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(xué)環(huán)境與市政工程學(xué)院,甘肅 蘭州,730070；2.甘肅省黃河水環(huán)境重點(diǎn)實(shí)驗(yàn)室,甘肅 蘭州 730070)

用共沉淀法和液相還原法制得Fe3O4負(fù)載nZVI復(fù)合材料(Fe3O4-nZVI),以Fe3O4-nZVI為非均相催化劑與H2O2耦合構(gòu)建類Fenton體系,將其用于降解廢水中的氯酚(CPs),如,對(duì)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(jié)果表明,Fe3O4-nZVI是可通過磁回收循環(huán)利用的納米級(jí)復(fù)合材料,且可促進(jìn)Fe2+和Fe3+循環(huán)并增加HO·的產(chǎn)量.動(dòng)力學(xué)擬合參數(shù)表明,Fe3O4-nZVI類Fenton體系對(duì)3種CPs的降解過程均符合擬一級(jí)動(dòng)力學(xué)方程(2>0.9).在25℃,Fe3O4和nZVI物質(zhì)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(yōu)化條件下,Fe3O4-nZVI類Fenton體系對(duì)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?yàn)?-CP>2,4-DCP>2,4,6-TCP,表明隨著苯環(huán)上氯取代基的增加,CPs的降解速率隨之減小.淬滅實(shí)驗(yàn)和質(zhì)譜結(jié)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(jī)理

氯酚類化合物(CPs)廣泛用于工農(nóng)業(yè)生產(chǎn),包括對(duì)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(huán)境中滯留期長,且能遠(yuǎn)距離遷移,許多國家都將CPs列入了優(yōu)先污染物黑名單[3-6].近年來,去除或降解CPs的工藝不斷發(fā)展,如吸附法、生物法、還原法和高級(jí)氧化法(AOPs)[7].附法對(duì)低濃度CPs廢水處理效果較好,但對(duì)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(huì)產(chǎn)生脫附,造成二次污染[9-10].陳永祥等[11]考察生物反應(yīng)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(xué)還原法中零價(jià)鐵單獨(dú)還原的去除率僅為40%[13].相比而言,AOPs降解CPs效果最好,是因?yàn)锳OPs產(chǎn)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(zhì)[14].

根據(jù)產(chǎn)生的自由基可分為以SO4·-或HO·為主導(dǎo)的AOPs.在特定條件下,SO4·-可被能量激發(fā)或被過渡金屬催化過一硫酸鹽或過硫酸鹽來產(chǎn)生,但在實(shí)際應(yīng)用中可能產(chǎn)生有毒有害物質(zhì),而基于HO·的AOPs發(fā)展更早更成熟[15-16],其包括類Fenton法[17-18].決定類Fenton法的關(guān)鍵因素是如何獲取高活性和可持續(xù)性催化材料[19].實(shí)踐中,納米零價(jià)鐵(nZVI)已廣泛應(yīng)用于CPs的類Fenton催化降解[20-21].nZVI因存在納米效應(yīng)、表面效應(yīng)、量子效應(yīng)和宏觀量子隧道效應(yīng),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(fā)生團(tuán)聚結(jié)塊,且在降解CPs的過程中會(huì)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(yīng)的持續(xù)進(jìn)行[23].為了提高nZVI的穩(wěn)定性和分散性,防止和減緩其團(tuán)聚和氧化,因此對(duì)nZVI進(jìn)行改性是非常必要的[24].

