Remediation Measures for Radioactively Contaminated Areas

The restoration of contaminated soil with radionuclides can be achieved with the application of physicochemical methods such as soil washing, soil flushing, and electrokinetic remediation. Alternatively, some confining or immobilization methods (e.g. vitrification, solidification/stabilization, passive or reactive barriers) can be used to confine the contaminants in the specific site avoiding the dispersion of the radionuclides in the surrounding environment. These technologies can be applied alone or in different combinations to achieve the low regulatory limits for radioactive elements. Ex-situ methods such as soil washing or electrokinetic remediation permit better control of the remediation conditions, and therefore they result in high removal efficiency in short time. These methods require excavation of the contaminated soil and transportation to the treatment facility. Thus, the affected area can be used for different purposes while the remediation progresses in the treatment facility. Unfortunately, the excavation and transportation is expensive and it can only be used in relatively small areas. For large areas affected by radionuclide contamination, in-situ processes such as soil flushing or electrokinetic remediation can be applied with different success that mainly depends on soil characteristics and radionuclide mobilization.

Sorption method is a prospective technique for rehabilitation of radioactively contaminated lands including decontamination of natural waters and soils from radionuclides and decrease of radionuclides transfer into agricultural vegetation. Sorbents being used for rehabilitation should meet special demands. This chapter systematizes data on characteristics of surface-modified ferrocyanide sorbents based on natural aluminosilicates (glauconite and clinoptilolite) that is important for using these sorbent for rehabilitation of radioactively contaminated lands. Furthermore, the chapter gives the evaluation of distribution coefficients of cesium and strontium at various concentration ranges, static exchange capacity, capacity of frayed edge site (FES), selectivity of sorption in presence of K⁺ and Cs/Na separation factors, radiocesium interception potentials in presence of potassium (RIP(K)), chemical stability of sorbents against cesium leaching, ecological safety of sorbents use as well as cesium transfer decrease factors for the case of cesium transfer from a soil to an agricultural vegetation. The high effectiveness of the NPF-Cl and NPF-Gl sorbents is shown.

A theory of competitive sorption as an instrument for predicting the conditions for desorption of radionuclides, Sr(II) in particular, from “contaminated solid–water” systems is formulated in this chapter. Typical isotherms of competitive sorption are presented. Based on the sorption isotherm we analyze the possibility of desorption of traces of Sr(II) and Sr-90 from ion-exchange materials. The role of humic acid (HA) in Sr(II) mass transfer in river water and the possibilities of strontium ion desorption from solids with the help of HA solutions are discussed in detail. We describe finally new competitive sorption techniques based on concurrent sorption of the aimed microelement from the contaminated material to the sorbent placed into a specially made pocketed membrane mini-reactor.

Metal and radioactive wastes are generated from industrial, domestic and anthropogenic activities. Management of such wastes is a challenging ecological task. Several steps like minimization, pretreatment, sorption, characterization etc. needs been done before the final disposal of these wastes. Absorption and adsorption have been successfully employed in removal and remediation of various pollutants such as heavy metals and radionuclides waste materials from the contaminated sites. Different types of adsorbents are used in the process, which vary with the metals. Among them, biological, chemical and nanosorption methods are well known and are exploited for environmental remediation. In biological method different life forms like plants, algae, fungi and bacteria are effectively used. Chemical approaches require the use of different chemicals and chemical-conjugates as functional adsorbents. Nanosorption is a recent technique, where nanoparticles, nanocomposites, core/shell nanoparticles as well as nanotubes are employed as adsorbent for removal, transformation, sorption and detection of all types of pollutants including noxious radioactive wastes from soil, air and water.

