馬欣程,徐紅霞,孫媛媛,施小清,吳吉春

氯代烴污染場地生物自然衰減修復(fù)研究進展

馬欣程,徐紅霞*,孫媛媛,施小清,吳吉春**

(南京大學(xué)地球科學(xué)與工程學(xué)院,表生地球化學(xué)教育部重點實驗室,江蘇 南京 210023)

隨著監(jiān)測自然衰減技術(shù)的不斷發(fā)展和日趨成熟,其在氯代烴污染場地修復(fù)領(lǐng)域中的應(yīng)用越來越廣泛和深入.本文在簡要回顧監(jiān)測自然衰減技術(shù)來源和發(fā)展的基礎(chǔ)上,圍繞主導(dǎo)氯代烴去除的生物自然衰減過程,綜合探究了影響監(jiān)測自然衰減效率的污染物、生物和環(huán)境因素,概述了評估氯代烴污染場地自然衰減能力的實地監(jiān)測和數(shù)值模擬手段,并基于室內(nèi)模擬實驗和場地實際應(yīng)用現(xiàn)狀,分析了其與強化衰減技術(shù)聯(lián)合應(yīng)用的發(fā)展趨勢.今后需要在復(fù)合污染體系的自然衰減特征、生物降解調(diào)控機理研究、數(shù)值模擬及同位素分析等手段的應(yīng)用等方面進一步深入研究,以期為氯代烴污染場地的綠色修復(fù)提供參考.

氯代烴；監(jiān)測自然衰減；強化衰減；場地應(yīng)用

隨著我國經(jīng)濟發(fā)展和城市化進程的加快,場地污染問題日益凸顯,氯代烴是其中最常見的有機污染類型之一.據(jù)統(tǒng)計,2005～2019年間,我國163處有機污染場地地下水中鹵代有機物的檢出比例高達84%,其中又以氯代烷烴(84%)、氯代苯類(46%)和氯代烯烴類(33%)等氯代烴的出現(xiàn)頻率最高[1];美國環(huán)保署(EPA)公布的114種優(yōu)先控制的有機污染物中,氯代烴的占比達22%[2];我國生態(tài)環(huán)境部《土壤環(huán)境質(zhì)量建設(shè)用地土壤污染風險管控標準(試行)》[3](GB 36600-2018)劃定的45項污染物指標中,氯代烴有21種,占比近一半.

大多數(shù)氯代烴具有“三致”效應(yīng),直接威脅生態(tài)環(huán)境和人體健康,三氯乙烯(TCE)、四氯乙烯(PCE)等已被我國列入68種優(yōu)先控制污染物“黑名單”中[4].氯代烴一般難溶于水且密度比水大,屬于重非水相液體(DNAPLs)污染物.DNAPLs自地表泄漏后,在重力、粘滯力和毛管壓力的綜合作用下向下運移,最終滯留在含水層底部,而DNAPLs的低水溶性和難降解性使其能夠向地下水體發(fā)生緩慢且持久的溶出,成為長期污染源[5],對地下水環(huán)境安全造成極大危害.

近年來,國內(nèi)外學(xué)者廣泛開展了氯代烴污染場地的修復(fù)研究,提出了氣相抽提、熱脫附、原位化學(xué)氧化、原位生物修復(fù)和監(jiān)測自然衰減等系列修復(fù)技術(shù).相較于其他原位修復(fù)技術(shù),監(jiān)測自然衰減(MNA)具有成本低、可有效減少污染物的暴露和對場地環(huán)境的擾動等特點,極具發(fā)展前景.

MNA技術(shù)日趨成熟且應(yīng)用前景廣闊,本文在梳理MNA技術(shù)發(fā)展歷程的基礎(chǔ)上,聚焦國內(nèi)外針對氯代烴污染場地MNA修復(fù)的研究進展, 圍繞主導(dǎo)氯代烴去除的生物自然衰減過程,總結(jié)了影響MNA修復(fù)效率的污染物、生物和環(huán)境因素,以及MNA場地應(yīng)用的最新進展,并對該領(lǐng)域未來需要進一步探索的研究方向進行了展望.

1 MNA技術(shù)的來源與發(fā)展

1.1 起源

以氯代烴為代表的有機污染物泄漏進入土壤-地下水環(huán)境后,固相介質(zhì)的吸附、污染物的生物降解以及地下水中的稀釋、擴散作用等天然過程可使污染物發(fā)生自然衰減,其中生物降解是降低污染物濃度、減輕污染物環(huán)境風險的重要途徑.自然衰減在污染場地中普遍存在,隨污染物性質(zhì)及其所處環(huán)境條件的變化,自然衰減強度表現(xiàn)出空間和時間上的顯著差異[6].