固體負(fù)載是nZVI改性的主要方法[25-26].黃超等[27]發(fā)現(xiàn)有機(jī)膨潤土改性nZVI類Fenton體系對(duì)2,4-DCP的表觀降解率在5min就達(dá)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(dǎo)HO·有效降解了4-CP.由此可見,負(fù)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(yīng)活性和穩(wěn)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(yīng)體系提供更多地Fe2+.以上研究可知Fe3O4可有效改善nZVI易團(tuán)聚易氧化的缺點(diǎn).但是,對(duì)于復(fù)合催化材料促進(jìn)Fe0、Fe2+和Fe3+的循環(huán)和增加HO·的產(chǎn)量中發(fā)揮的重要作用沒有予以充分闡述,且缺乏由污染物構(gòu)效關(guān)系出發(fā)來揭示Fenton反應(yīng)對(duì)多種典型氯酚的降解機(jī)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(fù)載nZVI磁性復(fù)合材料(Fe3O4-nZVI),并對(duì)其進(jìn)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(gòu)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(duì)CPs降解效率的影響.通過自由基淬滅實(shí)驗(yàn)和質(zhì)譜分析探究了Fe3O4-nZVI類Fenton體系對(duì)CPs的降解機(jī)理與途徑,并探明降解效率與CPs的構(gòu)效關(guān)系,為CPs廢水的處理提供參考和依據(jù).

1 材料與方法

1.1 實(shí)驗(yàn)材料

對(duì)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(yī)藥集團(tuán));30%H2O2、無水乙醇(天津大茂試劑公司);氨水(煙臺(tái)市雙雙化工公司),以上均為分析純.甲醇(山東禹王集團(tuán))為色譜純;高純氮?dú)?咸陽偉麗氣體制造公司).

1.2 實(shí)驗(yàn)方法

1.2.1 Fe3O4-nZVI的制備 在N2氛圍下分兩步制得Fe3O4-nZVI復(fù)合材料.首先采用共沉淀法制備Fe3O4.準(zhǔn)確稱取物質(zhì)的量比2:1的FeCl3·6H2O和FeSO4?7H2O,加適量去離子水溶解后轉(zhuǎn)入三口燒瓶,并開啟攪拌機(jī)攪拌,同時(shí)用恒壓漏斗將過量氨水緩慢勻速滴入三口燒瓶中,鐵鹽發(fā)生共沉淀反應(yīng)生成黑色顆粒,待反應(yīng)完全后用磁鐵分離出固體物質(zhì),并用去離子水反復(fù)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(yīng)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?dāng)嚢柚镣耆芙?然后將上述制備好的Fe3O4納米顆粒加入并進(jìn)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(yīng)30min,得Fe3O4-nZVI復(fù)合材料[32],反應(yīng)如式(2)所示:

1.2.2 CPs降解實(shí)驗(yàn) 采用批平衡實(shí)驗(yàn)法,通過單因素變化優(yōu)化降解參數(shù).在N2保護(hù)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(diào)節(jié)體系pH值,然后置于恒溫振蕩箱以180r/min振蕩,在反應(yīng)5,10,30,60,120,180min時(shí)加入淬滅劑甲醇終止反應(yīng)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(shí)驗(yàn) 在25℃,Fe3O4和nZVI物質(zhì)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(shí)驗(yàn)結(jié)束后,利用強(qiáng)磁鐵將Fe3O4-nZVI進(jìn)行分離、干燥、洗滌,然后將其投加到第2組CPs溶液中,實(shí)驗(yàn)控制條件與第1次完全相同,以此類推重復(fù)利用Fe3O4-nZVI共10次.

1.3 分析計(jì)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(shù):進(jìn)樣量10μL,流速0.1mL/min,柱溫35℃.4-CP和2,4-DCP流動(dòng)相甲醇和水的體積比為70:30, 2,4,6-TCP流動(dòng)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(chǎn)物用高效液相色譜質(zhì)譜聯(lián)用儀(LC-MS, Agilent 1290+6460C, USA)測定,測定離子源為負(fù)離子ESI,色譜柱為C18反相色譜柱(1.8m, 2.1mm×100mm, Waters),進(jìn)樣量為10μL,流動(dòng)相分別為甲醇和水.

CPs的表觀降解率()根據(jù)式(3)計(jì)算,降解動(dòng)力學(xué)數(shù)據(jù)用擬一級(jí)動(dòng)力學(xué)方程式(4)擬合.

