DOI: 10.1007/s10967-007-1120-2
Journal of Radioanalytical and Nuclear Chemistry, Vol. 274, No.2 (2007) 341–343
Leaching of uranium, radium and thorium from vertisol soil by ground water V. A. Pulhani,* S. Dafauti, A. G. Hegde Environmental Studies Section, Health Physics Division, Bhabha Atomic Research Centre, Mumbai–400085, India (Received September 18, 2006)
Shallow land burial is routinely used for the disposal of low-level radioactive waste. Natural processes causing leaching of radionuclides can lead to contamination of surrounding ground water and soil by the radionuclides. The comparative leachability of radionuclides U(nat), 226Ra, 228Ra and Th(nat) from the soil of a radioactive waste disposal site, by ground water was evaluated. The probability of leaching was obtained in the following order Ra (|77%) > U (|40%) > Th (|20%). Observed ratios (OR) were calculated to correlate leachability of radionuclides to that of major cations Ca2+ and Mg2+. The leaching of the radionuclides was seen to be dependent on Ca2+ and SO42– leached from the soil. This study provides sitespecific leachability of radionuclides, that can be used as indicator of the tendency for migration or retention in soil. It can play an important role during an unforeseen accident like breach of containment at the waste disposal site leading to contamination of soil and ground water and causing hazard to public via drinking water route.
Introduction Shallow land burial is routinely used for the disposal of low-level radioactive wastes. Radionuclides present in disposed waste could come in contact with ground water in the case of a breach in the geological containment. Infiltration of ground water can cause leaching of radionuclides leading to localized contamination of surrounding soil and ground water. The migration and retention of radionuclides is a geochemical phenomenon involving the interaction of local ground water with solid components. The leachability of the radionuclides depends upon a number of factors like the properties of soil, ground water and the chemical composition of the waste buried. Solubility of uranium and others is very site specific and reported1 to be the net result of competition between complexation with aqueous ligands like carbonates, hydroxyls, phosphates, sulphates, organics, etc., precipitation and sorption on solid phases. Thorium forms complexes with simple inorganic ligands and humic substances in natural organic matter2 (NOM). BEDNAR et al.3 suggest that complexation with NOM is a likely factor affecting dissolved aqueous thorium concentrations besides sorption during its migration. Radium leachability and migration are mostly reported for mixed cakes, through clay liners and phosphogypsum wastes at waste disposal site.4–6 All these studies report various behavior depending on individual site characteristics and radionuclide chemistry. Hence, speciation of the radionuclides with organic and inorganic ligands is required to understand their solubility and migration and predict the environmental impact of these soils on water quality. Calcium and magnesium are important components of soil and ground water participating in many complexation and precipitation reactions. They are
present in macro amounts in soil and water as compared to radionuclides and compete with them for complexation with the ligands. They thus regulate the behavior of radionuclides in the soil-water system. They can offer clues to the speciation of important radionuclides expected to have the same chemical behavior, e.g., calcium and radium. To assess the probability and extent of leachability and migration, it is essential to collect site-specific data for the radionuclides and study the major factors affecting these processes. The ratio of the concentration of radionuclides to the calcium concentration leached in water to that in the soil is known as observed ratio (OR) and can be represented by the following equation: OR = (CRn/CCa)water/(CRn/CCa)soil where CRn is the concentration of radionuclide in water and soil in Bq/l and Bq/kg, respectively, CCa is the concentration of calcium in water and soil in mg/l and or mg/kg, respectively. Thus, the concentration of the radionuclides leached gets normalized with respect to the concentration of calcium and gives an idea about the discrimination in leachability between them.7,8 The effect of Ca2+ and other major cations present in soil like Mg2+ can also be compared, by calculating the observed ratio (OR) for the leached radionuclides. This paper presents the work carried out to evaluate the leaching and migration of 238U, 226Ra, 228Ra and 232 Th from a known low-level radioactive contaminated soil. The soil studied is calcareous, belongs to black cotton group having pH range from neutral to alkaline.9,10 The comparative leachability of these radionuclides by ground water collected from boreholes in the same area and the effect of other cations and anions in the soil has been studied. This study provides site-specific leachability of the radionuclide, which can
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V. A. PULHANI et al.: LEACHING OF URANIUM, RADIUM AND THORIUM
be used as an indicator of its tendency for migration or retention. It can play an important role during an unforeseen accident like breach of containment at the waste disposal site leading to contamination of ground water and causing hazard to public via drinking water route. Experimental Leaching experiments were carried out with soils having known low level contamination. The bore well water from the same area was used as leachate. Preliminary experiments were carried out to design and standardize the conditions for the leaching experiment. A saturation of leached activity was obtained after about 7 hours of contact time. In three replicate experiments, about 10 g of soil collected was dried, powdered, taken into polythene containers and each was shaken with 100 ml of the ground water sample for 7 hours. The leachate was filtered with 0.42 Pm filter paper and collected. The soil and water were analyzed for thorium, uranium and radium content using a standard radiochemical separation procedure.11 All reagents used during the experiment were of Analar grade. The soil samples were treated with 8N HNO3 to extract the radionuclides from the matrix. This soil extract was made up in 8N HNO3 and loaded on Dowex-1 anionic resin preconditioned in the same medium. Uranium and radium are not retained in the anion resin in 8N HNO3. Thorium remains on the resin and was eluted with 8N HCl. Radium and uranium were reloaded on the resin in 8N HCl, radium is not retained and gets separated from uranium. Uranium was then eluted with 1N HNO3. Uranium and thorium were electroplated in ammonium sulphate medium and measured by D-spectrometry. 226Ra was estimated by gross D-counting after coprecipitation on barium sulphate. 228Ra was estimated
by gross E-counting of the daughter 228Ac coprecipitated on LaF3 and in secular equilibrium with it. The anionic and cationic contents of water and soil were also determined using specific ion chromatography. Minimum detection limits for the radionuclides using the above technique was for U 0.04 Bq/kg, for Th 0.1 Bq/kg, for 226Ra 0.1 Bq/kg and for 228Ra 26 Bq/kg. Results and discussion Table 1 gives the concentrations of U(nat) , Th(nat) , and 228Ra in the low level active soils before and after subjecting them to leaching with ground water, along with the activity in the ground water sample. The results show that Ra is appreciably leached out (77%) followed by U with around 40% leaching, but relatively Th is retained in the soil. The major cations and anions that got leached into the ground water from soil are given in Table 2. The soil studied here belongs to black cotton group and is highly calcareous and alkaline with about 40,000 Pg of CO32– and 30,000 Pg of Ca2+. This leads to the formation of negative carbonate and tricarbonato complexes of uranium, which are to highly mobile, and hence, higher solubility of uranium in the ground water. Uranium migration in soil is reported to occur by colloidal transport and carbonate complexation.12 ELLESS and LEE13 obtained 21% leaching of uranium from carbonate rich soils. They report that uranyl carbonate complexes are the major species (|90%) of the dissolved uranium in the presence of excess of carbonates, though hydroxides and sulphates are also formed, but only upto 0.4–1%. Thorium forms most insoluble thorium hydroxide and oxy-carbonate at this soil pH in presence of carbonates. Hence, only 20% of thorium was leached into ground water. 226Ra
Table 1. Activity (in Bq/kg) in soil before and after leaching 226Ra
Sample 1 2 3 4 Ground water*
228Ra
Before After 89.2 r 11.9 28.6 r 1.2 62.9 r 10.6 14.4 r 4.7 70.6 r 10.8 17.3 r 1.7 122 r 13 51 r 7