20世紀80年代中期,研究人員發(fā)現(xiàn)石油烴在有氧和厭氧的地下環(huán)境中均能被生物降解[7],由此開始了對自然衰減環(huán)境機制的研究.1990年,美國國家應(yīng)急計劃中首次提出“自然衰減”概念[8],促使學(xué)者通過對污染羽的檢測分析,將自然衰減技術(shù)應(yīng)用于有機污染場地的修復(fù)實踐.Gallego等[9]在受游輪重油泄漏污染影響的西班牙海岸進行了2a的監(jiān)測,證實了生物修復(fù)和自然衰減的有效性.陳余道等[10]對淄博市大武水源地石油污染泄漏區(qū)地下水中苯及NO3-、SO42-等電子受體濃度變化的長期監(jiān)測結(jié)果表明,研究區(qū)含水層中的污染物苯發(fā)生了自然衰減,且生物降解是其衰減的主要途徑.

1.2 監(jiān)測自然衰減

自20世紀90年代以來,場地修復(fù)技術(shù)的發(fā)展呈現(xiàn)出低成本化、無害化和充分利用自然過程的趨勢.研究人員開始有計劃地對場地自然衰減過程進行監(jiān)測調(diào)控,由此發(fā)展出MNA技術(shù).1999年,美國環(huán)保署正式將有計劃監(jiān)控下的自然衰減修復(fù)方法稱作監(jiān)測自然衰減(MNA)[11],標志著MNA技術(shù)逐漸成熟,并成為污染場地“第二代”管理和修復(fù)方法[12].MNA指通過實施有計劃的監(jiān)控策略,依靠污染場地中自然發(fā)生的物理、化學(xué)及生物等衰減作用,使土壤和地下水中污染物濃度和總量、毒性、遷移性等在合理的時間范圍內(nèi)降低到風險可接受水平[13].因此MNA也被稱為本能修復(fù)、自然恢復(fù)或自然同化.

目前,美國、英國和荷蘭等多個發(fā)達國家已經(jīng)制定了石油烴和氯代有機物等污染物的MNA技術(shù)準則,形成了具有較強規(guī)范性和操作性的技術(shù)體系[7].美國空軍環(huán)境中心和環(huán)保署先后出臺并規(guī)范了污染場地汽油類污染物和氯代有機物自然衰減監(jiān)測的技術(shù)方案[14-15].2000年,英國環(huán)境部發(fā)布了自然衰減的通用規(guī)程,荷蘭也于同年編制了有機氯溶劑的自然衰減可持續(xù)評價方案.此后,美國環(huán)保署陸續(xù)發(fā)布了甲基叔丁基醚(MTBE)(2005年)和無機污染物(2007年)的自然衰減監(jiān)測指南[16-17],并針對自然衰減過程評價建立了一系列數(shù)字化的標準指導(dǎo)[18-19].歐洲許多其他國家如法國、德國和丹麥也在積極推動針對氯代溶劑、苯系物(BTEX)、多環(huán)芳烴、重金屬等多種污染物質(zhì)的自然衰減技術(shù)方案[20].

近年來MNA技術(shù)在我國污染場地修復(fù)和風險管控方面的應(yīng)用逐漸起步.2014年,MNA技術(shù)被納入環(huán)境保護部發(fā)布的《污染場地修復(fù)技術(shù)目錄(第一批)》[13].生態(tài)環(huán)境部于2018年4月發(fā)布的《污染場地地下水修復(fù)技術(shù)導(dǎo)則》[21]中也將MNA技術(shù)納入其中.基于MNA低效率、低成本和低環(huán)境風險的特點,導(dǎo)則指出該技術(shù)適用于易降解有機物的原位修復(fù).

1.3 發(fā)展:與強化衰減聯(lián)合應(yīng)用

氯代烴污染場地的MNA修復(fù)效率通常較低,在實際應(yīng)用中常與強化衰減(EA)聯(lián)合應(yīng)用以提高效率.EA是指通過向污染場地添加具有特定功能的微生物(生物強化)、O2和營養(yǎng)物質(zhì)(生物刺激)等人為強化措施以維持或促進污染物的降解過程.EA作為污染源修復(fù)技術(shù)和MNA技術(shù)之間的“橋梁”而得到廣泛應(yīng)用[22].

污染場地修復(fù)技術(shù)的選取既需考慮修復(fù)技術(shù)的可操作性和穩(wěn)定性,也要權(quán)衡處理周期和修復(fù)成本.李元杰等[23]對MNA和EA修復(fù)技術(shù)的成本-效益進行了對比,與MNA平均4a的處理周期和29萬美元/場地的處理成本相比,EA的處理周期縮短至266～768d,平均處理成本增加至19美元/m3,但與熱處理、曝氣、化學(xué)清除等修復(fù)技術(shù)相比仍處于較低水平.Mcguire等[24]2006年報道了美國21處DNAPLs污染場地的微生物強化衰減修復(fù)效果,在各自的處理周期內(nèi),地下水中以三氯乙烯(TCE) 、四氯乙烯(PCE)為代表的含氯有機污染物濃度平均下降了62%,且在處理周期結(jié)束至少1a后,含氯有機物的平均去除率從77%繼續(xù)上升至96%.考慮到污染場地的成本控制和風險管控,MNA與EA的聯(lián)合運用既能維持較低的修復(fù)費用,又能適當縮短修復(fù)時間,與當前污染場地綠色修復(fù)的理念和趨勢相契合,受到國內(nèi)外學(xué)者越來越多的重視.鑒于場地氯代烴污染現(xiàn)狀,有必要對氯代烴MNA的研究進展進行梳理總結(jié),厘清研究現(xiàn)狀和未來發(fā)展方向,為綠色修復(fù)提供參考.

