黃雪征,張永祥,張大勝,朱新鋒,李厚運(yùn)

石墨烯負(fù)載鐵鎳復(fù)合材料去除水中的2,4-二氯酚

黃雪征1,2,3*,張永祥2,張大勝4,朱新鋒1,李厚運(yùn)1

(1.河南城建學(xué)院,河南省水體污染防治與修復(fù)重點(diǎn)實(shí)驗(yàn)室,河南 平頂山 467000；2.北京工業(yè)大學(xué)城市建設(shè)學(xué)部,北京 100124；3.南陽理工學(xué)院土木工程學(xué)院,河南 南陽 473000；4.河北水利科學(xué)院研究院,河北 石家莊 050051)

通過液相還原法成功制備了石墨烯負(fù)載納米鐵鎳復(fù)合材料,該材料可高效快速地吸附水中的2,4-二氯酚(2,4-DCP)并對其進(jìn)行脫氯.微觀形貌分析結(jié)果表明,粒徑為80～150nm的球形 Fe/Ni納米顆粒成功插入石墨烯片層,并主要分布在石墨烯片層邊緣和褶皺處,Fe/Ni顆粒團(tuán)聚現(xiàn)象明顯減少,更多活性位點(diǎn)暴露出來.XRD分析和FTIR分析表明,納米零價(jià)鐵(nZVI)通過Fe-O鍵成功嵌入石墨烯(rGO)中,且Fe/Ni納米顆粒結(jié)晶度較差,外圍包覆有無定形的鐵氧化物沉淀.探討了不同制備條件如碳鐵比、鎳化率、氧化石墨烯(GO)還原程度對材料去除2,4-二氯酚(2,4-DCP)性能的影響.綜合考慮材料制備成本及對2,4-DCP的吸附脫氯性能,Fe/Ni@rGO復(fù)合材料的最優(yōu)制備條件為:石墨烯與Fe質(zhì)量比1:2,鎳負(fù)載率5%,硼氫化鈉與鐵鹽的物質(zhì)的量比為5:1.研究表明5種材料對2,4-DCP的去除率遵循如下順序:Fe/Ni@rGO復(fù)合材料>Fe/Ni>Fe @rGO復(fù)合材料>石墨烯> nZVI.儲存穩(wěn)定性試驗(yàn)和循環(huán)試驗(yàn)表明,與Fe/Ni雙金屬相比,Fe/Ni@rGO材料具有穩(wěn)定的反應(yīng)活性和較高的重復(fù)利用價(jià)值.研究結(jié)果表明Fe/Ni@rGO復(fù)合材料對2,4-DCP的去除為吸附和脫氯協(xié)同作用的結(jié)果.

石墨烯；2,4-二氯酚；納米鐵；脫氯；吸附

氯酚類化合物(CPs) 廣泛用于木材、紡織和紙張等工業(yè)的防腐,并用做殺蟲劑,殺菌劑等,在世界范圍內(nèi)已經(jīng)使用數(shù)十年[1-2].氯酚類物質(zhì)的大量生產(chǎn)和廣泛應(yīng)用,使得氯酚類化合物通過吸附、滲透、淋濾等作用由地表進(jìn)入地下水,從而給地下水及土壤造成了一定的污染,特別在一些造紙廠、化工廠的土壤及地下水中氯酚污染嚴(yán)重.由于氯酚類化合物很強(qiáng)的毒性、持久性、致突變性和致癌性,美國環(huán)保署在1977年頒布的"清潔水法"修正案中將11種氯酚化合物列為環(huán)境優(yōu)先污染物名單.氯酚化合物被列入"中國環(huán)境有限污染物黑名單",成為需要優(yōu)先處理的污染物[3].

