Comparison of the effects of chemical surfactant and bio-surfactant on montmorillonite colloid-mediated transport of levofloxacin through saturated porous media

Abstract

The transport behavior of levofloxacin (LEV, a typical fluoroquinolone antibiotic) in aquifers may be affected by clay particles (e.g., montmorillonite colloids) and surfactants, which are widespread in aquatic environments. Little is known about the influence of different surfactants (e.g., chemical and bio-surfactants) on the LEV mobility in the presence of clay colloids. In this study, sodium dodecylbenzene sulfonate (SDBS) and rhamnolipid (Rha) were chosen as typical chemical surfactants and bio-surfactants, respectively. The combined roles of montmorillonite colloids and different surfactants on LEV mobility in saturated porous media under different pH conditions (5.0–9.0) were investigated. Generally, montmorillonite colloids promote LEV transport because of the colloids' high mobility and the montmorillonite binding capacity toward LEV. Meanwhile, the enhanced effects decreased with increasing pH because of the declined adsorption of LEV to colloids. Interestingly, colloid-facilitated transport of LEV was enhanced by the presence of surfactants. These observations mainly stem from the enhanced mobility of montmorillonite colloids and the increased binding abilities of colloids for LEV induced by surfactants (via the bridging effects of surfactants). Interestingly, compared with Rha, SDBS exhibited a superior effect on montmorillonite colloid-facilitated LEV transport. This is because more LEV is adsorbed onto clay colloids in the presence of SDBS owing to greater bridging effects and additional π–π stacking interactions. As a result, an increased amount of colloid-associated LEV may penetrate the columns. This study provides a fresh understanding of the diverse impacts of ubiquitous surfactants on colloid-mediated transport of antibiotics in subsurface environments.