環(huán)境專業(yè)外文翻譯-電氣化學促進tio2光催化降解有機廢水的方法

1、<p>　　中文3450字，2450英文單詞，1.4萬英文字符</p><p>　　出處：Chen J, Liu M, Zhang J, et al. Photocatalytic degradation of organic wastes by electrochemically assisted TiO2 photocatalytic system.[J]. Journal of Environme

2、ntal Management, 2004, 70(1):43-47.</p><p>　　電氣化學促進TiO2光催化降解有機廢水的方法</p><p><b>　　摘要</b></p><p>　　利用納米TiO2光催化降解有機廢水的研究已經(jīng)有很長的時間，其目的在于找出適合處理廢水的方法，但其反應(yīng)緩慢的過程在實際應(yīng)用中受到很大的限制。在這種情

3、況下，人們設(shè)計出一種利用裝有三級穩(wěn)壓器的組合電池，通過電氣化學的方法促進TiO2光催化降解。這組合系統(tǒng)有效的促進光催化降解有機廢水。連續(xù)處理半小時后，對玫瑰精的最大吸收值達90％以上。對于紡織廢水中的COD和BOD5的分別減少93.9％和88.7％。而且，由于COD/BOD5比減少到1.2-2.1之間，紡織廢水的生物降解能力也相對地提高。以上結(jié)果看出，此組合系統(tǒng)可以有效的處理有機廢水或者在生物處理預(yù)處理環(huán)節(jié)能夠脫色降解。</p&g

4、t;<p>　　關(guān)鍵詞：TiO2，玫瑰精，活性羥基，電解，廢水</p><p><b>　　文章概要</b></p><p><b>　　闡述</b></p><p><b>　　實驗部分</b></p><p><b>　　化學試劑</b>

5、</p><p><b>　　設(shè)備儀器</b></p><p><b>　　光解反應(yīng)</b></p><p><b>　　方法步驟</b></p><p><b>　　實驗討論</b></p><p>　　電化學產(chǎn)生活性羥基促進Ti

6、O2光催化活性</p><p>　　對比不同模擬條件下光催化降解羅丹紅</p><p>　　光催化降解羅丹紅的特性</p><p><b>　　廢水處理</b></p><p><b>　　結(jié)論</b></p><p><b>　　致謝</b></

7、p><p><b>　　參考文獻</b></p><p><b>　　闡述</b></p><p>　　紡織廢水是色度高，COD高而生物降解能力低的廢水。利用鋁鹽或鐵鹽不能充分降低生物需氧量和減少凝聚物。臭氧和氧化次氯酸可以有效的脫色，但是成本高，運行操作難和容易產(chǎn)生如余氯等二次污染物，由此而不能使用。</p>

8、<p>　　在過去的十年，研究表明，納米TiO2可以在適度條件下光解氧化有機廢水生成無機物質(zhì)（如CO2, H2O等）而不產(chǎn)生二次污染([Hagfeldt and Gratzel, 1995]; [Serpone and Pelizzetti, 1989])。但是，不僅是TiO2的光解效益，而且的光解性不能在最優(yōu)化的情況下有效的運用([Kawai and Sakata, 1980])。對于這個不利條件的主要因素就是光生電子和空穴

9、的再結(jié)合( [Li and Li, 2002])。為了提高光解能力，進行了很多有意思的工作。通過利用在TiO2的電子層上施加陽極電壓，或者利用各種貴金屬沉淀物改變TiO2([Hiramoto et al., 1990]; [Viswanathan et al., 1990])，金屬離子或者氧化物等等方法，都被證明是有希望的方法。</p><p>　　添加氧化劑理論上，或者更有效的方法。([Bandala et a

10、l., 2002])，但是，由于這種方法的投入成本高，而且會產(chǎn)生剩余氧化物等二次污染，所以在實際中難以應(yīng)用。過去，我們嘗試在適合條件下把H2O2介入TiO2的光催化單位當中，用以促進光催化進行。幸好，光化學有三個作用能促進這種方法。（1），H2O2在電化學過程中光解產(chǎn)生的離子比O2快很多，以至產(chǎn)生的OH和更多的光生電子進行反應(yīng)，所以光催化反應(yīng)加快。（2），這種方法不用增加反應(yīng)物，所以建造和運行管理費用沒有很明顯的增加。（3），如果溶液中

