UV-LED/NaClO工艺对水中对乙酰氨基酚的降解

李博强,马晓雁,李青松,廖文超,陈彦洁,陈国元,李国新

UV-LED/NaClO工艺对水中对乙酰氨基酚的降解

李博强1,2,马晓雁1,李青松2*,廖文超3,陈彦洁2,4,陈国元2,李国新2

(1.浙江工业大学建筑工程学院,浙江 杭州 310014；2.厦门理工学院水资源环境研究所,福建 厦门 361005；3.深圳技术大学健康与环境工程学院,广东 深圳 518118；4.青岛大学环境科学与工程学院,山东 青岛 266022)

采用NaClO、UV-LED和 UV-LED/NaClO工艺去除水中的对乙酰氨基酚(AAP),考察了NaClO投加量、pH值和腐殖酸(HA)等因素对UV-LED/NaClO工艺去除AAP的影响,研究了UV-LED/NaClO去除AAP过程中OH·、UV-LED、NaClO 和氯自由基RCS(Cl·,Cl2·-,ClO·)的贡献值,评估了AAP降解过程中溶液急性毒性的变化.结果表明,UV-LED/NaClO可以有效降解.AAP.反应90min后,UV-LED、NaClO和UV-LED/NaClO工艺对AAP的去除率分别为4.42%、93.61%和100%.AAP的降解符合拟一级反应动力学模型(2=0.9967). AAP的去除随着NaClO投加量的增大而增加,中性条件有利于AAP的降解,HA对AAP去除具有抑制作用,HCO3-和NO3-可略微促进AAP的去除.当NaClO投加量为1mg/L, OH·、UV-LED、NaClO和RCS各组分对AAP去除的相对贡献率分别为0.82%、0.66%、33.78%和64.74%,UV-LED/NaClO工艺可以有效的降低溶液的急性毒性.

对乙酰氨基酚；UV-LED/NaClO；动力学；贡献值；急性毒性

对乙酰氨基酚(AAP)是一种典型的药品和个人护理品(PPCPs),广泛应用于感冒药、解热药和镇痛药等常用非处方药中[1].由于AAP的广泛使用及其在环境中的不完全降解,地表水、地下水和饮用水等环境水体中均有检出[2-5].研究表明AAP具有内分泌干扰效应,会对生物造成遗传毒性、肝毒性等负面环境效应[6],因此有必要对水体中AAP的去除进行研究.

自由氯和臭氧氧化等可以有效降解水中AAP,但易生成毒性较高的消毒副产物[7-9].紫外(UV)和氯常用于水处理消毒,研究表明,NaClO在紫外辐照下能产生OH·、RCS(Cl·,Cl2·-,ClO·等)和其他强氧化性粒子[10-11],污染物能通过UV/NaClO协同去除且有较高的矿化度[12-13].同时UV/NaClO工艺在30min内可降解99%以上AAP[14].作为一种新型紫外光源,紫外发光二极管(UV-LED)可以发射210nm到可见光的辐射[15],具有使用寿命长、光子效率高、无毒及增加光电反应器设计的灵活性等优点[16-18].研究表明中性和碱性时UV-LED/NaClO工艺对罗硝唑(含甲基和氨基)的去除优于低压汞灯和NaClO联合工艺[19],UV-LED和NaClO的联合工艺能更好的去除污染物.但关于UV-LED协同NaClO去除AAP的研究尚未见相关报道,因此,本文采用UV-LED/ NaClO对AAP的去除进行研究.

本研究采用NaClO、UV-LED和UV-LED/ NaClO工艺去除水中的AAP,考察了NaClO投加量、pH值和腐殖酸(HA)等因素对UV-LED/NaClO工艺去除AAP的影响,研究了UV-LED/NaClO工艺去除AAP过程中OH·、UV-LED、NaClO和RCS(Cl·, Cl2·-,ClO·等)的贡献值,评估了AAP降解过程中急性毒性的变化.以期为UV-LED/NaClO工艺去除控制水体中微量污染物提供理论依据.