(3)

式中:0和C分別為CPs的初始濃度和反應(yīng)時(shí)刻的濃度,mg/L;為反應(yīng)時(shí)間,min;為擬一級(jí)動(dòng)力學(xué)反應(yīng)速率常數(shù),min-1.

2 結(jié)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(yīng)所致.Tan等[33]利用EDS對(duì)復(fù)合材料Fe0@Fe3O4進(jìn)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(jié)構(gòu).圖1(b)為Fe3O4-nZVI的X射線能譜圖(EDS),由圖看出,Fe3O4-nZVI復(fù)合材料中含有Fe、O、C、B和S等元素,且圖中出現(xiàn)了3個(gè)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(jié)合能分別為710.31、530.57和284.8eV.圖1(d)表明,Fe2p1/2和Fe2p3/2的結(jié)合能分別為723.98和710.98eV,證實(shí)了Fe2+和Fe3+的存在,表明材料中存在Fe3O4[35-36].結(jié)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(fā)生反應(yīng)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(xiàn)明顯特征峰,分別對(duì)應(yīng)于Fe3O4標(biāo)準(zhǔn)卡片JCPDS PDF#06-0696的(220)、(311)、(400)、(511)和(440)的晶面,這進(jìn)一步證明復(fù)合材料中存在Fe3O4,與XPS表征結(jié)果一致.Fe3O4-nZVI衍射峰與標(biāo)準(zhǔn)Fe3O4的峰位置基本吻合,表明材料的負(fù)載未改變Fe3O4的晶型結(jié)構(gòu).與nZVI標(biāo)準(zhǔn)卡片對(duì)比發(fā)現(xiàn),在2為44.71°出現(xiàn)特征寬峰,表明Fe3O4-nZVI復(fù)合材料中的nZVI以無定形態(tài)存在[37].

圖1(f)是nZVI、Fe3O4和Fe3O4-nZVI的振動(dòng)樣品磁強(qiáng)計(jì)分析圖(VSM).當(dāng)磁場強(qiáng)度為0Oe時(shí),三種材料的磁化強(qiáng)度均為0emu/g,均未出現(xiàn)磁滯現(xiàn)象.nZVI、Fe3O4和Fe3O4-nZVI的飽和磁化強(qiáng)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(fù)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(yīng)體系對(duì)2,4-DCP去除的影響

以2,4-DCP為目標(biāo)污染物,在25℃,pH=3,初始濃度為40mg/L,反應(yīng)時(shí)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(duì)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(duì)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?yàn)樵贖2O2體系中,H2O2對(duì)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(qiáng)還原作用下,發(fā)生還原脫氯,但nZVI易團(tuán)聚、易氧化的缺點(diǎn)又反過來阻礙了其對(duì)2,4-DCP的去除效果[38],因此2,4-DCP的表觀去除率仍然達(dá)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(yīng)體系對(duì)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(fù)合體系對(duì)2,4-DCP的表觀去除率都有較大提高.這是由于在Fe3O4-nZVI復(fù)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(zhì)能力,加速了2,4-DCP的去除效率;在nZVI-H2O2復(fù)合體系下,nZVI對(duì)2,4-DCP進(jìn)行脫氯去除,同時(shí)nZVI被H2O2氧化為Fe2+構(gòu)成Fenton體系,產(chǎn)生了HO·對(duì)2,4-DCP進(jìn)行氧化降解,因此表觀去除率提高,但由于部分nZVI發(fā)生團(tuán)聚和氧化,使得部分nZVI失效,削弱了其對(duì)污染物的去除能力;在Fe3O4-H2O2復(fù)合體系下,同時(shí)存在2,4-DCP的吸附及HO·的氧化降解,反應(yīng)初期,H2O2與Fe2+反應(yīng)產(chǎn)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(fù)合體系對(duì)2,4-DCP的表觀去除率最高,達(dá)到了98.98%.說明Fe3O4-nZVI可以有效改善nZVI易團(tuán)聚、易氧化現(xiàn)象,使2,4-DCP更有效地吸附在Fe3O4-nZVI復(fù)合材料表面,Fe3O4中的Fe2+和nZVI與H2O2反應(yīng)產(chǎn)生大量HO·對(duì)2,4-DCP進(jìn)行更徹底的降解.因此,后續(xù)實(shí)驗(yàn)選取Fe3O4-nZVI-H2O2組成的類Fenton體系對(duì)CPs進(jìn)行降解研究.

