Enhanced phosphate removal by thermally pretreated waste oyster shells

Waste oyster shells were used to remove phosphate and the effects of thermal pretreatment were studied. Thermal pretreatment under different conditions (400 °C for 0.5 h, 400 °C for 2 h, and 700 °C for 2 h) shifted the isoelectric point (pH_IEP) of oyster shells to more alkaline range, while surface area decreased from 3.47 to 0.52 m²/g when pretreated at 700 °C for 2 h. Phosphate adsorption was enhanced when using pretreated oyster shells. Adsorption fitted well with Langmuir isotherm model. Both the maximum monolayer coverage (Q₀) and the adsorption constant (b) increased when using oyster shells preheated at 400 °C. The values of Q₀ and b increased significantly to 5.332 mg/g and 0.728 L/mg, respectively, at pH 10.59 when using oyster shells preheated at 700 °C for 2 h, which could be attributed to the formation of calcium oxide (CaO) as evidenced by analysis of X-ray photoelectron spectroscopy (XPS). Both adsorption and precipitation processes were involved in the reactions as supported by analysis of loaded oyster shells by XPS, and hydroxyapatite (Ca₅(PO₄)₃(OH)) was formed on surfaces of oyster shells.