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Bioremediation of radioactive waste
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- Bioremediation of radioactive waste - Wikipedia.
Radioisotopes in microbiology. Aside from bioreduction, biosorption, bioaccumulation and biomineralization, which are bacterial strategies for natural attenuation of radioactive contamination, there are also human methods that increase the efficiency or speed of microbial processes. This accelerated natural attenuation involves an intervention in the contaminated area to improve conversion rates of radioactive waste, which tend to be slow. There are two variants: biostimulation and bioaugmentation. Biostimulation is the addition of nutrients with trace elements , electron donors or electron acceptors to stimulate activity and growth of natural indigenous microbial communities.
However, many of these microorganisms Geobacter , Shewanella or Desulfovibrio exhibit resistance genes to heavy metals that limit their ability to bioremediate radionuclides. In these particular cases, a carbon source such as ethanol is added to the medium to promote the reduction of nitrate at first, and then of uranium. A number of geophysical techniques have been used to monitor the effects of in situ biostimulation trials including measurement of: spectral ionization potential , self potentials , current density , complex resistivity and also reactive transport modelling RTM , which measures hydrogeological and geochemical parameters to estimate chemical reactions of the microbial community.
Bioaugmentaton, on the other hand, is the deliberated addition to the environment of microorganisms with desired traits to accelerate bacterial metabolic conversion of radioactive waste. They are often added when necessary species for bioremediation do not exist in the treatment place. Omics, especially genomics and proteomics, allow identifying and evaluating genes , proteins and enzymes involved in radionuclide bioremediation, apart from the structural and functional interactions that exist between them and other metabolites.
Genome sequencing of various microorganisms has uncovered, for example, that Geobacter sulfurreducens possess more than coding regions for c-type cytochromes involved in bioremediation radionuclide, or that NiCoT gene is significantly overexpressed in Rhodopseudomonas palustris and Novosphingobium aromaticivorans when grown in medium with radioactive cobalt.
From this information, different genetic engineering and recombinant DNA techniques are being developed to generate specific bacteria for bioremediation. Some constructs expressed in microbial species are phytochelatins , polyhistidines and other polypeptides by fusion-binding domains to outer-membrane-anchored proteins. Besides, through insertion of genes from other species it has been achieved that it can also precipitates uranyl phosphates and degrades mercury by using toluene as an energy source to grow and stabilize other priority radionuclides. Directed evolution of bacterial proteins related to bioremediation of radionuclides is also a field research.
YieF enzyme, for example, naturally catalyzes the reduction of chromium with a very wide range of substrates. Following protein engineering , however, it has also been able to participate in uranyl ion reduction. The use of plants to remove contaminants from the environment or to render them less harmful is called phytoremediation. In the case of radionuclides, it is a viable technology when decontamination times are long and waste are scattered at low concentrations.
Some plant species are able to transform the state of radioisotopes without suffering toxicity concentrating them in different parts of their structure, making them rush through the roots, making them volatile or stabilizing them on the ground. As in bacteria, plant genetic engineering procedures and biostimulation —called phytostimulation — have improved and accelerate these processes, particularly with regard to fast-growing plants.
In phytoextraction also phytoaccumulation, phytosequesteration or phytoabsorption  plants carry radioactive waste from the root system to the vascular tissue and become concentrated in the biomass of shoots. It is a technique that removes radionuclides without destroying the soil structure, with minimal impact on soil fertility and valid for large areas with a low level of radioactivity. Its efficiency is evaluated through bioaccumulation coefficient BC or total removal of radionuclides per m 2 , and is proven to attract cesium , strontium , technetium , cerium , plutonium , americium , neptunium and various radioisotopes of thorium and radium.
