Release of deposited MnO₂ nanoparticles from aqueous surfaces

Changes in solution chemistry and transport conditions can lead to the release of deposited MnO₂ nanoparticles from a solid interface, allowing them to re-enter the aqueous environment. Understanding the release behavior of MnO₂ nanoparticles from naturally occurring surfaces is critical for better prediction of the transport potential and environmental fate of MnO₂ nanoparticles. In this study, the release of MnO₂ nanoparticles was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D), and different environmental surface types, solution pH values and representative macromolecular organics were considered. MnO₂ nanoparticles were first deposited on crystal sensors at elevated NaNO₃ concentrations before being rinsed with double-deionized water to induce their remobilization. The results reveal that the release rate of MnO₂ depends on the surface type, in the decreasing order: SiO₂ > Fe₃O₄ > Al₂O₃, resulting from electrostatic interactions between the surface and particles. Moreover, differences in solution pH can lead to variance in the release behavior of MnO₂ nanoparticles. The release rate from surfaces was significantly higher at pH 9.8 that at 4.5, indicating that alkaline conditions were more favorable for the mobilization of MnO₂ in the aquatic environment. In the presence of macromolecular organics, bovine serum albumin (BSA) can inhibit the release of MnO₂ from the surfaces due to attractive forces. In presence of humic acid (HA) and sodium alginate (SA), the MnO₂ nanoparticles were more likely to be mobile, which may be associated with a large repulsive barrier imparted by steric effects.