2 氯代烴自然衰減

污染場地中氯代烴的自然衰減是物理、化學(xué)、生物等過程綜合作用的復(fù)雜結(jié)果,既包括對流、彌散、揮發(fā)、稀釋、吸附等非破壞性作用,也包括生物降解、非生物降解等破壞性作用[25],其中破壞性作用不僅能夠降低污染物的濃度,而且能夠分解污染物,實現(xiàn)氯代烴的真正去除.氯代烴在地層中活性礦物介導(dǎo)下的還原降解是其非生物降解的主要過程.以氯代烷烴和氯代烯烴的自然衰減為例,水解作用和脫鹵化氫作用是兩者非生物降解過程中最重要的兩種反應(yīng)機制,且降解速率受到礦物種類、pH值、共存金屬離子、天然有機物和礦物形態(tài)等多種因素的影響[26].氯代烴的生物降解在厭氧和好氧的環(huán)境條件下均可進行,成本較低,且能夠?qū)⒙却鸁N轉(zhuǎn)化為CO2和水等無害物質(zhì),被認為是地下含水層中氯代烴自然衰減最重要的一個環(huán)節(jié)[27].針對污染場地中氯代烴非生物自然衰減的最新研究進展已有學(xué)者予以專門總結(jié)[28-29],但純化學(xué)的自然轉(zhuǎn)化過程在氯代烴的各類自然衰減作用中并不常見[30].鑒于生物降解在氯代烴徹底無毒害化轉(zhuǎn)化過程中所起的核心作用[31-32],下文主要圍繞氯代烴生物自然衰減(生物降解)的研究進展進行專門總結(jié),按降解機理、影響要素予以分述.

2.1 氯代烴生物降解機理

氯代烴的生物降解一般可分為直接氧化、好氧共代謝和還原脫氯3種類型.幾種典型氯代烴在這3種降解途徑下的中間產(chǎn)物、最終產(chǎn)物如表1所示.

表1 典型氯代烴在不同生物降解途徑下的降解產(chǎn)物

注:-表示目前未見相關(guān)報道.

好氧條件下,大多數(shù)氯代烴能夠以糖類、乳酸鹽等生長基質(zhì)作為電子供體,以O(shè)2、硝酸鹽、硫酸鹽等作為電子受體,在加氧酶的作用下進行共代謝降解;少部分低氯取代烴如氯乙烯、二氯甲烷、1,2-二氯乙烷等能作為唯一碳源和能源被微生物直接利用,發(fā)生直接氧化.雖然氯代烴的好氧生物降解速率相對較快,但在自然地下水環(huán)境中氯代烴降解最為常見的途徑是厭氧還原脫氯.在厭氧微生物作用下,氯代烴中的氯可通過水解作用或親核反應(yīng)逐步被氫取代,高氯取代烴轉(zhuǎn)化為低氯取代烴或不含氯物質(zhì)[50].

2.2 氯代烴生物降解的影響因素

污染場地中氯代烴的生物降解受到多種因素的影響,包括氯代烴的污染特征、微生物的生長特性及場地的環(huán)境條件等.

2.2.1 污染物因素 氯代烴污染物自身的分子結(jié)構(gòu)和理化性質(zhì)決定了其生物降解脫氯進行的難易程度、反應(yīng)速率及中間產(chǎn)物等.一般而言,厭氧環(huán)境中,氯代烴的氯代程度越低越難發(fā)生降解,而好氧環(huán)境中則正好相反[51].PCE等全氯取代的氯代烴具有較高的還原脫氯傾向,可導(dǎo)致中間產(chǎn)物-DCE和VC在場地中的累積.盡管存在少量關(guān)于PCE好氧生物降解的文獻報道[34-35],研究人員認為非全氯取代的TCE、-DCE、CA等在好氧、厭氧條件下均可發(fā)生降解[52].好氧條件下,VC、一氯甲烷和二氯甲烷等含有一個或兩個氯取代基的低氯取代烴能以直接氧化的方式被微生物直接代謝,高氯取代烴則進行好氧共代謝降解[53].Schmidt 等[38]首次報道了TCE能在SF混合菌群的作用下發(fā)生穩(wěn)定且持久的直接氧化降解,是目前證明TCE能夠被直接氧化降解的唯一研究[54].多種氯代烴共存時,高氯取代烴對低氯取代烴還原脫氯的競爭抑制作用也會影響脫氯程度.Distefano等[55]發(fā)現(xiàn)PCE和TCE的存在會抑制VC向最終產(chǎn)物乙烯的轉(zhuǎn)化,從而導(dǎo)致中間產(chǎn)物的富集,影響PCE的完全脫氯效率.

