网络首发时间: 2019-05-17 10:48
稀有金属 2021,45(01),93-105 DOI:10.13373/j.cnki.cjrm.xy19040039
铀污染的微生物修复技术研究进展
钟娟 刘兴宇 张明江 闫潇 胡学武
摘 要:
铀矿冶行业的迅速发展造成了铀尾矿(渣)的大量堆积,对周围土壤和地下水的污染日益严重,对人类健康和社会安全造成潜在威胁,铀污染的治理已成为亟待解决的环境问题。针对铀污染的修复技术层出不穷,实践证明只使用传统物理化学方法修复铀污染有时很难达到理想的修复效果,并且修复成本通常非常昂贵。而微生物修复技术的出现,为铀污染的修复提供了更绿色,经济,高效,稳定的手段,引起了研究学者们的广泛关注,具有较大的研究价值和广阔的应用前景。首先对现阶段铀污染的修复技术进行了简单介绍,并重点综述了铀污染的微生物修复技术及其在国内外研究发展现状,接着介绍了铀污染微生物修复作用机制以及环境因子对铀污染修复效果的影响,最后分析了目前铀污染微生物修复技术存在的问题,并提出未来有待深入研究的方向,为铀污染的修复提供了新思路。
关键词:
铀污染 ;微生物修复 ;生物还原 ;修复机制 ;影响因素 ;
中图分类号: X172;X591
作者简介: 钟娟(1995-),女,湖南衡阳人,硕士研究生,研究方向:环境污染微生物修复,E-mail:15707332838@163.com;; *刘兴宇,教授,电话:010-82241312,E-mail:wellwoodliu@163.com;
收稿日期: 2019-04-19
基金: 国家自然科学基金项目(U1402234,41573074); 国家重点研发计划项目(2018YFC1802702,2018YFC1801803); 广西科学研究与技术开发计划项目(桂科AB16380287,桂科AB17129025); 国防科工局核设施退役及放射性废物治理科研项目(科工二司[2018]1521号)资助;
Research Progress of Bioremediation Technology for Uranium Contamination
Zhong Juan Liu Xingyu Zhang Mingjiang Yan Xiao Hu Xuewu
National Engineering Laboratory of Biohydrometallury,GRIMAT Engineering Institute Co.,Ltd
Abstract:
Anthropogenic activities such as uranium mining activities,primarily associated with decades of nuclear fuel production andweapon making,had accumulated a huge amount of uranium tailings(residue)and abundant hazardous uranium waste.Thus,caused anincreasingly serious uranium contamination in surrounding soil and under ground water,which posed a potential threat to the humanhealth and social security around the world.In particular,radioactive uranium commonly present as the uranyl cation,is highly solubleand mobile under oxidizing conditions and poses great danger to human health.Therefore,it is significant to focus on developing efficientremediation and long-term remediation strategies of increasingly severe uranium contamination.Various methods for the remediation ofuranium were emerged in an endless stream.Traditional physical and chemical remediation approaches based on pump and treat practice,such as lime neutralization,anion exchange,activated aluminum and biosorption,were not only prohibitively expensive but could also belimited by poor extraction efficiency,inhibitory competing ions and massive waste production.Besides,bringing the radioactive contami-nants up to the surface could increase health and safety risks for cleanup workers and the public.So,there was a great need for cost-effec-tive alternatives to treat uranium-contaminated groundwater and prevent its further migration and spread through the deep subsurface.However,the emergency of bioremediation technologies provided an eco-friendly,high-efficiency,stably to solve this problem.Bioreme-diation technologies had attracted extensive attention of researchers,which had great research value and broad application prospect.It wasfound that under the stress of uranium,microorganisms used the deposit or other external substances to gain energy for metabolism in orderto grow and survive,and deposited the dissolved U(VI)through biological reduction,biomineralization and other ways,reducing the con-centration of U(VI)in the solution.At present,the mechanisms of microbial remediation of uranium contamination mainly included biore-duction,biomineralization,biosorption and bioaccumulation.Uranium bioreduction had been proposed as a bioremediation technique,stimulated by adding an electron donor to promote enzymaticreduction of aqueous U(VI)to insoluble U(IV).The speciation of bioreduceduranium was often stated to be uraninite(UO2).Bioreduction was widely used in field tests for its high efficiency,low cost and simple oper-ation.However,potential concerns associated with the use of bioreduction as a remediation technique from whether reduced U(IV)wouldbe stable over long time periods,especially when the environment changed,such as the presence of oxygen and nitrate,solidified U(IV)would be reoxidized to the dissolved U(VI).Biomineralisation referred to the process by which metals precipitated with microbially gener-ated ligands such as sulfide or phosphate,or ascarbonates or hydroxides in response