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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 ∼25% and ∼50% breakthrough of the injected IONP mass, respectively. Moreover, preflushing media with 50 mg L −1 rhamnolipid further increased IONP mass breakthrough by ∼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. 

The effects of nanoscale silver (nAg) particles on subsurface microbial communities can be influenced by the presence of biosurfactants, which have been shown to alter nanoparticle surface properties. Batch and column studies were conducted to investigate the influence of rhamnolipid biosurfactant (1–50 mg L −1 ) on the stability and mobility of silver nanoparticles (16 ± 4 nm) in batch reactors and water-saturated columns with three solution chemistries: pH = 4 and dissolved oxygen concentration (DO) = 8.8 mg L −1 , pH = 7 and DO = 8.8 mg L −1 , pH = 7 and DO = 2.0 mg L −1 . In batch studies, the presence of rhamnolipid (2–50 mg L −1 ) reduced nAg dissolution by 83.3–99.1% under all pH and DO conditions. Improved nAg stability was observed when rhamnolipid was present in batch reactors at pH = 7 ± 0.2, where the hydrodynamic diameter remained constant (∼50 nm) relative to rhamnolipid-free controls (increased to >230 nm) in 48 hours. Column experiments conducted at pH 4.0 ± 0.2 demonstrated that co-injection of nAg with rhamnolipid (2, 5 and 50 mg L −1 ) decreased Ag + breakthrough from ∼22% of total applied mass in rhamnolipid-free columns to less than 8.1% in the presence of rhamnolipid and altered the shape of the nAg retention profile from a hyper-exponential to a uniform distribution. Column experiments performed at pH 7.0 ± 0.2 and DO levels of either ∼2.0 or ∼8.8 mg L −1 showed that co-injection of 5 mg L −1 and 50 mg L −1 rhamnolipid increased nAg mass breakthrough by 25–40% and ∼80%, respectively, enhancements in nAg stability and mobility were attributed to rhamnolipid adsorption on nAg surfaces, which effectively slowed the oxidation and thus release of Ag + , and adsorption of rhamnolipid on the porous medium, which competed for nAg attachment sites. These results indicate that the presence of rhamnolipid significantly influenced nAg dissolution and mobility under dynamic flow conditions. A mathematical model based on modified filtration theory (MFT) accurately reproduced nAg transport and retention behavior when aggregation and reaction processes were minimal and when rhamnolipid was present, providing a tool to predict the effects of biosurfactants on nAg transport in porous media.