| Fluoride is one of the essential trace elements for human.However,excessive intake of fluorine can cause various irreversible diseases.Hydroxyapatite（HAP）has excellent stability,biocompatibility and no secondary pollution,and is often used to remove fluoride ions（F-）from water.However,the application of HAP in water treatment is limited due to its small specific surface area and less active sites,which leads to its low fluoride removal capacity.To address these issues,nano-hydroxyapatite（nHAP）with uniform particles and good dispersion was prepared by EDTA-assisted hydrothermal method.nHAP was modified with Al（OH）3 by coprecipitation to obtain a new fluoride removal material Al（OH）2-nHAP with large specific surface area,many surface active sites and large fluoride removal capacity.The preparation conditions were optimized by taking the adsorption properties of nHAP and Al（OH）2-nHAP for F-as indexes.The Al（OH）2-nHAP obtained under the optimum preparation conditions was characterized and analyzed.The adsorption properties,kinetics and thermodynamics of F-on Al（OH）2-nHAP were investigated and compared with that of nHAP.Furthermore,the adsorption mechanism of Al（OH）2-nHAP on F-is explored.The main conclusions are as follows:The mesoporous material Al（OH）2-nHAP with large specific surface area,many active sites and high purity was successfully synthesized by modifying nHAP with Al（OH）3.Al（OH）2-nHAP was characterized by XRD,TEM,FTIR and BET.The results showed that:compared with nHAP,the average pore size of Al（OH）2-nHAP modified by Al（OH）3 decreased from 23nm to 16 nm,the specific surface area increased from 71.29 m2/g to 112.39 m2/g,and the surface hydroxyl content increased accordingly,which provided more active sites for F-adsorption.The effects of adsorbent dosage,solution pH value,coexisting anions(Cl-,NO3-,SO42-,CO32-,PO43-)and other variables on the fluoride removal performance of Al（OH）2-nHAP were investigated.The results showed that the amount of Al（OH）2-nHAP was only 1/5 of nHAP dosage when the initial concentration of F-was above90%;With the increase of solution pH,the fluoride removal performance of Al（OH）2-nHAP increased gradually.When the pH value is lower than 9,the adsorption effect of Al（OH）2-nHAP on F-is stable,and the adsorption capacity keeps above11.08mg/g.Cl-,NO3-and SO42-have no obvious effect on the fluoride removal capacity of nHAP and Al（OH）2-nHAP.However,CO32-and PO43-are prone to hydrolysis in aqueous solution,which causes a large amount of OH-to compete with F-for adsorption sites in the solution,resulting in a decrease in fluorine removal capacity.Al（OH）2-nHAP has good recycling ability,high stability,high safety and no secondary pollution.NaOH solution with pH=12 was selected as desorption solution.After four regeneration cycles,the removal rate of F-by Al（OH）2-nHAP increased from 68.32%to 81.32%compared with nHAP.The dissolution experiments of Al3+、Ca2+showed that the concentrations of Al3+,Ca2+and F-in aqueous solution were0.01mg/L,2.11mg/L and 0.56mg/L,respectively,which were far below the national standards.The maximum adsorption capacity of Al（OH）2-nHAP reached 194.20mg/g at 318K,pH=7 of solution and 200 mg/L of F-initial concentration,which was 6.6 times of the maximum adsorption capacity of nHAP under the same conditions.Meanwhile,the time required for Al（OH）2-nHAP to reach adsorption equilibrium was shortened from 150 minutes to 60 minutes compared with nHAP.Pseudo-second-order model and Freundlich model can better describe the adsorption process of F-on Al（OH）2-nHAP,which indicates that the adsorption process of F-on Al（OH）2-nHAP is mainly controlled by chemical adsorption,and the multi-layer adsorption occurs on the heterogeneous surface of Al（OH）2-nHAP,which is a spontaneous endothermic reaction.The adsorption of F-by Al（OH）2-nHAP mainly includes electrostatic attraction and ion exchange.After modification with Al（OH）3,the content of surface active hydroxyl groups of Al（OH）2-nHAP increased,and it has strong ion exchange effect.In addition,the surface of Al（OH）2-nHAP was protonated and the positive electricity was enhanced at pH=34,which produced strong electrostatic attraction between Al（OH）2-nHAP and negative F-,and the combined ion exchange effect made it have better fluoride removal performance.The surface of Al（OH）2-nHAP is deprotected in the range of pH from 9 to 12.At the same time,the removal rate of F-decreases due to the existence of large amounts of OH-,which are similar to F-in charge and ion radius,and competitive active sites with F-in the solution.The electrostatic attraction of Al（OH）2-nHAP to F-decreases when the pH value is 5-9,and ion exchange plays a leading role.The fluoride removal capacity of Al（OH）2-nHAP reached 11.37 mg/g at pH=7,which was 4.9 times higher than that of unmodified nHAP,indicating that Al（OH）2-nHAP was a good fluoride removal agent. |
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