Species like common heather or amaranths are able to concentrate cesium, the most abundant radionuclide in the Chernobyl Exclusion Zone. In the same way, bok choy and mustard greens can concentrate times more uranium than other species. Rhizofiltration is the adsorption and precipitation of radionuclides in plant roots or absorption thereof if soluble in effluents. It has great efficiency in the treatment of cesium and strontium , particularly by algae and aquatic plants , such as Cladophora and Elodea genera, respectively.
It is the most efficient strategy for bioremediation technologies in wetlands ,  but must have a continuous and rigorous control of pH to make it an optimal process. From this process, some strategies have been designed based on sequences of ponds with a slow flow of water to clean polluted water with radionuclides. The most promising plants for rhizofiltration are sunflowers. Phytovolatilization involves the capture and subsequent transpiration of radionuclides into the atmosphere.
It does not remove contaminants but releases them in volatile form less harmful. Despite not having too many applications for radioactive waste, it is very useful for the treatment of tritium , because it exploits plants' ability to transpire enormous amounts of water. The treatment applied to tritium shielded by air produces almost no external radiation exposure, but its incorporation in water presents a health hazard when absorbed into the body uses polluted effluents to irrigate phreatophytes.
Phytostabilization is an specially valid strategy for radioactive contamination based on the immobilization of radionuclides in the soil by the action of the roots.
Environmental isotopes in biodegradation and bioremediation.
This can occur by adsorption, absorption and precipitation within root zone, and ensures that radioactive waste can not be dispersed because soil erosion or leaching. It is useful in controlling tailings from strip and open pit uranium mines, and guarantees to retrieve the ecosystem.
Several fungi species have radioactive resistance values equal to or greater than more radioresistant bacteria; they perform mycoremediation processes. It was reported that some fungi had the ability of growing into, feeding, generating spores and decomposing pieces of graphite from destroyed reactor No.
They were called radiotrophic fungi. Since then, it has been shown that some species of Penicillium , Cladosporium , Paecilomyces and Xerocomus are able to use ionizing radiation as energy through the electronic properties of melanins. Current research on bioremediation techniques is fairly advanced and molecular mechanisms that govern them are well known.
There are, however, many doubts about the effectiveness and possible adversities of these processes in combination with the addition of agrochemicals. In soils, the role of mycorrhizae on radioactive waste is poorly described and sequestration patterns of radionuclides are not known with certainty.
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Longevity effects of some bacterial processes, such as maintenance of uranium in insoluble form because of bioreductions or biomineralizations, are unknown. There are not clear details about the electronic transfer from some radionuclides with these bacterial species either. Another important aspect is the change of ex situ or laboratory scale processes to their real application in situ , in which soil heterogeneity and environmental conditions generate reproduction deficiencies of optimal biochemical status of the used species, a fact that decreases the efficiency.
This implies finding what are the best conditions in which to carry out an efficient bioremediation with anions, metals, organic compounds or other chelating radionuclides that can compete with the uptake of interest radioactive waste. Finally, the potential of GMOs is limited by regulatory agencies in terms of responsibility and bioethical issues. Their release require support on the action zone and comparability with indigenous species. Multidisciplinary research is focused on defining more precisely necessary genes and proteins to establish new free-cell systems, which may avoid possible side effects on the environment by the intrusion of transgenic or invasive species.
From Wikipedia, the free encyclopedia. Main article: Radioactive contamination. Main article: Radioactive waste.
Main article: Biotransformation. Main articles: Biosorption , Bioaccumulation , and Biomineralization. Main articles: Biostimulation and Bioaugmentation. Main articles: Genetic engineering and Omics. Main article: Phytoremediation. Main article: Phytoextraction. Main article: Rhizofiltration. Main article: Phytovolatilization. Main article: Phytostabilization. Main article: Mycoremediation. Biology portal Technology portal Nuclear technology portal. List of environment topics Living machines Dutch standards Actinides in the environment Restoration ecology Uranium mining debate Radiobiology Nuclear power.
Bioremediation of metals and radionuclides: What it is and how it works PDF 2nd edition.