BOOK OF ABSTRACTS 27 IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 Monday, 24 August 2026 / Hall B+C Analysis and Characterization SL7 Quantifying the Contribution of Carbonyl Groups to the Optical and Photochemical Properties of Dissolved Organic Matter Garrett McKay, Ahmad Beyhaqil, Ruiya Zhoul 1 Zachry Department of Civil & Environmental Engineering, Texas A&M University, 3136 TAMU, College Station, TX 77843, gmckay@tamu.edu Carbonyl groups–particularly, aldehydes, ketones, and quinones–are well-known to play critical roles in the reactivity of humic substances (HS) and natural organic matter (NOM) in natural and engineered systems. Indeed, in early work by Leenheer et al., it was estimated based on 13C-NMR measurements that “about one ketone group per monocyclic aromatic ring” is present in both humic and fulvic acids. Carbonyl-containing compounds are particularly important for the absorbance, fluorescence, and photochemical properties of these materials. Quinones have received special attention given their reversible redox couple and importance as electron shuttles in biogeochemical systems. Prior reserach by McKnight and others [2) has suggested that fluorescence spectroscopy may be used to probe the redox state of NOM, which has been met with multiple conflicting reports [3,4]. The above example from carbonyl chemistry highlights a key unresolved challege in the field of HS and NOM photochemitry: we do not as of yet have a way to quantify the contribution of individual molecular species or even groups of related species to DOM optical spectra or photochemistry. In this presentation, I will demonstrate how we attribute over 50% of DOM visible absorbance to carbonyl-containing compounds using a combination of chemical and electrochemical reduction techniques. We further specify that quinones contribute, on average, <30% to the absorbance of HS and NOM. The methods involve the in parallel and in series use of sodium borohydride and direct electrochemical reduction, followed by measurement of absorbance and fluorescence spectra. Detailed electrochemical reduction studies illuminate the fact that only soil humic substances show quantitatively meaningful changes in fluorescence spectra during reduction, whereas aquatic HS and NOM do not. In addition, we use sodium borohydride reaction at different reaction times and doses to elucidate the impact of carbonyls to the photochemical production of singlet oxygen, hydroxyl radical, and triplet excited state NOM and HS. Overall, the results and conclusions drawn from them clarify the role of carbonyl groups in HS and NOM optical properties in photochemistry, which will be of great interest to the scientific and engineering community researching HS and NOM
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