Integrated Treatment of Soil Washing Wastewater Using Optimised Colloidal Gas Aphrons Column Flotation: Evaluation of Synthetic and Natural Aphrons-Generating Surfactants

Soil washing is effective for remediating hydrocarbon-contaminated soil but generates wastewater containing residual petroleum hydrocarbons and surfactants that requires treatment before discharge or reuse. This study developed an integrated remediation approach that combined sodium dodecyl sulphate (SDS)-enhanced soil washing with colloidal gas aphrons (CGA) column flotation for post-treatment of the resulting wastewater. CGAs were generated using SDS and Soapnut (natural surfactant) and the process was investigated using a three-factor Box–Behnken design comprising surfactant concentration, pump setting and flotation time. Performance was evaluated using total petroleum hydrocarbon (TPH) removal, turbidity, electrical conductivity (EC), total dissolved solids (TDS) and ph. Across the experimental runs, CGA gebnerated from SDS and Soapnut achieved average TPH removals of 51.4% and 64.6%, respectively. The highest observed removal for SDS was 67.8% at 0.20% w/v, a 60% pump setting and 5 min, whereas Soapnut achieved 76.2% at 0.05% w/v, a 60% pump setting and 15 min. The SDS quadratic model was statistically significant and demonstrated strong predictive performance (F = 20.26, p = 0.002, R² = 0.973; adjusted R² = 0.925). Numerical optimisation predicted 71.0% removal at 0.050% w/v SDS, a 60% pump setting and 15 min. The Soapnut quadratic model was not statistically significant (F = 2.35, p = 0.180, R² = 0.809; adjusted R² = 0.464), although it predicted 77.5% removal at approximately 0.074% w/v, a 79.25% pump setting and 14.5 min. Soapnut also generally produced lower residual EC and TDS. Overall, Soapnut demonstrated potential as a natural aphrons-generating alternative. Integrating soil washing with CGA flotation can reduce secondary wastewater pollution and improve the sustainability of hydrocarbon remediation operations while supporting safer wastewater discharge or reuse within integrated treatment systems.