孫博宇,韓東梅,馬偉芳*

rGO@nZVI-BC修飾的PRB增強(qiáng)糖皮質(zhì)激素還原轉(zhuǎn)化性能研究

孫博宇1,韓東梅2,馬偉芳1*

(1.北京林業(yè)大學(xué)環(huán)境科學(xué)與工程學(xué)院,北京 100083；2.北京節(jié)能環(huán)保中心,北京 101160)

為制備環(huán)境功能材料rGO@nZVI-BC,以此材料構(gòu)建可滲透反應(yīng)墻(PRB)實(shí)現(xiàn)對(duì)地下水中含氟糖皮質(zhì)激素(FGCs)的有效阻控.結(jié)果表明,阻控過(guò)程可用改進(jìn)Yoon-Nelson模型描述,在rGO@nZVI-BC和土壤系統(tǒng)中吸附速率和生物降解速率常數(shù)分別為0.485和0.035d?1、0.233和0.029d?1.提升阻控效能的主要機(jī)理是強(qiáng)化吸附和生物降解功能.其中運(yùn)行初期以吸附作用占主導(dǎo),貢獻(xiàn)比為76.12%左右.生物強(qiáng)化機(jī)制主要為nZVI為GCs還原提供了電子供體,從而增強(qiáng)了還原脫氟功能,該部分貢獻(xiàn)比約為87.06%,并伴隨著脫羥基、氧化側(cè)鏈降解和開(kāi)環(huán)等降解.此外具有還原性脫鹵屬的功能性微生物(和)物種豐度的增加進(jìn)一步索證了功能材料rGO@nZVI-BC的效能.本研究為地下水中GCs的污染阻控提供了一種有效的方法.

可滲透反應(yīng)墻；地下水；含氟糖皮質(zhì)激素；還原降解

地下水是水資源系統(tǒng)中最重要的組成部分,然而,隨著人類(lèi)活動(dòng)的加劇導(dǎo)致對(duì)地下水的需求不斷增加和地下水污染風(fēng)險(xiǎn)提升,造成了嚴(yán)重的地下水污染[1-2].其中糖皮質(zhì)激素(GCs)作為一種重要的類(lèi)固醇激素,主要通過(guò)人類(lèi)和牲畜的尿液和糞便排泄物、醫(yī)院、制藥廠(chǎng)廢水的排放等途徑進(jìn)入環(huán)境中[3].中國(guó)是最大的醫(yī)藥產(chǎn)品生產(chǎn)國(guó)和消費(fèi)國(guó),目前,在中國(guó)各種環(huán)境介質(zhì)均檢測(cè)到GCs.在南海歡呼灣地區(qū)檢測(cè)到2種GCs(皮質(zhì)醇和潑尼松龍)濃度可達(dá)40ng/L[4].北京清河及沿河淺層地下水中檢測(cè)到GCs濃度分別為476ng/L和65ng/L[5-6].北京的永定河和天津南部的排水河中水化可的松的濃度為4.96～ 8.32ng/L[6-7].很多報(bào)道表明即使在環(huán)境濃度水平之下,GCs也會(huì)影響動(dòng)物的性腺發(fā)育,導(dǎo)致畸形生長(zhǎng)等不利影響[8].此外,含氟糖皮質(zhì)激素(FGCs)的毒性作用比天然GCs高數(shù)百倍.因此,我們應(yīng)對(duì)FGCs的生態(tài)毒性給予更多的關(guān)注.

在進(jìn)行地下水治理的過(guò)程中,傳統(tǒng)的抽出凈化的方式使用的最為廣泛[9],但是在管理的過(guò)程中耗費(fèi)的各項(xiàng)費(fèi)用過(guò)高,并且過(guò)分的抽取地下水容易出現(xiàn)地下水位的下沉.可滲透反應(yīng)墻(PRB)作為污染地下水修復(fù)的前景的技術(shù)[10],其特點(diǎn)是運(yùn)行和維護(hù)成本低、效率高、避免地下水大量流失.理論上,PRB系統(tǒng)內(nèi)的污染物去除主要發(fā)生在反應(yīng)介質(zhì)區(qū)域.因此,主要目標(biāo)是將污染物質(zhì)帶入這個(gè)反應(yīng)區(qū),然后利用吸附、沉淀和還原等過(guò)程來(lái)有效地去除和穩(wěn)定污染物.由于零價(jià)鐵(ZVI)是負(fù)標(biāo)準(zhǔn)還原電位(0(Fe2+/Fe0) = -0.4402V),因此常常作為優(yōu)良的電子供體被廣泛應(yīng)用于處理地下水中的各種有毒污染物[11-12].納米級(jí)ZVI(nZVI)(粒徑<100nm)的還原能力大大增加,因?yàn)榘殡S粒徑減小其表面積增加巨大. nZVI可以通過(guò)還原或吸附來(lái)去除污染物,已被用于有效降解多種氯化有機(jī)物和固定重金屬[13-14].然而, ZVI的主要局限性是由于其固有的鈍化層而導(dǎo)致的低反應(yīng)性[15].為了提高其反應(yīng)速率并減少鈍化,可以制備納米零價(jià)鐵(nZVI)并將其負(fù)載到多孔材料上,還原氧化石墨烯(rGO)和生物炭(BC)則為不錯(cuò)的選擇.石墨烯(GP)具有比表面積大、良好的導(dǎo)電性和強(qiáng)機(jī)械強(qiáng)度而被開(kāi)發(fā)為一種新型催化劑載體[16].氧化石墨烯(GO)和rGO是兩種石墨烯衍生物.GO已被證明是一種良好的載體,并用于去除有機(jī)染料[17].與GO相比,rGO具有更高的導(dǎo)電率(約3個(gè)數(shù)量級(jí))、更高的表面積、高效的載流子遷移率[18-19].因此將nZVI裝載在rGO載體上不僅可以提高吸附能力,還能防止nZVI的聚集,提高反應(yīng)性能.BC是一種天然材料,去除重金屬、非金屬和新型污染物方面已經(jīng)得到了很好的研究[20-21].鑒于其巨大的表面積,我們可以將其作為載體,來(lái)負(fù)載其他材料,從而起到良好的分散作用.

