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.