11、有殘余H2O2，它會自發(fā)分解為H20和O2，避免產(chǎn)生二次污染。</p><p><b>　　實驗部分</b></p><p><b>　　化學試劑</b></p><p>　　P25型TiO2 ( 30 nm微粒)購自德國，以銳鈦礦型為主（ 79%銳鈦礦型和21%金紅石型) 。以降解125 mmol/l的羅丹紅溶液（溶解在

12、0.2mol/l、PH＝4.0的磷酸鹽緩沖液）來評價電化學促進TiO2的光催化學活性。實驗中的廢水 (COD=3320 mg/l, BOD5=1540 mg/l)是從在染色工藝提取取出的含氯液體。</p><p><b>　　儀器設(shè)備</b></p><p>　　所有的實驗都是在根據(jù)需要設(shè)計出來的設(shè)備而進行的，這個設(shè)備由溫度調(diào)節(jié)裝置、三極電池裝置和紫外燈組成（見下圖）

13、。溫度調(diào)節(jié)裝置有一根圓柱形耐熱玻璃管，管內(nèi)裝有100ML含有0.1%（重量比）TiO2的廢水懸浮物。利用調(diào)節(jié)溫度裝置夾層中循環(huán)流動的40℃的水來維持懸浮液的溫度。電解裝置由碳極（陰極）、Pt棒（陽極）和氯化亞汞離子溶液組成。把11W，λ=253.7 nm波長的無臭氧紫外燈固定在恒溫裝置的中央，對TiO2進行輻射。另外，用空壓泵對溶液進行曝氣以補償消耗的氧氣，用磁性攪拌器攪拌確保TiO2粒子均勻分布在懸浮液中。</p>

14、;<p><b>　　圖1. 組合的裝置</b></p><p>　?。╝）、11w紫外燈；（b）、空氣管，與空氣泵連接；</p><p>　?。╟）、SCE；（d）、碳電極；（e）、Pt電極；（f）磁力攪拌器</p><p><b>　　光解反應(yīng)</b></p><p>　　將50毫

15、克的TiO2放入50毫升的廢水中，靜置30分鐘。反應(yīng)開始前在空氣中暴露15分鐘并保持實驗的空氣流通。然后，在碳棒上加0.75 V的電壓，同時打開紫外燈對懸浮液光照半小時。在規(guī)定的時間間隔內(nèi)進行取樣，將樣品在11,000轉(zhuǎn)/分的速度下進行離心分離后分析。</p><p><b>　　方法步驟</b></p><p>　　用紫外可見分光光度計測量溶液樣品的吸光率。根據(jù)水

16、和廢水的標準曲線法可以分別計算出紡織廢水中的COD 和 BOD5。([American Public Health Association (APHA), 1989]).反應(yīng)過程中生成的H2O2用標準高錳酸鉀溶液參考滴定算出濃度。[Teffery et al., 1978]</p><p><b>　　結(jié)果討論</b></p><p>　　電化學中產(chǎn)生的活性羥基促進T

17、iO2光催化活性</p><p>　　光解實驗中的半導體詳細的相關(guān)構(gòu)造已經(jīng)被研究證明。([Fox and Dulay, 1993]; [Hoffmann et al., 1995]; [Howe, 1998]).通常認為，反應(yīng)是在紫外光(λ<385 nm)下TiO2激發(fā)能帶隙而開始。在光電子激發(fā)下，電子從原子帶轉(zhuǎn)移到TiO2電子帶，從而引發(fā)電子空穴/洞(Eq. (1))。電子和電子空穴擴散到TiO

