地球科学进展, 2020, 35(10): 1052-1063 DOI: 10.11867/j.issn.1001-8166.2020.084

综述与评述

含铁介质稳定砷与根际微生物的相互作用

殷怡童,, 罗锡明,

中国地质大学(北京)海洋学院,北京 100083

Interactions Between Stabilized Arsenic by Fe-based Media and Soil Microbes in the Rhizosphere

Yin Yitong,, Luo Ximing,

School of Ocean Sciences,China University of Geosciences(Beijing),Beijing 100083,China

通讯作者: 罗锡明(1967-), 男,河北保定人,教授,主要从事海岸带地质环境、场地污染修复研究. E-mail:luoxm@cugb.edu.cn

收稿日期: 2020-07-15   修回日期: 2020-09-20   网络出版日期: 2020-11-30

基金资助: 中国地质调查局地质调查项目“莱州—莱西一带矿集区生态修复支撑调查”.  DD20208080
国家自然科学基金项目“纳米零价铁在含水层中的腐蚀过程与产物演化对三氯乙烯去除的影响机理研究”.  41572229

Corresponding authors: Luo Ximing (1967-), male, Baoding City, Hebei Province, Professor. Research areas include geological environment of the coastal zone, site contamination remediation. E-mail:luoxm@cugb.edu.cn

Received: 2020-07-15   Revised: 2020-09-20   Online: 2020-11-30

作者简介 About authors

殷怡童(1997-),女,浙江定海人,硕士研究生,主要从事污染修复研究.E-mail:kaqichuan@163.com

YinYitong(1997-),female,DinghaiCity,ZhejiangProvince,Masterstudent.Researchareasincludepollutionremediation.E-mail:kaqichuan@163.com

摘要

对原位修复后植物复植区土壤中被稳定砷的稳定性进行评估是环境风险评价的重要内容,含铁介质稳定砷与根际微生物的相互作用是环境风险评价的关键。根据近5年的相关研究进展,从含铁介质修复砷污染土壤、砷污染土壤中的微生物群落结构特征及根际微生物参与的含铁介质中砷的释放过程3个方面来进行综述。综述发现,含铁介质是稳定土壤中砷的重要材料,但修复效果易受到环境变化和微生物作用的影响。在含铁介质稳定砷之后,土壤中的微生物群落会受到修复试剂的影响进而形成新的群落结构,而在继续进行植物复植后,由于根际环境作用,微生物群落结构还会发生进一步的演化从而最终形成新的微生物群落结构。在这个演化过程中,微生物与所接触的砷和铁的界面反应是影响土壤中砷的稳定性的重要因素,因此研究含铁介质稳定砷与根际微生物的相互作用具有重要意义。此外,对植物复植后根际微生物与含铁介质稳定砷的稳定性之间的关系进行了展望,旨在为更好地认识土壤复杂环境中砷的迁移提供参考。

关键词: 含铁介质 ; 砷污染土壤 ; 稳定化 ; 微生物群落结构 ; 微生物还原

Abstract

Assessment of the stability of stabilized arsenic in soils in revegetated areas after in-situ restoration has been regarded as a key component in environmental risk assessment, while the interaction between stabilized arsenic by Fe-based media and soil microbes in the rhizosphere is also critical for environmental risk assessment. According to the relevant research progress in the last five years, the paper elucidated three key aspects of this problem: Arsenic-contaminated soil remediation with iron containing media, structural characterization of microbial communities in arsenic-contaminated soil, and the arsenic-releasing process that involves the participation of rhizosphere microorganisms on the Fe-based media. This review finds that Fe-based media is an essential material for stabilizing arsenic in soil, but its impact is strongly affected by circumstance change and microbial action. After arsenic stabilization, microbial communities in the soil would be changed by Fe-based reagents, then the further evolution of microbial community structure will occur during continuing revegetation because of rhizosphere effect, and finally new microbial communities will form. In this process, the interface reaction between rhizosphere microbes and iron containing media with arsenic is an important factor affecting the stability of arsenic in soils. Therefore, it is of great significance to understand this interaction in the future. Furthermore,the reaction between rhizosphere microorganisms in revegetated areas and the stability of stabilized arsenic was also discussed. The purpose of this paper was to provide more information for the mobility of arsenic in the complex soil environment.

