氯酚類污染物在Fe3O4-nZVI類Fenton體系中的降解

鐘金魁,李聞青,謝亞瑞,李 靜,委嘉禾

氯酚類污染物在Fe3O4-nZVI類Fenton體系中的降解

鐘金魁1,2*,李聞青1,謝亞瑞1,李 靜1,委嘉禾1

(1.蘭州交通大學環境與市政工程學院,甘肅 蘭州,730070；2.甘肅省黃河水環境重點實驗室,甘肅 蘭州 730070)

用共沉淀法和液相還原法制得Fe3O4負載nZVI復合材料(Fe3O4-nZVI),以Fe3O4-nZVI為非均相催化劑與H2O2耦合構建類Fenton體系,將其用于降解廢水中的氯酚(CPs),如,對氯苯酚(4-CP)、2,4-二氯苯酚(2,4-DCP)及2,4,6-三氯苯酚(2,4,6-TCP).SEM、EDS、XPS、XRD和VSM表征結果表明,Fe3O4-nZVI是可通過磁回收循環利用的納米級復合材料,且可促進Fe2+和Fe3+循環并增加HO·的產量.動力學擬合參數表明,Fe3O4-nZVI類Fenton體系對3種CPs的降解過程均符合擬一級動力學方程(2>0.9).在25℃,Fe3O4和nZVI物質的量比1:1,溶液初始pH=3,H2O2濃度15mmol/L,CPs初始濃度40mg/L,Fe3O4-nZVI投加量0.6g/L的優化條件下,Fe3O4-nZVI類Fenton體系對4-CP、2,4-DCP和2,4,6-TCP的降解率分別為96.28%、98.77%和98.03%.3種CPs的降解速率順序為4-CP>2,4-DCP>2,4,6-TCP,表明隨著苯環上氯取代基的增加,CPs的降解速率隨之減小.淬滅實驗和質譜結果表明HO·氧化和取代脫氯是CPs降解的主要方式.

Fe3O4-nZVI；類Fenton；氯酚；降解機理

氯酚類化合物(CPs)廣泛用于工農業生產,包括對氯苯酚(4-CP)、2,4-二氯苯酚(2,4-DCP)和2,4,6-三氯苯酚(2,4,6-DCP)等[1-2].CPs來源廣、毒性大、水溶解度低,難生物降解,在環境中滯留期長,且能遠距離遷移,許多國家都將CPs列入了優先污染物黑名單[3-6].近年來,去除或降解CPs的工藝不斷發展,如吸附法、生物法、還原法和高級氧化法(AOPs)[7].附法對低濃度CPs廢水處理效果較好,但對高濃度CPs廢水的處理效率較低,鐘少芬等[8]利用活性炭可吸附80%水中氯酚,但吸附的污染物可能會產生脫附,造成二次污染[9-10].陳永祥等[11]考察生物反應柱去除率僅為30%,說明生物法也可將CPs降解,但微生物馴化周期長、選擇性高[12].此外,在化學還原法中零價鐵單獨還原的去除率僅為40%[13].相比而言,AOPs降解CPs效果最好,是因為AOPs產生的自由基可將CPs氧化分解,最終礦化為CO2和H2O等小分子物質[14].

根據產生的自由基可分為以SO4·-或HO·為主導的AOPs.在特定條件下,SO4·-可被能量激發或被過渡金屬催化過一硫酸鹽或過硫酸鹽來產生,但在實際應用中可能產生有毒有害物質,而基于HO·的AOPs發展更早更成熟[15-16],其包括類Fenton法[17-18].決定類Fenton法的關鍵因素是如何獲取高活性和可持續性催化材料[19].實踐中,納米零價鐵(nZVI)已廣泛應用于CPs的類Fenton催化降解[20-21].nZVI因存在納米效應、表面效應、量子效應和宏觀量子隧道效應,而被廣泛用于去除水中的CPs[22].但是,由于nZVI自身的磁性引力、范德華引力,比表面過剩Gibbs自由能比較高,極易發生團聚結塊,且在降解CPs的過程中會因氧化作用生成鐵的氫氧化物和氧化物,覆蓋在nZVI表面形成鈍化層,從而阻礙反應的持續進行[23].為了提高nZVI的穩定性和分散性,防止和減緩其團聚和氧化,因此對nZVI進行改性是非常必要的[24].