在實際污染場地中,氯代烴常和苯系物共存造成復(fù)合污染,二者不同的理化特性將導(dǎo)致更為復(fù)雜的污染羽分布與自然衰減過程.以石油為代表的苯系物可作為TCE脫氯的共代謝基質(zhì),在石油污染地下水發(fā)生自然衰減的同時,TCE的生物降解作用也能夠被強化[56].Zhang等[57]對苯系物和氯代脂肪烴復(fù)合污染地下水自然衰減過程中微生物群落組成的監(jiān)測結(jié)果表明,苯系物能強化還原脫氯菌的富集,脫氯菌、苯系物降解菌和發(fā)酵菌表現(xiàn)出協(xié)同作用,共同促進氯代烴還原脫氯反應(yīng)的進行.

此外,氯代烴和無機污染物(如高氯酸鹽、重金屬陽離子)形成的場地復(fù)合污染現(xiàn)象并不鮮見.這些共存無機污染物對氯代烴脫氯降解的影響不可忽視,如高氯酸鹽的存在就被發(fā)現(xiàn)不利于氯代烴的還原脫氯.溫麗蓮[58]利用富集培養(yǎng)得到的高氯酸鹽降解菌群探究高氯酸鹽和TCE的相互作用時發(fā)現(xiàn),濃度高于0.1mmol/L的高氯酸鹽會降低TCE還原速率,并認為TCE降解中間產(chǎn)物VC至最終產(chǎn)物乙烯的還原過程被抑制的主要原因在于高氯酸鹽還原反應(yīng)的優(yōu)先發(fā)生.Amos等[59]則認為,高氯酸鹽在微生物還原過程中產(chǎn)生的O2會抑制脫氯菌的生長,進而影響TCE等氯代烴的脫氯效率.重金屬離子對氯代烴降解的影響與重金屬離子的類型、濃度有關(guān).李明堂[60]利用微宇宙系統(tǒng)模擬典型氯代烴類污染物的生物降解時發(fā)現(xiàn),系統(tǒng)中Cd2+和Hg2+的濃度達到10mg/L時會對氯苯降解產(chǎn)生顯著抑制,且Hg2+的抑制作用更強.

截至目前,相關(guān)共存無機污染物對氯代烴生物降解影響的研究結(jié)論不盡一致.有學(xué)者研究了硫酸鹽和硫化物對氯代烴還原脫氯的影響,結(jié)果正好相反.郭瑩等[61]發(fā)現(xiàn)SO42-對TCE的降解具有明顯的促進作用,且TCE的降解率隨SO42-與TCE濃度比的增加而升高.然而,李燁等[62]發(fā)現(xiàn)硫酸鹽的還原作用會抑制PCE的脫氯速率,且抑制效果與硫酸鹽的初始濃度成正比,未加入硫酸鹽時PCE的22d去除率可達100.0%,但硫酸鹽初始濃度為100mg/L時PCE的去除率僅為47.79%.

2.2.2 生物因素 微生物的種類和數(shù)量直接影響著污染場地中氯代烴的生物降解.不同微生物具有不同的代謝底物范圍和底物親和性,降解氯代烴的效率和范圍也有所差異[63].楊洪江等[64]從化工廠排污口污泥中馴化分離所得7株氯苯好氧降解菌的降解能力差異明顯,菌株KD237對氯苯的24h降解率達60.78%,遠遠高于菌株KD232的7.84%.已有研究發(fā)現(xiàn),假單胞菌屬[39]、甲基彎曲菌屬[65]、紅球菌屬[66]、勞爾氏菌屬[67]、亞硝化單胞菌屬[68]、伯克氏菌屬[69]、埃希氏菌屬[70]、不動桿菌屬[71]等多個菌屬均被發(fā)現(xiàn)可實現(xiàn)氯代烴的好氧降解,但其目標污染物以低氯代烴為主.以厭氧或缺氧狀態(tài)為主的地下環(huán)境中,還原脫氯菌是重要的氯代烴生物降解菌.目前發(fā)現(xiàn)的氯代烯烴還原脫氯菌主要可分為2類[72]: (1)、等能將高氯取代烴降解為二氯乙烯(DCE)的菌屬;(2)能在降解高氯取代烴的基礎(chǔ)上,將中間產(chǎn)物DCE、VC等低氯取代烴進一步降解為乙烯的菌屬——,這也是迄今為止唯一已知的能夠?qū)⒑认N完全脫氯至不含氯終產(chǎn)物乙烯的微生物類群[73].Hendrickson等[74]在2002年對北美和歐洲24處PCE等氯代烴污染場地的調(diào)查結(jié)果表明,氯代烴的不完全脫氯與場地中屬微生物的缺乏密切相關(guān).實際地下水環(huán)境中,多種脫氯菌往往同時存在,表現(xiàn)出比單菌更好的降解效果[75].Pinellas混菌、KB-1混菌、Victoria混菌等[76]是常見的具有還原脫氯能力的混合菌群.Major等[77]在某PCE污染場地開展的靜態(tài)批實驗和場地原位實驗均表明,向-DCE等還原脫氯中間產(chǎn)物積累的污染場地中添加少量KB-1(0.02%)混合菌群就能實現(xiàn)PCE的快速、完全脫氯,在微生物活躍區(qū)域PCE的半衰期僅為數(shù)小時.