Keywords: Fe-based media ; Arsenic-contaminated soil ; Stabilization ; Microbial community structure ; Microbial reduction

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本文引用格式

殷怡童, 罗锡明. 含铁介质稳定砷与根际微生物的相互作用. 地球科学进展[J], 2020, 35(10): 1052-1063 DOI:10.11867/j.issn.1001-8166.2020.084

Yin Yitong, Luo Ximing. Interactions Between Stabilized Arsenic by Fe-based Media and Soil Microbes in the Rhizosphere. Advances in Earth Science[J], 2020, 35(10): 1052-1063 DOI:10.11867/j.issn.1001-8166.2020.084

1 引 言

砷(As)是地壳中含量排名第20位的元素,是一种天然存在的致癌准金属,已被列为全球关注的10种主要有毒物质之一,长期暴露于砷污染条件下对人类有害12。砷是245种以上矿物的组成部分,其中包括砷酸盐(60%)、硅酸盐(20%)以及硫化物和磺酸盐(20%)化合物3。火山喷发、岩石矿化和森林火灾释放到环境中的砷是砷的自然来源;使用农药、杀虫剂、含磷酸盐的肥料、造纸用纸浆、化石燃料、采矿和冶炼过程释放的砷是砷的人为来源4

目前,人为活动已显著增加了土壤中砷的浓度,这些砷可能通过农作物直接危害人类的健康,因此土壤中砷的修复已成为全球关注的焦点问题。目前已提出多种方法来修复土壤中的砷,包括物理、化学和生物修复技术5。其中通过结合化学修饰试剂进行原位固定/稳定化修复砷的技术虽然不能减少土壤中总砷的含量,但可以通过络合、吸附和沉淀降低污染物的迁移率、生物有效性、浸出性和生物利用度,是一种较现实且具成本效益的方法6。已有的评估发现,在多种可以稳定砷的材料中,含铁介质是最有效的材料之一。

含铁矿物在自然界储备丰富,因此有良好的应用前景。但被含铁介质稳定的砷也易受到环境变化和微生物作用的影响而再次活化7,尤其土壤是一种既包含固相又包含液相的复杂基质8。当含铁介质应用于砷污染土壤来进行修复时,它们的独特性质可能会直接(相互作用)或间接(环境因素)地对土壤中的微生物群落结构造成影响,同时,植物复植后由于根际作用也会在一定程度上改变土壤环境和微生物群落结构。相对的,土壤中微生物群落在一定条件下会与含铁介质和砷发生反应从而导致砷的再释放。

土壤修复的目的包括降低污染物在土壤环境中的迁移风险、生态风险、健康风险,以及改善受污染的土壤以便于后期恢复植被,因此为了更好地评估植物复植后土壤中被化学稳定砷的稳定性,结合近年来国内外研究现状(图1),我们发现与砷污染相关的关键词主要包括固定/稳定化、含铁介质、土壤微生物群落结构等。内容涉及了含铁介质修复砷、砷污染土壤微生物群落和微生物反应等。因此本文从含铁介质修复砷污染土壤、砷污染土壤中的微生物群落结构特征以及根际微生物参与含铁介质中砷的释放过程反应3个方面进行综述。

