Photodegradation of Pollutants in the Hydrophobic Cores of Dissolved Organic Matter: When Is It Important?

In natural surface waters, hydroxyl radicals (OH) and singlet oxygen (O) are known to occur not only in the bulk aqueous phase but also within the hydrophobic cores of dissolved organic matter (DOM). In these DOM sites, OH and O reach steady-state concentrations that are orders of magnitude higher than those in bulk water, which can enhance photodegradation of hydrophobic pollutants. In analogy with previous works, here, we use a two-phase reactivity model to address the importance of the phenomenon. The model requires much care to identify which variables are referred to the total solution volume (bulk water + DOM), the volume of the water bulk, and most importantly, that of the DOM phase. We suggest that DOM-phase partitioning could significantly affect the photodegradation of pollutants having an octanol-water partition coefficient log > 3. In the case of OH, differences should be expected between irradiation of solutions containing organic matter alone as the only OH source and irradiation of natural water samples where OH would also be generated by photolysis of nitrate and nitrite.