土壤還原過程對氯代有機污染物還原脫氯的影響與機制

馮曦，朱敏，何艷,*

1. 浙江大學(xué)土水資源與環(huán)境研究所，杭州3100582. 浙江省農(nóng)業(yè)資源與環(huán)境重點實驗室，杭州310058

土壤還原過程對氯代有機污染物還原脫氯的影響與機制

馮曦1,2，朱敏1,2，何艷1,2,*

1. 浙江大學(xué)土水資源與環(huán)境研究所，杭州3100582. 浙江省農(nóng)業(yè)資源與環(huán)境重點實驗室，杭州310058

自然環(huán)境中，大多數(shù)氯代有機污染物厭氧還原脫氯反應(yīng)是與土壤環(huán)境中一些生源要素的生物化學(xué)還原過程相伴生。有機污染物的種類、生物有效性以及毒性能夠顯著影響這些生源要素的轉(zhuǎn)化，反過來，土壤中活躍的氧化還原反應(yīng)也可以顯著影響有機污染物的動力學(xué)轉(zhuǎn)化過程。本文從氧化還原順序上綜述了反硝化過程、鐵還原過程、硫酸鹽還原過程和產(chǎn)甲烷過程對氯代有機污染物厭氧還原脫氯過程的影響與作用機制，旨在為氯代有機污染物在厭氧環(huán)境中還原脫氯的過程與機理的進一步研究、以及還原脫氯與微生物介導(dǎo)的生源要素氧化還原過程的耦合作用機制的揭示提供參考。

還原脫氯；反硝化；鐵還原；硫還原；產(chǎn)甲烷

Received14 January 2017accepted30 April 2017

Abstract: Under anaerobic conditions, reductive dechlorination of chlorinated organic pollutants is thought to be coupled to biogeochemical processes of some biogenic elements in soil. The variety, bio-availability and toxicity of organic pollutants can significantly affect the transformation of the biogeochemical processes. Correspondingly, soil natural active redox processes can also significantly affect the dynamics of organic pollutant transformation. Based on the classical redox reactions sequence, the effects and mechanisms of denitrification process, dissimilatory iron reduction process, sulfate reduction process and methanogenic process on reductive dechlorination of chlorinated organic pollutants were reviewed and discussed in this paper. We also provided reference for the further study to disclose more in-depth mechanism regarding reductive dechlorination and their interaction with the co-occurring natural redox processes in soil under anaerobic environment.Keywords: reductive dechlorination; denitrification; dissimilatory iron reduction; sulfate reduction; methanogenesis

氯代有機物(chlorinated organic compounds,COCs)指的是氯原子取代了脂肪烴、芳香烴及其衍生物中的一個或幾個氫原子后的產(chǎn)物，其種類繁多，主要包括氯代烴(chlorinated hydrocarbon)、有機氯農(nóng)藥(organochlorine pesticides,OCPs)、氯代苯(chlorinated biphenyls,CBs)、多氯聯(lián)苯(polychlorinated biphenyls, PCBs)、氯代苯酚(chlorinated phenols,CPs)等，被廣泛應(yīng)用于化工、醫(yī)藥、農(nóng)藥、制革等行業(yè)。大多數(shù)COCs具有較低的水溶性和較高的辛醇-水分配系數(shù)，一旦進入環(huán)境，很難在短時間內(nèi)自然降解，并能夠進行長距離遷移和沉積，容易在生物體內(nèi)聚集，可通過食物鏈積累，造成嚴(yán)重的環(huán)境污染和長期的生態(tài)破壞后果。

COCs與一般的芳香族化合物相比，其可生化性大大降低，這是因為氯原子引起了化合物分子結(jié)構(gòu)特性的改變，所以脫氯是COCs生物降解的關(guān)鍵步驟。根據(jù)脫氯過程中的電子得失，其機理分為氧化脫氯和還原脫氯[1]。但在好氧條件下高氯代有機污染物很難被微生物利用[2]，氯離子會阻礙雙加氧酶對苯環(huán)的攻擊，使得大部分好氧微生物降解高氯代污染物的能力和效率要低于低氯代有機污染物[3]。在缺氧條件下，一旦氯原子被脫去，其產(chǎn)物就對好氧的2,3-和3,4-雙氧化酶等變得更為敏感[4]，使氯代有機污染物形成容易進一步被好氧礦化的物質(zhì)。例如氯代烯烴可以通過一系列還原脫氯作用，形成烯烴，進而轉(zhuǎn)化為甲烷[5]；而對于氯代芳烴，氯原子強烈的吸電子性使芳烴上電子云密度降低，在缺氧條件下，電子云密度較低的苯環(huán)在酶作用下很容易受到還原劑的親核攻擊，顯示出較好的缺氧生物降解性[6]。因此，氯代有機污染物的缺氧脫氯越來越引起了研究者們的興趣。