2.3 H2O2濃度對(duì)CPs降解的影響

在25℃,Fe3O4與nZVI物質(zhì)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(duì)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(yīng)時(shí)間的增加而逐漸增大,約在120min趨于平緩.在實(shí)驗(yàn)條件控制范圍內(nèi), CPs的表觀降解率隨H2O2濃度的增加而增大.在反應(yīng)180min,H2O2濃度為15mmol/L時(shí),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(dāng)H2O2濃度超過15mmol/L,CPs的表觀降解率不增反降.這是因?yàn)殡S著H2O2濃度的增加,Fe3O4-nZVI催化H2O2產(chǎn)生大量強(qiáng)氧化性HO·,CPs被逐漸氧化降解,當(dāng)H2O2濃度超過一定范圍后,溶液中的HO·與H2O2發(fā)生反應(yīng)產(chǎn)生過氧羥基(HO2·),反應(yīng)見式(5),產(chǎn)生的HO2·再進(jìn)一步與HO·反應(yīng)生成H2O和O2,自由基反應(yīng)終止,反應(yīng)見式(6),以上反應(yīng)使體系中的HO·產(chǎn)生耗損,降低了HO·與CPs的反應(yīng)活性,導(dǎo)致Fe3O4-nZVI類Fenton體系對(duì)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(dá)到最佳降解效果,后續(xù)實(shí)驗(yàn)選擇H2O2濃度為15mmol/L.

(6)

2.4 溶液pH值對(duì)CPs降解的影響

在25℃,Fe3O4和nZVI物質(zhì)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(duì)CPs的降解影響如圖4所示.由圖看出,pH值對(duì)CPs降解影響非常明顯.在pH=3時(shí),Fe3O4-nZVI類Fenton體系對(duì)2,4,6-TCP、2,4-DCP和4-CP的表觀降解率達(dá)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(xù)增大pH到9時(shí), 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?yàn)樗嵝詶l件有利于維持Fe2+和HO·生成,CPs降解效果就更好,同時(shí)加速nZVI的析氫腐蝕促進(jìn)CPs脫氯[40].當(dāng)增大pH,體系中的H+減少,此時(shí)Fe2+易沉淀且加速nZVI表面鈍化,阻止nZVI進(jìn)一步釋放Fe2+.然而,在較低的酸度如pH=2的條件下,會(huì)發(fā)生nZVI的過度腐蝕和Fe3O4溶解,這導(dǎo)致Fe3O4-nZVI的損失并抑制了CPs脫氯[41].因此后續(xù)實(shí)驗(yàn)選擇最佳pH=3.

2.5 Fe3O4-nZVI投加量對(duì)CPs降解的影響

在25℃,Fe3O4和nZVI物質(zhì)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(duì)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(duì)3種CPs的表觀降解率隨反應(yīng)時(shí)間的延長先增大,然后在120min逐漸趨于平緩.當(dāng)投加量為0.2g/L時(shí),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(dāng)投加量增加到0.6g/L時(shí),2,4,6-TCP、2,4-DCP和4-CP的表觀降解率達(dá)到最大,分別為94.36%、96.39%和98.05%.但繼續(xù)增大投加量至0.8g/L時(shí),CPs的表觀降解率均出現(xiàn)下降;當(dāng)增加到1g/L時(shí),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(duì)(a) 2,4,6-TCP;(b)2,4-DCP;(c) 4-CP降解的影響