目前有關(guān)FGCs在水環(huán)境中的去除及降解行為還研究尚淺,且去除效果令人擔(dān)憂(yōu).對(duì)于此,本文通過(guò)液相還原法合成rGO@nZVI-BC復(fù)合材料,該材料不僅能夠提高吸附能力,還可以提高電子供體的濃度,促進(jìn)微生物對(duì)FGCs的還原脫氟[8].對(duì)于FGCs來(lái)說(shuō),生物還原脫鹵是一種有效的解毒途徑.眾所周知的環(huán)境激素(EDC)生物降解屬,即鞘氨醇單胞菌屬、假單胞菌屬和脫硫單胞菌屬,對(duì)FGCs有很好的除去作用,具體包括氧化、氫化、水解和脫氟等作用[8,22-23]. FGCs的生物還原脫氟需要兩個(gè)電子.因此,可用電子給體的數(shù)量直接影響去除效果.rGO@nZVI-BC中的nZVI可用于通過(guò)氧化增加電子供體的數(shù)量,來(lái)強(qiáng)化對(duì)FGCs的生物降解.因此,以rGO@nZVI-BC修飾的PRB有利于增強(qiáng)對(duì)FGCs還原和吸附作用,因?yàn)檫@種rGO@nZVI-BC復(fù)合材料對(duì)FGCs而言,不僅可以為微生物還原脫鹵提供電子供體,還可以為微生物提供吸附電位和結(jié)合位點(diǎn),這有助于污染物吸附和降低土壤的生物毒性[24].然而,鮮有研究報(bào)道以rGO@nZVI-BC修飾的PRB來(lái)處理地下水中微量有機(jī)污染物污染的阻控效果及機(jī)理研究.因此,本研究對(duì)于PRB處理FGCs在地下水原位修復(fù)過(guò)程中具有重要指導(dǎo)意義.

本文將曲安西龍(TRA)作為污染物質(zhì),來(lái)進(jìn)行PRB處理TRA污染地下水的研究.具體目標(biāo)如下:(i)確定以rGO@nZVI-BC修飾的PRB對(duì)于TRA的去除效能;(ii)闡明TRA的代謝途徑;(iii)確定強(qiáng)化微生物對(duì)TRA的去除效能研究.

1 材料與方法

1.1 材料

GO是通過(guò)改進(jìn)的hummers法制備的.1g石墨粉在冰浴的條件下與50mL H2SO4和1g NaNO3反應(yīng)30min,后加入6g KMnO4攪拌30min.后撤掉冰浴,滴加50mL去離子水,使反應(yīng)物溫度維持在35～40℃.滴加完畢后迅速轉(zhuǎn)移到95℃水浴中繼續(xù)磁力攪拌加熱30min,后加入200mL 3% H2O2,200mL 10% HCl,靜置過(guò)夜,后8000rmp離心清洗,至pH值呈中性后,呈黃褐色膠體.后冷凍干燥24h,得到GO.

rGO@nZVI-BC復(fù)合材料是通過(guò)液相還原法制備的.將0.5g GO分散于150mL去離子水中,超聲2h得到GO分散液,將溶液放置于三口燒瓶中.后加入100mL 0.05mol/L的FeSO47H2O溶液和2g椰殼生物炭(購(gòu)買(mǎi)自鞏義市淼源水處理材料有限公司),樣品磁力攪拌2h.后緩慢加入100mL 1mol/L的NaBH4溶液,反應(yīng)4h(式 (1)).用無(wú)水乙醇,去離子水洗滌產(chǎn)物,然后冷凍干燥12h,得到rGO@nZVI-BC.

Fe2++2BH4-+6H2O=Fe0↓+2H2BO3-+2H++7H2↑ (1)

1.2 實(shí)驗(yàn)方法

1.2.1 實(shí)驗(yàn)裝置 本研究中使用PRB模擬柱來(lái)模擬吸附-生物降解過(guò)程及對(duì)TRA污染地下水的去除性能.該系統(tǒng)由儲(chǔ)水罐、PRB反應(yīng)器和輸水裝置組成.PRB填料的高度為3cm,上下各為6cm的土壤,如圖1所示.本實(shí)驗(yàn)裝置放置于相對(duì)封閉的有機(jī)玻璃罩之內(nèi),共設(shè)置2組反應(yīng)器,作為土壤+功能材料(rGO@nZVI-BC)實(shí)驗(yàn)組(rGO@nZVI-BC質(zhì)量占比為40%)及100%土壤對(duì)照組.使用滅菌處理平行設(shè)置吸附控制實(shí)驗(yàn).用蒸餾水配置100μg/L的TRA,流速為0.1cm/d,整個(gè)實(shí)驗(yàn)過(guò)程在黑暗條件下進(jìn)行,定期采集樣品,測(cè)定TRA、氟離子和總鐵離子的濃度.

