IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 BOOK OF ABSTRACTS 198 Poster Session / Soil Organic Matter, Caustobiolites, and Biochar Hall A P2.6 Reductive Properties of Soil Humic Acid and Its Oxidative Reactivity Toward Radical Formation Taiki Yatsu1,2, Xue Chu3, Kohji Maeda3, Yasuro Fuse3 1 JNC Engineering Co., Ltd., 5-1, Okawa, Kanazawa-Ku, Yokohama, 236-8605, Japan, t.yatsu@ jnc-eng.co.jp 2 Graduate School of Science and Technology, Kyoto Institute of Technology, Kyoto, Japan 3 Department of Molecular Chemistry and Engineering, Kyoto Institute of Technology, Kyoto, Japan Humus‑amended activated sludge has been observed to generate hydroxyl radicals (•OH) during the transition from anaerobic to aerobic conditions[1]. Under anaerobic conditions, humicreducing microorganisms interact with humic substances that function as electron mediators, forming reduced humic substances. The reduced humic substances are then capable of transferring electrons to Fe(III), thereby facilitating Fe(II) formation. During the subsequent aerobic phase, hydrogen peroxide (H2O2) production is presumed to increase, and sustained generation of ·OH radicals is likely to occur via Fenton or Fenton-like reactions within the redox cycle. However, the specific chemical contribution of humic substances themselves to these redox processes occurring within activated sludge remains unclear. Therefore, this study focused on humic acid extracted from humus soil (HSHA). By combining electrochemical measurements and chemical reduction experiments, we evaluated (i) its reduction characteristics, defined as its electron-accepting ability, and (ii) oxidation reactivity of the reduced form upon subsequent exposure to oxygen. Cyclic voltammetry (CV) was performed using ethyl viologen (EtV) as an electron mediator in the mediating electrochemical reduction method to evaluate the electron-accepting ability of HSHA. Since humic substances often exhibit slow direct electron transfer with working electrodes, electrontransfer mediation by electrochemically reversible molecules such as EtV enables sensitive tracking of redox-active sites distributed across a wide potential range[2]. Under these conditions, HSHA in the presence of EtV exhibited a remarkable enhancement of cathodic current relative to EtV alone, suggesting that HSHA can accept electrons from the one‑electron‑reduced species of EtV (E1/2 = −0.72 V vs. Ag/AgCl). Furthermore, chronoamperometry (CA) measurements revealed that HSHA alone displays measurable reductive currents at applied potentials around −0.8 to −0.9 V vs. Ag/AgCl. The combined results of CV and CA measurements suggest that HSHA contains electrochemically active reduction sites capable of accepting
RkJQdWJsaXNoZXIy NDA4Mjc=