Enhanced Remediation of Weathered Petroleum-Contaminated Desert Soils Using Hybrid Ultrasonic–Mechanical Soil Washing

Weathered petroleum contamination in desert soils resists conventional remediation because aged hydrocarbons are strongly sorbed under arid conditions. A hybrid ultrasonic–mechanical soil-washing system with a biodegradable saponin biosurfactant was evaluated on crude oil-contaminated sand from the Kuwaiti desert. Operating conditions were screened one factor at a time, and mechanical, ultrasonic and combined washing compared over cycles (n = 6). Total petroleum hydrocarbons (TPH) were determined gravimetrically, as an operationally defined measure of solvent-extractable material. Under the selected conditions (50 °C, 1000 rpm, 20 min), combined washing removed 89.9 ± 0.9% of TPH after three cycles, against 75.5 ± 1.5% and 72.3 ± 1.8% for mechanical and ultrasonic washing alone. A fourth cycle raised cumulative removal to 97.0 ± 0.1%, leaving 9985 ± 325 mg/kg. This places the soil at the Kuwaiti action level of 10,000 mg/kg rather than reliably below it: three of six replicates exceeded that value, and all six fell below only after a fifth cycle. Electron microscopy and turbidity indicated rougher surfaces, less hydrocarbon coating and greater disaggregation. The proposed cavitation-based mechanisms are inferred rather than demonstrated; the process transfers hydrocarbons into the wash effluent rather than degrading them, so effluent management is integral to field application.