Catalytic oxidation of arsenite by zero-valent metals in the presence of polyoxometalate

地下水或表面水體中的砷(As)對人體的不利影響已是全球關注的議題之一，尤其是毒性高的As(III)，過去已有許多氧化或移除As(III)的方法，而其中又以Fenton反應最受到重視，其主要是利用Fe(Ⅱ)與過氧化氫在酸性條件下，產生具有強氧化能力的氫氧自由基(‧OH)來氧化As(Ⅲ)。此外，最近也有以零價鐵粉(ZVI)在酸性環境下做為Fenton反應的電子源，雖然在通氣下其亦具有不錯的氧化能力，但由於鐵粉較不穩定，故實際操作及應用上仍有其缺陷。本實驗擬藉由零價鋁片(ZVAl)做為最初電子來源，藉以營造較佳的氧化條件，因ZVAl具有比ZVI更高的氧化還原電位及在環境中較穩定等優點，而前人研究亦證實，ZVAl於低pH及通氣的環境下也可進行Fenton-like反應產生OH radical，故此系統應也具氧化As(III)之能力，因此，本研究除了以ZVAl取代ZVI作為H2O2 (Fenton反應之前趨物)來源，瞭解ZVAl對As(Ⅲ)的氧化外，亦將嘗試添加Fe(Ⅱ)來增進Fenton反應之效率，並促進As(Ⅲ)的氧化。然而，與ZVI一樣，ZVAl表面常含有一層金屬氧化層而減緩甚或抑制其反應速率，故一種聚合金屬鹽類Polyoxometalates (POM, 其為極強的Bronsted 酸)亦將被利用於ZVAl系統中，來移除ZVAl表面氧化層並促進ZVAl系統中之電子轉移效率，並進而增強此系統對As(Ⅲ)的氧化，此外，亦將探討POM回收再利用之可行性，及如何利用生活中常見之鋁罐代替ZVAl進行As(III)之氧化移除。 結果顯示，在無POM系統中，未經6 M HCl清洗表面之ZVAl/O2系統在180 min反應時間內均無法將As(Ⅲ)氧化，而ZVAl/Fe(Ⅱ)/ O2系統在pH 1.0及2.0分別於90及195 min內即可氧化80μM As(Ⅲ)。當加入POM後，ZVAl/O2系統在180 min內可將20μM As(Ⅲ)氧化，而ZVAl/Fe(Ⅱ)/ O2系統在pH 1.0及2.0分別於75及120 min則可將80μM As(Ⅲ)氧化，代表POM可促進ZVAl表面的氧化層溶解並提升電子轉移之效率。當As(Ⅲ)完全氧化後，將各反應之溶液pH值提升至6.0，Fe/Al會發生沉澱、吸附並同時移除溶液中的As(Ⅴ)，然而，於含POM系統下，POM會因為pH的上升而解離為不同型態，且會與As(Ⅴ)競爭Fe/Al氧/氫氧化物的吸附位置，故無法完全移除As，此為將來要利用POM於ZVAl系統來處理As時要解決的一項缺點。酸化於pH 6解離之POM後，因部分POM則會以催化能力略低之型態如[PW11O39]7-存在，故以回收之POM來進行ZVAl系統催化氧化As(Ⅲ)的能力會略為下降。市面回收的鋁罐，經簡單的處理後進行80μM As(III)的氧化，發現其於通氣的環境下在30 min內可氧化72% 的As(III)，當加入Fe(Ⅱ)後，同濃度的As(Ⅲ)可於30 min內完全氧化，此證實鋁罐中的Al具有短時間內氧化As(III)的能力，故其或許將來可應用於未開發國家進行飲用水中As氧化移除的一個簡便的技術。Arsenic has received much scientific concerns because it is toxic and commonly found in many water bodies, which may serve as drinking or irrigation water. Inorganic forms of arsenic, including As(III) and As(V), are carcinogenic, and they are considered more toxic than organic arsenic. Thus, the developments of some treatment technologies, such as Fenton or Fenton-like reactions, in the elimination of inorganic As from waters are desperate for decreasing its hazard to ecosystems. Zero-valent iron (ZVI) can serve as a precursor to initiate the Fenton reactions upon its oxidation under an aerobic environment. Fenton reaction induced by ZVI would produce strong oxidants for converting As(III) to As(V), which is then adsorbed or co-precipitated with the Fenton products of iron-hydroxides upon pH adjustments. In the current study, zero-valent aluminum (ZVAl) instead of ZVI were used to provide a better efficiency for As(III) oxidation because ZVAl/O2 system can induce a Fenton-like reaction and ZVAl exhibits a higher reduction potential than that of ZVI. Polyoxometalets (POM) was applied to accelerate As conversion in the ZVAl/O2 system because it can serve as a bronsted acid and an electron shuttle for removing oxide layers on ZVAl and promoting electron transfer among the reactants, respectively. Results showed that no discernible changes in As(III) concentration were observed when As(III) was reacted with ZVAl for 180 min under an aerobic solution even at pH 1. However, with the addition of 1 mg/L Fe(II) or 0.1 mM POM (check the validity of the values), 80 mM As(III) would be rapidly oxidized by ZVAl at pH ≦ 2 because a Fenton-like and catalytic reaction were promoted by Fe(II) and POM, respectively, on the ZVAl surfaces. Upon As(Ⅲ) oxidation, the solutions containing As(Ⅴ), Fe(Ⅲ), Al(Ⅲ) or POM was adjusted to pH 6.0 to co-precipitate As(V), Fe(III), and Al(III). In the meantime, POM was decomposed to low molecular weight anionic cluster such as [PW11O39]7-. Thus, the decomposed form of POM can be separated from the solid phases and be recycled after proceeding an acidification process for further use. Aluminum can could substitute ZVAl as a cheap electron donor for promoting As(Ⅲ) oxidation. It was found that 72% of added As(III) (ca. 80 mM) would be oxidized by ZVAl/O2 system within 120 min at pH 1.0, and 80 mM As(III) could be completely oxidized by ZVAl/Fe(Ⅱ)/O2 system within 30 min with the same experimental conditions.