Application of Dephosphorization Slag to Marine Surface Sediment to Increase the Organic Carbon Content

Fixation of carbon dioxide as organic matter in marine sediment has attracted attention as a blue carbon. Steel slag, a by-product of the steel industry, can be used to improve the sediment environments through mechanisms, such as suppression of hydrogen sulfide generation; however, the effect of steel slag on increasing the carbon storage is not well known. This study showed that the addition of dephosphorization slag to sediment accelerated increase in the sedimentary organic matter when the hypoxic overlying water was aerated. Adding the iron (Fe) powder or ferrous oxide (FeO) to the sediment as the major components of the slag, the former accelerated increase in the sedimentary organic matter too. The relative abundances of sulfur-oxidizing bacteria, including Sulfurovum and Sulfurimonas, increased together with that of Hydrogenovibrio, Mariprofundus, Magnetovibrio, and Gallionella. The phenomena might be related to sulfur and/or Fe oxidation in the sediment, and these bacteria might cause increase in the sedimentary organic matter. Furthermore, addition of the Fe powder suppressed elution of the dissolved organic carbon, potentially resulting from inhibition of oxygen transfer from the surface to the lower layers of the sediment to suppress aerobic degradation.