to localized alkaline conditionsat the cell surface.Adding inorganic phosphate directly into the contaminated area could also precipitate U(VI).However,due to its high activity,it was like-ly to precipitate rapidly with metal ions,resulting in a decrease in the permeability coefficient of the adding point.Compared with bioreduc-tion,biomineralization of U(VI)phosphate was observed over a wide p H range and in the presence of high uranium and nitrate concentra-tions,and might be a complementary approach to bioreduction.The cost of organophosphate was the biggest obstacle to the wide applica-tion of biomineralization remediation technology.In addition,some researchers believed that the rapid precipitation of metals on the cellsurface could hinder the cell metabolism and reduced the remediation effect.Biosorption described the passive uptake of uranium to thesurface of living or dead microbial cells.Despite the potential for bacteria to biosorb uranium,it was unlikely to be useful in the context ofbioremediation.Problems associated with biosorption were that desorption from cell surfaces could be as rapid assorption,and other cat-ions competed for binding site.Cell surfaces could also quickly become saturated,preventing further biosorption.Sorbed material couldbe re-released to solution when cells died and decomposed.Microbial cells were also able to accumulate a broad range of metalions via“bioaccumulation”mechanisms.Although of academic interest,there was scant evidence suggesting bioaccumulation of uranium wouldbe a viable technique for bioremediating contaminated land or water.Uranium remediation rates and effects had been extensively studied,with most studies conducted in microcosms containing suspended cells.However,the results of remediation rates and effects between stud-ies or between different microbial species were often difficult to predict because of the large variation in the experimental conditions.Manyphysical,geochemical and biological factors could affect the remediation effects,including aqueous uranyl speciation and uranium initialconcentration,cell concentration,temperature and p H,electron donor,bicarbonate,competing electron acceptors and other compounds.Besides,these factors and their effects were often closely coupled.The mobility and bioavailability of uranium varied with the type andcomplexity of the complex formed by uranium.And other culture parameters could substantially influence the reduction effects,becausethey influenced U speciation in aqueous environments.Thus,an understanding of U speciation was pivotal when considering the designand operation of bioremediation systems for U(VI)removal.In addition,many of the parameters,such as p H,temperature and high con-centrations of other metal ions could have direct effects on the growth and activity of U(VI)-remediation microorganisms.Overall,it wasclear that microbial remediation had a significant impact on uranium contamination across a wide range of environments and would be im-portant in managing contaminated land sites.But there were also some shortcomings that needed to be solved urgently.The long-term sta-bility of the bioreduced U(IV)solids was still questionable and further research was needed to underst and the mechanisms that led to theformation of most stable U(IV)products to effectively decrease U(IV)reoxidation and ensure longevity and environmental safety.Re-search worth further exploration in the future included:(1)Screening and cultivating the strains with higher remediation efficiency;(2)In the premise of high effect of remediation,choose cheaper carbon and phosphorus sources to promote the remediation of uranium contam-ination by target strains;(3)Molecular biological research methods were used to understand the metabolic pathways and figured out thesuccession of microbial community in the whole process of remediation.(4)Combined remediation method was adopted to make up for thepossible deficiency of single remediation method,so as to obtain better remediation effect and shorten remediation period.
