Transport Behaviors of Colloidal Manganese Dioxide in Aqueous Media: Effects of Ionic Specificity of Monovalent Cations

The influence of ionic strength on the transport behaviors of natural or engineered colloids has been widely studied in recent years, and the explanatory classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory has also been employed in many studies. However, ion-specific effects were barely applied to interpret the contradiction between the experimental observations and classical DLVO predictions when nanoparticle or colloidal aggregation and deposition took place in different cationic solutions. In this study, the effects of ionic specificity of the first-group monovalent cations (i.e., Li+, Na+, K+, and Rb+), and the influence of representative polysaccharides (alginate) and proteins (bovine serum albumin (BSA), on the aggregation and deposition behaviors of colloidal manganese dioxide (MnO2) onto silica (SiO2) and iron (Fe3O4) surfaces were systematically studied by employing time-resolved dynamic light scattering (DLS) and a quartz crystal microbalance with dissipation monitoring (QCM-D), respectively. Experimental results indicated that the additive cations with a poorer hydration degree (e.g., Rb+) were more efficient in promoting the aggregation, hindering the deposition kinetics of colloidal MnO2. This could be quantitatively interpreted with the modified DLVO theory, through the introduction of additional short-range repulsion, which was generated from a hydration force between colloids and interfaces. Moreover, BSA was more efficient in hindering the aggregation of colloidal MnO2 than alginate. Furthermore, BSA could enhance the deposition of MnO2 while alginate exhibited the opposite effect. These results revealed the significance of ionic specificity in the aggregation and deposition behaviors of MnO2 colloids and provided insights into understanding the role of counterion hydration in the transport and fate of engineering nanoparticles (ENPs) in an aquatic environment.