| Fluoride removal by adsorption is a kind of widely used method, and the research of adsorbents with high adsorption capacity such as metal oxides is increasing. Mostly, the adsorption mechanism is always studied through all kinds of characterization. Based on the surface chemical properties of adsorbent, the relationship between surface acid-base properties of adsorbent and its adsorption capacity is studied in this paper.In this test, modified activated alumina powder adsorbent was prepared by impregnation in aluminum sulfate and ferric sulfate solutions, respectively. The zero point of proton charge, pHzpc of unmodified adsorbent and two kinds of modified adsorbents was obtained by acid-base titration experiment. Then, their surface acid-base properties were analyzed, and surface acidity was caculated. Adsorption test was carried out to determine the adsorption equilibrium datas at different pH. The relationship between surface acidity and adsorption isotherm parameters was analyzed, and the related properties of three kinds of adsorbents were compared in this paper. For unmodified activated alumina adsorbent, the studies have shown that its pHzpc was8.8, and the surface of adsorbens had both acid and alkali properties. Surface acidity increases with the decrease of pH value, and adsorption isotherm data was in accord with the Freundlich model. Model parameter k, which presented adsorption capacity, added with the increase of surface acidity, and at the same time, n decreased, which stated that adsorption strength between adsorbent and adsorbate became enhanced with the increase of surface acidity. Adsorption distribution coefficient Kd was gradually reduced with the increase of the equilibrium concentration and pH, and then tended to be stable, which showed that the migration ability of fluoride ion to the surface of adsorbent was more and more weak.For two kinds of modified adsorbent, the concentrations of metal salt solution were0.5%,1.0%,1.5%and2.5%respectively. Results have shown that pHzpc of adsorbent modified by aluminum sulfate were6.3,4.9,4.3and6.3respectively, and pHzpc of adsorbent modified by ferric t were6.5,5.5,4.5and6.5respectively. Surface acid properties strengthened with the increase of metal ion load firstly, and then it tended to the same. Surface acidity increased with the decrease of pH value, and the relationship of modified adsorbent between adsorption isotherm parameters and surface acidity had the similar trend with that of unmodified activated alumina adsorbent. Also, for each kind of modified adsorbent, their adsorption distribution coefficient Kd was gradually decrease with the increase of the equilibrium concentration and pH, and tend to be balance. At low concentration, fluoride ion had the the strong migration ability to the surface of adsorbent.By comparing three kinds of absorbents, we found that pHZPC of modified adsorbents were both smaller than that of unmodified adsorbent, which was the result of adsorption of metal ions on the surface. Surface acid properties of the modified adsorbent were greater than unmodified adsorbent. At the same concentration, Surface acid properties of adsorbent modified by aluminum sulfate were stronger than adsorbent modified by ferric sulfate, and after calculation, surface acidity of aluminum sulfate modified adsorbent was larger than that of ferric sulfate modified. At the same time, the capcacity of fluoride ion on aluminum sulfate modified adsorbent was higher than that on ferric sulfate modified adsorbent. For different adsorbents, under the same surface acidity, adsorption capacity was always equal, which showed that the surface acidity is a main factor affecting the adsorption capacity. |
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