固體負載是nZVI改性的主要方法[25-26].黃超等[27]發現有機膨潤土改性nZVI類Fenton體系對2,4-DCP的表觀降解率在5min就達99%;Li等[28]將nZVI顆粒被介孔碳包裹并嵌入碳球體中,選擇性誘導HO·有效降解了4-CP.由此可見,負載型nZVI類Fenton體系可有效降解CPs.除了用以上材料改性nZVI,謝欣卓等[29]用Fe3O4改性nZVI材料證明其具有很好的反應活性和穩定性.王正新[30]用Fe3O4改性nZVI材料成功降解了水中2,4-DCP,為類Fenton反應體系提供更多地Fe2+.以上研究可知Fe3O4可有效改善nZVI易團聚易氧化的缺點.但是,對于復合催化材料促進Fe0、Fe2+和Fe3+的循環和增加HO·的產量中發揮的重要作用沒有予以充分闡述,且缺乏由污染物構效關系出發來揭示Fenton反應對多種典型氯酚的降解機理和途徑.

基于上述分析,研究利用共沉淀法和液相還原法制得Fe3O4負載nZVI磁性復合材料(Fe3O4-nZVI),并對其進行了SEM、EDS、XPS、XRD和VSM表征.然后以Fe3O4-nZVI為非均相催化劑與H2O2耦合構建Fe3O4-nZVI類Fenton體系,降解水中典型CPs,如,4-CP、2,4-DCP和2,4,6-TCP,考察H2O2濃度、Fe3O4-nZVI投加量、初始pH值和可回收利用性等因素對CPs降解效率的影響.通過自由基淬滅實驗和質譜分析探究了Fe3O4-nZVI類Fenton體系對CPs的降解機理與途徑,并探明降解效率與CPs的構效關系,為CPs廢水的處理提供參考和依據.

1 材料與方法

1.1 實驗材料

對氯苯酚(4-CP)、2,4-二氯苯酚(2,4-DCP)、2,4,6-三氯苯酚(2,4,6-DCP)、FeCl3?6H2O、FeSO4?7H2O(上海阿拉丁生化公司);硼氫化鈉(NaBH4)、HCl、NaOH(中國醫藥集團);30%H2O2、無水乙醇(天津大茂試劑公司);氨水(煙臺市雙雙化工公司),以上均為分析純.甲醇(山東禹王集團)為色譜純;高純氮氣(咸陽偉麗氣體制造公司).

1.2 實驗方法

1.2.1 Fe3O4-nZVI的制備 在N2氛圍下分兩步制得Fe3O4-nZVI復合材料.首先采用共沉淀法制備Fe3O4.準確稱取物質的量比2:1的FeCl3·6H2O和FeSO4?7H2O,加適量去離子水溶解后轉入三口燒瓶,并開啟攪拌機攪拌,同時用恒壓漏斗將過量氨水緩慢勻速滴入三口燒瓶中,鐵鹽發生共沉淀反應生成黑色顆粒,待反應完全后用磁鐵分離出固體物質,并用去離子水反復清洗3～5遍,即得Fe3O4納米顆粒,將制備好的Fe3O4干燥備用[31],反應如式(1)所示:

然后通過液相還原法制備Fe3O4-nZVI.首先將FeSO4?7H2O置于三口燒瓶中,加入去離子水攪拌至完全溶解,然后將上述制備好的Fe3O4納米顆粒加入并進行超聲攪拌,使溶液和顆粒物分散均勻,并用恒壓漏斗勻速滴加過量的NaBH4溶液,待NaBH4添加完備后高速攪拌反應30min,得Fe3O4-nZVI復合材料[32],反應如式(2)所示:

1.2.2 CPs降解實驗 采用批平衡實驗法,通過單因素變化優化降解參數.在N2保護下,將Fe3O4- nZVI、H2O2和CPs按一定濃度比例置于50mL具塞三角瓶中,用0.1mol/L HCl和0.1mol/L NaOH調節體系pH值,然后置于恒溫振蕩箱以180r/min振蕩,在反應5,10,30,60,120,180min時加入淬滅劑甲醇終止反應取樣,過0.22m膜,用超高效液相色譜儀(UPLC)測定濾液中CPs含量.考察影響因素有pH值(2,3,5,7,9)、H2O2濃度(5,10,15,20,30mmol/L)、Fe3O4-nZVI投加量(0.2,0.4,0.6,0.8,1.0g/L).

1.2.3 回收利用實驗 在25℃,Fe3O4和nZVI物質的量比1:1,溶液初始pH=3,H2O2濃度15mmol/L,CPs初始濃度40mg/L,Fe3O4-nZVI投加量0.6g/L的條件下降解CPs.第1次降解實驗結束后,利用強磁鐵將Fe3O4-nZVI進行分離、干燥、洗滌,然后將其投加到第2組CPs溶液中,實驗控制條件與第1次完全相同,以此類推重復利用Fe3O4-nZVI共10次.

1.3 分析計算方法

CPs濃度用超高效液相色譜儀(UPLC, Water Acquity UPLC I-Class, USA)測定,色譜柱為C18反相色譜柱(1.7μm, 2.1mm×100mm, Waters),具體測定參數:進樣量10μL,流速0.1mL/min,柱溫35℃.4-CP和2,4-DCP流動相甲醇和水的體積比為70:30, 2,4,6-TCP流動相甲醇和水的體積比為80:20;4-CP、2,4-DCP和2,4,6-TCP檢測波長分別為285、285和295nm.CPs降解10min和180min后的產物用高效液相色譜質譜聯用儀(LC-MS, Agilent 1290+6460C, USA)測定,測定離子源為負離子ESI,色譜柱為C18反相色譜柱(1.8m, 2.1mm×100mm, Waters),進樣量為10μL,流動相分別為甲醇和水.

CPs的表觀降解率()根據式(3)計算,降解動力學數據用擬一級動力學方程式(4)擬合.

(3)

式中:0和C分別為CPs的初始濃度和反應時刻的濃度,mg/L;為反應時間,min;為擬一級動力學反應速率常數,min-1.

2 結果與討論

2.1 Fe3O4-nZVI的表征

由圖1(a)可知,粒徑在50～200nm之間的Fe3O4- nZVI顆粒呈鏈狀均勻分布,這可能是磁偶極子的相互作用和納米效應所致.Tan等[33]利用EDS對復合材料Fe0@Fe3O4進行描述,芯體顆粒(Fe3O4)被小的球形顆粒(nZVI)所包圍,形成以nZVI為殼,以Fe3O4為核的結構.圖1(b)為Fe3O4-nZVI的X射線能譜圖(EDS),由圖看出,Fe3O4-nZVI復合材料中含有Fe、O、C、B和S等元素,且圖中出現了3個Fe元素的特征峰,是由于Fe原子核外層的電子向其他電子層躍遷釋放的能量不同所致[34].

圖1(c)、(d)是Fe3O4-nZVI的X射線光電子能譜圖(XPS).圖1(c)表明Fe3O4-nZVI表面元素有Fe,O和C,且Fe2p,O1s和C1s的結合能分別為710.31、530.57和284.8eV.圖1(d)表明,Fe2p1/2和Fe2p3/2的結合能分別為723.98和710.98eV,證實了Fe2+和Fe3+的存在,表明材料中存在Fe3O4[35-36].結合能為706.88eV的峰并不明顯,說明nZVI可能在制備、清洗和干燥過程中與O2發生反應后,其表面覆蓋了一層鐵氧化物所致.