自然環(huán)境中，大多數(shù)氯代有機污染物厭氧還原脫氯反應(yīng)在土壤環(huán)境中是與反硝化反應(yīng)、鐵還原反應(yīng)、硫酸鹽還原反應(yīng)、產(chǎn)甲烷反應(yīng)等重要的生物化學(xué)過程相伴生。有機污染物的種類、生物有效性以及毒性能夠顯著影響這些生源要素的轉(zhuǎn)化，反過來，土壤中活躍的氧化還原對也可以顯著影響氯代有機污染物的動力學(xué)轉(zhuǎn)化過程[10]。隨著土壤環(huán)境中的生物地球化學(xué)循環(huán)過程等熱點問題的提出，土壤生源要素氧化還原循環(huán)參與條件下的有機污染物消減的調(diào)控過程及作用機制引起了國內(nèi)外廣大學(xué)者的關(guān)注。本文介紹了反硝化過程、鐵還原過程、硫酸鹽還原過程和產(chǎn)甲烷過程對氯代有機污染物厭氧還原脫氯過程的影響與作用機制，旨在為氯代有機污染物在厭氧環(huán)境中還原脫氯的過程與機理的進一步研究、以及還原脫氯與微生物介導(dǎo)的生源要素氧化還原過程的耦合作用機制的揭示提供參考。

1 反硝化過程與還原脫氯過程(Denitrification process and reductive dechlorination process)

2 異化鐵還原與還原脫氯過程(Dissimilatory iron reduction process and reductive dechlorination process)

鐵，作為自然環(huán)境中含量最為豐富的變價金屬元素，其對地球生物化學(xué)循環(huán)過程有著深遠的意義。自然界中，特別是在厭氧的土壤或沉積物環(huán)境下，鐵物種通常以難溶性Fe(III)氧化物的形式存在，而這些含變價元素的鐵氧化物在這些厭氧環(huán)境中也是天然存在的氧化劑[30]。大量的研究結(jié)果表明，土壤中Fe(III)的還原主要是由特定的異化鐵還原微生物驅(qū)動，在異化型鐵還原菌的異化作用中，F(xiàn)e(III)被用作電子受體，還原產(chǎn)生的Fe(II)可以偶聯(lián)多種有機物的氧化還原。所以說Fe(III)還原是某些土壤和沉積物中有機質(zhì)分解中的一個重要過程，它們在氯代有機污染物的還原轉(zhuǎn)化過程中扮演著重要的角色[31]。

當(dāng)電子供體限制的條件下，土壤中可作為最終電子受體同時存在的Fe(III)和COCs為獲取有限的電子而表現(xiàn)為競爭關(guān)系。Paul等[32-33]發(fā)現(xiàn)三價鐵礦物的存在抑制了三氯乙烯(TCE)的厭氧還原脫氯，當(dāng)然，這也與鐵的存在形式、數(shù)量以及環(huán)境因子如pH等有關(guān)。另外，也有研究發(fā)現(xiàn)，F(xiàn)e(III)還原產(chǎn)生的吸附態(tài)Fe(II)可以強化化學(xué)的還原脫氯反應(yīng)過程。吸附于鐵氧化物表面的Fe(II)由于中心周圍的電子云密度增加，而且吸附態(tài)Fe(II)的氧化還原電位明顯低于溶解態(tài)Fe(II)，提高了脫氯轉(zhuǎn)化的自發(fā)性[34-36]。同時，還原產(chǎn)生的二價鐵也可以作為還原劑將COCs還原，反應(yīng)式為：Fe2++ R-Cl → Fe3++ R+Cl-，即實現(xiàn)了鐵氧化物還原溶解反應(yīng)與化學(xué)脫氯反應(yīng)的交互[37]。