圖1 可滲透反應(yīng)屏障(PRB)

1.2.2 表征方法 將GO,rGO@nZVI, rGO@nZVI- BC樣品安裝在碳帶上,并使用Phenom ProX 掃描電子顯微鏡(SEM)(SU 8020,日立,日本)進(jìn)行形態(tài)分析.通過(guò)X射線(xiàn)衍射儀(XRD)(Bruker Company, Germany)分析物相組成的晶型結(jié)構(gòu).通過(guò)傅里葉變換紅外(FTIR)光譜儀(Bruker Vertex 80)分析官能團(tuán).通過(guò)X射線(xiàn)光電子能譜(XPS)(Thermo Scientific Escalab 250Xi)表征表面元素含量和表面官能團(tuán).在表面積和孔隙度分析儀(ASAP,2460,麥克,USA)上測(cè)量表面積和孔結(jié)構(gòu).使用電化學(xué)工作站(CHI 660E)進(jìn)行電化學(xué)交流阻抗(EIS)表征.

1.2.3 分析方法 TRA的濃度采用高效液相色譜(HPLC)測(cè)定,配備C18分析HPLC柱(100.0mm × 2.1mm; particle size of 1.7 μm; Waters, USA) 以及一個(gè)1200個(gè)二極管陣列探測(cè)器,其波長(zhǎng)為241nm (Agilent 1290, Agilent Corporation, USA).用于TRA分析的流動(dòng)相為40%乙腈,流速為1mL/min.該方法具有良好的可靠性和準(zhǔn)確性,TRA及其代謝物的回收率為98%～106%.采用超高效液相色譜和高分辨率四極飛行時(shí)間質(zhì)譜儀(UHPLC-QTOF) (Agilent 6530Q-TOF, Millford, USA)分析了rGO@nZVI-BC去除TRA過(guò)程中的主要脫氟中間體.采用離子色譜法(IC3000,Dionex,USA)測(cè)定溶液中的氟離子濃度;用原子吸收法(GFA-6880,SHIMADZU,USA)測(cè)定溶液中總鐵的濃度.土壤樣品采集于潮白河沿岸的土壤.按“S”型采集20～40cm土層土壤,過(guò)篩之后保存,根據(jù)《土壤農(nóng)化分析》測(cè)定分析土壤有機(jī)質(zhì)、全N、全P、全K含量.哈希PH計(jì)測(cè)定土壤pH,烘干法測(cè)定土壤含水量,用環(huán)刀法測(cè)定土壤容重.土壤的理化性質(zhì)如表1所.

表1 土壤的理化性質(zhì)

采用改進(jìn)Yoon-Nelson模型分析了rGO@nZVI-BC和土壤中TRA的穿透模型(式(2)).

式中:1是吸附速率常數(shù),1/d;2是生物降解速率常數(shù),1/d;為穿透一半吸附材料所需要的時(shí)間,d;為運(yùn)行時(shí)間,d;0為入口污染物的濃度,μg/L;為出口污染物的濃度,μg/L.

1.2.4 微生物的分析方法 在反應(yīng)器運(yùn)行30d和60d時(shí),分別采集0.5g土壤樣品,采用標(biāo)準(zhǔn)強(qiáng)力土壤DNA提取試劑盒(OMEGA,CA,USA)提取DNA.所有DNA溶液均于-20℃保存.通過(guò)使用引物515FmodF(5～GTGYCAGCMGCCGCGGTAA～3)和806RmodR(GGACTACNVGGGTWTCTAAT)靶向16S rRNA基因的V4～V5區(qū)域來(lái)分析細(xì)菌群落.使用QIIME軟件包進(jìn)行序列分析,使用內(nèi)部Perl腳本測(cè)量-和-多樣性.

1.2.5 統(tǒng)計(jì)數(shù)據(jù)分析方法 本研究中的所有實(shí)驗(yàn)均為一式三份.使用SPSS軟件(Version 10.0, SPSS Inc., Chicago, IL, USA)進(jìn)行方差分析.研究采用典型的Rs(RctCPE)等效電路模型,利用ZView 3.0軟件擬合交流阻抗.

2 結(jié)果與分析

2.1 功能材料的特性表征

2.1.1 形貌特征分析 利用SEM對(duì)材料的形貌進(jìn)行了分析,如圖2所示,由于GO獨(dú)特的材料特性,其表面分布有大量褶皺和空穴,這些褶皺和空穴負(fù)載nZVI提供了大量的位點(diǎn)[25](圖2a);nZVI以突出的鏈狀團(tuán)聚體的形式存在[26],GO起到了分散劑的作用,nZVI基本上均勻分散在半透明的褶皺上(圖2b);rGO@nZVI均勻分布在生物炭的表面和孔隙中(圖2c).

