紫外活化過硫酸鈉去除水體中的三氯卡班

駱靖宇,李學艷,李青松,姚寧波,陸保松,李國新,陳國元,廖文超,高乃云(.蘇州科技大學環境科學與工程學院,江蘇 蘇州 5009；.廈門理工學院水資源環境研究所,福建 廈門 604；.同濟大學污染控制與資源化研究國家重點實驗室,上海 0009；4.浙江工業大學建筑工程學院,浙江 杭州 004)

紫外活化過硫酸鈉去除水體中的三氯卡班

駱靖宇1,2,李學艷1,李青松2*,姚寧波1,2,陸保松2,4,李國新2,陳國元2,廖文超2,高乃云3(1.蘇州科技大學環境科學與工程學院,江蘇 蘇州 215009；2.廈門理工學院水資源環境研究所,福建 廈門 361024；3.同濟大學污染控制與資源化研究國家重點實驗室,上海 200092；4.浙江工業大學建筑工程學院,浙江 杭州 310014)

采用紫外活化過硫酸鈉(UV/PS)降解三氯卡班(TCC).考察了UV、PS和UV/PS聯用工藝去除TCC的效果,研究了PS投加量、反應初始pH值和腐殖酸(HA)等因素對UV/PS降解TCC的影響,推測了UV/PS工藝中TCC可能的降解途徑,并對比了UV/PS和UV/H2O2工藝對TCC的去除效果和經濟性.研究表明:UV與PS聯用能夠高效去除TCC,其降解過程符合擬一級動力學模型(R2≥0.95);擬一級反應速率常數k隨著PS投加量的增加先增大再減小,在PS投加量為250μmol/L時,k達到最大值0.0810min-1;偏酸性條件(pH=6.0)有利于TCC的降解;HA對TCC的降解有抑制作用,抑制作用與HA的濃度呈正相關; GC/MS鑒定表明, TCC降解過程中主要的中間產物有異氰酸4-氯苯酯和對氯苯胺,其可能的降解途徑為 TCC分子結構中與酮羰基相連的 C-N鍵斷裂,脫氯,經過一系列的反應形成對氯苯胺和異氰酸 4-氯苯酯;UV/PS降解TCC過程中溶液中脫氯反應導致Cl-濃度增加;與UV/H2O2工藝相比, 相同條件下UV/PS工藝中k值增大了96.65%,單位電能消耗量提高了97%.

UV/PS；三氯卡班；硫酸根自由基；中間產物

三氯卡班(triclocarban, TCC)是一種典型的藥品和個人護理用品(PPCPs),廣泛應用于抗菌香皂、洗手液、化妝品和消毒劑中[1].其親水性低,親脂性強(pH=7.0時,logKow為4.9)[2],物理化學性質穩定,自然環境中較難降解.研究表明 TCC對一些藻類和老鼠、魚類、蝸牛具有慢性毒性影響,還會導致人類癌癥、生殖功能障礙和發育異常等健康問題[3-8].近年來,不同水體中均檢測出了TCC[9-15].我國五大水系的TCC檢出率達到了的100%[16-17].目前,水體中的TCC已引起國內外學者專家廣泛關注.

Gledhill[18]和Ying等[19]分別對TCC的生物降解做了相關的研究,發現生物去除TCC的效果不佳,且耗時長、條件嚴格;Sirés等[20]探究了電芬頓法對TCC的去除,去除效果受pH值影響大.傳統的水處理技術難以有效地去除水中的TCC[13,21-24],因此,亟待探尋水體中TCC經濟有效去除的方法.UV活化PS具有反應條件溫和、自由基產生快速、氧化性強且穩定、無選擇性、操作簡單等特點,逐步受到廣大學者的關注.

實驗采用UV活化PS的工藝來去除水中的TCC,對比了UV、PS和UV/PS聯用三種工藝對TCC的去除效果,考察了 PS投加量、反應初始pH值、腐殖酸(HA)和溫度等因素的影響,探討了TCC在UV/PS工藝中可能的降解途徑,以期為實際應用中UV/PS降解水中TCC提供理論指導和實驗基礎.

1 實驗部分

1.1 試劑與儀器

三氯卡班(TCC)(德國 Dr.Ehrenstorfer公司,純度＞99.5%);過硫酸鈉(PS)(AR,≥98.0%);腐殖酸(HA)(Tech,美國 Sigma-Aldrich);異氰酸 4-氯苯酯(德國 Dr.Ehrenstorfer公司,99.5%);對氯苯胺(德國Dr.Ehrenstorfer公司,≥98.0%),HCl、NaOH均為分析純;甲醇、乙腈(HPLC級,德國 Merck); 30%過氧化氫(H2O2)(AR)實驗室用水為 Mili-Q超純水(≤18.2M?).

