Removal of Phosphate Using Ettringite Synthesized from Industrial By-products

Abstract

Removal of phosphate using ettringite synthesized from industrial by-products was evaluated in this study. Ettringite was synthesized by combining basic oxygen furnace (BOF) slag, alum, and flue-gas-desulfurization (FGD) gypsum at molar ratio ([Ca]:[Al]:[SO₄]) of 3:2:3, pH 11.74, and 28 °C for 24 h. Kinetic study showed that the adsorption of phosphate by ettringite reached equilibrium after 24 h and could be represented by pseudo-second-order kinetic model. Equilibrium adsorption study revealed that pH was the most important factor, and removal efficiency increased with increasing pH. The mode of phosphate removal could be divided into two regions. At lower initial phosphate concentration (< 300 mg/L), experimental results fitted well with both Langmuir and Freundlich isotherm models. However, the adsorption density increased linearly with increasing phosphate concentration when at higher initial phosphate concentration (> 300 mg/L), implying that phosphate was mainly removed by surface precipitation. Judging from X-ray diffraction (XRD) analysis and PHREEQC simulation, the main calcium phosphate precipitate is hydroxyapatite (HAP). This study demonstrated that it is feasible to synthesize highly effective adsorption material using industrial by-products for phosphate removal.