Keyword:
uranium contamination; bioremediation; biological reduction; remediation mechanisms; influence factor;
Received: 2019-04-19
随着各国核电的大力发展,铀资源需求不断增加,铀矿的开采、冶炼迅速发展起来,全球铀产量以惊人的速度增长,随之而来的是铀尾矿(渣)的大量堆积
[1 ]
。据不完全统计,全世界铀尾矿总产量已高达200多亿吨。我国的铀尾矿等固体废物堆放场约有200处,分布在14个省区30多个地区
[2 ]
。结合我国实际国情,有关部门制定了《铀矿冶设施退役治理环境管理规定》、《核安全与放射性污染防治“十三五规划”及2025年远景目标》、《中华人民共和国核安全法》等一系列与铀矿冶退役治理有关的标准和法律。
堆积的铀尾矿(渣)经风化、降雨淋滤、微生物等共同作用下,其中的放射性物质和重金属离子不断浸出,并随雨水和扬尘迁移扩散,进入周边土壤甚至渗入地下水。在被污染水体中,铀通常以铀酰离子(UO2 2+ )形态存在,UO2 2+ 能和其他阳离子以及碳酸根、磷酸根、硫酸根等形成各种盐类化合物
[3 ]
。这类形态铀的化合物通常溶解度高,极易随地下水迁移流动,对人类健康和环境造成长期潜在威胁。铀对人体健康的危害主要表现在重金属化学毒性和放射性辐射危害两种形式,其中化学毒性对人体的危害远大于放射性毒害作用。进入人体的铀主要蓄积于肝脏、肾脏和骨骼中,可引起急性或慢性中毒,诱发多种疾病或导致突变、畸变甚至癌变
[4 ]
。因此,铀污染的修复和防控已成为亟待解决的环境问题。
微生物修复铀污染技术是一种起步较晚但发展潜力巨大的修复技术。微生物主要通过生物还原、生物矿化等方式与铀发生相互作用,改变铀的赋存状态,降低铀在环境中的迁移率,以减轻其毒害作用
[5 ]
。该法具有广阔的应用前景,近年来备受研究学者关注。本文首先介绍了铀污染修复技术现状,接着重点综述了铀污染的微生物修复技术,包括铀与微生物相互作用机制以及铀污染微生物修复影响因素,最后对微生物修复技术面临的机遇与挑战进行了展望,以期为我国铀污染的研究和修复治理提供理论依据。
1 铀污染修复技术研究概述
铀污染的修复技术大致分为物理、化学以及生物法3种。传统物理化学法是指覆土法、化学还原沉淀法、离子交换法、石灰中和法、吸附法等,生物法主要包括植物法和微生物法
[6 ,7 ]
。目前,我国铀矿冶废物管理实践中,一般首先用石灰中和尾矿(渣),提升其p H值,减少铀和部分污染物的溶出,从源头控制污染物的产生。对已产生的渗水经收集后主要采取化学沉淀法和吸附法的方式处理。这些处理方式见效快,并能将污染物从渗水中彻底去除。但也存在一定的弊端,如石灰中和法存在反酸、铀持续溶出的问题;沉淀法沉淀物量大、效果不稳定;吸附法去除效率低、选择性弱。
微生物修复是生物修复技术的重要组成部分,是指利用天然存在的或者培养的功能微生物,在适宜的环境条件下,通过微生物的非代谢性生物吸附和代谢性氧化还原作用的一种新型修复技术
[8 ]
。铀污染的微生物修复技术主要是通过往污染环境中加入电子供体及其他化学物质促进污染地区特定微生物的生长,以加快污染核素的还原和固定。这些微生物包括土著菌、外加菌株以及基因工程菌。微生物修复具有成本低,操作简单,对环境搅动性小,无二次污染等优点。另外,微生物对金属的固化效果稳定且可持续修复,使尾渣不再产生新的溶解态U(VI),从源头上解决铀的溶出问题,降低潜在环境污染风险。
目前,在我国已构建了特定功能菌的筛选技术,筛选出了能还原、吸附铀的微生物
[9 ]
。研究了通过控制有机碳源和有机磷源的加入刺激微生物对U(VI)的还原和矿化以及微生物治理技术关键环境因子的影响效果
[10 ,11 ,12 ]
。建立了菌根真菌和螯合剂等强化植物根际修复铀污染的技术
[8 ]
。总体上,国内外微生物修复铀污染的研究工作主要集中在(1)筛选和驯化特异性高效微生物菌株;(2)通过外加物质提高功能微生物在污染区域中的活性;(3)铀与微生物之间的相互作用和微生物对铀的成矿机理;(4)修复过程参数的优化和温度、p H、碳源等关键因子的调控等方面。