圖2 掃描電子顯微鏡圖像(′8000)

Fig.2 Scanning electron microscope images of

(a) GO; (b) rGO@nZVI;(c) rGO@nZVI-BC

2.1.2 晶格結(jié)構(gòu)分析 GO、rGO@nZVI和rGO@nZVI-BC在25°C下10°到80°范圍內(nèi)的X射線(xiàn)衍射圖,如圖3所示.

圖3 GO,rGO@nZVI和rGO@nZVI-BC的X射線(xiàn)衍射圖像

GO在10.36°處可以清楚地觀(guān)察到衍射峰,該衍射峰對(duì)應(yīng)于純GO樣品的(002)面[27].由于GO表面存在含氧官能團(tuán),因此意味著GO可以為裝載納米材料提供很大的空間[28].在rGO@nZVI和rGO@nZVI-BC的44.9°處可以看到nZVI的清晰衍射峰.此外,rGO@nZVI的XRD圖在33.16°、35.38°處出現(xiàn)了不同程度的特征峰,對(duì)應(yīng)于氧化鐵的衍射峰[29],但峰型較小,說(shuō)明氧化程度不明顯.而且rGO@nZVI中10.36°處最強(qiáng)烈的GO峰的缺失證實(shí)了GO被完全剝離.對(duì)于rGO@nZVI-BC復(fù)合材料,氧化鐵的特征峰出現(xiàn)了較明顯的減弱,26°左右的寬峰被指定為具有無(wú)定形結(jié)構(gòu)的碳材料中的(002)平面[30],44.9°處的衍射峰對(duì)應(yīng)于nZVI的(110)平面[31],這表明rGO@nZVI顆粒成功沉積在BC表面.

2.1.3 官能團(tuán)分析 在500～4000cm-1的波長(zhǎng)范圍內(nèi)對(duì)GO、rGO@nZVI和rGO@nZVI-BC復(fù)合材料進(jìn)行傅里葉紅外光譜分析,如圖4所示.

圖4 GO,rGO@nZVI和rGO@nZVI-BC的傅里葉變換紅外光譜圖

顯然,裸GO光譜在3384cm-1、1724cm-1、1610cm-1、1216cm-1和1062cm-1處顯示出明顯的特征吸收峰,分別對(duì)應(yīng)于-OH、-C=O、-C=C、-O=O和-CO鍵的伸縮振動(dòng)[32].可以觀(guān)察到,這些峰在rGO@nZVI的光譜中大幅度減弱甚至消失,推測(cè)在nZVI的制備過(guò)程中GO中的含氧官能團(tuán)被有效地還原,也可能發(fā)生了金屬配位作用,例如-C=O和-CO參與了金屬鍵合[33].此外,與GO相比, rGO@nZVI在621cm-1附近的特征峰主要?dú)w因于-FeO的伸縮振動(dòng)[32].說(shuō)明GO與nZVI的結(jié)合主要通過(guò)Fe-O鍵來(lái)完成[34].說(shuō)明在rGO@nZVI-BC的圖譜上可以看到GO和nZVI的特征吸收峰,這些結(jié)果表明rGO@nZVI已成功加載到BC上.

表2 土壤,GO,rGO@nZVI和rGO@nZVI-BC的表面積和孔隙度

2.1.4 比表面積及孔容孔徑分析 由表2可知, GO、rGO@nZVI和rGO@nZVI-BC的比表面積大于土壤的比表面積,rGO@nZVI-BC的比表面積最大,為648.940m2/g.此外,由于鏈狀聚集體nZVI沉積在GO表面,rGO@nZVI的比表面積增加了1.17倍.rGO@nZVI的總孔容略低,這是由于負(fù)載在rGO表面的nZVI阻塞了微孔,微孔體積與GO相比減少了44.78%[35].在負(fù)載到BC上時(shí),與rGO@nZVI相比,微孔體積增加了7.76倍.同時(shí),由于rGO@nZVI的負(fù)載,rGO@nZVI-BC的孔徑增加了0.955nm.rGO@nZVI- BC的形態(tài)發(fā)生了變化,呈現(xiàn)出具有大孔徑的蓬松材料[36].

圖5 rGO@nZVI-BC和BC的交流阻抗譜

(a)全光譜rGO@nZVI-BC;(b)rGO@nZVI-BC的鐵的Fe2p譜圖;(c)rGO@nZVI-BC的C1s譜圖;(d)rGO@nZVI-BC的O1s譜圖

2.1.5 電化學(xué)交流阻抗分析 rGO@nZVI-BC和BC的電化學(xué)交流阻抗分析表明rGO@nZVI-BC功能材料具有良好的電化學(xué)性能,如圖5所示.根據(jù)擬合, rGO@nZVI-BC和BC的電荷傳遞電阻分別為0.709和4.740Ω.在低頻率區(qū)域范圍內(nèi),交流阻抗譜越接近90°,說(shuō)明電化學(xué)性能越接近理想.對(duì)比兩條曲線(xiàn)可以發(fā)現(xiàn),rGO@nZVI-BC的曲線(xiàn)在低頻區(qū)最接近垂直,表明電解質(zhì)離子可以最快地與rGO@nZVI-BC材料充分接觸,展現(xiàn)了其良好的電子傳遞效能.

