Synthesis And Fluoride Ions Adsorption Study Of Magnetic Hydroxyapatite

The adsorption method has been used to treat water pollution because of many advantages.Most adsorbents are difficult to recycle,which result in secondary water pollution.So environmentally-friendly magnetic hydroxyapatite（M-HA）was prepared to adsorb fluoride ion.The effect of preparation and adsorption conditions were studied on the adsorption of fluoride ion by M-HA.M-HA were used to characterize by X-ray diffraction（XRD）,scanning electron microscope（SEM）,Fourier infrared spectroscopy（FT-IR）,X-ray photoelectron spectroscopy（XPS）and vibrating sample magnetometer（VSM）.The adsorption characteristics were studied through thermodynamic and kinetic models.Research indicates:（1）The M-HA synthesis at 80℃,Ca:P=1.87,Fe:P=0.7,and aging 40℃for 12 hours showed excellent performance in the adsorption of fluoride ions.The amount of calcium and iron had a large impact on the adsorption performance of the material by fluoride.（2）M-HA had a smaller size than hydroxyapatite（HA）because Fe₃O₄ improved the agglomeration properties and the morphology wasn’t changed significantly after adsorption by SEM.The binding energy characteristic peak of F can be seen in the XPS spectrum,suggesting that the adsorption was chemisorption.XRD results showed that the characteristic peak intensity of the Fe₃O₄ crystal plane increased with the increase of the Fe:P ratio,and the characteristic peak intensity of the apatite crystal plane decreased with the increase of the Fe:p ratio.It indicated that Fe₃O₄ partly entered into the HA lattice,causing the M-HA crystal structure to change.M-HA was different from HA on the hydroxyl sites of participating in adsorption by FT-IR.M-HA was a soft magnetic material and the saturation magnetization was 21.26emu/g of Fe:P=0.7 easy to solid-liquid separation.（3）M-HA can achieve a removal rate of more than 95%in 30 minutes and more than99%in 300 minutes when the adsorption conditions were controlled to be 40℃,initial pH=2.0 and initial F^-concentration of 20mg/L.The pH tolerance range was large and the acidic conditions were more favorable for M-HA.The removal rate was still more than 66%in weakly alkaline conditions（pH=8.0）.The hydroxyl group was replaced to cause the pH to increase after adsorption.Anions had significant difference effect on M-HA adsorption of fluoride ions.The inhibition capacity was PO₄^3->CO₃^2-and the promotion capacity was NO₃^-≈Cl^->SO₄^2-.（4）The adsorption of F^-by M-HA was more suitable for the pseudo second order dynamic kinetic and the Langmuir isotherm adsorption model.The adsorption equilibrium time was 240min and the maximum adsorption capacity was 14.202mg/g.The adsorption of F^-was mainly monolayer chemical adsorption,which was spontaneous,entropy increasing and endothermic.M-HA had strong stability and reusability and 0.3mol/L NaOH desorption 4 times still had about 80%removal rate.