孔青青,張祥丹,李富華,呂文英,劉國光,林曉璇,陳 平

(廣東工業(yè)大學(xué)環(huán)境科學(xué)與工程學(xué)院,廣東 廣州 510006)

不同形態(tài)氮對(duì)水環(huán)境中氯貝酸光降解的影響

孔青青,張祥丹,李富華,呂文英,劉國光*,林曉璇,陳 平

(廣東工業(yè)大學(xué)環(huán)境科學(xué)與工程學(xué)院,廣東 廣州 510006)

在模擬太陽光照射條件下,研究了不同濃度的NO3、NO2和NH4+對(duì)氯貝酸(Clofibric acid ,CA)在水環(huán)境中光降解的影響,并通過對(duì)光屏蔽系數(shù)的計(jì)算和對(duì)活性基團(tuán)的淬滅實(shí)驗(yàn)對(duì)它們的影響機(jī)理進(jìn)行了考察.通過模擬水體pE值變化,考察了無機(jī)氮離子對(duì)CA光降解的復(fù)合影響.結(jié)果表明,在NO3、NO2和NH4+存在時(shí),CA的光降解均符合擬一級(jí)動(dòng)力學(xué)方程.由于抑制·OH的產(chǎn)生,并與CA競(jìng)爭(zhēng)吸收光子,NO2和NO3對(duì)CA的光降解均具有抑制作用,且NO3對(duì)CA光降解的抑制作用強(qiáng)于NO2,NH4+則對(duì)CA的光降解無明顯影響.隨著水環(huán)境中pE值的改變, CA的光降解速率也發(fā)生改變. NO2和NH4+、NO2和NO3同時(shí)存在均對(duì)CA光降解的影響具有拮抗作用,而且NO2和NO3之間的拮抗作用強(qiáng)于NO2和NH4+之間的拮抗作用.

pE值；無機(jī)氮；氯貝酸；光降解；機(jī)理

近年來,藥物及個(gè)人護(hù)理品(PPC Ps)潛在的環(huán)境風(fēng)險(xiǎn)備受關(guān)注. CA為一種常見的PPC Ps類物質(zhì),是降血脂藥物的活性代謝產(chǎn)物.該類藥物使用劑量高,每人每天可達(dá)1～2g[1].藥物被人體攝入后,超過60%將通過人體排泄被排出體外,最終進(jìn)入污水處理廠[2].然而,污水處理廠對(duì)CA的去除效率并不高,使其在環(huán)境中積累[3-7],檢測(cè)濃度在ng/L～μg/L級(jí)水平[8].

研究表明,在太陽光照射下,CA可發(fā)生直接和間接光降解[9].同時(shí),環(huán)境中存在的很多因素,也會(huì)影響CA的光降解.氮元素是自然水體中的重要組成部分,隨著水體氧化還原性的變化,無機(jī)氮可在NO3、NO2及NH4+等形態(tài)之間轉(zhuǎn)化[10].但是,關(guān)于無機(jī)氮元素對(duì)CA光解的影響作用目前尚缺乏研究.

本文以800W氙燈作為光源模擬太陽光,研究水環(huán)境中無機(jī)氮形態(tài)變化對(duì)CA光降解的影響.

1 材料與方法

1.1 實(shí)驗(yàn)試劑和儀器

試劑: CA(＞97%純度,梯希愛(上海)化成工業(yè)發(fā)展有限公司);乙腈(色譜純,美國ACS恩科化學(xué));硫酸、氫氧化鈉、硝酸鈉、亞硝酸鈉、硫酸鈉(分析純,廣州化學(xué)試劑廠);冰乙酸(分析純,成都科試);實(shí)驗(yàn)用水均為超純水.

儀器:XPA-7型多試管同時(shí)攪拌光化學(xué)反應(yīng)儀及配套800W氙燈(南京胥江機(jī)電廠); Smart2 Pure 超純水/純水一體化系統(tǒng)(德國TKA); AL104型電子天平(梅特勒-托利多儀器(上海)有限公司); PHS-3C型pH計(jì)(上海精密科學(xué)儀器有限公司雷磁儀器廠); LC-20A 高效液相色譜儀(日本島津); UV-2450紫外可見分光光度計(jì)(日本Shimadzu); AS20500BDT-I超聲波清洗器(天津奧特賽恩斯儀器有限公司).