2.1.6 元素的定性定量及固體表面分析 XPS分析用于表征rGO@nZVI-BC表面的元素組成和鐵、氧和碳的價(jià)態(tài),如圖6所示.rGO@nZVI-BC的表面元素是鐵、碳和氧(圖6a).由C(1S)分峰圖(圖6c)看出,結(jié)合能在284.7,285.8和289.1eV分別對(duì)應(yīng)C-C、C-O、和O-C=O.由O(1S)分峰圖(圖6d)看出,結(jié)合能在531.0,532.5和533.4eV分別對(duì)應(yīng)O2-、OH-和H2O.由Fe(2p)分峰圖(圖6b)看出,結(jié)合能在706.7、711.2和713.1eV分別對(duì)應(yīng)nZVI、Fe2+、和Fe3+[37-38].因此,結(jié)果證明了nZVI的存在及其外層被氧化為不同的氧化態(tài),這與XRD的結(jié)果一致.而且,BC中GO的存在已被證明有助于阻止鐵被氧化成更多的氧化物.XPS結(jié)果表明rGO@nZVI成功負(fù)載,形成穩(wěn)定的生物炭復(fù)合材料.

2.2 rGO@nZVI-BC修飾的PRB對(duì)TRA的強(qiáng)化去除效能及機(jī)制

2.2.1 rGO@nZVI-BC修飾的PRB對(duì)TRA去除效能研究 隨著時(shí)間的延長(zhǎng),TRA的出水濃度逐漸增加,穿透過(guò)程呈“S”型曲線(xiàn),如圖7所示.在整個(gè)穿透過(guò)程中,rGO@nZVI-BC表現(xiàn)出比土壤更強(qiáng)的去除能力.rGO@nZVI-BC系統(tǒng)中污染物的高去除效果不僅是由于吸附容量的增加(rGO@nZVI-BC對(duì)TRA的飽和吸附容量為46.816μg/g,而土壤則僅為0.628μg/g,rGO@nZVI-BC的飽和吸附容量比土壤系統(tǒng)高74.55倍.rGO@nZVI-BC材料具有較大的比表面積和表面吸附活性點(diǎn)位.GO含有大量含氧官能團(tuán),使其富含高負(fù)電荷密度,增強(qiáng)了層間靜電排斥效應(yīng)從而降低了因范德華力吸引導(dǎo)致的層間團(tuán)聚,因此可為nZVI顆粒提供穩(wěn)定的位點(diǎn),以防止其氧化和聚集,而且其巨大的表面積增強(qiáng)了對(duì)污染物的吸附能力[39].與此同時(shí),BC的加入不僅增強(qiáng)其吸附性能,而且也增加了其穩(wěn)定程度),還歸因于促進(jìn)生物降解和電子轉(zhuǎn)移效率的微生物豐度和多樣性的增加[40]. rGO@nZVI-BC系統(tǒng)的穿透一半吸附材料所需要的時(shí)間(τ)高于土壤系統(tǒng),同時(shí)土壤系統(tǒng)也較早達(dá)到飽和,這可以從改進(jìn)Yoon-Nelson穿透模型中獲得,如表3所示.與土壤系統(tǒng)相比,rGO@nZVI-BC吸附速率常數(shù)和生物降解速率常數(shù)均高于土壤系統(tǒng),從改進(jìn)Yoon-Nelson模型看出,土壤系統(tǒng)的k1和k2分別為0.233和0.029d?1,rGO@nZVI-BC系統(tǒng)的k1和k2分別為0.485和0.035d?1.吸附率和生物降解率分別提高了2.08倍和1.21倍.其中rGO@nZVI-BC滅菌實(shí)驗(yàn)組的吸附速率為0.362d?1,且反應(yīng)器運(yùn)行100d時(shí)出水濃度已接近進(jìn)水濃度,證明完全穿透,也從側(cè)面證明了rGO@nZVI-BC系統(tǒng)中的生物降解作用.因此,Yoon-Nelson模型適用于柱試驗(yàn)中污染物突破曲線(xiàn)的模擬和預(yù)測(cè).rGO@nZVI-BC與土壤相比,對(duì)地下水中的TRA具有更加長(zhǎng)期穩(wěn)定的去除效果,同時(shí)也利于強(qiáng)化土壤修復(fù)能力[41].為了確定rGO@nZVI-BC體系中電子傳輸?shù)幕钚?在以下實(shí)驗(yàn)中研究了脫氟作用和鐵氧化反應(yīng).

