BOOK OF ABSTRACTS 101 IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 Thursday, 27 August 2026 / Hall C NOM and Aquatic Systems SL47 Hydroxyl Radical Production Associated with Sediment Redox Cycling in the Bottom Turbid Layer of Lake Biwa Raiga Shiota1, Xue Chu2, Kazuhide Hayakawa3, Yasuro Fuse2 1 Graduate School of Science of Technology/ Master‘s Program of Functional Chemistry, 606-0951, m25674020@edu.kit.ac.jp 2 Faculty of Molecular Chemistry and Engineering, Kyoto Institute of Technology 3 Lake Biwa Environmental, Research Institute In the northern basin of Lake Biwa, a bottom turbid layer (BTL) formed by resuspended sediments persists throughout most of the year except during the spring overturn. Within this layer, anaerobic (reducing) and aerobic (oxidizing) microenvironments coexist in close proximity. Redox-active species—including Fe, Mn, and quinone moieties associated with sedimentary humic acids—are expected to migrate across this redox interface and establish a coupled three-component cycling system. In this system, MnO2 oxidizes hydroquinone (H2Q) to quinone (Q) as Mn is reductively dissolved; Q in turn mediates electron transfer to Fe(III) oxyhydroxides, generating Fe²⁺; and Fe²⁺ reacts with H2O2—formed during reoxidation steps—to produce hydroxyl radicals (•OH) via Fenton chemistry. If operative, this Mn–Q–Fe cycle would sustain continuous •OH generation in the BTL. The present study aims to verify this hypothesis through laboratory simulation experiments and to develop a deployable in situ •OH trapping sampler for field measurements in Lake Biwa. Humic acids were extracted from Lake Biwa sediments and dissolved at 100 mg/L. To simulate redox oscillations in the BTL, oxidizing and reducing conditions were alternately imposed by purging with air, nitrogen, or nitrogen containing hydrogen sulfide. Hydroxyl radicals were trapped using terephthalic acid and quantified by fluorescencedetection HPLC as hydroxyterephthalic acid. Under these conditions, •OH concentrations of up to 2.98 μmol/L were detected, with production particularly elevated during transitions from aerobic to anaerobic conditions. Addition of catalase substantially suppressed •OH generation, confirming that H2O2 acts as a direct precursor. Addition of bulk iron phases (FeO and Fe(NO3)3) further enhanced •OH production, consistent with the release of trace Fe²⁺ from these solids and subsequent Fe²⁺–driven Fenton reactions. A critical component of the proposed cycle is the role of Mn. Because the reduction potential of MnO2/Mn²⁺ exceeds that of the quinone/hydroquinone couple under environmentally relevant pH conditions, Mn reduction by H2Q is thermodynamically favorable and was expected to accelerate the quinone
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