18、2表面上后，分別與O2、水、羥化物反應(yīng)，生成O2－ (Eq. (2), [Ishibashi et al., 2000 and Ishibashi et al., 1998]) 和√OH－(Eq. (3), [Schwarz et al., 1997]).一般認為，有機光解物主要是OH－。</p><p>　　TiO2 + hγ → hvb++e? （1）</p&

19、gt;<p>　　O2+ecb? → O22? （2）</p><p>　　h + +H2O→OH-+H+ （3）</p><p>　　但是，O2的溶解度小引起在懸浮液中的濃度很低，大部分光解生成的電子會跟電子空穴從新組合，導致有效電子空穴減少。這種在TiO2內(nèi)部的快速的再結(jié)合作用阻礙

20、著TiO2在廢水中的光解作用。另一方面，雖然OH－可以由O2生成，但這不是一個直接的過程，從而導致光解電子產(chǎn)生的OH－量少。所有的這些因素都減少電子空穴/空穴比的利用效率和紫外燈照射下的光解效率。表2顯示出在不同的電氣化學、紫外燈和TiO2的組合下逐漸產(chǎn)生的H2O2濃度的情況。在不同的電子、電子/紫外燈、電子/ TiO2、電子/ TiO/紫外燈條件下，對應(yīng)H2O2的濃度分別為8.60, 8.15, 7.21和3.89 mmol/l。但在

21、TiO/紫外燈條件下沒有H2O2生成([Kim et al., 2002])。對比在不同模式電解下產(chǎn)生的H2O2濃度，可以看出在光活性TiO2大大增加(Eq. (6), [Bandala et al., 2002])。整個化學反應(yīng)見表3。</p><p>　　O2+2H++2e?→H2O2 (4)</p><p><b&g

22、t;　　(5)</b></p><p>　　H2O2+H++e?→OH -+H2O (6)</p><p>　　與O2(Eo=1.23 V)相比，H2O2具有更高的氧化電位(Eo=1.77 V)，從而更容易更快的在光解中產(chǎn)生電子。而且在一下兩種條件下產(chǎn)生一定量的OH－：（1）、H2O2存儲更多的電子，接收更多的空穴與吸附在水表明的羥基反

23、應(yīng)生成OH－。（2）H2O2能與光解產(chǎn)生的電子直接反應(yīng)生成OH－。</p><p>　　圖 2. H2O2 反應(yīng)裝置 (a) EC; (b) EC/UV; (c) EC/TiO2; (d) EC/UV/TiO2.</p><p>　　圖3 電化學生成羥自由基促進氧化鈦光催化活性示意圖</p><p>　　對比不同模擬條件下光催化降解羅丹紅</p>&l

24、t;p>　　為了評價電氣化學在組合系統(tǒng)的效率，我們在對不同的EC, TiO2和UV組合下針對羅丹紅降解進行了研究。用在552nm（最大吸光度）條件下的吸光度的減少速度來評價對羅丹紅的降解效率和不同組合模式的處理容量。最初的羅丹紅溶液吸光度為2.432，在不同模式下半小時候處理后的羅丹紅溶液的吸光度見表1：</p><p>　　表1不同TiO2, EC 和UV組合模式下半小時后羅丹紅溶液在552nm波長吸光度

25、</p><p>　　對比不同組合可以看出，光電化學過程中產(chǎn)生的H2O2，因其氧化容量不好而未能很好的降解羅丹紅。但是，H2O2可以很好的促進TiO2的光化學反應(yīng)過程，通過產(chǎn)生具有更強的氧化容量的活性電子OH－，反應(yīng)過程見公式(6)。</p><p>　　光催化降解羅丹紅的特性</p><p>　　當羅丹紅溶液在TiO2光催化下進行光電化學反應(yīng)時，反應(yīng)中對光的吸收范

26、圍見圖4。從圖中可以看出，吸收范圍有兩種改變。一是吸光度快速減少；另外是在最大吸光度處有小小偏移。偏移的部分是由染色的破壞而產(chǎn)生的。沒有產(chǎn)生其他的最高點，表示羅丹紅被逐漸的降解成為其他的有機物。</p><p>　　有機化合物一般情況下會被捕捉的電子氧化分解或者與活性羥基反應(yīng)。根據(jù)實驗條件，由于羅丹紅溶液的濃度不高，少量的羅丹紅吸附在TiO2表面上，與電子空穴的反應(yīng)弱于羅丹紅與激發(fā)態(tài)羥基的反應(yīng)。</p>