表1 土壤中主要還原反應(yīng)的熱力學(xué)順序(pH 7.0，25 ℃)aTable 1 Thermodynamic sequence of main reduction processes in soil (pH 7.0, 25 ℃)a

在微生物的參與下，鐵還原過程與氯代有機污染物還原脫氯過程的關(guān)系顯得更為復(fù)雜。厭氧土壤環(huán)境中，參與還原脫氯過程的微生物主要還是以氯代有機污染物為電子受體進行能量代謝的脫氯菌，而在鐵還原條件下，鐵還原菌作為主要優(yōu)勢微生物也會相對抑制脫氯菌活性以及還原脫氯過程，主要也是通過對電子供體的競爭而產(chǎn)生抑制作用[32, 38]。一些鐵還原菌可以直接以有機氯作為電子受體氧化電子供體進行脫氯呼吸，為有機氯化合物污染的原位修復(fù)工作提供了一條新途徑。例如，Anaeromyxobacter dehalogenans strain 2CP-C除了可以利用Fe(III)作為電子受體外，還可以通過鄰位脫氯過程獲得能量生長[39]。Desulfuromonas chlorethenica也可以同時利用Fe(III)還原和四氯乙烯以及三氯乙烯作為電子受體進行生長[40]。作為富鐵土壤環(huán)境中的典型菌屬，梭菌屬微生物Clostridium sp.也常在氯代化合物污染區(qū)域活躍[36, 41-42]。同時具有脫氯呼吸作用的異化鐵還原菌還包括Anaeromyxobacter dehalogenes、Desulfitobacterium frappieri、Desulfitobacterium hafniense、Desulfitobacterium metallireducens、Desulfuromonas michigansis、Trichlorobacter thiogenes等[43-46]。

由于異化鐵還原反應(yīng)可以利用的電子供受體種類多種多樣，利用Fe(II)與Fe(III)之間的這種異化還原轉(zhuǎn)化過程來進行多種有機污染環(huán)境的修復(fù)也是一直以來的研究熱點。有研究表明，典型的異化鐵還原菌——希瓦氏菌(Shewanella)能夠通過還原Fe(III)過程來提高三氯、四氯、五氯等高氯代有機化合物的脫氯轉(zhuǎn)化效果[47-48]。Fe(Ⅲ)微生物還原過程成為聯(lián)系2個過程的“紐帶”：一方面，它可以以有機氯化合物作為電子受體直接還原脫氯；另一方面，它可為化學(xué)還原脫氯過程提供生物源Fe(Ⅱ)。我們前期通過馴化不同的功能菌群發(fā)現(xiàn)，鐵還原菌群可以通過耦合鐵氧化物的還原顯著提高五氯酚(PCP)的還原轉(zhuǎn)化，而且對PCP還原轉(zhuǎn)化能力的不同主要由菌群群落結(jié)構(gòu)不同導(dǎo)致，其中，我們發(fā)現(xiàn)另一類典型的異化鐵還原菌——地桿菌(Geobater)在該還原脫氯的過程中起重要作用[49]。此外，在鐵氧化物和腐殖質(zhì)充足的條件下，發(fā)酵型鐵還原菌C. beijerinckiistrain Z可以將大量的Fe(III)還原為Fe(II)，生成的吸附態(tài)Fe(II)可以顯著促進PCP的脫氯過程。Wu等[50](2010)研究發(fā)現(xiàn)鐵還原菌Comamonas koreensis CY01可以利用水鐵礦、針鐵礦、纖鐵礦或者赤鐵礦作為最終電子受體，同時促進了有機氯污染物2,4-二氯苯氧基乙酸(2,4-D)的還原。由此，鐵氧化物界面發(fā)生了鐵還原菌介導(dǎo)的異化還原溶解，并使得有機氯發(fā)生了鐵物種介導(dǎo)的化學(xué)脫氯反應(yīng)，這就形成了生物脫氯與化學(xué)脫氯的耦合反應(yīng)過程(圖1)，進一步揭示了氯代有機污染物的生物與非生物脫氯的交互反應(yīng)以及缺氧的污染區(qū)域中高氯代污染物的自然消解過程。

圖1 有機氯脫氯轉(zhuǎn)化的鐵還原菌-鐵氧化物界面交互反應(yīng)機制示意圖[51]Fig. 1 The mechanism describing the dechlorination of organochlorine in the dissmilatory iron-reducing system of dissimilatory iron reduction bacteria (DIRB) and iron oxide[51]