图1

图1   以砷为关键词的共现网络图

Fig.1   A co-occurrence network with arsenic as the key word


2 含铁介质修复砷污染土壤

2.1 含铁介质的固化机理

含铁介质对土壤中砷的固定/稳定主要以2种方式进行:吸附、络合作用,沉淀或共沉淀作用。其中沉淀或共沉淀作用是利用砷氧阴离子与部分矿物发生铁砷沉淀或硫砷沉淀,以形成难溶的沉淀化合物或次生矿物9。然而在实际污染场地中,基于含铁介质的吸附和络合作用才是修复的主要方式。吸附可分为化学反应(形成内圈络合物)和物理吸附(形成外圈络合物)两类,有研究表明,AsO2-和AsO3-与铁氧化物的表面羟基(O-H)结合形成的内圈络合物,其稳定性远比通过氢键和静电吸附形成的外圈络合物稳定,因此可以达到更高的吸附能力1011。近年来新出现的含铁介质修复方法也多利用了吸附作用。零价纳米铁在土壤中形成独特的核—壳结构,可分别通过零价铁核和羟基氧化铁壳充当电子给体和吸附剂12。铈嵌入铁基氧化物合成Fe-Ce(FC),能显著拓宽其最优pH吸附范围至2~713。铁锰双金属氧化物(Fe-Mn)能通过直接吸附As(III)和将As(III)先氧化后吸附2种机制修复土壤砷污染14

2.2 近期常用含铁介质评估

含铁介质是有效稳定土壤中砷的材料之一,至今为止,由于其吸附性能,已经对多种含铁介质应用于修复被砷污染的土壤进行了广泛的研究。

在自然界中存在的各种铁矿物一直都是人们研究稳定砷的主要材料。自然界的铁(氢)氧化物根据不同的结晶程度,可以分成两大类含铁矿物:其中无定形或弱结晶型铁矿物包括水铁矿(ferrihydrite,5Fe2O3·9H2O或Fe5HO8·4H2O)、施威特曼石[schwertmannite,(Fe8O8(OH)8-2x(SO4x;1<X<1.75)]和微晶针铁矿(δ-FeOOH)等,而针铁矿(α-FeOOH)、赤铁矿(α-Fe2O3)、纤铁矿(γ-FeOOH)、磁赤铁矿(γ-Fe2O3)等都属于晶型铁矿物。

据先前的报道,无定形或弱结晶型铁矿物,尤其是水铁矿可以有效地稳定土壤中的砷15。这是因为水铁矿的非晶态结构具有较高的表面积(100~700 m2/g),因此它对砷具有很强的亲和力1617。根据Parfitt18的发现,可以推测铁氧化物对砷酸盐的吸附能力如下排列:无定形铁氧化物>针铁矿>赤铁矿。但也有结论认为在相同用量下,针铁矿的固砷效果要比水铁矿的固砷效果好9

无定形或弱结晶型的铁氧化物是当前最有效的稳定剂之一。但无定形或弱结晶型的铁氧化物并不总是稳定的,并且会受到环境的强烈影响,例如冻融循环会降低无定形氧化铁的含量;而它在土壤中也往往会逐渐转变为晶型铁氧化物,从而显示出较低的砷吸附能力19

在晶型铁矿物中,Salazar-Camacho等20的研究表明,在砷与针铁矿反应期间,针铁矿表面形成了砷酸盐表面络合物,这可以解释针铁矿对砷的高亲和力。而磁铁矿与赤铁矿的稳定效果相对针铁矿与水铁矿低得多,其固化效率为44%~45%21。在邵金秋等22的研究中,发现在赤铁矿、褐铁矿、菱铁矿、钛铁矿和磁铁矿等不同的天然含铁矿物中,褐铁矿对砷的吸附最强,但与合成铁基材料相比,其吸附能力仍然偏低。

铁(氢)氧化物的前体零价铁和铁盐[包括FeSO4、Fe2(SO43、FeCl3等]也已被证明具有良好的稳定砷的能力2324。然而铁盐的施用会导致土壤酸化并重新释放稳定的砷,因此还必须使用石灰来控制土壤的pH值,但这样同时也会增加其他重金属(例如Pb和Cd)的溶解度1523。当使用零价铁时,不会发生这种现象,有报道称零价铁对砷的稳定长期有效(可达15年)25

