The Role of Manganese Dioxide in the Natural Formation of Organochlorines

Organochlorines are important environmental contaminants that impact ecosystems, public health, and global warming. Organochlorines are generally regarded as anthropogenic compounds, but they also have natural sources. Here, we report that the commonly occurring manganese(IV) secondary mineral birnessite (δ-MnO2) was found to chlorinate guaiacol (2-methoxyphenol), a moiety in lignocellulose and hence plant-derived natural organic matter (NOM). Environmentally relevant concentrations of birnessite and chloride were incubated together with guaiacol for 18 min at pH 4.0, resulting in the formation of up to 118 μg L-1 of chloroguaiacols. The extent of chlorination was dependent on the chloride concentration. Conducting these reactions under anoxic conditions had little effect on chloroguaiacol formation, suggesting that oxygen is not required in the MnO2-mediated chlorination of guaiacol. This study demonstrates that ubiquitous MnO2 catalyzes the formation of chlorinated NOM, revealing a globally important source of natural organochlorines that has previously been overlooked. The MnO2-mediated mechanism can account for the previous detection of organochlorines in forest soils, where accumulation of manganese is known to occur during the decomposition of wood and leaf litter.