Effects of steelmaking slag addition on growth of marine phytoplankton

Abstract

Culture experiments of a natural phytoplankton assemblage collected from coastal surface water and a mono-specifically cultured diatom were conducted to investigate the possibility of steelmaking slags to use as a nutrient resource for marine phytoplakton. Two kinds of slags, a decarburization slag and a dephosphorous slag, were tested in batch cultures under freely drifting pH. Leaching experiment to determine dissolution of nutrient elements from the slags demonstrated that phosphate and reactive silicate are released into seawater, while rate of their dissolutions does not simply depend on amount of the slags. Excess addition of the slags (3300 mg//) decreased phosphate in the seawater probably due to precipitation with calcium of main component of the slags. Growth of natural phytoplankton assemblage was evidently enhanced by addition of the slags at 33 mg//. However, excess addition (3300 mg//) suppressed the growth of phytoplankton due to not only decreased phosphate and silicate but also increased pH to 10. This shift to alkaline might decrease considerably solubility of iron (essential nutrient for phytoplankton growth), although organic ligands existed. These results indicate that slag enrichment would be effective at a lower dose to avoid pH increase. Although in our study 33 mg// was confirmed to be the best dose, solubility of the elements and pH increase might be different with different slags. Therefore, the best dose should be determined for each slag before application.