Hydroxyapatite (HAP) is one of the fluoride adsorbents to remove fluoride in water, but F - is hard to enter the lattice of HAP caused by spatial confinement and competitive adsorption, which leads to low defluoridation efficiency. In this study, Ca-deficient regulation method was used to create ion channels during HAP synthesis to overcome the inhibition caused by spatial confinement effect, which increased the defluoridation capacity by 50 % from 1.56 to 2.34 mg/g (Initial F - concentration: 5 mg/L). The effects of dosage, pH, initial F - concentration and co-existing ions on defluoridation process were evaluated and the dosage of 1.4 g/L under neutral condition was the economic condition. The adsorption performance followed the Pseudo-second-order kinetic model and Freundlich isotherm model, which was a spontaneous and endothermic process. The results of molecular mechanics and density functional theory (DFT) calculations showed ion channels near the F - adsorption sites on HAP surface were generated due to the calcium deficiency, which effectively promoted F - into the lattice. The defects also changed the adsorption energy and sites of HF and F - , which reduced the competitive adsorption. In addition, the catalytic decomposition of HF molecules occurred on the surface of HAP was discovered for the first time.
Haoyang Fu,Hongfei He,Muhammad Usman,Qingze Chen,Minwang Laipan,Yixuan Yang,Runliang Zhu,Limei Cai
In this work, we developed a novel magnetic bimetallic Al/Fe (oxyhydr)oxide adsorbent through a facile and cost-effective method and explored its potential to adsorb fluoride in water. Its synthesis involved corrosion of natural magnetite in aluminium chloride solution, followed by titration with NaOH solution for in-situ synthesis of Al/Fe (oxyhydr)oxide-coated magnetite (Mag@Al2Fe). Characterization data indicated a uniform coating of Al/Fe (oxyhydr)oxide on magnetite, and the resulting composite possessed large specific surface area (∼90 m2/g) and good magnetic property. In batch adsorption experiments, the isotherm and kinetic data fitted well to the Langmuir model and pseudo-second-order model, respectively. The maximum adsorption capacity of Mag@Al2Fe is 26.5 mg/g, which was much higher than natural magnetite (0.44 mg/g). Moreover, this material retained high adsorption capacity toward fluoride within a wide pH range (3.0–8.0) and offered facile magnetic separation from water. Influence of competing ions was also evaluated which showed that the presence of Cl− and NO3− posed negligible interference, while HCO3− and SO42− had negative effects on fluoride adsorption. Thermodynamic investigations revealed that fluoride adsorption was exothermic and spontaneous. The observed increase in solution pH and formation of Al–F and Fe–F bonds (as indicated by XPS analysis) after fluoride adsorption suggested the major adsorption mechanism of ligand exchange. Besides, the adsorption/desorption cycle studies demonstrated the well-retained performance of Mag@Al2Fe for repeated application after regeneration by 0.5 mol/L NaOH solution. Facile synthesis, high defluoridation, lower cost, and quick separation of Mag@Al2Fe indicates its promising potential for drinking water defluoridation.
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