1.2 CA溶液的配制

用乙腈配制1000.00mg/L 的CA母液,置于4℃的冰箱中備用.實(shí)驗(yàn)移取1.250mL的CA母液到250mL 棕色容量瓶中,配制5.00mg/L的反應(yīng)液.配置反應(yīng)液時(shí),首先通入高純氮?dú)鈱⒁译娲蹈?然后加入超純水搖勻,使用超聲振蕩輔助溶解,用1%的硫酸和氫氧化鈉溶液調(diào)節(jié)pH值至(7±0.1);最后定容.

1.3 光解實(shí)驗(yàn)

光解實(shí)驗(yàn)在光化學(xué)反應(yīng)儀中進(jìn)行,光源為800W的氙燈.準(zhǔn)確移取25mL的CA溶液到50mL的具塞石英試管中,置于光化學(xué)反應(yīng)儀中進(jìn)行光照,溫度控制在(25±1)℃,定時(shí)取樣,所得樣品采用高效液相色譜儀進(jìn)行分析.光解實(shí)驗(yàn)的同時(shí)進(jìn)行黒暗對(duì)照實(shí)驗(yàn),每組實(shí)驗(yàn)設(shè)置3次平行實(shí)驗(yàn),結(jié)果取平均值,標(biāo)準(zhǔn)偏差代表誤差線.

1.4 光屏蔽因子測(cè)定方法NO3和NO2引起的光衰減對(duì)CA直接光降解的影響由光屏蔽系數(shù)Sλ來評(píng)估[11].用紫外分光光度計(jì)掃描不同濃度的NO3和NO2溶液,掃描波長范圍為200～600nm.用獲得的各濃度的吸光度計(jì)算光屏蔽系數(shù),具體波長的光屏蔽系數(shù)計(jì)算如下[12]:

式中:l為光程, cm,本文中實(shí)驗(yàn)所用為1cm; ελ為 CA的摩爾吸光系數(shù), L/(cm·mol); [C]為CA的濃度, mol/L; aλ為某濃度NO3的消光系數(shù), cm-1.

本實(shí)驗(yàn)中所選光源為800W 氙燈,故本文中計(jì)算了NO3和NO2可吸收波長(200～400nm)的總屏蔽系數(shù)S∑λ, S∑λ通過下式計(jì)算:

式中:Lλ為光源在波長λ處相對(duì)光強(qiáng), %.

1.5 ·OH穩(wěn)態(tài)濃度的計(jì)算

以苯甲酸(BA)作為分子探針(k·OH,BA=5.9× 109M-1s-1)[13],通過BA的去除,利用公式(3)～(4)計(jì)算·OH的穩(wěn)態(tài)濃度[14].BA的濃度變化通過高效液相色譜儀檢測(cè).

式中:[BA]為BA的濃度, mmol/L; k·OH,BA為BA與·OH反應(yīng)的二級(jí)反應(yīng)速率常數(shù), min-1; [·OH]ss為·OH的穩(wěn)態(tài)濃度, mmol/L; kobs,BA為BA的降解速率常數(shù), min-1.

1.6 分析方法

采用高效液相色譜儀對(duì)CA和BA的濃度進(jìn)行測(cè)定,色譜柱為ZORBAX Eclipse XDB-C18 (4.6mm×150mm,5μm);流動(dòng)相由有機(jī)相和水相組成(V有機(jī):V水=55:45),其中有機(jī)相為體積比為1:1的乙腈和甲醇,水相為5mmol/L的KH2PO4和0.2%的冰乙酸溶液;流速為1mL/min.柱溫為35℃,進(jìn)樣體積10μL.檢測(cè)器為光電二極管陣列檢測(cè)器,CA的檢測(cè)波長為230nm,BA的檢測(cè)波長為228nm.

2 結(jié)果與討論

2.1 NO3對(duì)CA光解的影響

設(shè)置CA的濃度為5.00mg/L, NO3的添加濃度分別為0.00,0.01,0.05,0.10,0.20,0.50,0.80, 1.00mmol/L,調(diào)節(jié)溶液pH值為(7±0.1),在室溫條件下考察不同濃度的NO3對(duì)CA光降解的影響,結(jié)果如表1所示.由表1可以看出, NO3存在時(shí),CA的光降解符合準(zhǔn)一級(jí)動(dòng)力學(xué)方程, NO3-可抑制CA的光降解,且隨著NO3添加濃度的增加,抑制作用逐漸增大.這與Zhang等[10]在模擬太陽光照射下研究對(duì)雙氯酚酸光降解的影響結(jié)果相同.

