Ferrihydrite colloid-mediated transport of sulfamerazine in saturated porous media: Combined roles of rhamnolipid and solution pH

Abstract

To date, there is still very little knowledge about the combined effects of typical biosurfactants and solution pH values on iron oxide colloid-mediated transport of sulfamerazine (SMZ, a typical sulfonamide antibiotic) in saturated porous media. Herein, the influences of rhamnolipid (a common biosurfactant in the environment) on SMZ mobility in the absence or presence of ferrihydrite (a model iron oxide) colloids in saturated quartz sand were explored. Generally, the rhamnolipid suppressed the transport of SMZ without ferrihydrite colloids owing to the bridging effect of this biosurfactant (i.e., rhamnolipid served as a bridging agent between SMZ and sand). Moreover, ferrihydrite colloids also inhibited SMZ mobility due to the deposition of colloid-associated SMZ. Surprisingly, the presence of rhamnolipid significantly influenced ferrihydrite colloid-mediated transport of SMZ, and this effect was highly dependent on the solution pH. Concretely, rhamnolipid enhanced the inhibitory effects of ferrihydrite colloids on SMZ transport at pH 5.0 because of the increased SMZ retention on the deposited colloids via the bridging effect of biosurfactant. At pH 7.0, the transport-inhibition effects of colloids were weakened by adding rhamnolipid, which was due to the colloid-associated SMZ transport and the promoting electrostatic repulsion between the free SMZ and sand/ the deposited colloids. Surprisingly, ferrihydrite colloids facilitated SMZ transport in the addition of rhamnolipid at pH 9.0. This phenomenon stemmed from the enhanced free SMZ mobility, the colloid-assisted mobility of SMZ, and the competitive deposition between SMZ– species and negatively charged colloids. Moreover, the two-site non-equilibrium model was performed to help interpret the controlling mechanisms for the synergistic effects of rhamnolipid and solution pH on colloid-mediated transport of SMZ. The outcomes of this study offer valuable knowledge about the functions of widespread biosurfactants in the environmental consequences of antibiotics in subsurface systems with iron oxide colloids.