Design And Application Of Porous Dephosphorizer Derived From PrFe Cyanometallate (CM) Framework

The upper reaches of the Yangtze River Basin are the areas with the highest concentration of"three phosphorus"industries（phosphate mines,phosphorus chemical enterprises and phosphogypsum reservoirs）,and the high phosphorus emission load is the focus of phosphorus pollution control in the basin.Phosphogypsum leachate has complex composition,high concentration and low p H,conventional flocculation and chemical precipitation treatment have the limitations of complex process and difficult sludge recovery treatment,and the adsorption method with simple process,fast and efficient process is favored.The latest study found that the porous bimetal oxides prepared by the"self-template"conversion strategy using cyanometallate（CM）framework as the precursor have high selectivity for phosphate,which is an ideal phosphorus removal material with low energy consumption and high efficiency,suitable for acid wastewater treatment,and has good alkali buffering performance.Based on this,this study prepared PrFe cyanometallate（CM）framework precursors（PrFe-T CM,T is the synthesis temperature）by regulating the synthesis temperature（5℃～85℃）,and prepared a series of PrFe bimetal oxides（PrFe-T）after aerobic roasting at500℃,and it was found that 20℃and 30℃were the critical temperatures for precursor synthesis.Taking PrFe-20 and PrFe-30 as representatives,the influence of synthesis temperature on the micromorphology,structural characteristics and surface properties of materials was analyzed by SEM,XRD,TG-DTG,etc.,and the adsorption effect and influencing factors of phosphorus were systematically studied,and the phosphorus removal mechanism was discussed by combining the material characterization before and after adsorption,and the influence mechanism of precursor synthesis temperature on the phosphorus removal performance of PrFe bimetal oxides was elucidated.PrFe-20 with outstanding adsorption performance was used for phosphogypsum leachate treatment and achieved good results,which provided theoretical reference and technical support for the treatment of high-concentration acid phosphogypsum leachate.Its main conclusions are as follows:（1）Structural optimization in the preparation stage of precursor is an effective strategy for designing porous PrFe bimetal oxides high-performance phosphorus removal materials.The phosphorus removal performance of PrFe bimetal oxides was significantly correlated with the synthesis temperature of the precursor,and PrFe-5,PrFe-10 and PrFe-20 had strong adsorption and comparable performance on phosphorus,which was significantly higher than that of PrFe-30,PrFe-45,PrFe-65,PrFe-85,and the adsorption performance was increased by 2.80～6.51 times.Therefore,PrFe-20 and PrFe-30 were screened at the critical point of adsorption performance to further explore the influence mechanism of precursor synthesis temperature on the structural characteristics and phosphorus removal performance of PrFe bimetal oxides.（2）The precursor synthesis temperature has an important influence on the pore opening process and oxidation product composition of high-temperature roasting.The surface micromorphology,crystal structure and functional groups of the precursors PrFe-20 CM and PrFe-30 CM were similar,but there were significant differences in structural size.Compared to PrFe-30 CM（10μm,2.12 m²/g）,PrFe-20 CM monomers are longer（20-30μm）and SA smaller（0.53 m²/g）.After high-temperature roasting,PrFe-20 and PrFe-30 can better inherit the structure of the precursor,and the precursor forms an air flow to rush out during the high-temperature oxidation pyrolysis process to achieve porosity,and the SA of the material increases sharply,and PrFe O₃is generated at the same time.The SA of PrFe-20 CM increased by 36.8 times（19.49 m²/g）after opening,but the SA increased by only 3.15 times（6.68 m²/g）after PrFe-30 CM opening,and the pore formation difference was attributed to the C≡N bond decomposition and coordination water loss of PrFe-20 CM being higher than that of PrFe-30 CM.Correspondingly,PrFe-20 is more abundant mesoporous（20～50 nm）,and PrFe-30 contains more by-product Pr₂O₂CO₃.（3）PrFe bimetal oxides has outstanding selectivity for phosphorus,and its phosphorus removal efficiency is closely related to the synthesis temperature of the precursor and the p H of the solution.The saturated adsorption capacity of PrFe-20 for phosphorus is as high as90.02 mg P/g,which is three times that of PrFe-30（29.29 mg P/g）,which is comparable to the difference in specific surface area.The PrFe bimetal oxides shows outstanding selectivity and temperature stability for phosphorus,and the ambient temperature and the coexistence of fulvic acid and inorganic ions(Cl^-,NO₃^-,SO₄^2-,CO₃^2-)has no significant effect on the adsorption performance of PrFe-20 and PrFe-30.p H is a key environmental factor affecting the phosphorus removal efficiency of PrFe bimetal oxides,and the phosphorus removal performance decreases significantly with increased p H in the range of 2.5～10.5（PrFe-20,97.01→21.80 mg P/g;PrFe-30,45.54→8.39 mg P/g）.The acidic conditions are conducive to the adsorption of phosphorus by the oxide of PrFe,and the material has outstanding alkali buffering.（4）The main mechanism of phosphorus removal by PrFe bimetal oxides is the formation of stable PrPO₄with phosphate by ligand exchange at the exposed Prsites.The larger adsorption capacity of PrFe-20 is attributed to the richer development of mesopores and more exposed Prsites.The precursor synthesis temperature also has an effect on the phosphorus removal mechanism of PrFe bimetal oxides,and XPS and FTIR characterization show that PrFe-20 can be exchanged with phosphate through surface-OH and CO₃^2-ligands,while PrFe-30 is mainly exchanged with phosphate ligands by CO₃^2-and phosphate ligands.（5）PrFe-20 has outstanding treatment ability for phosphogypsum leachate,and ambient temperature has little effect on adsorption performance（standard deviation±6.03%）.Under the same reaction conditions,the phosphorus removal ability of PrFe-20 in phosphogypsum leachate was better than that of the deionized water system,and the maximum adsorption capacity obtained by the experiment was 121.0 mg P/g,which was 1.34 times the saturated adsorption capacity of the deionized water system.Under the non-oscillating condition of room temperature,the optimal dosage was linearly positively correlated with the initial concentration of phosphogypsum leachate（y=0.0127x+0.0044,R²=0.999）,the alkali buffering performance was outstanding（p H,3.12～4.25→>6）,and the adsorption product was stable and not easy to desorption.