Reinforced adsorption mechanism of fluorine ions by calcium-depleted hydroxyapatite and application in the raffinate from the vanadium industry

Abstract

Calcium-deficient hydroxyapatite (Ca-d HAP), an adsorbent of high defluoridation capacity with lattice defect, was prepared from vanadium industrial waste neutralization slag (NS). The adsorption performance was compared to commercially available stoichiometric HAP (S-HAP) under different solution conditions, and Ca-d HAP was superior to S-HAP. The specific surface area of Ca-d HAP was considerable (118.32 m2/g), which was 3.176 times larger than that of S-HAP (37.25 m2/g), and this was the main reason for the strong adsorption performance of Ca-d HAP. Meanwhile, due to the presence of minor impurities, such as Al and Fe, in NS, the doping of Al and Fe reduced the crystallinity after Ca-d HAP synthesis, which provided additional active sites for binding to other ions. In addition, combined with characterization analysis, it was speculated that some Ca ions in Ca-d HAP were replaced by Na ions with stronger ion-exchange properties, enhancing its adsorption performance. The adsorption mechanism of Ca-d HAP on F- ions was further investigated by discussing the surface potential, initial pH, adsorbent dosage, initial fluorine concentration, adsorption thermodynamics, and kinetics. Results showed that the Langmuir isothermal model well predicts the adsorption behavior of Ca-d HAP on F- ions at pH 3.5, with a maximum adsorption capacity of 277.78 mg/g, which is larger than most HAP adsorbents reported in the literature. In addition, the percentage adsorption of Ca-d HAP on F- ions was > 99 % in the raffinate and the NaF solution. In addition, the percentage adsorption of Ca-d HAP on F- ions was > 99 % both in the raffinate and in the NaF solution. The adsorption of F- ions by Ca-d HAP is a heat absorption process, with the ΔG° < 0 at various temperatures, demonstrating the reaction's spontaneity of the reaction. The pseudo-second-order model better described the adsorption kinetics data at 303 to 333 K, and the adsorption was regulated by chemical reactions. Overall, Ca-d HAP with lattice defects has a large ion-exchange rate and specific surface area, which is the key reason for the high adsorption performance of Ca-d HAP. Therefore, this technology provides a prospect for treating acidic wastewater with high fluorine content in the vanadium industry.