Study On The Removal Of Phosphate And Arsenic In Water By Cerium Doped Metal-organic Frameworks

At present,China is facing serious water pollution problem,especially the pollution of phosphorus and arsenic.Phosphorus in sewage and agricultural runoff is considered to be the main cause of eutrophication,which and the causing harmful algal blooms（HAB）cause enormous economic and environmental damage.Arsenic is omnipresent in the environment and is highly poisonous to all living forms.Direct drinking or indirect ingesting arsenic-contaminated food may be fatal to human health.Therefore,the remediation of phosphorus and arsenic in the water environment has become a focus of many institutions.In this paper,Ce-Ui O-66-NH₂ and Ce-MIL-101-NH₂（Fe）were prepared by solvothermal method.The physicochemical characteristics of Ce-Ui O-66-NH₂ and Ce-MIL-101-NH₂（Fe）were determined by means of XRD,TEM,BET,SEM,FTIR and XPS.Then,two kinds of MOFs were respectively applied to the adsorption of phosphate and As（V）in water.The main contents and results of this paper are as follows:（1）Cerium was introduced into zirconium-based MOF（Ui O-66-NH₂）to construct a new series of Ce-Ui O-66-NH₂ materials with different Ce doping proportions,and phosphate adsorption experiments were carried out in aqueous solution.The results showed that the adsorption isotherms of Ce-Ui O-66-NH₂ for phosphate were more consistent with the Langmuir model,and were consistent with the experimental results of the material optimization.0.75Ce-Ui O-66-NH₂ showed the best adsorption capacity,which was 211.86 mg/g.In addition,the adsorption of phosphate by 0.75Ce-Ui O-66-NH₂ occurred quickly and reached equilibrium at60 min.The pseudo-second-order kinetics could well describe the adsorption behavior,thus confirming that the adsorption process was monolayers chemisorption.Some of the interfering anions in the solution have little effect on the adsorption of phosphate,and this excellent selectivity is conducive to the recovery of high purity phosphate through the regeneration of0.75Ce-Ui O-66-NH₂ adsorbent.Thermodynamic experiments show that the adsorption reaction of phosphate at 0.75Ce-Ui O-66-NH₂ is endothermic and spontaneous.The adsorption capacity of0.75Ce-Ui O-66-NH₂ to phosphate only dropped by 19%after five adsorption/desorption experiments,and 0.75Ce-Ui O-66-NH₂ also has a higher phosphate removal ability in the actual water containing low concentration of phosphorus,which indicates that it is of great significance for the actual water purification.Furthermore,The adsorption mechanism analysis showed that the terminal hydroxyl groups involved in the formation of metaloctahedral might play a key role in efficient phosphate adsorption due to the formation of Al/Fe-O-P via ligand exchange.At the same time,Amino-functionalized MOFs adsorbents further enhance phosphate removal by providing an additional positive surface charge.（2）A series of Ce-MIL-101-NH₂ materials with different Ce doping ratios were prepared by introducing cerium into Fe based MOF（MIL-101-NH₂）,which were applied to the adsorption of phosphate and As（V）.In the separate adsorption system containing only phosphate or As（V）,isothermal adsorption experiments showed that Ce-MIL-101-NH₂ showed better adsorption capacity of phosphate and As（V）than MIL-101-NH₂,of which 1Ce-MIL-101-NH₂ had the highest adsorption capacity,reaching 341.5mg/g for phosphate and 249 mg/g for As（V）.The Langmuir model could better describe the two adsorption processes.In addition,kinetic experiments show that the adsorption rate of phosphate by 1Ce-MIL-101-NH₂ adsorbent is faster than that of As（V）,and the quasi-second-order kinetic model can fit the data of the two adsorption processes well.Based on the hypothesis of the rate-limiting step corresponding to this model,it can be proved that the adsorption of phosphate or As（V）on 1Ce-MIL-101-NH₂ is chemical adsorption by sharing or exchanging electrons between the adsorbent and the absorbate.Thermodynamic experiments show that the adsorption of phosphate or As（V）in 1Ce-MIL-101-NH₂ is endothermic and spontaneous.At the same time,the prepared 1Ce-MIL-101-NH₂ material simultaneously adsorbs phosphate and As（V）.Due to the existence of competitive adsorption,the adsorption capacity of both phosphate and As（V）decreased.By comparison,the adsorption capacities of1Ce-MIL-101-NH₂ on phosphate and As（V）were 167.36 mg/g and 87.55 mg/g,respectively,with phosphate prevailing.In addition,1Ce-MIL-101-NH₂ is highly selective for phosphate and arsenic in actual water bodies with competing anions.By TEM-mapping characterization analysis of the1Ce-MIL-101-NH₂ after adsorption,it can be clearly observed that the spine-like structure appears on the surface of the rod-like adsorbent,which makes it rougher.Meanwhile,it can be found from the EDS spectrum that P and As elements are evenly distributed on the surface of the adsorbent.Furthermore,FTIR and X-ray photoelectron spectroscopy（XPS）characterization indicated that the main mechanism of phosphate and As（V）capture by 1Ce-MIL-101-NH₂ involved the production of an inner-sphere complex of Fe/Ce-O-P and electrostatic attraction.The1Ce-MIL-101-NH₂ adsorbent was applied to the actual water of Yangzong coastal spring containing phosphate and arsenic.The results showed that the adsorbent could maintain excellent and rapid efficiency in removing phosphate and arsenic from the spring water,which further confirmed the application potential of 1Ce-MIL-101-NH₂.