為了降低氯酚廢水對地下水體的污染,眾多研究者分別采用活性炭吸附法、揮發(fā)法、溶劑萃取法、化學(xué)氧化法和好氧/厭氧生物降解法、化學(xué)還原法等多種處理技術(shù)對氯酚廢水進(jìn)行了研究[4-8].吸附法投資少、操作成本低、操作簡便,為去除有機(jī)污染物,特別是氯酚類化合物有效可行的成熟技術(shù)[9].但是污染物僅僅是實(shí)現(xiàn)了相轉(zhuǎn)移,并沒有從根本上對氯酚污染物去除,且存在吸附劑重復(fù)利用困難、僅適用于低濃度廢水等缺點(diǎn),污染物脫附后容易造成二次污染.氧化法處理成本高,且難以有效處理作為電子受體的含氯有機(jī)污染物[10].化學(xué)還原法采用活潑金屬,通過還原脫氯的方式,逐級脫氯,降低氯酚污染物的毒性,進(jìn)一步開環(huán),生成低毒易降解的物質(zhì),所采用的還原劑金屬有鎂、鋁、鐵,鎂、鋁反應(yīng)活性雖強(qiáng),但表面易形成鈍化層,不利于反應(yīng)的進(jìn)一步進(jìn)行,且鋁對環(huán)境的污染毒性較大.1994年加拿大滑鐵盧大學(xué)Gillham教授首次采用金屬鐵屑實(shí)地修復(fù)地下水[11].隨著研究的進(jìn)一步深入,人們發(fā)現(xiàn)零價(jià)鐵粒度的減小會導(dǎo)致鐵顆粒比表面積增大,反應(yīng)活性增強(qiáng).1997年Wang等[12]首次采用液相還原法合成了納米零價(jià)鐵顆粒,并將納米零價(jià)鐵漿液直接注入到污染含水層用以處理三氯乙烯,開創(chuàng)了納米零價(jià)鐵在環(huán)境修復(fù)領(lǐng)域的應(yīng)用研究.納米零價(jià)鐵由于其環(huán)境友好、還原能力強(qiáng)和成本低廉等受到了人們的廣泛關(guān)注,被廣泛應(yīng)用于氯代烴、重金屬、多氯聯(lián)苯、有機(jī)染料、硝酸鹽等的去除研究中[13-16].但納米零價(jià)鐵在應(yīng)用中存在易團(tuán)聚、易鈍化、遷移能力較差等缺陷.為提高納米鐵微粒的穩(wěn)定性和遷移能力,有必要對納米鐵進(jìn)行改性.納米鐵改性材料來源廣泛,涵蓋無機(jī)物、礦石、天然或合成聚合物等[17-22].

本研究通過引入過渡金屬鎳作為脫氯加氫催化劑,與納米零價(jià)鐵組合形成納米雙金屬體系,同時(shí)以還原氧化石墨烯作為載體,將納米鐵鎳雙金屬顆粒負(fù)載在石墨烯片層上,研制出具有協(xié)同效應(yīng)的石墨烯負(fù)載納米雙金屬體系,比較分析材料與nZVI、石墨烯、石墨烯負(fù)載鐵、Fe/Ni納米雙金屬的吸附-脫氯性能.通過調(diào)整制備過程中C:Fe質(zhì)量比、鎳負(fù)載率(Ni與Fe的質(zhì)量比)、氧化石墨烯還原度等因素制備系列石墨烯負(fù)載納米鐵鎳雙金屬材料,通過對2,4-DCP的去除效果優(yōu)選最佳制備工藝條件,并考察最優(yōu)工藝下制備的石墨烯負(fù)載雙金屬型納米鐵復(fù)合材料的儲藏穩(wěn)定性能以及重復(fù)利用性能,揭示材料去除2,4-DCP的反應(yīng)機(jī)理.

1 材料與方法

1.1 試劑與儀器

試劑:六水硫酸鎳,鹽酸,2, 4-二氯酚,2-氯酚,4-氯酚,苯酚,硼氫化鈉,七水硫酸亞鐵等(以上試劑均為分析純).氧化石墨烯購自深圳穗衡石墨烯科技公司,厚度2nm左右,片層直徑0.4 ～10mm,純度>99%.

儀器:JJ-1型精密定時(shí)電動攪拌器(北京中興偉業(yè)儀器有限公司),HY-6型雙層調(diào)速多用振蕩器(江蘇省金壇市榮華儀器制造有限公司), SHIMADZU LC-2030C 3D型高效液相色譜(島津企業(yè)管理(中國)有限公司),JEM2100F型透射電子顯微鏡(日本電子株式會社),BT100-1L型流量型蠕動泵(保定蘭格恒流泵有限公司儀舊本理學(xué)公司), D8advance bruker型X射線衍射儀(布魯克(北京)科技有限公司), Autosorb iQ型吸附脫附儀(美國康塔儀器公司), FD-1A-50型冷凍干燥機(jī)(上海比朗儀器制造有限公司)

1.2 材料制備

取0.28g氧化石墨烯粉末加入100mL去離子水中,超聲20min,制得分散的氧化石墨烯懸浮液并轉(zhuǎn)移至三口燒瓶中,充N2鼓泡15min后將一定量的FeSO4×7H2O加入到溶液中繼續(xù)攪拌300min,然后將適量濃度的NaBH4溶液通過蠕動泵勻速緩慢滴加到溶液中,滴加完畢后繼續(xù)攪拌反應(yīng)60min,再向三口燒瓶中加入一定量的硫酸鎳水溶液.機(jī)械攪拌下持續(xù)反應(yīng)30min.再用無水乙醇快速真空抽濾洗滌3次,冷凍干燥24h后密封保存.通過調(diào)整氧化石墨烯和鐵的質(zhì)量、鎳與鐵的質(zhì)量比、氧化石墨烯的還原程度等制備一系列石墨烯負(fù)載雙金屬型納米鐵復(fù)合材料.以上操作均在氮?dú)猸h(huán)境下進(jìn)行.