The aim of the research was an assessment of the possibility of use of biochar (the biomass which has undergone pyrolysis processing) and soil-improving additives on its basis for decrease in the transfer of cesium-137 to crop production. The objects of the research were the peat soil polluted by ¹³⁷Cs, biochar from the wood of hard-wood broadleaved tree, compost, mineral sorbent bergmeal and mangold. Influence of biochar on decrease the stock in soils ¹³⁷Cs in the dissolved and exchange forms, which determine its mobility and biological availability, was noted. Two mechanisms of influence of biochar on transfer of ¹³⁷Cs in plants are revealed: the former is connected with essential increase in concentration of available potassium in the soil, and the latter, a slower one – with the transformation of ¹³⁷Cs to physical and chemical forms which are inaccessible for absorption by the root systems of plants.

Large-scale contamination of agricultural lands resulted from global fallout and accidents on Southern Urals («Mayak” PA 1957) and Chernobyl NPP (1986) required the development, testing and application of remediation measures purposed to prevent the entrance of ⁹⁰Sr into human diet. A wide range of remediation methods was developed in agriculture. Their purpose is to ensure the reduction of radionuclide mobility in soil and, as follows, decrease of its migration rates in the soil-plant system. The basis of action of soil-based remedial techniques is formed by the processes of radionuclide sorption and fixation in soils, strengthening of competitiveness of element analogues and changes of soil properties. Remediation methods are based on current agricultural technologies aimed to heighten soil fertility and crop yield. Data on the effectiveness of application of mechanical and physicochemical methods of remediation of agricultural lands are described. The possibility of application of various natural and artificial sorbents for reduction of ⁹⁰Sr mobility in soils and decrease of its assimilation by plants is shown. Some mechanisms, which are the scientific basis for development of methods for remediation of agricultural lands, were described.

The release of radionuclides in the environment is of major concern. Radionuclides arise mainly from nuclear power production, nuclear accidents, nuclear weapons testing, uranium mining and processing and nuclear waste disposal. Remediation of the affected areas is urgently needed, since the presence of these contaminants represents a major human and environmental health concern. Bioremediation and phytoremediation have been considered the eco-friendly alternative to the environmentally problematic remediation of contaminated soils, recovering functions that make some future uses possible. Available studies demonstrate that both methodologies, combined or not with other chemical or physically-based strategies (to mitigate the availability of radionuclides), have the potential to be used inexpensively and effectively in the restoration of contaminated environments. In addition, bioremediation and phytoremediation are environment friendly procedures, and are therefore more accepted by regulatory bodies. In this chapter, the main problems associated to each type of radioactively contaminated site will be identified and the most relevant bioremediation and phytoremediation techniques to deal with this type of contamination, will be reviewed. Furthermore, the advantages and disadvantages of using such techniques will also be discussed with a perspective of identifying knowledge gaps and highlighting new approaches to deal with this challenging issue. The reasons for the existence of few applications in real scenarios of contamination will also be analyzed.

Up to date, a large number of different methods of radionuclide immobilization in the environment have been developed. Among them, the biological methods for decontamination of radioactively contaminated areas are of great importance. They are mostly concentrated on phytoremediation of the environment using plants. These processes can potentially be used for bioremediation of radioactively contaminated areas.

It was demonstrated that phytoremediation is carried out through a number of mechanisms, such as phytotransformation (phytodegradation), phytoextraction, phytostabilization, phytovolatilization and rhizofiltration. The best in terms of phytoremediation are the plants with extensively developed root system able to perform rhizofiltration or those with well-developed aboveground biomass that is important for phytoextraction. Therefore the plant species with high radionuclide accumulation factors and considerable biomass are applied.

The chapter presents prospective of use of modified sorbents based on various supports (nonwoven filtering fabrics, aluminosilicates) for decontamination of natural water with various salt content being contaminated by natural and anthropogenic radionuclides (cesium, strontium, uranium, radium, lead, bismuth and thorium). For these sorbents, radionuclides distribution coefficients vary from n·10³ to n · 10⁵ L kg⁻¹. The results of development of Individual Water Filters (IWF) for decontamination of radioactive drinking water are presented. It is experimentally found that resource of these filters exceeds 4000 L, whereas decontamination factors remain more than 10. It is shown that the IWF-5, IWF-7, IWF-8, IWF-10 filters can be regenerated and reused. The questions of sanitary attestation of the sorbents and IWF as well as radiation safety of their using are discussed.