圖7 rGO@nZVI-BC修飾的PRB去除TRA的穿透曲線(xiàn)

表3 改進(jìn)Yoon-Nelson穿透模型模擬參數(shù)

2.2.2 rGO@nZVI-BC修飾的PRB對(duì)TRA脫氟效能研究 在rGO@nZVI-BC系統(tǒng)中,氟離子的濃度隨著反應(yīng)時(shí)間的增加而增加.這清楚地表明nZVI增強(qiáng)了TRA的還原脫氟作用,如圖8所示.還原脫鹵作用是TRA降解的重要代謝途徑[42].在此過(guò)程中,nZVI氧化為生物還原脫鹵提供了所需的電子供體.氟離子的濃度增加到0.78 μg/L,從而占總TRA脫氟的87.06%.總鐵濃度逐漸達(dá)到3.21mg/L,后鐵離子濃度下降,當(dāng)運(yùn)行到100d時(shí),鐵離子濃度下降到1.23mg/L.表明此時(shí)nZVI已消耗殆盡.溶解鐵的量與nZVI的氧化強(qiáng)度密切相關(guān),它代表了rGO@nZVI- BC和微生物之間的電子轉(zhuǎn)移強(qiáng)度[22].足夠的供電子底物的存在是增強(qiáng)TRA還原脫氟的關(guān)鍵因素.因此,nZVI是一種將TRA進(jìn)行高效降解的還原劑[38].雖然生物降解是TRA去除的主要機(jī)制,但吸附和生物降解這兩種機(jī)制對(duì)TRA去除的貢獻(xiàn)需要進(jìn)一步研究.

圖8 PRB系統(tǒng)中鐵的氧化和TRA的脫氟作用

2.2.3 rGO@nZVI-BC修飾的PRB對(duì)TRA去除的歸趨貢獻(xiàn)分析 測(cè)定土壤中TRA濃度,可以看出吸附和生物降解作用作為T(mén)RA去除的貢獻(xiàn)機(jī)制,在反應(yīng)初期,吸附作用為主要的去除作用,如圖9所示.

在以rGO@nZVI-BC為填料的PRB反應(yīng)墻的情況下,吸附作用占主導(dǎo),占比76.12%左右.而同一時(shí)期的土壤系統(tǒng)則已發(fā)生了滲漏,吸附作用僅為37.12%.這些結(jié)果與TRA的穿透曲線(xiàn)動(dòng)力學(xué)的分析一致,吸附發(fā)生在早期而生物降解在后期發(fā)揮了重要作用,因?yàn)樯锝到庵饾u取代吸附成為主要貢獻(xiàn)機(jī)制,吸附能力趨于迅速飽和.環(huán)境中多環(huán)有機(jī)化合物如PAHs、EDCs和PCBs的長(zhǎng)期去除機(jī)制是生物降解[43].此外,rGO@nZVI-BC系統(tǒng)中的nZVI為T(mén)RA還原提供了電子供體,尤其是氟的去除[36,44].為了分析增強(qiáng)的脫氟作用與nZVI之間的關(guān)系,我們提出了主要的代謝途徑,并在后續(xù)的實(shí)驗(yàn)中分析了微生物群落結(jié)構(gòu)的演變.

圖9 rGO@nZVI-BC去除TRA過(guò)程中的貢獻(xiàn)解析占比

2.2.4 TRA代謝途徑解析 TRA微生物代謝產(chǎn)物是根據(jù)源自母體化合物的元素組成、質(zhì)荷比、生化反應(yīng)途徑分析確定的.

基于本研究中出現(xiàn)的代謝產(chǎn)物,推測(cè)了三種主要的TRA生物降解途徑,即脫氟作用、脫羥基作用和氧化側(cè)鏈降解作用,如圖9所示.主要的降解途徑為還原脫氟(途徑Ⅰ),從C9的環(huán)B開(kāi)始?xì)浠?接受兩個(gè)電子形成一種還原性脫氟低毒代謝產(chǎn)物2 (C21H28O6,/=376. 19;(8S, 10R, 11S, 13S, 14S, 16R, 17S)-11, 16,17-trihydroxy-17-(2-hydroxyacetyl)-10, 13-dimethyl-6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17-dodecahydro-3H-cyclopenta[a]phenanthren-3-one).后C17之間的碳鏈斷開(kāi)以至D環(huán)上的支鏈斷裂,生成三羥基酮,代謝產(chǎn)物3(C19H26O4,/=318.18;(8S, 10R, 11S, 13S, 14S, 16R, 17S)-11, 16, 17- trihydroxy-10, 13-dime-thyl-6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17-dodecahydro-3H-cyclopenta[a] phenanthren-3-one).代謝物3進(jìn)一步脫羥基生成羥基酮,代謝產(chǎn)物10(C19H26O2,/=286. 19,8S, 10R, 11S, 13S, 14S)-11-hydroxy-10, 13-dimethyl-6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17-dodecah-ydro-3H- cyclopenta[a] phenanthren-3-one),此反應(yīng)類(lèi)似于雌二醇的生物降解機(jī)制[45].第二個(gè)代謝途徑(途徑II)主要以脫羥基作用為主.通過(guò)C17的D環(huán)脫羥基生成代謝產(chǎn)物4(C21H27FO5,/=378. 18;8S, 9R, 10S, 11S, 13S, 14S, 16R, 17R)-9-fluoro-11, 16-dihydroxy-17- (2-hydroxyacetyl)-10, 13-dimethyl-6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17-dodecahydro-3H-cyclopenta [a]phenanthren-3-one),該反應(yīng)類(lèi)似于對(duì)四環(huán)素的生物降解[46].后推測(cè)通過(guò)水解作用生成代謝產(chǎn)物5,后通過(guò)脫氟脫羥基作用生成代謝產(chǎn)物10.

