Radionuclide contamination, treatment

Phytoextraction has the potential to remediate many metal and radionuclide contaminated sites using a less invasive form of treatment than traditional methods such as escavation and disposal. There are four factors that influence or determine the ability of phytoextraction to effectively remediate a metal contaminated site 1). Site arability and plant biomass yields 2) metal solubility and availability for uptake 3) the ability of the plant to accumulate metals in the harvestable plant tissues and 4) regulatory criteria. 

The United States Department of Energy (DOE) manages approximately 1.9 billion cubic meters of radionuclide contaminated environmental media and 4.1 million cubic meters of stored, contaminated waste at 150 different sites located in 30 different states (i, 2). This environmental legacy is a result of the massive industrial complex responsible for defense related and non-defense related research, development and testing of nuclear weapons, nuclear propulsion systems and commercial nuclear power systems. Cleaning up the environmental legacy is expected to cost several hundred billion dollars over the next 5 to 7 decades. To reduce costs and speed remediation efforts the DOE has invested in waste treatment and environmental remediation research. 

Many of the sources of soil and groundwater contamination occur at or near the land surface. With this in mind, source removal implies the removal, destruction, or isolation of contaminated soil which would otherwise represent a long-term source of contaminants to groundwater. Where these approaches are feasible and cost effective, they are clearly preferable to attempt to destroy contaminants in-place. Table VI provides a selective siunmaiy of soil and vadose zone treatment methods ranging from excavation and removal to contaminant treatment in place. Above-ground treatments favor total contaminant destrac-tion rather than isolation of contaminants except in the cases of radionuclides for which only time will cause to dissipate the radiation. 

At locations where more than a few thousand liters of radionuclide-contaminated water were released (particularly with high concentrations), the long-lived radionuclides are expected to be present In the saturated soils and (to an unknown extent) In the groundwater directly below and downgradlent from the release location. These locations Include the 116-N-2 radioactive chemical waste treatment and storage tank, the 118-N-l spacer storage silos, the 1304-N EOT, and the 1314-N LWLS. 

INTERNATIONAL ATOMIC ENERGY AGENCY, Assessment and Treatment of External and Internal Radionuclide Contamination, IAEA-TECDOC-869, Vienna... 

Pollution of soils and waters by human activities is an important and widespread problem. This pollution by, organic and inorganic substances can affect individual organisms, human populations, and ecosystems, each in its own unique way. In particular former military installations, often used for weapons production and nuclear power plants represent a ongoing and substantial threat to environment and human health because of the specific pollutants that can be released Solvents, explosives, fuels, radionuclides, heavy metals, and metalloids all have been identified in the environment around these installations. Remediation technologies for these contaminated sites have been developed based on conventional systems utilising physical and chemical treatments, such as excavation and incineration, pump-and-treat methods, ultraviolet oxidation, soil washing, etc. 

The SSM/TEVE system is best suited for removing VOCs, and has been shown to reduce the average soil concentration of VOCs by 90%. The shallow soil mixing technology is also used to mix various solidification/stabilization slurries into the soil for the treatment of inorganic contaminants (including radionuclides). (Refer to the Geo-Con in situ solidification/stabilization process). 

Apatite, a natural calcium fluoride phosphate, can adsorb low to moderate levels of dissolved metals from soils, groundwater, and waste streams. Metals naturally chemically bind to the apatite, forming extremely stable phosphate phases of metal-substituted apatite minerals. This natural process is used by UFA Ventures, Inc., and is called phosphate-induced metals stabilization (PIMS). The PIMS material can by used in a packed bed, mixed with the contaminated media, or used as a permeable barrier. The material may be left in place, disposed of, or reused. It requires no further treatment or stabilization. Research is currently being conducted on using apatite to remediate soil and groundwater contaminated with heavy metals, and the technology may also be applicable to radionuclides. The technology is not yet commercially available. 

The WRS soil washing process (WSWP) is a commercially available, ex situ technology for the treatment of soils and sludges contaminated with organics, heavy metals, radionuclides, and combinations of contaminants. 

Although the process requires the addition of a phosphate donor, such as glycerol-2-phosphate, it may be a valuable tool for cleaning water contaminated with radionuclides. An alternative mode of uranium precipitation is driven by sulfate-reducing bacteria such as Desulfovibrio desutfuricansy which reduce U(VI) to insoluble U(IV). When combined with bicarbonate extraction of contaminated soil, this may provide an effective treatment for removing uranium from contaminated soil (85). 

The availability of pure water will certainly be one of the major environmental issues of the 21s1 Century. Water contamination can originate from domestic, agricultural, agroindustrial or industrial activities, and accidental damages. The major pollutants are heavy metals, radionuclides, ammonia, nitrates and organic compounds. Health problems are associated with each of them, such as leukaemia, saturnism..., as well as modifications of the eco-system, e.g. entrophication of lakes and rivers. The nature of water treatments obviously depends on the kind of contaminants, and zeolite-based processes are of great concern in this field. 

A particular aspect of water treatment is the rehabilitation of accidentally contaminated soils by radionuclides. This is well illustrated by the works carried out after the Cernobyl catastrophe. The incorporation of clinoptilolite into contaminated soils reduced the transport of heavy metals and radionuclides from soils into ground water and biomass (7). Union Carbide s IONSIV EE-95 (CHA) and A-51 zeolites (LTA) with excellent Cs+/Na+ and Sr2+/Na+ selectivities, respectively, have also been employed for decontamination of high activity level water in the reactor containment building from Cs+ and Sr2+ after the accident at Three Miles Island (5). The radioisotope loaded zeolites were then transformed into glasses for ultimate disposal. 

For example, in fission product effluents where radio-contaminants having longer half-lives are present, the emphasis should be on very high volume reduction with permissible/acceptable decontamination factors so that the permeate could be directly discharged. For effluents contaminated with radionuclides of short half-lives, a good decontamination with reasonable volume reduction may be acceptable because the concentrate could be stored tiU the activities decay before discharge. The radioactive effluents requiring treatment may vary with respect to the type of radionuclide, its chemical nature, concentration, pH, concentration of inactive solutes, and presence of suspended matter. 

The experiments have proved that membrane distillation can be applied for radioactive wastewater treatment. In one-stage installation the membrane retained all radionuclides and decontamination factors were higher than those obtained by other membrane methods. The distillate obtained in the process was pure water, which could be recycled or safely discharged into the environment. It seems the process can overcome various problems of evaporation such as corrosion, scaling, or foaming. There is no entrainment of droplets, which cause the contamination of condensate from thin-film evaporator. Operation at low evaporation temperature can decrease the volatility of some volatile nuclides present in the waste, such as tritium or some forms of iodine and ruthenium. The process is especially economic for the plants, which can utilize waste heat, e.g., plants operating in power and nuclear industry. 

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