The Adsorption Properties And Reaction Mechanisms For The Simultaneous Removal Of Chromium And Phosphate From Aqueous Solution By Using The Nano-sized Composite Material Of Fe⁰-Fe₃O₄

The heavy metal element chromium and the aquatic nutrient element phosphorus enter the water body in large quantities,which will cause serious harm to human health and water safety.In recent years,due to the widespread use of chromium（Cr）and phosphorus-containing materials or agents,a large amount of chromium-containing and phosphorus-containing wastewater enters the water environment,and may cause compound pollution of chromium and phosphorus.Compared with other methods,the adsorption method has attracted much attention because of its high efficiency and strong adaptability.In this paper,adsorption method is used to simultaneously remove chromium and phosphorus pollutants from water.In this paper,the nano-Fe⁰-Fe₃O₄ composite material was prepared by the co-precipitation method,and the adsorption and removal performance of the two pollutants under the Cr（VI）/P single pollution system and the Cr（VI）&P composite pollution system were explored.The nano-Fe⁰-Fe₃O₄ composites before and after adsorption were characterized by SEM,FTIR,EDS-Mapping and XPS,and the mechanism of Cr（VI）/P removal by nano-Fe⁰-Fe₃O₄ composites was explored.The main results are as follows:（1）Nano-Fe⁰-Fe₃O₄ composite material is a superparamagnetic material;the average particle size is between 20-100nm;it has a high specific surface area（35.43m²/g）.The isoelectric point is about 3.8;there are abundant metal hydroxyl groups on the surface of the material.（2）Pseudo-second-order kinetic model may better describe the adsorption kinetics process of Cr（VI）and P on nano-Fe⁰-Fe₃O₄ composites under different experimental conditions,suggesting that the adsorption type is chemical adsorption.The isotherm data of Cr is fitted better with the Fredilich model,which indicates that capacitary reaction is multi-molecular layer adsorption.The adsorption isotherm model of P is more consistent with the Langmuir model,indicating that the adsorption of P by the nano-Fe⁰-Fe₃O₄composite material is a monolayer adsorption.At the same time,the increase in temperature increases the adsorption capacity of the adsorbent for Cr（VI）&P,indicating that the adsorption reaction process for Cr（VI）&P is an endothermic process.（3）In the single pollution system and the Cr（VI）&P composite pollution system,when the pH of the solution is 2～10,with the increase of pH,the adsorption capacity of nano-Fe⁰-Fe₃O₄ composite material to Cr gradually decreases,and when the pH=8,the adsorption capacity is basically not change again.In a single P pollution system,when the pH rises from 2 to 3,the adsorption capacity of the nano-Fe⁰-Fe₃O₄ composite material drops significantly,and when the pH=6,the adsorption capacity basically no longer changes.In the Cr（VI）&P composite pollution system,the adsorption capacity of nanometer Fe⁰-Fe₃O₄ composite material to P gradually decreases with the increase of pH,and the adsorption capacity basically does not change when pH=8.In single Cr pollution and Cr（VI）&P compound pollution system,Ca²⁺among the six kinds of coexisting ions has a positive effect on Fe⁰-Fe₃O₄ adsorption and removal of chromium,and the rest have inhibitory effects and the order of the inhibitory effects is CO₃^2->SO₄^2->F^->Mg²⁺>Cl^-.In the single P pollution and Cr（VI）&P compound pollution system,the presence of four anions and two cations inhibits the adsorption of phosphorus by the nano-Fe⁰-Fe₃O₄adsorbent to varying degrees,and the order of inhibition is F^->CO₃^2->SO₄^2->Cl^->Mg²⁺>Ca²⁺.（4）The adsorption and removal mechanism of Fe⁰-Fe₃O₄ composite material for Cr is carried out by the direct adsorption of Cr（VI）and the adsorption and fixation of Cr（III）generated by the reduction of part of Cr（VI）on nano-Fe⁰.There is also mutually reinforcing role between these two pathways.The removal mechanism of P is that under the action of electrostatic attraction between the surface of the material and the phosphate,the protonated phosphate replaces a single-coordinated surface hydroxyl group(≡Me-OH^-0.5)（Me means Fe）,And forming a monodentate complex(≡Me-OPO₃^2-)on the material surface so as to be adsorbed...