纳米铁材料因其高比表面积以及对砷的高吸附而受到广泛关注。当Fe3O4的粒径从300 nm减小至12 nm时,对As(III)和As(V)的吸附容量增加约200倍26。Yan等27机械活化了褐铁矿,发现减小粒径并增加比表面积后,褐铁矿结晶度降低,表面活性位增加,矿物相转化为无定形氧化铁物质。纳米颗粒具有更大的表面积/质量值和原子结构差异,从而导致反应活性发生变化,并且反应活性通常会大大提高28。大多数纳米铁研究是针对水样进行的29,但其在土壤修复中的作用最近引起了关注3031。纳米级零价铁(nZVI)是最常用的纳米材料,纳米氧化铁诸如Fe2O3NP也逐渐应用于土壤砷修复。

除了单一的含铁介质外,近几年也有研究者将含铁氧化物与其他介质结合在一起,从而改善了在土壤中稳定砷的效果,例如使用进行铁修饰改性的活性炭负载纳米二氧化钛32、铁锰双金属氧化物33以及铁铈双金属氧化物13等,除此之外也有使用含铁工业副产品进行固砷的。

铁氧化物及其前体、纳米铁、新兴含铁材料等均可作为修复砷污染的含铁介质,表1中列出了其稳定砷的效果,因为纳米铁材料和各类新兴含铁材料具有相对较高的吸附能力,因此有着广阔的应用前景。而自然界中存在着大量的自然含铁矿物,因此通过将自然含铁矿物机械活化以应用于稳定砷有可能成为新的选择。

表1   含铁介质修复土壤效果案例

Table 1  Cases of soil remediation effect by Fe-based media

土壤类型土壤pH砷浓度/(mg/kg)含铁介质用量修复效率/%
矿区石灰性始成土[34]7.63 197施威特曼石(FeOS)10%99
矿山壤砂土[15]8.2247水铁矿5%91
金锑矿的沉积区[24]4.6479合成水铁矿3%84
金锑矿的沉积区[24]4.6479氯化铁+石灰1∶171
金锑矿的沉积区[24]4.6479零价铁粉末1%90
砷污染土壤[27]7.4228机械活化褐铁矿10%78
矿山边缘土壤[35]6.32 548磁赤铁矿纳米颗粒5%99
棕地子区域A[31]6.470 200零价纳米铁10%92
棕地子区域B[31]6.425 900零价纳米铁10%91~95
自然高砷土壤[36]6.115 910零价铁纳米粉1%60
砷污染土壤[13]3.61 400

铁铈氧化物

铁铈氧化物

铁铈氧化物

2%

2%

2%

84~98
砷污染土壤[13]7.2200
砷污染土壤[13]5.0800

新窗口打开| 下载CSV


由此可见,含铁介质是良好的稳定砷材料。但向砷污染土壤中添加的含铁介质的稳定效率及稳定性会受到各种土壤环境化学和生物因素的影响,因此在实际应用前,应对修复位点的土壤环境进行调研,了解土壤类型、土壤pH、总砷浓度以及砷污染土壤中的微生物群落结构等信息。

3 砷污染土壤中的微生物群落结构特征

3.1 砷污染土壤及修复后土壤中微生物群落

对相关研究进行总结,在被砷污染的土壤样品中存在一些优势细菌(图237~45

图2

图2   砷污染土壤优势菌

Fig.2   Dominant bacteria in arsenic-contaminated soil


而对于污染土壤中的真菌群落,Zygomigota、担子菌门(Basidiomycota)、子囊菌门(Ascomycota)是相对优势的门,最丰富的属是毛霉属(Mucor)、被孢霉属(Mortierella)、Cryptococcus(隐球酵母属)和Pseudotomentella46

