UV-LED/NaClO工藝對水中對乙酰氨基酚的降解

李博強,馬曉雁,李青松,廖文超,陳彥潔,陳國元,李國新

UV-LED/NaClO工藝對水中對乙酰氨基酚的降解

李博強1,2,馬曉雁1,李青松2*,廖文超3,陳彥潔2,4,陳國元2,李國新2

(1.浙江工業大學建筑工程學院,浙江 杭州 310014；2.廈門理工學院水資源環境研究所,福建 廈門 361005；3.深圳技術大學健康與環境工程學院,廣東 深圳 518118；4.青島大學環境科學與工程學院,山東 青島 266022)

采用NaClO、UV-LED和 UV-LED/NaClO工藝去除水中的對乙酰氨基酚(AAP),考察了NaClO投加量、pH值和腐殖酸(HA)等因素對UV-LED/NaClO工藝去除AAP的影響,研究了UV-LED/NaClO去除AAP過程中OH·、UV-LED、NaClO 和氯自由基RCS(Cl·,Cl2·-,ClO·)的貢獻值,評估了AAP降解過程中溶液急性毒性的變化.結果表明,UV-LED/NaClO可以有效降解.AAP.反應90min后,UV-LED、NaClO和UV-LED/NaClO工藝對AAP的去除率分別為4.42%、93.61%和100%.AAP的降解符合擬一級反應動力學模型(2=0.9967). AAP的去除隨著NaClO投加量的增大而增加,中性條件有利于AAP的降解,HA對AAP去除具有抑制作用,HCO3-和NO3-可略微促進AAP的去除.當NaClO投加量為1mg/L, OH·、UV-LED、NaClO和RCS各組分對AAP去除的相對貢獻率分別為0.82%、0.66%、33.78%和64.74%,UV-LED/NaClO工藝可以有效的降低溶液的急性毒性.

對乙酰氨基酚；UV-LED/NaClO；動力學；貢獻值；急性毒性

對乙酰氨基酚(AAP)是一種典型的藥品和個人護理品(PPCPs),廣泛應用于感冒藥、解熱藥和鎮痛藥等常用非處方藥中[1].由于AAP的廣泛使用及其在環境中的不完全降解,地表水、地下水和飲用水等環境水體中均有檢出[2-5].研究表明AAP具有內分泌干擾效應,會對生物造成遺傳毒性、肝毒性等負面環境效應[6],因此有必要對水體中AAP的去除進行研究.

自由氯和臭氧氧化等可以有效降解水中AAP,但易生成毒性較高的消毒副產物[7-9].紫外(UV)和氯常用于水處理消毒,研究表明,NaClO在紫外輻照下能產生OH·、RCS(Cl·,Cl2·-,ClO·等)和其他強氧化性粒子[10-11],污染物能通過UV/NaClO協同去除且有較高的礦化度[12-13].同時UV/NaClO工藝在30min內可降解99%以上AAP[14].作為一種新型紫外光源,紫外發光二極管(UV-LED)可以發射210nm到可見光的輻射[15],具有使用壽命長、光子效率高、無毒及增加光電反應器設計的靈活性等優點[16-18].研究表明中性和堿性時UV-LED/NaClO工藝對羅硝唑(含甲基和氨基)的去除優于低壓汞燈和NaClO聯合工藝[19],UV-LED和NaClO的聯合工藝能更好的去除污染物.但關于UV-LED協同NaClO去除AAP的研究尚未見相關報道,因此,本文采用UV-LED/ NaClO對AAP的去除進行研究.

本研究采用NaClO、UV-LED和UV-LED/ NaClO工藝去除水中的AAP,考察了NaClO投加量、pH值和腐殖酸(HA)等因素對UV-LED/NaClO工藝去除AAP的影響,研究了UV-LED/NaClO工藝去除AAP過程中OH·、UV-LED、NaClO和RCS(Cl·, Cl2·-,ClO·等)的貢獻值,評估了AAP降解過程中急性毒性的變化.以期為UV-LED/NaClO工藝去除控制水體中微量污染物提供理論依據.