27、;<p>　　類似的實驗可以顯示出，在沒有光電化學輔助的條件下TiO2進行的光解至少要延長2個小時才能達到相同的降解效果。結(jié)果還表面電子空穴與空穴之間的再組合是光解反應(yīng)的一個主要阻礙因素，而H2O2起到很大的作用，較少再結(jié)合和促進TiO2的光解活性。</p><p><b>　　廢水處理</b></p><p>　　紡織廢水中的COD和BOD5在降解過程

28、中有序的較少，處理所得數(shù)據(jù)見圖5。從圖中可以看出，預(yù)處理后的廢水富含COD和BOD5并且COD/BOD5高。這比率顯示出廢水的生物降解能力，高比率表示廢水不利于生物降解能力。作為一個預(yù)處理氧化過程，TiO2光解可以有效地將廢水中的有機物轉(zhuǎn)化為無機物。利用三極光電化學組合輔助納米TiO2可以應(yīng)用于廢水處理。從圖5看出，0.5小時后紡織廢水中的COD和BOD5的處理效率達93.9％和88.7％。同時，COD/BOD5比從2.1減少到1.2。

29、則表明廢水的生物降解能力得到提高。</p><p>　　圖4 電氣化學促進TiO2光催化降解COD和BOD5</p><p>　　圖5電化學輔助的TiO2光催化過程中COD和BOD5的減少</p><p><b>　　結(jié)論</b></p><p>　　文章中指出，通過三極光電化學反應(yīng)裝置良好反應(yīng)環(huán)境，H2O2被引入TiO

30、2的光解反應(yīng)中，此組合系統(tǒng)更有效果地促進光解反應(yīng)是有機廢水無機化。這個效果由以下兩個因素引起：（1）、H2O2比O2捕獲電子的速度快，而且有更多的電子空穴與吸附的水和羥基反應(yīng)，產(chǎn)生更多的OH－。（2）、H2O2與電子之間的反應(yīng)消耗光電化學產(chǎn)生的電子，同時產(chǎn)生OH－。所以O(shè)H－的產(chǎn)生量和整個系統(tǒng)的電子空穴/空穴比效率大大地改變了。從組合系統(tǒng)中連續(xù)取樣半個小時，可以看出很好的處理效果。羅丹紅的最大吸光度減少至少90％，紡織廢水中的COD和B

31、OD5的處理效率達93.9％和88.7％。COD/BOD5從2.1減少到1.2，生物降解能力得到有效的改良。</p><p>　　總結(jié)上述，光電化學能促進TiO2光解系統(tǒng)有效地使有機廢水無機化、消除放射性污染和在預(yù)處理階段進行褪色，促進生物降解。</p><p><b>　　致謝</b></p><p>　　本項目是在上海納米光催化研究科技委員

32、會的支持下完成(0114nm072)。</p><p><b>　　參考文獻：</b></p><p>　　Alfano et al., 2000. O.M. Alfano, D. Bahnemann, A.E. Cassano, R. Dillert and R. Goslich, Photocatalysis in water environments using

33、 artificial and solar light. Catal. Today 58 2/3 (2000), pp. 199–230. SummaryPlus | Full Text + Links | PDF (1614 K) </p><p>　　American Public Health Association (APHA), 1989. American Public Health Associat

34、ion (APHA), Standard Methods for the Examination of Water and Wastewater. (seventeenth ed.), (1989). </p><p>　　Bandala et al., 2002. E.R. Bandala, S. Gelover, M.T. Leal, C. Arancibia-Bulnes, A. Jimenez and C

35、.A. Estrada, Solar photocatalytic degradation of Aldrin. Catal. Today 76 (2002), pp. 189–199. </p><p>　　Fox and Dulay, 1993. M.A. Fox and M.T. Dulay, Heterogeneous photocatalysis. Chem. Rev. 93 1 (1993), pp.