从国内铀污染防治现状来看,微生物修复技术在含铀污染物的治理方面具有良好的应用前景。采用微生物修复技术已在国内多个非铀金属矿污染治理中获得应用,但在铀污染治理中的应用还属于研究阶段,尚未有工程实践。而国外已就微生物修复技术开展了多次工程试验。例如,Anderson等
[13 ]
和吴唯民等
[14 ]
分别在美国科罗拉多州和橡树岭开展铀污染修复试验,通过往污染地区注入醋酸和乙醇原位刺激异化金属还原菌的生长,研究发现U(VI)的浓度均下降,并达到预期标准。
近年来,基于微生物的生物处理技术正在兴起,形成了与常规的物理、化学法并存,相互补充、共同完善、协调发展的新格局。微生物修复技术在污染防治方面显示出明显的技术优势,采用微生物技术防控铀污染具有良好应用前景。
2 微生物修复铀污染的作用机制研究进展
研究发现,在铀的胁迫下,微生物为了生长和存活而不断适应环境变化,利用矿床或外加物质进行新陈代谢的同时,通过生物还原、生物矿化等方式使溶解态的U(VI)沉积下来,减少溶液中U(VI)的浓度。目前微生物修复铀污染的作用机制主要包括生物还原、生物矿化、生物吸附以及生物富集4种,如图1所示。
2.1 生物还原
生物还原是在缺氧条件下,微生物利用醋酸盐、乳酸盐、H2 等作为电子供体,将易迁移的U(VI)还原为较稳定的U(IV),还原得到的U(IV)主要以晶质铀矿(UO2 )的形态存在
[16 ]
。20世纪60年代,Woolfolk和Whiteley
[17 ]
证明了微生物还原U(VI)的能力,然而直到30年后Lovely等
[18 ]
才首次提出利用铁还原菌将地下水中的可溶性U(Ⅵ)转化为稳定的、溶解度低的U(Ⅳ),进而防止其迁移扩散的设想。
研究表明,铁还原菌如Geobactermetallireducens,硫酸盐还原菌如Desulfovibriovulgaris和Desulfovibriodesulfuricans strain G20等微生物均可还原U(VI)
[19 ]
。硫酸盐还原菌(SRB)的生长主要是利用有机质还原SO4 2- 为H2 S。然而若环境中SO4 2- 不足以提供充足的电子受体时,SRB可通过生物代谢将U(VI)还原为U(IV),并从中获取代谢所需能量。Zhou等
[20 ]
研究发现实验5 h内Desulfovibrio vulgaris对U(VI)的还原率高达80%,同时蛋白质的含量增加了19%,证实了D.vulgaris能够在还原U(VI)的同时促进自身的生长。另外在接近中性环境下,U(VI)与Fe3+ 有相似的氧化还原电位,若Fe3+ 供给不足或环境中存在大量U(VI)的条件下,铁还原菌(Fe RB)也能将U(VI)代谢性还原为U(IV)
[21 ]
。此外,发酵细菌、嗜酸菌、粘细菌等也可用于铀的还原
[22 ]
。U(VI)的生物还原产物通常为UO2 ,但也有研究人员发现以其他形态存在的U(VI)的还原产物。例如,Thermoterrabacterium ferrireducens可以还原U(VI)为[(NH4 )(UO2 )(PO4 )·3H2 O],而Shewanellaputrefaciens CN32能将固体矿物中的U(VI)还原为(UO3 ·2H2 O)
[10 ]
。
图1 微生物修复铀污染的作用机制示意图
Fig.1 Schematic diagram of bioremediation mechanisms of uranium contamination
[15]
尽管研究表明微生物可以还原U(VI),但其还原机理尚未完全掌握。大多数研究表明直接酶促反应是介导U(VI)还原的主导机制
[23 ,24 ]
,但也有人认为U(VI)的还原是由Fe2+ ,H2 等介导的非生物还原作用
[25 ]
。生物还原修复效率高,成本低,操作简单,被广泛应用于现场试验中。但生物还原后的U(VI)可能难以长时间维持稳定,特别是当环境发生变化时,例如有氧气和硝酸盐存在时固化的U(IV)会重新氧化为溶解态的U(VI)。
2.2 生物矿化
生物矿化,也称生物沉淀,指微生物通过细胞表面局部碱化作用使U(VI)沉淀生成碳酸盐或氢氧化物
