陳家斌,魏成耀,房 聰,周 露,胡金梅,郝尚斌,張黎明,王忠明,黃天寅(蘇州科技大學(xué)環(huán)境科學(xué)與工程學(xué)院,江蘇 蘇州 215009)

碳納米管活化過二硫酸鹽降解偶氮染料酸性橙7

陳家斌,魏成耀,房 聰,周 露,胡金梅,郝尚斌,張黎明,王忠明,黃天寅*(蘇州科技大學(xué)環(huán)境科學(xué)與工程學(xué)院,江蘇 蘇州 215009)

采用碳納米管(CNT)活化過二硫酸鹽(PS)降解偶氮染料酸性橙7(AO7).考察了PS濃度、CNT投加量、初始pH值、溫度等反應(yīng)條件對AO7降解效果的影響.結(jié)果表明,當(dāng)初始pH為7、n(PS)/n(AO7)為20、CNT投加量為0.2g/L時,AO7在反應(yīng)480min后可以被完全脫色去除.隨著PS劑量、CNT投加量和溫度的升高,AO7的去除率也逐漸增加,中性條件下最有利于AO7的去除.AO7降解反應(yīng)主要發(fā)生在CNT表面,且反應(yīng)活化能Ea為46.76kJ/mol.通過紫外-可見分光光譜、氣相色譜—質(zhì)譜(GC-MS)和TOC分析表明,AO7分子偶氮鍵和萘環(huán)結(jié)構(gòu)斷裂,生成含苯環(huán)類物質(zhì),最終礦化為C O2和H2O.

碳納米管；過硫酸鹽；偶氮染料；降解

紡織印染行業(yè)廢水排放占我國工業(yè)廢水排放總量的比重逐年遞增.其中偶氮染料廢水色度大、難降解、毒性強(qiáng),不經(jīng)處理直接排放會造成嚴(yán)重的環(huán)境污染[1-3].偶氮染料廢水的處理方法分為生物法和物化法,偶氮染料分子具生物毒性,因此難以好氧生物處理;而厭氧生物處理工藝能有效破壞偶氮染料分子偶氮鍵,但是較長的水力停留時間是生物處理的限制性因素[4-6].物化法主要是利用物理或化學(xué)作用脫色,如吸附、絮凝、離子交換、膜分離、氧化等[7-8].其中高級氧化方法由于產(chǎn)生氧化能力強(qiáng)的活性自由基,不僅能夠?qū)ε嫉玖嫌行撋?也具有較好的礦化效果,在處理偶氮染料廢水時顯示出潛在的優(yōu)越性[9-10].

近年來,基于硫酸根自由基(SO4-·)的高級氧化技術(shù)逐漸引起人們的關(guān)注[11].硫酸根自由基具有較高的氧化還原電位,相比羥基自由基(OH·),具有更長的半衰期,在中性或堿性條件下仍有較好的氧化能力[12].過二硫酸鹽(PS)和過一硫酸鹽(PMS)是兩種常用的產(chǎn)生 SO4-·的氧化劑,它們在常溫下較穩(wěn)定,而在過渡金屬離子[13]、紫外光[14]、堿[15]、加熱等作用下能夠活化產(chǎn)生·[16].其中過渡金屬離子被認(rèn)為是最常用的活化劑,然而重復(fù)利用性以及潛在的毒性限制了過渡金屬離子活化的應(yīng)用[17].而碳質(zhì)材料表面含有的羰基等含氧官能團(tuán)具有催化功能,能夠有效催化 O3和H2O2氧化降解污染物[18-19].最近,有學(xué)者發(fā)現(xiàn)活性炭和活性炭纖維表面的=O、—OH及—OOH等基團(tuán)能夠有效活化PS或PMS[20-22].碳質(zhì)材料有較好的重復(fù)利用性,無金屬離子排放,對環(huán)境無二次污染物排放,在催化氧化降解偶氮染料中具有潛在的優(yōu)勢[23].