1 材料與方法

1.1 實驗試劑與儀器

AAP(德國Dr.Ehrenstorfer公司,純度>99.9%);腐殖酸(HA)(Tech,美國 Sigma-Aldrich);甲醇、乙腈(HPLC級,德國Merck)、硝基苯(NB)、叔丁醇(TBA)(HPLC級,上海安譜);次氯酸鈉(CP,活性氯35.2%);Na2S203·5H2O、NaHCO3、NaNO3、HCl和NaOH均為分析純;BioFix?Lumi Multi-Shot凍干細菌及激活液;實驗室用水均為Mill-Q超純水(￡18.2MΩ).

LC-20A高效液相色譜儀(shimadzu,日本); BioFix??Lumi生物毒性儀(Macherey-Nagel,德國);CL200余氯計(ExStik,上海三信儀表廠);純水機(Milipore,美國),DZF-6050真空干燥箱(上海精宏實驗設備有限公司),HJ-6A型磁力恒溫攪拌器(江蘇金壇崢嶸儀器),pH計(Eutevch,美國),UV-LED燈(深圳微紫科技有限公司,波長278nm,單個100mW).

1.2 實驗方法

試驗在一個置于磁力攪拌器上的燒杯(2L)中進行,UV-LED/NaClO工藝中光源采用9個UV-LED燈并聯(單個100mW,并聯后900mW)組成,外置石英套管.燒杯內放置濃度為250μg/L的AAP溶液,通過0.1mol/L的NaOH和HCl調節pH值,然后投加一定量的NaClO溶液(0.5~1.5mg/L),同時啟動攪拌,打開UV-LED燈,開啟反應.設定時間取出10mL水樣,所取水樣經過1.0mol/L的硫代硫酸鈉淬滅過膜后,測定AAP濃度和溶液急毒性.所有實驗重復三次,取其平均值.利用方差分析給出實驗數據的誤差棒.

1.3 分析方法

AAP和NB濃度采用HPLC進行測定.流動相為乙腈和水(AAP為20:80,NB為65:35),流速均為1.0mL/min,柱溫分別為40和35℃,AAP和NB的UV檢測波長分別為243和262nm,進樣量為10μL.方法的相對標準偏差為0.4904%,線性范圍為5~ 1000μg/mL,檢測限為3.359μg/L.

1.4 急毒性的試驗方法

發光細菌急性毒性測試方法參考UNE-EN- ISO 11348-32007[20].待測水樣經2%的NaCl鹽化后,水樣pH值調到6~8,然后采用生物毒性儀進行毒性測定,培養時間設定為30min,急毒性分析結果的表示形式是以抑制百分比和增強百分比來表示水樣受污染程度,見式(1).

2 結果與討論

2.1 UV-LED、NaClO和UV-LED/NaClO對AAP的去除

如圖1所示,實驗反應90min后單獨UV-LED和單獨NaClO氧化對AAP的去除分別為4.42%和93.61%.然而,相同時間內UV-LED/NaClO對AAP的去除可達100%. UV-LED輻射、單獨NaClO氧化和UV-LED/NaClO工藝的擬一級動力學常數分別為0.0006,0.0305和0.0903min-1,UV-LED/ NaClO降解AAP的擬一級動力學常數分別是NaClO和UV-LED的3倍和151倍.實驗表明UV- LED/NaClO工藝對AAP的去除具有協同作用.

UV-LED對AAP的去除主要是UV-LED輻射產生的OH·參與反應[21],NaClO能夠通過氧化作用去除AAP.UV-LED/NaClO工藝中溶液中的HOCl/OCl-在紫外光激發下產生了高活性氧化自由基(如OH·和RCS(Cl·,Cl2·-,ClO·))[10,13,22-23],主要的反應式如式(2)~(7).OH·、Cl·和Cl2·－的氧化還原電位分別為2.8,2.47和2.0V,具有強氧化性[14,24],氯自由基可以有效地去除含芳香環和富電子的有機污染物[14].隨著UV-LED輻照時間的增加,溶液中高活性氧化自由基含量逐漸增加,因此在UV-LED/NaClO工藝中不僅有UV-LED對AAP的輻射作用,還有NaClO的直接氧化和溶液中產生的活性自由基的氧化作用,三者共同作用促進了AAP的降解產生了協同效應.