圖1(e)是nZVI、Fe3O4和Fe3O4-nZVI的X射線衍射圖(XRD).由圖可知,Fe3O4-nZVI在2為30.34°、35.46°、43.09°、57.06°和62.62°處出現明顯特征峰,分別對應于Fe3O4標準卡片JCPDS PDF#06-0696的(220)、(311)、(400)、(511)和(440)的晶面,這進一步證明復合材料中存在Fe3O4,與XPS表征結果一致.Fe3O4-nZVI衍射峰與標準Fe3O4的峰位置基本吻合,表明材料的負載未改變Fe3O4的晶型結構.與nZVI標準卡片對比發現,在2為44.71°出現特征寬峰,表明Fe3O4-nZVI復合材料中的nZVI以無定形態存在[37].

圖1(f)是nZVI、Fe3O4和Fe3O4-nZVI的振動樣品磁強計分析圖(VSM).當磁場強度為0Oe時,三種材料的磁化強度均為0emu/g,均未出現磁滯現象.nZVI、Fe3O4和Fe3O4-nZVI的飽和磁化強度值(Ms)分別為173.14、81.45和105.52emu/g,表明在外磁場作用下,Fe3O4-nZVI復合材料具有良好的磁分離性能,為材料的回收和再利用提供了可能.

2.2 不同反應體系對2,4-DCP去除的影響

以2,4-DCP為目標污染物,在25℃,pH=3,初始濃度為40mg/L,反應時間為180min條件下,不同材料體系(H2O2、Fe3O4、nZVI、Fe3O4-nZVI、nZVI- H2O2、Fe3O4-H2O2和Fe3O4-nZVI-H2O2)對2,4-DCP表觀去除率如圖2所示.

由圖看出,H2O2、Fe3O4和nZVI 3種單一體系對2,4-DCP均具有一定的去除效果,但總表觀去除率均小于70%.這是因為在H2O2體系中,H2O2對2,4-DCP的氧化降解效果并不高.在Fe3O4和nZVI單一體系中,固體納米材料由于較高的比表面積和表面自由能可吸附去除水中的2,4-DCP.吸附在nZVI表面的2,4-DCP在nZVI強還原作用下,發生還原脫氯,但nZVI易團聚、易氧化的缺點又反過來阻礙了其對2,4-DCP的去除效果[38],因此2,4-DCP的表觀去除率仍然達不到理想效果.

圖1 Fe3O4-nZVI的(a)SEM;(b)EDS;(c)XPS;(d)XPS Fe2p;(e)XRD和(f)VSM圖

Fig 1 (a)SEM; (b)EDS; (c)XPS; (d)XPS Fe2p; (e)XRD and (f)VSM spectrum of Fe3O4-nZVI

圖2 不同反應體系對2,4-DCP降解的影響

(a)H2O2; (b)Fe3O4; (c)Fe3O4+H2O2; (d)nZVI; (e)Fe3O4-nZVI; (f)nZVI+H2O2; (g)Fe3O4-nZVI +H2O2

與單一材料相比,Fe3O4-nZVI、nZVI-H2O2和Fe3O4-H2O23種復合體系對2,4-DCP的表觀去除率都有較大提高.這是由于在Fe3O4-nZVI復合體系下,2,4-DCP被大量迅速地吸附于Fe3O4-nZVI表面,提高了nZVI與2,4-DCP之間的傳質能力,加速了2,4-DCP的去除效率;在nZVI-H2O2復合體系下,nZVI對2,4-DCP進行脫氯去除,同時nZVI被H2O2氧化為Fe2+構成Fenton體系,產生了HO·對2,4-DCP進行氧化降解,因此表觀去除率提高,但由于部分nZVI發生團聚和氧化,使得部分nZVI失效,削弱了其對污染物的去除能力;在Fe3O4-H2O2復合體系下,同時存在2,4-DCP的吸附及HO·的氧化降解,反應初期,H2O2與Fe2+反應產生HO·,但HO·的壽命極短,因此2,4-DCP表觀去除率降低[39].

與以上體系相比,Fe3O4-nZVI-H2O2三元復合體系對2,4-DCP的表觀去除率最高,達到了98.98%.說明Fe3O4-nZVI可以有效改善nZVI易團聚、易氧化現象,使2,4-DCP更有效地吸附在Fe3O4-nZVI復合材料表面,Fe3O4中的Fe2+和nZVI與H2O2反應產生大量HO·對2,4-DCP進行更徹底的降解.因此,后續實驗選取Fe3O4-nZVI-H2O2組成的類Fenton體系對CPs進行降解研究.