36、 341–357. </p><p>　　Hagfeldt and Gratzel, 1995. A. Hagfeldt and M. Gratzel, Light induced redox reactions in nanocrystalline systems. Chem. Rev. 95 1 (1995), pp. 49–68. </p><p>　　Hidaka et al.,

37、1995. H. Hidaka, Y. Asai, J.C. Zhao, K. Nohara, E. Pelizzetti and N. Serpone, Photoelectrochemical decomposition of surfactants on a TiO2/TCO Particulate film electrode assembly. J. Phys. Chem. 99 20 (1995), pp. 8244–824

38、8. </p><p>　　Hidaka et al., 1999. H. Hidaka, K. Ajisaka, S. Horikoshi, T. Oyama, J.C. Zhao and N. Serpone, Photodecomposition of amino acids and photocurrent generation on TiO2/OTE electrodes prepared by pul

39、se laser deposition. Catal. Lett. 60 (1999), pp. 95–98. </p><p>　　Hiramoto et al., 1990. M. Hiramoto, K. Hashimoto and T. Sakata, An ultramicrostructured photoelectrode system composed of TiO2 single crystal

40、 and layered SiO2 and Pt thin Film. Chem. Lett. 8 (1990), pp. 1343–1346. </p><p>　　Hoffmann et al., 1995. M.R. Hoffmann, S.T. Martin, W. Choi and D.W. Bahnemann, Environmental applications of semiconductor p

41、hotocatalysis. Chem. Rev. 95 1 (1995), pp. 69–96. </p><p>　　Howe, 1998. R.F. Howe, Recent development in photocatalysis. Dev. Chem. Engng Miner. Process. 6 1/2 (1998), pp. 55–84. </p><p>　　Ishib

42、ashi et al., 1998. K. Ishibashi, Y. Nosaka, K. Hashimoto and A. Fujishima, Time-dependent behavior of active oxygen species formed on photoirradiated TiO2 films in air. J. Phys. Chem. B 102 12 (1998), pp. 2117–2120. <

43、/p><p>　　Ishibashi et al., 2000. K. Ishibashi, A. Fujishima, T. Watanabe and K. Hashimoto, Generation and deactivation processes of superoxide formed on TiO2 film illuminated by very weak UV light in air or wat

44、er. J. Phys. Chem. B 104 20 (2000), pp. 4934–4938. </p><p>　　Kawai and Sakata, 1980. T. Kawai and T. Sakata, Photocatalytic hydrogen production from liquid methanol and water. J. Chem. Soc., Chem. Commun. 15

45、 (1980), pp. 694–695. </p><p>　　Kim et al., 2002. Y.C. Kim, S. Sasaki, K. Yano, K. Ikebukuro, K. Hashimoto and I. Karube, A flow method with photocatalytic oxidation of dissolved organic matter using a solid

46、-phase (TiO2) reactor followed by amperometric detection of consumed oxygen. Anal. Chem. 74 (2002), pp. 3858–3864.</p><p>　　Li and Li, 2002. F.B. Li and X.Z. Li, The enhancement of photodegradation efficienc

47、y using Pt–TiO2 catalyst. Chemosphere 48 10 (2002), pp. 1103–1111. SummaryPlus</p><p>　　Linsebigler et al., 1995. A.L. Linsebigler, G. Lu and J.T. Yates, Photocatalysis on TiO2 surfaces: principles, mechanis

48、ms, and selected results. Chem. Rev. 95 3 (1995), pp. 735–758. Full Text via CrossRef </p><p>　　Miguel et al., 2002. R. Miguel, S. Victor, E. Santiago and P. Cesar, Photo-Fenton treatment of a biorecalcitran

49、t wastewater generated in textile activities: biodegradability of the photo-treated solution. J. Photochem. Photobiol. A: Chem. 151 1–3 (2002), pp. 129–135. </p><p>　　Oturan et al., 2001. M.A. Oturan, N. Otu

50、ran, C. Lahitte and S. Trevin, Production of hydroxyl radicals by electrochemically assisted Fenton's reagent: application to the mineralization of an organic micropollutant, pentachlorophenol. J. Electroanal. Chem.

