BIOREMEDIATION OF RADIOACTIVE WASTE

           A.  Farguell, L. Pérez i C. Turné

           Universitat de Girona, 2018

Bioremediation of
radioactive waste

Application of microorganisms for
decontamination of potentially radioactive areas.

Abstract

Radioactive waste is a global problem which is difficult and expensive to treat. Geobacter sulfurreducens and Exiguobacterium acetylicum CR1 showed positive results as a bioremediators. Their metabolism can remove radioactive uranium, technetium and cesium from the medium using different mechanisms.

Introduction

Global nuclear activity begun 60 years ago with nuclear weapons and power plants and since then, it has not stopped increasing. This kind of industrial activities produces a huge quantity of radioactive waste, which are not easy to manage. Because of the high financial and environmental costs associated with invasive physical and chemical cleanup strategies this kind of techniques supposes, there has been an increased interest in microorganisms to developing cost-effective bioremediation approaches for decontamination of sediments and waters impacted by nuclear waste. (1)

Radioactive environments and its microbial life

The ubiquitous distribution of microbial life is well known, but only recently have we been able to obtain an indication of the level of microbial colonisation of radioactive environments.

There are some isolated places as a pool storing nuclear materials in Spain, or a vadose zone sediments contaminated with high level nuclear waste at the US which makes an idoneous place to study these microorganisms.

Despite the difficulties of working with highly radioactive environmental samples in the laboratory, several isolates were obtained using water and sediments from these areas.

These results would seem to suggest that the radioactive burden of several nuclear waste types is not necessarily inhibitory to all microbial life. (1)

Radionuclides [1]

The inventory of radionuclides generated during the past six decades of operating fission reactors is long and includes 237Np, Pu isotopes, Am, 3H, 14C, 85Kr, 90Sr, 99Tc, 129I and 137Cs in addition to uranium (for example, 235U). However, most studies have addressed to cesium, uranium and its fission product, technetium, which are usually the most abundant in radioactively contaminated environments. (1,4)

Due to this, in this article we show two microorganisms with different mechanisms to bioremediate these three different radionuclides.

Geobacter sulfurreducens

A 2011 research at Michigan State University found Geobacter sulfurreducens, an iron oxide reduction bacterium, and precipitate uranium from groundwater. The biological mechanism behind this reaction remains uncertain.

The experiment showed that this microbe has a pili expression which induces a protective cellular mechanism against uranium. First, the pile promotes an extracellular reduction of Uranium IV (soluble) into Uranium IV (insoluble). Then, the pili induces a periplasmic deposition of the Uranium IV.

The experiment studied four strains of the bacteria: Wild Type incubated at 25ºC (WT_p+), Wild Type incubated at 30ºC (WT_p-), a mutant pili-deficient (PilA^-) and a genetically complemented strain  (pRG5::pilA). The importance of studying two different conditions of the Wild Type  WT_p- and WT_p+ is to induce or prevent pili assembly. (2)

In year 2000, another research was testing Geobacter sulfurreducens bioremediation activity, and they found two technetium (or Tc(VII)) reduction and precipitation mechanisms used by this bacteria.

The first mechanism they found was a coupling the oxidation of hydrogen to the enzymatic reduction of Tc (VII) to Tc (IV), leading to the precipitation of technetium at the periphery of the cell. They proved it using a cell suspension of G. sulfurreducens supplied with hydrogen, acetate or fumarate as  electron donors to see the Tc (VII) reduction and removal from solution as an insoluble precipitate. (3)

Secondly, an indirect Fe(II)-mediated mechanism was also identified. Acetate, although not utilized efficiently as an electron donor for direct cell-mediated reduction of technetium, supported the reduction of Fe(III), and the Fe(II) formed was able to transfer electrons abiotically to Tc(VII). However, this second mechanism was significantly less efficient than the first one. (3)

Exiguobacterium acetylicum CR1

Another recent study from Inha University, Republic of Korea, in 2018, shows the Exiguobacterium acetylicum CR1 applicability to environmental cesium uptake bioremediation.

It was known some bacteria can remove cesium from radioactively contaminated water, but this radionuclide also produces gamma emissions, which can induce transformations and mutations to this microorganism and damage them. So, this research team was looking for a multi-stress radioactive-tolerant bacterial strain which can resist gamma radiation and remove cesium as well. (4)

Firstly, they isolated six different cesium resistant Exiguobacterium acetylicum CR1 stains from around a nuclear power plant in Ulsan City, Republic of Korea. Then, they prove their gamma radiation resistance and cesium adsorption getting the following results.

As a result, it was proved that this bacterial strain is not affected by gamma radiation and also, its efficiency to remove cesium after gamma radiation exposure even increase.

Here is a hypothetical mechanism of cesium reduction by E. actetylicum CR1 using the gamma radiation proposed by O. Yeong et al. (4)

Conclusions

In our opinion, bioremediation is a good way to reduce radioactive waste. Overall, it is an economic, efficient and environmental-friendly way to erase the problematic waste. Moreover, it can raise the popular opinion about this energy source that now has a lot of detractors. But, it’s also true that, to encourage this new technology, more research and public broadcast is needed.

Bibliography

1.           Lloyd JR, Renshaw JC. Bioremediation of radioactive waste: Radionuclide-microbe interactions in laboratory and field-scale studies. Curr Opin Biotechnol. 2005;16(3 SPEC. ISS.):254–60.

2.        Cologgi DL, Lampa-Pastirk S, Speers AM, Kelly SD, Reguera G. Extracellular reduction of uranium via Geobacter conductive pili as a protective cellular mechanism. Proc Natl Acad Sci [Internet]. 2011;108(37):15248–52. Available from: http://www.pnas.org/cgi/doi/10.1073/pnas.1108616108

3.           Lloyd JR, Sole VA, Van Praagh C V, Lovley DR. Direct and Fe(II)-mediated reduction of technetium by Fe(III)-reducing bacteria. Appl Environ Microbiol [Internet]. 2000 Sep [cited 2018 Dec 8];66(9):3743–9. Available from: http://www.ncbi.nlm.nih.gov/pubmed/10966385

4.           Oh SY, Heo NS, Shukla S, Kang SM, Lee I, Lee H, et al. Multi-stress radioactive-tolerant Exiguobacterium acetylicum CR1 and its applicability to environmental cesium uptake bioremediation. J Clean Prod [Internet]. 2018;205:281–90. Available from: https://doi.org/10.1016/j.jclepro.2018.09.077

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[1] It is the correct way to name radioactive isotopes in radioactive waste terms.