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In eutrophic waters, such as Mikawa Bay, Japan, anoxic bottom water develops in summer. This causes H₂S release into seawater due to sulfate reduction in the sediment, leading to environmental problems. The addition of Fe to sediments is a method used to improve the sediment environment. However, this method, which was devised from a natural mechanism, has not yet been quantitatively evaluated. In this study, we aim to quantitatively evaluate the suppressive effect of Fe on H₂S release. First, we developed a sediment model that focuses on S and Fe. We then attempted to reproduce the observations and experiments on H₂S dynamics using the model. Consequently, the proposed model was able to reproduce field S and Fe dynamics in sediments even under anoxic conditions. Additionally, the model could describe the difference in H₂S release depending on the amount, type, and time of iron-based additives. Finally, we conducted predictive calculations for one year assuming 637 g/m² of iron materials, and quantitatively evaluated the effects of adding iron materials to the sediments in terms of S and Fe cycles. We found iron-based additives suppressed H₂S release by 14 mmol/m²/d compared to the case without additives.