圖1 UV-LED、NaClO和UV-LED/NaClO對AAP的去除

[AAP]=250μg/L, [NaClO]=1mg/L, pH=(7.0±0.2)

HClO/OCl-+?OH·+Cl· (2)

HO·+HClO?H2O+ClO· (3)

ClO-+HO·?OH-+ClO· (4)

Cl·+HClO?H++Cl-+ClO· (5)

Cl·+OCl-?Cl-+ClO· (6)

2.2 UV-LED/NaClO去除AAP的影響因素

2.2.1 NaClO投加量對AAP去除的影響 NaClO濃度影響氯自由基的產率[14],因此考察了NaClO投加量對AAP去除的影響,結果見圖2.NaClO投加量分別為0.5,0.75,1.0,1.25和1.5mg/L,擬一級動力學常數分別為0.0165,0.0484,0.0903,0.1331和0.1433min-1.實驗中AAP的去除隨著NaClO投加量的增加而增加,增加NaClO投加量可以生成更多的OH·和RCS[25].實驗中表現為AAP的降解速率加快.然而,當NaClO投加量超過1mg/L時,AAP的降解速率增加幅度變小.可能是因為NaClO過量時,大量的游離氯會與Cl·和OH·發生反應(式(3)和(5)),同時隨著NaClO濃度的增加,NaClO吸收的紫外光逐漸達到飽和,使RCS的生成量也逐漸達到飽和[26].這與Dong等[27]采用UV/NaClO工藝降解氯霉素得到的規律類似.

圖2 NaClO對UV-LED/NaClO降解AAP的影響

[AAP]=250μg/L, pH=(7.0±0.2)

2.2.2 pH值對AAP去除的影響 pH值影響紫外/氯工藝中氧化自由基的存在形式[28],因此考察pH值對AAP去除的影響,結果見圖3.pH值為3.02,5.05, 7.03、9.10、11.09時,反應90min后的去除率分別為39.05%、84.28%、99.48%、99.16%、69.44%,AAP的去除隨著pH值的升高先增加再降低.不同pH值對AAP的去除均符合擬一級動力學模型(2> 0.97)(圖3),擬一級動力學常數先由0.0085min-1增加到0.0903min-1然后降低至0.0178min-1.中性條件下AAP的去除效果最佳.

pH值影響NaClO在溶液中主要存在形式[29].當溶液pH值為3~5時,溶液中NaClO的主要存在形式為HClO,溶液中的H+可以與Cl·和水合電子eaq·反應生成還原性較低的·H和Cl-(式8)[30],降低對AAP的去除.同時當溶液過酸時由于攪拌作用有部分的Cl2溢出[28],導致溶液pH值為3.02時的去除效果相比pH值為5.05時有所降低;當溶液pH值為中性時,溶液中NaClO的主要存在形式為HClO和少量的OCl-,溶液主要是通過HOCl和少量的OCl-發生光解反應(式(2)~式(7)),產生更多的強氧化自由基進而有效的去除AAP;當溶液pH值9~11時,溶液中NaClO的主要存在形式為OCl-,溶液主要是通過OCl-發生光解反應(式(2)和(4)),由于OCl-和HOCl與HO·的反應速率常數為8.8×109和8.46×102L/(mol×s)[31],因此OCl-能較快的與HO·反應,導致溶液中的氧化自由基減少,從而降低對AAP去除.當溶液pH值大于9.71時,AAP電離為AAP-(pa,AAP=9.71)[1],由于電荷之間的排斥作用,AAP-與ClO-之間的反應受阻,使溶液pH值為11.09時的去除效果相較于pH值為9.10時有所降低.Deng等[32]在利用UV協同NaClO去除環丙沙星過程中具有類似的結論.