1.3 批試驗(yàn)

在室溫下,在250mL的具塞錐形瓶中加入245mL調(diào)節(jié)好pH值的去離子水,然后通過移液槍加入5mL 1000mg/L 2,4-DCP儲備液,即可配制初始濃度為20mg/L的2,4-DCP溶液,在通氮?dú)鈼l件下,加入一定量的制備好的石墨烯負(fù)載雙金屬型納米鐵復(fù)合材料,將其置于臺式恒溫水浴振蕩器中振蕩,調(diào)節(jié)振蕩速率為200r/min,設(shè)置溫度為25℃,使材料與溶液充分接觸.間隔一定時(shí)間(5,10,20,45,60,90,120, 150,180,240,300min)用一次性注射器取樣2mL左右,并用孔徑0.45μm的濾膜過濾,濾液存放于高效液相自動進(jìn)樣器瓶中,然后用高效液相色譜儀測出剩余2,4-DCP及產(chǎn)物的峰面積,通過外標(biāo)法計(jì)算出2,4-DCP及產(chǎn)物的濃度.2,4-DCP及其降解產(chǎn)物鄰氯苯酚(2-CP)、對氯苯酚(4-CP)和苯酚的測定均采用高效液相色譜儀(島津LC2030C)測定.采用反相色譜柱SHIMADZU ODS-SP Column(250′4.6mm)分離,L-2420型紫外檢測器測定2,4-DCP溶液的濃度,流動相采用(CH3OH):(H2O)=60:40,流動相流速1.0mL/min;進(jìn)樣量為20μL,柱溫40℃,檢測波長均為280nm.

2,4-DCP的去除率按如下公式計(jì)算:

式(1)中:2,4-DCP,0是2,4-DCP初始時(shí)刻的濃度,mg/L;2,4-DCP,t是反應(yīng)進(jìn)行至?xí)r刻2,4-DCP的濃度,mg/L;是2,4-DCP的去除率,%.

2 結(jié)果與討論

2.1 材料的表面形貌

石墨烯負(fù)載納米鐵鎳復(fù)合材料與2,4-DCP反應(yīng)前后的電鏡掃描譜圖如圖1所示.從圖中可以看出反應(yīng)前球形的納米鐵鎳顆粒主要分布在石墨烯的片層邊緣或鑲嵌于褶皺處,并被石墨烯片層包裹,顆粒團(tuán)聚較少.這是因?yàn)檠趸然倌軋F(tuán)數(shù)量主要分布在片層邊緣,而Fe2+與氧化石墨烯的絡(luò)合主要是通過羧基絡(luò)合的,所以所產(chǎn)生的納米鐵鎳金屬顆粒主要分布在片層邊緣.反應(yīng)后分散在石墨烯上的球形納米雙金屬鐵鎳顆粒消失,出現(xiàn)針狀晶體,顆粒表面呈團(tuán)簇狀聚集體.根據(jù)反應(yīng)推測,該針狀晶體為鐵的氧化物γ-羥基氧化鐵[23].

圖1 不同反應(yīng)階段石墨烯負(fù)載納米鐵鎳復(fù)合材料的SEM圖像

從圖2a可以看出,由于自身磁性和納米尺度效應(yīng),純納米Fe/Ni顆粒具有明顯的鏈狀聚集體結(jié)構(gòu),甚至聚集為片狀,顆粒大部分呈球形,單個(gè)顆粒不能清晰分辨出來.圖2b為石墨烯負(fù)載納米鐵鎳復(fù)合材料的TEM圖.圖中的黑色球形顆粒即為納米鐵鎳金屬顆粒,片狀的透明層狀物質(zhì)即為石墨烯,呈現(xiàn)為具有皺褶和折疊邊緣的二維薄層.納米鐵鎳顆粒均勻分散在石墨烯表面,無明顯團(tuán)聚現(xiàn)象,相較于納米Fe/Ni材料,納米Fe/Ni金屬顆粒團(tuán)聚現(xiàn)象明顯降低.從圖2c可以看出,負(fù)載在石墨烯上的納米鐵鎳顆粒呈明顯的核殼構(gòu)型,外層表面比較疏松,呈非晶態(tài).這種松散的結(jié)構(gòu)可能是Fe3O4,其導(dǎo)電能力較強(qiáng),有利于內(nèi)核鐵原子的電子轉(zhuǎn)移,表面可觀察到一些致密的小顆粒,結(jié)合圖3分析可知,這些小顆粒為Ni納米顆粒,沉積在納米鐵表面形成小突起,從而抑制了納米鐵核與空氣中氧氣的直接接觸,從而減少了nZVI的氧化和消耗.圖2d的高分辨TEM照片表明內(nèi)部顏色較深部分即為Fe核,尺寸約為10nm左右,外部被非晶態(tài)的鐵氧化物覆蓋.可以在納米顆粒上觀察到清晰的晶格條紋,說明Fe/Ni納米顆粒的核結(jié)晶性較好.其晶格間距約為0.248nm,這一結(jié)果與Fe (JCPDS 06-0696)的(110)晶面十分吻合[24].