圖1 CA、、、紫外可見吸收光譜圖Fig.1 UV-Vis spectra comparison of CA、、and

式中:k NO3為光敏化作用引起的CA降解速率常數(shù), min-1; kW為CA在純水中的光降解速率常數(shù),min-1.

計(jì)算結(jié)果如表1所示,可以看出在本實(shí)驗(yàn)所添加的NO3濃度范圍內(nèi), kNO3均小于0,且隨著濃度的增大而減小,說明在本研究中,溶液中的存在可能抑制了·OH的產(chǎn)生.分別向反應(yīng)液中加入50mmol/L和100mmol/L的異丙醇(·OH的淬滅劑),驗(yàn)證存在時(shí)·OH的產(chǎn)生,結(jié)果如圖2所示.添加50mmol/L和100mmol/L的異丙醇時(shí),CA的光降解速率常數(shù)分別為0.03310min-1和0.03380min-1,利用公式(10)計(jì)算0.1mmol/L存在時(shí)·OH的貢獻(xiàn)率為1.46%,利用公式(3)計(jì)算0.1mmol/L存在時(shí)·OH的穩(wěn)態(tài)濃度為1.41×10-15mol/L.

表1 對(duì)CA光降解的影響Table 1 Effect ofon photodegradation of CA

表1 對(duì)CA光降解的影響Table 1 Effect ofon photodegradation of CA

濃度(mmol/L) kobs(min1) 半衰t1/2(min) R2 S∑200-400kWS∑200~400(min1)kNO3(min-1) 抑制率η (%) 0.00 0.04110 16.86 0.9999 1.00000 0.04110 0.00000 0.00 0.01 0.03630 19.09 0.9999 0.97379 0.04002 -0.00372 11.68 0.05 0.03530 19.63 0.9992 0.97269 0.03998 -0.00468 14.11 0.10 0.03430 20.20 0.9989 0.97230 0.03996 -0.00566 16.55 0.20 0.03330 20.81 0.9993 0.97172 0.03994 -0.00664 18.78 0.50 0.03140 22.07 0.9990 0.96667 0.03973 -0.00833 23.41 0.80 0.03010 23.02 0.9970 0.96563 0.03968 -0.00959 26.83 1.00 0.02740 25.29 0.9997 0.95610 0.03929 -0.01190 33.17

在純水中分別添加50mmol/L和100mmol/L的異丙醇, 此時(shí), CA的光降解速率常數(shù)為0.03479min-1和0.03476min-1(如圖3). 計(jì)算得·OH對(duì)純水中CA光降解的貢獻(xiàn)率為15.43%, ·OH的穩(wěn)態(tài)濃度為4.24×10-11mol/L.

圖2 存在時(shí)異丙醇對(duì)CA光降解的影響Fig.2 Effect of isopropanol onphotodegradationof CA with

圖3 不同量的異丙醇對(duì)CA光降解的影響Fig.3 Effect of different amounts of isopropanol on photodegradation of CA

表2 對(duì)CA光降解的影響Table 2 Effect ofon photodegradation of CA

表2 對(duì)CA光降解的影響Table 2 Effect ofon photodegradation of CA

濃度(mmol/L) kobs(min1) 半衰t1/2(min) R2 S∑200-400kWS∑200-400(min1)kNO2(min-1) 抑制率η (%) 0.00 0.04110 16.86 0.9999 1.00000 0.04110 0.00000 0.00 0.01 0.03867 17.59 0.9999 0.95357 0.03919 -0.00052 4.14 0.05 0.03856 18.78 0.9999 0.95067 0.03907 -0.00051 10.22 0.10 0.03795 18.99 0.9999 0.94947 0.03902 -0.00107 10.22 0.20 0.03685 19.58 0.9998 0.93605 0.03847 -0.00162 13.66 0.50 0.03382 20.44 0.9994 0.93505 0.03843 -0.00461 17.32 0.80 0.03345 22.07 0.9980 0.93204 0.03831 -0.00486 23.41 1.00 0.03334 23.26 0.9994 0.93128 0.03828 -0.00494 27.32