在门水平上,发现在砷污染较轻的位置,酸杆菌门(Acidobacteria)、绿弯菌门(Chloroflexi)、芽单胞菌门(Gemmatimonadetes)和疣微菌门(Verrucomicrobia)相对要丰富得多,其中Chloroflexi与总砷浓度呈负相关;而放线菌门(Actinobacteria)和蓝细菌(Cyanobacteria)在污染严重位置明显富集,且Actinobacteria、变形菌门(Proteobacteria)和浮霉菌门(Planctomycetes)与总砷浓度呈正相关3739。在纲水平上浮霉菌纲(Planctomycetia)和α-变形杆菌纲(Alphaproteobacteria)与总砷浓度呈正相关,β-变形杆菌纲(Betaproteobacteria)与总砷浓度呈负相关,因此在高砷胁迫下,属于Alphaproteobacteria的细菌被确定为受污染土壤中的主要微生物群落,而Betaproteobacteria则将在土壤中消失3741。希瓦氏菌属(Shewanella)和Steroidobacter常见于砷含量较高的土壤中而梭菌科(Clostridiaceae)、地杆菌属(Geobacter)、节细菌属(Anthrobacter)、紫色杆菌属(Janthinobacterium)、假单胞菌属(Pseudomonas)与总砷浓度呈正相关38~4147。在真菌中,随着砷污染的增加,Basidiomycota的相对丰度趋于增加46

在土壤修复过程中,对于添加含铁介质后土壤中的微生物群落结构,发现氧化铁会影响砷污染土壤的微生物群落例如拟杆菌门(Bacteroidetes)37。经过赤泥处理的砷污染土壤中增加了Proteobacteria和Bacteroidetes的相对丰度,微降低了厚壁菌门(Firmicutes)和Planctomycetes的相对丰度,并显著降低了Acidobacteria的相对丰度48

相较于铁矿物,近年来纳米铁对土壤微生物造成的影响受到更多的关注。对于纳米氧化铁,在一项研究中,发现纳米氧化铁不会显著影响微生物生物量C和N,但会影响C/N微生物值并增加代谢商49。但也有研究发现,向砷污染土壤中添加纳米氧化铁后土壤总微生物(细菌、真菌和放线菌)的生物量会降低,并且认为纳米氧化铁是通过调节土壤pH值来影响土壤微生物群落组成50

零价纳米铁颗粒是更常应用的土壤修复材料,对土壤微生物的影响似乎较为复杂。零价纳米铁的添加虽然可以通过引起氧化胁迫来抑制金属(类金属)污染土壤中丛枝菌根真菌的发育和功能,但也会增加土壤中Shewanella的丰度,零价纳米铁对沙壤土土壤微生物的抑制作用相较于黏土壤土更加明显,这是因为土壤有机质的作用85152。添加泡沸石负载的零价纳米铁后,属于Firmicutes的优势属芽孢杆菌属(Bacillus)、类芽孢杆菌属(Paenibacillus)和环脂酸芽孢杆菌属(Alicyclobacillus)的相对丰富度显着增加,相对的,抑制了革兰氏阴性细菌(黄单胞菌科(Xanthomonadaceae)、马赛菌属(Massilia)和溶杆菌属(Lysobacter))的生长12。最终部分细菌被淘汰并推动了本地细菌群落的重建,新的微生物群落在土壤复植后又经历新的演化过程,最终形成新的种群结构。复植后土壤微生物群落的演化在一定程度上是影响稳定化砷重新释放的重要因素。

3.2 砷污染土壤及修复后植物根际微生物群落

根际是连接植物、土壤和微生物的纽带,根际土壤作为土壤的一个重要的子系统,也因此导致其特性与土壤基质完全不同36。植物复植会对已有的土壤环境微生物群落结构造成影响,且由于根际效应,与植被区非根际土壤相比,根际土壤中具有更高的微生物群落。