碳納米管(CNT)由于具有比表面積大、優(yōu)良的熱電傳導(dǎo)性等優(yōu)點,在各種催化反應(yīng)中具有廣泛的應(yīng)用[24].最近,作者相關(guān)研究表明 CNT能夠高效活化PMS降解偶氮染料[25].然而,與PMS相比,PS價格更加低廉,且在常溫下更穩(wěn)定,更容易保存和運(yùn)輸,在實際工程中運(yùn)用更廣泛[26],因此,研究CNT活化PS降解偶氮染料具有更重要的意義.本文研究了CNT活化PS降解偶氮染料酸性橙AO7,分析各種反應(yīng)條件對AO7降解的影響,并對其降解產(chǎn)物和礦化程度進(jìn)行了分析.

1 材料與方法

1.1 材料與試劑

碳納米管(CNT)購于南京先豐納米材料科技有限公司,過硫酸鈉(Na2S2O8, PS)購于Sigma-Aldrich;酸性橙 7(AO7)購于國藥集團(tuán)化學(xué)試劑有限公司, 圖1為化學(xué)結(jié)構(gòu).分析純甲醇、叔丁醇、苯酚、硫酸、氫氧化鈉、亞硝酸鈉均購于國藥集團(tuán)化學(xué)試劑有限公司.實驗用水為去離子水.

圖1 酸性橙7的化學(xué)結(jié)構(gòu)式Fig.1 Molecule structure of Acid Orange 7

1.2 降解實驗

CNT活化PS降解AO7實驗在250mL錐形瓶中進(jìn)行,反應(yīng)溶液在磁力攪拌器上持續(xù)攪拌,初始pH值采用硫酸和氫氧化鈉調(diào)節(jié).往AO7和PS反應(yīng)溶液中加入 CNT后,啟動反應(yīng),在預(yù)定的時間點取樣,并迅速加入過量的亞硝酸鈉溶液猝滅反應(yīng),隨后經(jīng)過 0.45μm的濾膜過濾,濾過液采用UV-Vis分光光度計測其濃度.

1.3 分析方法

使用 Mapada UV-1600 (PC)紫外可見分光光度計,于AO7最大吸收波長484nm處測定濾液的吸光度,代入標(biāo)準(zhǔn)曲線求得濃度c. TOC采用總有機(jī)碳分析儀(TOC-LCPH,島津)測定.

2 結(jié)果與討論

2.1 CNT活化PS性能

圖2 不同體系A(chǔ)O7和PS的降解效果Fig.2 Degradation of AO7and PS in different systems[AO7]=20mg/L; [CNT]=0.2g/L; pH=7.0; T=298K; n(PS)/n(AO7)=20/1

從中圖2(a)可看出,PS單獨存在時,AO7幾乎不降解,說明常溫下 PS穩(wěn)定,不會活化產(chǎn)生自由基.CNT單獨存在時,AO7在480min時去除率為50%,說明CNT可以有效吸附AO7,并且該吸附主要發(fā)生在初始30min內(nèi).PS和CNT共同存在時,AO7的去除效率快速增加,360min時幾乎完全去除,表明CNT可以有效活化PS產(chǎn)生活性自由基,從而快速脫色去除 AO7.為了進(jìn)一步表明CNT對PS的活化,監(jiān)測了反應(yīng)過程中PS的降解,如圖 2(b)所示.PS單獨存在時不分解,而加入CNT后,PS分解較明顯,初始20min時約40%的PS分解,而30min后PS分解較慢,這和AO7的降解趨勢一致,進(jìn)一步說明了CNT/PS體系中PS活化產(chǎn)生自由基促使 AO7氧化降解.CNT由于具有高比表面積、sp2雜化碳原子結(jié)構(gòu)等特性,在各種催化反應(yīng)過程中呈現(xiàn)較強(qiáng)的活性[27].CNT曾被報道可以高效活化過一硫酸鹽(PMS)產(chǎn)生硫酸根自由基,進(jìn)而高效氧化降解苯酚[28].與 PMS相比,PS分子結(jié)構(gòu)對稱而更加穩(wěn)定,但仍可以被CNT表面的sp2雜化碳或者其它官能團(tuán)活化產(chǎn)生硫酸根自由基,進(jìn)而氧化降解偶氮染料AO7.