Cl·+eaq+H+?·H+Cl-(8)

圖3 pH值對UV-LED/NaClO降解AAP的影響

[AAP]=250μg/L, [NaClO]=1mg/L

2.2.3 腐殖酸對AAP去除的影響 腐殖酸(HA)是自然界中豐富的大分子有機質,廣泛存在于自然水體中[33].因此利用HA模擬水體中的天然有機質,考察了HA對去除AAP的影響,結果見圖4.AAP的去除隨著HA投加量的增加而降低.AAP去除的擬一級動力學常數隨著HA濃度的增加逐漸降低.當HA的投加量分別為0,1,3,5,7和9mg/L時,擬一級動力學常數從0.0903降低至0.0550,0.0297,0.0230,0.0155, 0.0132min-1.

HA不僅可以增加水的色度影響光的透光率,還可以消耗溶液中的NaClO(其擬一級動力學常數為3×10-5s-1)[34],更為重要的HA可以與AAP競爭溶液中的Cl·和OH·等氧化粒子[35],從而降低對AAP去除.實驗中HA對AAP的去除具有抑制作用,這與Tang等[36]利用UV/NaClO降解降固醇酸的實驗中發現隨著溶液中HA的濃度增多對降固醇酸的去除率降低的規律相同.

2.2.4 HCO3-和NO3-對AAP去除的影響 HCO3-和NO3-廣泛存在于地表水中,研究表明它們可能會影響紫外線高級氧化工藝中有機物的去除[37-38].因此投加不同濃度的HCO3-和NO3-,考察了兩種陰離子對AAP去除的影響,結果如圖6所示.AAP去除的擬一級動力學常數值隨著HCO3-和NO3-濃度的增加逐漸增大.實驗中,HCO3-和NO3-投加量為0時,反應40min后AAP的去除率為97.43%,當HCO3-投加量為5,20,50和100mmol/L時,對AAP的去除率分別增加至98.39%、98.48%、99.08%和99.23%.當NO3-的投加量為0.1,0.5,1.0和2.0mmol/L時,對AAP的去除分別增加為97.59%、97.79%、98.36%和98.45%.因此水體中HCO3-和NO3-對AAP的去除均有促進作用.

圖4 HA對UV-LED/NaClO降解AAP的影響

[AAP]=250μg/L, [NaClO]=1mg/L, pH=(7.0±0.2)

圖5 HCO3-和NO3-對UV-LED/NaClO降解AAP的影響

[AAP]=250μg/L, [NaClO]=1mg/L, pH=(7.0±0.2)

Tan等[39]和Luo等[40]在研究UV/H2O2等工藝去除安替比林和阿特拉津的實驗中觀察到HCO3-對降解過程有抑制作用,然而Fan等[41]和Liu等[42]在過硫酸鹽和UV活化過硫酸鹽工藝去除磺胺甲嘧啶和氧四環素的實驗中表明HCO3-可以促進目標物的去除.本實驗觀察到HCO3-可以促進AAP的降解.其主要原因是HCO3-可與RCS和OH·快速反應形成CO3·-,其反應過程如(式(9)和式(10))[31].同時由于AAP結構中含有苯胺,CO3·-可與含有苯胺結構的物質發生雙分子反應[43],從而增強了AAP的去除.NO3-經過光解后會產生OH·[44],OH·可以降解AAP,同時也可與HOCl反應產生ClO·(式3),因而增加NO3-的濃度可以促進AAP的降解.

HCO3-+OH·?H2O+CO3·-=8.5×106L/(mol×s) (9)

HCO3-+Cl·?H++Cl-+CO3·-=2.2×108L/(mol×s) (10)

2.3 UV-LED/NaClO降解AAP過程中各組分貢獻率

叔丁醇(TBA)能與OH·和Cl·快速反應,同時還可以與Cl2·-反應,常用于測定RCS和OH·貢獻值的研究中[45-46].因此投加不同濃度的TBA,考察了在UV-LED/NaClO去除AAP過程中活性自由基對去除率的影響,結果見圖6.TBA的投加量為0,0.1,0.5和1mmol/L時,AAP去除的擬一級動力學常數分別為0.0903,0.0543,0.0379和0.0345min-1.表明TBA對AAP的去除有抑制作用.由此可知在UV-LED/ NaClO去除AAP的過程中Cl·,Cl2·-和HO·有著一定貢獻量.但是在加入較高濃度的TBA后去除率仍大于單獨NaClO對AAP的去除,這表明AAP的去除過程中可能還有其他的RCS的作用.

