Reduction-induced aggregation and/or dissolution of MnO2 colloids by organics

The reduction-induced aggregation of MnO2 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 MnO2 colloids in aqueous solutions. The reduction-induced aggregation of MnO2 colloids was observed in the presence of catechol, 4-methylguaiacol, guaiacol, 4-methylphenol, or 4-nitroguaiacol. The dissolution of MnO2 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 MnO2 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 MnO2 colloids was observed after a slow, slight size decrease. The autocatalytic reduction of MnO2 might be responsible for their "autocatalytic dissolution". For all tested organics, the increase of surrounding pH hindered the reduction-induced aggregation of MnO2 colloids, which might indicate a slow reduction of MnO2 by organics at the corresponding pH. (C) 2015 Elsevier B.V. All rights reserved.