| Fluoride is a kind of trace elements which is widely but unevenly distributed in rock,soil and water.For both human and animal,fluoride is an important constituent.A large number of evidence indicates that suitable level of fluoride can prevent people from dental caries and enhance the development of strong bones.However,elevated fluoride may provoke fluorosis—a kind of chronic systemic diseases.This is to say,excessive intake of fluoride may cause dental fluorosis,or even skeletal fluorosis--a serious bone abnormality.There is no treatment to cure fluorosis.Drinking water is a main source of human intake of fluoride,thus,when assessing water quality for human consumption,fluorine content in drinking water is recognized as a very important physicochemical factor.According to WHO standards,optimum dose of fluoride in drinking water is 0.5-1.5mg/l,once the concentration excesses the upper limit of fluoride,there will be some harmful health influences on human beings.Endemic fluorosis in China is a common phenomenon,high fluoride groundwater affected area is distributed in a wide range,in addition to Shanghai,there are wards in China's other provinces,municipalities and autonomous regions,serious illness regions always focus on the remote western and poverty-stricken areas,drinking water type fluorosis area covers approximately 2.2million km2 with the population of 41.94 million.Defluorination from aqueous solution can be achieved by various physiochemical methods,there are three major strategies so far: precipitation,adsorption,membrane exchange.Among them,adsorption is the most frequently-used method which is the most effective methodology for removing ionic contaminants from drinking water.And the study of fluorine removal by adsorption focuses on the exploitation and development of highly efficient,available adsorbents.Common adsorbents are as follows: activated alumina,zeolite,activated carbon,clay and hydroxyapatite,etc.The hydroxyapatite is a kind of low cost biomaterials which is easy to regenerate and doesn't produce secondary pollution,so it is concerned as an ideal adsorbent for fluoride removal.Much efforts have been devoted to the fluoride removal of hydroxyapatite.Typically,defluorination capacity of hydroxyapatite is not small compared with other adsorbents such as zeolite or activated carbon.However,heavy specific gravity?Small surface area and dense internal structure characteristics of hydroxyapatite lead the contact area to became smaller and hinder the fluoride removal effect.Some literatures about modified HAP have been reported.In this study,pure HAP was synthesized by co-precipitation method.,AC-HAP composites ?CNT-HAP composites were synthesized by using in-suit synthesis method based on co-precipitation method.The scanning electron microscopy ?X-ray powder diffractometer ? Fourier transform infrared spectrometer ?BET were used to characterize the samples.The effect of different elements on defluorination were examined by adsorption experiments including adsorption thermodynamic ? adsorption kinetics?effect of pH on defluorination? effect of co-existed anions on defluorination to reveal fluoride removal mechanism.It is concluded that: the CNT-HAP composites realized the ordered selfassembly of the hydroxyapatite on the AC and CNT template materials,then the large specific surface area Uniform,well-mixed composites were obtained.Besides,it was found the better performance of AC-HAP composites and CNT-HAP composites in removing fluoride Compared with pure phase HAP,and CNT-HAP composites was the most ideal material in defluorination. |
PDF Full Text Request