Separation And Recovery Of Phosphate Anions By A Novel NiFe-LDH/rGO Hybrid Film Based On Electroactivity-variable Valence

The water environment crisis has become a global problem,and the eutrophication of water caused by excessive discharge of phosphorus has become one of the main environmental problems attracting much attention.Eutrophication of water will not only lead to the growth of algae in the water,but also make the water oxygen content drop sharply,affecting the survival of fish,shellfish and other aquatic organisms.In daily life,however,the growth of crops need rely on a large amount of phosphorus resource input,the preparation of all kinds of refractory materials is also inseparable from the use of phosphorus compounds,to extend the shelf life of food,phosphorus additive also plays a considerable role,and with the development of new energy field,phosphorus resource demand is growing.Therefore,it is very important to separate and recycle phosphate ions in water.Electrically switched ion exchange（ESIX）is a new separation technology,which can achieve selective separation and recovery of target ions by adjusting the redox potential on electrically active membrane materials.In the process of recycling,there is no need to add any regeneration reagent,only through the application of appropriate reverse potential,membrane regeneration can be achieved.The premise of ESIX is to prepare electrically active ion exchange materials,which have sufficient adsorption sites for target ions at oxidation（or reduction）potential and can easily desorbed target ions at opposite reduction（or oxidation）potential.Based on the above conditions,the layered bimetallic hydroxides based on nickel and iron were selected in this experiment to achieve selective separation and recovery of phosphate anion.In this study,different types of NiFe-LDH materials were prepared by means of oil bath,hydrothermal and electrodeposition to investigate the superiority of phosphate anions in the adsorption process.The experimental results showed that the NiFe-LDH material prepared by oil bath showed better dispersion and hydrophilicity,and the adsorption capacity of phosphate ions reached 86.27 mg·g^-1 at 0.8 V in ESIX system.In addition,the NiFe-LDH material prepared by the oil bath method also showed good stability of adsorption and desorption.On the basis of the first stage,in order to compensate the electrical activity of materials,the NiFe-LDH/rGO hybrid film was prepared by doping rGO in oil bath,which was used to treat phosphate anions in water.The morphology,crystal structure,types of functional groups and energy state distribution of the materials were measured by SEM,XRD,FT-IR and XPS.The experimental results show that nickel and iron ions in the ESIX system can realize the valence transition of ions by adjusting the film electrode potential,which plays a good auxiliary role in the separation and recovery of phosphate anions.At a voltage of 0.8 V,the adsorption capacity of the film material for phosphorus is about 270 mg·g^-1.At the same time,under the control of-0.8 V reverse potential,it also shows good desorption performance.In addition,the hybrid film material shows excellent selectivity for phosphate anions in the presence of coexisting anions,and can maintain good adsorption capacity after 10 cycles.The p H measurement showed that the hybrid film material also showed ideal adsorption capacity in medium and alkaline solutions,which provided a certain reference for the industrial application of the hybrid film in the future.