鈦酸鋇壓電臭氧化降解水中的硝基苯

莊 瑋,楊 婧,龔冰柔,鄭 瑩,趙 純

鈦酸鋇壓電臭氧化降解水中的硝基苯

莊 瑋1,2,楊 婧,龔冰柔,鄭 瑩,趙 純*

(1.重慶大學(xué)環(huán)境與生態(tài)學(xué)院,重慶 400030；2.四川省城鄉(xiāng)建設(shè)研究院,四川 成都 610043)

將臭氧(O3)體系與壓電(PE)體系相結(jié)合提出了壓電臭氧化(PE-O3)體系,探究了該體系對難降解有機污染物硝基苯(NB)的降解效果,考察了轉(zhuǎn)速、O3濃度、鈦酸鋇(BT)投加量和初始pH值對NB去除的影響.此外,探討了PE-O3體系降解NB過程中存在的活性物質(zhì),并分析了反應(yīng)機理.結(jié)果表明:PE-O3體系對NB的降解體現(xiàn)出明顯的協(xié)同效應(yīng)(協(xié)同系數(shù)高達5.04),在15min內(nèi)對NB的去除率高達85.37%,反應(yīng)符合一級反應(yīng)動力學(xué)規(guī)律,為0.1256min-1.此外,PE-O3體系在120min內(nèi)對NB實現(xiàn)了74.06%的礦化.隨著磁力轉(zhuǎn)子轉(zhuǎn)速的增加,體系反應(yīng)速率提升,當(dāng)轉(zhuǎn)速提高到1500r/min時,反應(yīng)速率常數(shù)可達到0.1446min-1.反應(yīng)速率隨體系中BT濃度和O3濃度的增加而增加,但一定程度后,增長趨勢變緩.NB降解速率隨pH值的增加而增大,當(dāng)pH值為9.0時,在15min后體系中的NB降解率達85.69%.反應(yīng)過程中產(chǎn)生的是降解NB的主要活性物質(zhì).

鈦酸鋇；壓電臭氧化；硝基苯；壓電；臭氧

硝基苯(NB)在石油化學(xué)工業(yè)、燃料、化學(xué)原料和焦化工業(yè)中的廣泛應(yīng)用導(dǎo)致了大量含硝基苯的廢水排放到環(huán)境中[1].NB具有潛在的致癌、致畸和致突變性[2],被列為中國的58種優(yōu)先污染物之一[3].但是傳統(tǒng)的水處理工藝難以有效降解NB.

臭氧作為一種清潔高效的氧化劑在水處理高級氧化過程中被廣泛研究[4-5],但單獨臭氧體系對難降解污染物的去除率低,難以徹底礦化難降解污染物[6-7].為了實現(xiàn)對污染物的高效降解,研發(fā)了多種基于臭氧的高級氧化技術(shù),其中電催化臭氧化體系(E-O3)因?qū)ξ廴疚锶コ芰姟⒌V化率高等優(yōu)點受到廣泛關(guān)注[8-10].但該體系對電極性質(zhì)要求較高,且電極、臭氧、反應(yīng)液之間的傳質(zhì)條件限制了該體系去除污染物的效果[11-12].

目前已有通過電化學(xué)反應(yīng)法,利用臭氧高效降解硝基苯的實驗研究.Wang等[13]研究比較了以顆粒活性炭(GAC)為三維電極電解、臭氧氧化以及電解、GAC與臭氧(E-GAC-O3)聯(lián)用對硝基苯(NB)的去除礦化效果,證實了電解法、活性炭法和臭氧法相結(jié)合可以有效降低電能的投入,降低水處理費用.

壓電材料可以將外部機械能轉(zhuǎn)化為電能和化學(xué)能,這一過程被稱為壓電效應(yīng).近年來,基于壓電效應(yīng)的壓電催化過程由于對水中有機污染物的有效降解而引起了廣泛關(guān)注.但壓電催化過程產(chǎn)生?OH的能力較弱,因此只能對部分易降解的污染物進行高效降解.

