Abstract
Abstract The reduction-induced aggregation of MnO 2 colloids was examined in the presence of seven organics in order to understand the influence of organic structure on the dissolution and/or aggregation of MnO 2 colloids in aqueous solutions. The reduction-induced aggregation of MnO 2 colloids was observed in the presence of catechol, 4-methylguaiacol, guaiacol, 4-methylphenol, or 4-nitroguaiacol. The dissolution of MnO 2 colloids was enhanced in the presence of organics with strong complexing functional groups, i.e., catechol, 4-methylguaiacol, and guaiacol. Conversely, negligible Mn dissolution was observed in the presence of weak complexation agents, i.e., 4-methylphenol, or 4-nitroguaiacol. However, significant dissolution of MnO 2 was achieved, while no aggregation was observed in the presence of vanillic acid, which contains the strong complexation functional group carboxyl. In the presence of citric acid, a rapid, significant decrease in the hydrodynamic diameter of MnO 2 colloids was observed after a slow, slight size decrease. The autocatalytic reduction of MnO 2 might be responsible for their “autocatalytic dissolution”. For all tested organics, the increase of surrounding pH hindered the reduction-induced aggregation of MnO 2 colloids, which might indicate a slow reduction of MnO 2 by organics at the corresponding pH.
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