Effect of Carbonated Steelmaking Slag on the Growth of Benthic Microalgae

Calcium oxide in steelmaking slag may increase the pH when the slag is applied to marine environments such as the creation of an artificial tidal flat. Since sharp increases in pH are supposed to have negative impacts on the growth of benthic microalgae, carbonation of the steelmaking slag surface with CO2 is effective in alleviating the increase in pH and turbidity of seawater attributed to the formation of magnesium hydroxide. Even though the pH was increased by the non-carbonated slag, growth of Nitzschia laevis was significantly enhanced, when compared to that of the carbonated slag. In batch experiments (without N.laevis), the amounts of dissolved silicate, phosphate and iron from the non-carbonated slag were higher than those from the carbonated slag, indicating that the release of these elements might be the main factor responsible for enhancing the growth of this algal species. However, the released phosphate from the non-carbonated slag was initially low, a fact that could be attributed to suppression of phosphate dissolution under higher pH condition. Therefore, this study elucidated that the nutrients released from the non-carbonated slag enhanced the growth of N.laevis well, whereas sharp increase in pH may have inhibited the growth directly. On the contrary, even though the carbonated slag was effective in alleviating the pH increase, it was less effective in enhancing the growth of this algal species because released nutrients from the carbonated slag were low.