1 材料与方法

1.1 实验试剂与仪器

AAP(德国Dr.Ehrenstorfer公司,纯度>99.9%);腐殖酸(HA)(Tech,美国 Sigma-Aldrich);甲醇、乙腈(HPLC级,德国Merck)、硝基苯(NB)、叔丁醇(TBA)(HPLC级,上海安谱);次氯酸钠(CP,活性氯³5.2%);Na2S203·5H2O、NaHCO3、NaNO3、HCl和NaOH均为分析纯;BioFix®Lumi Multi-Shot冻干细菌及激活液;实验室用水均为Mill-Q超纯水(£18.2MΩ).

LC-20A高效液相色谱仪(shimadzu,日本); BioFix®®Lumi生物毒性仪(Macherey-Nagel,德国);CL200余氯计(ExStik,上海三信仪表厂);纯水机(Milipore,美国),DZF-6050真空干燥箱(上海精宏实验设备有限公司),HJ-6A型磁力恒温搅拌器(江苏金坛峥嵘仪器),pH计(Eutevch,美国),UV-LED灯(深圳微紫科技有限公司,波长278nm,单个100mW).

1.2 实验方法

试验在一个置于磁力搅拌器上的烧杯(2L)中进行,UV-LED/NaClO工艺中光源采用9个UV-LED灯并联(单个100mW,并联后900mW)组成,外置石英套管.烧杯内放置浓度为250μg/L的AAP溶液,通过0.1mol/L的NaOH和HCl调节pH值,然后投加一定量的NaClO溶液(0.5~1.5mg/L),同时启动搅拌,打开UV-LED灯,开启反应.设定时间取出10mL水样,所取水样经过1.0mol/L的硫代硫酸钠淬灭过膜后,测定AAP浓度和溶液急毒性.所有实验重复三次,取其平均值.利用方差分析给出实验数据的误差棒.

1.3 分析方法

AAP和NB浓度采用HPLC进行测定.流动相为乙腈和水(AAP为20:80,NB为65:35),流速均为1.0mL/min,柱温分别为40和35℃,AAP和NB的UV检测波长分别为243和262nm,进样量为10μL.方法的相对标准偏差为0.4904%,线性范围为5~ 1000µg/mL,检测限为3.359µg/L.

1.4 急毒性的试验方法

发光细菌急性毒性测试方法参考UNE-EN- ISO 11348-32007[20].待测水样经2%的NaCl盐化后,水样pH值调到6~8,然后采用生物毒性仪进行毒性测定,培养时间设定为30min,急毒性分析结果的表示形式是以抑制百分比和增强百分比来表示水样受污染程度,见式(1).

2 结果与讨论

2.1 UV-LED、NaClO和UV-LED/NaClO对AAP的去除

如图1所示,实验反应90min后单独UV-LED和单独NaClO氧化对AAP的去除分别为4.42%和93.61%.然而,相同时间内UV-LED/NaClO对AAP的去除可达100%. UV-LED辐射、单独NaClO氧化和UV-LED/NaClO工艺的拟一级动力学常数分别为0.0006,0.0305和0.0903min-1,UV-LED/ NaClO降解AAP的拟一级动力学常数分别是NaClO和UV-LED的3倍和151倍.实验表明UV- LED/NaClO工艺对AAP的去除具有协同作用.

UV-LED对AAP的去除主要是UV-LED辐射产生的OH·参与反应[21],NaClO能够通过氧化作用去除AAP.UV-LED/NaClO工艺中溶液中的HOCl/OCl-在紫外光激发下产生了高活性氧化自由基(如OH·和RCS(Cl·,Cl2·-,ClO·))[10,13,22-23],主要的反应式如式(2)~(7).OH·、Cl·和Cl2·－的氧化还原电位分别为2.8,2.47和2.0V,具有强氧化性[14,24],氯自由基可以有效地去除含芳香环和富电子的有机污染物[14].随着UV-LED辐照时间的增加,溶液中高活性氧化自由基含量逐渐增加,因此在UV-LED/NaClO工艺中不仅有UV-LED对AAP的辐射作用,还有NaClO的直接氧化和溶液中产生的活性自由基的氧化作用,三者共同作用促进了AAP的降解产生了协同效应.