此外，一些對鐵還原過程產(chǎn)生影響的中間物質(zhì)也會間接影響到有機氯還原脫氯的過程，在這一多過程耦合的過程中，提高鐵還原速率是強化還原脫氯的關(guān)鍵。溶鐵螯合劑的使用一方面可增加反應(yīng)體系中生物可利用Fe(Ⅲ)的濃度，另一方面還可以提高Fe(Ⅲ)與鐵還原菌直接接觸的機率，進而提高鐵還原的速率[52-53]。電子穿梭體如腐殖質(zhì)、類腐殖質(zhì)物質(zhì)如anthraquinone-2,6-disulfonate (AQDS)等[54-56]，以及現(xiàn)在新興的污染修復(fù)材料如可導(dǎo)電的石墨烯、生物炭等[57-58]，提高了電子的運轉(zhuǎn)效率從而強化了COCs還原脫氯過程。

3 硫酸鹽還原與還原脫氯過程(Sulfate reduction process and reductive dechlorination process)

目前，硫酸鹽還原作用能否促進氯代有機物的還原消減仍然是一個有爭議的話題，硫酸根作為電子受體對還原脫氯影響的兩面性主要取決于硫酸鹽形式和濃度、微生物群落結(jié)構(gòu)的差異以及非生物還原脫氯對電子供體的競爭作用等眾多因素[64]。Chang等[65](2008)和Yuan等[66](2011)研究發(fā)現(xiàn)添加硫酸鹽可使氯代烴和多環(huán)芳烴的降解率分別提高25.5%和50%～77.8%。Yoshida等[67](2007)對稻田土壤中PCP的消減研究發(fā)現(xiàn)，外源添加的硫酸鹽濃度在20 mmol·L-1的時候會抑制PCP還原，而10 mmol·L-1的時候卻不產(chǎn)生抑制作用。而Ehlers等[68](2006)在分批補給反應(yīng)器中研究硫還原條件下硫酸鹽濃度對2,4,6-三氯酚(2,4,6-TCP)還原消減的影響時發(fā)現(xiàn)，當(dāng)硫酸根濃度在110 mg·L-1時增強了2,4,6-TCP的還原轉(zhuǎn)化，而在900mg·L-1時卻抑制了其還原脫氯。我們前期研究也發(fā)現(xiàn)，外源添加硫酸鹽對PCP還原脫氯過程的影響還與土壤類型有關(guān)，在黃斑田與黑土中表現(xiàn)為抑制作用，而對紅壤和潮土中的PCP脫氯則起到了一定的促進作用，且前者的抑制作用隨硫酸根濃度的增加而增強[17]。我們認(rèn)為這種抑制作用可能由兩方面造成，一是外源電子受體與PCP同作為電子受體的形成競爭，二是硫酸根的氧化性會提高環(huán)境氧化還原電位初始值并減緩了其下降速度。因此，在缺氧條件下，土壤本底條件、電子供體的種類和數(shù)量以及硫酸根濃度等均會影響氯代有機污染物的還原脫氯過程。

圖2 硫酸鹽還原菌的代謝過程[63]Fig. 2 The metabolic process of sulfate reducing bacteria[63]

Drzyzga等[69](2001)發(fā)現(xiàn)，在以乳酸鹽為碳源且無硫酸鹽作為電子受體的恒化器培養(yǎng)條件下，硫還原菌Desulfovibrio sp.與脫氯菌Desulfitobacterium frappieri TCE1仍可以共存，作者認(rèn)為二者可以通過氫氣和電子傳遞形成互營關(guān)系并降解四氯乙烯。也有研究發(fā)現(xiàn)，在以丁酸鹽(H2釋放基質(zhì))作為碳源的厭氧環(huán)境中，SRB會通過對H2的競爭來抑制四氯乙烯的還原脫氯，而且二者競爭的激烈程度取決于氫氣的釋放和消耗速率[70]。但也有研究表明，硫酸鹽還原反應(yīng)對H2的競爭并不是抑制脫氯的原因[71]。此外，有研究發(fā)現(xiàn)SRBs代謝還原產(chǎn)生的產(chǎn)物H2S反過來也會對微生物產(chǎn)生不利的影響，當(dāng)H2S的濃度達到547 mg·L-1時會完全抑制SRBs的生長[72]。而硫酸鹽還原過程雖然會與還原脫氯過程爭奪電子，但當(dāng)硫酸鹽被SRB還原成H2S后，其也可能作為強還原性物質(zhì)強化氯代化合物還原脫氯過程[73]。因此，碳源以及一些代謝中間產(chǎn)物如H2和H2S也可能成為影響硫酸鹽還原過程和還原脫氯過程之間關(guān)系的因素之一。

