Abstract:
The removal efficiency of hydroxyapatite (HAP ) to fluoride was low due to its easy agglomeration during the synthesis process. Based on this, a clean, simple, green and environmentally fiendly electrochemcial synthesis method was applied to improve the fluoride removal efficiency. Polyethylene glycol (PEG), a non ionic surfactant with strong hydrophilicity and excellent dispersion, was added to the mixed support electrolyte for preparing HAP. A new type of HAP composite (PEG/HAP) was prepared on the surface of copper sheet as the working electrode. By contrast to HAP, the crystal structure, pore size, specific surface area, surface morphology, elemental proportion, and functional groups of PEG/HAP were analyzed to reveal the instrinsic mechnism of the higher fluoride removal efficiency of PEG/HAP than HAP. The results show that PEG/HAP and HAP have the same crystal plane structure characteristics, elements and chemical bonds, while the proportion of various elements as well as the absorption peaks and intensity of hydroxyl and phosphate ion functional groups in PEG/HAP have certain differences compared to HAP. PEG transformed HAP from a short rod-shaped surface morphology to a porous and porous structure that facilitated the exchange and adsorption of fluoride ions. Average pore size of PEG/HAP decreased from 16.58 nm of HAP to 11.93 nm, and its specific surface area increased from 24.29 m
2/g OF HAP to 29.83 m
2/g. Although the adsorption types of PEG/HAP and HAP were both IV type H3 hysteresis loop, and their mesoporous distribution ranges were consistent, the number of micropores and mesopores in PEG/HAP were significantly higher than those in HAP. Although the adsorption reactions of both materials for fluoride ions exhibited entropy increase, endothermic, and spontaneous process characteristics with the adsorption isotherm model conforming to Langmuir-Freundlich, the intra particle diffusion rate constant of PEG/HAP was slightly higher than HAP. Therefore, the maximum adsorption capacity of PEG/HAP for fluoride ions can reach 9.56 mg/g, which was higher than that of HAP of 8.36 mg/g. Compared with 4 times of recycle regeneration for removing fluoride ions of HAP, PEG/HAP can arrived at 6. In addition, the presence of PEG did not affect the change trend of preparation parameters such as electrolyte pH on the adsorption capacity of HAP for fluoride ions. However, PEG increased the adsorption capacity of HAP for fluoride ions. All coexisiting ions such as Cl
−, NO
3 −, SO
4 2−, and CO
3 2− did not interfere with the adsorption of fluoride ions for PEG/HAP and HAP.