Radioactive substances occur naturally in the environment and they emit small amounts of radiations. However, anthropogenic activities produce high levels of radioactive materials that are released into the environment causing pollution. The most common radioactive metals are Uranium, Plutonium, Polonium, Radium, Thorium and Cesium. Among these, Uranium is the most frequently and naturally occurring radioactive substance and possesses weak radioactivity properties. Therefore it is important to focus on mitigating its pollution and uranium bioremediation is one of them.
Furthermore, it needs to be emphasised that Uranium is found in varying quantities in rocks, water, air, soil, plants, animals and humans. Although primarily used as fuel for generating electricity globally, diverse anthropogenic activities have resulted in depletion of this element. However, increasing contamination of the environment with Uranium radioactivity has called for stringent measures of disposal and efficient clean-up technologies. When compared with physical or chemical techniques, microbial remediation systems are efficient, economical and reliable. Further, sequestration of radioactive contaminants in-situ is feasible using microbial remediation procedures.
Uranium has three radio-isotopes, 234U, 235U and 238U, with identical physical and chemical characteristics, although with different radioactive properties. It is released into the environment due to the weathering of rocks and mining of Uranium while also present in water bodies naturally. It also occurs in soils formed from the parent materials containing Uranium (1). The table below lists major sources of Uranium contamination.
|Natural Sources of Radiation||Man Made Sources of Radiation|
|· Cosmic rays from outer space and Sun’s surface
· Radionuclides occurring in Earth’s crust
· Intake of terrestrial radionuclides via inhalation or ingestion
· Indoor exposure to Radon such as in underground mines
|· Mining and milling of Uranium
· Fuel enrichment and fabrication
· Nuclear reactor operation and nuclear weapons tests
· Fuel reprocessing involving recovering of radioactive substances such as Plutonium and Uranium
· Radionuclides which are dispersed globally
· Disposal and transport of solid wastes containing radioactive substances
Table: Natural and Man Made sources of exposure
Need for Uranium bioremediation
It is therefore important to note that upon exposure beyond certain threshold levels, radiation proves to be harmful to living organisms. It produces deleterious effects at the molecular level by damaging the living cells. This results in genetic changes or cell death and if the cell mechanisms fail to repair such modifications, it could lead to cancer and may be passed to the affected person’s offspring (2). Owing to its radioactive as well as its chemical toxicity, excessive exposure to uranium can lead to lung cancer, kidney damage, diminished bone growth and changes in fertility (4).
This establishes the significance of remediating Uranium contaminated sites, either by chemical, physical and biological methods. Although physical and chemical methods produce satisfactory results, they involve the use of chemicals and other synthetic products. This often leads to secondary damage to water and soil, as opposed to restoration of a natural balanced state through bioremediation.
Microbes involved in Uranium bioremediation
Furthermore, microbes are natural inhabitants of different water and soil systems, capable of utilizing and adapting to a wide range of energy sources. Hence, they can remediate contaminated sites using metabolic activities that actively and passively interact with the radioactive elements. The removal of harmful radionuclides can be facilitated by solubilization using enzymes, biosorption or biodegradation, which are unique to different groups of microbes (5). In addition Uranium occurs primarily as salts of Uranyl ion in the toxic environments and most of the bacteria bring about remediation by reducing U(VI) to U(IV). Many species of bacteria possess the ability to bring about this reduction, including sulfate reducing and Fe (III) reducing bacteria (6). Bacteria belonging to the genus Geobacter, Desulfosporosinus, Closteridium have also been found to remove toxic uranium from water sources (7,8). The table below provides information on some of the microbial species’s uranium bioremediation capabilities.
|Shewanella putrefaciens||Reductive precipitation of U(VI) to U(IV) which acts as electron acceptor, and gets precipitated to UO2 (9)||Hass & Northup (2004)|
|Geobacter sulfurreducens||Direct enzymatic reduction using acetate as electron donor and U(VI) as electron acceptor (10)||Llyod et al. (2003)|
|Cystoseira indica||Biosorption involving passive uranium uptake on the surface of bacterial cells due to attraction of U cations by the electronegatively charged membranes (11)||Khani et al. (2005)|
|Synechococcus elongates||Biosorption involving passive uranium uptake on the surface of bacterial cells due to attraction of U cations by the electronegatively charged membranes (12)||Acharya et al. (2009)|
|Synechococcus sp.||Adsorption independent of metabolism involving association of Uranium with Extracellular Polysaccharides (EPS) (12)||Acharya et al. (2009)|
|Anabaena torulosa||Bioaccumulation involving sequestration of Uranium by polyphosphates on cell surface (13)||Acharya et al. (2013)|
Table: Microbes involved in bioremediation of Uranium
Mode of Action of Microbes involved in Uranium bioremediation
Microbes interact with uranium in multiple ways, which can be advantageous in uranium bioremediation applications. One is biosorption in which metal cations are adsorbed on the negatively charged surface of microbial cells. Also characterized by presence of metal binding ligands. Arthrobacter sp. and Bacillus sp. contain high Uranium sorption abilities as shown by Tsuruta (14). Microorganisms are also found to interact with Uranium ions and immobilize them by a process called biomineralization. They facilitate the binding of ions via biofilms on the surface of their cells, which aid in the process of precipitation. The remediation can also be carried out by process of bioreduction which involves either direct or indirect enzymatic reduction of radionuclides, thereby converting the aqueous U (VI) form into insoluble U (IV) (15).
The U (VI) shares biochemical similarities with Fe(III), hence, certain microbes utilise it as substitute to facilitate their metabolic processes. Such bacteria can substitute Fe(III) with U(VI) during bioreduction of Fe(III) into Fe(II) and hence remove U(VI) from the environment. These bioreduction processes involve the use of Cytochrome c enzyme by bacteria such as Geobacter and Shewanella species to facilitate the transfer of electrons from cytoplamic membrane to the outer membrane. Generally, microbes can accumulate a wide range of metal ions through bioaccumulation, especially essential elements important for their survival. Uranium is taken up into the microbial cells in conditions of Uranium toxicity, which leads to increase in membrane permeability. These ions are accumulated into the microbial cells and are removed from the local environment (16).
Need to find better ways of implementation
In conclusion, uranium pollution is a serious problem because of its radioactive nature and its ability to remain in the environment for long periods of time. Similarly, exposure to high levels of Uranium can cause damage to living organisms at genetic level leading to genetic defect and cancers. The bioremediation of Uranium using microbes is a cost effective and nature-friendly alternative for the detoxification of the environment. Scholars are studying the process of bioreduction in order to devise better ways for implementation of the method. Although species capable of extracting uranium from the environment have been studied in the laboratory, in-situ and feasibility studies have not been confirmed for practical applications. Moreover, it is important to address the dangers of reversible solubilization of uranium with changing physicochemical conditions for their applicability.
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