紫外活化过硫酸钠去除水体中的三氯卡班

骆靖宇,李学艳,李青松,姚宁波,陆保松,李国新,陈国元,廖文超,高乃云(.苏州科技大学环境科学与工程学院,江苏 苏州 5009；.厦门理工学院水资源环境研究所,福建 厦门 604；.同济大学污染控制与资源化研究国家重点实验室,上海 0009；4.浙江工业大学建筑工程学院,浙江 杭州 004)

紫外活化过硫酸钠去除水体中的三氯卡班

骆靖宇1,2,李学艳1,李青松2*,姚宁波1,2,陆保松2,4,李国新2,陈国元2,廖文超2,高乃云3(1.苏州科技大学环境科学与工程学院,江苏 苏州 215009；2.厦门理工学院水资源环境研究所,福建 厦门 361024；3.同济大学污染控制与资源化研究国家重点实验室,上海 200092；4.浙江工业大学建筑工程学院,浙江 杭州 310014)

采用紫外活化过硫酸钠(UV/PS)降解三氯卡班(TCC).考察了UV、PS和UV/PS联用工艺去除TCC的效果,研究了PS投加量、反应初始pH值和腐殖酸(HA)等因素对UV/PS降解TCC的影响,推测了UV/PS工艺中TCC可能的降解途径,并对比了UV/PS和UV/H2O2工艺对TCC的去除效果和经济性.研究表明:UV与PS联用能够高效去除TCC,其降解过程符合拟一级动力学模型(R2≥0.95);拟一级反应速率常数k随着PS投加量的增加先增大再减小,在PS投加量为250µmol/L时,k达到最大值0.0810min-1;偏酸性条件(pH=6.0)有利于TCC的降解;HA对TCC的降解有抑制作用,抑制作用与HA的浓度呈正相关; GC/MS鉴定表明, TCC降解过程中主要的中间产物有异氰酸4-氯苯酯和对氯苯胺,其可能的降解途径为 TCC分子结构中与酮羰基相连的 C-N键断裂,脱氯,经过一系列的反应形成对氯苯胺和异氰酸 4-氯苯酯;UV/PS降解TCC过程中溶液中脱氯反应导致Cl-浓度增加;与UV/H2O2工艺相比, 相同条件下UV/PS工艺中k值增大了96.65%,单位电能消耗量提高了97%.

UV/PS；三氯卡班；硫酸根自由基；中间产物

三氯卡班(triclocarban, TCC)是一种典型的药品和个人护理用品(PPCPs),广泛应用于抗菌香皂、洗手液、化妆品和消毒剂中[1].其亲水性低,亲脂性强(pH=7.0时,logKow为4.9)[2],物理化学性质稳定,自然环境中较难降解.研究表明 TCC对一些藻类和老鼠、鱼类、蜗牛具有慢性毒性影响,还会导致人类癌症、生殖功能障碍和发育异常等健康问题[3-8].近年来,不同水体中均检测出了TCC[9-15].我国五大水系的TCC检出率达到了的100%[16-17].目前,水体中的TCC已引起国内外学者专家广泛关注.

Gledhill[18]和Ying等[19]分别对TCC的生物降解做了相关的研究,发现生物去除TCC的效果不佳,且耗时长、条件严格;Sirés等[20]探究了电芬顿法对TCC的去除,去除效果受pH值影响大.传统的水处理技术难以有效地去除水中的TCC[13,21-24],因此,亟待探寻水体中TCC经济有效去除的方法.UV活化PS具有反应条件温和、自由基产生快速、氧化性强且稳定、无选择性、操作简单等特点,逐步受到广大学者的关注.

实验采用UV活化PS的工艺来去除水中的TCC,对比了UV、PS和UV/PS联用三种工艺对TCC的去除效果,考察了 PS投加量、反应初始pH值、腐殖酸(HA)和温度等因素的影响,探讨了TCC在UV/PS工艺中可能的降解途径,以期为实际应用中UV/PS降解水中TCC提供理论指导和实验基础.

