李國(guó)亭 , 馮艷敏, 范金周, 柴曉琪, 何小爽, 梅 靜

(1.華北水利水電大學(xué) 環(huán)境與市政工程學(xué)院,河南 鄭州 450045; 2.河南省建筑科學(xué)研究院有限公司,河南 鄭州 450053)

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納米二氧化鈦吸附去除對(duì)苯醌的實(shí)驗(yàn)研究

李國(guó)亭1, 馮艷敏1, 范金周2, 柴曉琪1, 何小爽1, 梅 靜1

(1.華北水利水電大學(xué) 環(huán)境與市政工程學(xué)院,河南 鄭州 450045; 2.河南省建筑科學(xué)研究院有限公司,河南 鄭州 450053)

TiO2光催化氧化芳香類(lèi)有機(jī)污染物的過(guò)程中會(huì)產(chǎn)生一系列的中間氧化產(chǎn)物,如對(duì)苯醌和鄰苯二酚等.考慮到這些中間產(chǎn)物的高毒性,很有必要研究在光催化過(guò)程中TiO2對(duì)這些中間產(chǎn)物的吸附性能.本研究中,使用納米TiO2吸附去除對(duì)苯醌,研究了吸附劑用量和溶液pH值等條件對(duì)吸附效果的影響,并著重對(duì)吸附過(guò)程進(jìn)行熱力學(xué)分析.研究結(jié)果表明,中性條件有利于對(duì)苯醌的吸附去除.Langmuir和Redlich-Peterson模型能更好地模擬吸附過(guò)程,對(duì)苯醌的最大吸附量達(dá)到342.7 mg/g.熱力學(xué)分析表明:吸附反應(yīng)的焓變?yōu)?51.096 kJ/mol,吉布斯自由能變從288 K下的-20.338 kJ/mol增至308 K下的-18.196 kJ/mol,說(shuō)明該吸附過(guò)程是放熱、自發(fā)過(guò)程.

吸附;對(duì)苯醌;TiO2;吸附等溫線;熱力學(xué)

目前高級(jí)氧化技術(shù)被廣泛應(yīng)用于水和廢水中有機(jī)物的去除,并因此越來(lái)越受到世界各國(guó)的關(guān)注.高級(jí)氧化技術(shù)的特征:在水溶液中產(chǎn)生最活潑的羥基自由基HO·(2.72V/NHE)及H2O2等強(qiáng)氧化劑[1].例如,在以TiO2為催化劑的光催化氧化過(guò)程中產(chǎn)生大量的氧化性物質(zhì)如H2O2,HOO·和·O2等,這些氧化性物質(zhì)在整個(gè)氧化過(guò)程中一直存在[2-3]，因此,高級(jí)氧化過(guò)程中產(chǎn)生的氧化性物質(zhì)能有效去除病原微生物[4-6]和大量的有機(jī)污染物.在反應(yīng)時(shí)間充分的情況下,有機(jī)污染物可被礦化為CO2,H2O和無(wú)機(jī)鹽[6-9].但由于高級(jí)氧化技術(shù)的成本較高,目前僅被用于部分氧化而不是完全礦化.

使用高級(jí)氧化技術(shù)氧化芳香類(lèi)污染物的過(guò)程中會(huì)產(chǎn)生一系列的中間氧化產(chǎn)物,如鄰苯二酚、對(duì)苯醌和有機(jī)酸等[10-12].Santos等研究了濕式氧化苯酚的過(guò)程中產(chǎn)生的中間體的毒性,苯酚、鄰苯二酚、對(duì)苯二酚和對(duì)苯醌的EC50值分別為(16.7±4.2),(8.32±2.7),0.041,0.1 mg/L[13],表明對(duì)苯二酚和對(duì)苯醌的毒性分別達(dá)到它們母體苯酚的2到3個(gè)數(shù)量級(jí).因此,盡管高級(jí)氧化法對(duì)有機(jī)污染物有較強(qiáng)的去除能力,但通過(guò)延長(zhǎng)反應(yīng)時(shí)間將產(chǎn)生的中間產(chǎn)物全部氧化為CO2和H2O是不現(xiàn)實(shí)的[14].所以有必要在高級(jí)氧化處理設(shè)施后采用后續(xù)處理技術(shù)以去除高毒性中間氧化產(chǎn)物.

