低濃度過氯酸根溶液在改質活性碳表面之吸附與電吸附特性研究

Abstract

本研究是針對含有低濃度的過氯酸根水溶液，以活性碳做為吸附劑，利用吸附原理將過氯酸根從水中移除，為了提升活性碳吸附的處理速率，研究中利用活性碳表面改質以及電吸附，提升活性碳的吸附效率與吸附量，同時藉由吸附與質傳模型的應用，試著求取模型參數，以瞭解活性碳吸附過氯酸根時的吸附特性。 首先探討活性碳的物性與電化學性質，利用SEM觀察活性碳表面的微觀情形，發現當活性碳預載CTAC分子(Cetyl-Trimethyl-Ammoninum Chloride，氯化十六烷基三甲基銨) ，CTAC分子均勻分散於表面並呈現棉花團狀，而無電鍍鎳則在表面形成一層緻密的鎳金屬化合物，顆粒大小約15-20 nm。經由表面電位的測量，可以發現在低濃度過氯酸根時，未處理前活性碳表面零價點為4.38，預載CTAC的活性碳表面則零價點為5.30，無電鍍鎳的活性碳表面零價點為5.97。利用循環伏安法做電位掃瞄，可以發現在 -1.0 V到 +1.0 V中，活性碳床電極並不會與溶液中的過氯酸鹽產生氧化還原反應，可做為電吸附參數使用。 利用Langmuir isotherm吸附模型可以瞭解活性碳過氯酸根溶液的平衡關係，當溶液中只存在過氯酸根時，活性碳在25°C的平衡吸附量為17.87 mg/g-GAC，當溶液溫度逐漸升高時，平衡吸附量則逐漸降低。當溶液中存在硫酸根離子時，會與過氯酸根產生競爭性吸附，使得平衡吸附量下降，當硫酸根離子濃度為0.1M時，平衡吸附量則下降到10.09 mg/g-GAC。當活性碳表面預載CTAC分子時，平衡吸附量提高到31.34 mg/g-GAC/CTAC，但活性碳表面無電鍍上鎳化合物時，平衡吸附量卻反向降低至16.50 mg/g-GAC/E-Ni。最後可求得活性碳對過氯酸根之吸附熱為–27.57 kJ/mol，屬於物理吸附。 在填充床操作上，利用濃度對時間的變化，配合pseudo-first-order動態吸附模型，可以得知吸附量與動態速率常數。當pH值控制在較低值時，因為較高的表面電位有助於動態平衡吸附量的增加；當水中存在硫酸根離子時，過氯酸根會受到競爭吸附而使得動態平衡吸附量下降；在不同表面組成時，以表面預載CTAC之活性碳的動態平衡吸附量最多；藉由施加正電位可以有效提升吸附效率及動態平衡吸附量，主要原因是因為正電位可利用靜電力吸引負離子加速靠近吸附表面，同時也利用靜電相吸的原理增加了吸附表面吸附過氯酸根的能力。因此利用活性碳吸附時，以低pH值、低同離子濃度、高活性碳表面電位及施加正電位，都將提升吸附效率與動態平衡吸附量。

The activated carbon was used as adsorbent to remove perchlorate anions from the solution in this study. In order to increase the efficiency of adsorption and adsorbed amount, the surface modification of activated carbon and electrosorption process were used. The characteristics of perchlorate adsorption by activated carbon had been investigated by proposed adsorption model. The SEM images showed that the CTAC molecules were deposited on the surface and the shape was cotton-like. The surface of E-Ni activated carbons was covered densely and the particle size were about 15-20 nm. The point of zero charge were investigated that 4.38 for granular activated carbon, 5.30 for CAC-preloaded activated carbon and 5.97 for E-Ni activated carbons. The Langmuir isotherm model was used for the equilibrium relation between the activated carbon and perchlorate anions. The saturated adsorbed amount of perchlorate was 17.87 mg/g-GAC which anion in the solution was perchlorate only. When 0.1 M of sulfate anions were in the solution, the equilibrium adsorbed weight was decreased to 10.09 mg/g-GAC because of competing adsorption. The saturated adsorbed amount of GAC/CTAC increased to 31.34 mg/g-GAC/CTAC. The heat of adsorption for perchlorate was estimated to be -27.57 kJ/mol and was classified as physical adsorption. In order to understand the kinetic adsorption phenomena, the pseudo-first-order kinetics adsorption model was used to characterize the circulating adsorption operations. Lower pH value induced a higher zeta potential of activated carbons and the adsorbed weight of activated carbons was increased from 5.616 mg/g-GAC to 6.246 mg/g-GAC. With the presence of 0.1 M sulfate anions, theadsorbed amount was decreased. The applied positive potential on the activated carbons could increase the adsorbed weight and kinetic adsorption rate constant effectively because the electrostatic attraction increase the moves of anions to surface. The study showed that low common anion concentration, higher zeta potential and positive electric potential applied could improve the adsorbed amount and decrease the adsorption time.