圖2 材料的TEM及HRTEM圖像

圖3 石墨烯負(fù)載鐵鎳材料的EDS面掃圖

2.2 石墨烯負(fù)載納米鐵鎳復(fù)合材料反應(yīng)前后的XRD譜圖

圖4 氧化石墨烯,納米鐵鎳,反應(yīng)前后石墨烯負(fù)載納米鐵鎳的X射線衍射圖

從圖4中可以看出,氧化石墨烯GO的衍射峰在2=11.6°處有一個(gè)最強(qiáng)特征峰(002)[25].通過布拉格方程計(jì)算其層間距為0.761nm,高于普通石墨粉的層間距0.337nm,這是因?yàn)槭劢?jīng)過氧化之后引入了大量的含氧官能團(tuán),含氧官能團(tuán)的插入增加了石墨烯層間距,為金屬納米顆粒的負(fù)載提供了空間.在納米Fe/Ni雙金屬 XRD譜圖中,在2=44.7°處出現(xiàn)了一個(gè)明顯的寬鈍峰,對應(yīng)于α-Fe(JCPDS. 06-0696)的(110)衍射面[26].峰型較寬,強(qiáng)度較弱,這是由于納米鐵外殼層的氧化,呈無定形態(tài),所以沒有觀察到氧化鐵峰的存在,說明納米鐵結(jié)晶度較低,晶粒尺寸較小,這和納米鐵鎳的TEM圖觀察相符.由于鎳金屬與鐵金屬的衍射峰十分接近[27],而且鎳金屬的含量相對鐵而言很少,所以未檢測到明顯的鎳的衍射峰.石墨烯負(fù)載納米鐵鎳復(fù)合材料反應(yīng)前后在2=12.5°處均有很弱的衍射峰,說明氧化石墨烯特征峰消失,氧化石墨烯基本還原,未出現(xiàn)明顯的石墨烯特征峰(2=25.1°和2=43.0°)[28],這可能是由于復(fù)合材料中石墨烯的無序堆積和團(tuán)聚較少所致,這在石墨烯負(fù)載納米鐵鎳復(fù)合材料的SEM圖中得到了證實(shí).反應(yīng)前的石墨烯負(fù)載納米鐵鎳復(fù)合材料XRD譜圖在2=44.7°處出現(xiàn)了一個(gè)較弱的寬峰,說明納米鐵已經(jīng)成功插入石墨烯片層結(jié)構(gòu)上,在63.1°處的較小的峰對應(yīng)于Fe(200)的特征衍射峰,而納米鐵鎳的XRD譜圖中沒有觀察到此峰,說明納米鐵顆粒負(fù)載到石墨烯上后結(jié)晶度有所提高[29].由于Fe/Ni@rGO復(fù)合材料中Ni的含量較低,沒有觀察到Ni的衍射峰[30].反應(yīng)前的石墨烯負(fù)載納米鐵鎳復(fù)合材料在2=30.7°和2=35.0°處可以觀察到磁赤鐵礦Fe3O4/-Fe2O3的較弱的衍射峰,納米鐵的氧化峰強(qiáng)度普遍都很弱,說明了納米鐵外層的氧化層呈無定形狀態(tài)[31].與反應(yīng)前的Fe/Ni@rGO復(fù)合材料相比,反應(yīng)后的Fe/Ni@rGO復(fù)合材料XRD譜圖2=44.7°處寬峰消失,在2=30.7°和2=35.0°處磁赤鐵礦峰強(qiáng)增大,并在2=53.8°和2=62.8°處出現(xiàn)纖鐵礦-羥基氧化鐵特征峰[32],表明零價(jià)納米鐵經(jīng)過反應(yīng)后得以氧化,生成Fe3O4/-Fe2O3和-羥基氧化鐵.

圖5 氧化石墨烯和石墨烯負(fù)載鐵鎳復(fù)合材料的紅外譜圖

2.3 FTIR分析

從圖5中可以看出,在3445cm-1附近氧化石墨烯有一個(gè)較寬、較強(qiáng)的吸收峰,這歸屬于-OH的伸縮振動峰, 1650cm-1處的峰對應(yīng)于C=O基團(tuán)的伸縮振動,1411cm-1處的峰對應(yīng)于羧基O=C-O基團(tuán)的伸縮振動[33],1222和1065cm-1的峰分別對應(yīng)于環(huán)氧基C-O-C和烷氧基C-O的振動吸收峰[34-35],說明GO至少存在-OH,-COOH, C-O-C,C=O四種類型含氧官能團(tuán),說明GO高度親水.對于Fe/Ni@rGO復(fù)合材料紅外譜圖, -OH、C=O、O-C=O的峰強(qiáng)相對于GO峰強(qiáng)出現(xiàn)了較大程度的降低,環(huán)氧基C-O-C和烷氧基C-O峰消失,表明材料中含氧親水極性官能團(tuán)減少,GO被成功還原成石墨烯rGO,這也解釋了Fe/Ni@rGO材料表面高度疏水的原因[36],復(fù)合材料的強(qiáng)疏水性能有利于對有機(jī)物2,4-DCP分子的吸附.同時(shí)可以看到Fe/Ni@rGO材料在1121cm-1出現(xiàn)Ni-O峰[37],在587cm-1出現(xiàn)Fe-O峰[38],表明石墨烯與nZVI的結(jié)合主要通過Fe-O鍵來完成,表明nZVI顆粒己成功嵌入石墨烯中.

