Reversible solubilization of typical polycyclic aromatic hydrocarbons by a photoresponsive surfactant

For the purpose of surfactant-enhanced remediation (SER) as a means of removing polycyclic aromatic hydrocarbons (PAHs) from polluted soils, a new method of reversibly solubilizing PAHs has been developed based on modulating the surface activity of the photoresponsive surfactant 4-butylazobenzene-4′-(oxyethyl)trimethylammonium bromide (AZTMA). AZTMA undergoes reversible isomerization between trans and cis isomers upon irradiation with ultraviolet (UV) and visible lights. The considerable disparity in critical micelle concentrations (CMCs) between trans- and cis-AZTMA implies that the surface activity of this surfactant may be easily manipulated. Kinetic studies have indicated that the solubilization mechanism of PAHs by AZTMA is similar to that of the conventional surfactants and can be described by an adsorption–desorption model. The molar solubilization ratios (MSRs) of pyrene, phenanthrene and acenaphthene in 10 mmol L−1 trans-AZTMA solution were 0.66 × 10−2, 3.92 × 10−2 and 5.02 × 10−2, respectively, much higher than those with cis-AZTMA. More than 70% of the solubilizate in trans-AZTMA solution could be released through irradiation with 365 nm UV light at an appropriate surfactant concentration. The presented results provide a theoretical foundation for reversible surfactant-enhanced remediation (RSER) based on a photoresponsive switchable surfactant.