Byproducts of the anammox-hydroxyapatite coupling process — Characterization and its adsorption capacity for Cd(II)

Abstract

The anammox-HAP coupling process can simultaneously achieve high-efficiency nitrogen removal and phosphorus recovery from sewage. The composition and physical properties of the byproducts (aHAP) collected in the anammox-HAP process were analyzed. In addition, the application potential and mechanisms for Cd(II) adsorption by aHAP in wastewater were investigated. The results show that aHAP is mainly composed of hydroxyapatite and calcium carbonate, having granular structure with honeycombs and pores. The aHAP shows an adsorption capacity of 21.10–34.80 mg/g for Cd(II) removal, and the adsorption behavior is consistent with pseudo-second-order and intra-particle diffusion models, showing that the rate-limiting step is controlled by the chemical reaction of surface complexation, ion exchange, and precipitation. In addition, the remaining anammox biomass in aHAP can bind Cd(II). The aHAP also shows a pH buffer capacity when the initial pH ranges between 4.10 and 8.46, and the final pH can be stabilized in the range of 6.3 to 8.3, which is suitable for anammox reactions. Collectively, the anammox-HAP coupling process can not only be used to achieve high-efficiency nitrogen and phosphorus removal, but its aHAP byproduct shows good adsorption properties and pH buffering capacity, which meets the requirements of green chemistry and has high economic and environmental benefits.