2.4 碳鐵比對Fe/Ni@rGO復(fù)合材料性能的影響

圖6 不同碳鐵比對去除2,4-DCP的影響

鎳化率5%,反應(yīng)溫度30 °C,[2,4-DCP]0=20mg/L,初始 pH=5.0

2.5 鎳負(fù)載率對Fe/Ni@rGO復(fù)合材料性能的影響

Fe/Ni雙金屬體系已被證明可以有效去除含氯有機(jī)物,在加氫脫氯過程中,2,4-DCP吸附在Ni催化劑的活性中心上會生成Ni-Cl鍵,Ni表面的原子氫通過加氫脫氯反應(yīng)取代氯原子,參與C-Cl鍵的斷裂,從而得到脫氯產(chǎn)物.大量的研究[42-44]也充分證實(shí)了過渡金屬鎳的負(fù)載量對雙金屬體系中氯代烴脫氯效率的影響.

圖7 鎳負(fù)載率對2,4-DCP的去除率和苯酚產(chǎn)率的影響

C:Fe=1:2,反應(yīng)溫度30°C,[2,4-DCP]0=20mg/L,pH=5.0

從圖7可以看出,不同鎳含量復(fù)合材料對2,4- DCP的去除率排序?yàn)?5%>14%>9%> 3%,苯酚的產(chǎn)率排序?yàn)?9% >5% >14% >3%.2,4-DCP的加氫脫氯作用主要是由雙金屬催化劑上吸附的活性原子氫(H*)脫氯引起的,氫原子和鎳以類氫化物的形式存在于鎳的表面[45].當(dāng)鎳化率小于9%時(shí),2,4-DCP的脫氯效率隨著鎳化率的上升而增大.這是由于鎳能夠使納米鐵腐蝕所產(chǎn)生的氫氣分解成活性氫原子并附著在鎳的表面,活性氫原子的強(qiáng)還原能力使得2,4-DCP的脫氯效率增大,鎳化率越高,材料的脫氯效率就越高,此時(shí)鎳化率是材料脫氯反應(yīng)的控制因素.當(dāng)鎳化率大于9%時(shí),雙金屬中鎳的含量過高,較高含量的鎳覆蓋在納米鐵顆粒的表面,不利于納米鐵的電子轉(zhuǎn)移,最終降低雙金屬催化劑的脫氯效率.從圖中可以明顯看出5%的鎳含量對2,4-DCP的去除效果最優(yōu).

2.6 氧化石墨烯還原度對Fe/Ni@rGO材料的影響

圖8 硼氫化鈉投加量對2,4-二氯酚去除率和苯酚產(chǎn)率的影響曲線圖

C:Fe=1:2,反應(yīng)溫度30°C,[2,4-DCP]0=20mg/L,pH=5.0

Shin等[46]通過調(diào)整NaBH4溶液的投加量制備了不同還原程度的rGO,結(jié)果表明硼氫化鈉與亞鐵離子的物質(zhì)的量比越大,氧化石墨烯的還原就越徹底,高的C/O比意味著rGO具有更多被修復(fù)的共扼結(jié)構(gòu)和相對少的含氧官能團(tuán),具有相對高的導(dǎo)電性.由此可通過調(diào)整NaBH4物質(zhì)的量來改變生成的氧化石墨烯的還原程度,從而對材料的吸附性能和導(dǎo)電性能產(chǎn)生較大影響[47],并由此影響復(fù)合材料對2,4-DCP的吸附及脫氯效果.從圖8a可以看出,當(dāng)投加的NaBH4物質(zhì)的量是亞鐵離子2倍時(shí),此時(shí)材料對2,4- DCP的去除能力強(qiáng)于其他兩種還原程度的材料,這是因?yàn)殡m然此時(shí)材料的還原程度最小,脫氯還原能力較弱,但負(fù)載材料上存在的羧基、羥基、羰基等含氧官能團(tuán)較多,可與酚類化合物形成氫鍵,吸附能力較強(qiáng).但此投加量僅僅還原了部分亞鐵離子,且對氧化石墨烯的還原程度較低,且低還原度的氧化石墨烯導(dǎo)電能力很弱,不利于材料的脫氯反應(yīng),這從圖8(b)可以得到證實(shí).綜合考慮材料的吸附脫氯性能以及制備成本,選用NaBH4與亞鐵離子物質(zhì)的量比值為5:1的反應(yīng)條件制備材料為宜.

