Rhamnolipids govern iron oxide nanoparticle deposition, mobility and release in subsurface-relevant aqueous systems - a QCM-D study

Abstract

Biosurfactants such as rhamnolipids (RLs) are increasingly recognized as critical regulators of nanoparticle (NP) fate in natural and engineered subsurface systems. Here, we systematically evaluate how RL adsorption influences the deposition, mobility, and release behavior of surface-functionalized superparamagnetic iron oxide nanoparticles (IONPs), using a library of IONPs with distinct coatings including CTAB (cationic), SDS (anionic), and oleic acid (anionic). Deposition kinetics were quantified on silica (SiO2), alumina (Al2O3), and polystyrene (PS) surfaces across a range of ionic strengths (0-850 mM NaCl) using real-time quartz crystal microbalance with dissipation monitoring (QCM-D). RL adsorption varied with surface type, with rigid, electrostatically bound monolayers formed on positively charged Al2O3, while softer, hydrated multilayers were observed on PS and SiO2. These interfacial films strongly modulated IONP deposition. On PS, CTAB-IONPs showed higher mean deposition on RL-predeposited surfaces, with the response depending on RL headgroup structure, whereas SDS- and oleic acid-coated IONPs showed enhanced mobility and partial desorption/restructuring of preadsorbed RL under salt-free conditions. In contrast, RL predeposition on Al2O3 significantly suppressed SDS-IONP deposition, decreasing attachment by nearly 170-fold at 10 mM NaCl. Classical DLVO theory could not fully account for these observations, suggesting the role of hydration, steric, and orientation-dependent interactions consistent with extended DLVO frameworks. RL preadsorption also altered the critical deposition concentration (CDC) of CTAB-IONPs, reducing it by nearly half. The addition of RLs also drove apparent release of pre-deposited anionic "soft-coated" NPs, consistent with surfactant-mediated remobilization. Taken together, these results show that biosurfactant-modified interfaces are dynamic regulators of nanoparticle retention, which is necessary for predicting IONP transport and designing remediation strategies in complex environmental media.