51、507 (2001), pp. 96–102. SummaryPlus </p><p>　　Paola et al., 2002. A.D. Paola, E. García-López, S. Ikeda, G. Marcì, B. Ohtani and L. Palmisano, Photocatalytic degradation of organic compounds i

52、n aqueous systems by transition metal doped polycrystalline TiO2. Catal. Today 75 1–4 (2002), pp. 87–93. </p><p>　　Schwarz et al., 1997. P.F. Schwarz, N.J. Turro, S.H. Bossmann, A.M. Braun, A.-M.A. Abdel Wah

53、ab and H. Du¨rr, A new method to determine the generation of hydroxyl radicals in illuminated TiO2 suspensions. J. Phys. Chem. 101 36 (1997), pp. 7127–7134. Full Text via CrossRef </p><p>　　Serpone and

54、Pelizzetti, 1989. N. Serpone and E. Pelizzetti, Photocatalysis—Fundamentals and Application. , Wiley/Interscience, New York (1989). </p><p>　　Teffery et al., 1978. G.H. Teffery, J. Bassett, J. Mendham and R.

55、C. Denney, Vogel's Textbook of Quantitative Chemical Analysis. , Longman, UK (1978). </p><p>　　Viswanathan et al., 1990. B. Viswanathan, U.D. Mary and R.P. Viswanath, Photocatalytic dehydrogenation of me

56、thanol on Pt/TiO2. Indian J. Chem. Sect. A 29 11 (1990), pp. 1138–1139. </p><p>　　Yamashita et al., 2002. H. Yamashita, M. Harada, J. Misaka, M. Takeuchi, K. Ikeue and M. Anpo, Degradation of propanol dilute

57、d in water under visible light irradiation using metal ion-implanted titanium dioxide photocatalysts. J. Photochem. Photobiol. A: Chem. 148 1–3 (2002), pp. 257–261.</p><p>　　Photocatalytic degradation of org

58、anic wastes by electrochemically assisted TiO2 photocatalytic system</p><p>　　Junshui Chen, Meichuan Liu, Jidong Zhang, Xiangyang Ying and Litong Jin, Department of Chemistry, East China Normal University,

59、Shanghai, 200062, China Received 11 October 2002;  revised 30 August 2003;  accepted 23 September 2003. ; Available online 17 December 2003. </p><p><b>　　Abstract</b></p>

60、<p>　　Photocatalytic degradation of organic wastes with nanosized titanium dioxide particles has been studied for a long time in order to offer an appropriate method for wastewater treatment, but its practical appli

61、cation is greatly limited by the slow process. In this work, an electrochemically assisted TiO2 photocatalytic system was set-up by combining a TiO2 photocatalytic cell with a three-electrode potentiostatic unit. The com

62、posite system revealed high photocatalytic activity towards organic was</p><p>　　Author Keywords: Titanium dioxide; Rhodamine 6G; Hydroxyl radical; Electrolysis; Wastewater </p><p>　　Article Out

63、line</p><p>　　1. Introduction </p><p>　　2. Experimental </p><p>　　2.1. Chemical reagents </p><p>　　2.2. Apparatus </p><p>　　2.3. Photoreactivity </p>

64、<p>　　2.4. Characterization techniques</p><p>　　3. Results and discussion </p><p>　　3.1. Hydroxyl radical generation in electrochemically assisted TiO2 photocatalytic system </p>&

65、lt;p>　　3.2. Comparison of rhodamine 6G degradation in different combination patterns </p><p>　　3.3. Photocatalytic characteristics of rhodamine 6G degradation </p><p>　　3.4. Wastewater treatm

66、ent</p><p>　　4. Conclusion </p><p>　　Acknowledgements </p><p>　　References</p><p>　　1. Introduction</p><p>　　Textile dye wastewater (TDW) is well known to

