Adsorption of switchable surfactant mixed with common nonionic surfactant on montmorillonite: Mechanisms and arrangement models

To improve understanding of adsorption behavior of mixed surfactants on typical clay components in soil for potential applications of surfactant-enhanced remediation (SER) technologies during remediation of hydrophobic organics-contaminated soil, the adsorption mechanisms and arrangement models of an electrochemical switchable cationic surfactant (11-Ferrocenylundecyl) trimethylammonium bromide (FTMA), mixed with the nonionic surfactant Tween 80 on montmorillonite were investigated. The mixed system in the presence of Tween 80 remains electrochemically reversible. The adsorption isotherms of the pure FTMA and FTMA-Tween 80 mixed system on montmorillonite were Langmuir type. The major mechanism of pure FTMA adsorption is via cation exchange as the total capacity is close to the cation exchange capacity (CEC) of montmorillonite. With the addition of Tween 80, the cation exchange of FTMA would be weakened by the function of hydrogen bonding. As the added concentration of FTMA and Tween 80 are 2874 mg/L and 50 times of critical micelles concentration of Tween 80, adsorption capacity of FTMA and mixed surfactants decrease from 280.6 to 235.2 mg/g and 400 to 298 mg/g. Moreover, the adsorption capacity of Tween 80 linearly decreases from 118.7 to 62.7 mg/g. Meanwhile, as the amount of FTMA increases from 0.4 to 1.0 times CEC of montmorillonite, the arrangement model of FTMA in the interlayer of montmorillonite changes from a flat monolayer, lateral bilayer to pseudotrilayer. When mixed with Tween 80, the interlayer space of montmorillonite increased significantly, and it showed a gradual increasing trend of interlayer space as the concentration of Tween 80 increased. The results of the present study show that the combined use of cationic and nonionic surfactants can reduce the adsorption loss of surfactants during remediation of polluted soil and then be conducive to the application of SER technologies.