Study On Preparation, Characterization And Sulfate, Fluoride Adsorption By Layer Double Hydroxides

Sulfate and fluoride contamination in groundwater has wide distribution inChina and has been recognized as potential health problems."Standards forDrinking Water Quality"(GB5749-2006) sets the limits of sulfate concentrationof250mg/L, and fluoride concentration of1.0mg/L. Conventional sulfate andfluoride treatment methods have the disadvantage of low removal efficiency,long process units, high operation and maintenance cost. Adsorption process isone of the promising methods in terms of cost, simplicity of design andoperation. Among various adsorption materials used for anion removal of water,the layer double hydroxide offers attractive attention due to its higher anionadsorption ability. Therefore, calcined Mg/Fe layer double hydroxides were usedto treat the sulfate and fluoride anion, starch-stabilized Mg/Al layered metalhydroxide used for fluoride adsorption. The main research contents andconclusions are as follows:(1) A qualified Mg/Fe layer double hydroxides (Mg/Fe LDHs) was formedby co-precipitation method and Mg/Fe CLDH was gained after calcinations at 400℃. The characterization of Mg/Fe LDHs and Mg/Fe CLDH were analyzedby XRD, FT-IR, TG-DSC, BET, SEM, ICP and elemental analysis. It shows thatMg/Fe LDH has the typical lateral structure, while Mg/Fe CLDH is formed asmetal oxides. Mg/Fe CLDH shows the higher specific surface area and totalpore volume than Mg/Fe LDH. SEM images show the particles of Mg/Fe CLDHare formed by the aggregation of very small platy crystallites. The preliminaryexperiment shows that Mg/Fe CLDH with Mg/Fe molar of3:1and calcinedtemperature of400℃has the efficient sulfate adsorption ability. Data ofequilibrium experiments in the sulfate adsorption by Mg/Fe CLDH in simulativewater were fitted well to Langmuir isotherm, indicating adsorption process ismonolayer adsorption. The value of KLbeyond zero points out a favorableadsorption process. The kinetic data demands the applicability of the pseudosecond-order kinetic model in the sulfate removal by Mg/Fe CLDH insimulative water. The rate constant with a low value reflects a fast reaction. Thethermodynamic feasibility of adsorbent-anion interaction in simulative waterwas studied by the value of free energy△G0, standard enthalpy△H0, andstandard entropy△S0. The value of△G0was found to be negative (-20.67KJ/mol,-22.02KJ/mol and-23.02KJ/mol, respectively), indicating feasibilityand spontaneity of adsorption. The positive values of△H0（15.01KJ/mol）confirms the endothermic nature of the sorption process. The positive value of△S0（14.18J/(mol·K)）indicates increasing randomness during the sorptionprocess. The value of the activation energy is74.25kJ/mol, suggesting that the adsorption proceeded by a chemisorption process. The intra-particle diffusionadsorption shows that kinetics of anions onto adsorbent may be controlled bythree steps named boundary layer diffusion, particle surface adsorption, andintra-particle diffusion, indicating adsorption is a complicated process. Theadsorption mechanism was determined by XRD, FT-IR analysis. The LDHcharacteristic reflections reappeared in the Mg/Fe CLDH after adsorption,indicating that the structure “memory effect” existed in the preparedadsorbents and its original layered double hydroxide structure was reconstructedby intercalation of sulfate ions into interlayer region after adsorption. FT-IRshows that sulfate anions were effectively adsorbed by the adsorbent. Sulfateadsorption capacity decreased slowly with pH increase, but the declination is toosmaller. The equilibrium pH was around11, indicating that structurereconstruction existed in the adsorption process.(2) A novel, simple Mg/Al LDHs （S-LDH） were prepared withstarch-stabilized. The characterization of Mg/Al LDHs and S-LDH wereanalyzed by XRD, FT-IR, TG-DSC, SEM and laser granulometer. It shows thatS-LDH has smaller diameter and more uniform pore size than that withoutstarch-stabilized. The preliminary experiment shows that Mg/Al LDHs withstarch content of10mg has the higher adsorption ability in fluoride removal thanthat without starch content. Data of equilibrium experiments in the fluorideremoval by S-LDH in simulative water were fitted well to Langmuir isotherm,while data of equilibrium experiments in the fluoride removal by S-LDH in natural water were fitted well to Freundlich isotherm, indicating mutual ions hadstrong influence on fluoride removal in natural water. The thermodynamicfeasibility of adsorbent-anion interaction in simulative water indicates feasibilityand spontaneity of adsorption, the endothermic nature of the sorption processand randomness during the sorption process. The kinetic data indicates theapplicability of the pseudo second-order kinetic model. The mechanism offluoride removal on S-LDH includes chemical adsorption and surfaceadsorption.(3) Mg/Fe CLDH adsorbent was synthesized for simultaneous removal ofsulfate and fluoride. Influence of pH variance shows that pH value had higherinfluence on fluoride than on sulfate. Data of equilibrium experiments in thesulfate and fluoride removal by the CLDH in simulative water were fitted wellto Langmuir isotherm. The kinetic data meet the pseudo second-order kineticmodel. The adsorption mechanism involved surface adsorption, ion exchangeinteraction and original LDH structure reconstruction by rehydration of mixedmetal oxides and concomitant intercalation of fluoride and sulfate ions into theinterlayer region.