminant. On-line (FT-IR) estimates of the aged soil concentrations were 1.3–4.4 times higher than off-line concentrations obtained by gas chromatography/mass spectrometry. The discrepancies were primarily ascribed to post-aging losses that occurred during depressurization and subsequent sample handling. FT-IR spectroscopy is shown to be a powerful tool for monitoring soil loading behavior and for developing artificial aging protocols.