Mn effect on manganese oxides (MnO) nanoparticles aggregation in solution: Chemical adsorption and cation bridging.

Manganese oxides (MnO) and Mn usually co-exist in the natural environment, as well as in water treatments for Mn removal. Therefore, it is necessary to investigate the influence of Mn on the stability of MnO nanoparticles, as it is vital to their fate and reactivity. In this study, we used the time-resolved dynamic light scattering technique to study the influence of Mn on the initial aggregation kinetics of MnO nanoparticles. The results show that Mn was highly efficient in destabilizing MnO nanoparticles. The critical coagulation concentration ratio of Mn (0.3 mM) to Na (30 mM) was 2, which is beyond the ratio range indicated by the Schulze-Hardy rule. This is due to the coordination bond formed between Mn and the surface O of MnO, which could efficiently decrease the negative surface charge of MnO. As a result, in the co-presence of Mn and Na, a small amount of Mn (5 μM) could efficiently neutralize the negative charge of MnO, thereby decreasing the amount of Na, which mainly destabilized nanoparticles through electric double-layer compression, required to initiate aggregation. Further, Mn behaved as a cation bridge linking both the negatively charged MnO and humic acid, thereby increasing the stability of the MnO nanoparticles as a result of the steric repulsion of the adsorbed humic acid. The results of this study enhance the understanding of the stability of the MnO nanoparticles in the natural environment, as well as in water treatments.