| Arsenic and fluorine in groundwater are seriously above the regulation limit in some areas due to the influence of hydrogeological conditions and human activities,which affects ecological security and human health.The development of traditional adsorption methods and membrane separation methods is limited by the treatment performance,application forms and costs.Therefore,it is urgent to develop efficient and economic treatment technologies.This thesis proposes an integrated dynamic membrane technology of"Adsorption+Filtration"for fluorine and arsenic removal in water.A hydrophilic hydroxyapatite adsorbent with the capacity for arsenic and fluorine was synthesized by hydrothermal homogeneous co-precipitation method,improved by modification to form Al-HAP,investigated to explore its adsorption performance and adsorption mechanism.Then,Al-HAP was combined with ultrafiltration membrane to form a dynamic layer with function of adsorption and filtration on the surface of ultrafiltration membrane.The preparation conditions of dynamic membrane were preliminarily explored by filtration experiments.Firstly,HAP nanomaterials with arsenic and fluorine removal ability were prepared by hydrothermal homogeneous co-precipitation method.In view of the weak fluorine removal performance of HAP,the adsorbent Al-HAP was prepared by modifying HAP.The influence of adsorption material dosage,initial p H,initial concentration of pollutants,adsorption reaction time and coexisting ions on the adsorption performance of Al HAP were analyzed and discussed.The results show that Al-HAP can stably and efficiently remove fluoride from water,and the adsorption efficiency after modification is nearly 30%higher than that of HAP.When the initial concentration of fluoride ion is 200 mg/L,the adsorption capacity of the material reaches 56.44 mg/g.The characterization results show that the main body of Al-HAP is still HAP with the same position and structure of functional groups,but with huge specific surface area which provides more active sites,indicating that Al-HAP is suitable as an adsorption material.Langmuir and pseudo-second-order kinetic models are more suitable for describing the adsorption behavior of fluoride ions on Al-HAP,which demonstrates that it is more consistent with monolayer adsorption,and chemisorption is dominant in the adsorption process.Thermodynamic data indicates that the reaction is a spontaneous endothermic reaction.Main mechanism of removal of F~-by Al-HAP is ion exchange with hydroxyl group on the surface.Secondly,a dynamic membrane system was constructed by Al-HAP with aluminum salt flocs pre-coated on the surface of ultrafiltration membrane for the removal of fluorine and arsenic in water.The influence of preparation conditions on the performance of dynamic membranes were investigated and furtherly optimized by response surface methodology.The results show that Al-HAP dynamic membrane has extremely high rejection rate and large membrane flux,with the rejection rate of fluoride ion as 96.97%,and the rejection rate of arsenite as 99.5%.The dynamic layers formed by the pre-coated dynamic membranes under different loadings show different adsorption and retention capacities.The dynamic membrane has a high flux and pollutant removal rate at a loading of 10 mg/cm~2.With the fluoride ion concentration of 5 mg/L,the dynamic layer Al-HAP loading of 50 mg/cm2,and the transmembrane pressure difference(TMP)of 0.01 MPa,the threshold of the Al-HAP dynamic membrane fluorine removal capacity is 100 L/m~2.This study shows that Al-HAP is an potential adsorption material suitable for preparing dynamic membrane,which can effectively improve the rejection rate of fluoride and arsenite by ultrafiltration membrane,slow down the membrane surface pollution,which provide technical support for the removal of molecular with small ion radius such as arsenite and fluoride by filter membrane with large pore size. |
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