Study On Preparation Of Transition Metal Composite Modified Sepiolite And Its Phosphate Adsorption Properties

It is of great significance to control the eutrophication of water body,balance the aquatic ecosystem and maintain the normal production and life of residents to control the phosphorus-rich wastewater into water body.Therefore,the removal of phosphate from waters has always been a hot research topic.Adsorption method has been widely used because of its merits of convenient and effective use,fast processing speed,no secondary pollution and resource recovery.Sepiolite,as a kind of clay mineral with rich mineral resources,relatively low price and environmental protection and non-toxic,has a unique fibrous crystal structure and 2:1layered pore structure,and has a high specific surface area and excellent ion exchange ability,was widely used in the field of adsorption.Metal oxides have the advantages of good adsorption performance,small compound particles and insoluble,while the transition metal doping other metal oxides to form bimetal composite oxides to improve the adsorption performance of the adsorbent by using the synergistic effect of the two is a common modification method.Therefore,in this study,sepiolite was modified by introducing transition metal doped metal oxides to form a mixture of fibrous crystal structure and bimetal oxide,so as to strengthen the adsorption performance of sepiolite to phosphate.The adsorption kinetics,adsorption isotherm,adsorption thermodynamic model and response surface model were used to describe the adsorption process of phosphate and optimize the adsorption conditions.Combined with BET,SEM,XRD and FTIR,the surface morphology and structure changes of sepiolite before and after modification were deeply analyzed.The effects of adding metal ratio,p H value and coexisting ions on phosphate adsorption were investigated.The dynamic adsorption performance of the adsorbent to remove phosphorus containing wastewater was investigated in order to provide reference for its practical application in the treatment of rich phosphorus wastewater.The main conclusions of this study are as follows:（1）Transition metal composite modification of sepiolite（H⁺-Zn Al-Sep、H⁺-Fe Al-Sep and H⁺-Mn Al-Sep）was prepared by transition metal(Zn²⁺、Fe³⁺、Mn²⁺)and Al³⁺under the action of alkali solution.It was analyzed by BET,SEM,XRD and FTIR.The results show that the crystal structure of sepiolite is not damaged by the bimetal composite modification,and the specific surface area of H⁺-Zn Al-Sep、H⁺-Fe Al-Sep and H⁺-Mn Al-Sep is greatly increased,which because the bimetallic oxides themselves have rich mesoporous structures after forming fine metal compounds on the sepiolite surface.Moreover,the interweaving of metal oxides and sepiolite increases the dispersity and specific surface area of sepiolite,which increases the number of effective active adsorption sites on the surface and is conducive to the adsorption of phosphate ions.In addition,the metal oxides on the surface of H⁺-Fe Al-Sep and H⁺-Mn Al-Sep mainly exist in the form of amorphous substances.H⁺-Zn Al-Sep has relatively obvious characteristic diffraction peak of zinc oxide（Zn O）,and the lattice vibration peak of metal cation（M-O and M-OH）is added in all three.It shows that sepiolite is successfully modified by metal in the form of oxide.（2）The factors affecting the adsorption properties of H⁺-Zn Al-Sep、H⁺-Fe Al-Sep and H⁺-Mn Al-Sep phosphate were studied respectively.The results showed that the phosphate adsorption performance of the three was greatly affected by the proportion of prepared metal and the p H value of simulated wastewater.The optimal ratio of H⁺-Zn Al-Sep was Zn²⁺:Al³⁺=4:1,the optimal ratio of H⁺-Fe Al-Sep was Fe³⁺:Al³⁺=1:4,the optimal addition ratio of H⁺-Mn Al-Sep is Mn²⁺:Al³⁺=2:1.The optimal p H value of phosphate adsorption is 3,and the adsorption reaction is greatly affected by CO₃^2-and SO₄^2-,so the adsorbent is more suitable for acidic phosphorus-rich wastewater,and the incorporation of CO₃^2-and SO₄^2-should be avoided in the process of phosphorus removal by the adsorbent.（3）The phosphate adsorption properties of H⁺-Zn Al-Sep、H⁺-Fe Al-Sep and H⁺-Mn Al-Sep were studied.The adsorption of phosphate by H⁺-Zn Al-Sep、H⁺-Fe Al-Sep and H⁺-Mn Al-Sep is more aligned with pseudo-second-order kinetics and Elovich equation.Chemisorption is the decisive step in the adsorption process.The intra-particle diffusion model shows that H⁺-Zn Al-Sep has faster reaction rate and adsorption capacity.The regression model can predict the equilibrium adsorption capacity of H⁺-Zn Al-Sep well.The optimal adsorption conditions are p H 3.2,temperature 40℃and initial concentration 70mg·L^-1,the equilibrium adsorption capacity of H⁺-Zn Al-Sep can reach 82.99 mg·g^-1.（4）The adsorption of phosphate by H⁺-Zn Al-Sep、H⁺-Fe Al-Sep and H⁺-Mn Al-Sep is a process of spontaneous endothermic entropy increase,mainly due to the ligand exchange of phosphate and-OH on the sepiolite surface to form compact surface complexes and electrostatic attraction.The H⁺-Zn Al-Sep fibers interweave with Zn-Al oxide lamellar structure,which increases the dispersity,thus increasing the number of active adsorption sites on the surface and improving the adsorption performance.（5）The dynamic adsorption of phosphate by H⁺-Zn Al-Sep was investigated.The higher the dosage of H⁺-Zn Al-Sep,the slower the flow rate of the solution,the lower the initial concentration of the solution,and the shorter the penetration time and depletion time of H⁺-Zn Al-Sep,that is,the higher the dynamic phosphate adsorption performance of the adsorbent.Therefore,H⁺-Zn Al-Sep has the potential to be used in phosphorus-rich wastewater..