Study On Preparation Of Layered Polymetallic Hydroxides And Its Adsorption For Phosphate

Human activities lead to the enrichment of nitrogen and phosphorus in water under the effect of runoff,especially the discharge of phosphorus makes the eutrophication of water more severe,the degradation of ecosystem,and the reduction of self-purification capacity of water.However,most of the existing phosphorus removal adsorbents are non-selective,and there are some problems such as low phosphorus removal effect and poor recovery effect.Therefore,it is pressing to develop a kind of cheap,environmentally friendly and high phosphorus removal adsorption material.Layered bimetallic hydroxides（LDHs）are layered materials composed of some of the most common and abundant metal hydroxides.In this review,recent advances in the characterization,synthesis,structure and phosphate removal of LDHs are reviewed.Different layered polymetallic hydroxides（multiple-LDHs）were prepared by changing the metal type and metal mole ratio,and their phosphorus removal performance was compared.Binary LDHs,ternary LDHs and quaternary LDHs with better adsorption performance were screened out,and their morphology,composition and structure were characterized and analyzed.The adsorption behavior of phosphate by LDHs-based adsorbent was studied,the influence of various factors was studied,the possible phosphorus removal mechanism was discussed,and the optimal material was modified with Fe₃O₄ to improve the adsorption performance and separate from the adsorption system.The concrete conclusions are as follows:（1）This study found that Zn Al-LDHs(n(Zn²⁺):n(Al³⁺)=3:1),Mg Zn Al-LDHs(n(Mg²⁺):n(Zn²⁺):n(Al³⁺)=1:3:1),Mg Zn Al Fe-LDHs(n(Mg²⁺):n(Zn²⁺):n(Al³⁺):n(Fe³⁺)=2:6:1:1)phosphorus removal effect is good.The prepared adsorbents were characterized by scanning electron microscopy（SEM）,Fourier transform infrared spectroscopy（FTIR）,specific surface area analysis（BET）and X-ray diffraction analysis（XRD）.The three adsorbents all had typical LDHs lamellar structure and characteristic absorption peaks.Multiple LDHs were successfully prepared,and the interlamellar channels and specific surface area of LDHs increased with the introduction of metals.（2）The single factor influence experiment on the adsorption and phosphorus removal of Zn Al-LDHs,Mg Zn Al-LDHs and Mg Zn Al Fe-LDHs shows that p H has a great influence on the adsorption process..With the increase of p H,the adsorption amount decreases.The common anions NO₃^-,Cl^-,CO₃^2-and SO₄^2-in water have a certain interference effect on the adsorption and phosphorus removal of multiple LDHs.The Elovich model and quasi-second-order kinetic adsorption model can be used to describe the phosphate adsorption process,indicating that the adsorption is chemical adsorption,and the order of adsorption rate is:Mg Zn Al Fe-LDHs>Mg Zn Al-LDHs>Zn Al-LDHs;Zn Al-LDHs,Mg Zn Al-LDHs and Mg Zn Al Fe-LDHs conform to Sips isothermal adsorption model,and the maximum fitting adsorption capacity can reach 148.01 mg·g^-1,181.58 mg·g^-1 and 212.50 mg·g^-1.The adsorption process is endothermic reaction,which is irreversible.（3）Fe₃O₄-LDHs-Ⅱ(n（Fe₃O₄）:n(Mg²⁺)=0.02)was prepared by composite modification of Fe₃O₄ and Mg Zn Al Fe-LDHs.The Fe₃O₄-LDHs-Ⅱwas characterized by SEM/XRD/FTIR/VSM.The results showed that the introduction of Fe₃O₄ did not destroy the original structure,and the diffraction peak of Fe₃O₄ and LDHs structure was obvious,and the crystallinity was good.The magnetic intensity of Fe₃O₄-LDHs-Ⅱwas significantly lower than that of Fe₃O₄,but it did not affect the recovery of adsorbent from the adsorption system by magnetism.（4）The p H of Fe₃O₄-LDHs-Ⅱhas a great influence on the adsorption effect,and the adsorption adaptability is better under acidic conditions.The process of phosphate adsorption conforms to the Elovich model and quasi-second-order kinetic model.The maximum fitting adsorption capacity of Fe₃O₄-LDHs-Ⅱphosphate by Sips isothermal adsorption model was 213.17 mg·g^-1.The regression model established by BBD response surface analysis method is significant,and more than 99.01%of the response value can be explained by the model,which can be used to optimize phosphate adsorption conditions.