Preparation And Characterization Of Apatites As Environmental Mineral Materials

Apatite-type environmental mineral materials have a promising prospect in treatment of heavy metal ion pollutions. The treatment of Cd2+-bearing waste water by apatite has been extensively studied as a key project for the high toxicity of Cd2+. The Cd2+sorption capacity is a suitable indicator of environmental function for apatite. This dissertation focused on exploring effective methods for preparation of apatites with high environmental function through studying the effects of preparation methods on their composition, structure and C2+sorption properties.The natural apatite samples were the powders with some gangue minerals of directly grinded phosphorite. Three industrial and eleven natural types of phosphorite were collected for this research. Their processing properties related to the grinding characteristics were clarified. Natural apatite in phosphorite was carbonate-substituted fluorapatite, and principally occurred in the form of aggregate (collophane grain) of nano-submicron sized particles. The composition of phosphorite had appeared differentiation in grinding process. The degree of the differentiation was affected by the processing properties of phosphorite samples, such as mineral component, grain size, intergrowth texture and hardness. Main gangue minerals, including dolomite, quartz, layered aluminosilicate minerals and brown hematite, tended to enrich in fine powders of phosphorite. The finer samples showed lower Cd2+sorption capacity. The Cd2+sorption properties of phosphorite sample intrinsically depended on its particle size, mineral composition and the structure of collophane. A standard was established for phosphorite as mineral material. The massive siliceous phosphorite was selected for it contained abundant nano-structured carbonate-substituted fluorapatite and few carbonate gangue minerals. The sorption capacity for Cd2+of the selected sample was over35%higher than those industrial types.Coprecipitation was selected to prepare hydroxyapatite as environmental mineral material based on a comparative study for three typical liquid precipitation methods. The mechanisms of coprecipitation reactions were revealed by experiments plus simulated calculation. In coprecipitation process, the primary precipitate was CaHPO4·2H2O, and hydroxyapatite was formed by hydrolyzing of CaHPO4·2H2O. Ultrafine hydroxyapatite particles could be produced by quickly elevating the pH value of coprecipitation reaction to raise the rate and density of hydroxyapatite nucleation, and to decline the critic radius of the crystal nuclei. Well dispersed nano-structured hydroxyapatite powders with high specific surface area had been obtained by optimizing the processing conditions including the rate and method of feeding raw materials, the initial concentration of Ca2+in the mixed solution of calcium nitrate and phosphoric acid, the pH value of the slurry before aging, the aging temperature and time. The Cd2+sorption capacity of so prepared sample was over38%higher than those samples in the comparative study.Three kinds of ion-substituted hydroxyapatite,[Ca5-xSrx(PO4)3OH],[Sr5-xBax(PO4)3OH] and [Ca10(PO4)6-x(SiO4)x(OH)2-x], had been prepared by coprecipitation. The crystal cell parameters of cation-substituted hydroxyapatites displayed a positive linear relationship to the content of larger cation in the samples.Ca-Sr-hydroxyapatite showed favorable structures for sorption of Cd2+, while Sr-Ba-hydroxyapatite was not suitable for treatment of Cd2+bearing waste water.The mechanism of SiO44-substitution to PO43-in hydroxyapatite was elucidated through a study on the thermal stability and structural evolution of silicate-substituted hydroxyapatite. Silicon could enter the crystal lattice of hydroxyapatite in the form of SiO44-and HSiO43-in coprecipitation reaction. Si2O76-occurred in calcined samples by polycondensation of SiO44-or HSiO43-. Incorporation of silicate effectively inhibited the crystal growth, decreased the crystal size and increased the specific surface area of hydroxyapatite in calcining process. Silicate-substituted hydroxyapatite showed75%higher Cd2+sorption capacity than hydroxyapatite in the samples calcined at900℃for2h.The Cd2+sorption capacity of apatite tended to increase with the increase of initial pH of simulated waste water. The reaction process of Cd2+removal from aqueous solutions by apatite highly fitted with the pseudo-second order kinetic model. The characters of Cd2+sorption on hydroxyapatite and silicate-substituted hydroxyapatite showed high correlation with Langmuir adsorption isotherm. These phenomena reflected that ion exchange was the main mechanism of aqueous Cd2+removal by apatite-type environmental materials.