Removal Of Phosphorus And Algae From Water By Magnesium-Iron Layered Double Hydroxides And It Utilization

With the development of economy,water pollution is becoming increasingly severe and eutrophication is increasing.Layered Double Hydroxides（LDHs）can effectively remove phosphate and algae cells from waster water because of their large specific surface area,exchange of anions between layers,unique memory effect and positive charge on the surface.In this paper,the performance of MgFe-LDH to remove phosphate and Chlorella vulgaris was studied,and the MgFe-LDH-Chlorella vulgaris waste was recycled by heat treatment to prepare a new type of magnesium-iron based carbon nanomaterial(Mg_x Fe_1-xO_y/C).And it was studied the adsorption performance and mechanism of Mg_xFe_1-x-x O_y/C on nitrogen under high temperature and pressure.The main results of the study are as follows:（1）The effects of different calcination temperatures on the adsorption of phosphorus by MgFe-LDH were studied.The structural changes of MgFe-LDH and its calcined products were analyzed by XRD,BET and TG-DSC,and the effect of pH value and coexisting ions on the adsorption performance.On this basis,the adsorption phosphate mechanism of MgFe-LDH and its calcined products was illustrated by means of adsorption kinetics and isotherm data fitting,and means of XRD,XPS,SEM and TEM.The results show that when the calcination temperature is 450℃,MgFe-450 has the largest phosphate adsorption capacity of 28.3 mg/g,the pH value of 4¹0 has little effect on the adsorption of phosphate.The greater the concentration of coexisting ions,which increases the adsorption competitiveness,the order of influence of its adsorption capacity is:CO₃^2->SO₄^2->NO₃^->Cl^-.The adsorption kinetics of MgFe-LDH was fitted with the pseudo-second-order kinetics model.When the initial phosphate concentrations are 20 mg/L and 100 mg/L,the kinetics of MgFe-450 adsorption of phosphate was fitted with the pseudo-second-kinetic model.However,the initial phosphorus concentration of 800 mg/L is divided into two stages:the first stage is that the kinetic data of the first 25 min is best fitted by the pseudo second-order kinetic model;and the second stage is fitted with the pseudo second-order kinetic model after 25 min.The MgFe-LDH isotherm data is fitted well with the Freundich model.The MgFe-450 adsorption isotherm is divided into two stages:the first stage is a lower initial phosphate concentration（20 mg/L–200 mg/L）,which is better fitted with the Langmuir model;the adsorption isotherms of phosphate concentration（200 mg/L–800 mg/L）is fitted well with with the Sips model.MgFe-LDH dominates the adsorption process with anion exchange,while MgFe-450contributes to reconstruction via the unique memory effect at lower phosphate concentrations and the surface precipitation at higher concentrations.（2）The effects of MgFe-LDH dosage,pH value,shaking time and oscillation speed on the removal of Chlorella vulgaris were studied.Chlorella vulgaris and phosphate was simultaneously removed for MgFe-LDH.The results showed that the dosage was 1.0 g/L,the pH was 4-6,the shaking time was 60 min,and the removal rate of Chlorella vulgaris reached the maximum at 94.6%.MgFe-LDH can simultaneous effectively removal of Chlorella vulgaris and phosphate.Zeta potential,XRD,microscopy characterization methods were used to illustrate the removal mechanism of Chlorella vulgaris by MgFe-LDH.MgFe-LDH has a strong electrostatic adsorption effect on Chlorella vulgaris with negative surface charge and can effectively remove Chlorella vulgaris.（3）The MgFe-450 after removing Chlorella vulgaris was recovered by heat treatment（temperature was 800℃）,and a new type of MgxFe1-xOy/C was successfully prepared.The structure and properties of Mg_xFe_1-xO_y/C were analyzed by means of XRD,XPS,SEM,TEM and Raman.The adsorption properties of Mg_xFe_1-xO_y/C under high temperature and pressure were studied.The results show that Mg_xFe_1-xO_y/C obtained by heat treatment have characteristic diffraction peaks of MgFe₂₊₃O₄,and the binding energy of Fe 2p shifts to the low energy direction,indicating that Fe has undergone redox reaction during heat treatment.The morphology of MgxFe1-xOy/C exhibits a polycrystalline layer structure with inconsistent size,and has typical characteristics of graphene-like materials.The four elements of Mg,Fe,C and O are well distributed uniformly.In order to further explore the mechanism of nitrogen adsorption,a temperature cycle experiment was carried out and the effect of temperature on the adsorption of nitrogen was investigated.As the temperature increases,the adsorption capacity of nitrogen increases.When the adsorption temperature is 800℃,the adsorption capacity of nitrogen reaches a maximum value（5.0%）at an adsorption time of 150 min.The temperature cycling experiment of nitrogen adsorption and desorption showed that the adsorption of nitrogen by Mg_xFe_1-xO_y/C is a reversible cyclic process.