光催化劑TiO2是一種穩(wěn)定、無(wú)腐蝕性、環(huán)保、廣泛存在且價(jià)格合適的材料,被廣泛應(yīng)用于各種有機(jī)污染物的光催化氧化過(guò)程中,但氧化過(guò)程中產(chǎn)生的中間體會(huì)吸附在TiO2的表面并導(dǎo)致其光催化活性降低.如果光催化氧化過(guò)程中使用的TiO2沒(méi)有被回收或再生,其表面吸附的氧化中間體將緩慢釋放到水體中,對(duì)環(huán)境產(chǎn)生持續(xù)的不利影響.但目前氧化中間體在TiO2顆粒上的吸附和轉(zhuǎn)移鮮有報(bào)道,被吸附的氧化中間體向環(huán)境轉(zhuǎn)移的程度到目前并不清楚.所以,很有必要探索氧化中間產(chǎn)物在TiO2上的吸附行為.本研究以高級(jí)氧化過(guò)程中常見(jiàn)的中間體對(duì)苯醌為目標(biāo)污染物,經(jīng)納米TiO2吸附去除,研究對(duì)苯醌的吸附熱力學(xué)和動(dòng)力學(xué)過(guò)程,以及吸附劑用量和溶液酸堿性對(duì)吸附的影響.

1 實(shí)驗(yàn)部分

對(duì)苯醌，化學(xué)純,北京化學(xué)試劑有限公司.納米TiO2(商業(yè)用TiO2,P-25),德國(guó)Degussa公司.其他試劑均為分析純.實(shí)驗(yàn)用水為去離子水.

稱(chēng)取一定量的對(duì)苯醌溶于去離子水中并超聲5 min,配制質(zhì)量濃度為1 000 mg/L的對(duì)苯醌溶液作為母液,室溫下保存于棕色瓶中,使用時(shí)用去離子水稀釋母液獲得所需濃度的溶液.

對(duì)苯醌的吸附去除是在錐形瓶中以分批實(shí)驗(yàn)的形式進(jìn)行.動(dòng)力學(xué)實(shí)驗(yàn)中,將200 mg納米TiO2加入到1 000 mL質(zhì)量濃度為5 mg/L的對(duì)苯醌溶液中.在pH值影響實(shí)驗(yàn)和熱力學(xué)實(shí)驗(yàn)中,將10 mg納米TiO2加入到50 mL質(zhì)量濃度為5 mg/L的對(duì)苯醌溶液中,并在135 r/min的轉(zhuǎn)速下于搖床中恒溫振蕩24 h至吸附平衡.除特殊說(shuō)明外,溫度控制在298 K.通過(guò)加入HCl或NaOH溶液來(lái)調(diào)節(jié)溶液pH值.除了討論pH值對(duì)對(duì)苯醌吸附效果的影響實(shí)驗(yàn)外,其他所有溶液酸堿性均控制為中性.

用0.45 μm的濾膜過(guò)濾吸附后的溶液,收集過(guò)濾液用于測(cè)定分析.使用UVmini-1240紫外分光光度計(jì)在254 nm波長(zhǎng)處測(cè)定對(duì)苯醌的吸光度.

2 結(jié)果與討論

2.1 納米TiO2的質(zhì)量對(duì)吸附效果的影響

分別向50 mL質(zhì)量濃度為5 mg/L的對(duì)苯醌溶液中加入5～100 mg不等的TiO2,研究TiO2質(zhì)量對(duì)對(duì)苯醌吸附效果的影響,結(jié)果如圖1所示.

圖1 中性條件下TiO2 的質(zhì)量對(duì)對(duì)苯醌吸附效果的影響

可知,當(dāng)所加TiO2的質(zhì)量分別為5,10,20 mg時(shí),對(duì)苯醌的平衡吸附量達(dá)到16.8,9.1,4.3 mg/g,隨著吸附劑質(zhì)量的增大,對(duì)苯醌的單位吸附量降低.這是因?yàn)楸M管吸附位點(diǎn)仍未飽和,可供吸附的有效位點(diǎn)總數(shù)也隨著TiO2的質(zhì)量的增多而增大,但由于未利用吸附位的增多,單位質(zhì)量吸附劑對(duì)對(duì)苯醌的吸附量仍然下降.后續(xù)實(shí)驗(yàn)中每50 mL溶液中加入TiO2的質(zhì)量為10 mg.

