Effect of rhamnolipid biosurfactant on transport and retention of iron oxide nanoparticles in water-saturated quartz sand

Although prior studies have investigated the effects of solution constituents, including dissolved organic matter and synthetic polymers, on nanoparticle mobility in porous media, far less attention has been directed toward evaluating the impacts of biosurfactants secreted by microorganisms on the transport and retention behavior of nanomaterials. The objective of this study was to explore the influence of rhamnolipid, a biosurfactant associated with biofilms, on the transport and retention of iron oxide nanoparticles (IONPs) in a water-saturated quartz sand. Column experiments were conducted using aerobic medium (ionic strength = 50.4 mM) or 10 mM NaCl as background electrolyte at a pore velocity of 0.43 m per day and pH 6.8 +/- 0.2. In aerobic medium columns, nearly all introduced nanoparticles were retained when IONPs were injected alone, whereas the presence of 10 mg L-1 or 50 mg L-1 rhamnolipid resulted in similar to 25% and similar to 50% breakthrough of the injected IONP mass, respectively. Moreover, preflushing media with 50 mg L-1 rhamnolipid further increased IONP mass breakthrough by similar to 30%. Similar enhancement of nanoparticle mobility by 50 mg L-1 rhamnolipid was also measured in lower ionic strength (10 mM NaCl) columns. Mathematical models that incorporated nanoparticle filter ripening and biosurfactant competitive adsorption successfully reproduced experimental observations. Modeling results predicted an order-of-magnitude decrease in IONP filter ripening rate coefficient and a three-fold drop in average IONP retention capacity in the presence of rhamnolipid, consistent with a stabilizing effect and competition for surface sites. These findings demonstrate that rhamnolipid biosurfactant can potentially enhance nanomaterial stability and mobility in subsurface environments and that these effects should be considered when evaluating the impact of biological process on nanoparticle fate and transport in porous media.