2.7 五種材料對2,4-DCP去除效果比較研究

從圖9可以看出,石墨烯、Fe@rGO復(fù)合材料和Fe/Ni@rGO復(fù)合材料對2,4-DCP的去除效果在反應(yīng)最初的20min遠(yuǎn)優(yōu)于Fe/Ni和nZVI,隨著反應(yīng)的進(jìn)行,nZVI、石墨烯和Fe@rGO復(fù)合材料對2,4-DCP的去除率趨于平緩.Fe/Ni和Fe/Ni@rGO復(fù)合材料對2,4-DCP的去除率穩(wěn)步上升,最終在反應(yīng)進(jìn)行至 240min時(shí)達(dá)到95%以上的去除率.在反應(yīng)初始階段,材料對2,4-DCP的去除以吸附為主.Fe@rGO復(fù)合材料和Fe/Ni@rGO復(fù)合材料兩者均以石墨烯為載體,具有多孔結(jié)構(gòu),比表面積大,其特有的大p環(huán)可通過p-p作用力吸引2,4-DCP分子,所以吸附能力遠(yuǎn)強(qiáng)于nZVI和Fe/Ni對2,4-DCP分子的吸附能力.石墨烯材料比表面積大,由于其表面沒有納米金屬顆粒的干擾,參與形成大p鍵的碳原子數(shù)目更多,其形成的共軛體系比Fe@rGO復(fù)合材料要大,所以在吸附達(dá)到平衡時(shí)其對2,4-DCP的去除率高于Fe@rGO復(fù)合材料[48].由于純納米鐵易團(tuán)聚,外層形成致密的氧化鐵外殼,阻礙內(nèi)層鐵核的電子轉(zhuǎn)移,再加上2,4-DCP為芳香性氯代物,和苯環(huán)相連的C-Cl鍵難以斷裂,脫氯反應(yīng)活化能高達(dá)138.91kJ/mol[49],單純的nZVI還原能力一般,很難發(fā)生脫氯反應(yīng),所以nZVI和Fe@rGO復(fù)合材料主要通過吸附作用去除2,4-DCP.在Fe/Ni和Fe/Ni@rGO體系中,反應(yīng)20min后2,4-DCP去除率能夠持續(xù)增大,這是因?yàn)榘l(fā)生了脫氯反應(yīng).隨著脫氯反應(yīng)的進(jìn)行,對Fe/Ni體系來說,固相中2,4-DCP逐漸脫氯生成苯酚,其占據(jù)的吸附位點(diǎn)得到釋放,其空出的吸附位點(diǎn)對液相中的2,4-DCP分子產(chǎn)生新的吸引力,所以整個(gè)反應(yīng)過程中2,4-DCP去除速率相對比較穩(wěn)定;而對Fe/ Ni@rGO體系來說,經(jīng)過最初的20min的吸附,液相中的大部分2,4-DCP分子被吸附至材料表面,并通過孔道擴(kuò)散至Fe/Ni顆?；钚晕稽c(diǎn)上發(fā)生脫氯反應(yīng),生成產(chǎn)物苯酚釋放至溶液中,但后續(xù)2,4-DCP濃度越來越低,故去除速率逐漸趨緩.

圖9 不同材料去除2,4-DCP的濃度變化曲線圖

C:Fe=1:2,反應(yīng)溫度30°C,[2,4-DCP]0=20mg/L,初始pH=5.0

2.8 儲存穩(wěn)定性能及重復(fù)利用性能

從圖10a可以看出,在空氣中暴露90d后,Fe/ Ni@rGO復(fù)合材料對2,4-DCP的去除率仍達(dá)到92.9%,而納米Fe/Ni材料空氣中暴露90d后,其對2,4-DCP的去除率降至52.8%.這說明Fe/Ni@rGO復(fù)合材料具有比n-Fe/Ni更高的抗氧化性能和催化活性,Fe/Ni@rGO復(fù)合材料不僅能阻止nZVI的聚集,而且有助于復(fù)合材料保持較高的反應(yīng)活性.由此可見,Fe/Ni@rGO復(fù)合材料抗氧化性能強(qiáng),可在開放環(huán)境中存放三個(gè)月時(shí)間而保持反應(yīng)活性基本不變.

從圖10b可以看出,2,4-DCP在5個(gè)循環(huán)試驗(yàn)中的去除率分別為100%,98.5%,96.8%,85.1%,80.7%,從第4次循環(huán)試驗(yàn)開始,2,4-DCP去除率明顯下降,這可能是由于隨著2,4-DCP降解反應(yīng)的進(jìn)行,nZVI在還原降解反應(yīng)過程中不斷發(fā)生腐蝕,生成的Fe3O4和Fe2O3等氧化物沉淀覆蓋了材料表面的活性位點(diǎn),導(dǎo)致降解速率降低.與Fe/Ni@rGO復(fù)合材料相對應(yīng),納米Fe/Ni雙金屬作為還原劑在2,4-DCP第一輪循環(huán)試驗(yàn)中性能很好,但到第五輪降解試驗(yàn)中只能去除40.2%的2,4-DCP,其持久活性較差,說明其隨著反應(yīng)的進(jìn)行,容易被氧化失活.綜上所述,Fe/Ni@rGO復(fù)合材料反應(yīng)活性持久,可循環(huán)性強(qiáng).

