铀污染的微生物修复技术研究进展

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。

Research Progress of Bioremediation Technology for Uranium Contamination

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.