Kill two birds with one stone: Solubilizing PAHs and activating PMS by photoresponsive surfactants for the cycle remediation of contaminated groundwater

Polycyclic aromatic hydrocarbons (PAHs) represent a dangerous threat to the groundwater environment. PAHs are difficult to be effectively removed from groundwater because of their hydrophobicity. In this study, a combination of a Gemini photosensitive surfactant (N1, N2-bis[4-[4-[(4-butylphenyl) azo] phenoxy] butyl]-N1, N2-tetramethylethane-1,2-diammonium bromide, AzoPBT) with peroxymonosulfate (PMS) is developed to treat PAHs-contaminated groundwater. To investigate the reaction mechanism of the PMS/AzoPBT process, we chose naphthalene (Nap) and phenanthrene (Phe) as target pollutants. The quenching experiments, electron paramagnetic resonance (EPR) spectroscopy and the identified degradation products indicated that singlet oxygen could contribute to the degradation of PAHs. A tentative solution for Phe degradation was proposed based on the degradation products. The role of surfactant micelles in the PMS/AzoPBT process is further explored. The positively charged micelles in the PMS/AzoPBT process escalate the removal of PAHs by electrostatically polymerizing Br- and HSO5- in the solution. Although increasing the number of micelles can enhance the solubilization of PAHs by surfactant solutions, the encapsulation of PAHs by micelles can "protect" PAHs, which hinders the removal of PAHs. The PMS/AzoPBT process can be applied in a broad range of pH values. In the process of recycling AzoPBT to activate PMS to remediate groundwater, PAHs can be completely removed in the first cycle, and 51.4 % of them can be removed after four times of surfactant use. The combination of photoresponsive surfactants and PMS achieves efficient oxidation of PAHs without additional activators while increasing the solubility of PAHs in groundwater. This novel technology of PAHs-contaminated groundwater treatment may provide insights into the degradation mechanism of the PMS/AzoPBT process.