Impact of halides on the photoproduction of reactive intermediates from organic matter.

The excitation of dissolved organic matter (DOM) from sunlight produces a range of reactive intermediates, including triplet-excited state dissolved organic matter ((3)DOM*), hydroxyl radical (HO(•)), and singlet oxygen ((1)O2). These intermediates are important for the inactivation of pathogens and for the degradation of trace organic contaminants (OC) within natural and engineered systems. However, halides found in the background matrix can alter the photoproduction rates by promoting or quenching the formation of these intermediates. Apparent quantum yields (Φ(a)) for (1)O2, HO(•), and steady state (3)DOM* concentrations photoproduced from DOM isolates were determined with varying concentrations of chloride and bromide. Fluorescence quantum yields were measured as well to probe the photophysics of the system. The maximum fluorescence quantum yield (ΦF) decreased with the addition of halides, representing a quenching of the excited singlet state of DOM. In contrast, the steady state concentrations for (3)DOM* were enhanced, suggesting intersystem crossing from the singlet state to the triplet state was increased by the presence of halides. The Φ(a) for (1)O2 was increased with the addition of halides, which was expected following the (3)DOM* results because the mechanism for (1)O2 production occurs through the inactivation of (3)DOM* by dissolved oxygen. Although HO(•) production would be expected to follow (1)O2, the opposite trend was seen, which suggests the formation of HO(•) does not occur through the same precursor. Understanding the impact of specific quenchers on DOM could be a key to understanding the true formation potential for reactive intermediates and is especially important in estuaries and wastewater impacted aquatic systems.