2.2 PS濃度對AO7降解的影響

圖3 CNT/PS體系中PS濃度對AO7降解的影響Fig.3 Effect of PS concentration on the degradation ofAO7in the CNT/PS system[AO7]=20mg/L; [CNT]=0.2g/L; pH=7.0; T=298K

圖3為不同PS濃度對AO7降解的影響.當(dāng)n(PS)/n(AO7)由1/1增加為5/1時,AO7的降解速度顯著加快,480min時AO7的去除率由55%提高到80%.當(dāng)n(PS)/n(AO7)進(jìn)一步增加時,AO7的去除率同樣增加,但增加程度減小,如 n(PS)/ n(AO7)由20/1增加到50/1時,AO7在480min時的去除率僅從 90%增加到 94%.由此可見,隨著n(PS)/n(AO7)初始濃度的增大,活化降解AO7速率加快,降解完全所需時間越短.盡管有文獻(xiàn)表明,在活化PS的反應(yīng)體系中,當(dāng)PS濃度過高時,會產(chǎn)生大量的自由基相互促滅,進(jìn)而降低污染物的降解速率[29].但是這一現(xiàn)象在本研究中并沒有出現(xiàn),CNT由于比表面積大,可以提供足夠多的表面反應(yīng)活性位點,活化 PS產(chǎn)生活性自由基,進(jìn)而氧化降解AO7.

2.3 CNT投加量對AO7降解的影響

圖4 CNT/PS體系中CNT投加量對AO7去除的影響Fig.4 Effect of CNT dosage on the removal of AO7 in the CNT/PS system[AO7]=20mg/L; pH=7.0; T=298K; n(PS)/n(AO7)=20/1

由圖4可知,隨著CNT的投加量增大,AO7的吸附和降解速率都顯著加快.在單獨投加CNT時,當(dāng)投加量由0.04g/L增加到0.1g/L時,240min時AO7的吸附去除率由5%提升到25%,當(dāng)投加量進(jìn)一步增加到0.2g/L和0.4g/L時,AO7的吸附去除率為58%和90%.在CNT活化PS的反應(yīng)體系中,投加量的增加對 AO7降解影響更加明顯,投加量為0.2g/L時,AO7在180min時可完全去除;當(dāng)投加量進(jìn)一步增加到 0.4g/L時,AO7在30min內(nèi)就可以完全去除.隨著CNT濃度的升高,可以提供更多的活性位點吸附AO7,同時可以活化PS產(chǎn)生自由基降解AO7.

2.4 反應(yīng)溫度對AO7降解的影響

圖5 不同的反應(yīng)溫度對CNT/PS體系降解AO7的影響Fig.5 Effect of reaction temperature on the degradation of AO7in the CNT/PS system[AO7]=20mg/L;[CNT]=0.2g/L; pH=7.0; n(PS)/n(AO7)=20/1

溫度是影響 PS活化的重要因素,于是考查了不同溫度下CNT活化PS降解AO7的效果.如圖5所示,溫度對AO7的降解有重要的影響,隨著溫度的升高,AO7去除效率逐漸升高.30℃時,AO7在90min的去除率只有90%,當(dāng)溫度升高到40℃時,AO7在90min內(nèi)完全去除.而溫度繼續(xù)升高到 50℃和 60℃時,AO7完全去除所需要的時間分別為60min和30min.這可能是因為在高溫條件下CNT更容易活化PS產(chǎn)生硫酸根自由基,并且 AO7分子在高溫下容易克服反應(yīng)活化能

[30].對不同溫度下 AO7降解趨勢進(jìn)行動力學(xué)模擬,結(jié)果發(fā)現(xiàn)AO7降解能較好的符合一級降解動力學(xué),反應(yīng)動力學(xué)常數(shù) k分別為 0.02588、0.07554、0.10896、0.14457min-1,隨著溫度的升高,AO7降解速率越快.采用Arrhenius公式對不同溫度下k進(jìn)一步擬合,計算出反應(yīng)活化能Ea為 46.76kJ/mol.