图1 UV-LED、NaClO和UV-LED/NaClO对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的影响因素

2.2.1 NaClO投加量对AAP去除的影响 NaClO浓度影响氯自由基的产率[14],因此考察了NaClO投加量对AAP去除的影响,结果见图2.NaClO投加量分别为0.5,0.75,1.0,1.25和1.5mg/L,拟一级动力学常数分别为0.0165,0.0484,0.0903,0.1331和0.1433min-1.实验中AAP的去除随着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].实验中表现为AAP的降解速率加快.然而,当NaClO投加量超过1mg/L时,AAP的降解速率增加幅度变小.可能是因为NaClO过量时,大量的游离氯会与Cl·和OH·发生反应(式(3)和(5)),同时随着NaClO浓度的增加,NaClO吸收的紫外光逐渐达到饱和,使RCS的生成量也逐渐达到饱和[26].这与Dong等[27]采用UV/NaClO工艺降解氯霉素得到的规律类似.

图2 NaClO对UV-LED/NaClO降解AAP的影响

[AAP]=250μg/L, pH=(7.0±0.2)

2.2.2 pH值对AAP去除的影响 pH值影响紫外/氯工艺中氧化自由基的存在形式[28],因此考察pH值对AAP去除的影响,结果见图3.pH值为3.02,5.05, 7.03、9.10、11.09时,反应90min后的去除率分别为39.05%、84.28%、99.48%、99.16%、69.44%,AAP的去除随着pH值的升高先增加再降低.不同pH值对AAP的去除均符合拟一级动力学模型(2> 0.97)(图3),拟一级动力学常数先由0.0085min-1增加到0.0903min-1然后降低至0.0178min-1.中性条件下AAP的去除效果最佳.

pH值影响NaClO在溶液中主要存在形式[29].当溶液pH值为3~5时,溶液中NaClO的主要存在形式为HClO,溶液中的H+可以与Cl·和水合电子eaq·反应生成还原性较低的·H和Cl-(式8)[30],降低对AAP的去除.同时当溶液过酸时由于搅拌作用有部分的Cl2溢出[28],导致溶液pH值为3.02时的去除效果相比pH值为5.05时有所降低;当溶液pH值为中性时,溶液中NaClO的主要存在形式为HClO和少量的OCl-,溶液主要是通过HOCl和少量的OCl-发生光解反应(式(2)~式(7)),产生更多的强氧化自由基进而有效的去除AAP;当溶液pH值9~11时,溶液中NaClO的主要存在形式为OCl-,溶液主要是通过OCl-发生光解反应(式(2)和(4)),由于OCl-和HOCl与HO·的反应速率常数为8.8×109和8.46×102L/(mol×s)[31],因此OCl-能较快的与HO·反应,导致溶液中的氧化自由基减少,从而降低对AAP去除.当溶液pH值大于9.71时,AAP电离为AAP-(pa,AAP=9.71)[1],由于电荷之间的排斥作用,AAP-与ClO-之间的反应受阻,使溶液pH值为11.09时的去除效果相较于pH值为9.10时有所降低.Deng等[32]在利用UV协同NaClO去除环丙沙星过程中具有类似的结论.

Cl·+eaq+H+®·H+Cl-(8)

图3 pH值对UV-LED/NaClO降解AAP的影响

[AAP]=250μg/L, [NaClO]=1mg/L

2.2.3 腐殖酸对AAP去除的影响 腐殖酸(HA)是自然界中丰富的大分子有机质,广泛存在于自然水体中[33].因此利用HA模拟水体中的天然有机质,考察了HA对去除AAP的影响,结果见图4.AAP的去除随着HA投加量的增加而降低.AAP去除的拟一级动力学常数随着HA浓度的增加逐渐降低.当HA的投加量分别为0,1,3,5,7和9mg/L时,拟一级动力学常数从0.0903降低至0.0550,0.0297,0.0230,0.0155, 0.0132min-1.