許多氯代烴污染場地自身即存在具有氯代烴降解能力的土著菌群.在缺乏氯代烴降解菌的污染場地內(nèi),土著微生物也可能在長時間的污染脅迫下,發(fā)生選擇性的富集和遺傳改變從而產(chǎn)生對氯代烴針對性的降解作用.但土著微生物的代謝活性往往較低,對氯代烴的降解速率和程度有限,可通過進一步的定向培養(yǎng)以強化土著微生物的降解性能或引入特定降解功能菌來促進降解.如表2所示,已有的室內(nèi)實驗研究已經(jīng)證實引入氯代烴降解功能菌進行強化衰減的巨大潛力.因此,篩選培育降解能力較強的氯代烴降解菌是目前MNA與EA聯(lián)合應(yīng)用研究的重點任務(wù)之一.

表2 引入氯代烴降解功能菌的強化衰減實驗研究

2.2.3 環(huán)境因素 氯代烴污染場地的氧化/還原條件、營養(yǎng)物質(zhì)、溫度、pH值等環(huán)境因素會影響脫氯功能菌的活性,進而影響氯代烴的生物降解.

(1)氧化/還原條件:在一定的氧化還原電位下,氯代烴會在微生物的作用下進行有次序的降解,即在污染源附近以甲烷產(chǎn)生和SO42-還原過程為主,電子受體CO2和SO42-消耗盡后,將進一步進行鐵錳還原和去硝酸根的生物降解.如圖1所示,隨著與污染源距離的增加,污染羽不同區(qū)域可利用的電子受體差異明顯,在氯代烴污染區(qū)下游可形成由還原環(huán)境向氧化環(huán)境轉(zhuǎn)變的時空變化[82-83].

氯代烴還原脫氯積累的中間產(chǎn)物(低氯代烴)可從厭氧區(qū)域向下游運移至好氧區(qū)域,被好氧微生物進一步降解,從而有可能實現(xiàn)氯代烴的完全脫氯,對于場地修復(fù)具有重要意義.基于此,近年來研究人員廣泛開展了以氯代烴連續(xù)厭氧-好氧生物降解過程為核心的室內(nèi)強化衰減修復(fù)研究(表3),初步證實了氯代烴在連續(xù)厭氧-好氧生物降解過程中完全礦化的可行性,為氯代烴污染場地的徹底修復(fù)提供了新思路.

(2)營養(yǎng)物質(zhì):微生物在降解氯代烴的過程中需要碳源、氮源等多種營養(yǎng)物來維持自身代謝繁殖.高氯取代烴的降解一般以共代謝方式進行,因此保證地下環(huán)境中固有或添加的共代謝基質(zhì)為氯代烴降解菌提供充足的電子是場地MNA和EA實施的關(guān)鍵之一.Semprini[91]的研究表明,苯酚、甲苯等芳香烴適合用作TCE等氯代烯烴的共代謝基質(zhì),甲烷、丙烷等氣態(tài)烷烴則更適合于用作氯代烷烴(如氯化甲烷和乙烷)的共代謝基質(zhì).唐詩月等[92]對比了甲醇、乙醇、葡萄糖、酵母浸膏以及乳酸鈉5種共代謝基質(zhì)對PCE降解速率的影響,發(fā)現(xiàn)其對PCE降解的促進程度依次為酵母浸膏>葡萄糖≈甲醇>乳酸鈉>無共代謝基質(zhì)>乙醇.值得指出的是,共代謝基質(zhì)濃度過高時,可能抑制脫氯關(guān)鍵酶的活性;共代謝基質(zhì)濃度過低時,則很難產(chǎn)生充足的酶來催化共代謝反應(yīng).Mu等[93]利用甲苯作為共代謝基質(zhì)進行強化微生物修復(fù)的研究表明,TCE/甲苯濃度比的增加會導(dǎo)致降解菌生長的遲滯.Futamata等[94]考察苯酚對TCE好氧生物降解的影響時發(fā)現(xiàn),苯酚濃度為0.75g/L時脫氯菌能表現(xiàn)出最高的降解活性.

圖1 氯代烴污染場地中微生物反應(yīng)過程示意

表3 氯代烴連續(xù)厭氧-好氧生物降解實驗研究

除上述充當氯代烴共代謝基質(zhì)的碳源外,場地中還需要充足的無機營養(yǎng)鹽供給功能微生物生長所需的N、P及其他各種礦物元素.污染場地的強化衰減修復(fù)應(yīng)注意選取適當?shù)臓I養(yǎng)鹽形態(tài)及比例,以起到進一步促進氯代烴降解的積極作用.Palumbo等[95]發(fā)現(xiàn),將磷酸三乙酯和一氧化二氮以氣態(tài)形式注入TCE污染場地,更有利于營養(yǎng)鹽的傳輸和提高TCE的去除.針對不同C:N:P比值對氯代烴去除影響的室內(nèi)研究表明[96],n(C):n(N):n(P)=100:26.7: 1.8～4.8時,TCE和-DCE的好氧生物降解效果最佳.

