Release of Phosphorus and Silicon from Steelmaking Slag and Their Effects on Growth of Natural Phytoplankton Assemblages

Abstract

Some coastal seas in post-industrialized countries are in the process of oligotrophication as a consequence of a rapid reduction in the load of certain nutrients such as phosphorus. Much worse is that this can sometimes be accompanied by noxious phytoplankton blooms due to an imbalance in the elemental ratio in seawater. Oligotrophication also leads to a reduced fishery production through a decrease in primary production and a change in community structure of the ecosystem. Slag produced as a waste bi-product during steelmaking, contains significant amounts of phosphorus and silicon. Both phosphorus and silicon form nutrient salts, phosphate and silicate, which enhance the growth of marine phytoplankton. Experimental studies were carried out to measure both the release of phosphate and silicate from steelmaking slag, and the increase of the growth rate of natural marine phytoplankton assemblages from this release. The appropriate concentration of slag addition for the growth of phytoplankton was considered to be around 100 mgl^-1 among the concentrations set in the present study, because sufficient growth was not obtained with 50 mg l^-1, and sharp increase in pH with 500 mgl^-1 addition inhibited the growth of phytoplankton. Diatom was the taxonomic group dominated in cell numbers after the addition of the slag. Silicate released from the slag appeared to support the growth of this group. In the group of diatoms, the growth response was different for individual species, suggesting that the species which will appear as a result of slag addition could be controlled with the characteristic and amount of slag applied. Discussion is made on the importance of controlling the coastal seas to conserve the precious coastal marine ecosystems as we have done so in rural areas of terrestrial ecosystems where human and nature coexist.