Sustainable decontamination of crude oil-contaminated soil using Sapindus mukorossi extract: A green intervention to synthetic surfactant treatment

Remediating crude oil-contaminated sites is vital for environmental recovery. Surfactant based remediation strategies have gained importance in the management of such sites. Combining biogenic surfactant flushing with conventional methods is noted to be effective than using a single approach. Plant-based surfactants, such as saponins from Sapindus mukorossi, are easier to produce than microbial biosurfactants which require stringent control of conditions. However, research on their application in crude oil contaminated site remediation is limited. This study is an exploration of Sapindus mukorossi extract (saponin extract) for flushing crude oil from contaminated sand. It was observed that the dissolution of crude oil was controlled by its diffusion from sand. A pseudo first order model was used as an empirical description of the removal process. The saponin extract demonstrated a higher solubility enhancement factor (1.32) and better removal of asphaltenes compared to water and favored synthetic surfactant sodium dodecyl sulphate (SDS). The optimal duration of flushing was 22 h, 2 h and 106 h for distilled water (DW), sodium dodecyl sulfate (SDS) and saponin extract respectively. The dominant mechanism of flushing was inferred, based on the experimental data, to be mobilization. This affirms that a low concentration of saponin extract would render satisfactory flushing. The emulsion forming ability of saponin extract was more pronounced with crude oil- water mixtures having higher proportion of water. A lab-scale setup simulated continuous soil flushing, examining the effects of aqueous phase composition, flow rate, and flushing duration. Saponin extract flushed 60% of the crude oil from contaminated sand. Given its recognized renewability, biodegradability, low toxicity and cost, and the laboratory scale effectiveness demonstrated in the current study, saponin extract may serve as a preliminary intervention to reduce initial contaminant loads and render the soil matrix amenable for subsequent synthetic surfactant action and/or natural microbial degradation. Such an approach might contribute to the reduction in consumption of synthetic surfactants in remediation. These results indicate the potential of the approach under controlled conditions and motivate further evaluation for real remediation scenarios.