Sequential solvent extraction as a tool for evaluating hydrocarbons speciation in soil after electrochemical treatment

Highlights: • The soil interphases were hypothesized based on physicochemical characterization. • A Sequential Solvent Extraction was devised to reach specific soil interphases. • Controlling the electric field allowed the restriction of electrochemical phenomena favoring surface processes. • The speciation scheme permitted identifying which interphases were altered by the presence of the electric field. • Alterations on soil interphases led to a change in hydrocarbon's speciation, which was extrapolated to environmental fate. Soluble and total extractable concentrations used for predicting contaminants' environmental fate may lead to uncertainties due to the lack of understanding of soil-contaminants interactions. The present study focuses on the influence of a controlled electric field on the distribution of polycyclic aromatic hydrocarbons in soil samples evaluated through a speciation scheme. Soil samples were spiked with 25,000 mg (hexadecane, phenanthrene, and pyrene 100:1:1 w/w) per kg of soil, and speciation of hydrocarbons was determined by employing a novel Sequential Solvent Extraction procedure, resulting in five fractions: soluble, pseudosoluble, desorbable, extractable, and sequestered. The distribution of hydrocarbons was then changed through the application of an electric field (72 h, 0.708 mA cm⁻², 2.95 ± 0.13 V cm⁻¹), which modified the interactions in the soil-water interface. The electrochemical treatment significantly increased the pyrene soluble, desorbable and sequestered fractions by 340, 1.3 and 19-fold (p < 0.05); the hexadecane soluble fraction increased in 6-fold (p < 0.05) and the phenanthrene desorbable fraction increased in 1.3-fold (p < 0.05). The use of the speciation scheme proposed in this study provides a wider view of hydrocarbons distribution in soils, rather than using water-soluble or total extractable concentrations. Finally, this speciation scheme is proposed as a tool to evaluate the environmental fate of organic contaminants in soils.