2.2 溶液pH值對(duì)對(duì)苯醌吸附效果的影響

在溶液酸堿性對(duì)納米TiO2吸附對(duì)苯醌的影響室驗(yàn)中,溶液的pH值控制在2.0～11.0,結(jié)果如圖2所示.

圖2 溶液平衡pH值對(duì)對(duì)苯醌吸附效果的影響

由圖2可知,溶液酸堿性對(duì)對(duì)苯醌的吸附影響明顯.在酸性溶液中,隨著溶液pH值的增大,對(duì)苯醌的吸附量顯著升高,而當(dāng)溶液pH值從7.0增大到11.0時(shí),對(duì)苯醌的吸附量急劇下降,當(dāng)pH值為7.0時(shí),對(duì)苯醌的吸附量最大,達(dá)6.4 mg/g.由于納米TiO2的零電勢(shì)點(diǎn)是6.8,所以,當(dāng)pH值小于6.8時(shí),TiO2帶正電,反之則帶負(fù)電,而對(duì)苯醌通常被認(rèn)為是非極性或具有弱極性，可推測(cè)對(duì)苯醌在納米TiO2上的吸附與溶液的pH值關(guān)系不大.但是,上述實(shí)驗(yàn)結(jié)果表明，對(duì)苯醌在TiO2上的吸附受pH值的影響很大.據(jù)此可以推測(cè),對(duì)苯醌的極性足以影響它的帶電特性,在酸性條件下對(duì)苯醌顯正電性,在堿性條件下顯負(fù)電性.所以,對(duì)苯醌和納米TiO2之間的靜電斥力在酸性和堿性條件下較強(qiáng),而在中性條件下則較弱,因此,在pH值為7.0左右時(shí)對(duì)苯醌的吸附量最大.后續(xù)實(shí)驗(yàn)中,溶液pH值均控制在7.0左右.

2.3 吸附熱力學(xué)

本研究用Langmuir,Freundlich,Temkin和Redlich-Peterson模型擬合對(duì)苯醌的吸附過(guò)程,各模型的數(shù)學(xué)表達(dá)式如下.

Langmuir等溫式[15]:

(1)

式中:Ce為平衡濃度,mg/L;kL為與吸附質(zhì)和吸附劑之間親和力相關(guān)的平衡常數(shù),L/mg;qm為最大吸附量,mg/g.

Freundlich 等溫式[16]:

(2)

式中:kF為Freundlich常數(shù);n為自由度.

Temkin等溫式[17]:

qe=A+BlnCe.

(3)

式中:A和B均為不均勻因子.

Redlich-Peterson等溫式[18]:

(4)

298 K時(shí)對(duì)苯醌在納米TiO2上的吸附實(shí)驗(yàn)點(diǎn)及非線性擬合結(jié)果如圖3所示.

圖3 對(duì)苯醌在TiO2 上的吸附熱力學(xué)(T=298 K)

由圖3可知,吸附平衡的數(shù)據(jù)經(jīng)4種吸附等溫線(Langmuir,Freundlich,Temkin和Redlich-Peterson等溫式)擬合,相關(guān)系數(shù)分別為0.915,0.836,0.873和0.968.對(duì)比可知,Langmuir和Redlich-Peterson等溫式能較好地?cái)M合實(shí)驗(yàn)結(jié)果.為了進(jìn)行進(jìn)一步對(duì)比,288 K和308 K下的數(shù)據(jù)也由上述4種等溫式擬合,擬合結(jié)果同樣是Langmuir和Redlich-Peterson等溫式的擬合效果較好,結(jié)果見(jiàn)表1.

吉布斯自由能變(ΔG0,J/mol)、焓變 (ΔH0,J/mol)和熵變(ΔS0,J/(mol·K))這些熱力學(xué)參數(shù)可以分析溫度對(duì)吸附的影響,吉布斯自由能變的計(jì)算公式為

ΔG0=-RTlnK0.

(5)

式中:R為氣體摩爾常數(shù),8.314 J/(mol·K);T為熱力學(xué)溫度,K;K0為平衡吸附常數(shù).