圖10 提出的TRA代謝途徑

途徑I分別占rGO@nZVI-BC和土壤系統(tǒng)地下水中TRA衰減的87.10%和20.01%.代謝途徑分析結(jié)果表明, rGO@nZVI-BC的加入改善了TRA的還原降解,尤其是還原脫氟作用.氧化分解途徑(途徑Ⅲ)源于D環(huán)C16和C17處羥基的氧化,根據(jù)氧化程度的不同,生成代謝產(chǎn)物6(C19H21FO4, m/z=332. 14; (8S, 9R, 10S, 11S, 13S, 14S)-9-fluoro-11-hydroxy- 10, 13-dimethyl-7, 8, 9, 11, 12, 13, 14, 15- octahydro-3H-cyclopenta [a] phenanthrene-3, 16, 17(6H, 10H)- trione)跟代謝產(chǎn)物7(C19H23FO4,m/ z=334. 16;(8S, 9R, 10S, 11S, 13S, 14S)-9-fluoro-11, 16-dihydroxy-10, 13-dimethyl-7, 8, 9, 10, 11, 12, 13, 14, 15, 16- decahydro-3H-cyclopenta[a] phenan- threne-3, 17 (6H)-dione).代謝產(chǎn)物6和代謝產(chǎn)物7進(jìn)一步發(fā)生羥基化、氫化和脫氟作用形成代謝產(chǎn)物10.代謝產(chǎn)物10通過(guò)苯環(huán)加成和A,C和D環(huán)開(kāi)環(huán)進(jìn)一步降解,生成代謝產(chǎn)物11(C16H30O4,/=286.21; 2-hydroxy-3-((S)-1-hydroxybutyl)-4-propylcyclohexylpropionate).隨后代謝產(chǎn)物11的側(cè)鏈在環(huán)B的C8和C9位置發(fā)生斷裂,生成cyclohexane-1, 2-diol(代謝產(chǎn)物12, C6H12O2,/=116.08).代謝產(chǎn)物12進(jìn)一步氧化開(kāi)環(huán)形成6-hydroxyhexanoic acid(代謝產(chǎn)物13,C6H12O3,/=132.08).功能微生物群落結(jié)構(gòu)和多樣性在TRA生物降解過(guò)程中發(fā)揮了重要作用[19].為了分析微生物的去除代謝與rGO@nZVI-BC促進(jìn)作用之間的關(guān)系,在以下實(shí)驗(yàn)中分析了微生物群落的演變.

2.3 強(qiáng)化微生物對(duì)TRA的去除效能研究

2.3.1 微生物多樣性和豐富度的變化 培養(yǎng)30d和60d后,收集兩個(gè)反應(yīng)器中的微生物樣品進(jìn)行分析.在V4～V5區(qū)域,從5個(gè)PRB反應(yīng)介質(zhì)層土壤樣本中檢索到16Sr RNA基因序列,序列范圍為29676到33894.這些序列進(jìn)一步聚類(lèi)為1888～2223個(gè)OUT,如表3所示.在所有樣本中,OTU隸屬于36門(mén)105綱557屬的已知細(xì)菌.由于添加了TRA,各個(gè)體系的微生物群落豐富度和多樣性較對(duì)照CK有所下降.這樣的結(jié)果也證明了TRA的內(nèi)分泌干擾特性會(huì)對(duì)天然土壤中的微生物多樣性和豐度產(chǎn)生負(fù)面影響.進(jìn)一步對(duì)添加了rGO@nZVI-BC的微生物多樣性和豐富度分析表明,與土壤體系相比,Shannon指數(shù)和Chao1指數(shù)分別增加了約1.08～1.11倍和1.14～1.16倍,同時(shí)對(duì)比30d和60d的樣本,微生物多樣性隨著生物降解反應(yīng)時(shí)間的增加而增加,這也說(shuō)明了微生物群落多樣性和豐度有隨時(shí)間恢復(fù)的趨勢(shì).