另外，一些硫還原菌本身也可能具有還原脫氯功能，它們在氯代有機化合物消減過程中起著不可忽視的作用，這些微生物能夠通過呼吸代謝PCBs、OCPs等氯代有機污染物，并從中獲取自身生長所需能量[74]。已有大量研究通過篩選分離SRB或其富集物進行了還原脫氯研究，證實這些具有硫還原功能的微生物也能夠直接參與脫氯[75-76]。微生物的生物脫氯代謝機制可以分為共代謝和脫氯呼吸，前者表現(xiàn)為微生物電子傳遞過程不以氯代有機物作為最終電子受體進行脫氯反應(yīng)，而后者則需要以之作為最終電子受體[77]，而具有脫氯功能的SRB究竟是以共代謝還是脫氯呼吸機制對進行脫氯這一關(guān)鍵問題目前尚不明確。

4 產(chǎn)甲烷過程與還原脫氯過程(Methanogenic process and reductive dechlorination process)

產(chǎn)甲烷(methanogensis)是一類可以將無機物或有機物在厭氧發(fā)酵條件下轉(zhuǎn)化為甲烷和二氧化碳的過程。甲烷產(chǎn)生的過程主要可分為4步：(1)水解過程：在好氧、厭氧或兼性微生物的作用下，土壤或其他環(huán)境中沉積的高分子有機物水解為單分子(如糖苷、脂肪酸和氨基酸等)；(2)酸化過程：在兼性或嚴(yán)格厭氧微生物的作用下，單分子化合物進一步酸解為酸性的小分子化合物(如甲酸、乙酸、丙酸等)；(3)乙酸生成過程：厭氧細(xì)菌或同型乙酸細(xì)菌將酸性小分子化合物繼續(xù)轉(zhuǎn)化為乙酸和其他小分子H2和CO2；(4)甲烷形成過程：產(chǎn)甲烷菌利用少數(shù)幾種C1、C2化合物(如CO2、HCOOH、CH3OH等)與H2還原生成甲烷[78]。產(chǎn)甲烷的發(fā)生要求體系中的氧化還原電位處于一個比較低的水平，而很多研究表明高氯代化合物還原脫氯過程的還原條件非常的嚴(yán)格，一般也只會發(fā)生在強還原條件下，因此，產(chǎn)甲烷環(huán)境也比較利于氯代有機物實現(xiàn)還原脫氯和進一步降解，產(chǎn)甲烷條件下氯代有機污染物的厭氧生物降解過程和機制也引起了諸多學(xué)者的關(guān)注。

產(chǎn)甲烷菌是一種嚴(yán)格厭氧菌，在生物發(fā)酵池、稻田、濕地，甚至一些溫泉、海底熱水噴口等與氧氣隔絕的極端環(huán)境中，幾乎都有產(chǎn)甲烷菌的存在。由于產(chǎn)甲烷菌處于有機物厭氧降解末端，但如果沒有產(chǎn)甲烷菌分解有機酸產(chǎn)生甲烷的平衡作用，必然導(dǎo)致有機酸的積累使得發(fā)酵環(huán)境酸化，其可能最主要的作用就是產(chǎn)物抑制作用的消除，使還原脫氯反應(yīng)能夠連續(xù)進行。再者，產(chǎn)甲烷菌所能利用的基質(zhì)有限，需要不產(chǎn)甲烷菌先將復(fù)雜有機物分解為簡單化合物，因此，前面幾步間接參與產(chǎn)甲烷過程的其他微生物也可能影響還原脫氯過程[79-80]。