將臭氧體系與壓電催化相結(jié)合,利用壓電過程為臭氧反應(yīng)提供能量,可能是一種更具吸引力的類似電催化臭氧化的新技術(shù).通過水力驅(qū)動的壓電處理技術(shù)也能有效利用水處理流程中富余的水力能量.同時,粉末狀催化劑相較于電極板而言具有更好的傳質(zhì)能力.因此,本研究將臭氧體系(O3)與壓電體系(PE)相結(jié)合提出了壓電臭氧化體系(PE-O3).本文將系統(tǒng)的研究壓

電臭氧化體系(PE-O3)對NB的降解,以及轉(zhuǎn)速、BT投加量、O3的濃度、初始pH值對PE-O3體系降解NB的影響,并揭示其反應(yīng)機理.

1 材料與方法

1.1 試驗材料

硝基苯(AR)、硝酸銀(AR)、超氧化物歧化酶(200u/mg)、過氧化氫酶(200m/mg)、全氟磺酸(～5%)購于阿拉丁試劑有限公司;鈦酸鋇(BaTiO3,99.99%),購于成都市麥卡希化工有限公司;甲醇、叔丁醇、草酸銨、無水乙醇、無水硫酸鈉均為分析純,購于成都市科龍化工試劑廠;硫酸和氫氧化鈉購于重慶川東化工(集團)有限公司;氧氣(99%)購于重慶瑞信氣體有限公司.

1.2 試驗方法

試驗過程:所有的試驗都在圓柱形反應(yīng)器內(nèi)進行(PTFE材質(zhì),底部直徑7cm,高12cm)反應(yīng)液有效體積為300mL,反應(yīng)裝置被展示在圖1中.將反應(yīng)器置于磁力攪拌器上后投入轉(zhuǎn)子,將轉(zhuǎn)速調(diào)至200r/min,保持五分鐘,該步驟是為了讓BT對NB達到吸附平衡.然后利用臭氧發(fā)生器(3S-X,北京同林)將純氧制成臭氧,并將臭氧氣體臭氧氣體通入反應(yīng)液中,同時將磁力攪拌器轉(zhuǎn)速調(diào)整至需要的轉(zhuǎn)速.在一個典型的實驗過程中:氣體臭氧濃度為14.0mg/L,氣體臭氧流量為300mL/min,BT投加量為3.0g/L,污染物NB濃度為10mg/L,反應(yīng)液初始pH為9.6,磁力攪拌轉(zhuǎn)速為1000r/min.開始計時,取樣時間點一般為0、1、3、5、7、10、15min.在預(yù)先設(shè)定的時間間隔用注射器取樣2.0mL,而后采用0.22μm的PTFE濾膜將其過濾至小瓶內(nèi),然后用移液槍量取1.5mL過濾后的樣品至預(yù)先加入了50μL甲醇的液相進樣瓶內(nèi).所有采集的樣品都被儲藏在4℃條件下,以待測試.

電化學(xué)測試:將150mg BT投加至0.2mL全氟磺酸溶液和1.8mL乙醇的混合液中,然后將其置于超聲中分散20min,待其完全分散即得BT懸濁液.然后,將BT懸濁液均勻地涂抹在清洗并晾干的ITO玻璃基質(zhì)上.最后,將被涂覆的ITO玻璃置于室溫下干燥12h,即得BT電極.ITO電極也以同樣的方法被制得,唯一不同的地方在于未添加BT.

制得BT和ITO電極后即可進行電化學(xué)測試,以測量壓電過程中產(chǎn)生的瞬態(tài)壓電電流.使用三電極體系的電化學(xué)工作站測定各個體系中的壓電電流,三個電極分別為:Ag/AgCl參比電極、Pt對電極和BT(或ITO)工作電極.

圖1 壓電催化、臭氧氧化和壓電催化臭氧氧化處理NB的反應(yīng)器示意

1.3 分析方法

NB濃度檢測:采用高效液相色譜法(HPLC)測定本研究中硝基苯(NB)的濃度,采用美國Waters公司的高效液相色譜,配備了COSMOSIL3C18-MS-II (5μmparticlesize, 4.6×150mm, Nacalai Tesque, Inc., Japan)型號的色譜柱,流動相為水、乙腈和甲醇(55:35:15,//),流動速度為1.0mL/min,檢測波長為263nm,進樣量為10 μL.