2.5 初始pH值對AO7降解的影響

圖6 初始pH對CNT/PS體系降解AO7的影響Fig.6 Effect of initial pH on the degradation of AO7 in the CNT/PS system[AO7]=20mg/L; [CNT]=0.2g/L;T=298K; n(PS)/n(AO7)=20/1

pH值也是影響PS活化的重要因素;同時偶氮染料廢水水質(zhì)復(fù)雜,pH變化范圍廣,因此研究pH對活化PS降解偶氮染料AO7具有重要的意義.由圖6看出,在pH為3～9范圍內(nèi),AO7降解都能取得較好的效果,其中中性條件下降解效果最好,說明CNT活化PS氧化體系適于偶氮染料廢水的處理.在酸性條件下,由于 CNT的表面零電荷點(pHpzc)為 7.2,CNT表面主要帶正電,有利于吸附陰離子染料AO7,CNT表面的活性位點吸附AO7后不利于進(jìn)一步活化 PS產(chǎn)生活性自由基,因而不利于污染物的降解.在堿性條件下,CNT表面主要帶負(fù)電,與過硫酸鹽陰離子靜電相斥,不利于 PS陰離子吸附進(jìn)而被活化;同時,溶液中的氫氧根離子會猝滅活化產(chǎn)生的·,降低 AO7被氧化降解的機(jī)會.因此,在酸性或者堿性條件下,AO7在CNT活化PS體系中的降解效率都會降低.

2.6 抑制劑的影響

在 PS活化過程中,通常會產(chǎn)生硫酸根自由基和羥基自由基,為了鑒別哪種自由基在污染物降解中起作用,通常會采用甲醇和叔丁醇分別作為自由基抑制劑,通過污染物降解抑制情況來判別[31].甲醇由于含有α－H,與·和HO·都有較高的二級反應(yīng)常數(shù),對兩者都有良好的淬滅效果;叔丁醇與HO·二級反應(yīng)速率常數(shù)是·的1000倍以上,只對HO·具有良好的猝滅效果[32-33].

圖7 CNT/PS體系中自由基抑制劑對AO7的降解Fig.7 Degradation of AO7 in the CNT/PS system in the presence of different radical scavengers[AO7]=20mg/L; [CNT]=0.2g/; pH=7.0; T=298K; n(PS)/n(AO7)=20/1

如圖7(a)、(b)所示,在CNT活化PS降解AO7反應(yīng)體系中分別加入甲醇和叔丁醇后,發(fā)現(xiàn)AO7的降解并未受很大影響,這與以往的文獻(xiàn)報道結(jié)果不一致.這是因為甲醇和叔丁醇是相對親水性物質(zhì),在均相反應(yīng)體系中可以擴(kuò)散均勻,是一種良好的猝滅劑[34],但是在非均相反應(yīng)體系中,不容易擴(kuò)散到固體催化劑,如 CNT的表面,不能猝滅CNT表面的自由基.因此,根據(jù)以上結(jié)果可以得出CNT活化PS產(chǎn)生的自由基主要存在于CNT的表面.為了驗證這一結(jié)論,進(jìn)一步采用苯酚作為抑制劑,考查對 AO7降解的抑制情況.相對于甲醇和叔丁醇,苯酚是疏水性物質(zhì),很容易擴(kuò)散到CNT表面,并且苯酚與·和HO·都有較高的二級反應(yīng)常數(shù),對兩者都有良好的淬滅效果[22].如圖7(c)所示,加入苯酚后,AO7的降解被明顯的抑制,并且抑制程度隨著苯酚加入濃度的升高而增強(qiáng).加入苯酚/AO7物質(zhì)的量比為1000:1時,AO7的去除趨勢與吸附趨勢一致,表明AO7的降解被完全抑制.由以上自由基抑制實驗表明,CNT活化PS降解AO7主要發(fā)生在CNT的表面,并且·和HO·都在AO7的降解中起重要作用.

2.7 AO7紫外可見光譜和TOC變化

由圖8可知,AO7主要有484nm和310nm特征吸收峰,分別代表 AO7分子結(jié)構(gòu)中的偶氮鍵以及萘環(huán)結(jié)構(gòu)[35-36].隨著降解反應(yīng)的進(jìn)行,這兩個特征峰強(qiáng)度逐漸降低,表明偶氮鍵和萘環(huán)斷裂,生成其它小分子物質(zhì).與此同時,在 250nm左右形成一個明顯的吸收特征峰,該特征峰強(qiáng)度隨著反應(yīng)的進(jìn)行而逐漸增強(qiáng).250nm特征峰對應(yīng)苯環(huán)結(jié)構(gòu)中的π→π*躍遷,說明反應(yīng)過程中,萘環(huán)被硫酸根自由基氧化斷裂后,生成一系列含苯環(huán)的物質(zhì).