2.3.2 屬水平微生物群落結(jié)構(gòu)的分析 通過(guò)分析具有降解復(fù)雜有機(jī)化合物潛力的10種細(xì)菌的屬,可以看出其相對(duì)豐度與CK相比有明顯變化,如圖11所示.rGO@nZVI-BC系統(tǒng)中主要的兩個(gè)屬包括和,它們屬于,具有降解糖皮質(zhì)激素和內(nèi)分泌干擾的化學(xué)物質(zhì)[47-48].在培養(yǎng)了60d后,其相對(duì)豐度與CK對(duì)比分別增加了30.56倍(0.55%～16.87%)和5.12倍(2.21%～11.32%)且與TRA的脫氟率成正相關(guān)(=0.97,=0.95;<0.05).是受污染土壤中多環(huán)芳烴的生物降解劑,可能有助于去除TRA[49].會(huì)生物降解有毒有機(jī)化合物,如多環(huán)芳烴和EDCs[49].因此,被認(rèn)為是TRA降解的促成因素之一.經(jīng)過(guò)30d和60d的馴化,相對(duì)豐度為7.37-10.21%的成為rGO@nZVI-BC系統(tǒng)中第三大優(yōu)勢(shì)屬.研究表明對(duì)重金屬和有機(jī)質(zhì)污染具有較強(qiáng)的抗性,是一種潛在的土壤生物修復(fù)細(xì)菌[50].第四豐富的屬,即,屬于微桿菌科.該菌屬以多環(huán)有機(jī)化合物(如五溴二苯醚或氫化可的松)作為碳源參與還原脫鹵[51].連續(xù)運(yùn)行60d后, rGO@nZVI-BC的相對(duì)豐度比CK高9.76倍(1.57%～ 15.35%).的相對(duì)豐度與TRA脫氟率呈正相關(guān)(=0.92,<0.05).在rGO@nZVI-BC系統(tǒng)中,和屬于.與CK相比,其相對(duì)豐度分別增大了5.99倍(0.86%～5.12%)和74.70倍(0.15%～ 11.5%).作為土壤中常見(jiàn)的菌屬,常作為復(fù)雜有機(jī)污染物的降解聚生體[52].同時(shí)也有助于去除多環(huán)和鹵素有機(jī)物,如呋喃有機(jī)化合物、六氯環(huán)己烷和氯化聯(lián)苯[48].屬含有一個(gè)加氧酶編碼基因,該基因與TRA脫鹵后的有效氧化和開(kāi)環(huán)有關(guān). rGO@nZVI-BC中第七豐富的屬,的相對(duì)豐度在連續(xù)運(yùn)行60d后,比CK高2.58倍(1.82%～ 4.69%).因?yàn)樗梢陨锝到夥枷阕寤衔颷53-54]. Pseudomonas的相對(duì)豐度與rGO@nZVI-BC系統(tǒng)的脫氟率呈正相關(guān)(=0.98;<0.05).屬于Burkholderiaceae的兩個(gè)屬,即Hydrogenophaga和Paraburkholderia,可以降解TRA[55-56].連續(xù)運(yùn)行60d后,與還原性脫鹵降解率呈正相關(guān)(=0.93,=0.98;<0.05).經(jīng)過(guò)30d和60d的馴化,rGO@nZVI-BC系統(tǒng)中相對(duì)豐度為1.28～2.41%的Hyphomicrobium成為第十優(yōu)勢(shì)屬.Hyphomicrobium是一種對(duì)二甲基亞砜和多環(huán)芳烴有良好降解效果的菌屬,其豐度與TRA脫氟率呈正相關(guān)[57](=0.98;<0.05).一般來(lái)說(shuō),微生物向具有降解或電子轉(zhuǎn)化功能的群落的進(jìn)化及其協(xié)同作用有助于有效去除TRA.

表3 rGO@nZVI-BC修飾的PRB系統(tǒng)中細(xì)菌種群多樣性指數(shù)

圖11 rGO@nZVI-BC修飾的PRB系統(tǒng)在屬水平上的微生物群落結(jié)構(gòu)

3 結(jié)論

3.1 制備的rGO@nZVI-BC材料具有核-殼結(jié)構(gòu)和較高的比表面積特性,同時(shí)以rGO@nZVI-BC修飾的PRB可以有效強(qiáng)化TRA的固定和生物降解.

3.2 rGO@nZVI-BC功能材料的核殼結(jié)構(gòu)不僅提高了吸附容量,而且提供了生物降解所需要的電子,從而促進(jìn)了TRA的還原脫氟效能.與土壤系統(tǒng)相比,生物降解作用提高了約1.21倍.

3.3 在rGO@nZVI-BC系統(tǒng)中,微生物群落演化為具有還原脫鹵、水解和氧化降解功能的微生物,從而來(lái)強(qiáng)化微生物對(duì)TRA的去除效能研究.

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Enhancement of glucocorticoid reduction transformation by PRB modified with rGO@nZVI-BC.

SUN Bo-yu1, HAN Dong-mei2, MA Wei-fang1*

(1.School of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, China；2.Beijing Energy Conservation and Environmental Protection Center, Beijing 101160, China)., 2022,42(7)：3212～3223

In order to synthesize an environmentally useful material, rGO@nZVI-BC, and to design a permeable reactive barrier (PRB) employing this material for the effective removal of fluorinated glucocorticoids (FGCs) from groundwater. The results indicated that the inhibitory process could be represented using an improved Yoon-Nelson model, with adsorption and biodegradation rate constants of 0.485 and 0.035d-1, 0.233 and 0.029d-1, respectively, in rGO@nZVI-BC and soil systems. The primary mechanism for increasing the efficacy of the process is by strengthening the adsorption and biodegradation activities. At the first stage of operation, adsorption was found to be the dominant mechanism, accounting for approximately 76.12% of the total removal. The mechanism of bioaugmentation is that nZVI acts as an electron donor for the reduction of GCs, thus, enhancing the defluorination reduction function. This component contributed to approximately 87.06% of the total, which is accompanied by dehydroxylation, oxidative side chain degradation, and ring-opening degradation. Additionally, the increased relative abundance of the functional bacteria belonging to the reductive dehalogenation taxa (,, and) indicated the efficiency of rGO@nZVI-BC. This research provides a practical strategy for preventing and controlling the contamination of groundwater.

permeable reactive barrier；groundwater；fluoroglucocorticoid；reductive degradation

X703,X523

A

1000-6923(2022)07-3212-12

孫博宇(1997-),男,河北衡水人,北京林業(yè)大學(xué)碩士研究生,主要從事PRB介質(zhì)材料研究.

2021-12-27

國(guó)家自然科學(xué)基金資助項(xiàng)目(51678052)

* 責(zé)任作者, 教授, mpeggy@163.com