Effects of calcium on phosphorus recovery from wastewater by vivianite crystallization: Interaction and mechanism analysis

Abstract

Phosphorus recovery by vivianite from different types of wastewater has attracted considerable attention recently. However, the existence of calcium (Ca) would unavoidably affect the formation of vivianite. In this work, the effect of calcium on vivianite crystallization was investigated. The pH of the solutions with low Ca concentration (1–10 mg/L) increased by only 0.06 compared to the conditions without Ca, while a more visible change occurred at high concentrations. Besides, higher removal rate of phosphorus was obtained at higher Ca concentration and the residual amount of Fe in the supernatant increased from 2.01 mg/L in control to 27.29 mg/L at 100 mg Ca/L, indicating competition between Ca and Fe ions. Furthermore, microscopic observation revealed that Ca caused the surface of vivianite to be covered by fine particles, resulting in the inhibition of vivianite growth. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis demonstrated the fine particles covering in the surface of vivianite crystals were mainly amorphous Ca3(PO4)2 and the formation of Fe-phosphate (s) also led to the decrease of the recovery rate and purity of vivianite. The zeta potential of the recovered product indicated that electrostatic adsorption existed in the process and complemented with surface adsorption to become the main routes of Ca binding. The results obtained herein will provide a theoretical basis for understanding the interference of calcium with vivianite crystallization.