Aggregation kinetics of manganese dioxide colloids in aqueous solution: Influence of humic substances and biomacromolecules

In this work, the early stage aggregation kinetics of manganese dioxide (MnO₂) colloids in aqueous solution and the effects of constituents of natural organic matter (i.e., Suwannee River fulvic acid (SRFA), Suwannee River humic acid (SRHA), alginate, and bovine serum albumin (BSA)) were investigated by time-resolved dynamic light scattering. MnO₂ colloids were significantly aggregated in the presence of monovalent and divalent cations. The critical coagulation concentrations were 28, 0.8, and 0.45 mM for NaNO₃, Mg(NO₃)₂, and Ca(NO₃) ₂, respectively. The Hamaker constant of MnO₂ colloids in aqueous solution was 7.84 × 10^-20 J. All the macromolecules tested slowed MnO₂ colloidal aggregation rates greatly. The steric repulsive forces, originated from organic layers adsorbed on MnO₂ colloidal surfaces, may be mainly responsible for their stabilizing effects. However, the complexes formed by alginate and Ca²⁺ (>5 mM) might play a bridging role and thus enhanced MnO₂ colloidal aggregation instead. These results may be important for assessing the fate and transport of MnO₂ colloids and associated contaminants.