Photochemical production of singlet oxygen from particulate organic matter

Dissolved organic matter is established as one of the most relevant photosensitizers in aquatic environments, producing singlet oxygen (¹O₂) alongside other photochemically produced reactive intermediates. While the production of ¹O₂ from DOM has been well studied, the relative importance of particulate organic matter (POM) to the overall ¹O₂ production is less well understood. POM is known to play an important role in pollutant fate through the sorption and transport of hydrophobic pollutants. If POM is directly involved in ¹O₂ production, sorbed molecules would be expected to undergo enhanced photodegradation. In this work, synthetic POM was prepared by coating silica particles with commercial humic acid. The photochemical behavior of these POM samples was compared to dissolved commercial humic acids (DOM). Suspended natural sediment was also studied to test the environmental relevance of the synthetic POM model. Synthetic POM particles appear to simulate well the ¹O₂-production of suspended sediment. The ¹O₂ concentrations experienced by POM-sorbed probe molecules was up to 30% higher than experienced by DOM-sorbed ones, even though the aqueous concentration of ¹O₂ in irradiated POM suspensions was much lower than the analogous DOM solutions. These results were interpreted with a reaction-diffusion model, which suggested that the production rate of ¹O₂ by POM is lower than DOM, but the loss of ¹O₂ from the POM-phase is also lower than DOM. Based on the experimental results of this study, calculations were conducted to estimate the impact of removing POM on ¹O₂-mediated processes. These calculations indicate that compounds with a log K_oc value near 4 will be most affected by removal of POM and that the magnitude of the effect is proportional to the fraction of the total organic matter represented by POM. This study demonstrates that particles can play an important role in the degradation of organic compounds via aquatic photochemistry.