Ester linked cationic gemini surfactants for biodegradable soil washing of PAH contaminated sites

Polycyclic aromatic hydrocarbons (PAHs), including naphthalene, phenanthrene, and pyrene, are persistent and toxic soil contaminants that pose a threat to human health and ecological integrity. Traditional remediation approaches are often limited by high costs, poor sustainability, and suboptimal efficacy. Biodegradable surfactants provide a viable and environmentally friendly approach for surfactant-enhanced soil washing. Novel ester-linked cationic gemini surfactants (12-E2-12, 14-E2-14, and 16-E2-16) were synthesized via an eco-friendly protocol and structurally confirmed by FTIR and ¹H-NMR spectroscopy. Key physicochemical parameters were systematically determined. Critical micelle concentrations (CMC) exhibited an inverse correlation with alkyl chain length: 0.00597 mM for 12-E2-12, 0.00492 mM for 14-E2-14, and 0.00364 mM for 16-E2-16. Biodegradability was rigorously evaluated by Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) and after 5 days, Biochemical Oxygen Demand (BOD5) to COD ratios of 0.59, 0.36, and 0.31, respectively were recorded, exceeded the 0.21 threshold limit indicative of favourable microbial degradability. Hemolysis remained below 50% even at 2500 µg/mL across all surfactants, with a chain-length-dependent increase (12-E2-12 < 14-E2-14 < 16-E2-16). Antimicrobial efficacy against Staphylococcus aureus displayed both concentration- and chain-length dependency; notably, 16-E2-16 generated inhibition zones of 12 mm and 14 mm at 5 mM and 10 mM, respectively, whereas shorter-chain counterparts exhibited negligible activity under identical conditions. Soil washing experiments were performed on soils artificially contaminated with naphthalene, phenanthrene, and pyrene (initial loading 10⁻³ mol/g) using single and binary surfactant systems. The gemini surfactant 16-E2-16 exhibited strong adsorption (> 98.7%) onto the soil, whereas the non-ionic Brij 58 showed minimal retention (25.6%). Equimolar 16-E2-16 + Brij 58 mixtures reduced surfactant adsorption to ~ 80% and lowered the composite CMC to 0.0013–0.0016 mM. This binary system achieved the highest PAH removal efficiencies, reaching up to 43.8% for individually spiked PAHs and 6.8–25.5% in co-contaminated soils, outperforming single-surfactant controls. The results demonstrate that ester-linked gemini/Brij 58 composites offer synergistic micellization, reduced surfactant loss, and enhanced PAH solubilization, making them promising biodegradable agents for soil remediation.