TOC測定方法:利用TOC-VCPH分析儀(Shimadzu Co., Japan)測定了反應(yīng)過程中溶液的TOC.

采用X射線衍射(XRD, Cu Kα source, 40kV- 40mA, Spectris Pte. Ltd)以7°min-1的掃描速率在20°到80°的2范圍內(nèi)分析了相組成和晶體結(jié)構(gòu),并使用室溫下配備的波長為532nm的100mW激光器進行了拉曼光譜分析,以進一步分析BT的相態(tài)(HORIBA Jobin Yvon S.A.S company, France).用X射線光電子能譜(XPS,ESCALAB250Xi,Thermo, USA)分析了BT中元素的變化及其化學(xué)狀態(tài).

2 結(jié)果與討論

2.1 NB在不同體系中的降解

由圖2(a)所示,在15min內(nèi),PE體系對NB幾乎沒有去除效果,O3體系對NB的去除率為29.88%, PE-O3體系對NB的去除率為85.37%.此外,根據(jù)圖2(b)看出,PE-O3體系、單獨PE體系和單獨O3體系的一級反應(yīng)動力學(xué)常數(shù)分別為12.56×10-2min-1、0.08 ×10-2min-1和2.41×10-2min-1.其中,PE-O3體系的擬一級反應(yīng)動力學(xué)常數(shù)明顯大于單獨PE體系和單獨臭氧體系的一級反應(yīng)動力學(xué)常數(shù).臭氧體系和壓電體系之間存在明顯的協(xié)同作用,協(xié)同系數(shù)達到5.04.與NB降解相似,在PE-O3體系在120min內(nèi)對NB的礦化率(74.06%)遠高于PE體系(1.20%)和O3體系(24.93%)的礦化率(圖2(c)).

根據(jù)以上實驗結(jié)果可知,PE-O3體系可以實現(xiàn)對NB更有效的降解和礦化,這可能與更多活性物質(zhì)(尤其是?OH)的生成有關(guān)[14].

2.2 影響因素

2.2.1 轉(zhuǎn)速的影響 外部機械力的改變會影響壓電材料壓電勢的大小,進而影響體系對污染物的降解效果[15].因此,水力條件是PE-O3體系降解NB的一個重要影響因素.實驗結(jié)果如圖3(a)所示,NB單位時間降解量隨轉(zhuǎn)子轉(zhuǎn)速變化而改變.如,PE-O3體系對NB的降解率隨轉(zhuǎn)速增加而提高.反應(yīng)進行至15min后,隨著轉(zhuǎn)速從0r/min增長到1500r/min,PE- O3體系對NB的降解率從54.33%增長到90.29%.

轉(zhuǎn)速對NB降解速率的影響被展示在圖3(b)中,當(dāng)轉(zhuǎn)速為0r/min時,值僅為5.17×10-2min-1.隨著轉(zhuǎn)速的不斷增大,PE-O3體系對NB的降解率隨之提高,值也不斷增大.當(dāng)轉(zhuǎn)速為1500r/min時,值可達14.46×10-2min-1.易知,PE-O3體系降解NB的值與轉(zhuǎn)速呈正相關(guān).該結(jié)論與Feng等[16]的研究結(jié)論一致,即提高轉(zhuǎn)速有利于壓電體系對污染物的降解.這可能一方面是轉(zhuǎn)速的增加提高了傳質(zhì)效果[17],另一方面轉(zhuǎn)速的提高使壓電材料產(chǎn)生的壓電勢提高,產(chǎn)生了更多的自由載流子[18],進而產(chǎn)生了更多的自由基(如?OH)[11,19].

