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Disappearing of benthic seaweeds in coastal areas has gradually become a serious environmental problem worldwide. One possible reason is iron deficiency in seawater. As a major byproduct generated in steelmaking industry, steelmaking slag is rich in Fe and has the potential to restore seaweeds by supplying soluble Fe in seawater. The authors aim to develop a sustainable approach for ecosystem restoration in coastal areas by utilizing steelmaking slag. This work investigates the releasing behavior of elements from synthesized steelmaking slags in seawater and clarifies the effects of slag composition, carbonation and usage of gluconic acid. Slag with larger CaO/SiO 2 ratio (= 3.0) has difficulty in releasing Fe into seawater directly, due to drastic increase of pH. Combination of slag carbonation and gluconic acid usage is necessary and effective in boosting release of Fe. Slag with smaller CaO/SiO 2 ratio (= 1.0) would not cause drastic increase of pH in seawater and thus releasing of Fe is easier. These results have proved the potential value and feasibility of using steelmaking slag as an underwater iron fertilizer.

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