鐵/亞鐵氰化錯合離子於氧化鋁表面競爭吸附特性之研究

Abstract

過去競爭吸附研究的對象多為陰、陽離了或有機物。由於土壤中磷酸鹽的吸附量對植物生長的影響很大，因此早期的競爭吸附多以磷酸監與其他有機物為對象；近年來各式污染物充斥，傳統的水處理程序常無法滿足人們對水質的要求，因此利用活性碳優越的吸附能力，去除水中微量的污染物成為近年來的研究重點之一。然而，對於錯合離子的競爭吸附研究卻相當少見。本研究以鐵╱亞鐵氰化錯合物（Ferricyanide/ferrocyanide complexs）作為競爭吸附之研究對象。 氰化物為常見的電鍍工業的污染物，具有高毒性，排放至環境中易易地表中含量甚豐的鐵反應，形成鐵╱亞鐵氰化錯合物，而兩者在自然界中由於電子數的相互轉移，必然同時存在，因此有必要探究其競爭吸附行為。再者，選擇天然土壤中含量甚豐的r-氧化鋁作為吸附劑。本研究藉由等溫批次試驗，觀察鐵╱亞鐵氰化錯合物於r-氧化鋁表面之競爭吸附行為。並將過去文獻針對Ferricyanide及Ferrcyanide之個別吸附所發展的修正型Langmuir等溫式，應用於競爭吸附，計算競爭吸附之外觀平衡常數Kappp，再嘗試三層械進行模擬，求得固有平衡常數（Kint），經由兩種模式的比較，瞭解Ferricyanide/Ferricyanide於r-氧化鋁表面進行競爭吸附的反應的機構。 實驗結果顯示，帶負四價的Ferricyanide競爭力大於負三價之Ferricyanide，而兩者於r-氧化鋁表面之吸附百分比均隨溶液之PH值及錯合離子濃度增加而減少，與單獨吸附結果相同。 以等溫吸附實驗配合修正型Langmuir模式，求出Ferricyanide及Ferricyanide個別之最大吸附量（Cmax）及Kapp，且得出標準焓變化值為負，可知競爭吸附與單獨吸附系統的熱力學參數，得出標準變化值為正，即系統之亂度隨競爭吸附之進行而增加，與單獨吸附相同。 以HYDRAQL電腦程式進行三層模式之模擬，模擬值與實驗數據有極佳的相關性，除了求出競爭吸附之Kint，並得知Ferricyanide及Ferricyanide於競爭吸附系統，分別以ALOH2+--Fe(CN)63-、ALOH2+--HFE(CN)63-之形式於r-氧化鋁表面進行吸附，屬於外層吸附。另外，將吸附後的r-氧化鋁進行紅外光光譜分析，發現Ferricyanide及Ferricyanide的吸收鋒位置與未吸附的純固體樣品相同，顯示吸附後離子結構並未改變，可推論Ferricyanide/Ferricyanide之競爭吸附應是以外層吸附的形態進行。

Most laboratory studies of competitive adsorption have been focused on the behavior of anions, cations, and organic substances on soil or metal oxides. The earliest research was the competitive adsorption between phosphate and other organic compounds on soil because of the importance of phosphate in the growth of plants. Much attention has also been drawn to the competitive adsorption of pollutants on the activated carbon, an efficient adsorbent for trace contaminants in water. In this study, we chose ferricyanide and ferrocyanide as the adsobates due to the limited information on the competitive adsorpton of complex ions. Cyanide compounds is one of the common pollutants from the electroplating plants, which form ferricyanide and ferrocyanide complexs with the iron soon after reaching natureal environment. Both complex ions exist in equilibrium in nature because of the exchange of charge. therefore, it is important to understand the competitive adsorption of ferrocyanide and ferrocyanide. We chose one of the major metal oxides on the earth surfaces, r-Al2O3,as the adsorbent, because of its importance in transfering pollutants in mature soil. The goal of this study was to determine whether the adsorption occurred in the inner-sphere or the outer-sphere through the analysis of the thermodynamic parameters of the competitive adsorption between ferricyanide and ferrocyandie under various PH, temperature and ion condition with the modified Langmuir isotherm and the TLM. The experimental result shows that the adsorption percentages of ferricyandie and ferrocyanide on r-AL2O3 decreased whith both increasing PH and initial concentration of complex ions, a similar observation found on the individual adsorption. Ions with higher electric charge are always more competitive in adsorption. The modified Langmuir isotherm can accurtely predict the competitive adsorption, and then the apparent constant (Kapp) and maximum adsorption capacity can be derived. The thermodynamic parameters of the competitive adsorption can be derived using the Gibbs-Helmholtz equaton. The increase in entropy is the same as those on the individual adsorption. The intrinsic constant (Kint) was derived by simulating the Triple-Layer Model with HYDRAQL program. Comparing the experimental data and the simulation of TLM, we found that ferricyanide and ferrocyanide existed in AlOH2+--Fe(CN)63- and AlOH2+--HFe(CN)63- on the surface of r-Al2O3,which indicated the outer-sphere complexation. Infrared Spectroscopy (IR) was used to confirm the adsorption mechanism on the oxide surface. The result shows that the chemical composition of ferricyanide and ferrocyanide didn't change after the adsorption, which supports the theory of outer-sphere complexation.