Sequential remediation of crude oil-contaminated beach sand using a bio-based microemulsion: a sustainable alternative to surface washing agents

Oil spills in coastal environments pose significant threats to ecosystem integrity and biodiversity, while conventional surface washing agents (SWAs) may present environmental compatibility limitations and incomplete contaminant removal. This study evaluated a sequential remediation strategy using a bio-based Winsor IV microemulsion composed of 60 %wt pine oil, 30 %wt isopropyl alcohol/saponified coconut oil (C/T = 10), and 10 %wt distilled water. The formulation was assessed for crude oil removal from contaminated beach sand through a complete 2³ factorial design, considering contact time (15-30 min), microemulsion volume (1-2 mL), and sand mass (0.5-1.0 g), corresponding to microemulsion-to-sand ratios of 1 to 4 mL·g⁻¹. The strategy was designed to optimize the initial microemulsion treatment and then evaluate the contribution of aqueous washing to the sequential process. After microemulsion treatment, the sand was rinsed with water and subjected to three consecutive hexane extractions for residual petroleum quantification by spectrophotometry. Petroleum removal in the factorial experiments, accounting for the combined microemulsion-plus-rinse sequence, ranged from 61.51 to 91.37%, with a mean efficiency of 78.96%. ANOVA identified sand mass and microemulsion volume as the most influential factors, with a significant interaction, showing that performance depends strongly on the microemulsion-to-sediment ratio. Response-surface analysis indicated maximum removal at 2 mL of microemulsion per 0.5 g of sand (4 mL·g⁻¹) and contact times above 20 min. Batch assays at a 2:1 (mL·g⁻¹) ratio and 30 min confirmed high efficiency (93.4 ± 2.5%). Fixed-bed column experiments (100 g sand; flow rate 5 mL·min⁻¹) achieved 92.3 ± 1.2% removal after a single microemulsion pass, increasing to 96.5 ± 0.8% after the subsequent aqueous rinse, an incremental gain of ~4.2 percentage points from washing alone. Additionally, 96% of the microemulsion volume was recovered after the column assay, indicating reuse potential. These results indicate that residual surfactant and cosurfactant species retained in the sand are reactivated upon dilution, promoting further mobilization and solubilization of residual hydrocarbons without new chemical agents, evidence of a synergistic mechanism in which water acts not only as a rinsing medium but also as a functional second washing stage. The column-scale results further support the operational feasibility of microemulsion recovery and reuse, reinforcing the potential for reduced chemical demand. Overall, the proposed sequential strategy combines high petroleum-removal efficiency with reduced chemical demand and a bio-based formulation, supporting its potential as a more environmentally compatible alternative to conventional SWAs for petroleum-contaminated coastal sediments.