圖10 材料儲存穩(wěn)定性及重復(fù)利用性能試驗(yàn)分析

[2,4-DCP]=20mg/L,投加量,1.0g/L,溫度30°C,pH=5.0,反應(yīng)時(shí)間300min

2.9 Fe/Ni@rGO復(fù)合材料去除2,4-DCP反應(yīng)機(jī)理分析

Fe/Ni@rGO復(fù)合材料去除2,4-DCP是一個(gè)吸附及脫氯協(xié)同作用的結(jié)果.在反應(yīng)前,Fe/Ni@rGO 復(fù)合材料表面有大量空置的吸附位點(diǎn),因而具有較大的表面吉布斯自由能.當(dāng)投加入2,4-DCP溶液中,復(fù)合材料的大π鍵與2,4-DCP分子的苯環(huán)形成p-p堆積作用,石墨烯表面也含有部分含氧官能團(tuán),其和2,4-DCP分子之間也存在氫鍵作用,從而對2,4-DCP分子產(chǎn)生較強(qiáng)的吸附作用,如圖11所示.同時(shí)溶液不斷震蕩下形成的紊流擴(kuò)散加劇了2,4-DCP分子從溶液本體向復(fù)合材料表面的擴(kuò)散.2,4-DCP分子被吸附至材料表面后,通過材料介孔擴(kuò)散至Fe/Ni@rGO復(fù)合材料的吸附活性位點(diǎn)及反應(yīng)活性位點(diǎn)Fe/Ni顆粒附近,與Fe/Ni顆粒發(fā)生加氫還原脫氯反應(yīng).

圖11 石墨烯和2,4-DCP分子之間π-π作用力與氫鍵示意

2,4-DCP分子在 Fe/Ni雙金屬納米顆粒表面發(fā)生加氫脫氯反應(yīng),具體反應(yīng)路徑如下:

Fe/Ni雙金屬納米顆粒表面脫氯反應(yīng)示意圖如圖12 所示.在反應(yīng)過程中,石墨烯作為載體,有效分散了納米Fe/Ni雙金屬顆粒,降低了納米顆粒的團(tuán)聚,同時(shí)由于其強(qiáng)吸附作用和強(qiáng)導(dǎo)電能力,通過吸附和脫氯協(xié)同效應(yīng)增強(qiáng)對2,4-DCP的反應(yīng)活性.Fe作為還原劑釋放出電子,生成氫氣,同時(shí)提供電子給石墨烯,由于單層石墨烯π電子的自由移動特性,電子迅速傳導(dǎo)給石墨烯表面吸附的2,4-DCP分子,參與到2,4-DCP的加氫脫氯還原反應(yīng)中.在Fe/Ni雙金屬體系中,Ni與Fe可形成原電池效應(yīng),加速Fe的腐蝕,導(dǎo)致析氫速度加快,抑制nZVI顆粒氧化層的形成.Ni作為過渡金屬,具有空軌道,可與2,4-DCP中的氯原子提供的孤對電子成鍵,形成過渡絡(luò)合物Ni…Cl…R(式2),削弱C-Cl鍵,降低脫氯反應(yīng)活化能[50].同時(shí)鎳作為儲氫金屬,可吸附H2,并在嵌入的晶格中形成強(qiáng)還原性物質(zhì) Ni×2H*(式3),并釋放出活性氫原子 H*(式4),與吸附在Ni上的2,4-DCP發(fā)生加氫脫氯反應(yīng)(式5).金屬鎳的加入改變了 2,4-DCP的反應(yīng)途徑,2,4-DCP不再直接通過接受零價(jià)鐵與Fe2+釋放的電子來實(shí)現(xiàn)脫氯還原,而是和活性氫原子發(fā)生催化加氫脫氯還原反應(yīng),從而大大提高了脫氯效率[51].

圖12 Fe/Ni雙金屬納米顆粒表面脫氯反應(yīng)示意

3 結(jié)論

3.1 成功研制了石墨烯負(fù)載納米鐵鎳復(fù)合材料.綜合考慮材料制備成本及對2,4-DCP的吸附脫氯性能,Fe/Ni@rGO復(fù)合材料的最優(yōu)制備條件為:石墨烯與Fe質(zhì)量比1:2,鎳負(fù)載率5%,硼氫化鈉與鐵鹽的物質(zhì)的量比為5:1.