[26 ]
,或U(VI)与微生物酶促作用生成的配位体如磷酸盐,草酸盐等共沉淀生成HUO2 PO4 ,[Ca(UO2 )2 (PO4 )2 ]等稳定的配合物
[27 ]
。20世纪90年代初,人们首次发现Serratia能将U(VI)以磷酸铀酰盐矿物的形态沉淀下来,当时的做法是加入甘油磷酸盐后,发现Citrobacter sp.(后被重新分类为Serratia)分泌出的磷酸酶能分解该有机磷酸盐,释放出的无机磷酸盐与U(VI)反应生成胞外HUO2 PO4
[28 ]
。而美国橡树岭实验室研究发现在Ca2+ 浓度比较高的情况下会形成[Ca(UO2 )2 (PO4 )2 ]沉淀。另外,丝状真菌如Aspergillus niger尽管在含铀培养基中生长缓慢,但由于其发达的菌丝也能沉淀大量的磷酸铀酰矿物。Liang等
[29 ]
在改性察氏培养基中加入硝酸铀酰和有机磷酸盐,发现在Aspergillus niger和Paecilomycesjavanicus表面可以观察到铀的磷酸盐沉淀。
直接向污染区域加入无机磷酸盐也能沉淀U(VI),但由于其非常活泼,很可能与金属离子迅速生成沉淀,导致加入点的渗透系数下降,阻碍磷酸根的继续迁移
[30 ]
。而通过微生物酶促反应水解有机磷的过程是缓慢进行的,从而降低了注入位置被堵塞的风险。此外,细胞能成为沉淀成核核心,因此该法沉淀铀的效率更高。相比于生物还原,生物矿化能在更宽的p H值范围内、较高的氧气和硝酸盐浓度下修复铀污染
[31 ]
。同时生成的磷酸铀酰矿物在较宽的p H值范围内其溶解度低,能长期保持稳定。目前阻碍生物矿化修复技术广泛应用的最大障碍是有机磷酸盐的成本过高
[22 ]
。另外,有研究学者认为细胞表面金属的快速沉淀原则上会阻碍细胞的代谢,降低修复效果。
2.3 生物吸附
生物吸附是指铀被动的吸附在细胞表面。生物吸附通常非常迅速,且与细胞的生命代谢无关。古细菌和细菌比表面积大,细胞壁表面含有的官能团(如羟基,羧基,巯基等)带负电荷,可以将铀酰阳离子吸附在细胞壁表面
[32 ,33 ]
。目前芽孢杆菌、地杆菌、梭状芽孢杆菌、链霉菌和节杆菌等已证明可以作为铀的生物吸附剂,其中芽孢杆菌已多次用于铀污染的吸附。Gorman-Lewis等
[34 ]
研究了枯草芽孢杆菌对铀的吸附能力,结果表明在任何实验条件下都能观察到铀在细胞表面的大量吸附。生物吸附法比较适合处理中低浓度的重金属废水。从电镀废水中分离出的Pseudomonas MGF-48,在p H为6.5时,该菌在5 min内可以吸收86%的铀
[35 ]
。另外研究表明放线菌、酵母菌和丝状真菌等也具有生物吸附铀的潜力。例如,Zinjard等
[36 ]
从石油污染海水中分离出的酵母菌在p H为7.5时能够吸附溶液中50%的铀。
虽然微生物具有吸附铀的潜力,但将这项技术应用于铀污染修复却难以实现。这是因为环境中的其他阳离子也会竞争细胞膜表面吸附位点,这些位点很容易达到饱和,从而阻止了吸附的继续进行。同时若细胞凋亡或分解,吸附的离子会被重新释放。这也是尽管生物吸附可以去除水溶液中的铀,但其几乎没有工业应用的原因。
2.4 生物富集
微生物可以通过生物富集吸收多种重金属离子进入细胞体内。然而铀不同于细胞代谢所需的微量元素(如Fe,Zn等),可以通过一定的运输途径在细胞内富集。研究报导称铀在细胞体内的富集可能是因为铀的毒性破坏了细胞膜的通透性
[37 ]
。此外,也没有直接证据表明微生物体内存在铀的转运蛋白,因此生物富集铀被认为是与细胞生长代谢无关的
[38 ]
。但是一旦铀在细胞内积累,细胞可能会通过多种方式来固定铀,例如聚磷酸盐对铀的螯合作用,这是一种被动的金属耐受机制,能提高细胞对铀的耐受性
[39 ]
。这种机制在许多菌株中可以观察到,如A.ferroxidans
[40 ]
和Sphingomonas sp.S15~S1
[41 ]
。尽管研究学者对生物富集开展了相关研究,但基本没有证据表明生物富集修复铀污染是否可行。
最后,综合本节所述内容对铀污染微生物修复作用机制的原理、优缺点以及研究现状进行了归纳总结,如表1所示。
3 环境因子对铀污染微生物修复效果的影响
