Effects of humic acid and surfactants on the aggregation kinetics of manganese dioxide colloids

The aggregation of common manganese dioxide (MnO2) colloids has great impact on their surface reactivity and therefore on their fates as well as associated natural and synthetic contaminants in engineered (e.g. water treatment) and natural aquatic environments. Nevertheless, little is known about the aggregation kinetics of MnO2 colloids and the effect of humic acid (HA) and surfactants on these. In this study, the early stage aggregation kinetics of MnO2 nanoparticles in NaNO3 and Ca(NO3)(2) solutions in the presence of HA and surfactants (i.e., sodium dodecyl sulfate (SDS), and polyvinylpyrrolidone (PVP)) were modeled through time-resolved dynamic light scattering. In the presence of HA, MnO2 colloids were significantly stabilized with a critical coagulation concentration (CCC) of similar to 300 mmol.L-1 NaNO3 and 4 mmol.L-1 Ca(NO3)(2). Electrophoretic mobility (EPM) measurements confirmed that steric hindrance may be primarily responsible for increasing colloidal stability in the presence of HA. Moreover, the molecular and/or chemical properties of HA might impact its stabilizing efficiency. In the case of PVP, only a slight increase of aggregation kinetics was observed, due to steric reactions originating from adsorbed layers of PVP on the MnO2 surface. Consequently, higher CCC values were obtained in the presence of PVP. However, there was a negligible reduction in MnO2 colloidal stability in the presence of 20 mg.L-1SDS.