Study On Phosphate Removal From Water By Polypyrrole Modified Red Mud

Phosphate pollution is one of the important causes of eutrophication.Among the remediation technologies of phosphate polluted water,adsorption method has attracted more and more attention because of its advantages of simple operation,low cost,high efficiency and no toxic and harmful substances.In this study,a new polypyrrole modified red mud was prepared by chemical oxidation polymerization with ferric sulfate as oxidant to adsorb and remove phosphate from water.First,the optimum preparation conditions of polypyrrole modified red mud were determined by response surface analysis.Under this condition,the effects of adsorption time,initial phosphate concentration,pH,ionic strength and coexisting ions on the properties of the adsorbent were investigated,and the fitting parameters of adsorption kinetics,adsorption isotherm and adsorption thermodynamics in the process of phosphate removal were obtained.Then,this study through the analysis of the specific surface area,scanning electron microscope,X-ray energy spectrum,transmission electron microscopy,X-ray photoelectron spectroscopy,Fourier transform infrared spectroscopy,X-ray diffraction instrument,the determination of the content of amorphous iron/aluminum,multiple means such as sulfate content determination,the morphology,chemical characteristics of adsorbent and phosphate adsorption ways are analyzed.Finally,the feasibility of phosphate removal by polypyrrole modified red mud was investigated through the analysis of recycling performance,dynamic phosphorus removal performance and heavy metal leaching toxicity.The main conclusions are as follows:（1）Using Fe₂（SO₄）₃ as oxidant,polypyrrole modified red mud prepared by oxidation polymerization of pyrrole can effectively remove phosphate in water.According to single factor experiment and response surface analysis optimization,the optimal adsorption effect of phosphate can be obtained under the preparation conditions of red mud dosage of 1.5 g/100mL,ferric sulfate concentration of 0.05 mol/L,pyrrole concentration of 0.11mol/L,the model predicted that the equilibrium adsorption capacity can reach 20.61 mg/g.（2）The adsorption process can be described by quasi-second-order kinetic equation,which is a chemical adsorption process in which the adsorption rate is proportional to the quadratic power of the number of remaining adsorption sites,and can be divided into three successive stages:outer surface diffusion,inner particle diffusion and adsorption equilibrium.The equilibrium adsorption capacity of polypyrrole modified red mud for phosphate is proportional to the initial phosphate concentration,and the isothermal adsorption process can be expressed by Langmuir equation.The maximum phosphorus adsorption capacity of polypyrrole modified red mud is 21.74,20.25 and 19.53 mg/g（P）at 15,25 and 35℃,respectively.The adsorption thermodynamics study shows that the adsorption removal of phosphate by polypyrrole modified red mud is a process of increasing disorder（ΔS₀>0）,exothermic（ΔH₀<0）and spontaneous（ΔG₀<0）.（3）Polypyrrole modified red mud has obvious practical advantages in environmental adaptability.It can maintain good adsorption performance when the pH of the solution is4.0～10.0,and the application effect is best in acidic and neutral solutions.The coexisting substances have little effect on the adsorbent performance,among which the presence of CO₃^2-,SO₄^2-and SiO₃^2-has a weak inhibition effect,and the coexistence of low concentration humic acid can improve the adsorption capacity at equilibrium.When the initial phosphate concentration is low,the adsorption effect is not sensitive to the change of ionic strength,but when the initial phosphate concentration is high,the equilibrium adsorption capacity increases with the increase of ionic strength.（4）The surface of polypyrrole modified red mud is covered with a large number of spherical pyrrole particles of different sizes and clustered together,which are loaded on the surface of red mud in the form of coating.The modification of polypyrrole can increase the specific surface area and average pore volume of red mud,decrease the average pore size and significantly increase the proportion of mesopores（2～50 nm）.The content of amorphous iron in polypyrrole modified red mud is significantly higher than that of the original red mud before modification,and it has stronger phosphate adsorption capacity.The adsorption pathways in order of contribution are as follows:ion exchange,ligand exchange and electrostatic attraction,of which the contribution of ion exchange and coordination reaction are 64.68%and 26.31%,respectively.（5）Polypyrrole modified red mud has good recycling performance.For the polypyrrole modified red mud with adsorption equilibrium,0.1 mol/L Na OH solution is used for desorption and 0.025 mol/L H₂SO₄ solution is used for cleaning,which can keep the phosphate removal rate above 80%after four cycles.The dynamic phosphorus removal performance of polypyrrole modified red mud shows that the dynamic adsorption process of phosphate is in line with Thomas model,and the maximum phosphorus adsorption capacity within the study range is positively correlated with influents flow rate and influents phosphorus concentration.The toxic leaching concentration of major heavy metals in polypyrrole modified red mud is lower than the limit set in HJ 1090-2020,which is an environmentally friendly phosphorus removal adsorbent.On the premise of effectively controlling the release of heavy metals and other harmful substances,this study applies red mud,a bulk solid waste,as an adsorption carrier in the field of phosphate pollution control of water bodies.The research content runs through the sustainable development concept of"waste treating waste".The research results can provide some data support for promoting comprehensive utilization of solid waste,improving resource utilization efficiency and improving water environment quality.