這是因?yàn)橥都恿枯^小時(shí),Fe3O4-nZVI能夠參與吸附CPs的活性位點(diǎn)較少,少量的Fe3O4-nZVI與H2O2反應(yīng)無法產(chǎn)生大量的HO·,因此CPs的表觀降解率較小.當(dāng)增大投加量時(shí),體系中的反應(yīng)活性位點(diǎn)得以補(bǔ)充,Fe3O4-nZVI能夠與H2O2充分反應(yīng)產(chǎn)生大量的HO·降解CPs.但當(dāng)投加量超過0.6g/L,繼續(xù)增大投加量,過量的Fe3O4-nZVI溶出的Fe2+和Fe3+與HO·和HO2·反應(yīng)(見式(7)～(9)),這對(duì)HO·產(chǎn)生了耗損作用,同時(shí)該體系也促進(jìn)了Fe0、Fe2+和Fe3+的高效循環(huán)[42].為使Fe3O4-nZVI和H2O2均得到充分利用,實(shí)驗(yàn)選擇Fe3O4-nZVI的優(yōu)化投加量為0.6g/L.

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

2.6 Fe3O4-nZVI的重復(fù)利用性能

由圖6可知,Fe3O4-nZVI在重復(fù)利用的過程中,隨著重復(fù)利用次數(shù)增加,Fe3O4-nZVI類Fenton體系對(duì)2,4,6-TCP、2,4-DCP和4-CP的表觀降解率均表現(xiàn)出緩慢下降趨勢(shì)(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(fù)利用的過程中,Fe3O4-nZVI表面的活性吸附位點(diǎn)逐步減少,其表面催化活性隨之下降.另外,經(jīng)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(chǎn)物,Fe3O4-nZVI在降解污染物的過程中會(huì)析出Fe2+, Fe2+的消耗使得Fe3O4-nZVI在重復(fù)利用過程中的活性也會(huì)降低[43-44].但是重復(fù)利用次數(shù)達(dá)到5次時(shí),CPs的表觀降解率依然可達(dá)到85%以上,甚至重復(fù)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(fù)利用性能較高的磁性復(fù)合材料,可以降低污染物修復(fù)的成本,這一結(jié)果與VSM的表征結(jié)果是一致的.

圖6 Fe3O4-nZVI重復(fù)利用次數(shù)對(duì)CPs降解的影響

2.7 降解動(dòng)力學(xué)

Fe3O4-nZVI類Fenton體系對(duì)降解CPs的最佳H2O2濃度、Fe3O4-nZVI投加量及pH值等因素的擬一級(jí)動(dòng)力學(xué)參數(shù)如表1所示,相關(guān)系數(shù)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(jí)動(dòng)力學(xué)方程.由表1可知,在25℃,Fe3O4和nZVI物質(zhì)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(jí)動(dòng)力學(xué)速率常數(shù)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(huán)上氯原子數(shù)目影響Fe3O4-nZVI類Fenton體系對(duì)CPs的降解速率,即CPs上面的氯代原子數(shù)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(yīng)、-電子誘導(dǎo)效應(yīng)和π-電子共軛效應(yīng)共同作用的結(jié)果[45].因此,鄰位和對(duì)位的取代氯原子越多,氧化抑制作用越強(qiáng),且苯環(huán)上的氯代原子與苯環(huán)形成p-π共軛體系,可以鈍化苯環(huán)的鄰對(duì)位,此時(shí)氯取代基的鄰位和對(duì)位處的電子云密度較高,進(jìn)攻基團(tuán)HO·優(yōu)先進(jìn)入其鄰位和對(duì)位[46].因此Fe3O4-nZVI類Fenton體系對(duì)CPs的降解速率不僅與苯環(huán)上取代基的數(shù)量有關(guān)而且與取代基的位置也有一定關(guān)系.

