The Adsorption Of Fluoride Ion By Hydrotalcite-like Compounds

Some people lived in villages and counties in Shanxi are still usedgroundwater as the source of drinking water. Most of groundwater arefluoride exceeded, which is harmful to hman health. Adsorption method is thepossible way to treat micro-polluted water due to its convinence and reliability.To remove the excessive fluoride ion in groundwater to meet the purpose ofdrinking water standard, the main research on Mg/Al/Fe hydrotalcite-likecompounds, its calcined products and different dispersion of hydrotalcite-likecompounds were investigated for the preparation, characterization. Adsorptionprocess of fluoride ion and adsorption mechanism were also discussed. Themain conclusion is as follows:Mg/Al/Fe hydrotalcite-like compounds were synthesized by co-precipitationmethod and then calcination with different temperature to form calcinedhydrotalcite-like materials. The adsorption materials were characterized by XRD,FT-IR, SEM and other methods, and the characterization results show that thesynthesized materials have the typical layered structure of hydrotalcite structure,clear and complete. The influence of calcination temperature, adsorbent dosage,initial pH value of adsorption time is studied and adsorption kinetics, adsorptionisotherm and adsorption thermodynamics are also investigated. Research showsthat500℃was the best calcined temperature. The best adsorbent dosage valuewas4g/L, the adsorption equilibrium point was180min. In the initial solutionpH range for the3-8the adsorption capacity had no change. Calcined Mg/Al/Fe hydrotalcite adsorption was fitted well with the pseudo-second order kinetic, thereaction activation energy Ea=14.63kJ/mol. Thermodynamics showed that thereaction was fitted well with Langmuir isotherm model, the reaction was aspontaneous and endothermic process. The entropy of the process was increased.The adsorption of fluoride ion would be inhibited when the co-existent ionswere existed in the water. The hydrotalcite-like materials could be usedrepeatly via calcined recovery method.The modified Mg/Al like compound was prepared by different modificationmaterials during the synthesis.Change the Mg/Al molar ratio in the process ofsynthesis to prepareddifferent cation ratio of hydrotalcite. The adsorptionmaterials were characterized by XRD, FT-IR, SEM, TG-DSC and sizedistribution. The characterization results show thatthe average particle size ofmodified Mg/Al hydrotalcite like compounds was420.7nm. The effect ofdifferent modified materials on the adsorption was studied. And the contents ofthe dosage, reaction time, reaction temperature, adsorption kinetics andthermodynamics of the adsorbent were also carried out.The research indicatedthat10g/L was used as a dispersing agent the best adding concentration ofglucose,4g/L was the best adsorbent dosage,180min was the equilibriumadsorption site, ML-2equilibrium adsorption capacity was better than the othertwo materials. The kinetics of adsorption showed that the three kinds ofmaterials were all fitted the pseudo-second order kinetic, the activation energieswere: Ea1=31.30kJ/mol, Ea2=7.08kJ/mol, Ea3=17.88kJ/mol. Characteristics ofthe reaction process werefitted the physical adsorption and chemical adsorption,where ML-1and ML-3type material were under the influence of internaldiffusion control. Adsorption thermodynamics showed that the adsorptionprocess of ML-1type material accorded with Freundlich adsorption isothermmodel, adsorption process of ML-2and ML-3type material accorded withLangmuir isothermal adsorption model. The process of ML-1was spontaneous; three kinds of materials were well fitted with the process of exothermicentropy decreassation in adsorption.Modified materials were synthesised with sodium dodecyl sulfate,hexadecyl sulfonate and starch as dispersant. The influence of the Mg/Al molarratio, adsorbent amount and reaction time was used to compare differentmodified adsorbent. The results showed that the starch as a dispersant has thehigher fluoride ion removal ability than sodium dodecyl sulfate and hexadecylsulfonate as dispersant, showing that it can be used as an excellent adsorbent inwater treatment. Characterization of methods such as XRD, FT-IR results showsthat the synthesized material still has the typical kind of hydrotalcite layeredstructure, and the dispersant did not involve in the synthesis process of modifiedmaterials.