2.2.2 O3濃度的影響 O3濃度對基于臭氧的高級氧化工藝有重要的影響.如圖3所示, O3濃度從3.0mg/L增加至18.0mg/L時,反應(yīng)15min后,PE-O3體系對NB的降解率從32.14%提高至89.02%,NB去除率與臭氧濃度呈正相關(guān).此外,值隨著O3濃度的增加而增加.當(dāng)O3濃度從3.0mg/L增至14.0mg/L時,值從2.63 × 10-2min-1提高至12.56×10-2min-1.這是因為O3濃度的提高使更多的O3參與反應(yīng),進而生成了更多的活性自由基,提高了反應(yīng)速率.而當(dāng)O3濃度進一步提高到18.0mg/L時,NB降解率僅提高了3.33%,值也僅增至14.30 × 10-2min-1.這可能是因為,在此條件下,O3濃度不再是反應(yīng)的限制因素,壓電效應(yīng)產(chǎn)生的自由載流子成為了限制體系反應(yīng)速率的因素.過量的O3不能完全與有限的自由載流子反應(yīng),造成體系降解NB的增長速率減緩.綜上,選擇14.0mg/L的O3為最佳O3濃度進行下一步實驗.

2.2.3 BT投加量的影響 壓電材料BT的濃度也會影響PE-O3體系對NB的降解效果.如圖4所示,提高BT的投加量有利于PE-O3體系對NB的降解.當(dāng)BT投加量從0.7g/L增加至3.0g/L時,反應(yīng)15min后,NB的去除率從48.71%增加至85.69%,值從4.60×10-2min-1增加至12.56×10-2min-1.當(dāng)BT投加量繼續(xù)提高至6.0g/L時,PE-O3體系對NB的降解率略有提高,為89.90%,值為14.87×10-2min-1.這是由于BT投加量的增加能夠增加壓電效應(yīng)的活性位點,進而產(chǎn)生更多活性氧化物質(zhì),提高體系對污染物的降解效果[20].然而,進一步提高BT的投加量可能導(dǎo)致過量的自由載流子的自我淬滅,使PE-O3體系降解污染物的增長速率減緩.Wu等[21]研究也發(fā)現(xiàn),壓電催化體系中,存在壓電材料的最佳投加量,投加過量的壓電材料無法進一步提高壓電催化體系對污染物的降解效果,甚至產(chǎn)生抑制作用.因此,本文選取的BT最佳投加量為3.0g/L.

2.2.4 pH值的影響 在各類臭氧高級氧化工藝中,pH值是一個重要的影響因素[22-23].使用0.1mol/L的NaOH和H2SO4溶液調(diào)節(jié)反應(yīng)液的初始pH至3.0、5.0、7.0、9.0、11.0,以探究不同pH值下PE-O3體系對NB的降解效果.如圖5(a)所示,反應(yīng)15min后,當(dāng) pH值從3.0提高到9.0,PE-O3體系對NB的降解速率隨著pH的增大而增加.其中,NB的去除率從35.67%增加至85.69%,值從2.96×10-2min-1增加至12.52×10-2min-1.當(dāng)pH值進一步提高至11.0后, PE-O3體系對NB的降解率相對pH值為9.0時略有提高(1.49%),為87.18%,值為13.20×10-2min-1.酸性條件下,O3的氧化還原電位比堿性條件下[24],導(dǎo)致臭氧在酸性條件下更難活化.此外,研究表明[25],堿性條件下,體系中的OH-能夠與O3反應(yīng)產(chǎn)生?OH(式2-1和2-2),促進污染物的降解.因此,提高PE-O3體系的pH值有利于NB的降解.

