Multiple roles of humic acid in the photogeneration of reactive bromine species using a chemical probe method

Highlights: • Photogeneration of RBrS in HA solution was quantified by a chemical probe method. • RBrS concentrations were 3–5 orders of magnitude lower than those in model system. • The multiple roles of HA in the RBrS photogeneration were firstly illustrated. • 0.12%–0.70% of ³HA* would be transformed into RBrS in HA-involved solution. • Structures of HA affected RBrS generation and altered transformation pathways. Photosensitization of natural organic matter (NOM) is an important natural source of reactive bromine species (RBrS) in the environment. Up to now, quantitative information about RBrS was mainly based on model sensitizers. Whether the behavior of model compounds could represent those of complex NOM remains unknown. In this study, we employed a chemical probe (3,5-dimethyl-1-H-pyrazole) to measure RBrS in humic acid (HA)-containing solutions and investigated their influential factors. The formation rate, decay rate constant, steady-state concentration, and lifetimes of RBrS were 3.87(±0.16) × 10⁻¹³ mol L⁻¹·s⁻¹, 1.99(±0.20) × 10⁴ s⁻¹, 2.04(±0.13) × 10⁻¹⁷ mol L⁻¹, and 5.06(±1.05) × 10⁻⁵ s, respectively. Measured steady-state concentrations of RBrS were 3–5 orders of magnitude lower than those in model sensitizer system. Results showed that HA drove the RBrS generation, and about 0.12–0.70% of triplet-state HA (³HA*) would be transformed into RBrS. HA structures strongly affected this process. Phenolic-like groups suppressed the formation, while aromatic ketone-like moieties facilitated it. Last, HA also altered the transformation pathways. The contribution of ·OH dependent and direct oxidation pathways was almost equal, while the direct oxidation was predominant in the model system. Thus, careful consideration should be taken into photochemical formation of RBrS in NOM-involved solution, due to their complexity and multiple roles.