67、contain strong color, high chemical oxygen demand (COD) and low biodegradability ([Miguel et al., 2002]). Biological oxidation and coagulation by aluminum or iron salts could not treat it adequately. Ozone and hypochlori

68、te oxidation are efficient decolorization methods, but they are not desirable because of the high cost for equipment and operating, and the secondary pollution arising from the residual chlorine ( [Alfano et al., 2000];

69、[Hoffmann et a</p><p>　　Over the past several decades, studies have revealed that nanosized TiO2 particles can photocatalytically oxidize many organic wastes into inorganic substances (such as CO2, H2O, etc.

70、) under moderate conditions, without any serious secondary pollution ([Hagfeldt and Gratzel, 1995]; [Serpone and Pelizzetti, 1989]). However, not only the photo efficiency or activity but also the photo response of TiO2

71、is not suitable for direct application in environmental optimization ([Kawai and Sakata, 1980]). </p><p>　　Addition of oxidants (usually bromate or hypochlorite) is also a theoretically, even more efficient

72、way ; however, the method is inhibited from practical application for its disadvantages such as higher cost for reagents, and possible secondary pollution from the residual oxidants. In this paper, we attempt to introdu

73、ce H2O2 into TiO2 photocatalytic cell through an environmentally desirable way to improve the photocatalytic process. Fortunately, electrochemistry offers such a method, which was </p><p>　　2. Experimental&l

74、t;/p><p>　　2.1. Chemical reagents</p><p>　　P25 TiO2 (30 nm primary crystal size) was purchased from Degussa, which contains predominantly anatase (79% anatase and 21% rutile as determined from X-ra

75、y diffraction). Degradation of 125 mmol/l Rhodamine 6G (chemical grade, dissolved in 0.2 mol/l phosphate buffer solution with a pH of 4.0) solution was used to evaluate the catalytic ability of the electrochemically assi

76、sted TiO2 photocatalytic system. TDW (COD=3320 mg/l and biochemical oxygen demand, BOD5=1540 mg/l) used in the experiments was</p><p>　　2.2. Apparatus</p><p>　　All experiments were carried out i

77、n the experimental set-up (Fig. 1), which consisted of a thermostatic reactor, a three-electrode potentiostatic unit and an ultraviolet (UV) lamp. The reactor was served by a cylindrical Pyrex glass vessel of 100 ml, whi

78、ch contained the suspension, i.e. wastewater with 0.1% (w/w) TiO2 added. The temperature of the suspension was kept constant by circulation of 40 °C water in the interlayer of the reactor. The electrolysis unit cons

79、isted of a carbon electrode (ca</p><p>　　Fig. 1. Set-up of the experiment. (a) 11-W UV lamp, (b) air pipe, through which a air pump connected (c) SCE, (d) carbon electrode, (e) Pt net electrode, (f) magnetic

80、 stirrer, CB and VB represent conduction and valence band, respectively.</p><p>　　2.3. Photoreactivity</p><p>　　Fifty milligrams of TiO2 was suspended in 50 ml of wastewater and sonicated for 30

81、 min. The resulting suspension was saturated with air for 15 min before the reaction started and kept aerated throughout the experiments. Then a potential of ?0.75 V (vs. SCE) was applied on the carbon electrode. Simulta

82、neously the UV lamp was switched on to illuminate the suspension for 0.5 h. Samples were withdrawn at regular intervals, centrifuged for 6 min at the speed of 11,000 rpm and finally analyzed. </p><p>　　2.4.