圖6 TBA對UV-LED/NaClO去除AAP的影響

[AAP]=250μg/L, [NaClO]=1mg/L, pH=(7.0±0.2)

硝基苯(NB)在紫外協同NaClO工藝中僅與OH·發生反應[32],因此選用NB作為OH·的探針來研究UV-LED/NaClO去除AAP的過程中OH·的貢獻值.假設在去除過程中各自由基的濃度保持穩定不變,OH·和RCS的貢獻值可以通過式(11)和式(12)計算所得.

AAP'=UV-LED'+chlorine'+OH·-AAP[OH·]+RCS-AAP' (11)

NB'=d-NB'+vol-NB'+OH·-NB[OH·] (12)

式中:AAP￠和NB￠是UV-LED/NaClO去除AAP和NB過程中的擬一級動力學常數.UV-LED￠表示單獨UV-LED去除AAP的擬一級動力學常數,chlorine￠和RCS-AAP￠是NaClO和RCS氧化導致AAP降解的擬一級動力學常數,OH·-AAP和OH·-NB是UV-LED/ NaClO工藝中產生的OH·與AAP和NB反應的二級反應速率常數.d-NB￠和vol-NB￠分別是對NB直接光解和NB揮發的擬一級動力學常數.

[AAP]=250μg/L, [NB]=250μg/L, [NaClO]=1mg/L, pH=(7.0±0.2)

由圖7可知,NB的降解過程均符合擬一級動力學模型.UV-LED/NaClO降解NB的擬一級動力學常數為0.0026min-1,NB揮發作用和單獨UV-LED去除NB的擬一級動力學常數分別為0.0010和0.0002min-1.已知OH·-NB=3.9×109(mol×s)/L[30],因此由式(12)可計算出OH·的濃度為2.3×10-11mol/L.由圖1可知,AAP￠=0.0903min-1,chlorine￠=0.0305min-1,UV-LED￠=0.0006min-1,同時已知OH·-AAP=1.7×109L/ (mol×s)[47],由此可通過式(11)得出RCS降解AAP的擬一級動力學常數為0.0585min-1.因為UV-LED￠= 0.0006min-1,OH·-AAP[OH·]=0.0007min-1,chlorine￠= 0.0305min-1,RCS-AAP￠=0.0585min-1,因此單獨UV- LED、OH·、NaClO和RCS對AAP降解的相對貢獻率分別為0.66%、0.82%、33.78%和64.74%.

2.4 AAP降解過程中溶液急性毒性的變化

改變NaClO的投加量,考察了UV-LED/NaClO和NaClO降解AAP過程中溶液急性毒性的變化.結果見圖8.當NaClO投加量為1.0mg/L時,單獨NaClO氧化去除AAP的過程中溶液急毒性從25%逐漸增加至45%.相同取樣時間,在UV-LED/NaClO降解AAP過程中,投加不同濃度NaClO時溶液的急毒性均呈先略微升高,再逐漸降低的趨勢.NaClO投加量為1.5mg/L,反應90min后溶液的相對抑制率降低至10%.表明UV-LED/NaClO工藝能更有效的降低溶液的急性毒性,并且當NaClO濃度越高時,溶液的急性毒性降低的更為顯著.

圖8 UV-LED/NaClO和NaClO降解AAP過程中溶液急毒性的變化

[AAP]=250μg/L, pH=(7.0±0.2)

Bedner等[48]在研究NaClO去除AAP的降解產物中發現AAP可生成毒性為其58倍和25倍的1,4-苯醌和N-乙酰-對-苯醌亞胺(NAPQI),同時發現隨著反應的進行,1,4-苯醌的濃度逐漸升高,NAPQI的濃度先升高后降低.因此實驗中單獨NaClO無法有效去除AAP降解過程中生成的1,4-苯醌類等消毒副產物,導致了溶液急性毒性的升高.加入了UV- LED輻射后,隨著反應的進行,溶液中高活性自由基逐漸增加,因為OH·和RCS具有高氧化還原電位,同時OH·的氧化作用無選擇性能迅速降解廣泛的有機物[13],因此會導致此類消毒副產物去除進而降低溶液的急性毒性.Vogna等[49]在研究UV/H2O2去除AAP的實驗中發現OH·可有效降解1,4-苯醌.

