Evaluating the Microheterogeneous Distribution of Photochemically Generated Singlet Oxygen Using Furfuryl Amine

Singlet oxygen (¹O₂) is an important reactive species in natural waters produced during photolysis of dissolved organic matter (DOM). Prior studies have demonstrated that ¹O₂ exhibits a microheterogeneous distribution, with [¹O₂] in the interior of DOM macromolecules ∼30 to 1000-fold greater than in bulk solution. The [¹O₂] profile for DOM-containing solutions has been determined mainly by the use of hydrophobic probes, which are not commercially available. In this study, we employed a dual-probe method combining the widely used hydrophilic ¹O₂ probe furfuryl alcohol (FFA) and its structural analogue furfuryl amine (FFAm). FFAm exists mainly as a cation at pH <9 and was therefore hypothesized to have an enhanced local concentration in the near-DOM phase, whereas FFA will be distributed homogeneously. The probe pair was used to quantify apparent [¹O₂] in DOM samples from different isolation procedures (humic acid, fulvic acid, reverse osmosis) and diverse origins (aquatic and terrestrial) as a function of pH and ionic strength, and all samples studied exhibited enhanced reactivity of FFAm relative to FFA, especially at pH 7 and 8. To quantify the spatial distribution of [¹O₂], we combined electrostatic models with Latch and McNeill's three-phase distribution model. Modeling results for Suwannee River humic acid (SRHA) yield a surface [¹O₂] of ∼60 pM, which is ∼96-fold higher than the aqueous-phase [¹O₂] measured with FFA. This value is in agreement with prior reports that determined 1-3 orders of magnitude higher [¹O₂] in the DOM phase compared to bulk solution. Overall, this work expands the knowledge base of DOM microheterogeneous photochemistry by showing that diverse DOM isolates exhibit this phenomenon. In addition, the dual-probe approach and electrostatic modeling offer a new way to gain mechanistic insight into the spatial distribution of ¹O₂ and potentially other photochemically produced reactive intermediates.