表1 Fe3O4-nZVI類Fenton體系降解CPs的擬一級(jí)動(dòng)力學(xué)擬合參數(shù)

2.8 自由基淬滅

以2,4-DCP為代表,在25℃,Fe3O4和nZVI物質(zhì)的量比1:1, pH=3,H2O2濃度15mmol/L,CPs初始濃度40mg/L條件下,實(shí)驗(yàn)組加入自由基淬滅劑甲醇(CH3OH),空白組不加淬滅劑,實(shí)驗(yàn)結(jié)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(nèi)呈快速上升趨勢(shì),且在180min達(dá)到97.66%,此時(shí)實(shí)驗(yàn)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(xiàn)大幅度降低,說明HO·是Fe3O4-nZVI類Fenton體系降解CPs的主導(dǎo)活性物質(zhì).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(jìn)而增大了HO·與其接觸的路徑,增大了降解速率.另外,nZVI的強(qiáng)還原性可以對(duì)2,4-DCP進(jìn)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(chǎn)物及途徑

根據(jù)LC-MS實(shí)驗(yàn)數(shù)據(jù),結(jié)合自由基淬滅實(shí)驗(yàn)推測Fe3O4-nZVI類Fenton法對(duì)CPs的降解產(chǎn)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(gè)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(duì)位取代基羥基化脫氯轉(zhuǎn)化為二氯對(duì)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(jìn)一步被礦化為CO2和H2O.路徑(2)也分三個(gè)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(jìn)一步脫氯生成一氯苯酚,直至脫氯生成苯酚;苯酚會(huì)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(huán)階段,HO·攻擊苯環(huán),苯二醌開環(huán)轉(zhuǎn)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(duì)位的氯自由基生成2-氯對(duì)苯二酚,繼續(xù)羥基化可得到苯四酚和2,6-二羥基對(duì)苯醌等物質(zhì);第Ⅱ階段為開環(huán)階段,HO·繼續(xù)攻擊苯環(huán)使其開環(huán)生成丙烯二酸、乙酸和丙酸等小分子酸后;進(jìn)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(duì)氯苯酚(4-CP)和苯酚,2-CP和4-CP也可進(jìn)一步脫氯生成苯酚,HO·進(jìn)一步氧化苯酚生成對(duì)苯二酚、鄰苯二酚和間苯二酚,酚類物質(zhì)進(jìn)一步轉(zhuǎn)化為對(duì)苯二醌和鄰苯二醌;第Ⅱ階段為開環(huán)階段,HO·繼續(xù)對(duì)苯環(huán)進(jìn)行攻擊,苯二醌和苯二酚開環(huán)轉(zhuǎn)化為順丁烯二酸和乙二酸等小分子酸;進(jìn)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(dǎo)地位[50-52].

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

圖9 Fe3O4-nZVI類Fenton體系對(duì)2,4-DCP降解途徑示意

3 結(jié)論

3.1 Fe3O4-nZVI球形磁性復(fù)合材料可促進(jìn)Fe2+和Fe3+的循環(huán),增加HO·的產(chǎn)量.

3.2 在25℃,CPs初始濃度40mg/L,Fe3O4和nZVI物質(zhì)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(yōu)化條件下,Fe3O4- nZVI類Fenton體系對(duì)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(huán)上氯原子數(shù)目的增加,相應(yīng)的Fe3O4- nZVI類Fenton體系對(duì)CPs的降解速率常數(shù)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(zhuǎn)化為單氯酚,再進(jìn)一步轉(zhuǎn)化為酚類化合物,酚類化合物氧化為醌,最后開環(huán)轉(zhuǎn)變?yōu)樾》肿铀?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(shù)和土壤污染修復(fù).發(fā)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(huán)境科學(xué), 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(xué)基金資助項(xiàng)目(22166022);甘肅省科技計(jì)劃項(xiàng)目(20JR2RA002);蘭州交通大學(xué)創(chuàng)新創(chuàng)業(yè)項(xiàng)目(CXXL20230087)

* 責(zé)任作者, 教授, zhongjk@mail.lzjtu.cn