氯代烴污染場地自然衰減修復(fù)的順利進行首先需要降解微生物的存在,而實際場地中相關(guān)營養(yǎng)物質(zhì)的缺乏常成為污染物降解過程的限制因素,為此添加共代謝基質(zhì)、營養(yǎng)鹽等營養(yǎng)物質(zhì)進行生物刺激,是提高污染場地內(nèi)微生物氯代烴降解能力的有效手段.如表4所示,國內(nèi)外學(xué)者廣泛開展了氯代烴在不同營養(yǎng)物質(zhì)作用下的強化衰減室內(nèi)模擬研究,證明了添加營養(yǎng)物質(zhì)進行強化衰減的積極效果.值得注意的是,土著微生物由于對污染環(huán)境適應(yīng)性更強,與外源引入的降解功能菌相比,在污染場地中更易保持競爭優(yōu)勢,也因此成為生物刺激的主要作用對象.

(3)溫度和pH值:作為氯代烴污染場地自然衰減的核心過程,微生物降解的反應(yīng)速率受控于環(huán)境溫度和pH值.溫度變化不但能夠改變微生物細胞的生長速率和細胞成分,還會對降解關(guān)鍵酶的活性等造成影響.盧曉霞等[99]分別研究了12℃及20℃時PCE、-DCE、VC等氯代烴的微生物降解過程,結(jié)果表明,氯代烴的降解速率在20℃條件下較快,當溫度下降至12℃時,PCE能完全脫氯,但反應(yīng)速率明顯下降. Pietari[100]認為25～30℃為PCE厭氧降解的最適溫度,溫度過高(>35℃)會抑制微生物的降解性能甚至使得反應(yīng)停止.一般地,溫度在一定范圍內(nèi)升高可促進氯代烴降解菌活性,但過高的溫度也會導(dǎo)致降解酶活性降低甚至失活[101].此外,溫度也會影響含水層介質(zhì)對污染物的吸附特性,進而影響TCE的修復(fù)效率[102-103].研究溫度對氯代烴生物降解的影響有助于根據(jù)污染場地環(huán)境溫度的季節(jié)性變化來制定適宜的修復(fù)策略.

表4 添加營養(yǎng)物質(zhì)進行生物刺激的強化衰減實驗研究

pH值是影響微生物生命活動的主要環(huán)境條件之一.不同環(huán)境pH值會造成細胞膜以及酶、核酸等大分子所帶電荷的差異,進而影響細胞對營養(yǎng)物質(zhì)的吸收和細胞活性.李燁[104]以厭氧污泥作為接種物研究PCE在不同厭氧環(huán)境中的生物降解時發(fā)現(xiàn),在中性或微堿性條件下(pH=6-8),該菌種的降解活性較高, PCE的16d去除率達到90%以上.實際修復(fù)中,調(diào)節(jié)適宜的土壤-地下水酸堿環(huán)境可對氯代烴污染場地MNA和EA修復(fù)工程的實施效果產(chǎn)生積極影響.

3 氯代烴污染場地監(jiān)測自然衰減及強化衰減修復(fù)的評估和應(yīng)用現(xiàn)狀

大量的室內(nèi)實驗研究表明,氯代烴能夠在生物降解等自然衰減作用下轉(zhuǎn)化為乙烯、乙烷或其他對環(huán)境無害的物質(zhì).但由于功能微生物的分布、活性和場地水文地質(zhì)條件的復(fù)雜性等實際問題,MNA和EA技術(shù)在實際場地中的應(yīng)用效果可能會與室內(nèi)模擬結(jié)果存在差距.如何評估并提高該技術(shù)在氯代烴污染場地中應(yīng)用的有效性仍值得探索.

3.1 場地自然衰減評估

MNA不是消極不作為的場地管理,其實施需要場地調(diào)查、監(jiān)測系統(tǒng)構(gòu)建、自然衰減有效性驗證和風險評價等一系列專業(yè)技術(shù)的保障[8].判定氯代烴污染場地的自然衰減程度是實施MNA修復(fù)及判斷是否需要與EA聯(lián)合應(yīng)用的前提和關(guān)鍵.一般需根據(jù)污染場地的水文地質(zhì)條件和水化學(xué)特征等資料,綜合分析氯代烴濃度、Cl-等降解中間產(chǎn)物及O2、NO3-和SO42-等電子受體濃度的變化來預(yù)測和判定氯代烴污染羽的衰減趨勢,但當場地內(nèi)存在復(fù)合污染或水化學(xué)條件時空變化劇烈等情形時,預(yù)測結(jié)果的準確性會受到影響.近期的研究表明,借助微生物菌群研究定量測定氯代烴的生物降解速率,并結(jié)合穩(wěn)定同位素分析來指示氯代烴的生物降解途徑,可實現(xiàn)其自然衰減的早期識別[25].

