The OH radical is the most reactive oxidant produced in natural waters, while low-molecular-weight (LMW) carbon compounds, e.g., carbonyls (formaldehyde), can be readily taken up by bacteria and therefore can stimulate carbon cycling in natural waters. Photoproduction of these species from DOM in natural waters is well documented. However, the responsible chromophoric sites within DOM remain unknown. A series of substituted phenol model compounds were examined to test whether these structures could act as sources of OH radicals and LMW carbonyl compounds. We found that many of these phenolic compounds, especially compounds with hydroxyl and carboxyl groups in para positions (e.g., 4-hydroxybenzoic acid and 2,4-dihydroxybenzoic acid) produce OH radicals with quantum yields up to ~1.0x10^-2 in the wavelength range 280-340 nm. The formation of OH radicals was confirmed by competition kinetics and reaction with methane. Regarding photoproduction of LMW carbonyl compounds, aromatic compounds with alkoxy groups directly bonded to the ring had the highest quantum yields. Methoxy aromatics gave rise to only formaldehyde, while ethoxy species gave rise to only acetaldehyde. Possible formation pathways will be presented. We conclude that phenolic structures, which may derive from polyphenolic DOM such as lignins, tannins and humic substances, play a key role in the photoreactivity of DOM and as a direct source of reactive species in natural waters.

Insights into the photoproduction sites of hydroxyl radicals in DOM in natural waters

ACS Spring Meeting 2021

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