HA不仅可以增加水的色度影响光的透光率,还可以消耗溶液中的NaClO(其拟一级动力学常数为3×10-5s-1)[34],更为重要的HA可以与AAP竞争溶液中的Cl·和OH·等氧化粒子[35],从而降低对AAP去除.实验中HA对AAP的去除具有抑制作用,这与Tang等[36]利用UV/NaClO降解降固醇酸的实验中发现随着溶液中HA的浓度增多对降固醇酸的去除率降低的规律相同.

2.2.4 HCO3-和NO3-对AAP去除的影响 HCO3-和NO3-广泛存在于地表水中,研究表明它们可能会影响紫外线高级氧化工艺中有机物的去除[37-38].因此投加不同浓度的HCO3-和NO3-,考察了两种阴离子对AAP去除的影响,结果如图6所示.AAP去除的拟一级动力学常数值随着HCO3-和NO3-浓度的增加逐渐增大.实验中,HCO3-和NO3-投加量为0时,反应40min后AAP的去除率为97.43%,当HCO3-投加量为5,20,50和100mmol/L时,对AAP的去除率分别增加至98.39%、98.48%、99.08%和99.23%.当NO3-的投加量为0.1,0.5,1.0和2.0mmol/L时,对AAP的去除分别增加为97.59%、97.79%、98.36%和98.45%.因此水体中HCO3-和NO3-对AAP的去除均有促进作用.

图4 HA对UV-LED/NaClO降解AAP的影响

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

图5 HCO3-和NO3-对UV-LED/NaClO降解AAP的影响

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

Tan等[39]和Luo等[40]在研究UV/H2O2等工艺去除安替比林和阿特拉津的实验中观察到HCO3-对降解过程有抑制作用,然而Fan等[41]和Liu等[42]在过硫酸盐和UV活化过硫酸盐工艺去除磺胺甲嘧啶和氧四环素的实验中表明HCO3-可以促进目标物的去除.本实验观察到HCO3-可以促进AAP的降解.其主要原因是HCO3-可与RCS和OH·快速反应形成CO3·-,其反应过程如(式(9)和式(10))[31].同时由于AAP结构中含有苯胺,CO3·-可与含有苯胺结构的物质发生双分子反应[43],从而增强了AAP的去除.NO3-经过光解后会产生OH·[44],OH·可以降解AAP,同时也可与HOCl反应产生ClO·(式3),因而增加NO3-的浓度可以促进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过程中各组分贡献率

叔丁醇(TBA)能与OH·和Cl·快速反应,同时还可以与Cl2·-反应,常用于测定RCS和OH·贡献值的研究中[45-46].因此投加不同浓度的TBA,考察了在UV-LED/NaClO去除AAP过程中活性自由基对去除率的影响,结果见图6.TBA的投加量为0,0.1,0.5和1mmol/L时,AAP去除的拟一级动力学常数分别为0.0903,0.0543,0.0379和0.0345min-1.表明TBA对AAP的去除有抑制作用.由此可知在UV-LED/ NaClO去除AAP的过程中Cl·,Cl2·-和HO·有着一定贡献量.但是在加入较高浓度的TBA后去除率仍大于单独NaClO对AAP的去除,这表明AAP的去除过程中可能还有其他的RCS的作用.

图6 TBA对UV-LED/NaClO去除AAP的影响

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

硝基苯(NB)在紫外协同NaClO工艺中仅与OH·发生反应[32],因此选用NB作为OH·的探针来研究UV-LED/NaClO去除AAP的过程中OH·的贡献值.假设在去除过程中各自由基的浓度保持稳定不变,OH·和RCS的贡献值可以通过式(11)和式(12)计算所得.

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过程中的拟一级动力学常数.UV-LED¢表示单独UV-LED去除AAP的拟一级动力学常数,chlorine¢和RCS-AAP¢是NaClO和RCS氧化导致AAP降解的拟一级动力学常数,OH·-AAP和OH·-NB是UV-LED/ NaClO工艺中产生的OH·与AAP和NB反应的二级反应速率常数.d-NB¢和vol-NB¢分别是对NB直接光解和NB挥发的拟一级动力学常数.

