| Years of China groundwater quality analysis showed that excessive pollutants have the characteristics of classical ionized features.Among them,the fluoride ion and hardness are major excessive pollutants.However,in many rural areas,groundwater is still directly used as drinking water.Long-term drinking high-fluorine and high-hard water is harmful for human health."Hygienic Standards for Drinking Water"?GB5749-2006?sets the limiting values of fluoride concentration and hardness of 1.0mg/L and450mg/L,respectively.Among the treatment methods for fluoride ion and hardness,the adsorption method has attracted much attention because of its simple operation and low cost.In this paper,the adsorption mechanism of the simultaneous removal of fluoride ion and hardness by calcined Mg/Al hydrotalcite?HTCs-400-Mg Al?was explored.The main results are as follows:Mg/Al hydrotalcite?HTCs-MgAl?was prepared by co-precipitation method and calcined at 400°C to obtain its calcined product?HTCs-400-Mg Al?.The prepared material was characterized by XRD,FT-IR and SEM.The results showed that the prepared hydrotalcite had obvious layered structure with good crystallinity and typical hydrotalcite-like characteristics.After calcination at high temperature,the layered structure disappeared and formed metal oxide structure.HTCs-400-Mg Al was used to remove fluoride ion and total hardness simultaneously in water.The factors including HTCs-400-Mg Al dosage,reaction time,initial pH value,initial concentration,temperature,rotation speed on the adsorption effect were studied.The results showed that there were positive correlations between HTCs-400-MgAl dosage and the adsorption capacity of fluoride ion and total hardness.Adsorption capacities of fluoride ion and total hardness reached adsorption equilibrium at 150min and 240min,respectively,and their equilibrium adsorption capacities reached 1.57mg/g and47.5mg/g,respectively.With the increase of the initial p H,the adsorption capacity of fluoride ion changed little,and that of total hardness increased.The initial concentration of ions in solution was positively correlated with the adsorption capacity of fluoride ion and total hardness.The Plackett-Burman tests were used to screen the main factors affecting the removal rates of fluoride ion and total hardness as pH value,reaction time and HTCs-400-Mg Al dosage.Temperature and rotation speed had little effect on fluoride ion and total hardness removal.The optimum adsorption conditions were obtained by Box-Behnken experiments:pH value 9.40,reaction time239.33min,and HTCs-400-MgAl dosage 5.61g/L.Under the optimal conditions,the removal rates of fluoride ion and total hardness reached97.55%and 58.09%,respectively.The adsorption kinetics and isothermal adsorption of fluoride and total hardness by HTCs-400-MgAl were studied,and the adsorption property was investigated by activation energy.The results showed that the adsorption process of fluoride ion and total hardness by HTCs-400-MgAl was more consistent with the pseudo-second-order kinetic model.Activation energy results were as follows:Eafluoride ion=53.9kJ/mol,Eatotal hardness=144.02kJ/mol,reflecting that the adsorption process was mainly controlled by chemical reaction.The intragranular diffusion model reflected that the adsorption process of fluoride ion and total hardness by HTCs-400-MgAl consisted of three stages:diffusion of fluoride ion or total hardness from the solution to the outer surface of the adsorbent,slow internal diffusion,and equilibrium adsorption.The adsorption process of fluoride ion by HTCs-400-MgAl was more in line with the Langmuir isotherm adsorption model.The adsorption process of total hardness by HTCs-400-MgAl was more in line with the Freundlich adsorption model,which was inhomogeneous multi-layer preferential adsorption.The reaction samples were characterized by XRD,FT-IR and EDS,and the adsorption kinetics was investigated by a coexistent solution of the fluoride ion combining with magnesium and calcium hardness,respectively.The adsorption mechanism was also analyzed by the changes in pH before and after the reaction.The results showed that the adsorption of fluoride ion and hardness by HTCs-400-MgAl were chemical adsorption,and the mechanism of fluorine ion removal was via to the“memory effect”of calcined hydrotalcite to complete the reconstruction of the material structure.When the adsorbent removed fluoride ion and magnesium hardness,the presence of magnesium ions promoted the adsorption of fluoride ion,resulting in more fluoride ion being adsorbed to the layers.On the one hand,magnesium ion was removed by forming metal oxide bonds with the Mg-Al ions of the material surface.On the other hand,magnesium ion could be removed by embedding into surface of the adsorbent.The removal of calcium hardness was much weaker than that of magnesium.The change in pH indicated that the calcium hardness was removed by form Ca?OH?2 precipitation with-OH,which was generated by fluoride ions entering into the interlayer of the material. |
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