在工矿复垦区,自然恢复根际微生物与种植植物根际微生物丰度和多样性具有很大差异53;且与没有植被的地点相比,经过植被恢复地点的细菌、真菌密度更高54。Tipayno等55强调在高度污染的土壤中引入植物后明显发生细菌群落的重组。Sun等56研究了铜矿尾矿中3种先锋植物的根际细菌群落的多样性和结构,发现根际土壤中的Alphaproteobacteria,δ-变形杆菌纲(Deltaproteobacteria)、Chloroflexi、Acidobacteria和Gemmatimonadetes显示出相对较高的相对丰度;相反,在裸露的尾矿中,γ-变形杆菌纲(Gammaproteobacteria)和Firmicutes占细菌群落的大部分。在富砷土壤中生长的蜈蚣草的根际中Proteobacteria、Actinobacteria和Chloroflexi的相对丰度增加57。甚至于在砷污染的水稻田中,由于铁斑的存在,水稻根部具有铁斑的部分、根际土壤和非根际土壤的微生物群落结构存在差异,根际土壤中以Acidobacteriales、黏球菌目(Myxococcales)和除硫单胞菌目(Desulfuromonales)为主,而根部铁斑部位的菌群富含假单胞菌目(Pseudomonadales)、伯克霍尔德氏菌目(Burkholderiales)、鞘脂单胞菌目(Sphingomonadales)和根瘤菌目(Rhizobiales)58

对于添加铁介质后的植物根际微生物的变化,有研究表明添加不同浓度纳米Fe3O4,丰度较高且变化较大的为Betaproteobacteria,其次为Cyanobacteria、Acidobacteria、Gemmatimonadetes和硝化螺旋菌门(Nitrospirae),其余菌群的相对丰度变化较小59。而对于真菌,有研究称Fe3O4 纳米颗粒会减少丛枝菌根真菌群落多样性并改变其组成,从而对丛枝菌根真菌与寄主植物之间的相互作用产生负面影响60

在使用含铁介质修复的砷污染土壤中种植植物,根据植物种类不同,根际微生物群落之间存在的差异也会很大。由于水稻是世界上最重要的农作物之一,且在水稻种植期间交替的水淹条件会极大地改变土壤的氧化还原条件并影响砷的形态,因此,近年来对砷污染稻田土壤的修复进行了大量深入研究。有研究称Fe-Mn-Ce改性生物炭增加了砷污染稻田土壤中土壤酶的活性,并极大地影响了根际微生物,包括Proteobacteria, Acidobacteria和Gemmatimonadetes的相对丰度61。在整个水稻种植期间,根际硫酸盐还原菌的相对丰度会随着钛石膏(主要由结晶石膏和无定形氢氧化铁组成)的添加而增加,但铁还原菌相对丰度变化不大62。而在添加了纳米零价铁改性生物炭的水稻根际,Geobacter的相对丰度增加但真菌中的Ascomycota的相对丰度降低63

综上所述,砷浓度和含铁介质的添加都会对土壤中的微生物群落结构造成影响,并且在植物复植后,由于复杂的根际行为,也会使微生物群落的结构发生改变。对于被污染土壤的修复区,复植后的非根际土壤及根际土壤微生物群落对研究环境中铁和砷的微生物反应以及被固定砷的稳定性有着重要意义,然而对这方面的研究文献较少,因此有必要展开相关工作,特别是根际微生物与固定砷的含铁介质在界面上的反应是揭示微生物与矿物界面反应机制的重要环节。

4 根际微生物参与含铁介质中砷的释放过程

4.1 含铁介质还原溶解释放砷

Fe(III)矿物的还原溶解是向环境中释放砷的一个重要途径,而微生物在铁循环中扮演着重要角色64。Fe(III)还原的生物机制主要归因于发酵或微生物呼吸的间接作用,主要涉及微生物包括发酵微生物、硫酸盐还原细菌和异化铁还原细菌,其中异化铁还原细菌一直是主要的研究对象65

在异化铁还原细菌存在下,氧化铁(羟基氧化物)的生物还原通常受微生物细胞外电子转移过程中的3种不同策略控制。第一种策略是直接接触机制,需要微生物与电子受体[例如,Fe(III)-氧化物]之间足够的接触面积,涉及铁还原酶和细胞色素6667。第二种策略是由电子穿梭控制的,它可以加速某些特定的氧化还原反应6468。第三种策略使用导电的纤毛(菌毛)介导的机制,其中纤毛在特定异化铁还原细菌的细胞与氧化铁(羟基氧化物)之间转移电子69ShewanellaGeobacter是已知最重要的异化铁还原细菌属,已经建立了微生物铁还原的生理生化模型70,其中Geobacter sulfreducens是将导电纤毛(菌毛)和细胞色素组合在一起的代表性物种71