2.3 H2O2濃度對CPs降解的影響

在25℃,Fe3O4與nZVI物質的量比1:1,pH=3, CPs初始濃度40mg/L,Fe3O4-nZVI投加量0.6g/L的條件下,H2O2濃度對CPs降解的影響如圖3所示.由圖看出,CPs的表觀降解率隨著反應時間的增加而逐漸增大,約在120min趨于平緩.在實驗條件控制范圍內, CPs的表觀降解率隨H2O2濃度的增加而增大.在反應180min,H2O2濃度為15mmol/L時,2,4,6- TCP、2,4-DCP和4-CP的表觀降解率分別為94.21%、97.35%和97.73%.當H2O2濃度超過15mmol/L,CPs的表觀降解率不增反降.這是因為隨著H2O2濃度的增加,Fe3O4-nZVI催化H2O2產生大量強氧化性HO·,CPs被逐漸氧化降解,當H2O2濃度超過一定范圍后,溶液中的HO·與H2O2發生反應產生過氧羥基(HO2·),反應見式(5),產生的HO2·再進一步與HO·反應生成H2O和O2,自由基反應終止,反應見式(6),以上反應使體系中的HO·產生耗損,降低了HO·與CPs的反應活性,導致Fe3O4-nZVI類Fenton體系對CPs的表觀降解率下降.因此,H2O2濃度過高或者過低都不利于CPs的表觀降解率,為達到最佳降解效果,后續實驗選擇H2O2濃度為15mmol/L.

(6)

2.4 溶液pH值對CPs降解的影響

在25℃,Fe3O4和nZVI物質的量比1:1,H2O2濃度15mmol/L,CPs初始濃度40mg/L,Fe3O4-nZVI投加量0.6g/L的條件下,溶液初始pH對CPs的降解影響如圖4所示.由圖看出,pH值對CPs降解影響非常明顯.在pH=3時,Fe3O4-nZVI類Fenton體系對2,4,6-TCP、2,4-DCP和4-CP的表觀降解率達到最大,分別為98.03%、98.39%和98.44%.隨著體系pH值增大,CPs的表觀降解率均快速下降.在pH=7的中性條件下, CPs降解率已下降到65%左右,繼續增大pH到9時, 2,4,6-TCP、2,4-DCP和4-CP的降解率分別下降到55.60%、54.54%和52.68%,因為酸性條件有利于維持Fe2+和HO·生成,CPs降解效果就更好,同時加速nZVI的析氫腐蝕促進CPs脫氯[40].當增大pH,體系中的H+減少,此時Fe2+易沉淀且加速nZVI表面鈍化,阻止nZVI進一步釋放Fe2+.然而,在較低的酸度如pH=2的條件下,會發生nZVI的過度腐蝕和Fe3O4溶解,這導致Fe3O4-nZVI的損失并抑制了CPs脫氯[41].因此后續實驗選擇最佳pH=3.

2.5 Fe3O4-nZVI投加量對CPs降解的影響

在25℃,Fe3O4和nZVI物質的量比1:1,pH=3, H2O2濃度15mmol/L,CPs初始濃度40mg/L的條件下,Fe3O4-nZVI的投加量對CPs的表觀降解率的影響如圖5所示.由圖看出,Fe3O4-nZVI類Fenton體系對3種CPs的表觀降解率隨反應時間的延長先增大,然后在120min逐漸趨于平緩.當投加量為0.2g/L時,2,4,6-TCP、2,4-DCP和4-CP的表觀降解率分別為85.1%、84.74%和85.06%;當投加量增加到0.6g/L時,2,4,6-TCP、2,4-DCP和4-CP的表觀降解率達到最大,分別為94.36%、96.39%和98.05%.但繼續增大投加量至0.8g/L時,CPs的表觀降解率均出現下降;當增加到1g/L時,2,4,6-TCP、2,4-DCP和4-CP的表觀降解率分別下降至83.16%、83.06%和86.97%.

