Reducing the Gap Between Chemical and Biological Assessment of Petroleum-Contaminated Soils: An FTIR-Based Method Using Aqueous Dimethyl Sulfoxide Extraction

Chemical and biological approaches are widely applied to assess petroleum hydrocarbon contamination in soils; however, their results are often difficult to compare due to the use of fundamentally different extraction media. Chemical analytical methods typically rely on non-polar organic solvents, whereas biological toxicity assessments are based on aqueous soil extracts, reflecting water-soluble fractions of contaminants. This methodological discrepancy complicates the integrated evaluation of soil contamination and the assessment of remediation effectiveness. This study proposes an FTIR-based approach for assessing petroleum hydrocarbon contamination in sandy podzolic soil based on extraction with aqueous dimethyl sulfoxide (DMSO). The proposed method aims to reduce the methodological gap between classical chemical monitoring and aqueous-based biological assessment frameworks. The extraction performance of distilled water and aqueous DMSO solutions with different concentrations was evaluated using model soil samples artificially contaminated with crude oil. Crude oil was used as a multicomponent contaminant, whereas “petroleum hydrocarbons” refers to the operationally defined hydrocarbon fraction extracted from the soil–oil system and detected by FTIR in the C–H stretching region. Hydrocarbon extraction efficiency was assessed based on characteristic C–H stretching absorption bands in the 2800–3100 cm−1 infrared region. Distilled water exhibited limited extraction capacity and rapidly reached saturation, resulting in weak and concentration-independent absorption responses. In contrast, aqueous DMSO increased the apparent solubility of hydrocarbons and yielded reproducible, concentration-dependent spectral responses. A 75% (v/v) aqueous DMSO solution was selected as a practical compromise extraction medium, increasing analytical sensitivity while remaining more compatible with aqueous conditions than conventional non-polar organic solvents. The proposed method provides a lower-hazard and methodologically coherent extraction approach for FTIR-based chemical monitoring of petroleum-contaminated soils and may facilitate improved comparability between chemical measurements and aqueous-based biological assessment approaches in integrated soil contamination and remediation studies.