| Summary: | 碩士 === 國立臺灣大學 === 環境工程學研究所 === 92 === The adsorption process applied in the removal of fluoride from aqueous solutions was investigated in this study. Commercial activated alumina (γ-Al2O3) and two novel magnetic alumina adsorbents, which were synthesized by precipitation method (MAP) and sol-gel method (MASG), were used as the alumina-type adsorbents. The physicochemical characteristics of three adsorbents and factors affecting the adsorption equilibrium and kinetics were further examined in this study.
The Langmuir and Freundlich isotherms were successfully used to predict the adsorption behavior of fluoride onto γ-Al2O3, MAP, and MASG, in which the monolayer adsorption capacity (qL) of MASG is the highest among three adsorbents. Furthermore, the linear isotherm also well described the adsorption behavior of fluoride onto both novel magnetic nano-adsorbents, MAP and MASG. Therefore, the novel magnetic nano-adsorbents, which were prepared by Fe3O4 coated with SiO2 and sequentially synthesized by means of precipitation and sol-gel methods in this study, can successfully be applied in the removal of fluoride from solutions. Regarding the adsorption kinetics of fluoride from solutions, among the tested kinetics models in this study (e.g. pseudo-first-order equation, pseudo-second-order equation and Elovich rate equation), both the pseudo-second-order equation and Elovich rate equation can well predict the adsorption kinetics of fluoride onto MASG in CFSTR.
The removal of fluoride by commercial Al2O3 was also carried out in the fixed-bed adsorber. The optimal operating conditions were the pH value of 4 for the fed fluoride solution, the regeneration agent (NaOH) of 0.01 N, and the neutralization pH of 4 by means of 3.16×10-4 N HNO3. Under the optimal conditions, the removal efficiency of fluoride from solutions was not reduced for at least fourteen runs. In addition, the fluoride adsorption capacity increased with decreasing pH of fluoride solution and can be divided into two parts: adsorbed by alumina and by aluminum hydroxide, in which the former might be the major mechanism in the whole adsorption process and the latter mainly influenced the adsorption in the plateau area.
The main composition and pH value of the aged scrubbing solution in the semiconductor manufacturing industries are fluoride of 40~ 60 mg/L, and 3.6, respectively. Therefore, on the basis of the data obtained from this study it is deduced that Al2O3, MAP and MASG possess high potential as the adsorbents for the application of adsorption process in the defluoridation from aged scrubbing solution.
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