GeobacterShewanella以外的属中铁还原机理的信息还很少。有研究显示,常规异化铁还原细菌在与水铁矿的孵育过程中展现出较高的Fe(III)还原速率,但与之相对的,这类细菌很难还原结晶型的氧化铁矿物,仅发现了较小量的Fe(III)还原65。因此近年有人开始研究发酵细菌与硫酸盐还原细菌对铁氧化物的还原作用。Gagen等72报道了Alphaproteobacterial纲Telmatospirillum属的铁还原菌基因组,称促进微生物发酵是还原结晶型铁氧化物的有效策略。

发现在富含砷的根际土壤中与Fe(III)还原(e.g., Geothrix fermentans Geobacter pickeringii, Carboxydocella ferrireducensGeobacter argillaceus, Geobacter toluenoxydans),SO42-还原(e.g., Desulfovibrio oryzae, Desulfurispora thermophila, Thermodesulfovibrio thiophilus, Thermodesulfovibrio aggregans)有关的一些优势种明显含量较高,表明可能在增强Fe(III)还原中起作用57。日本稻田中也发现某些Fe(III)还原的细菌,包括厌氧粘细菌(Anaeromyxobacter)和Geobacter,以及兼性Fe(III)还原的梭菌纲(Clostridia)7374

微生物生物还原铁的策略,除了上文提到的3种,根际环境也促进了另一种氧化铁(羟基氧化物)的生物还原策略,即通过铁螯合剂来增加Fe(III)溶解还原75。这是因为微生物产生的铁载体(siderophore)是低分子量(<1 500 Da)的Fe3 +特异性螯合剂76。大多数铁载体生产者仅限于植物根际77,在环境中具有异羟肟酸酯部分的铁载体可能会发生水解并生成羟胺基团,在此过程之后,将Fe(III)还原为Fe(II)78

4.2 砷从吸附位点解吸

砷从含铁介质上的还原解吸是砷的另一个释放途径。微生物参与砷的解吸过程一般都认为优先进行As(V)的还原,因为含铁介质对As(III)的吸附比对As(V)的吸附性低得多,并且As(III)迁移性较强,当As(V)被还原成As(III)后,As(III)易于从含铁介质上解吸下来。不过也有研究表明,Shewanella putrefaciens可能通过细菌磷酸基或羧酸基与铁矿物相互作用,促进As(V)解吸至可溶性相,然后再还原溶解的As(V)79

砷的微生物还原主要涉及2种机制,砷抗性系统和砷还原系统。砷抗性系统涉及的还原酶ArsC存在于细胞质中,对于进入细胞的水溶态As(V),能够将其还原并外排,属于解毒作用。砷还原系统的呼吸性砷酸盐还原酶Arr(由亚基ArrA和ArrB组成)位于细胞外周胞质中或作为膜结合的周质蛋白,在周质中具有催化位点80

因为2种酶在细胞中的分布位置不同,因此有研究认为仅具有ArsC的微生物不能直接还原被吸附在介质上的砷,而具有Arr的微生物可还原吸附在土壤或合成矿物上的砷81。但有研究发现土壤中ArsC相对丰度与As(III)浓度呈正相关42。Meharg等82提出,细胞质途径在土壤中起着更重要的作用,因为细胞质中砷的还原是由多种微生物在有氧和厌氧条件下进行的,而异化砷的还原仅是由少数微生物在厌氧条件下进行的。宏基因组学分析中所观察到的,土壤中arsC的丰度比arrA的丰度更高,使这一观点得到了支持83。然而另一方面,有研究提供了土壤DNA的证据,表明arrA基因的丰度和群落成员与土壤砷浓度有关,但arsC基因没有发现关系84