Large volumes of radioactive wastes of various activity levels and chemical composition are formed as a result of rehabilitation of radioactively contaminated lands. Deactivation works result in formation of both liquid and solid radioactive waste. Liquid radioactive waste after deactivation contains surfactants, complexing agents, inorganic acids, bases and salts as well as organic solvents. Solid radioactive waste includes radioactive soils, biomaterials, spent sorbents, etc. This chapter describes modern approaches to decontamination of deactivation solutions, radioactively contaminated soils, treatment of radioactive biological materials and spent sorption materials as well as disposal of radioactive waste after rehabilitation.

Mechanical strategies for remediating radiocesium contaminated soils, e.g. at farms, schoolyards, gardens or parks, lower air dose rates in one of two characteristic ways. The first is to physically remove radiocesium from the environment, for example by stripping topsoil and sending it for disposal. The second is to redistribute the radiocesium deeper within the ground, e.g. by mixing the topsoil or switching the positions of different soil layers, in order that soil attenuates radiocesium gamma rays before they reach the surface. The amount that air dose rates reduce because of remediation can be calculated using radiation transport methods. This chapter summarizes modelling results for the effect of topsoil removal (with and without recovering with a clean soil layer), topsoil mixing, and soil layer interchange on dose rates. Using measurements of the depth profile of ¹³⁴Cs and ¹³⁷Cs activity in soil at un-remediated sites across North East Japan, the potential effectiveness of remediation work was estimated considering remediation to different soil depths and different time lags after the accident. The results show that remediation performance would have been essentially constant irrespective of the time at which it was undertaken in the initial five year period following the fallout.

The paper gives estimated results of remediation efficiency in the radioactively contaminated territory by plowing soil. Efficiency of three tilling technologies has been estimated. The research showed the efficiency of soil plowing in terms of reduced surface contamination in the territory. Equivalent dose rate (EDR) levels and levels of β-particle flux density after plowing research areas, decreased, on average, by 56 and 87% respectively. Radionuclide migration is observed from top soil to underlying horizons. Maxima of activity concentrations in some research areas can occur in soil strata of 20–25 cm. The mechanical soil composition is found to change in remediated areas and humus content, which may point to the presence of both weathering processes and silt fraction and humus movement into the soil depth under gravity and by precipitation.

The paper also looks into the issue of radionuclide carry-over by plants onto the daylight soil surface and the impact of a plant root system type on this process, and an estimate is given to the influence of selected remediation technologies on the overgrowing process in remediated areas with vegetation. On the whole, it is found that soil plowing at a depth of 30 cm by overturning at 180^о is a preferable technology followed by sowing with steppe vegetation. The paper also gives information on other remediation measures taken in areas contaminated with radionuclides in the territory of Semipalatinsk test site.

Various peaceful applications of radioactive sources, the operation of nuclear facilities, radioactive waste management and accidents involving radioactive material as well as nuclear weapons testing (up to the 1960s) have resulted in some radioactive contamination of vast areas in different parts of the world. There is also a certain small contribution to this contamination from past practices, when no strict requirements had been introduced. Some of the affected areas, where the population may be exposed to higher levels close or even exceeding the present regulatory requirements or where the environment has been radioactively contaminated in excess of the introduced activity concentrations above the level considered now as being unjustifiable, require appropriate remediation actions in order to reduce the exposure and radioactive contamination levels in line with the relevant current national and international standards. In order to implement these standards and any other relevant requirements for assessing the impact of radioactively contaminated sites, the exposure of persons and the radioactive contamination of affected areas has to be quantified based on a sound generally accepted system of quantities and units. This is why the introduction of such a system is of primary importance to ensure consistent and unambiguous understanding and interpretation of the monitoring results expressed in well-defined quantities. The same approach should also be applied in recommending appropriate limits or reference levels ensuring the adequate radiation protection of people and the environment.