光電催化結合離子交換薄膜程序併同處理六價鉻與羧酸之研究

Synergistic Effects of Chromium(VI) Reduction and Carboxylic acid Oxidization by Photoelectrocatalysis Combined Ionic Exchange Membrane Processes

徐宏德(Hung-Te Hsu)

指導教授 ： 陳孝行

國立臺北科技大學/工程學院/工程科技研究所/博士(2013年)

摘要

本研究研發光電催化觸媒結合離子交換薄膜程序之處理技術，即以結合光電催化觸媒與陽離子薄膜程序併同處理六價鉻與羧酸及其反應機制之研究，研究內容為：以連續式光電催化觸媒結合陽離子交換薄膜併同處理六價鉻與EDTA之反應機制；不同羧酸基團數之電洞捕捉劑(EDTA(4)、檸檬酸(3)、水楊酸(1))於六價鉻還原效率之影響；比較自行製備改質含氮二氧化鈦(N-TO2)與商用二氧化鈦(DP-25 TO2)於併同處理六價鉻與奈磺酸反應機制探討；另擇一還原型污染物(硝酸鹽)於光電催化觸媒處理程序之還原效率與反應機制探討。
以光電催化觸媒結合陽離子交換薄膜併同處理Cr(VI)與EDTA，兩者皆可有效提升Cr(VI)還原與EDTA氧化之效率；且隨著電流密度增加，Cr(VI)與EDTA處理效率隨之提升，於電流密度為4.0 mA/cm2可達到最佳Cr(VI)與EDTA處理效率。Cr(VI)與EDTA之處理效率於酸性條件下優於鹼性條件；隨著水力停留時間增加，Cr(VI)與EDTA處理效率愈佳，於HRT 4 hr為此處理程序之臨界水力停留時間；光照強度愈大，光子數目越多，對於Cr(VI)與EDTA處理效率也愈高。
選用三種不同羧酸基數之電洞捕捉劑(EDTA(4)、檸檬酸(3)及水楊酸(1))於光電催化觸媒結合陽離子交換薄膜還原六價鉻之探討，電洞捕捉劑的羧基團數目越多，在酸性條件下帶正電的TiO2能吸附更多的羧基團於TiO2表面上，使表面羧基團吸附密度增加，能有效捕捉電洞而加速與羧酸進行氧化反應，以及提高光電催化的電子-電洞對量子產率，進而有較多電子與Cr(VI)進行還原反應。羧酸基數對於光電催化還原Cr(VI)處理效率：EDTA(4)＞檸檬酸(3)＞水楊酸(1)。
以高溫煅燒法製備N-TiO2光觸媒吸收波長於可見光(λmax為419 nm)，與商用DP-25 TO2光觸媒吸收波長於紫外光(λmax為254 nm)比較併同處理六價鉻與奈磺酸鹽之探討，應用N-TiO2光觸媒於六價鉻還原效率可提升12.5%，且光電催化反應時間可持續達62小時之六價鉻還原效率(95%以上)，較商用DP-25 TO2光觸媒之光電催化反應時間達48小時為較長之時效性。而且電洞捕捉劑(奈磺酸鹽)之硫元素，藉由光電催化觸媒氧化程序，將會氧化解離為硫酸根離子(SO42-)，並推算(NS as S)與(SO42- as S)之硫元素(S)轉換率趨近為100%，即(NS as S)與(SO42- as S)之S呈現質量平衡。
選擇另一還原型污染物(硝酸鹽)於光電催化觸媒處理程序之還原效率探討，由結果得知，以DP-25 TO2光觸媒之光電催化處理程序還原硝酸鹽處理效率較佳，且光電催化反應時間可持續達22小時之六價鉻還原效率(85%以上)。再者，選用四種電洞捕捉劑比較光電催化還原硝酸鹽處理效率：甲酸＞EDTA＞檸檬酸＞水楊酸；推測乃是甲酸與電洞反應形成還原力強之 CO2•-所致，可加速並提升硝酸鹽還原效率以及選擇性氮氣產量。於酸性條件下，硝酸鹽還原效率以及選擇性氮氣產量皆較鹼性條件佳；隨著水力停留時間增加以及光照強度愈大，對於硝酸鹽還原效率以及選擇性氮氣產量也愈高。