在厌氧呼吸中使用As(V)作为末端电子受体的含有呼吸砷酸盐还原酶(arr)的微生物称为异化砷酸盐呼吸原核生物(Dissimilatory Arsenate-Respiring Prokaryotes,DARPs)。DARPs在严格的厌氧条件下通过利用有机化学物质或无机化学物质作为末端电子给体(Terminal Electron Donor,TED)将As(V)还原为As(III),包括Shewanella,脱亚硫酸菌属(Desulfitobacterium),脱硫芽孢弯曲菌属(Desulfosporosinus),BacillusGeobacter和硫磺单胞菌属(Sulfurospirillum85~89。此外硫酸盐还原细菌可以利用通过硫酸盐还原过程所产生的硫化物来还原被吸附的As(V)90

由于富集砷,根际土壤中Deltaproteobacteria的相对丰度较高,且Deltaproteobacteria纲内的一些属因其在多种环境中对铁、砷和硫的还原作用而闻名577091。部分ShewanellaGeobacter和硫酸盐还原细菌被发现即能够还原As(V)也能够还原Fe(III),因此可能同时释放Fe(II)和As(III)9293,例如近期发现Shewanella putrefaciens同时还原了As(V)和水铁矿94。另一方面,发现氧化铁纳米颗粒可以用作促进土壤微生物种群,尤其是Geobacter呼吸的电子导管,并且可以增强Geobacter介导的水铁矿还原9596。因此,还需要进一步研究微生物与固定砷的含铁介质的界面反应。

图3

图3   砷释放途径

Fig.3   Pathways of arsenic release


5 展 望

由于砷污染土壤借由食物链影响着植物动物以及人类,因此修复被砷污染的土壤已经是当今关注的重点方向。使用含铁介质原位稳定砷污染土壤,被视为一种常用有效的修复方法。但由于多种因素影响,该方法固定的砷容易发生二次活化而再释放到环境中。前文内容大致从含铁介质固定砷、砷铁影响土壤中微生物以及微生物参与含铁介质中砷的释放3个方面进行了综述。通过对相关文献进行分析,发现当今对于污染土壤中的微生物群落的研究中,微生物主要作为被影响因子或者诊断污染状况的环境指标,通过改变其他环境条件从而导致微生物群落结构特征发生改变,并通过某类微生物相对丰度的变化和土壤环境中相关元素的变化来推测环境中发生的主要微生物反应,但由于土壤环境较为复杂,此类推测也存在问题。有文献研究了施用单种微生物对土壤环境中砷铁的影响,但仍缺乏有关复杂微生物群落(例如根际微生物)对土壤环境中砷铁作用的研究。同时,近几年对于砷迁移的研究对象主要为稻田土壤,缺乏对普遍土壤环境及较为常见植物根际环境的研究,并且在含铁介质固定砷方面大多为研究含铁介质固定砷的有效性以及通过多种评价方法例如植物指示法来评估土壤中砷的稳定性的短期实验,相对的,缺乏植物复植后对砷修复效果稳定性的长期研究。因此需要将含铁介质固定砷与复植根际微生物串联在一起,进一步提出复植根际微生物作用可能对固砷铁介质产生的影响。

因此今后的研究重点应该集中在以下3个方面:在稳定化后的土壤中进行植物复植时,根际微生物群落结构的演化过程;根际微生物群落结构演化过程中与含铁介质稳定砷的稳定化之间的关系;如果两者之间存在关系,是什么样的关系,涉及的实际作用机制是什么。

同时复植后土壤根际环境和微生物群落在砷和铁的地球环境命运中起着重要作用,一方面,微生物本身是砷和铁生物转化的主要动力,另一方面植物根际作用也会对砷铁的地球化学循环和微生物群落造成影响,因此对于植物根际—微生物—固砷含铁介质在土壤中的长期相互作用以及可能会对其他金属浓度造成影响的研究是极其有必要的。这有助于我们进一步评估植物复植后土壤中固砷的稳定性。

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