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

由图7可知,NB的降解过程均符合拟一级动力学模型.UV-LED/NaClO降解NB的拟一级动力学常数为0.0026min-1,NB挥发作用和单独UV-LED去除NB的拟一级动力学常数分别为0.0010和0.0002min-1.已知OH·-NB=3.9×109(mol×s)/L[30],因此由式(12)可计算出OH·的浓度为2.3×10-11mol/L.由图1可知,AAP¢=0.0903min-1,chlorine¢=0.0305min-1,UV-LED¢=0.0006min-1,同时已知OH·-AAP=1.7×109L/ (mol×s)[47],由此可通过式(11)得出RCS降解AAP的拟一级动力学常数为0.0585min-1.因为UV-LED¢= 0.0006min-1,OH·-AAP[OH·]=0.0007min-1,chlorine¢= 0.0305min-1,RCS-AAP¢=0.0585min-1,因此单独UV- LED、OH·、NaClO和RCS对AAP降解的相对贡献率分别为0.66%、0.82%、33.78%和64.74%.

2.4 AAP降解过程中溶液急性毒性的变化

改变NaClO的投加量,考察了UV-LED/NaClO和NaClO降解AAP过程中溶液急性毒性的变化.结果见图8.当NaClO投加量为1.0mg/L时,单独NaClO氧化去除AAP的过程中溶液急毒性从25%逐渐增加至45%.相同取样时间,在UV-LED/NaClO降解AAP过程中,投加不同浓度NaClO时溶液的急毒性均呈先略微升高,再逐渐降低的趋势.NaClO投加量为1.5mg/L,反应90min后溶液的相对抑制率降低至10%.表明UV-LED/NaClO工艺能更有效的降低溶液的急性毒性,并且当NaClO浓度越高时,溶液的急性毒性降低的更为显著.

图8 UV-LED/NaClO和NaClO降解AAP过程中溶液急毒性的变化

[AAP]=250μg/L, pH=(7.0±0.2)

Bedner等[48]在研究NaClO去除AAP的降解产物中发现AAP可生成毒性为其58倍和25倍的1,4-苯醌和N-乙酰-对-苯醌亚胺(NAPQI),同时发现随着反应的进行,1,4-苯醌的浓度逐渐升高,NAPQI的浓度先升高后降低.因此实验中单独NaClO无法有效去除AAP降解过程中生成的1,4-苯醌类等消毒副产物,导致了溶液急性毒性的升高.加入了UV- LED辐射后,随着反应的进行,溶液中高活性自由基逐渐增加,因为OH·和RCS具有高氧化还原电位,同时OH·的氧化作用无选择性能迅速降解广泛的有机物[13],因此会导致此类消毒副产物去除进而降低溶液的急性毒性.Vogna等[49]在研究UV/H2O2去除AAP的实验中发现OH·可有效降解1,4-苯醌.

3 结论

3.1 UV-LED、NaClO和UV-LED/NaClO 3种工艺对AAP有不同程度的去除效果,反应90min后,对AAP去除率分别为4.42%、93.61%和100%,UV- LED/NaClO工艺能有效的去除AAP.

3.2 在UV-LED/NaClO去除AAP工艺中,NaClO投加量、pH值、HA浓度和阴离子(HCO3-、NO3-)均影响AAP的反应速率.增加NaClO的投加量会增大AAP的降解率,HCO3-和NO3-有利于AAP的去除.并且在中性条件下更适宜AAP的降解.HA对AAP的去除具有抑制作用.

3.3 NaClO的投加量为1mg/L时,UV-LED/ NaClO降解AAP过程中单独UV-LED、OH、NaClO和RCS对AAP降解的相对贡献率分别为0.66%、0.82%、33.78%和64.74%.氯自由基占去除AAP的主导地位.

3.4 相比于单独投加NaClO,UV-LED/NaClO工艺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

李博强(1994-),男,安徽安庆人,浙江工业大学硕士研究生,主要从事水处理理论与技术方面研究.

2019-04-15

国家自然科学基金资助项目(51878582,51678527,51378446);福建省科技计划引导性资助项目(2017Y0079);福建省自然科学基金项目(2017J01491);福建省高校新世纪优秀人才支持计划项目(JA14227)

* 责任作者, 研究员, leetsingsong@sina.com