Study On Preparation And Removal Of Fluoride From Aqueous Solution Of Hydroxyapatite

Hydroxyapatite, as an important part of the human body, accounts for approximately 60% of the inorganic component of bone, exhibits good biocompatibility and bioactivity. Over the past decade, researchers have developed an increasingly strong interest in synthesized hydroxyapatite, so a large number of synthetic routes have been developed and the hydroxyapatite particles prepared by different synthetic methods possesses different characteristics. For example, the method dry, wet synthesis, the thermal decomposition method, all these can produce particles with good crystalline particles. In this passage, the traditional chemical precipitation and sol-gel methods are used to synthesize nanometer-structured hydroxyapatite to remove fluoride ions in aqueous solution. It reports the preparation of different-structured hydroxyapatites with these different synthesis methods and their effect on fluorine removal is significantly different. This shows that the different synthesis methods of hydroxyapatite have a great impact on fluorine ion adsorption.The results showed that at 25 ℃, adsorption of hydroxyapatite prepared by chemical synthesis method is 1.99mg/g. While being analyzed through transmission electron microscopy, it shows that the hydroxyapatite derived from chemical precipitation is mainly in the shape of disks and has an even distribution, remains stable in solution, with an average diameter of 50nm; X-ray diffraction analysis showed that the spectrum is almost consistent with the standard spectrum of hydroxyapatite; Compared with this, the adsorption of hydroxyapatite by sol-gel method is 1.75 mg/g, and the result of transmission electron microscopy analysis shows that the hydroxyapatite derived from the sol-gel method is shaped like a more uniform rod-shaped and also has an even distribution with its length ranging from 2μm-20μm, width of about 50nm. In addition, X-ray diffraction analysis showed that its spectrum is also consistent with the standard hydroxyapatite map. Above all, as is shown in transmission electron microscopy analysis, the two sample images obtained by synthetic methods have some differences. The hydroxyapatite prepared by the former method is a disk-like shape while the latter is in the shape of a rod. The sample is also finer as is analyzed in transmission electron microscopy and X-ray diffraction by the sol-gel. And this method also produces the samples with a relatively higher intensity indicating that the crystal structure is more orderly with a better degree of crystallinity.In addition, the paper further studied the relationship between the time and the absorption of hydroxyapatite, prepared by different synthetic methods, to fluoride ions in the solution. The results show that:although the adsorption kinetics of two synthetic methods comply with the Langmuir equation first-level, the amount of fluorine ion adsorption is not desirable. So it does not comply with a Langmuir first-level kinetics. But later, it is found that two synthetic methods comply with the Langmuir model quasi-two reactions, with the correlation coefficient R2 greater than 0.9990. Kinetic experiments also show that the two adsorption models of hydroxyapatite to fluoride ion are also consistent with Weber and Morris models. It is shown that the adsorption process can be described as an adsorbent-surface adsorption and pore diffusion adsorption process.The adsorbent-surface absorption in the first stage exhibits faster adsorption rate, which may be attributed to surface absorption of the hydroxyapatite caused by the diffusion of fluoride ions from the solution and the fitting straight lines all do not pass through the original point, indicating that diffusion is not the only step controlling adsorption process.While in the pore diffusion slow adsorption process, the adsorption rate decreased, possibly due to the slow diffusion of fluoride ions into the inside of hydroxyapatite. So it is clear that the traditional chemical synthesis of hydroxyapatite was prepared at a rate higher than that in the first stage of the sol-gel method. This is partly because the relatively large surface area and higher adsorption efficiency of hydroxyapatite as is prepared in chemical synthesis method; while in the second phase, the rate of hydroxyapatite prepared by sol-gel method is slightly higher than traditional chemical synthesis. The reason may be the gap size of the rod-like hydroxyapatite produced by the sol-gel method, is greater than that of the disk-shaped form prepared by the traditional chemical synthesis.