3.2 研究表明4種材料對2,4-DCP的去除效率遵循如下順序:Fe/Ni@rGO復(fù)合材料>Fe/Ni>Fe @rGO復(fù)合材料>nZVI.儲存穩(wěn)定性試驗(yàn)和循環(huán)試驗(yàn)表明,與Fe/Ni雙金屬相比,Fe/Ni@rGO復(fù)合材料對2,4-DCP的去除效率均有顯著提高,這表明Fe/Ni@rGO材料具有穩(wěn)定的反應(yīng)活性和較高的重復(fù)利用價(jià)值.

3.3 揭示了Fe/Ni@rGO復(fù)合材料去除2,4-DCP的反應(yīng)機(jī)理.Fe/Ni@rGO復(fù)合材料對2,4-DCP的去除為吸附和脫氯協(xié)同作用的結(jié)果,反應(yīng)初期主要為物理吸附作用,復(fù)合材料通過p-p作用力將2,4-DCP分子從溶液主體吸附至材料表面,隨后吸附在材料上的2,4-DCP分子通過孔道擴(kuò)散到達(dá)反應(yīng)活性位點(diǎn),與Ni催化劑形成過渡態(tài)絡(luò)合物Ni…Cl…R,同時(shí)Ni吸附氫氣并分解成活性氫原子,發(fā)生加氫脫氯反應(yīng),產(chǎn)生最終脫氯產(chǎn)物苯酚,并逐漸釋放至溶液中.

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Reduced graphene oxide supported Fe/Ni nanocomposites for 2,4-dichlorophenol removal.

HUANG Xue-zheng1,2,3*, ZHANG Yong-xiang2, ZHANG Da-sheng4, ZHU Xin-feng1, LI Hou-yun1

(1.Henan Province Key Laboratory of Water Pollution Control and Rehabilitation Technology, Henan University of Urban Construction, Pingdingshan 467000, China；2.Department of Urban Construction, Beijing University of Technology, Beijing 100124, China；3.School of Civil Engineering, Nanyang Institute of Technology, Nanyang 473000, China；4.Hebei Institute of Water Science, Shijiazhuang 050051, China)., 2023,43(12)：6352～6362

Reduced graphene oxide supported Fe/Ni nanocomposites were prepared for the rapid and effective adsorption and dechlorination of 2,4-dichlorophenol (2,4-DCP) by using liquid phase reduction method. The morphological characterization showed that the spherical Fe/Ni bimetallic nanoparticles with the size of 80～150nm were successfully inserted into the graphene sheets and mainly distributed at the edges and folds of the graphene sheets. The agglomeration of Fe/Ni nanoparticles decreased significantly. XRD patterns and FTIR analysis showed nZVI nanoparticles were successfully embedded into graphene through Fe-O bond, Fe/Ni bimetallic nanoparticles had poor crystallinity and amorphous iron oxide which covered the outer layer of nanoparticles. The effects of different preparation conditions such as carbon iron ratio, nickel loading and reduction degree of graphene oxide on the removal of 2,4-DCP were discussed. The optimum preparation conditions of the Fe/Ni@rGO composites are as follows: the mass ratio of graphene to Fe is 1:2, the Ni loading is 5%, and the molar ratio of NaBH4to Fe2+is 5:1. The adsorption and dechlorination perfermance of 2,4-DCP by nZVI, Fe/Ni, Fe@rGO composites and Fe/Ni@rGO composites were compared and analyzed. The results showed that the removal efficiency of 2,4-DCP by five materials followed the sequence: Fe/Ni@rGOcomposites>Fe/Ni>rGO>Fe@rGOcomposites>nZVI. However, the cycle test and storage stability test showed: compared with Fe/Ni bimetallic, Fe/Ni@rGO composites had stable reactivity activity and high reruse value. The results demonstrated the removal mechanism of 2,4-DCP by Fe/Ni@rGO composites was the synergistic effect of adsorption and dechlorination.

graphene；2,4-dichlorophenol；nanoscale zerovalent iron；dechlorination；adsorption

X52

A

1000-6923(2023)12-6352-11

黃雪征,張永祥,張大勝,等.石墨烯負(fù)載鐵鎳復(fù)合材料去除水中的2,4-二氯酚 [J]. 中國環(huán)境科學(xué), 2023,43(12):6352-6362.

Huang X Z, Zhang Y X, Zhang D S, et al. Reduced graphene oxide supported Fe/Ni nanocomposites for 2,4-dichlorophenol removal [J]. China Environmental Science, 2023,43(12):6352-6362.

2023-04-28

河南省科技攻關(guān)項(xiàng)目(202102310609);國家重點(diǎn)研發(fā)計(jì)劃子課題(2016YFC040140402);河南省高等學(xué)校重點(diǎn)科研項(xiàng)目(21A610009)

* 責(zé)任作者, 副教授, 58626472@qq.com

黃雪征(1978-),男,河南南陽人,副教授,博士,主要從事環(huán)境功能材料研究,場地污染治理修復(fù).發(fā)表論文30余篇.58626472@qq.com.