LC-20A高效液相色譜儀(Shimadzu,日本),自動進樣器(SIL-20A),檢測器(SPD-M20A);氣相色譜質譜聯用儀(GCMS-QP2010Ultra,日本島津);GC/MS自動進樣器(AOC-5000,日本島津),色譜柱(Rxi?-5ms: 30m×0.32mm×0.25μm,日本島津);離子色譜儀(戴安 ICS-1100);pH計(Eutevch,美國);HJ-6A型磁力恒溫攪拌器(江蘇金壇崢嶸儀器);HC-C18小柱(Anpel);紫外線光源(主波長254nm,楊紫特種紫外線光源,低壓汞燈,9W),紫外線強度計(TN-2365A,臺灣泰納).

1.2 實驗方法

實驗前開啟紫外燈預熱5min,然后置于燒杯中,保持紫外燈的位置相對燒杯固定,燒杯邊緣位置(圖中距離燒杯底部 8cm處的燒杯壁)的光強為11.5μW/cm2;投加一定量的PS溶液后打開紫外燈開始反應,分別在0、5、10、20、30、45、60min時取樣,隨即加入適量甲醇淬滅,經0.45μm的玻璃纖維濾膜過濾后分析.GC/MS產物鑒定前用固相萃取對樣品進行富集.反應裝置見圖1.

圖1 實驗裝置示意Fig.1 Schematic description of the reactor

1.3 分析方法

HPLC 條件:色譜柱為 Inertsil?ODS-SP(250mm×4.6mm,5μm);流動相為乙腈:水=65:35 (V:V),流動相流速為 1.0mL/min;檢測波長為265nm;進樣體積為10μL; S/N＞3.

GC/MS條件:載氣為高純度氦氣,90kPa;進樣量為1μL;無分流進樣方式;進樣口溫度為280℃;爐溫控制:初始溫度為60℃,保留時間安3min,然后以 5℃/min升溫至 150℃,持續 5min,然后以10℃/min升溫至280℃,持續3min;MS離子化溫度為 250℃;接口溫度為 280℃;采用 Scan掃描:質荷比 m/z起始為 50,終止為 600,掃描時間為0～40min.

2 結果與分析

2.1 UV、PS和UV/PS工藝對TCC的降解

UV、PS和UV/PS工藝對TCC的降解結果如圖2所示.

圖2 PS、UV和UV/PS對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

實驗表明,60min內單獨PS對TCC的去除率小于 14%;單獨 UV對 TCC的去除增加至61.52%;相同條件下,UV/PS聯用對TCC的去除可達99.95%,表明UV/PS聯用工藝具有協同作用,可以更有效地降解TCC.TCC降解曲線的擬一級動力學擬合結果表明,UV/PS工藝中擬一級動力學常數k比單獨UV光降解的增大了4.6倍.

PS分子中含有過氧基且在水中可電離產生S2O82-,其氧化還原電位為2.01V,具有一定的氧化能力[25],因此,單獨PS對TCC有一定的降解能力;單獨 UV照射下,對 TCC降解起主要作用的是·O2[26].UV/PS工藝中,PS在UV的照射下,其中的O-O鍵會因吸收能量而斷裂,產生含有孤電子對的·SO4-,·SO4-不僅具有超強氧化性,還可以與水(H2O)或者氫氧根(OH-)反應生成羥基自由基(·OH),增加溶液中自由基的濃度,使得TCC能夠高效降解,其反應過程如方程(1)-(3)所示:

2.2 PS投加量的影響

實驗中考察了PS投加量對UV/PS降解TCC的影響,不同PS投加量下TCC的降解曲線的擬一級動力學擬合結果見圖3.

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

由圖3可知,在實驗范圍內,TCC的降解符合擬一級動力學(R2≥0.91).PS投加量從 0增加到0.25mmol/L,擬一級動力學常數k由0.0151min-1增大為0.0810min-1,PS投加量為0.25mmol/L時,實驗中60min TCC的降解率可達99.44%;繼續增加至0.75mmol/L,擬一級動力學常數k反而減小為0.0344min-1.因此,UV/PS工藝中TCC的去除并不是隨著PS投加量的增加而越高.

由式(1)～(3)可知,PS濃度的增加可以提高溶液中自由基·SO4-和·OH的穩態濃度,從而加速TCC的降解,因此,在投加量0～0.25mmol/L范圍內TCC的去除隨著PS濃度的增加而增加;但有研究表明PS也會消耗·SO4-和·OH,生成氧化性較弱的·S2O8-[式(5)和(6)],影響 TCC的降解(PS與·SO4-和·OH 反應的速率常數分別是 5.5× 105M-1s-1[27]和 1.4×107M-1s-1[28]),故實驗中當 PS投加量較大(0.5mmol/L和 0.75mmol/L)時 TCC的降解效果反而下降.

