Evaluation of the Mechanism of Phosphate Removal Using Oyster Shell Powder in Aqueous Environments

Eutrophication is caused by the inflow of nutrients, such as phosphorus and nitrogen, into closed waterbodies from wastewater. Calcination of oyster shells greatly increases their capacity for phosphate removal; however, information available on this mechanism and the capacity for phosphate removal under different initial pH values and temperatures is less. Herein, we investigated the utilization of oyster shells for phosphate removal under different pH and temperature conditions. Oyster shell powder (OSP) was calcined in a muffle furnace at temperature ranges of 200-1000 °C. Each OSP sample was added to a phosphate solution and the suspension was shaken under different pH and temperature conditions. The main component of OSP changed from CaCO₃ to CaO after calcination at approx. 800 °C. The amount of phosphate removal by the calcined OSPs at 800 and 1000 °C was higher than that removal by the other OSPs. Further, the amount of calcium elution from the OSPs calcined at 800 and 1000 °C was higher than that elution from the other OSPs. This was because the solubility of CaO was higher than that of CaCO₃. The amount of phosphate removal by the OSP and calcined OSPs at 200-600 °C was the highest at pH 5-7, and increased with increasing reaction temperature. These findings suggested that the mechanism of phosphate removal may involve adsorption in the OSP and OSPs calcined at 200-600 °C, whereas it is associated with coagulation settling and adsorption in the OSPs calcined at 800 and 1000 °C.