圖4 溶液pH對(a) 2,4,6-TCP;(b)2,4-DCP;(c) 4-CP降解的影響

這是因為投加量較小時,Fe3O4-nZVI能夠參與吸附CPs的活性位點較少,少量的Fe3O4-nZVI與H2O2反應無法產生大量的HO·,因此CPs的表觀降解率較小.當增大投加量時,體系中的反應活性位點得以補充,Fe3O4-nZVI能夠與H2O2充分反應產生大量的HO·降解CPs.但當投加量超過0.6g/L,繼續增大投加量,過量的Fe3O4-nZVI溶出的Fe2+和Fe3+與HO·和HO2·反應(見式(7)～(9)),這對HO·產生了耗損作用,同時該體系也促進了Fe0、Fe2+和Fe3+的高效循環[42].為使Fe3O4-nZVI和H2O2均得到充分利用,實驗選擇Fe3O4-nZVI的優化投加量為0.6g/L.

圖5 Fe3O4-nZVI投加量對(a) 2,4,6-TCP;(b)2,4-DCP;(c) 4-CP降解的影響

2.6 Fe3O4-nZVI的重復利用性能

由圖6可知,Fe3O4-nZVI在重復利用的過程中,隨著重復利用次數增加,Fe3O4-nZVI類Fenton體系對2,4,6-TCP、2,4-DCP和4-CP的表觀降解率均表現出緩慢下降趨勢(第一次降解后的降解率分別為96.28%、98.77%和98.03%).原因是在Fe3O4-nZVI材料重復利用的過程中,Fe3O4-nZVI表面的活性吸附位點逐步減少,其表面催化活性隨之下降.另外,經過分離、洗滌后的Fe3O4-nZVI表面可能依然存在污染物及其降解產物,Fe3O4-nZVI在降解污染物的過程中會析出Fe2+, Fe2+的消耗使得Fe3O4-nZVI在重復利用過程中的活性也會降低[43-44].但是重復利用次數達到5次時,CPs的表觀降解率依然可達到85%以上,甚至重復利用10次后,CPs表觀降解率可維持在66%以上.說明Fe3O4-nZVI材料是一種重復利用性能較高的磁性復合材料,可以降低污染物修復的成本,這一結果與VSM的表征結果是一致的.

圖6 Fe3O4-nZVI重復利用次數對CPs降解的影響

2.7 降解動力學

Fe3O4-nZVI類Fenton體系對降解CPs的最佳H2O2濃度、Fe3O4-nZVI投加量及pH值等因素的擬一級動力學參數如表1所示,相關系數2均大于0.9,說明Fe3O4-nZVI類Fenton體系降解CPs的過程符合擬一級動力學方程.由表1可知,在25℃,Fe3O4和nZVI物質的量比1:1,pH=3,H2O2濃度15mmol/L,CPs初始濃度40mg/L條件下,4-CP、2,4-DCP和2,4,6-TCP的擬一級動力學速率常數的平均值分別為0.1000±0.0040、0.0885±0.0067和0.0807±0.0124,即的順序是4-CP>2,4-DCP>2,4,6-TCP,說明苯環上氯原子數目影響Fe3O4-nZVI類Fenton體系對CPs的降解速率,即CPs上面的氯代原子數目越多,就越小,CPs的降解率就越小.這可能是由于空間位阻效應、-電子誘導效應和π-電子共軛效應共同作用的結果[45].因此,鄰位和對位的取代氯原子越多,氧化抑制作用越強,且苯環上的氯代原子與苯環形成p-π共軛體系,可以鈍化苯環的鄰對位,此時氯取代基的鄰位和對位處的電子云密度較高,進攻基團HO·優先進入其鄰位和對位[46].因此Fe3O4-nZVI類Fenton體系對CPs的降解速率不僅與苯環上取代基的數量有關而且與取代基的位置也有一定關系.

