Study On Mechanochemical Synthesis Of Layered Double Hydroxide For The Removal Of Phosphate From Aqueous Solution

The excessive emission of phosphorus would cause eutrophication in aquatic ecosystem and even threaten human health.Therefore,it is of great significance to remove phosphate from polluted water efficiently.As an excellent adsorbent,Layered double hydroxide（LDH）is widely used in water treatment because of its ion exchangeability and highly dispersed active sites.The coprecipitation is the most common method to prepare LDH with high crystallinity and good dispersion.However,a large amount of wastewater is produced in the preparation process and resulting in the secondary pollution.The preparation of LDH by mechanochemical method could not only reduce the production of wastewater greatly,but also reduce the energy consumption effectively,which has a wide application prospect in practical production.In this dissertation,the Ca-Al-Fe LDH（CAF-LDH）and Mg-Al LDH（LDH-MC）were synthesized successfully by the mechanochemical method and used to remove phosphate from aqueous solution efficiently.The adsorption performance and practical application of the adsorbents for phosphate were studied by the batch adsorption,recycling and coexisting anion influence experiments.The adsorption mechanisms of the adsorbents for phosphate in water were further studied by various characterization methods.The specific experimental results are as follows:（1）The CAF-LDH was prepared using Ca（OH）₂,Al（OH）₃ and Fe（NO₃）₃·9H₂O as starting materials and Ca-Al-Fe layered double oxide（CAF-LDO）was obtained by calcining CAF-LDH.Based on the characterization results of X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）,scanning electron microscopy（SEM）and Brunauer-Emmett-Teller（BET）for CAF-LDH and CAF-LDO.Both CAF-LDH and CAF-LDO contained abundant functional groups.The adsorption behavior of phosphate on CAF-LDH and CAF-LDO was studied by batch experiments.Within only 30 min,the adsorption process quickly reached equilibrium.The removal ratios of CAF-LDH and CAF-LDO for phosphate with the initial concentration of 25.0 mg/L were above 90%in p H 3-10.The isotherms and kinetics data better fit the Langmuir model and the pseudo-second-order kinetic equation,with the adsorption capacities of 43.31 and 50.79 mg/g,respectively.The phosphate was absorbed by CAF-LDH and CAF-LDO mainly via the electrostatic attraction and surface complexation.In addition,CAF-LDH and CAF-LDO also presented efficient regeneration,selectivity and potential application to remove phosphate in the real water environment.（2）The LDH-MC was prepared by the mechanochemical method from the Mg（OC₂H₅）₂and C₉H₂₁Al O₃ and compared with the LDH synthesized by hydrothermal method（LDH-HT）and commercial LDH（LDH-C）.The characterization results showed LDH-MC and LDH-HT presented high crystallinity and uniform hexagonal lamellar structure.The phosphate adsorption equilibrium of LDH-HT and LDH-MC was rapidly reached within 5 min.The adsorption process of LDH-MC,LDH-C and LDH-HT better fit the Langmuir and the pseudo-second-order kinetic model.The phosphate was absorbed by LDH-HT,LDH-C and LDH-MC via the electrostatic attraction,ion exchange and surface complexation.The experiments of adsorption-desorption cycle and coexisting anion influence showed the LDH-HT and LDH-MC had excellent regeneration and selective adsorption properties for aqueous phosphate.In this dissertation,the LDH materials were prepared by the mechanochemical method,which had the advantages of green,simple preparation and low energy consumption.The LDHs prepared by the mechanochemical method removed aqueous phosphate quickly,and provided theoretical reference for the green and large-scale industrial production of LDH materials.