2.3 反應(yīng)機理

根據(jù)目前的研究可知,壓電臭氧化體系中可能存在羥基自由基(?OH)、電子(e-)、空穴(+)、超氧自由基(?O2-)和過氧化氫(H2O2)等活性物質(zhì)[26],為了確定壓電臭氧化體系中的活性物質(zhì),選取叔丁醇(TBA)[27]、硝酸銀(AgNO3)[28]、草酸銨(AO)[29]、超氧化物歧化酶(SOD)[30]、過氧化氫酶(CAT)[30]分別對這六種物質(zhì)進行淬滅.由圖6試驗結(jié)果可知,當(dāng)TBA、AgNO3、AO、SOD和CAT被加入后,PE-O3體系去除NB的反應(yīng)速率常數(shù)分別為2.56,5.27, 10.81,11.28,12.37(×10-2min-1).而沒有淬滅劑時,PE- O3體系去除NB的反應(yīng)速率常數(shù)為12.56× 10-2min-1.TBA對PE-O3體系去除NB的抑制能力最強,說明?OH是最重要的活性物質(zhì).同時,超氧自由基(?O2-)、電子(e-)和空穴(+)也是壓電臭氧化體系中存在的活性物質(zhì),而過氧化氫(H2O2)幾乎不存在或不占主導(dǎo)作用.由此前的研究可知,?OH和+被廣泛證明可氧化降解NB[31].而e-可能是作為產(chǎn)生?OH的前驅(qū)物對試驗造成了影響.此外,?O2-在以往的研究中也被證實可以進一步產(chǎn)生?OH以降解有機污染物[32].

圖6 不同淬滅劑對NB降解速率的影響

壓電臭氧化過程起始于壓電效應(yīng)的激活,當(dāng)壓電材料受到外界機械壓力時會發(fā)生形變和極化,電子和空穴向材料兩端分離而形成內(nèi)建電場(式(3))[33].被檢測到的e-和+證明了這一反應(yīng)的發(fā)生.此外,電化學(xué)測試被廣泛應(yīng)用于壓電效應(yīng)的檢測和證明,測試結(jié)果被展示在圖7中.當(dāng)磁力攪拌出于關(guān)閉狀態(tài)時,ITO電極系統(tǒng)和BT電機系統(tǒng)內(nèi)均無感應(yīng)電流生成.而當(dāng)磁力攪拌打開后,BT電極系統(tǒng)內(nèi)立即檢測到感性電流的存在,而ITO系統(tǒng)卻沒有.這些結(jié)果表明水力驅(qū)動的方式可以有效激活BT的壓電效應(yīng).此外,PE-O3體系產(chǎn)生的壓電電流明顯高于PE體系,這表明臭氧促進了BT的電子和空穴分離過程,出現(xiàn)了更強的電子轉(zhuǎn)移反應(yīng),從而顯示出更強的感應(yīng)電流.這些結(jié)果表明了O2(式(4))和O3(式(5))的得電子反應(yīng)的發(fā)生.根據(jù)以往的文獻,推測了這些活性物質(zhì)分別可以發(fā)生一系列鏈?zhǔn)椒磻?yīng)(式(6)至(8)),從而生成?OH降解NB[34-35](式(12)).壓電效應(yīng)產(chǎn)生的+因其獨特的氧化性也可以在降解NB過程中發(fā)揮作用,+一方面可以直接氧化降解NB(式(9))[31],另一方面可以與反應(yīng)液中的水電離出的OH-發(fā)生反應(yīng)生成?OH(式(10)).

圖7 不同體系中的瞬態(tài)壓電電流測試

此外,在反應(yīng)體系中,O3可直接氧化NB(式2-11).并且,在弱堿性條件下(pH 9.8),O3和OH-反應(yīng)產(chǎn)生?OH(式(1)和(2))[25].

因此,PE-O3體系對NB的降解主要分為自由基過程和非自由基過程.其中,自由基過程的主要活性物質(zhì)為?OH,并可以通過以下四條路徑生成:1)O3的單電子還原(式(5)、式(7)和式(8))2)O2的單電子還原(式(4)和式(6)至式(8))3)O3和OH-的反應(yīng)(式(1)和式(2))和4)+和OH-的反應(yīng)(式(10)).而非自由基過程主要包括O3(式(11))和+(式(9))由對NB的直接氧化降解.