表1 Fe3O4-nZVI類Fenton體系降解CPs的擬一級動力學擬合參數

2.8 自由基淬滅

以2,4-DCP為代表,在25℃,Fe3O4和nZVI物質的量比1:1, pH=3,H2O2濃度15mmol/L,CPs初始濃度40mg/L條件下,實驗組加入自由基淬滅劑甲醇(CH3OH),空白組不加淬滅劑,實驗結果如圖7所示.由圖看出,空白組2,4-DCP的表觀降解率在180min內呈快速上升趨勢,且在180min達到97.66%,此時實驗組的表觀降解率為52.83%.與空白組相比,加入自由基淬滅劑后,2,4-DCP的降解率出現大幅度降低,說明HO·是Fe3O4-nZVI類Fenton體系降解CPs的主導活性物質.由于Fe3O4和nZVI都具有較大比表面積,可以有效地將污染物吸附在Fe3O4-nZVI表面,進而增大了HO·與其接觸的路徑,增大了降解速率.另外,nZVI的強還原性可以對2,4-DCP進行快速吸附,然后逐步脫氯后最終脫附釋放[47].

圖7 自由基淬滅

2.9 降解產物及途徑

根據LC-MS實驗數據,結合自由基淬滅實驗推測Fe3O4-nZVI類Fenton法對CPs的降解產物及途徑.

如圖8所示,Fe3O4-nZVI類Fenton法降解2,4,6-TCP的途徑分為2種方式.路徑(1)分3個階段:第Ⅰ階段為羥基化脫氯,2,4,6-TCP通過HO·取代氯自由基,對位取代基羥基化脫氯轉化為二氯對苯二酚,二氯乙酸和丙酸等小分子酸;第Ⅲ階段為礦化階段,小分子酸進一步被礦化為CO2和H2O.路徑(2)也分三個階段,第Ⅰ階段是脫氯階段,2,4,6-TCP脫氯生成二氯苯酚,二氯苯酚進一步脫氯生成一氯苯酚,直至脫氯生成苯酚;苯酚會被HO·氧化為苯二酚,苯二酚被氧化為苯二醌;第Ⅱ階段為開環階段,HO·攻擊苯環,苯二醌開環轉化為己二酸、乙二酸和順丁烯二酸等小分子酸;第Ⅲ階段為小分子酸被徹底礦化為CO2和H2O[48].

如圖9所示Fe3O4-nZVI類Fenton法降解2,4-DCP的途徑分為2種方式.路徑(1)第Ⅰ階段為羥基化階段,HO·羥基化2,4-DCP的鄰位和間位,生成4,6-二氯鄰苯二酚和4,6-二氯間苯二酚,也可能HO·通過取代對位的氯自由基生成2-氯對苯二酚,繼續羥基化可得到苯四酚和2,6-二羥基對苯醌等物質;第Ⅱ階段為開環階段,HO·繼續攻擊苯環使其開環生成丙烯二酸、乙酸和丙酸等小分子酸后;進入第Ⅲ階段.路徑(2)為脫氯方式,第Ⅰ階段是2,4-DCP脫氯可以直接生成鄰氯苯酚(2-CP)、對氯苯酚(4-CP)和苯酚,2-CP和4-CP也可進一步脫氯生成苯酚,HO·進一步氧化苯酚生成對苯二酚、鄰苯二酚和間苯二酚,酚類物質進一步轉化為對苯二醌和鄰苯二醌;第Ⅱ階段為開環階段,HO·繼續對苯環進行攻擊,苯二醌和苯二酚開環轉化為順丁烯二酸和乙二酸等小分子酸;進入第Ⅲ階段[49]Fe3O4-nZVI類Fenton法降解4-CP的途徑與2,4,6-TCP和2,4,-DCP相似,其降解途徑在圖9的路徑(2)中已有敘述. 綜上CPs的降解途徑主要通過HO·氧化和取代脫氯兩種方式,其中HO·氧化占主導地位[50-52].