2.2 NO2對(duì)CA 光解的影響設(shè)置CA 的濃度為5.00mg/L,NO2的添加濃度分別為0.00,0.01,0.05,0.10,0.20,0.50,0.80,1.00mmol/L,調(diào)節(jié)溶液pH 為(7±0.1),在室溫條件下考察不同濃度的NO2對(duì)CA 光降解的影響,結(jié)果如表2 所示.在NO2存在時(shí)CA 的光降解符合準(zhǔn)一級(jí)動(dòng)力學(xué)方程,NO2對(duì)CA 的光降解具有抑制作用,且NO2濃度越大,對(duì)CA光降解的抑制作用越強(qiáng).趙倩等[22]對(duì)鎮(zhèn)痛消炎藥安替比林光解效能與機(jī)制的研究也有相同的情況.

研究表明,NO2在較低濃度時(shí)可產(chǎn)生O·-,O·-可快速與H+反應(yīng)生成強(qiáng)氧化劑·OH(如式(11, 12))[23-24],促進(jìn)藥物的光降解;而在添加濃度較高時(shí)則對(duì)·OH表現(xiàn)出淬滅作用,抑制藥物的光降解(如式(8))[21].此外,由圖1可以看出在200～400nm之間與CA有很大的吸收重疊,可與CA競(jìng)爭(zhēng)吸收光子,對(duì)CA的光降解產(chǎn)生一定的抑制作用.同樣,考慮到對(duì)CA的光屏蔽作用,可利用式(9)對(duì)存在時(shí)CA的光降解進(jìn)行評(píng)估.

圖4 存在時(shí)異丙醇對(duì)CA光降解的影響Fig.4 Effect of isopropanol onphotodegradationof CA with

表3 NH4+對(duì)CA光降解的影響Table 3 Effect of NH4+on photodegradation of CA

2.4 pE值對(duì)CA光解的影響

由于細(xì)菌的硝化作用和氮肥的施用,自然水體中氮的含量高達(dá)10-3mol/L[25].在實(shí)際水體中,無機(jī)氮主要以和的形式存在,但在某些況下,也以中間氧化態(tài)存在.隨著水環(huán)境中氧化還原電位的不同,不同形態(tài)的N之間會(huì)發(fā)生轉(zhuǎn)換,如半反應(yīng)(13)和(14).本文通過改變不同形態(tài)N的添加濃度來模擬pE(pE用于衡量溶液接收或遷移電子的能力, pE=-lg e, e為電子活度)值改變引起的N的形態(tài)變化,以探索不同形態(tài)N對(duì)CA光降解的影響.

設(shè)定水體中氮的總濃度為1.00mmol/L,pH值設(shè)置為(7.0±0.1).含有和的半反應(yīng)為:

在較低的pE(pE＜5)值時(shí),N在水溶液中的存在形態(tài)主要為;在pE值等于6.5左右時(shí), N在水溶液中的存在形態(tài)主要為;在pE值大于7時(shí), N在水溶液中的存在形態(tài)主要為.

表4 不同pE值對(duì)應(yīng)的不同型態(tài)氮的濃度Table 4 Different pE values corresponding inorganic N concentration

表4列舉了不同pE值對(duì)應(yīng)的不同型態(tài)氮的濃度,表5列出了CA光降解的速率常數(shù)、半衰期及不同pE值條件下CA光降解的抑制率η. CA光降解的抑制率η的計(jì)算如式(17)所示.理論抑制率由3種不同形態(tài)氮單獨(dú)存在時(shí)的抑制率相加而得到.

式中:k0為純水中CA的光降解速率常數(shù),min-1; ki為、、存在時(shí)CA的光降解速率常數(shù), min-1.

表5 CA在不同pE值下的光降解動(dòng)力學(xué)Table 5 Photodegradationrate constants of CA with different inorganic N concentration

