Comparison of ultrasonic and conventional mechanical soil-washing processes for diesel-contaminated sand

The effect of ultrasound on the conventional mechanical soil-washing process was investigated. To determine the optimal frequency for maximum efficiency, tests were conducted with aluminum foils under four frequencies including 35, 72, 110, and 170 kHz. It is known that the physical effects generated during acoustic cavitation damage the foil by causing pits and holes. The sonication at 35 kHz resulted in maximum damage to the aluminum foil as compared to that observed at other frequencies. Based on these results, 35 kHz was selected for the ultrasonic soil-washing processes in this study. The optimal washing time was found to be 1 min, because there was no significant increase in the removal efficiency over 1 min for the three processes, mechanical, ultrasonic, and combined ultrasonicmechanical. It was also found that the combined process enhanced the performance of the soil-washing process significantly as compared to other two processes in terms of (i) diesel removal efficiency, (ii) process time, (iii) consumption of electric energy, and (iv) production of washing leachate. The efficiency of washing under ultrasonic processing conditions was similar to that observed with mechanical washing in the presence of small amounts of sodium dodecyl sulfate (SDS), suggesting that the ultrasonic washing process does not require external chemicals and can be considered as a "green" process.