铀污染微生物修复效果往往很难预测,在特定条件下或由特定微生物获得的结果不一定适用于其他条件或生物体。这主要是因为许多环境因素都能影响修复效果,包括温度、酸碱度、生物量、铀的浓度和赋存状态以及其他化合物的存在等等。
表1 铀污染微生物修复作用机制对比 下载原图
Table 1 Comparison of bioremediation mechanisms of uranium contamination
3.1 铀的浓度及其赋存形态
与其他酶促反应相似,当铀浓度增加时,铀的还原速率增加,但浓度过高时,铀的还原率反而明显降低,这是因为高浓度的铀对微生物有毒害作用。但铀对微生物的毒害作用强弱不仅取决于铀的浓度,还取决于微生物的种类以及铀的赋存状态等。研究表明不同的微生物对铀的耐受能力明显不同。例如,当铀的浓度分别达到50,1000,1190 mg·L-1 时能抑制Clostridium sp.ATCC 53464,Pseudomonas aeruginosa,Thermoterrabacterium ferrireducens的生长
[42 ]
。在不同环境中,铀可与不同物质形成多种络合物,其中以氧化铀酰水合物、磷酸铀酰、硅酸铀酰、硫酸铀酰和碳酸铀酰最为常见,这些络合物的溶解度存在明显差异
[43 ,44 ]
。铀的迁移率和生物可利用度因铀形成的络合物类型和复杂程度而异,而铀的生物可利用度又与其毒性密切相关,因此铀的赋存状态在一定程度上决定了铀的毒性强弱。Belli等
[45 ]
研究表明,铀对S.putrefaciens的毒性与以非碳酸盐形态存在的铀酰离子浓度直接相关,而总铀浓度不能直接用来预测铀的毒性。此外,他们还认为Ca的存在能降低铀的毒性,这是因为Ca?UO2 ?CO3 络合物的形成降低了铀的生物可利用度。
3.2 生物量、温度和p H值
各项研究表明,增加初始生物量浓度可以提高铀的去除率。例如,Chabalala和Chirwa的研究中
[46 ]
,初始生物量浓度高达9.3 g·L-1 ,这在一定程度上可以解释了实验24 h后铀的还原率高达100%的原因。另外,Senko等
[47 ]
的研究表明随着S.putrefaciens初始生物量的增加,U(VI)的还原速率也不断提高。然而,这并不意味着在实际应用中,应该加入过多的微生物。因为为了维持高的生物量浓度需要投入大量的营养源,而这无疑大大增加了修复成本。此外,过多的生物量可能会堵塞尾矿孔隙,特别是修复柱试验和现场试验中。
温度和p H值能显著影响微生物的酶活和铀的赋存状态,从而影响铀污染的修复效率。虽然温度的升高通常会加快化学反应的速率,但大多数微生物只能存活于某个特定的温度和p H值范围内。极端的p H值和温度会影响生物代谢活性。例如Boonchayaanant等
[48 ]
报道称当温度从20℃升高至30℃时,U(VI)还原速率提高,而在10℃时基本观察不到。较低或较高的p H会降低微生物活性,抑制U(VI)的还原。但研究发现某些细菌能在较低的p H值条件下还原U(VI),如Gao和Francis
[49 ]
从酸性污染物中分离出Clostridium sp.对较低的p H值具有较高的耐受性,能在p H值约为4时还原U(VI)。
从图2可以看出当p H值小于5或大于7.5时,铀主要以易溶解的UO2 2+ 或碳酸铀酰盐形态存在,造成溶液中U(VI)的浓度增加。同时在较低p H值下,由于H+ 体积较小、浓度较高而优先占据细菌细胞膜表面活性位点,从而阻止铀酰阳离子的还原或吸附。因此,大多数U(VI)的生物还原都需要p H值接近中性条件。此外,若保持p H值略低于中性环境(5.7~6.2),可能会抑制竞争微生物的生长,如产甲烷菌依靠代谢电子供体生长,但尚未证明其具有还原U(VI)的能力
[50 ]
。
3.3 电子供体
对于通过刺激土著微生物的生长,成功实现原位生物还原U(VI),选择合适的电子供体是必不可少的。在厌氧条件下,金属还原细菌对蛋白质、纤维素或长链脂肪酸等高分子有机化合物的代谢能力有限
[52 ]
