Impact of salinity on colloidal ozone aphrons in removing phenanthrene from sediments

Polycyclic aromatic hydrocarbons (PAHs) tend to adsorb and accumulate on sediments owing to their hydrophobicity and persistence. Salinity is the predominant factor determining the PAH partition between aqueous and solid phases in freshwater, estuaries and seawater. This study focuses on the impact of salinity on the phenanthrene (PHE) removal from sediments using an in situ and targeted remediation technology - colloidal ozone aphrons (COAs). The ozone-encapsulated colloidal aphrons exhibited increasing air holdup but decreasing stability with the salinity increasing from 0.5‰ to 35‰. The hydrophobic attraction between Tween-20-coated bubbles and the hydrophobic solid surface weakened at high salinities. The presence of inorganic ions in the aqueous phase could lead to the salting-out of nonionic compounds (PHE, Tween-20 and even ozone), hindering detaching and degrading PHE from the solid phase. Anyhow, COAs achieved high efficiencies of washing (88.0-90.2%) and oxidative degradation (74.0-76.5%) particularly for the hydrophobic sediments with highly concentrated PHE (200.4 μg/kg) over the investigated salinities. The flushing effect imposed by the bubble flow played an important role, which was not greatly influenced by salinity. Although the dissolved natural organic matter competed with PHE for COAs and led to low PHE removal, the efficiency was improved by successive COA addition.