由表5可以看出,當(dāng)pE值從4.82上升到6.15時(shí),CA的光降解速率逐漸降低,當(dāng)pE值從6.15上升至7.45時(shí),CA的光降解速率逐漸升高,當(dāng)pE值從7.15上升到8.15時(shí),CA的光降解速率又再次降低.這是因?yàn)樵诓煌膒E值條件下N的存在形式不同.當(dāng)pE值小于5時(shí),N主要以的形式存在;當(dāng)pE值在6.5左右時(shí),N主要以的形式存在;當(dāng)pE值大于7時(shí),N主要以的形式存在.前面實(shí)驗(yàn)結(jié)果已經(jīng)表明和對(duì)CA的光降解均有抑制作用,且的抑制作用大于,而則對(duì)CA光降解影響較小.因此,當(dāng)pE值從4.82上升到6.15時(shí),反應(yīng)液中濃度逐漸增大,即反應(yīng)液中不同形態(tài)N對(duì)CA光降解的總抑制作用隨濃度的增大而增大,故降解率逐漸降低;由表5中不同pE值條件下不同形態(tài)N對(duì)CA光降解的實(shí)際抑制率和理論抑制率對(duì)比可見,當(dāng)反應(yīng)液中和共同存在時(shí),實(shí)際抑制率小于理論抑制率,即說明和對(duì)CA光降解的影響具有拮抗作用,分析可知,和共存時(shí)會(huì)發(fā)生如式(18)的氨氧化反應(yīng)降低的濃度[26-27],從而減弱其對(duì)CA的抑制作用.當(dāng)pE值從6.15上升到7.45時(shí),反應(yīng)液中的濃度逐漸降低,轉(zhuǎn)化為抑制作用更強(qiáng)的,然而,CA的降解速率并沒有隨濃度的增大而降低.比較表5中和共存時(shí)對(duì)CA光降解的實(shí)際抑制率和理論抑制率可發(fā)現(xiàn),和對(duì)CA光降解的影響同樣具有拮抗作用,并且這種拮抗作用強(qiáng)于和之間的拮抗作用.這是因?yàn)楹凸餐嬖跁r(shí)的吸光度小于兩者單獨(dú)存在時(shí)的吸光度之和,削弱了與CA的光競(jìng)爭(zhēng),從而減弱了其對(duì)CA光降解的抑制(如圖5可見),所以當(dāng)pE值從6.15上升到7.45時(shí),CA的光降解速率逐漸升高,而當(dāng)pE值為8.15時(shí),反應(yīng)液中缺少了的存在,拮抗作用消失,CA的光降解速率又再次降低.

圖5 NO3和NO2共存紫外可見吸收光譜Fig.5 UV-Vis spectra comparison of co-existence of NO3and NO2

3 結(jié)論

3.3 pE值改變時(shí),會(huì)影響無機(jī)氮的存在形態(tài),進(jìn)而影響CA的光降解速率.當(dāng)pE值從4.82上升到6.15時(shí),CA的光降解速率逐漸降低,當(dāng)pE值從6.15上升到7.45時(shí),CA的光降解速率逐漸升高,當(dāng)pE值從7.15上升到8.15時(shí),CA的光降解速率又再次降低;

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致謝：本論文的完成得到了劉國光老師和張祥丹老師的悉心指導(dǎo),在此表示感謝!

Effect of different forms of nitrogen on the photodegradation of clofibric acid in water environment.

KONG Qing-qing, ZHANG Xiang-dan, LI Fu-hua, LV Wen-ying, LIU Guo-guang*, LIN Xiao-xuan, CHEN Ping
(Faculty of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, China). China Environmental Science, 2017,37(2)：584～591

This study examined the influence of NO3, NO2, and NH4+on the photodegradation of clofibric acid under the simulated sunlight in water. Their influence mechanisms were inspected by calculating light-screening factor and quenching experiments. The recombinational influence of inorganic nitrogen on the photodegradation of clofibric acid under various simulated pE values was also explored. Results showed that photodegradation of clofibric acid with the presence of NO3, NO2, and NH4+followed the pseudo-first-order kinetics. Both NO3and NO2inhibited the photodegradation of clofibric acid, while NH4+had no influence on clofibric acid removal. NO3showed much stronger inhibitory effect than that of NO2. Photodegradation conversion of clofibric acid changed with altering pE value. The co-existence of NH4+and NO2, or NO3and NO2in water displayed antagonismto influence on the photodegradation of clofibric acid. However, antagonismbetween NO and NO was stronger than that between NH+and NO.3242

pE value；inorganic nitrogen；clofibric acid；photodegradation；mechanism

X703

A

1000-6923(2017)02-0584-08

孔青青(1990-),女,河南周口人,廣東工業(yè)大學(xué)碩士研究生,主要研究方向?yàn)橛袡C(jī)污染物在水環(huán)境中的遷移轉(zhuǎn)化.

2016-05-24

國家自然科學(xué)基金資助項(xiàng)目(21377031)

* 責(zé)任作者, 教授, liugg615@163.com