# Probing the Microheterogeneous Distribution of Photochemically Produced Hydroxyl Radicals in Dissolved Organic Matter

## TL;DR

This study shows that hydroxyl radicals formed in dissolved organic matter are unevenly distributed and influenced by the organic matter's chemical structure.

## Contribution

The first demonstration of how aliphaticity and aromaticity affect the microheterogeneous distribution of hydroxyl radicals in DOM.

## Key findings

- Hydroxyl radical concentration in the DOM phase was 20–150 times higher than in the aqueous phase.
- A negative correlation was found between [•OH]DOM/[•OH]aq and aromaticity, and a positive correlation with aliphaticity.
- Size fractionation showed that both high- and low-molecular weight DOM components influence •OH microheterogeneity.

## Abstract

Dissolved organic
matter (DOM) is a major photosensitizer
in sunlit
surface waters that generates hydroxyl radicals (•OH). While •OH is believed to form within hydrophobic
DOM microdomains, its spatial distribution and phase-specific reactivity
remain poorly characterized among DOM from diverse environments. In
this study, we employed testosterone as a probe to quantify DOM-phase •OH concentration ([•OH]DOM) via hydrophobic partitioning. Across eight DOM isolates, [•OH]DOM was found to be 20–150 times
higher than aqueous phase concentration ([•OH]aq). To explore the underlying drivers of this microheterogeneity,
we evaluated [•OH]DOM/[•OH]aq in relation to DOM composition. We report, for the
first time, a negative correlation between [•OH]DOM/[•OH]aq and aromaticity and
a positive correlation with aliphaticity. Testosterone was further
employed to quantify [•OH]DOM/[•OH]aq for Suwannee River natural organic matter and humic
acid isolates size fractionated with a 3 kDa ultrafiltration membrane.
As expected, the <3 kDa showed little evidence of •OH microheterogeneity. In contrast, the >3 kDa fraction showed
lower
[•OH]DOM/[•OH]aq than the bulk fraction, suggesting that both high- and low-molecular
weight components are important for the formation of microheterogeneous •OH. Overall, these results establish the ubiquity of
microheterogeneous •OH formed during DOM photolysis
and suggest that the abundance of aliphatic and aromatic carbon are
important structural features governing this microheterogeneity.