The solubilization effects and microscopic mechanisms of a novel sugar-based Gemini surfactant for multiple DNAPLs

Surfactant-enhanced aquifer remediation (SEAR) is an effective technology for removing dense non-aqueous phase liquids (DNAPLs) from contaminated aquifers. However, multiple DNAPLs with distinct phases, types, and properties commonly coexist in groundwater, thereby decreasing the remediation efficiency. Although enhanced single DNAPL solubility has been achieved by synthesizing new gemini surfactants, the solubilization effect and mechanisms for multiple DNAPLs in aquifers need to be further studied as they are critical for the promotion of SEAR efficiency. In this study, the competitive and synergistic solubilization effects and microscopic mechanisms of a novel sugar-based gemini surfactant (SANG) are fully evaluated and revealed for five common contaminants (NAP, PCE, MCB, CTC, and NB) in groundwater. SANG demonstrates the highest solubilization capacity for halogenated hydrocarbons, especially for CTC with a molar solubilization ratio value of 3.61, followed by MCB and PCE. From the results of dynamic light scattering and cryo-transmission electron microscopy, it is clear that the SANG micelles swell from spherical micelles to vesicles or wormlike micelles in larger sizes after solubilization, which provides greater space and more solubilization sites. When two contaminants co-exist in the system, the solubilizing effect of NAP and NB is significantly enhanced through partitioning at the micellar core and surface region, respectively. In contrast, the co-solubilizing effects of PCE, MCB, and CTC are significantly inhibited due to competition in the intermolecular region of the micelles. Finally, a mass-transfer conceptual model named "MDAM" is established to describe the process of solubilizing DNAPLs for SANG. The aqueous-phase diffusion coefficient serves as the predominant factor dictating the kinetics of solubilization. Based on these findings, the comprehensive solubilization mechanisms of this gemini surfactant could be provided to improve the remediation efficiency for DNAPL-contaminated aquifers by SEAR technology.