1 实验部分

1.1 试剂与仪器

三氯卡班(TCC)(德国 Dr.Ehrenstorfer公司,纯度＞99.5%);过硫酸钠(PS)(AR,≥98.0%);腐殖酸(HA)(Tech,美国 Sigma-Aldrich);异氰酸 4-氯苯酯(德国 Dr.Ehrenstorfer公司,99.5%);对氯苯胺(德国Dr.Ehrenstorfer公司,≥98.0%),HCl、NaOH均为分析纯;甲醇、乙腈(HPLC级,德国 Merck); 30%过氧化氢(H2O2)(AR)实验室用水为 Mili-Q超纯水(≤18.2MΩ).

LC-20A高效液相色谱仪(Shimadzu,日本),自动进样器(SIL-20A),检测器(SPD-M20A);气相色谱质谱联用仪(GCMS-QP2010Ultra,日本岛津);GC/MS自动进样器(AOC-5000,日本岛津),色谱柱(Rxi®-5ms: 30m×0.32mm×0.25μm,日本岛津);离子色谱仪(戴安 ICS-1100);pH计(Eutevch,美国);HJ-6A型磁力恒温搅拌器(江苏金坛峥嵘仪器);HC-C18小柱(Anpel);紫外线光源(主波长254nm,杨紫特种紫外线光源,低压汞灯,9W),紫外线强度计(TN-2365A,台湾泰纳).

1.2 实验方法

实验前开启紫外灯预热5min,然后置于烧杯中,保持紫外灯的位置相对烧杯固定,烧杯边缘位置(图中距离烧杯底部 8cm处的烧杯壁)的光强为11.5μW/cm2;投加一定量的PS溶液后打开紫外灯开始反应,分别在0、5、10、20、30、45、60min时取样,随即加入适量甲醇淬灭,经0.45μm的玻璃纤维滤膜过滤后分析.GC/MS产物鉴定前用固相萃取对样品进行富集.反应装置见图1.

图1 实验装置示意Fig.1 Schematic description of the reactor

1.3 分析方法

HPLC 条件:色谱柱为 Inertsil®ODS-SP(250mm×4.6mm,5μm);流动相为乙腈:水=65:35 (V:V),流动相流速为 1.0mL/min;检测波长为265nm;进样体积为10μL; S/N＞3.

GC/MS条件:载气为高纯度氦气,90kPa;进样量为1μL;无分流进样方式;进样口温度为280℃;炉温控制:初始温度为60℃,保留时间安3min,然后以 5℃/min升温至 150℃,持续 5min,然后以10℃/min升温至280℃,持续3min;MS离子化温度为 250℃;接口温度为 280℃;采用 Scan扫描:质荷比 m/z起始为 50,终止为 600,扫描时间为0～40min.

2 结果与分析

2.1 UV、PS和UV/PS工艺对TCC的降解

UV、PS和UV/PS工艺对TCC的降解结果如图2所示.

图2 PS、UV和UV/PS对TCC的降解效果Fig.2 Removal of TCC by direct UV irradiation, PS oxidation alone, and UV/PS process [TCC]=400µg/L, [PS]=0.25mmol/L, pH=6.0

实验表明,60min内单独PS对TCC的去除率小于 14%;单独 UV对 TCC的去除增加至61.52%;相同条件下,UV/PS联用对TCC的去除可达99.95%,表明UV/PS联用工艺具有协同作用,可以更有效地降解TCC.TCC降解曲线的拟一级动力学拟合结果表明,UV/PS工艺中拟一级动力学常数k比单独UV光降解的增大了4.6倍.

PS分子中含有过氧基且在水中可电离产生S2O82-,其氧化还原电位为2.01V,具有一定的氧化能力[25],因此,单独PS对TCC有一定的降解能力;单独 UV照射下,对 TCC降解起主要作用的是·O2[26].UV/PS工艺中,PS在UV的照射下,其中的O-O键会因吸收能量而断裂,产生含有孤电子对的·SO4-,·SO4-不仅具有超强氧化性,还可以与水(H2O)或者氢氧根(OH-)反应生成羟基自由基(·OH),增加溶液中自由基的浓度,使得TCC能够高效降解,其反应过程如方程(1)-(3)所示:

2.2 PS投加量的影响

实验中考察了PS投加量对UV/PS降解TCC的影响,不同PS投加量下TCC的降解曲线的拟一级动力学拟合结果见图3.

