Reduction-induced aggregation of manganese dioxide colloids by guaiacol

It is well known that the most abundant oxidizing agent in aquatic environments, manganese dioxide (MnO₂), can be reduced to Mn(II) by organics containing electron-rich moieties such as phenol, aniline, and thiol. It is expected that the redox reactions between MnO₂ colloids and electron-rich moieties may significantly affect the aggregation of MnO₂ colloids. This work, for the first time, reports a potential role of MnO₂ colloid reduction in their aggregation through time-resolved dynamic light scattering with guaiacol (2-methoxyphenol) as a model phenolic monomer. Data on initial aggregation kinetics show that additive Mn(II) exhibit excellent ability to cause MnO₂ colloidal aggregation compared with other divalent ions such as Ca(II) and Mg(II), probably due to its much higher surface affinity. As expected, the introduction of guaiacol results in a reduction-induced aggregation of MnO₂ colloids. As the redox reactions proceeded, the hydrodynamic diameter of MnO₂ colloids first decreased slightly despite the decreased electrostatic repulsion between them, then an accelerated rate aggregation of MnO₂ colloids was observed accompanying a further decrease of electrostatic repulsion, and finally the MnO₂ aggregates settled out. The reduction-induced aggregation slowed down with the increase of pH or decrease of guaiacol concentration. Surface bound Mn(II) might play a primary role in decreasing the electrostatic repulsion between MnO₂ colloids, and thus result in their reduction-induced aggregation.