2.3 初始pH的影響

pH是影響高級氧化(AOPs)處理效果的重要參數.實驗中考察了pH對TCC去除的影響.不同pH值時TCC的去除擬合結果見圖4.

圖4 pH對UV/PS降解TCC的影響Fig.4 Effect of initial pH on TCC degradation by UV/PS process

實驗中 pH值為 4.0、6.0、7.0、8.0和 9.5時,60min內 TCC的去除率分別為 76.50%、99.44%、91.20%、91.71%和 74.74%.pH=6.0時TCC的去除率最高.

由圖 4可知,實驗范圍內,隨著 pH的增大,k先增大后減小,在 pH=6.0時,k達到最大值0.0810min-1.這與Saien等在考察pH對UV/PS去除水楊酸的影響時的研究結果相一致[29-31].

過硫酸根離子非催化反應的活化能為33.5Kcal,而酸催化反應的活化能為26.0Kcal[32],酸性條件下S2O82-會與H+發生酸催化反應[33-34],因此過硫酸根在酸催化反應中更易轉化為硫酸根自由基,產生更多的·SO4-[式(7)和(8)],促進TCC的降解.但強酸條件下,更易發生式9和10的反應,反而消耗·SO4-產生氧化性更弱的自由基[35],導致TCC的降解速率降低.因此,相比于中性和強酸條件,偏酸性條件更有利于TCC的降解.

堿性條件下·SO4-會與 OH-反應生成·OH(式3),但堿性環境中·OH的氧化性較弱[36],因此盡管堿性條件下有·OH產生,但效果沒有偏酸性和中性條件下好.有研究表明,堿性環境中大量生成的SO42-[式(3)]對·SO4-和·OH 均有抑制作用[31,37].因此,在一定 pH范圍內,偏酸性條件(本實驗為pH=6.0)下TCC的降解效果更好.

2.4 腐殖酸的影響

腐殖酸(HA)是天然水體中主要的有機物,因此本實驗采用 HA來模擬天然水體中的有機物(NOM),考察了HA對TCC去除的影響,結果如圖5所示.

實驗中 HA 濃度為 0,0.5,1.0,3.0,5.0mg/L時,TCC的去除率分別為 99.45%、96.99%、96.36%、89.03%和63.78%;另外由圖4可知,TCC降解的動力學常數 k隨著 HA濃度的增加由0.0810min-1減小到 0.0158min-1.表明 HA 對UV/PS降解TCC有著的抑制作用.

基于UV的高級氧化體系中HA有兩方面的影響,一方面,UV激發下NOM可以產生·OH、·O等活性自由基,促進污染物的降解;另一方面NOM中的各種不飽和官能團對UV有一定的吸收能力,削弱光的透射能力,同時會與目標污染物競爭自由基[38].

實驗中HA一直起抑制作用,原因可能是HA屏蔽了UV光輻射,降低了UV的活化作用;HA分子中的酚羥基、胺基、羧基等活性基團與目標污染物TCC競爭·SO4-和·OH等自由基,導致自由基的穩態濃度降低[39].

圖5 腐植酸對UV/PS降解TCC的影響Fig.5 Effect of humic acid on TCC degradation by UV/PS process

2.5 UV/PS降解TCC的反應途徑分析

TCC的初始濃度為 900μg/L, PS投加量0.25mmol/L,反應60min后取出溶液固相萃取富集500倍后經GC/MS鑒定,發現在9.97min和11.9min有兩個明顯的出峰(圖 6),特征離子碎片的質荷比分別為 m/z=63,90,125,153和 m/z= 65,92,127.經譜庫檢索鑒定為異氰酸 4-氯苯酯(1-chloro-4-isocyanato-benzen)和對氯苯胺(4-chloroaniline).

據此推斷出TCC的降解產物可能有異氰酸4-氯苯酯和對氯苯胺.TCC在UV/PS系統中可能的光降解途徑如圖7所示.

圖6 總離子色譜Fig.6 Total ion chromatogram (TIC)

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

根據降解過程中中間產物的生成及Cl-濃度的增加,可以推測TCC的降解路徑:TCC分子結構中酮羰基左側的C—N鍵(與二氯苯胺環相連)斷裂,形成異氰酸4-氯苯酯和3,4-二氯苯胺,然后3,4-二氯苯胺脫掉一個Cl形成對氯苯胺,這條降解途徑與丁世玲[26]的研究結果相似.還有一種可能的途徑:酮羰基兩側的 C-N鍵(分別與二氯苯胺環和對氯苯胺環相連)斷裂,經過一系列的反應形成主要中間產物對氯苯胺.TCC的降解過程中兩種降解途徑可能同時存在,異氰酸 4-氯苯酯和對氯苯胺等中間產物繼續被降解,生成其他物質,最終苯環開環轉化為CO2、H2O等[40-43].