。因此,U(VI)的生物还原通常优先选择低分子量有机物作为电子供体。目前研究发现包括氢气、乙醇等醇类、葡萄糖等糖类、醋酸盐等有机酸类、以及甲苯等芳香烃类化合物均能作为电子供体促进U(VI)的生物还原
[53 ]
。现场试验中广泛应用醋酸盐作为电子供体实现U(VI)的生物还原
[54 ,55 ]
。NˊGuessan等
[56 ]
研究发现当加入醋酸盐做电子供体,U(VI)的还原速率提高了,并且还原后的U(IV)再氧化速度大大减慢。然而,也有研究发现简单醇类,特别是甲醇和乙醇,是比醋酸盐更适宜的电子供体
[57 ]
。这说明电子供体的选择可能需要考虑一系列因素,包括污染区域原有微生物群落结构、物相组成以及地球化学条件等。例如,在某铀污染区域采用醋酸盐和葡萄糖对U(VI)生物还原的效果明显好于甲酸盐或乳酸盐
[58 ]
。而另一研究中却发现乙醇比醋酸盐或乳酸盐能更快地还原U(VI)
[57 ]
。
图2 U-CO2-H2O系U(VI)的赋存状态与p H值的关系
Fig.2 Distribution of U(VI)species as a function of p H in U-CO2 -H2 O system
[51]
采用简单的有机底物促进微生物原位修复铀污染技术上是可行的,但过度刺激注入点附近的微生物生长可能会引起污染区域局部堵塞,降低U(VI)的还原速率。研究发现,采用更复杂的电子供体如油酸盐、植物油、聚乳酸脂等应用于原位修复,其修复效果可能更好,因为它们电子释放速度慢,可以在更大范围内维持还原氛围
[53 ]
。
3.4 碳酸氢盐
研究表明,碳酸氢盐对微生物群落结构和U(VI)的去除率有显著影响。加入不同浓度的碳酸氢盐可导致溶液p H值和固液相中U(VI)的分配比例变化
[59 ]
。Lovehy和Phillips
[60 ]
在20世纪90年代首次提出将碳酸氢盐应用于铀污染土壤的治理,研究发现碳酸氢盐对U(VI)的脱附与后续微生物对U(VI)的还原相结合可以促进生物修复的效果。另外最近的现场试验也证明碳酸氢盐能促进U(VI)的还原。例如,Long等
[54 ]
发现向铀污染试验井中加入醋酸以及碳酸氢盐,其U(VI)的还原速率明显高于只加醋酸的对照组。但研究发现浓度过高的碳酸氢盐可能会降低微生物的活性,从而抑制U(VI)的还原。例如D.desulfuricans在碳酸氢盐浓度为30mmol·L-1 溶液中还原U(VI)的速率比100 mmol·L-1 的快
[60 ]
。
另外,若修复体系中还存在一定量的Ca时,Ca能与U(VI)和碳酸氢盐形成Ca?UO2 ?CO3 络合物,从而抑制Fe RB和SRB对U(VI)的还原
[61 ]
。
3.5 竞争性电子受体
硫酸盐、硝酸盐、锰和铁是铀污染区域含有的典型组分,同时也是厌氧生物生长的适宜电子受体。他们的存在可以延缓甚至阻止U(VI)还原。
SO4 2- 能对U(VI)的还原起重要作用。SO4 2- 能促进如可还原U(VI)的Desulfovibrio spp.的生长,也可以促进如Desulfobacter spp.的生长,它们不能还原U(VI)但可以与U(VI)还原菌竞争电子供体。Spear等
[62 ]
的研究表明硫酸盐的存在能提高U(VI)的还原率,这可能是因为SO4 2- 作为电子受体能促进SRB的生长,增强细胞活性。此外SO4 2- 还原生成的硫化物也具有还原U(VI)的能力
[63 ]
。但也有研究学者认为U(VI)的还原速度不受SO4 2- 浓度的影响,或SO4 2- 能抑制U(VI)的还原速度,这可能是由于SO4 2- 能与U(VI)竞争电子供体。
NO3 - 普遍存在于铀污染区域中,有研究表明当NO3 - 的含量高达50 mg·L-1 时能抑制D.desulfuricans对U(VI)的还原
[64 ]
。NO3 - 对U(VI)还原的影响主要表现在两方面:一是与U(VI)竞争电子供体,抑制U(VI)的还原,二是能使还原生成的U(IV)再氧化。Istok等
[65 ]
在一处NO3 - 浓度很高的含水层中利用碳源刺激土著菌还原U(VI),试验发现加入的碳源和NO3 - 很快被消耗,但这一阶段并未发现U(VI)的还原。直至NO3 - 被完全消耗并重新加入多种碳源后才发现U(VI)的还原。因此现阶段的现场修复试验,通常会先通过化学试剂淋洗
