Preparation Of Metal Oxide Nano-composites And Its Fluoride Removal Mechanism

The problem of high fluoride pollution in groundwater has long existed and continues to threaten the ecological environment and human health.Research on the removal of fluoride in water is particularly important.Among the many existing technologies for removing fluorine in water,the adsorption method has received extensive attention because of its good effect and low cost.The key to using adsorption to treat fluorine in water lies in the adsorption efficiency and use cost of the adsorbent materials.However,the existing conventional adsorbent materials have the disadvantages of slower adsorption rate,lower adsorption capacity and higher use cost.Nanomaterials have the characteristics of huge specific surface area with small size.The unsaturation of the surface atoms leads to the existence of a large number of surface functional groups and high chemical activity.In the process of surface interface,nano-adsorbed materials show high adsorption affinity and high adsorption capacity,and the application of nano-materials to remove harmful fluorine in water has broad application prospects.In this paper,three kinds of nano-adsorbed materials were synthesized through optimized preparation methods,and their adsorption effect and adsorption mechanism for fluorine were systematically studied.The main research contents are as follows:?1?After optimizing the preparation method,an iron-aluminum layered double hydroxide material?Fe-Al LDH?was prepared under the simple synthesis conditions of Fe/Al molar ratio of 3:1 and hydrothermal reaction at 120?for 12h,and studied the defluorination performance of the material.The results of adsorption kinetics studies on Fe-Al LDH indicate that the adsorption process conforms to the Langmuir model.The kinetic data conforms to the quasi-second-order kinetic model,and the defluorination process is mainly chemical adsorption.In the study of the effect of pH,the pH of the solution was increased from 3 to 12,and the efficiency of removing fluoride from the adsorbent was reduced from 93.57%to 55.63%,and the removal rate of more than 70%was maintained in a wider range of pH 5-10.In the experiment of the influence of coexisting ions,several common coexisting ions were studied:Cl^-,SO₄^2-,NO₃^-,HCO₃^-,and PO₃^3-.None of these ions have a significant effect on the removal of fluoride by Fe-Al LDH,indicating that the material has strong selective adsorption.According to analysis,it is believed that the fluorine adsorption is mainly due to the presence of carbonate ions between the structural layers of Fe-Al LDH,which can undergo ion exchange reaction with fluoride ions.?2?Iron-aluminum-nickel composite oxide nanomaterials?FAN?was synthesized by a simple preparation process which is co-precipitation method.When the molar ratio of Fe/Al/Ni is 1:1:1,the sample has the highest defluorination adsorption capacity.At pH=7,the adsorption process conforms to the Langmuir model.The maximum ideal adsorption capacity is 79.3mg/g,which is a certain advantage over other adsorbents.During the pH change from 2 to 11,the defluorination efficiency changes from high to low,and the environment with a pH greater than 8 and less than 5 is more favorable for FAN defluorination.HF generated under strong acid conditions inhibits the adsorption reaction,and a large amount under strong alkaline conditions OH^-and F^-form a competitive effect.The high concentration of PO₃^3-and HCO₃^-in many common coexisting ions has a significant effect on FAN fluoride removal.When the interference ion concentration is several times the fluoride ion concentration,the fluoride removal efficiency remains of 50%,indicating that it has a fluoride ion Stronger selectivity is stronger.?3?In this study,the idea of FAN adsorbent synthesis was continued,and the iron-aluminum-manganese composite metal hydroxide?FAM?with nanowire structure was prepared by coprecipitation.In the isothermal adsorption experiment,the adsorption of fluoride ion by FAM conforms to the Langmuir model,and the ideal adsorption capacity is 95.51mg/g.This material has a faster adsorption rate for fluorine.In the experiment of adsorption kinetics,adsorption equilibrium can be reached within 60 minutes.The adsorption process is mainly divided into the initial rapid adsorption stage caused by the concentration difference,and the later stage caused by saturation of the adsorption site.In the slow adsorption stage,the adsorption process conforms to the pseudo-second-order kinetic model,indicating that the adsorption reaction is essentially an irreversible chemical reaction.FAM adsorbs fluorine with a wide range of pH adaptation,and the removal rate of fluorine remains above 70%under the condition of pH=4⁹.High concentrations of PO₃^3-and HCO₃^-will have a certain effect on the adsorption of fluorine,and the fluorine removal efficiency can still reach 75%under the influence of coexisting ions.The main adsorption mechanism of FAM is:the electrostatic attraction to fluoride ions and the ion exchange reaction caused by the abundant hydroxyl groups on the surface.