O3+R?byproducts (11)

在大多數(shù)情況下,多相催化臭氧氧化涉及催化劑與臭氧之間的電子轉(zhuǎn)移,這可能改變催化劑中過渡元素的價態(tài),從而降低催化劑的穩(wěn)定性[36-37].X射線光電子能譜(XPS)可對晶體中各種元素化學(xué)狀態(tài)進行分析測量[38-39].對BT晶體在10次循環(huán)試驗前后的XPS進行測量.如圖8(b)所示,鈦(BT中唯一的過渡金屬)的化學(xué)狀態(tài)保持在Ti4+,沒有任何變化[40-41].此外,如圖8(a)和8(c)所示,鋇(Ba2+)和氧(O2-)的化學(xué)狀態(tài)都沒有變化[42-43].由此可見,參與氧化還原反應(yīng)的電子可能來自壓電反應(yīng)產(chǎn)生的內(nèi)置電場,能量由機械轉(zhuǎn)化為電能[44-45],因此活性元素的化學(xué)狀態(tài)不變.

3 結(jié)論

3.1 水力驅(qū)動的壓電體系與臭氧體系對硝基苯的降解表現(xiàn)出較好的協(xié)同效果,協(xié)同系數(shù)高達5.04,并在120min內(nèi)實現(xiàn)了對NB高達74.06%的礦化.

3.2 在PE-O3體系中,體系反應(yīng)速率與轉(zhuǎn)速呈正相關(guān).這可能是因為,一方面攪拌使得壓電效應(yīng)增強,更多的電子參與反應(yīng),另一方面,體系中反應(yīng)物充分混合,接觸面積增大,反應(yīng)更為迅速.

3.3 在PE-O3體系中,反應(yīng)速率常數(shù)隨著O3濃度和BT投加量的增大增加,但增大到一定程度以后,將不再是關(guān)鍵影響因素,增速將會變得緩慢.

3.4 在堿性條件下,O3和OH-反應(yīng)產(chǎn)生更多的活性自由基,但在較高pH下,O3含量不足,導(dǎo)致O3的壓電催化臭氧化與臭氧與OH-反應(yīng)兩個反應(yīng)互相制約,減緩了反應(yīng)速率的增加.

3.5 在PE-O3體系中,NB主要被?OH降解,同時也可被O3和+降解.

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Degradation of nitrobenzene from water by piezoelectric ozonation of barium titanate.

ZHUANG Wei, YANG Jing, GONG Bing-rou, ZHENG Ying, ZHAO Chun*

(College of Environment and Ecology, Chongqing University, Chongqing 400045, China)., 2021,41(10)：4654～4661

The single ozone process is difficult to degrade refractory pollutants. Although the electrolysis ozonation process has been proven to effectively degrade and mineralize refractory organic pollutants, it is limited by electrode materials and mass transfer. The piezoelectric ozonation (PE-O3) process was proposed by combined the ozone (O3) process with the piezoelectric (PE) process. The PE-O3process showed a significant synergistic effect (the synergy index = 5.04) on the degradation of nitrobenzene (NB). Besides, the degradation ratio of NB was 85.37% in PE-O3process within 15min, and the reaction conformed to pseudo first order (= 0.1256min-1). The TOC removal ratio was 74.06% in PE-O3process within 120min. As the rotation speed increased, the reaction rate increased. However, the reaction rate constant could reach 0.1446min-1when the rotation speed increased to 1500r/min. The reaction rate increased with the increase of BT and O3concentration in PE-O3process, but the increasing trend slowed down after a certain degree. Moreover, the degradation rate of NB increased with the increase of pH value. When the pH value is 9.0, the degradation rate of NB in the system reached 80% after 15min. Notably, the?OH produced during the reaction is the main active species to degrade NB.

barium titanate；piezoelectric ozonation；nitrobenzene；piezoelectricity；ozone

X52

A

1000-6923(2021)10-4654-08

莊 瑋(1995-),男,四川閬中人,重慶大學(xué)碩士研究生,主要從事基于臭氧的高級氧化研究.

2021-03-25

國家自然科學(xué)基金資助項目(22076015);重慶市自然科學(xué)基金(cstc2019jcyj-msxmX0463)

* 責(zé)任作者, 副教授, pureson@163.com