謝誌................................................................................................................................i 中文摘要...................................................................................................................... iii Abstract.........................................................................................................................v 目錄...........................................................................................................................vii 圖次..............................................................................................................................xi 表次.............................................................................................................................xiv 第一章 緒論................................................................................................................1 1-1 研究動機.............................................................................................................1 1-2 研究目的.............................................................................................................2 第二章 文獻回顧..........................................................................................................3 2-1 砷之特性.............................................................................................................3 2-1-1砷的基本性質...............................................................................................3 2-1-2砷的來源.......................................................................................................3 2-1-3砷的環境化學...............................................................................................4 2-1-4 砷的毒害.....................................................................................................8 2-1-5砷對植物的有效性.....................................................................................10 2-1-6 砷的汙染現況...........................................................................................10 2-1-7 砷的移除方法...........................................................................................11 2-2零價金屬之應用................................................................................................13 2-2-1零價鐵(ZVI)在環境上之應用...................................................................13 2-2-2零價鐵對砷之移除研究.............................................................................14 2-2-3零價鋁之應用.............................................................................................15 2-3Polyoxometalates之特性....................................................................................17 2-3-1 Polyoxometalates之催化應用...................................................................17 2-3-2 Polyoxometalates於暗反應下之氧化還原...............................................18 第三章 材料與方法....................................................................................................20 3-1 研究架構...........................................................................................................20 3-1-1 ZVI、ZVAl、ZVAl + Fe(Ⅱ)三種系統催化氧化As(Ⅲ)之比較...........21 3-1-2 Fe(Ⅱ)存在與否下，HNa2PW12O40(POM)催化零價鋁(ZVAl)氧化三價砷.............................................................................................................22 3-1-3 HNa2PW12O40(POM)催化氧化能力之探討...........................................23 3-1-4 HNa2PW12O40之回收再利用評估..........................................................24 3-2實驗試劑，器材與儀器.....................................................................................25 3-2-1 催化氧化實驗之試劑與材料...................................................................25 3-2-2 H2O2濃度之測定(比色法) ........................................................................25 3-2-2-1 三次蒸餾水之製備(用於H2O2之定量) ............................................25 3-2-2-2 H2O2之定量與配置............................................................................25 3-2-2-3 DPD比色法試劑之配製.....................................................................26 3-2-2-4 H2O2測定流程....................................................................................26 3-2-3 二價鐵比色法...........................................................................................26 