83、Characterization techniques</p><p>　　A Cary 50 Probe UV/VIS spectrophotometer (Varian, America) was used to record the absorbance data of the solution samples; COD and BOD5 of the TDW were measured according

84、 to the Standard Methods for Examination of Water and Wastewater ([American Public Health Association (APHA), 1989]). Concentration of H2O2 during the process was determined by titration with standard potassium permangan

85、ate according to the description in reference [Teffery et al., 1978]. </p><p>　　3. Results and discussion</p><p>　　3.1. Hydroxyl radical generation in electrochemically assisted TiO2 photocataly

86、tic system</p><p>　　The detailed mechanisms involved in semiconductor photocatalysis have been studied and reviewed ([Fox and Dulay, 1993]; [Hoffmann et al., 1995]; [Howe, 1998]). Usually it is considered th

87、at the process is initiated by the band-gap excitation of TiO2 under UV illumination (λ<385 nm). Excited by energetic photons, electrons are transferred from the valence band to the conduction band of TiO2, leadi

88、ng to the generation of electron/hole pairs (Eq. (1)). The electrons and holes diffuse to the surface</p><p>　　However, O2 exists in the suspension at a low concentration due to its low solubility in water;

89、quite a number of photogenerated electrons would recombine with photogenerated holes, causing a decrease in availability of photogenerated holes. The rapid unfavorable charge carrier recombination reaction in TiO2 has be

90、come a drawback for TiO2 photocatalysis in wastewater treatment. On the other hand, although √OH could be generated from O2√?, the process is not a direct one, resulting in low quantum</p><p>　　With three-el

91、ectrode potentiostatic unit introduced, H2O2 was generated through cathodic reaction (Eq. (4), [Oturan et al., 2001]) and the consumed O2 was compensated partly by anodic reaction (Eq. (5), [Oturan et al., 2001]). Fig. 2

92、 shows the gradually growing concentration of H2O2 in different combination patterns among electrochemical unit (EC), UV and TiO2. The steady concentration of H2O2 in EC, EC/UV, EC/TiO2 and EC/TiO2/UV system is 8.60, 8.1

93、5, 7.21 and 3.89 mmol/l, respectively. But no H</p><p>　　H2O2 traps the photogenerated electrons faster and easier for its higher oxidation potential (Eo=1.77 V) than O2 (Eo=1.23 V), which can prom

94、ote the quantum yields of √OH based on two reasons. (1) More electrons are trapped by H2O2, reserving more holes to react with adsorbed water surface hydroxyl group, producing √OH. (2) The reaction between H2O2 and photo

95、generated electrons can produce √OH directly. </p><p>　　Fig. 2. H2O2 accumulation in the reactor, (a) EC; (b) EC/UV; (c) EC/TiO2; (d) EC/UV/TiO2.</p><p>　　Fig. 3. Schematic presentation of the p

96、roduction of hydroxyl radicals from electrochemically assisted TiO2 photocatalytic system.</p><p>　　3.2. Comparison of rhodamine 6G degradation in different combination patterns</p><p>　　In orde

97、r to evaluate the efficacy of the electrochemical unit in the composite system, different combination patterns among EC, TiO2 and UV were studied towards R-6G degradation. And the decrease of absorbance at 552 nm (the ma

98、ximum absorption wavelength) was used to evaluate the degradation degree of R-6G and the handling capacity of different combination patterns. The original absorbance of R-6G solution was detected to be 2.432, and the abs

99、orbance of the end R-6G solution after continuous tre</p><p>　　Table 1. Absorbance of R-6G solution at 552 nm after continuous treatment for 0.5 h by different combination patterns among TiO2 powder, EC and

100、UV </p><p>　　Conclusion can be drawn by comparing the different combination patterns that H2O2, produced from electrolytic process, could not oxidize R-6G very well by itself for its insufficient oxidizing c

101、apacity; however, it can greatly promote the photocatalytic process of TiO2, suggesting the production of intermediate radicals with stronger oxidative capacity, OH-, based on the reaction shown in Eq. (6). </p>&

102、lt;p>　　3.3. Photocatalytic characteristics of rhodamine 6G degradation</p><p>　　When R-6G solution was treated by electrochemically assisted TiO2 photocatalytic system, the absorption spectra of the solut

103、ion during the treatment is shown in Fig. 4. As seen from the figure, the spectra showed two types of changes. One is a fast decrease in absorbance; the other is a slightly hypsochromic shift in the absorbance maximum. T