Design And Application Of Fluoride Adsorption By Dicalcium Phosphate Dihydrate And Rare Earth Based Organic Frameworks

The fluorine pollution in water seriously endangers the ecological environment and human health.The removal of fluoride ions is of great significance to the sustainable development of society.Adsorption is the most commonly used and effective technology for fluoride ions treatment,with the advantages of low cost,simple operation,high removal rate and environmental friendliness.In this paper,dicalcium phosphate dihydrate（DCPD）and cerium,lanthanum based organic frameworks（Ce-MOFs and La-MOFs）were prepared,and analyzed by XRD,SEM,FT-IR and XPS,et al characterization methods.The factors including initial concentration of fluoride ions,pH value,co-existing anions,adsorption time and temperature on fluoride ions removal were investigated,and the adsorption mechanism of fluoride ions was analyzed through kinetic and thermodynamic studies.The main contents were as follows:（1）DCPD was synthesized by precipitation method using Ca（NO₃）₂·4H₂O and（NH₄）₂HPO₄as raw materials at room temperature.It was found that DCPD has good fluoride ions adsorption performance in the solution of pH range of 4～10.With the increase of CO₃^2-or PO₄^3-concentration in solution,the adsorption of fluoride ions by DCPD was significantly inhibited.And the maximum adsorption capacity was 22.99 mg/g at 303 K.Through the analysis of the kinetic and thermodynamic studies,the adsorption process can be described by the Langmuir isotherm model and pseudo-first-order equation model.The adsorption of fluoride ions by DCPD is a spontaneous,endothermic and entropic process.In addition,the adsorption mechanism may exist ion exchange and electrostatic interaction.（2）Metal-organic framework materials were designed and synthesized to improve the adsorption performance of fluoride ions because of the relatively low adsorption performance of inorganic materials.Through optimized experimental conditions,Ce-MOF synthesized by hydrothermal method from Ce Cl₃·7H₂O and fumaric acid had the maximum adsorption capacity of 224.72 mg/g at 303 K,which was significantly higher than that of DCPD.The pH value was wider range and more applicable than DCPD,which was still of stable absorption of fluoride ions in 2～10 pH value.Kinetic and thermodynamic analysis showed that the adsorption behavior was more consistent with the Langmuir isotherm model and pseudo-second-order equation model,and the process was spontaneous,exothermic and entropic.（3）In order to further improve the adsorption performance of fluoride ions,used lanthanide precursor to replace cerium precursor,La-MOF synthesized by La（NO₃）₃·6H₂O and fumaric acid as raw materials through hydrothermal method had a maximum adsorption capacity of 272.48 mg/g at 303 K after experimental conditions optimization.There was a further improvement of adsorption capacity by La-MOF than Ce-MOF.The adsorption behavior of La-MOF can be described by the Langmuir isotherm model and pseudo-second-order equation model with a spontaneous,exothermic and entropy-increasing process,which was similar to Ce-MOF.And ion exchange and electrostatic interaction may be the adsorption mechanisms of fluoride ions by La-MOF and Ce-MOF.