In order to continuously supply nutrient elements such as Si, P and Fe into seawater for the multiplication of phytoplankton, steelmaking slag has always been utilized at coast. Yet Fe as the obligatory micronutrient element shows an extremely low solubility under the natural seawater condition. As one kind of the broadly existing organic ligands, gluconic acid is able to form a complex with iron in alkaline aqueous solution, by which the soluble iron will be stabilized and thus the solubility

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... f iron will be improved. In the current research, the influence of gluconic acid on the dissolution of Si, P and Fe as well as the variation of pH was investigated by using shaking experiment. Present results show that gluconic acid has little effect on the variation of pH and the increase of the dissolved Si and P concentrations, whereas gluconic acid enhances the dissolution of Fe greatly by forming the iron-gluconate complexes. However, the photoreduction reaction of iron-gluconate complex occurred during shaking in the day time results in a slight decline of the concentration of the soluble iron.