Bioremediation of radioactive waste
[1] These radioactive particles are by-products generated as a result of activities related to nuclear energy and constitute a pollution and a radiotoxicity problem (with serious health and ecological consequences) due to its unstable nature of ionizing radiation emissions.
The techniques of bioremediation of environmental areas as soil, water and sediments contaminated by radionuclides are diverse and currently being set up as an ecological and economic alternative to traditional procedures.
[3][4] The presence of radioactive waste in the environment may cause long-term effects due to the activity and half-life of the radionuclides, leading their impact to grow with time.
[2][8][9] The classification of radioactive waste established by the International Atomic Energy Agency (IAEA) distinguishes six levels according to equivalent dose, specific activity, heat released and half-life of the radionuclides:[13] Radioactive contamination is a potential danger for living organisms and results in external hazards, concerning radiation sources outside the body, and internal dangers, as a result of the incorporation of radionuclides inside the body (often by inhalation of particles or ingestion of contaminated food).
It is very difficult to estimate from the available data the total doses that can accumulate during specific stages of the life cycle (embryonic development or reproductive age), in changes in behavior or depending on environmental factors such as seasonality.
They are caused by the recruitment and retention of radioisotopes by bivalves, crustaceans, corals and phytoplankton, which then amounted to the rest of the food chain at low concentration factors.
[23] The biochemical transformation of radionuclides into stable isotopes by bacterial species significantly differs from the metabolism of organic compounds coming from carbon sources.
[2] Other radioactive actinides such as thorium, plutonium, neptunium and americium are enzymatically reduced by Rhodoferax ferrireducens, S. putrefaciens and several species of Geobacter, and directly form an insoluble mineral phase.
In the case of sulfate-reducing bacteria hydrogen sulfide is produced, promoting increased solubility of polluting radionuclides and their bioleaching (as liquid waste that can then be recovered).
Microbacterium flavescens, for example, grows in the presence of radioisotopes such as plutonium, thorium, uranium or americium and produces organic acids and siderophores that allow the dissolution and mobilization of radionuclides through the soil.
[4][24] From this knowledge exists a system that combines the degradation of radionuclide-citrate complex with subsequent photodegradation of remaining reduced uranyl-citrate (previously not biodegradated but sensitive to light), which allows for stable precipitates of uranium and also of thorium, strontium or cobalt from contaminated lands.
However, it is a technique that requires a high amount of biomass to affect bioremediation; it presents problems of saturation and other cations that compete for binding to the bacterial surface.
[3] Bioaccumulation refers to uptake of radionuclides into the cell, where they are retained by complexations with negatively charged intracellular components, precipitation or granules formations.
In Citrobacter and Serratia genera, this cleavage liberates inorganic phosphates (HPO42−) that precipitates with uranyl ion (UO22+) and cause deposition of polycrystalline minerals around the cell wall.
[4] Investigations of terrestrial and marine bacterial isolates belonging to the genera Aeromonas, Bacillus, Myxococcus, Pantoea, Pseudomonas, Rahnella and Vibrio have also demonstrated the removal of uranium radioisotopes as phosphate biominerals in both oxic and anoxic growth conditions.
[25] 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.
Ethanol is also used in soil injection systems with hydraulic recirculations: it raises the pH and promotes the growth of denitrifying and radionuclide-reducing bacteria, that produce biofilms and achieve almost 90% decrease in the concentration of radioactive uranium.
[3] 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.
[4][30] This technique has shown in field trials over the years that it does not offer better results than biostimulation; neither it is clear that introduced species can be distributed effectively through the complex geological structures of most subsurface environments or that can compete long term with the indigenous microbiota.
[1][26] 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 100 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.
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.
[citation needed] In phytoextraction (also phytoaccumulation, phytosequesteration or phytoabsorption)[34] plants carry radioactive waste from the root system to the vascular tissue and become concentrated in the biomass of shoots.
They are able to remove up to 95% of uranium of contaminated water in 24 hours, and experiments in Chernobyl have demonstrated that they can concentrate on 55 kg of plant dry weight all the cesium and strontium radioactivity from an area of 75 m2 (stabilized material suitable for transfer to a nuclear waste repository).
[33][34] 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.
[33][34] However, it has significant drawbacks such as large doses of fertilizer needed to reforest the area, apart from radioactive source (which implies long-term maintenance) remaining at the same place.
[citation needed] Several fungi species have radioactive resistance values equal to or greater than more radioresistant bacteria; they perform mycoremediation processes.
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.
Multidisciplinary research is focused on defining more precisely necessary genes and proteins to establish new cell-free systems which may avoid possible side effects on the environment by the intrusion of transgenic or invasive species.