除利用場地監(jiān)測數(shù)據(jù)進行自然衰減評估外,構(gòu)建場地概念模型,通過數(shù)值模擬建立起實驗?zāi)M與實地監(jiān)測之間的聯(lián)系,已成為定量化評估氯代烴污染場地自然衰減能力的重要手段,近年來受到的重視程度日益增加.COMSOL[105]、MT3DMS[106]、TMVOC[107]等多種數(shù)值模擬軟件均可被用于刻畫氯代烴在地下環(huán)境中的遷移轉(zhuǎn)化過程.劉明柱等[108]利用GMS軟件對我國北方某氯代烴污染區(qū)地下水中PCE和TCE的遷移轉(zhuǎn)化情況進行了數(shù)值模擬研究,模擬結(jié)果顯示1a內(nèi)2種氯代烴在含水層中僅發(fā)生微弱的生物降解,由此推測在較長時間內(nèi)該區(qū)域氯代烴的濃度可能仍保持在較高水平. Widdowson[109]通過SEAM3D模型重點模擬了TCE的生物降解過程,指出TCE的衰減程度受到水相和固相介質(zhì)中電子受體濃度的影響.而針對氯代烴污染物在某沖洪積扇中上部遷移轉(zhuǎn)化的數(shù)值模擬研究則表明[110],研究區(qū) PCE和TCE的衰減主要與氧化還原環(huán)境有關(guān),且PCE的生物衰減系數(shù)約為TCE的6倍.近年來,對氯代烴在復(fù)雜非均質(zhì)裂隙、巖溶含水層中自然衰減過程的數(shù)值模擬也取得了長足進展.Guo等[111]基于TMVOC模型準確刻畫了三氯甲烷和1,1,2-三氯乙烷在某裂隙巖溶含水層中的自然衰減過程,并推測由生物降解主導(dǎo)的自然衰減作用可在10～15a內(nèi)實現(xiàn)對污染區(qū)域的有效修復(fù).

利用數(shù)值模型可突破傳統(tǒng)監(jiān)測方法由于成本高昂導(dǎo)致數(shù)據(jù)稀缺的桎梏,能夠模擬并預(yù)測污染物在地下環(huán)境中的運移行為和濃度時空分布,從而為氯代烴的控制和修復(fù)提供支撐.而氯代烴在場地中的衰減是物理、化學(xué)和生物多過程共同作用的結(jié)果,如何將微觀尺度的過程機制研究與所監(jiān)測到的現(xiàn)場宏觀尺度污染物通量相結(jié)合以實現(xiàn)尺度提升是現(xiàn)階段亟需解決的難題之一.考慮到實際場地的時空非均質(zhì)性,實現(xiàn)復(fù)雜條件下污染物衰減多作用過程的耦合模擬和精準刻畫仍面臨巨大挑戰(zhàn).

3.2 實際應(yīng)用

盡管在修復(fù)過程中還存在一些需要深入研究解決的問題,包括如何保持脫氯菌群的活性、如何保證氯代烴的完全脫氯及如何維持合適的地下水氧化還原環(huán)境等,MNA和EA技術(shù)在氯代烴污染場地的應(yīng)用比例已顯著增加,并且在許多實踐中取得了成功.近些年來,國內(nèi)外學(xué)者在利用MNA 技術(shù)指導(dǎo)實際氯代烴污染場地的修復(fù)工作方面開展了大量研究工作,主要包括自然衰減過程中場地內(nèi)產(chǎn)生的氧化還原帶現(xiàn)象、氯代烴自然衰減時在含水層中的遷移轉(zhuǎn)化和影響機制、自然衰減模型的建立和自然衰減的強化技術(shù)研究等.目前,國內(nèi)對氯代烴污染場地的自然衰減修復(fù)以小試、中試為主,通過室內(nèi)實驗?zāi)M和野外監(jiān)測等來研究氯代烴自然衰減過程中發(fā)生的地球化學(xué)作用,估算衰減速率等.如表5所示,現(xiàn)階段氯代烴污染場地單獨實施MNA修復(fù)的數(shù)量較少,更多的是與EA修復(fù)的聯(lián)合應(yīng)用.

表5 氯代烴污染場地監(jiān)測自然衰減及強化衰減修復(fù)實例

續(xù)表5

注:-表示未見相關(guān)報道.

4 結(jié)論與展望

4.1 結(jié)論

4.1.1 在眾多氯代烴污染場地的修復(fù)技術(shù)中,由于經(jīng)濟、綠色且對環(huán)境擾動小等特點,MNA技術(shù)在針對低濃度氯代烴污染場地的修復(fù)中受到越來越多的重視.由于該技術(shù)僅依靠場地中自然發(fā)生的生物衰減等作用來實現(xiàn)對污染物的去除,MNA所需修復(fù)時間較長,故在實際應(yīng)用中,常與EA技術(shù)聯(lián)合以提高效率.

4.1.2 國內(nèi)外學(xué)者已在場地自然衰減修復(fù)的適用性評估、氯代烴的衰減機理和影響機制、衰減速率預(yù)測等方面取得了積極進展.

