鐵/亞鐵氰化錯合離子在氧化鋁表面吸附特性之研究

Abstract

摘要 氰化物為廣用之工業原料，於電鍍、選礦、照相、冶金、合成纖維及工業氣 體的清洗等產業均有氰化物廢水的產生。雖然氰化物在自然界的存在量不多，但它卻是列 為嚴格管制的劇毒化學品。工業界所謂的「氰鹽」包含簡單自由氰鹽與金屬錯合氰鹽。前 者係以CN-或HCN存在，對人體而言，致死量約為0.2~0.3克，毒性非常大，後者係指氰離 子與金屬離子配位鍵結後形成之錯合物。本研究使用之鐵氰化錯合離子(Ferricyanide, F e(CN)63-)及亞鐵氰化錯合離子(Ferrocyanide, Fe(CN)64-)，均屬較安定且毒性較低之錯 合物，但容易行照光反應重新分解釋放出劇毒之HCN。另一方面 本研究乃利用氧化鋁(g- Al2O3)作為吸附劑，改變溶液pH值、溫度與吸附質濃度，透過批次式實驗，以期能瞭解鐵 /亞鐵氰化錯合離子在g-Al2O3表面之吸附行為。並嘗試以三層模式預測吸附反應之平衡式 與固有平衡常數值(Kint, Intrinsic Constant)，或以考慮過質子競爭效應之修正型Lang muir吸附模式探討外觀平衡常數值(Kapp, Apparent Constant) 。藉由這兩種不同模式之 探討，期望能夠清楚該吸附反應之反應機構。 實驗結果顯示，g-Al2O3對鐵/亞鐵氰化 錯合離子的吸附百分比均隨溶液pH值及溶質濃度增加而減少。經由三層模式之模擬，顯示 g-Al2O3吸附Fe(CN)63-或Fe(CN)64-之表面反應機構均以形成AlOH2+--Fe(CN)63- 及AlOH2 +--HFe(CN)63- 之外層吸附反應假設式最能符合實驗結果。從能量光譜儀(EDS)與紅外光 譜儀(FTIR)之光譜鑑定也得到同樣外層吸附之證據。另一方面，以Langmuir模式求得之外 觀平衡常數值來比較三層模式之固有平衡常數值，發現兩者之間具有強烈之相關性，這也 印證了外層吸附反應的假設。 由於修正型Langmuir吸附模式能正確地獲得不同溫度及pH 條件下之外觀平衡常數。因此，應用Gibbs-Helmholtz方程式便能求得吸附反應之焓變化( Enthalpy Change, DHo)。結果顯示g-Al2O3對鐵/亞鐵氰化錯合離子的吸附過程均屬放熱 反應，且焓的變化DHo值約為-5 Kcal/mole。除了焓的變化外，本研究也嘗試探討其他反 應熱力學參數，結果發現吸附反應過程熵的變化(Entropy Change, DSo)均大於零，自由 能變化(Free Energy Change, DGo)也均為負值。顯示g-Al2O3對鐵/亞鐵氰化錯合物的吸 附過程屬於亂度增加且自發性的吸附反應過程。 利用等溫吸附實驗數據配合Langmuir模 式，計算在25℃、pH6及背景電解質濃度為0.01N時之最大吸附量(Cmax)，發現鐵氰化錯合 離子分別吸附在高嶺土(Kaolinite)、氧化鋁(g-Al2O3)及水合性氧化鐵(Fe(OH)3)表面的C max值分別為3.1*10-8、4.2*10-7及5.0*10-7 mol/m2。而亞鐵氰錯合離子則為5.8*10-8、 6.3*10-7及1.1*10-6 mol/m2。結果顯示Cmax值大小與這三種吸附劑的吸附位置密度(表面 氫氧官能基密度)大小排列順序相同，即Fe(OH)3>g-Al2O3> Kaolinite。 因為鐵/亞鐵氰 化錯合離子均是高電荷數之離子，對於自然水體中存在之濁度，具有一定程度的混凝效果 。因此本研究以不同價數離子為混凝劑進行混凝去穩定機構探討，觀察到磷酸根離子對g- Al2O3人工濁水混凝去穩定所需添加劑量與Cl-、SO42-、Fe(CN)63-及Fe(CN)64-的混凝實 驗明顯不同，得知Ferrocyanide與Ferricyanide離子在g-Al2O3表面反應機構可能為外層 吸附。另外依據Schultz-Hardy理論，本研究以90%的濁度去除率為例，發現Cl-、SO42-、 Fe(CN)63-及Fe(CN)64-四種不同價數陰離子之混凝所需劑量對數值與Schultz-Hardy理論 劑量之自然對數值之間存在良好之線 ABSTRACT Cyanide compound is commonly found as a waste water contaminant on s everal industrial processes such as electroplate, mining, photographic, metall urgical, synthetic fiber and industrial gas purification and so on. Although c yanide compound exists in small amount in the nature system, it is tightly con trolled and highly toxic chemical material. "Industrial cyanide salt" includes simple free cyanide and metal complex cyanide, the former exists in the form of CN- and HCN. Free cyanide is highly toxic, the le This proposed research p roject aims at the study of adsorption behavior of ferricyanide and ferrocyani de on the aluminum oxide (g-Al2O3) surface as a functions of pH and concentrat ion with batch experiments. Attempts to describe and predict the adsorption of ferrocyanide/ferricyanide by aluminum oxide particle will be treated surface sites as homogeneous groups with intrinsic constant using a triple-layer mode l or with a single adsorption constant incorporating proton competition effect using a modified The experiment results indicated, the adsorption percentag es of ferricyanide and ferrocyanide on g-aluminum oxide decreased with increas ing pH and initial concentration of ferricyanide and ferrocyanide. The triple- layer model was used for simulating ferricyanide and ferrocyanide adsorption o n g-aluminum oxide surface. Then, AlOH2+--Fe(CN)63- and AlOH2+--HFe(CN)63- hav e been shown with outer-sphere reaction products to be more agreeable with exp erimental adsorption observation than complexation with other The modified L angmuir isotherm model developed herein can accurately predict the apparent eq uilibrium constant at different pH values and temperature for ferrocyanide and ferricyanide adsorption onto aluminum oxide surface. In the experimental pH r ange of 5~7.5, adsorption capacity and apparent constants decrease as the temp erature increases. According to the Gibbs-Helmholtz equation, these apparent c onstants at various temperature and pH conditions can be applied to determine enthalpy changes (DHo) for t By batch experiments, under the condition of pH 6.0, 25℃, electrolyte concentration 0.01M, the maximum adsorption capacity of kaolinite, aluminum oxide and hydrous iron oxide are 3.1*10-8、4.2*10-7、5.0* 10-7 mol/m2 for ferricyanide and 5.8*10-8、6.3*10-7、1.1*10-6 mol/m2 for ferro cyanide adsorption。This results shown a strong relationship between the value s of adsorption capacity and the site density of oxide surface. From there highly valence, the ferrocyanide and ferricyanide ion have strong coagulation ability in natural water system. In our study, a series of batch test with al uminum oxide turbid water were performed to evaluate the coagulation effective ness of anion with different valence. There are dramatic difference in the coa gulation abilities of simple ions (Cl-、SO42-、Fe(CN)63-、Fe(CN)64-), and othe r species that interact chemically with the oxide surface (H2PO4-). The eviden ces infer that the mechanism of