[66 ]
或反硝化流化床
[67 ]
等方式除去污染区域的NO3 - ,再利用微生物去除铀。
根据表2给出的理论还原电位可知,Mn(IV)的还原电位最高,Fe(III)次之,U(VI)最末。因此当细菌以有机碳源或H2 作电子供体时,Mn(IV)和Fe(III)应该在U(VI)之前被还原。此外,根据热力学分析Mn(IV)和Fe(III)具有使还原的U(IV)重新被氧化的潜力。当p H值在5~8之间时,Fe(III)一般以针铁矿或水铁矿等形态存在,其溶解度很低,Fe(III)对U(IV)还原的影响可忽略不计。然而,当p H值小于5时,Fe(III)在沉积物中的浓度通常远高于U(VI),U(VI)的还原通常与Fe(III)的还原同时进行
[68 ,69 ]
。目前,Mn(IV)对U(IV)还原的影响研究较少。Fredrickson等
[70 ]
报道称锰氧化物能降低U(VI)还原速率,但U(VI)的总还原率并未减少。
表2 电极反应及理论还原电位(E0 ) 下载原图
Table 2 Electrode reactions and theoretical reduction po-tentials(E0 )
3.6 其他化合物
铀污染区域存在的金属离子,包括Zn,Ni,Cu,Ca和Mg等能抑制微生物修复铀污染。Zn,Ni和Cu的抑制作用可能是由于其对微生物的毒性所致。研究表明当Ni≥11.7 mg·L-1 ,Zn≥25 mg·L-1 和Cu≥15 mg·L-1 时均能完全抑制U(VI)的去除
[19 ]
。而Ca和Mg能与U(VI)和碳酸盐形成相对稳定且难溶的三元碳酸铀酰络合物,从而抑制U(VI)的修复。Brooks等
[71 ]
研究发现当溶液中存在Ca时,U(VI)的还原效率大大降低。出现这种现象的主要原因可能是当溶液中存在钙时,Ca2 UO2 (CO3 )3 会在溶液中占主要成分。Ca2 UO2 (CO3 )3 还原电势低,不易被还原,可作为一种竞争抑制剂,抑制其他形态的铀被还原。
研究学者对环境因子影响铀污染的微生物修复效率做了大量研究工作,本文选取部分有代表性实验,绘制了部分环境影响因子对铀还原率的影响,如表3所示。
表3 部分环境影响因子对铀还原率的影响 下载原图
Table 3 Influence of partial environmental factors on uranium reduction rates
4 存在的问题及展望
综上所述,微生物修复铀污染,因其成本低,操作简单,修复效果好,无二次污染等优点引起了研究学者的关注,取得了一些突破性的研究进展。但也存在着一些不足,亟待我们去解决。例如,微生物修复耗时相对较长、修复效果有一定的不确定性,并且当环境条件改变时,修复后的产物可能难以长时间维持稳定。
现阶段的研究主要集中在实验室小试研究,大规模现场修复案例还很少,有些技术在实验室虽有较好的效果,可一旦应用于实际工程,修复效果往往大打折扣。造成这种现象的主要原因:一是对微生物修复铀污染的作用机制研究还不够深入,未彻底弄清铀污染修复机理以及微生物在这一过程中是如何调控并发挥作用的;二是现场环境复杂多变,如污染物类型,温度、酸碱度,铀的赋存状态,微生物活性,其他化合物的存在等这一系列因素都会影响着修复效果。因此微生物修复要应用于现场试验,仍需进一步的研究和探索。未来值得深入探索的研究方向包括:
1.筛选、培育修复效率更高的优势菌株,必要时对其进行基因改造,提高其对环境的耐受性和修复效率。
2.在保证修复效果的前提下,寻求价格低廉的碳源和磷源,促进目标优势菌株对铀污染的修复。
3.运用分子生物学研究方法,了解铀污染修复过程中细胞代谢途径和调控机制,把握和调控修复全过程中的微生物群落演替,提高生物修复效果的稳定性。
4.研究采用联合修复方法,例如植物—微生物联合修复,渗透反应墙—微生物联合修复,电化学—微生物联合修复等方法,弥补单一修复方法可能存在的不足,获得更好的修复效果,缩短修复周期。
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