BOOK OF ABSTRACTS 199 IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 Hall A Poster Session / Soil Organic Matter, Caustobiolites, and Biochar electrons, plausibly associated with redox-active functional moieties such as quinone-type structures. To evaluate oxygen reactivity following reduction, HSHA was chemically reduced by sodium dithionite (E = −0.86 V) under a N2 atmosphere, followed by aeration in the presence of disodium terephthalate (TPA·2Na) as a hydroxyl radical probe[3]. Upon aeration, the accumulation of 2-hydroxyterephthalic acid (hTPA), the reaction product of •OH with TPA, increased relative to the dithioniteonly control, indicating that reduced HSHA undergoes oxygen-dependent oxidation in the dark under near-neutral pH conditions. These observations are consistent with prior reports showing that reducible humic substances can undergo oxidation by O2 in the dark to generate reactive oxygen species [4]. Although the detailed mechanistic pathway leading to radical generation and its quantitative efficiency remain subjects for future investigation, the present results demonstrate the intrinsic capacity of reduced HSHA to drive oxygen-dependent oxidative processes. References 1. T. Yatsu, T. Kwai, X. Xie, N. Namiki, C. Xue, Y. Fuse, 41st annual meeting of Japanese Humic Substances Society, Okayama, 2025. 2. M. Aeschbacher, M. Sander, R. P. Schwarzenbach, Environ. Chem. Soc., 2010, 44, 87-93. 3. S. E. Page, W. A. Arnold, K. McNeill, J Environ Monit., 2010, 12, 1658-1665. 4. S. E. Page, M. Sander, W. A. Arnold, K. McNeill, Environ. Sic. Chem. Soc., 2012, 46, 1590-1597.
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