4.1.3 目前相關(guān)研究較少涉及裂隙含水層、高濃度污染物的自然衰減,考慮到實際應(yīng)用中土壤組成、污染特征、場地水文地質(zhì)條件和氧化還原條件等環(huán)境因素的復(fù)雜性,相關(guān)研究仍存在一定局限性.

4.2 展望

4.2.1 目前,氯代烴污染場地的自然衰減研究大都基于實驗室小試和中試模擬,且主要模擬均質(zhì)和非均質(zhì)孔隙介質(zhì)情形,現(xiàn)場研究較少,針對裂隙、巖溶介質(zhì)的研究更少,難以全面反映實際場地水文地質(zhì)和地球化學(xué)等條件的高度復(fù)雜性.裂隙介質(zhì)和巖溶介質(zhì)相對孔隙介質(zhì)更為復(fù)雜,需要在提升相關(guān)現(xiàn)場監(jiān)測技術(shù)基礎(chǔ)上,進一步深入研究氯代烴類污染物在裂隙、巖溶介質(zhì)中的自然衰減規(guī)律以及影響因素和影響機制.

4.2.2 地下水系統(tǒng)中,以氯代烴為代表的有機污染物因其特殊的水理性質(zhì),常以多組分、多相態(tài)的形式存在,苯系物(BTEX)、重金屬等共存污染物對其生物降解、對流彌散、揮發(fā)、吸附等衰減作用的影響不可忽視.掌握復(fù)合污染體系的自然衰減規(guī)律,對于客觀反映環(huán)境中多種污染物共存的真實風險至關(guān)重要.

4.2.3 明確氯代烴生物降解過程中的關(guān)鍵酶、降解產(chǎn)物和代謝途徑對于優(yōu)化調(diào)控實際污染場地氯代烴的脫氯過程具有重要的指導(dǎo)意義.目前,部分氯代烴的生物降解作用機制尚未完全明晰, 需進一步考察微生物群落不同種屬對氯代烴的代謝機理、降解產(chǎn)物及其功能基因組信息.

4.2.4 數(shù)值模擬在污染場地自然衰減修復(fù)的適用性評估、修復(fù)效率預(yù)測等眾多環(huán)節(jié)中發(fā)揮重要作用,而建立準確的場地概念模型是該技術(shù)有效應(yīng)用的前提.在未來的氯代烴污染場地修復(fù)工作中,應(yīng)進一步重視數(shù)值模擬技術(shù)的應(yīng)用,加強對場地水文地質(zhì)和水化學(xué)條件等的調(diào)查、監(jiān)測和刻畫,實現(xiàn)場地概念模型的精準構(gòu)建.此外,不斷發(fā)展的分子生物學(xué)等先進技術(shù)使得掌握場地微生物群落組成、活動和功能的基本模式成為可能,未來有望將微生物信息耦合至數(shù)值模型中以提升模擬預(yù)測效果.

4.2.5 同位素等分析手段在氯代烴生物降解過程和性能評估方面具有廣闊的應(yīng)用前景.但目前,同位素技術(shù)對實際氯代烴污染場地MNA和EA修復(fù)的研究開展得還較少,今后應(yīng)更加重視氯代烴類污染物降解過程中單體穩(wěn)定同位素分餾模型的判定與選擇.

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Research progress on biotic natural attenuation for the remediation of chlorinated hydrocarbon-contaminated sites.

MA Xin-cheng, XU Hong-xia*, SUN Yuan-yuan, SHI Xiao-Qing, WU Ji-chun**

(Key Laboratory of Surficial Geochemistry of Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China)., 2022,42(11)：5285～5298

With the continuous development and the increasing maturity of Monitored Natural Attenuation (MNA), its application in the remediation of sites contaminated by chlorinated hydrocarbons is becoming more and more extensive in-depth. In this paper, we first conducted a brief review on the origin and development history of MNA. We then summarized contaminant, biological and environmental factors that affect the remediation efficiency of MNA, especially focusing on the biotic natural attenuation processes. Field monitoring and numerical simulation methods were also summarized to assess the natural attenuation capacity of contaminated sites. Afterwards, we analyzed development trend of the joint use of MNA and Enhanced Attenuation (EA) and discussed their current application status in laboratory studies and contaminated sites. To provide a reference for the green remediation of sites contaminated by chlorinated hydrocarbons, the natural attenuation characteristics of mixed pollutants, regulatory mechanism of biodegradation and the application of numerical simulation and isotope analysis should be further discussed.

chlorinated hydrocarbons；monitored natural attenuation；enhanced attenuation；site application

X523

A

1000-6923(2022)11-5285-14

馬欣程(1998-),女,江蘇徐州人,南京大學(xué)博士研究生,主要從事土壤-地下水中污染物遷移轉(zhuǎn)化及修復(fù)研究.

2022-04-06

國家自然科學(xué)基金資助項目(41730856,41877182);中央高?；究蒲袠I(yè)務(wù)費專項資金資助項目(0211-14380166)

*責任作者, 副教授, hxxu@nju.edu.cn; ** 教授, jcwu@nju.edu.cn