由圖9可知,反應(yīng)90min后,體系中TOC的去除率達(dá)到49%,因此CNT/PS體系對AO7不僅具有很好的脫色效果,同時也具有一定的礦化率.同時,隨著反應(yīng)的進(jìn)行,TOC的去除率先快后慢,在30min內(nèi)去除率占總?cè)コ实?94%,這主要是因為反應(yīng)過程中可能生成難礦化的中間產(chǎn)物,占據(jù)CNT活性位點,阻礙進(jìn)一步與PS接觸并活化PS產(chǎn)生硫酸根自由基,從而不利于體系進(jìn)一步的礦化.通過GC/MS對AO7降解過程中的降解產(chǎn)物進(jìn)行分析,檢測到的主要降解產(chǎn)物如表1所示,可以看出降解產(chǎn)物多以苯環(huán)為主要結(jié)構(gòu),這與光譜分析中結(jié)論一致.

圖8 AO7降解過程紫外可見光譜Fig.8 UV-Vis spectra changes for AO7 degradation[AO7]=20mg/L; [CNT]=0.2g/L;T=298K;n(PS)/n(AO7)=20/1

圖9 AO7降解過TOC的變化Fig.9 TOC changes for AO7 degradation[AO7]=20mg/L; [CNT]=0.2g/L; T=298K;pH=7.0;n(PS)/n(AO7)=20/1

表1 GC/MS測得AO7降解的降解產(chǎn)物Table 1 Degradation products of AO7 determined by GC/MS

3 結(jié)論

3.1 CNT能夠有效活化PS降解偶氮染料AO7,降解反應(yīng)發(fā)生在CNT表面, 并且·和HO·都在AO7的降解中起重要作用.

3.2 AO7的降解效果隨著PS的劑量、CNT投加量的升高而升高,高溫有利于 AO7的降解,且反應(yīng)活化能為49.74kJ/mol.

3.3 AO7降解過程中偶氮鍵和萘環(huán)結(jié)構(gòu)斷裂,生成含苯環(huán)類物質(zhì),最終礦化為CO2和H2O.

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Decolorization of acid orange 7 by persulfate activated by carbon nanotube.

CHEN Jia-bin, WEI Cheng-yao, FANG Cong, ZHOU Lu, HU Jin-mei, HAO Shang-bin, ZHANG Li-ming, WANG Zhong-ming, HUANG Tian-yin1*(School of Environmental Science and Engineering, Suzhou University of Science and Technology, Suzhou 215009, China). China Environmental Science, 2016,36(12)：3618～3624

Carbon nanotube (CNT) was used as an activator to activate persulfate (PS) to decolorize the azo dye, acid orange 7 (AO7). Effect of PS dosage, CNT loading, initial pH and reaction temperature on decolorization of AO7 was investigated. The results indicated that completely decolorization of AO7 was achieved within 480min when 0.2g/L of CNT and 20 of n(PS)/n(AO7) were present in the system. AO7 decolorization was increased with increasing PS dosage, raising CNT loading and elevating temperature, and neutral pH was most favorable for AO7 decolorization. The oxidative degradation of AO7 mainly took place on the surface of CNT and the activation energy (Ea) was determined to be 46.76kJ/mol. From the analysis of UV-Vis spectra, GC-MS and TOC during the reaction, the azo band and naphthalene ring of AO7 were proposed to be destroyed to generate other small intermediate containing benzene, and finally mineralized to CO2and H2O.

carbon nanotubes；persulfate；azo dye；degradation

X703.1

A

1000-6923(2016)12-3618-07

陳家斌(1986-),男,江西贛州人,講師,主要研究方向為抗生素等新興污染物的污染控制、環(huán)境行為和生態(tài)效應(yīng)研究.發(fā)表論文20余篇.

2016-04-20

國家自然科學(xué)基金項目(51478283);蘇州科技大學(xué)學(xué)術(shù)學(xué)位研究生科研創(chuàng)新計劃項目(SKCX15_026)

* 責(zé)任作者, 教授, huangtianyin111@sohu.com