图3 PS投加量对UV/PS降解TCC的影响Fig.3 Effect of PS concentration on TCC degradation by UV/PS process [TCC]=400µg/L, pH=6.0

由图3可知,在实验范围内,TCC的降解符合拟一级动力学(R2≥0.91).PS投加量从 0增加到0.25mmol/L,拟一级动力学常数k由0.0151min-1增大为0.0810min-1,PS投加量为0.25mmol/L时,实验中60min TCC的降解率可达99.44%;继续增加至0.75mmol/L,拟一级动力学常数k反而减小为0.0344min-1.因此,UV/PS工艺中TCC的去除并不是随着PS投加量的增加而越高.

由式(1)～(3)可知,PS浓度的增加可以提高溶液中自由基·SO4-和·OH的稳态浓度,从而加速TCC的降解,因此,在投加量0～0.25mmol/L范围内TCC的去除随着PS浓度的增加而增加;但有研究表明PS也会消耗·SO4-和·OH,生成氧化性较弱的·S2O8-[式(5)和(6)],影响 TCC的降解(PS与·SO4-和·OH 反应的速率常数分别是 5.5× 105M-1s-1[27]和 1.4×107M-1s-1[28]),故实验中当 PS投加量较大(0.5mmol/L和 0.75mmol/L)时 TCC的降解效果反而下降.

2.3 初始pH的影响

pH是影响高级氧化(AOPs)处理效果的重要参数.实验中考察了pH对TCC去除的影响.不同pH值时TCC的去除拟合结果见图4.

图4 pH对UV/PS降解TCC的影响Fig.4 Effect of initial pH on TCC degradation by UV/PS process

实验中 pH值为 4.0、6.0、7.0、8.0和 9.5时,60min内 TCC的去除率分别为 76.50%、99.44%、91.20%、91.71%和 74.74%.pH=6.0时TCC的去除率最高.

由图 4可知,实验范围内,随着 pH的增大,k先增大后减小,在 pH=6.0时,k达到最大值0.0810min-1.这与Saien等在考察pH对UV/PS去除水杨酸的影响时的研究结果相一致[29-31].

过硫酸根离子非催化反应的活化能为33.5Kcal,而酸催化反应的活化能为26.0Kcal[32],酸性条件下S2O82-会与H+发生酸催化反应[33-34],因此过硫酸根在酸催化反应中更易转化为硫酸根自由基,产生更多的·SO4-[式(7)和(8)],促进TCC的降解.但强酸条件下,更易发生式9和10的反应,反而消耗·SO4-产生氧化性更弱的自由基[35],导致TCC的降解速率降低.因此,相比于中性和强酸条件,偏酸性条件更有利于TCC的降解.

碱性条件下·SO4-会与 OH-反应生成·OH(式3),但碱性环境中·OH的氧化性较弱[36],因此尽管碱性条件下有·OH产生,但效果没有偏酸性和中性条件下好.有研究表明,碱性环境中大量生成的SO42-[式(3)]对·SO4-和·OH 均有抑制作用[31,37].因此,在一定 pH范围内,偏酸性条件(本实验为pH=6.0)下TCC的降解效果更好.

2.4 腐殖酸的影响

腐殖酸(HA)是天然水体中主要的有机物,因此本实验采用 HA来模拟天然水体中的有机物(NOM),考察了HA对TCC去除的影响,结果如图5所示.

实验中 HA 浓度为 0,0.5,1.0,3.0,5.0mg/L时,TCC的去除率分别为 99.45%、96.99%、96.36%、89.03%和63.78%;另外由图4可知,TCC降解的动力学常数 k随着 HA浓度的增加由0.0810min-1减小到 0.0158min-1.表明 HA 对UV/PS降解TCC有著的抑制作用.

基于UV的高级氧化体系中HA有两方面的影响,一方面,UV激发下NOM可以产生·OH、·O等活性自由基,促进污染物的降解;另一方面NOM中的各种不饱和官能团对UV有一定的吸收能力,削弱光的透射能力,同时会与目标污染物竞争自由基[38].

实验中HA一直起抑制作用,原因可能是HA屏蔽了UV光辐射,降低了UV的活化作用;HA分子中的酚羟基、胺基、羧基等活性基团与目标污染物TCC竞争·SO4-和·OH等自由基,导致自由基的稳态浓度降低[39].

