A novel TiO2/Fe0 composite as water treatment chemical: its synthesis, characterization and mechanism of photocatalysis
|關鍵字:||可見光奈米級二氧化鈦/零價鐵複合材料;偶氮染料;二氧化鈦;零價鐵;NTFC;azo dye;Titanium dioxide (TiO2);zerovalent iron (ZVI)|
|摘要:||零價鐵與二氧化鈦已被廣泛應用於還原與氧化有機污染物，零價鐵之主要限制為長期操作後，表面包覆氫氧化鐵氧化層，使得還原力變弱，二氧化鈦應用之主要限制為電子與電洞再回覆現象。本研究擬以二氧化鈦產生之標準電位差(-0.2 to 3.0 eV)延緩零價鐵表面氧化層之快速形成，並利用鐵為電子接受者，延緩二氧化鈦電子與電洞再回覆現象，依此理論開發新型奈米級二氧化鈦/零價鐵複合材料(nanoscale TiO2/Fe0 composite, NTFC)作 為水處理藥劑。本論文包括NTFC之製備、NTFC處理雙偶氮染料 (Acid Black-24, AB-24)、利用共反應法UV/NTFC/H2O2 提升總有機碳之去除、冷陰極管(CCFL)與可見光發光二極體(VIS-LED)作為光源之測試、並探討NTFC反應機制與AB-24降解之機制。
經由電子順磁共振光譜(EPR)與電化學分析法(EIA)證實，NTFC確實可以延緩零價鐵表面氧化與二氧化鈦再回覆現象，並且可以維持反應系統之二價鐵高濃度及產生雙氧水與自由基進行Fenton反應，進而形成TiO2結合奈米核-殼材料(TiO2-iron oxide/Fe0 core-shell)。本研究所提出之NTFC作用機制有三階段:第一階段為強還原反應，第二階段為氧化反應，第三階段為核-殼效應與賈法尼電池效應。本研究並測出NTFC處理含AB-24廢水的中間產物，進一步證實NTFC之作用機制。一開始主要為N=N斷鍵造成發色團與助色團快速脫色，將AB-24分解成小分子有機物，未進一步礦化之有磺酸與N=N官能基之分子，則吸附於零價鐵表面。|
A novel TiO2/Fe0 composite as water treatment chemical: Zerovalent iron (ZVI) and titanium dioxide (TiO2) have been considered potent reagents in the degradation of organic pollutants. However, the diminishing reactivity with time has obstructed their wide applications, due to the building-up of oxide layer on the surface of ZVI and the electron-hole recombination in TiO2. This study proposed to used the reduction potential of photocatalytic TiO2 (-0.2 to 3.0 eV) to retard the formation of the surface oxide layer and to retard the electron-hole recombination phenomena in TiO2. By this theory, a novel TiO2/Fe0 composite (NTFC) has been developed. This study is comprised of the synthesis of the NTFC, the testing and optimization of NTFC reactivity efficiency by using Acid Black-24 (AB-24) as the target compound, the enhanced degradation of TOC by the compound UV/NTFC/H2O2 system, the choice of light source in the UV/VIS/NTFC system, and the mechanisms of NTFC. The nanoscaled neutral TiO2 sol and nanoscaled zerovalent irons (nZVI) were prepared separately. Surface characterization confirmed that the NTFC is a composite of TiO2 and ZVI with partial core shell structure. The composite is with high specific surface area and microporous structure, with strong binding between TiO2 and ZVI. Moreover, The NTFC which has good strong binding structure strength can be recycled by magnets. Results showed that NTFC was highly efficient in degrading AB-24. The NTFC of highest reactivity was obtained when the TiO2 and Fe0 ratio was 1 to 10. And the optimal pH for reaction was discovered between 3 and 7. Experimental result with color removal of AB-24 showed that the NTFC can be excited by visible lights with the possibility of combining cold cathode fluorescent light (CCFL) and light emitted diode (LED). Tests by the combined UV/NTFC/H2O2 system indicated that the color of the AB-24 containing wastewater was removed by the Fe0 in the beginning and the TOC was degraded by UV/H2O2. The results from the analysis of Electron Paramagnetic Resonance (EPR) and Electrochemical impedance spectroscopy (EIS) confirmed that the NTFC can retard the formation of the oxide layers on the Fe0 and the recombination of electron-hole in TiO2. High concentration of ferrous ions can be maintained in the reaction, and the hydrogen peroxide and free radicals can be produced resulting in Fenton reaction. The TiO2-iron oxide/Fe0 core shell was formed, which resulted in the adsorption of pollutants and the Galvanic cell effect. The mechanism of NTFC reaction can be summarized as: (1) strong reduction in the first stage, (2) oxidation in the second stage, (3) adsorption on the core-shell and Galvanic cell effect in the final stage. The reaction of AB-24 in the NTFC began with the N=N bonds breaking which released the chromophores and auxochromic groups and resulted in decoloration. The moieties from AB-24 bond breaking containing functional groups such as sulfonic acid and N=N bonds were adsorbed on the surface of Fe0 ready for further treatment.
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