Optimising Hydrocarbon Extraction from Soil Using Mixed-Surfactant Systems

In industrial settings, one of the key environmental challenges is the remediation of soil contaminated by hydrocarbons. Washing the soil with surfactants mobilises and extracts these compounds, making them easier to treat. As it enables the recovery and reuse of soil within sustainable production processes, this technique is part of the circular economy. Soil-washing experiments using surfactants were carried out to determine whether a mixture of anionic and non-ionic surfactants could improve the remediation of soil contaminated by gasoline and diesel fuel compared to the use of a single surfactant. Four surfactants were used (non-ionic: polyoxyethylene lauryl ether and polyoxyethylene (80) sorbitan monooleate; anionic: sodium dodecylbenzenesulfonate and sodium dodecyl sulfate). The aliphatic and aromatic hydrocarbon fractions (C6-C8, C8-C10, C10-C12, C12-C16, C16-C21 and C21-C35) of gasoline and diesel fuel were analysed. Sodium dodecylbenzenesulfonate was selected for the purpose of preparing mixtures with the other two non-ionic surfactants, polyoxyethylene lauryl ether and polyoxyethylene (80) sorbitan monooleate. These surfactant mixtures demonstrated significantly higher removal rates than sodium dodecylbenzenesulfonate alone. Mixtures of sodium dodecylbenzenesulfonate and polyoxyethylene lauryl ether achieved hydrocarbon extraction of between 61% and 68%, while sodium dodecylbenzenesulfonate-polyoxyethylene (80) sorbitan monooleate mixtures obtained extraction of between 58% and 66%. Analysis of the gasoline and diesel hydrocarbon fractions indicated that smaller molecules desorb more easily than larger ones and that aromatics desorb more easily than aliphatics. Furthermore, the mixtures increased the extraction of both aliphatic and aromatic hydrocarbons, particularly the lighter compounds. The variation on removal rates within the hydrocarbon ranges may be related to the octanol-water partition coefficient (K_ow). These improvements with mixtures of anionic and non-ionic surfactants could be exploited to enhance the effectiveness of surfactant-flushing treatments and optimise the design of soil surfactant treatments.