Recognizing Functional Groups of MES/APG Mixed Surfactants for Enhanced Solubilization toward Benzo[ a ]pyrene.

Benzo[ a ]pyrene is difficult to remove from soil due to its high octanol/water partition coefficient. The use of mixed surfactants can increase solubility but with the risk of secondary soil contamination, and the compounding mechanism is still unclear. This study introduced a new approach using environmentally friendly fatty acid methyl ester sulfonate (MES) and alkyl polyglucoside (APG) to solubilize benzo[ a ]pyrene. The best result was obtained when the ratio of MES/APG was 7:1 under 6 g/L total concentration, with an apparent solubility ( S w ) of 8.58 mg/L and a molar solubilization ratio (MSR) of 1.31 for benzo[ a ]pyrene, which is comparable to that of Tween 80 (MSR, 0.95). The mechanism indicates that the hydroxyl groups (-OH) in APG form "O-H···OSO2 - " hydrogen bonding with the sulfonic acid group (-SO3 - ) of MES, which reduces the electrostatic repulsion between MES molecules, thus facilitating the formation of large and stable micelles. Moreover, the strong solubilizing effect on benzo[ a ]pyrene should be ascribed to the low polarity of ester groups (-COOCH3 ) in MES. Functional groups capable of forming hydrogen bonds and having low polarity are responsible for the enhanced solubilization of benzo[ a ]pyrene. This understanding helps choose suitable surfactants for the remediation of PAH-contaminated soils.