Effects of soil composition, surfactant structure and washing conditions on diesel contaminated soil removal efficiency using micellar solution

Abstract

Surfactant solutions are commonly used for the remediation of petroleum-contaminated soil due to their good petroleum removal performance, time-saving capability, and cost effectiveness. However, applying surfactants in excess concentrations could make oil recovery difficult. Moreover, residual surfactants in soil are toxic to microorganisms and plants. Thus, it is crucial to identify a suitable surfactant concentration for soil washing applications. The main objective of this study was to evaluate the effect of soil composition (sand, silt, clay and organic matter), surfactant structure (Tween 20, 40, 60 and 80, and Tergitol 15-S-7, 9 and 15). Subsequently, two surfactants from each series (Tween and Tergitol) were selected for diesel removal from diesel-contaminated soil by surfactant-assisted soil washing. Then the optimizing condition was examined based on physical factors, i.e., shaking speed, Liquid: Solid (L/S) ratio and time. The results showed that Tween surfactants with shorter carbon chain lengths required higher CMC for diesel removal from sand, while those with longer carbon chains needed higher CMC for clay cleanup. Tergitol surfactants with less ethoxylate group on the hydrophilic head have higher CMC in all soil texture. At a certain OM concentration, OM exhibited antagonistic effects with sand and silt, resulting in CMC reduction. In soil washing application, the mixture design shows that maximum diesel removal could be achieved from sand. Interestingly, there are high diesel removal efficiency from soil with highly clay, which surfactants exhibited the highest CMC. Based on FTIR results, the adsorbed surfactant could reduce the hydrophobicity of the clay surface, thus preventing the re-deposition of detached diesel. The soil with highly clay content was found in most area of Thailand. Therefore, Tween 80 and Tergitol 15-S-15, which have high diesel removal efficiency in clay, were selected. In optimizing physical condition test, effect of shaking speed and L/S ratio were tested. The result showed that shaking speed was more significant and the optimal physical condition was 3:1 L/S ratio with shaking speed at 100 rpm. At these conditions, time has no significant effect.