3 結論

3.1 UV-LED、NaClO和UV-LED/NaClO 3種工藝對AAP有不同程度的去除效果,反應90min后,對AAP去除率分別為4.42%、93.61%和100%,UV- LED/NaClO工藝能有效的去除AAP.

3.2 在UV-LED/NaClO去除AAP工藝中,NaClO投加量、pH值、HA濃度和陰離子(HCO3-、NO3-)均影響AAP的反應速率.增加NaClO的投加量會增大AAP的降解率,HCO3-和NO3-有利于AAP的去除.并且在中性條件下更適宜AAP的降解.HA對AAP的去除具有抑制作用.

3.3 NaClO的投加量為1mg/L時,UV-LED/ NaClO降解AAP過程中單獨UV-LED、OH、NaClO和RCS對AAP降解的相對貢獻率分別為0.66%、0.82%、33.78%和64.74%.氯自由基占去除AAP的主導地位.

3.4 相比于單獨投加NaClO,UV-LED/NaClO工藝能更有效的降低溶液的急性毒性.

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Degradation of acetaminophen in aqueous by UV-LED/NaClO process.

LI Bo-qiang1,2, MA Xiao-yan1, LI Qing-song2*, LIAO Wen-chao3, CHEN Yan-jie2,4, CHEN Guo-yuan2, LI Guo-xin2

(1.College of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou 310014, China；2.Water Resource and Environment Institute, Xiamen University of Technology, Xiamen 361005, China；3.College of Health Science and Environmental Engineering, Shenzhen Technology University, Shenzhen 518118, China；4.School of Environmental Science and Engineering, Qingdao University, Qingdao 266022, China)., 2019,39(11)：4681~4688

The degradation of acetaminophen (AAP) in aqueous solution by NaClO, UV-LED and UV-LED/NaClO was investigated, the influences of several factors such as NaClO dosage, pH values and humic acid (HA) on AAP degradation by UV-LED/NaClO process were discussed. Contributions of hydroxyl radical (OH·), UV-LED irradiation, NaClO and free chlorine radical RCS(Cl, Cl2·-, ClO·) AAP degradation also studied. Variation of acute toxicity of reacted solution was evaluated. The results indicated that AAP was removed effectively during UV-LED/NaClO process. After reaction of 90min, UV-LED irradiation, NaClO and UV-LED/NaClO processes for AAP removal were found to be 4.42%, 93.61% and 100%, respectively. The AAP degradation well fitted with the pseudo-first-order kinetics model (2=0.9967). The removal of AAP increased with the increasing of NaClO dosage, neutral condition was conducive to AAP degradation. AAP removal was inhibited in the presence of HA. HCO3-and NO3-could slightly promote the removal of AAP. When the dosage of NaClO was 1mg/L, the relative contribution rates of OH·, UV-LED, NaClO and RCS radicals in the degradation of AAP were 0.82%, 0.66%, 33.78% and 64.74%, respectively. The UV-LED/NaClO process can decrease the acute toxicity of the solution effectively.

acetaminophen；UV-LED/NaClO；kinetics；contribution；acute toxicity

X703

A

1000-6923(2019)11-4681-08

李博強(1994-),男,安徽安慶人,浙江工業大學碩士研究生,主要從事水處理理論與技術方面研究.

2019-04-15

國家自然科學基金資助項目(51878582,51678527,51378446);福建省科技計劃引導性資助項目(2017Y0079);福建省自然科學基金項目(2017J01491);福建省高校新世紀優秀人才支持計劃項目(JA14227)

* 責任作者, 研究員, leetsingsong@sina.com