The pH-sensitive polymerization process of phenol on hexagonal birnessite

Birnessite, a common manganese oxide present in soil, interacts with organic compounds through various mechanisms, including adsorption, oxidative decomposition, and oxidative polymerization. However, this polymerization pathway has been relatively underreported. This study focuses on the reaction pathways of phenols on hexagonal birnessite and the contribution of polymerization process. The results indicate that about 94 % of phenol can be effectively removed by birnessite from solution via polymerization and degradation processes under acidic conditions. When solution initial pH is 2.0, approximately 80 % of phenol undergoes the polymerization pathway, which is predominantly influenced by solution pH, while the other 20 % undergoes degradation pathway. The polymeric products and intermediates were further analyzed. Moreover, the polymerization mechanism was elucidated. Specifically, the zero-point charge (pH_PZC) of the hexagonal birnessite is 2.44. When the solution pH is below this pH_PZC, phenol is adsorbed to the surface of birnessite, subsequently undergoing a two-electron transfer process to generate phenoxonium ion intermediates, which subsequently condense into multimers. These findings provide new insights into the influence of birnessite on the fate of phenolic pollutants through the polymerization mechanism.