3-2-3-1 二價鐵比色法試劑之配製................................................................26 3-2-3-2 Fe(Ⅱ)測定流程..................................................................................27 3-2-4 氫化產生器-原子吸收光譜法測砷..........................................................27 3-2-4-1氫化產生器-原子吸收光譜法測砷之試劑.........................................27 3-2-4-2 As(Ⅲ)之測定......................................................................................27 3-2-5 反應之氣體...........................................................................................28 3-2-6 實驗器材與儀器...................................................................................28 3-3 實驗方法步驟...................................................................................................29 3-3-1 ZVI、ZVAl、ZVAl + Fe(Ⅱ)三種系統催化氧化As(Ⅲ)之比較..........29 3-3-1-1 ZVI於酸性環境下催化氧化As(Ⅲ) .................................................29 3-3-1-2 ZVAl於酸性環境下催化氧化As(Ⅲ) ..............................................29 3-3-1-3 ZVAl與Fe(Ⅱ) 於酸性環境下催化氧化As(Ⅲ) ..............................29 3-3-2 HNa2PW12O40(POM)催化ZVAl與二價鐵氧化三價砷.........................29 3-3-2-1 含POM之系統下ZVAl對As(Ⅲ)之氧化......................................29 3-3-2-2 含POM、ZVAl與Fe(Ⅱ)系統對As(Ⅲ)之氧化................................30 3-3-2-3 ZVAl於酸性環境下催化氧化產生H2O2之影響.............................30 3-3-2-4 POM對ZVAl於酸性環境下催化氧化產生H2O2之影響...............30 3-3-2-5 OH radical 產生之確認.....................................................................30 3-3-3 POM還原能力探討之實驗步驟...............................................................31 3-3-3-1 ZVAl還原POM..................................................................................31 3-3-3-2 還原態還原態POM(e-)於通氣條件下，氧化生成H2O2之濃度探討........................................................................................................31 3-3-3-3 還原態還原態POM(e-)與H2O2之反應............................................31 3-3-3-4 還原態還原態POM(e-)於通氣條件下，加入Fe(Ⅱ)對As(Ⅲ)的氧化........................................................................................................31 3-3-4 移除反應中砷及HNa2PW12O40回收再利用...........................................32 3-3-4-1 砷的移除............................................................................................32 3-3-4-2 含POM系統中砷的移除及POM之回收再利用............................32 3-3-4-3 鋁罐於酸性環境下催化氧化As(Ⅲ) ...............................................32 第四章 結果與討論....................................................................................................33 4-1 ZVI、ZVAl、ZVAl + Fe(Ⅱ)三種系統催化氧化As(Ⅲ)之比較.....................33 4-1-1 ZVI於通氧環境對As(Ⅲ)的氧化.............................................................33 4-1-2 ZVAl於通氧環境對As(Ⅲ)的氧化...........................................................36 4-1-3 ZVAl與Fe(Ⅱ)於通氧環境對As(Ⅲ)的氧化............................................39 4-2 HNa2PW12O40(POM)液相催化零價鋁(ZVAl)與添加之二價鐵氧化三價砷..42 4-2-1 POM催化氧化ZVAl氧化As(Ⅲ)..............................................................42 4-2-2 POM、ZVAl與二價鐵系統對As(Ⅲ)之氧化及POM於通O2下催化氧化ZVAl產生H2O2....................................................................................44 4-2-3 氧化劑之確認...........................................................................................50 4-3 POM催化氧化能力探討..................................................................................52 4-3-1 不同pH下POM催化ZVAl生成還原態POM(e-)...................................52 4-3-2 還原態POM(e-)於通O2系統中生成的H2O2濃度測定與添加Fe(Ⅱ)氧化As(Ⅲ) ..................................................................................................58 4-4移除反應中砷及HNa2PW12O40回收再利用及鋁罐對As(Ⅲ)的氧化.................68 4-4-1 砷的移除及POM的回收再利用..................................................................68 4-4-2 鋁罐對As(Ⅲ)之氧化....................................................................................76 第五章 結論................................................................................................................78 參考文獻......................................................................................................................8