關鍵字

六價鉻 ； 光電催化 ； 離子交換薄膜 ； 電洞捕捉劑 ； 硝酸鹽

並列摘要

A novel technology of photoelectrocatalysis (PEC) combining with cationic exchange membrane (CEM) was proposed for simultaneous reduction of chromium(VI) and oxidization of carboxylic acid. The objectives of this study were focused on: heterogeneous photoreduction of Cr(VI) in the presence of EDTA; effect of carboxyl groups number (EDTA (4), citric acid (3) and salicylic acid (1)) on the photoreduction of Cr(VI); photoreduction of Cr(VI) over different photocatalysts (DP-25 TiO2 and nitrogen-doped TiO2), and comparison photoreduction of Cr(VI)/ nitrate.
The application of CEM was used to enhance the efficiency for prevention of the re-oxidation of reduced chromium with the electron-hole pairs. The results showed that the optimum conversion efficiency occurred at 4 mA/cm2 with the presence of CEM. Higher conversion efficiencies were observed at lower pH due to the electrostatic attractions between positive charged Ti-OH2+, and negatively charged Cr(VI) and EDTA. The optimum TiO2 loading of 1 g/L was depended mainly on the acidic pH range, especially at higher HRT and irradiation intensity. In addition, higher EDTA/Cr(VI) molar ratio enhanced the reduction efficiency of Cr(VI).
Carboxylic acids (EDTA, citric acid and salicylic acid) generally experienced positive effects, deriving from their adsorption on TiO2 and susceptibility to photogenerated hole attack. The reduction of Cr(VI) was greatly enhanced under acidic pH, higher current density, HRT, light intensity and EDTA/Cr(VI) molar ratio. Furthermore, Cr(VI) reduction with various carboxyl group number in the presence of different carboxylic acids increases in the following order: 1 (SA) < 3 (CA) < 4 (EDTA).
Comparison photoreduction of Cr(VI) by using DP-25 TiO2 with UV light and N-TiO2 with visible light in PEC combined CEM was proposed for simultaneous oxidization of naphthalenesulfonate (NS) and reduction of Cr(VI) by electron-hole pairs. The results shows, both VL/N-TiO2 and UV/DP-25 TiO2 system were capable of reducing Cr(VI) and NS simultaneously, the photocatalytic activities of N-TiO2 under VL irradiation were improved than that of DP-25 TiO2 under UV irradiation. Moreover, longer exhaustion time for N-TiO2 (around 62 hours) compared to that of the DP-25 TiO2 under UV irradiation (around 48 hours). Complete Cr(VI) reduction was observed for pH 3 with N-TiO2 of 1 g/L, and initial NS concentration up to 50 mg/L, indicating NS plays the role of hole scavenger in the system. The generation of sulfate (as sulfur) is equivalent to the degradation of NS (as sulfur), representing the yield of sulfate is an excellent indicator for NS removal.
Comparison various oxidants (nitrate) in PEC combined CEM systems had been estimated in this study. The optimal process of nitrate reduction is by using DP-25 TiO2 under UV irradiation with aeration by argon. When the addition of formic acid was demonstrated to act as hole scavengers in the TiO2 surface to accelerate nitrate reduction. Under the same reaction condition, the reactivity of nitrate reductions by using PEC can be ranked as follows: formic acid > EDTA > CA > SA. Meanwhile, the reduction of nitrate and the selectivity to nitrogen were greatly enhanced under acidic pH, higher HRT and light intensity.

並列關鍵字

Hexavalent chromium ； Photoelectrocatalysis ； Ion exchange membrane ； hole scavenger ； Nitrate

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