Preparation and defluorination mechanism of a novel copolymerized hydroxyapatite–aluminium chloride material

Abstract

As an attempt to avoid the low defluoridation capacity of conventional adsorbents, this study offers a novel copolymerized hydroxyapatite–aluminum (HAP–PAC) adsorbent and evaluates its performance in fluoride removal of drinking water, and a possible fluoride removal mechanism is proposed based on the characterization results obtained by FTIR, XRD, BET, SEM and EDS. The results indicate that the copolymerization material prepared by co-crystallization with aluminum chloride is featured by the chemical combination between hydroxyapatite (HAP) and poly-aluminium chloride (PAC). A certain amount of Ca²⁺ and OH⁻ in the HAP crystal lattice are partially replaced by Al³⁺ and Cl⁻ doping from PAC, and the structure of the copolymerization material shows a uniform trend. In addition, mainly by chemical adsorption, the highest defluoridation capacity (DC) of the copolymerization material can reach up to 18.12 mg g⁻¹, an increase of 14.02 mg g⁻¹ as compared with conventional hydroxyapatites. When aluminum chloride is replaced by bauxite in the study, the highest DC is 13.72 mg g⁻¹ due to the co-occurrence of chemical and physical combinations and relatively lower homogeneous structure caused by the co-existence of HAP and PAC while electrostatic attraction and chemical adsorption are both involved. As for the material prepared by the physical mixing method, in which the combination between HAP and PAC is mainly dominated by physical effects, its maximum DC is 13.08 mg g⁻¹, with physical adsorption as its main form of defluoridation.