而在以發(fā)酵型碳源乳酸作為電子供體時，Wen等[84]發(fā)現(xiàn)，產(chǎn)甲烷過程產(chǎn)生的中間產(chǎn)物H2以及產(chǎn)甲烷菌產(chǎn)生的輔酶等促進了脫氯菌Dehalococcoides的生長，從而促進了TCE的生物降解。Chen等[85]也是發(fā)現(xiàn)，SRB和產(chǎn)甲烷菌是濕地中氯仿生物降解過程中的關(guān)鍵微生物，通過共代謝的作用促進了氯仿的生物脫氯過程。當(dāng)然，也有研究發(fā)現(xiàn)產(chǎn)甲烷過程對還原脫氯過程并無影響[86-88]，因此，產(chǎn)甲烷過程對氯代有機污染物厭氧還原脫氯過程的作用與前面提到的氮/鐵/硫還原過程類似，是多種因素影響下的綜合表現(xiàn)。

5 結(jié)論與展望(Conclusion and prospect)

綜上所述，厭氧條件下，在自然界中，有多種還原環(huán)境存在，如典型的反硝化環(huán)境、鐵還原環(huán)境、硫酸鹽還原環(huán)境以及產(chǎn)甲烷環(huán)境，很多研究發(fā)現(xiàn)一些難降解的三氯、四氯、五氯等高氯代有機化合物均能在這些還原環(huán)境中發(fā)生。土壤中天然存在的不同形態(tài)的氮、鐵、硫等活躍的氧化還原能夠顯著影響這些難降解有機污染物的還原脫氯過程，反過來，有機污染物的種類、生物有效性以及毒性能夠顯著影響這些生源要素的轉(zhuǎn)化。二者的關(guān)系可能表現(xiàn)為抑制、促進或無關(guān)，土壤本底條件、電子供/受/穿梭體的種類和數(shù)量、以及參與代謝的微生物及中間產(chǎn)物等均會影響氯代有機污染物的還原脫氯過程。

此外，在厭氧環(huán)境中，氯代有機污染物的厭氧還原轉(zhuǎn)化與碳、氮、鐵、硫等生源要素生物化學(xué)循環(huán)過程本質(zhì)上均是微生物呼吸代謝介導(dǎo)的電子傳遞過程，這種過程受到電子供、受體之間協(xié)調(diào)性和競爭性的影響。但目前對于這種過程如何協(xié)調(diào)厭氧環(huán)境中微生物與土壤環(huán)境之間的交互效應(yīng)、誘導(dǎo)微生物種群和關(guān)鍵功能菌的定向變化、并通過改變電子傳遞路徑最終影響氯代有機污染物在厭氧環(huán)境中的還原轉(zhuǎn)化過程和機制還缺乏系統(tǒng)認(rèn)識。特別是如何基于改變電子供/受/穿梭體的數(shù)量(濃度)、種類等途徑調(diào)控電子傳遞流向，協(xié)調(diào)污染物快速削減與鐵硫等還原性毒害物質(zhì)產(chǎn)生、以及與溫室氣體排放的關(guān)系，實現(xiàn)多贏的修復(fù)目標(biāo)，在未來的土壤污染控制與修復(fù)研究中顯得尤為重要。

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◆

EffectsandMechanismsofSoilRedoxProcessesonReductiveDechlorinationofChlorinatedOrganicPollutants

Feng Xi1,2, Zhu Min1,2, He Yan1,2,*

1. Institute of Soil and Water Resources and Environmental Science, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310058, China2. Zhejiang Provincial Key Laboratory of Agricultural Resources and Environment, Hangzhou 310058, China

10.7524/AJE.1673-5897.20170114004

2017-01-14錄用日期2017-04-30

1673-5897(2017)3-151-11

X171.5

A

國家自然科學(xué)基金項目(41322006, 41771269); 國家重點研發(fā)計劃項目(2016YFD0800207)；中組部青年拔尖人才支持基金

馮曦(1992-)，男，碩士研究生，研究方向為有機污染土壤修復(fù)，E-mail:13588221213@163.com

*通訊作者(Corresponding author)， E-mail: yhe2006@zju.edu.cn

馮曦, 朱敏, 何艷. 土壤還原過程對氯代有機污染物還原脫氯的影響與機制[J]. 生態(tài)毒理學(xué)報，2017, 12(3): 151-161

Feng X, Zhu M, He Y. Effects and mechanisms of soil redox processes on reductive dechlorination of chlorinated organic pollutants [J]. Asian Journal of Ecotoxicology, 2017, 12(3): 151-161 (in Chinese)