Preparation And Optimization Of Sr²⁺ Doped Hydroxyapatite Materials And Its Adsorption Property Research To Lead And Cadmium Ions

Heavy metals are one of the pollutants in industrial and mining wastewater.Because of the biological enrichment and toxicity of heavy metal ions,how to remove heavy metal ions in the wastewater is a serious environment problem to deal with.Hydroxyapatite is a new environmental functional material,which can be used as an adsorbent for heavy metal ions in wastewater without secondary pollution.It is one of a research direction of sewage pollution control.At present,the industrially prepared hydroxyapatite is applied commercially,but its preparation process is imperfect and adsorption capacity is inadequate,such problems as low activity and lack of lattice defects are always found,which will hinder the application of hydroxyapatite as industrial heavy metal adsorbent.To solve these problems,the preparation parameters of chemical precipitation method for hydroxyapatite were optimized in this paper,and on this basis,the surface modification and strontium-doped modification of hydroxyapatite were studied to further improve its adsorption capacity.The adsorption effect of modified hydroxyapatite to Pb²⁺and Cd²⁺was also investigated.Hydroxyapatite before and after modification was characterized by XRD,FITR,XPS,BET and SEM.The adsorption kinetics,adsorption thermodynamics and adsorption mechanism were discussed.The main findings are as follows:?1?Using Ca?NO₃?₂·4H₂O as the calcium source and?NH₄?₂HPO₄ as the phosphorus source,high purity hydroxyapatite was synthesized by chemical precipitation method.The effect of pH and calcination temperature for hydroxyapatite removal of Pb²⁺and Cd²⁺was stidied.The results showed that the optimum preparation parameters are solution pH value of 10.5 and calcination temperature of 300°C.Calcination affect the adsorbent properties by interfering the chemical activity and crystal structure of materials.When the calcination temperature is lower than 300°C,the adsorption performance is mainly improved for thermal activation;however,when the temperature is higher than 300°C,the adsorption performance is reduced for the lattice defects in hydroxyapatite are decreased.?2?Hydroxyapatite was surface-modified by using cationic surfactant CTAB.The results showed that the optimum CTAB addition ratio is 5%,and the modified hydroxyapatite is improved in particle size,specific surface area and porosity.The specific surface area is increased from 26.36m²/g to 67.79m²/g.The maximum adsorption capacity for Pb²⁺and Cd²⁺were increased from 416.2 mg/g and 113.6 mg/g to 494.2 mg/g and 135.2 mg/g,respectively.?3?The Ca²⁺in hydroxyapatite was replaced by Sr²⁺with Sr²⁺doping ratio of 5%,10%,15%,20%,40%,60%,80%and 100%in modification experiments.The results indicated that the optimum Sr²⁺doping ratio is 20%,and the products in preparation are mainly Ca₅?PO₄?₃OH and?Ca,Sr?₅?PO₄?₃OH.The particles of modified hydroxyapatite?SrHAP?are broken,the lattice defects are increased,and the specific surface area is also improved.The maximum adsorption capacities for Pb²⁺and Cd²⁺are 644.7 mg/g and 157.5 mg/g,respectively.?4?The adsorption kinetics and adsorption isotherm model were fitted to the adsorption of Pb²⁺and Cd²⁺by SrHAP.It is found that the pseudo-first order kinetic equation and the Langmuir isotherm model can be well used to describe the adsorption process.The adsorption mechanism is speculated as monolayer adsorption.?5?pH in solution will affect the adsorption mechanism of SrHAP to Pb²⁺and Cd²⁺.At the pH value of 2,the adsorption mechanism of SrHAP to Pb²⁺is dissolution-precipitation;when the pH value is between 3 and 6,the adsorption mechanism to Pb²⁺includes dissolution-precipitation and ion exchange;at the pH value of 7,the adsorption mechanism to Pb²⁺is mainly ion exchange.However,at the pH value of 2,SrHAP cannot effectively adsorb Cd²⁺,and when pH value is between 3 and 7,the adsorption mechanism of SrHAP to Cd²⁺is always ion exchange.