2.6 TCC降解過程中主要成分的變化

實驗考察了TCC的去除和降解產物的生成情況,結果見圖8.

TCC的快速降解發生在前30min,去除率達到 95.34%,之后反應速率逐漸減小,在 60min時TCC濃度已經低于檢出限.TCC降解產生異氰酸4-氯苯酯和對氯苯胺.在前15min異氰酸4-氯苯酯和對氯苯胺穩定增加至 142.45μg/L 和169.55μg/L,然后開始緩慢減少,在 60min時濃度降低至70.30μg/L和100.55μg/L.從TCC的降解路徑分析,對氯苯胺的產生量應該比異氰酸4-氯苯酯多,但其在反應過程中的濃度一直比后者低,這可能是因為TCC分子結構中的對氯苯胺環比二氯苯胺環降解更快[18]導致的.實驗中隨著反應的進行,TCC分子結構中的3個氯不斷脫離形成異氰酸 4-氯苯酯和對氯苯胺及大量 Cl-等,部分異氰酸4-氯苯酯和對氯苯胺降解也會有Cl-脫離,導致溶液中Cl-的濃度增大,在前15minCl-快速增加至242.73μg/L,之后Cl-的增加速率較之前有所下降,60min時 Cl-濃度達到 429.19μg/L,這表明UV/PS可以快速降解TCC并脫氯,進而可以降低溶液的毒性[44-45].

圖8 TCC降解過程中主要物質的濃度變化Fig.8 Concentration changes of main products during TCC degradation by UV/PS process

在降解過程中TCC與異氰酸4-氯苯酯和對氯苯胺的摩爾比并不是 1:1.可能是因為在異氰酸 4-氯苯酯和對氯苯胺形成的同時一部分已經被反應消耗;可能有其它反應途徑產生其它的中間產物,馮振濤等[46]的研究表明,有更為復雜的降解產物產生.

2.7 與UV/H2O2工藝比較

實驗中對比考察了UV/PS與典型的高級氧化工藝UV/H2O2去除TCC的效能,結果見圖9.

圖9 UV/PS、UV/H2O2對TCC處理效果比較Fig.9 Removal of TCC by UV/PS and UV/H2O2processes

H2O2與 PS的投加量均為 250mmol/L時, TCC的去除均遵循自由基反應的擬一級動力學,反應速率常數k分別是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 不同體系的單位電能消耗量和氧化劑成本Table 1 The electrical energy per order and oxidant costs in different systems

采用單位電能消耗量(EE/O)來評價兩種工藝的電能利用效率,其公式如下[47]:式中:EE/O:單位電能消耗量,kW·h/m3;P:紫外燈的功率,kW;V為時間t內處理溶液的體積,L;k為反應速率常數,min-1.

EE/O值越低,體系中的電能利用率越高,效率也越高[48].

將反應速率常數 k帶入公式得到兩個體系的單位電能消耗表1所示,可以看出,UV/PS具有更高的電能利用率.另外,氧化劑的成本方面, UV/PS也略低于UV/H2O2.

3 結論

3.1 UV活化PS工藝能有效去除水體中的TCC,降解過程符合擬一級反應動力學模型,UV輻射強度為 11.5μW·cm-2,PS投加量為 250μmol·L-1, pH=6.0時,60min的后,初始濃度為 400μg·L-1的TCC去除率可達99.44%.

3.2 TCC的去除隨PS投加量的增加和pH值的升高,先增強后減弱,偏酸性環境更有利于 TCC的降解,腐殖酸對 TCC的去除有抑制作用,且抑制作用與腐殖酸濃度呈線性關系.

3.3 UV/PS降解TCC的中間產物主要有異氰酸4-氯苯酯和對氯苯胺,可能是TCC分子結構中酮羰基兩側的C-N鍵斷裂產生的.

3.4 UV/PS聯用工藝比UV/H2O2工藝更加有優勢,其動力學常數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

國家自然科學基金項目(51378446,51678527,51408518);福建省科技計劃引導性項目(2017Y0079);福建省高校新世紀優秀人才支持計劃項目(JA14227);福建省自然科學基金項目(2016J01695);江蘇省企業研究生工作站合作項目;廈門市科技局項目(3502Z20131157,3502Z20150051)

* 責任作者, 副研究員, leetsingsong@sina.com

駱靖宇(1992-),男,江蘇南通人,蘇州科技大學碩士研究生,主要研究方向為水處理理論與技術.