Enhanced flushing mechanism of petroleum hydrocarbon contaminated sandy soil by air nano bubbles

At present, chemical flushing is an effective way to remediate petroleum hydrocarbon contaminated sites, but there are associated problems, such as high cost and secondary pollution. Nano bubbles (NBs) have large specific surface areas, negatively charged surfaces, and high stability. They have a strong adsorption effect on organic substances, such as petroleum hydrocarbons, and show great potential for organic-contaminated soil remediation. In this paper, column experiments were carried out to study the removal of petroleum hydrocarbon pollutants from sandy soil by air NBs. Tap water flushing was used as a control experiment to investigate the effects of the NB concentration, porous media particle size, pore flow rate, and pH on the removal efficiency of the organic pollutants. In addition, the differences in the removal rates based on the interfacial tension between oil and water were explained. The results showed that the relative dissolved oxygen value of NBs after passing through medium sand was higher than that of fine sand, and the relative dissolved oxygen value increased with an increase in pH. The higher the concentration of NBs and the pore flow rate, the more efficient removal of pollutants. The highest removal rate was achieved under weak alkalinity (pH = 7-8), whereas strong acidities and alkalinities were detrimental. After flushing the sand column for 8 h using water containing air NBs, the removal of contaminants adhering to medium and fine sands were 72.5% and 60.3%, which were 11.5% and 15.7% higher than those of tap water, respectively. NBs have a weak degradation effect on long-chain alkanes due to the production of a small amount of hydroxyl radicals, but the physical desorption is absolutely dominant. Air NB is a natural green medium, showing good potential in efficient remediation of petroleum hydrocarbon contaminated sites. The NBs have the best removal rate of petroleum hydrocarbon pollutants in porous media under weakly alkaline condition (pH = 7-8). Both strong acidity and strong alkalinity are not conducive to the flushing removal of pollutants.