表1 不同溫度下納米TiO2吸附對(duì)苯醌的熱力學(xué)模型參數(shù)

吸附過(guò)程中的熱力學(xué)平衡吸附常數(shù)K0是通過(guò)繪制ln(qe/Ce)對(duì)qe的坐標(biāo)圖(如圖4所示)并外推出截距來(lái)求得的[19],即為3個(gè)溫度下的直線與縱軸的交點(diǎn).

圖4 不同溫度下TiO2吸附對(duì)苯醌的ln(qe/Ce)對(duì)

ΔH0和ΔS0的值可由范特霍夫方程求出:

ΔG0=ΔH0-TΔS0.

(6)

納米TiO2吸附對(duì)苯醌的吉布斯自由能變隨溫度變化關(guān)系如圖5所示，其他熱力學(xué)參數(shù)具體值見(jiàn)表2.

由圖5和表2可知,吉布斯自由能變從288 K 下的-20.338 kJ/mol增至308 K下的-18.196 kJ/mol,焓變?yōu)?51.096 kJ/mol,說(shuō)明該吸附過(guò)程是一個(gè)自發(fā)的、放熱的吸附過(guò)程.

圖5 吉布斯自由能變 (熱力學(xué)計(jì)算)

溫度T/KlnK0吉布斯自由能變?chǔ)0/(kJ/mol)焓變?chǔ)0/(kJ/mol)熵變?chǔ)0/(J/(mol/K)2888．494-20．338-51．09106．672987．667-18．9963087．106-18．196

3 結(jié) 語(yǔ)

本研究選取具有高毒性的氧化中間產(chǎn)物對(duì)苯醌作為目標(biāo)污染物,探究了納米TiO2對(duì)它的吸附效果.研究發(fā)現(xiàn),對(duì)苯醌的吸附受pH值的影響較大,中性條件下的吸附效果較好.Langmuir和Redlich-Peterson模型能較好地模擬吸附過(guò)程,Langmuir模型擬合的最大吸附量達(dá)到342.7 mg/g.熱力學(xué)分析表明該吸附過(guò)程是自發(fā)、放熱過(guò)程.

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(責(zé)任編輯:蔡洪濤)

Experimental Research on Adsorption of Toxic P-benzoquinone from Water by Nano-TiO2

LI Guoting1, FENG Yanmin1, FAN Jinzhou2, CHAI Xiaoqi1, HE Xiaoshuang1, MEI Jing1

(1.School of Environmental and Municipal Engineering, North China University of Water Resources and Electric Power,Zhengzhou 450045, China; 2.Henan Provincial Academy of Building Research, Zhengzhou 450053, China)

During photocatalytic oxidation of aromatic pollutants by photocatalyst such as TiO2, a series of organic intermediates, such as catechol, quinones and organic acids, could be generated concurrently. Considering the high toxicity of these intermediates, it is essential to investigate the adsorption performances of these intermediates by TiO2. In this research, p-benzoquinone was selected as a target pollutant and subjected to the adsorptive removal by nano-TiO2. Effect of TiO2dosage and solution pH was investigated. Adsorption thermodynamics was emphatically studied. The results show that neutral solution pH is favorable for p-benzoquinone adsorption, Langmuir model and Redlich-Peterson model can better simulate the adsorption process, the maximal adsorption of p-benzoquinone is up to 342.7 mg/g by Langmuir model. Thermodynamic analysis shows that The Gibbs free energy increases from -20.338 kJ/mol at 288 K to -18.196 kJ/mol at 308 K, while the enthalpy is -51.096 kJ/mol. Thermodynamic analysis indicates that the adsorption process is spontaneous and exothermic.

Adsorption; p-benzoquinone; TiO2; adsorption isotherm; thermodynamics

2014-11-21

國(guó)家自然科學(xué)基金(51378205);河南省高等學(xué)校青年骨干教師資助計(jì)劃(2013GGJS-088);華北水利水電大學(xué)大學(xué)生創(chuàng)新實(shí)驗(yàn)計(jì)劃項(xiàng)目(HSCX2014054).

李國(guó)亭(1977—),男,河南葉縣人,副教授,博士,主要從事水處理物化處理技術(shù)及應(yīng)用方面的研究.

10.3969/j.issn.1002-5634.2015.01.018

X703.1

A

1002-5634(2015)01-0086-04