图5 腐植酸对UV/PS降解TCC的影响Fig.5 Effect of humic acid on TCC degradation by UV/PS process

2.5 UV/PS降解TCC的反应途径分析

TCC的初始浓度为 900μg/L, PS投加量0.25mmol/L,反应60min后取出溶液固相萃取富集500倍后经GC/MS鉴定,发现在9.97min和11.9min有两个明显的出峰(图 6),特征离子碎片的质荷比分别为 m/z=63,90,125,153和 m/z= 65,92,127.经谱库检索鉴定为异氰酸 4-氯苯酯(1-chloro-4-isocyanato-benzen)和对氯苯胺(4-chloroaniline).

据此推断出TCC的降解产物可能有异氰酸4-氯苯酯和对氯苯胺.TCC在UV/PS系统中可能的光降解途径如图7所示.

图6 总离子色谱Fig.6 Total ion chromatogram (TIC)

图7 TCC可能的光降解途径Fig.7 Proposed reaction pathway for TCC degradation by UV/PS process

根据降解过程中中间产物的生成及Cl-浓度的增加,可以推测TCC的降解路径:TCC分子结构中酮羰基左侧的C—N键(与二氯苯胺环相连)断裂,形成异氰酸4-氯苯酯和3,4-二氯苯胺,然后3,4-二氯苯胺脱掉一个Cl形成对氯苯胺,这条降解途径与丁世玲[26]的研究结果相似.还有一种可能的途径:酮羰基两侧的 C-N键(分别与二氯苯胺环和对氯苯胺环相连)断裂,经过一系列的反应形成主要中间产物对氯苯胺.TCC的降解过程中两种降解途径可能同时存在,异氰酸 4-氯苯酯和对氯苯胺等中间产物继续被降解,生成其他物质,最终苯环开环转化为CO2、H2O等[40-43].

2.6 TCC降解过程中主要成分的变化

实验考察了TCC的去除和降解产物的生成情况,结果见图8.

TCC的快速降解发生在前30min,去除率达到 95.34%,之后反应速率逐渐减小,在 60min时TCC浓度已经低于检出限.TCC降解产生异氰酸4-氯苯酯和对氯苯胺.在前15min异氰酸4-氯苯酯和对氯苯胺稳定增加至 142.45μg/L 和169.55μg/L,然后开始缓慢减少,在 60min时浓度降低至70.30μg/L和100.55μg/L.从TCC的降解路径分析,对氯苯胺的产生量应该比异氰酸4-氯苯酯多,但其在反应过程中的浓度一直比后者低,这可能是因为TCC分子结构中的对氯苯胺环比二氯苯胺环降解更快[18]导致的.实验中随着反应的进行,TCC分子结构中的3个氯不断脱离形成异氰酸 4-氯苯酯和对氯苯胺及大量 Cl-等,部分异氰酸4-氯苯酯和对氯苯胺降解也会有Cl-脱离,导致溶液中Cl-的浓度增大,在前15minCl-快速增加至242.73μg/L,之后Cl-的增加速率较之前有所下降,60min时 Cl-浓度达到 429.19μg/L,这表明UV/PS可以快速降解TCC并脱氯,进而可以降低溶液的毒性[44-45].

图8 TCC降解过程中主要物质的浓度变化Fig.8 Concentration changes of main products during TCC degradation by UV/PS process

在降解过程中TCC与异氰酸4-氯苯酯和对氯苯胺的摩尔比并不是 1:1.可能是因为在异氰酸 4-氯苯酯和对氯苯胺形成的同时一部分已经被反应消耗;可能有其它反应途径产生其它的中间产物,冯振涛等[46]的研究表明,有更为复杂的降解产物产生.

2.7 与UV/H2O2工艺比较

实验中对比考察了UV/PS与典型的高级氧化工艺UV/H2O2去除TCC的效能,结果见图9.

图9 UV/PS、UV/H2O2对TCC处理效果比较Fig.9 Removal of TCC by UV/PS and UV/H2O2processes

H2O2与 PS的投加量均为 250mmol/L时, TCC的去除均遵循自由基反应的拟一级动力学,反应速率常数k分别是0.04117min-1(UV/ H2O2)和 0.08096min-1(UV/PS),相比 UV/H2O2, UV/PS降解TCC的k增加了96.65%.