圖8 Fe3O4-nZVI類Fenton體系對2,4,6-TCP降解途徑示意

圖9 Fe3O4-nZVI類Fenton體系對2,4-DCP降解途徑示意

3 結論

3.1 Fe3O4-nZVI球形磁性復合材料可促進Fe2+和Fe3+的循環,增加HO·的產量.

3.2 在25℃,CPs初始濃度40mg/L,Fe3O4和nZVI物質的量比1:1,溶液初始pH=3,H2O2濃度15mmol/L, Fe3O4-nZVI投加量0.6g/L的優化條件下,Fe3O4- nZVI類Fenton體系對4-CP、2,4-DCP和2,4,6-DCP的最大降解率分別為98.05%、98.39%和98.03%.

3.3 隨著苯環上氯原子數目的增加,相應的Fe3O4- nZVI類Fenton體系對CPs的降解速率常數值隨之減小,即4-CP>2,4-DCP>2,4,6-TCP.

3.4 Fe3O4-nZVI類Fenton體系降解CPs主要以HO·氧化和取代脫氯為主,降解途徑為多氯酚轉化為單氯酚,再進一步轉化為酚類化合物,酚類化合物氧化為醌,最后開環轉變為小分子酸,直至形成CO2和H2O.

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Degradation of chlorophenolic pollutants in Fe3O4-nZVI Fenton-like system.

ZHONG Jin-kui1,2*, LI Wen-qing1, XIE Ya-rui1, LI Jing1, WEI Jia-he1

(1.School of Environmental and Municipal Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China；2.Key Laboratory of Yellow River Water Environment of Gansu Province, Lanzhou 730070, China)., 2023,43(11)：5746～5756

The ferriferrous oxide supported nano zero-valent iron (Fe3O4-nZVI) was prepared by co-precipitation and liquid phase reduction method, and used as heterogeneous catalysts with H2O2of Fenton-like systems to degrade chlorophenols (CPs) in wastewater, such as-chlorophenol (4-CP), 2,4-dichlorophenol (2,4-DCP) and 2,4,6-trichlorophenol (2,4,6-TCP). The characterisation results of SEM, EDS, XPS, XRD and VSM showed that Fe3O4-nZVI was a nanoscale magnetic composite that could be recycled by magnetic recovery and could promote the circulation of Fe2+and Fe3+and increase the yield of HO·. The fitting parameters of kinetics showed that the degradation process of three kinds of CPs in Fe3O4-nZVI Fenton-like system fitted the pseudo-first-order kinetics model (2>0.9). Under the optimization conditions, which were reaction temperature 25℃, Fe3O4: nZVI=1:1 (mol:mol), pH=3, H2O215mmol/L, CPs 40mg/L, and Fe3O4-nZVI 0.6g/L, the degradation rates of 4-CP, 2,4-DCP and 2,4,6-TCP by Fe3O4-nZVI Fenton-like systems were 96.28%, 98.77% and 98.03%, respectively. The degradation rates of three kinds of CPs were in the order of4-CP>2,4-DCP>2,4,6-TCP. The degradation rates of CPs decreased with the increase of chlorine atoms on the benzene ring. The quenching experiments and mass spectrometry results indicated that HO· oxidation and substitution dechlorination played a critical role on degrading CPs.

Fe3O4-nZVI；Fenton-like；chlorophenols；degradation mechanism

X703.1

A

1000-6923(2023)11-5746-11

鐘金魁(1968-),男,甘肅古浪人,教授,主要從事水處理技術和土壤污染修復.發表論文44篇.zhongjk@mail.lzjtu.cn.

鐘金魁,李聞青,謝亞瑞,等.氯酚類污染物在Fe3O4-nZVI類Fenton體系中的降解 [J]. 中國環境科學, 2023,43(11):5746-5756.

Zhong J K, Li W Q, Xie Y R, et al. Degradation of chlorophenolic pollutants in Fe3O4-nZVI Fenton-like system [J]. China Environmental Science, 2023,43(11): 5746-5756.

2023-03-15

國家自然科學基金資助項目(22166022);甘肅省科技計劃項目(20JR2RA002);蘭州交通大學創新創業項目(CXXL20230087)

* 責任作者, 教授, zhongjk@mail.lzjtu.cn