| 摘要: | 以數種化學修飾處理法改良銀合歡(Leucaena lucocephala)木質纖維質吸附能力，確實可提升銀合歡木質纖維質吸附去除水中鉛及鎘離子(Pb2+及Cd2+)量。化學修飾處理法包括磷酸化修飾、檸檬酸化修飾、磺酸化修飾、硫代硫酸鈉修飾及甲醛化修飾作用。
針對影響吸附劑吸附金屬離子量之各項特性進行探討，包括pH值、接觸時間、反應溫度、重金屬離子種類、受其他離子吸附競爭及脫附再生率等。各種吸附劑於最佳操作條件下對Pb2+及Cd2+吸附效能依序為：經磷酸化修飾(91.8 mg-Pb2+/g、61.2 mg-Cd2+/g ) > 經磺酸化修飾(46.2 mg-Pb2+/g、18.4 mg-Cd2+/g) > 經檸檬酸化修飾(43.8 mg-Pb2+/g、16.4 mg-Cd2+/g ) > 經甲醛化修飾(41.0 mg-Pb2+/g、19.2 mg-Cd2+/g) > 經硫代硫酸鈉修飾(30.8 mg-Pb2+/g、12.4 mg-Cd2+/g) > 未經修飾化吸附劑(16.8 mg-Pb2+/g、10.4 mg-Cd2+/g)。對Pb2+及Cd2+最佳吸附效果pH值相當接近約為6.5。吸附劑約在吸附60分鐘後可達穩定平衡吸附。
各種吸附劑對Pb2+及Cd2+離子的等溫平衡吸附實驗數據以Freundlich equation、Langmuir equation及Dubinin–Radushkevich equation三種等溫吸附平衡模式套用，可計算各種等溫吸附平衡常數及判斷屬於物理性或化學性吸附行為。以Pseudo first–order rate equation 、 Pseudo second–order rate equation 及 Intraparticle diffusion equation三種吸附動力模式進行評估，發現動力吸附行為在不受溫度、金屬離子濃度及金屬離子相互競爭之影響，以Pseudo second–order rate equation 最適於描述吸附動力行為。由計算出之吸附熱力學特性參數（標準自由能、焓值、及熵值之變化）顯示吸附反應溫度從15℃增至60℃時，吸附Pb2+反應為放熱之自發性反應，對吸附Cd2+則為吸熱之自發性反應。受雙離子相互競爭時Pb2+吸附競爭力大於Cd2+，並隨著混合金屬離子濃度升高，Pb2+競爭性也會增強。
The use of five chemical modification processes, including phosphorylation, sulfonation, sodium thiosulfate, citric acid,and formaldehyde modification, to modify the lignocellulosic substrate of Leucaena lucocephala into adsorbent which could actually enhance the removal efficiency of metal ions (Cd2+ and Pb2+) from aqueous solution.
Parameters that may affect the metal ions adsorption efficiency including solution pH, contact time, kinds of metal ions, initial metal concentration, competition metal ions and reaction temperature were conducted. The best adsorption efficiencies of Cd2+ and Pb2+ with adsorbents at optimal operation condition were sequenced as phosphorylation (91.8 mg-Pb2+/g, 61.2 mg-Cd2+/g ), sulfonation(46.2 mg-Pb2+/g, 18.4 mg-Cd2+/g), citric acid (43.8 mg-Pb2+/g, 16.4 mg-Cd2+/g ), formaldehyde modification (41.0 mg-Pb2+/g, 19.2 mg-Cd2+/g), sodium thiosulfate(30.8 mg-Pb2+/g, 12.4 mg-Cd2+/g), and nonmodified adsorbents (16.8 mg-Pb2+/g, 10.4 mg-Cd2+/g). The adsorption capacities of Cd2+ and Pb2+ depended on solution pH. The maximum adsorbed capacity of both heavy metals happened at pH 6.5. The adsorption equilibrium reached after 60 minutes of contact time with batch kinetic studies of each metal ion and different adsorbents.
The experimental isothermal equilibrium data were evaluated with Freundlich, Langmuir, and Dubinin–Radushkevich equation isotherm models to calculate the isothermal adsorprtion constants and to decide the adsorption behaviour being chemical or physical reaction. Three adsorption kinetic models, Pseudo first–order rate equation, Pseudo second–order rate equation, and Intraparticle diffusion equation, were used to evaluate the experimental data. Under the effects of reaction temperature, initial metal concentration and competition metal ions, Pseudo second–order rate equation model best fitted to experimental data. The thermodynamic parameters (enthalpy change, free energy change, and entropy change) for both metal ions adsorption into six adsorbents with increasing temperature from 15 to 60 ℃ indicated that the Pb2+ adsoption was exothermic and feasible, and the Cd2+ adsorption was endothermic and feasible. The competition of binary heavy metal ions into six kinds of adsorbents also studied. The binding strength of Pb2+ was stronger than Cd2+ and increased with the initial heavy metal ions concentration. |
