The aggregation kinetics of manganese oxides nanoparticles in Al(III) electrolyte solutions: Roles of distinct Al(III) species and natural organic matters

This study explored the aggregation kinetics of manganese oxides (MnO_x) nanoparticles in Al(III) electrolyte solutions. This is a common process in both water treatments and the natural environment. The results show that aggregation kinetics are Al(III) species-dependent. Without natural organic matters (NOM), ferron Al_a (monomeric Al(III)) and ferron Al_b (polymeric Al(III)) are the main species controlling the Derjaguin−Landau−Verwey−Overbeek (DLVO) type aggregation behavior of MnO_x at pH 5.0 and 7.2, respectively. Al_a and Al_b can neutralize and reverse the negative charge of MnO_x. Correspondingly, the attachment efficiency as a function of Al(III) concentrations contains three stages: destabilization, diffusion-limited, and re-stabilization stage. Interestingly, due to the tiny size of Al_b nanoclusters, they behave similar to free ions and do not induce heteroaggregation at pH 7.2. The influence of some model NOM (i.e., bovine serum albumin (BSA), Sigma humic acid (HA), and alginate) was also studied. At pH 5.0, alginate polymers, while Sigma HA and BSA cannot be, are linked by Al(III) to form alginate gel clusters which bridge MnO_x nanoparticles, and thus induce bridging flocculation. At pH 7.2, NOM induce the aggregation of Al_b nanoclusters to form NOM-Al(III) aggregates through charge neutralization effects. Consequently, highly enhanced aggregation rate, due to the heteroaggregation between these aggregates and MnO_x, was observed.