表1 不同体系的单位电能消耗量和氧化剂成本Table 1 The electrical energy per order and oxidant costs in different systems

采用单位电能消耗量(EE/O)来评价两种工艺的电能利用效率,其公式如下[47]:式中:EE/O:单位电能消耗量,kW·h/m3;P:紫外灯的功率,kW;V为时间t内处理溶液的体积,L;k为反应速率常数,min-1.

EE/O值越低,体系中的电能利用率越高,效率也越高[48].

将反应速率常数 k带入公式得到两个体系的单位电能消耗表1所示,可以看出,UV/PS具有更高的电能利用率.另外,氧化剂的成本方面, UV/PS也略低于UV/H2O2.

3 结论

3.1 UV活化PS工艺能有效去除水体中的TCC,降解过程符合拟一级反应动力学模型,UV辐射强度为 11.5μW·cm-2,PS投加量为 250μmol·L-1, pH=6.0时,60min的后,初始浓度为 400μg·L-1的TCC去除率可达99.44%.

3.2 TCC的去除随PS投加量的增加和pH值的升高,先增强后减弱,偏酸性环境更有利于 TCC的降解,腐殖酸对 TCC的去除有抑制作用,且抑制作用与腐殖酸浓度呈线性关系.

3.3 UV/PS降解TCC的中间产物主要有异氰酸4-氯苯酯和对氯苯胺,可能是TCC分子结构中酮羰基两侧的C-N键断裂产生的.

3.4 UV/PS联用工艺比UV/H2O2工艺更加有优势,其动力学常数k提高了96.65%.

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Degradation of triclocarban aqueous solution through UV irradiation-activated sodium persulfate process.

LUO Jing-yu1,2, LI Xue-yan1, LI Qing-song2*, YAO Ning-bo1,2, LU Bao-song2,4, LI Guo-xin2, CHEN Guo-yuan2, LIAO Wen-chao2, GAO Nai-yun3(1.School of Environmental Science and Engineering, Suzhou University of Science and Technology, Suzhou 215009, China；2.Water Resources and Environmental Institute, Xiamen University of Technology, Xiamen 361024, China；3.National Key Laboratory of Pollution Control and Reuse, Tongji University, Shanghai 200092, China；4.College of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou 310014, China). China Environmental Science, 2017,37(9)：3324～3331

Triclocarban (TCC) in aqueous solution was degraded by UV-activated persulfate. The removal efficiency of TCC by direct UV irradiation, PS oxidation alone, and UV/PS process was compared in this experiment. The effect of PS dosage, initial pH and HA on TCC degradation by UV/PS was investigated. The possible degradation approach and intermediates was proposed, meanwhile, the effect of degradation and economical efficiency for UV/PS were compared with UV/H2O2. The results showed that UV irradiation-activated sodium persulfate process could remove TCC efficiently and TCC degradation followed the pseudo-first order kinetic model well (R2≥0.95). The pseudo-first-order-constant k increased firstly and then decreased with the increase of PS dosage. The value of k reached a maximum of 0.0810min-1when the dosage of PS was 250µmol/L. Slightly acidic condition (pH=6.0) was better for TCC degradation. The removal of TCC was inhibited in the presence of HA, and the effect of inhibition was significantly positively correlated with the concentration of HA. 1-chloro-4-isocyanato-benzen and 4-chloroaniline were identified as the main intermediates by GC/MS. The possible degradation approach is that the C-N chemical bonds of the keto carbonyl group were broken during the degradation process, and thus 1-chloro-4-isocyanato-benzen and 4-chloroaniline was generated via the dechlorinationand other reactions. The concentration of Cl-was increased through the degradation process of TCC by UV/PS. Compared with UV/H2O2process, the pseudo-first-order-constant k and the electrical energy per order of UV/PS process increased by 96.65% and 97%, respectively.

UV/PS；triclocarban；sulfate radical；intermediates

X703

A

1000-6923(2017)09-3324-08

2017-02-28

国家自然科学基金项目(51378446,51678527,51408518);福建省科技计划引导性项目(2017Y0079);福建省高校新世纪优秀人才支持计划项目(JA14227);福建省自然科学基金项目(2016J01695);江苏省企业研究生工作站合作项目;厦门市科技局项目(3502Z20131157,3502Z20150051)

* 责任作者, 副研究员, leetsingsong@sina.com

骆靖宇(1992-),男,江苏南通人,苏州科技大学硕士研究生,主要研究方向为水处理理论与技术.