Phosphorus removal of coal-based bottom ash: Performance evaluation and mechanism exploration

Abstract

In this paper, the phosphorus removal efficiency and capability and pore structure and composition of coal-based bottom ash before and after phosphorus removal are studied to reveal the phosphorus removal mechanism. The results show that the phosphorus removal rate of bottom ash increases with the increase of pH value; When the pH value is 8, the phosphorus removal rate can reach more than 95%. The highest desorption rate is only 2.42% and the pseudo-second order model can be better fitted with the phosphorus removal process of bottom ash, indicating chemical adsorption is the control mechanism. The maximum phosphorus removal capacity is 5.54 mg/g. After phosphorus removal the fractal dimension and porosity decrease, and the specific surface area increases, indicating that the complexity of the pore structure decreases, and the roughness of the pore wall increases. The characteristic peak of brushite (CaHPO4·2H2O) in the XRD spectrum and HPO42− at 2362 cm−1, 985 cm−1 and 800-650 cm−1 appear, respectively. In the XRF results, P2O5/SiO2 ratio increases from 0.008 to 0.089, but SO3/SiO2 ratio decreases from 0.047 to 0.027. The phosphorus removal mechanism of coal-based bottom ash lies in its complex pore structure and the precipitation reaction between soluble Ca2+ and HPO4− in the sewage.