|Year : 2013 | Volume
| Issue : 3 | Page : 109-114
Concentration and activity ratio of thorium isotopes in surface soil around proposed uranium mining site in India
SK Srivastava1, K Vishwa Prasad1, AY Balbudhe1, P Padma Savithri1, PM Ravi2, RM Tripathi2
1 Department of Atomic Energy, Health Physics Unit, NFC, Health Physics Division, Bhabha Atomic Research Centre, Hyderabad, Andhra Pradesh, India
2 Health Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India
|Date of Web Publication||28-Jul-2014|
S K Srivastava
Health Physics Unit, NFC, Health Physics Division, BARC, PO. ECIL, Hyderabad 500 062, Andhra Pradesh
Source of Support: None, Conflict of Interest: None
Unconformity type of uranium deposits at Lambapur and Peddagattu located in Nalgonda District of Andhra Pradesh, India has been reported. Soil surveillance for isotopic thorium belonging to two different radioactive decay chains provide information on characterization of soil formation due to weathering of underlying host rocks. Thorium concentration and their isotopic activity ratio in localized soil samples were assessed. Thorium concentration and 230 Th/ 232 Th activity ratio were observed to be in the range of 15 ± 3 mg/kg to 132 ± 15 mg/kg and 0.21 ± 0.07-0.68 ± 0.05, respectively. A significant wide variation in thorium concentration and that of activity ratio for localized area soil indicates the soil development due to a different type of underlying host rocks. Activity ratio of 228 Th/ 232 Th in the soil samples of the study area are observed to vary from 0.87 ± 0.05 to 1.07 ± 0.15 with a mean of 0.96 ± 0.02, indicating secular equilibrium between 232 Th and its daughter product 228 Th. The study describes the application of isotopic thorium activity as a chronological indicator for geological characterization.
Keywords: Alpha spectroscopy, radioactive equilibrium, soil, thorium
|How to cite this article:|
Srivastava S K, Prasad K V, Balbudhe A Y, Savithri P P, Ravi P M, Tripathi R M. Concentration and activity ratio of thorium isotopes in surface soil around proposed uranium mining site in India. Radiat Prot Environ 2013;36:109-14
|How to cite this URL:|
Srivastava S K, Prasad K V, Balbudhe A Y, Savithri P P, Ravi P M, Tripathi R M. Concentration and activity ratio of thorium isotopes in surface soil around proposed uranium mining site in India. Radiat Prot Environ [serial online] 2013 [cited 2020 Feb 18];36:109-14. Available from: http://www.rpe.org.in/text.asp?2013/36/3/109/137474
| Introduction|| |
The high-grade and large-tonnage unconformity-related uranium deposits of Canada and Australia are now well-understood and documented. ,,, These vein-like uranium deposits occur as fracture/breccia-fillings in the lower proterozoic pelitic rocks (±graphite) close to the unconformity, under the cover of dominantly arenaceousMiddle Proterozoic strata.With increasing emphasis on unconformity type uranium deposits within intracratonic Middle Proterozoic basins, efforts toward identifying such basins in the Indian peninsular shield, since 1990, have encountered some success. The mid-proterozoic cuddapah basin has thus come up as a most promising target area in India. On the North-Western margin of this basin the Srisailam formation, the youngest member of the Cuddapah supergroup, directly overlies the basement granite of lower Proterozoic/Archaean age and forms a dominant plateau of more than 3000 km 2 . Among the numerous uraniferous anomalies located close to this unconformity, uranium deposits at Lambapur and Peddagattu located in Pedda Adsarlapalli Mandal of Nalgonda district Andhra Pradesh, India has been reported. The Srisailam sub-basin has a highly dissected topography resulting into the development of numerous isolated flat-topped hills rising up to 100-150 m above the ground level. Lambapur outlier is one such flat-topped hill, having a dimension of 1.8 km by 0.70 km, and exposes the unconformity between the Mahboobnagar granite and Srisailam formation.
Soils result from the weathering of rocks, which involves the destruction of rock forming minerals. Nuclides correlations present in the rocks carried through to the soil developed on them. Thus, characterization of soil represents one of the most relevant study areas for exploring geological setting of the environment. 238 U and 232 Th, head of primordial radionuclide decay series, have existed in the earth's crust throughout its history.  As weathering processes break the rock down it would seem likely for isotopic uranium or thorium that the correlations present in the rock would carry through to the soil. As 230 Th isotope belongs to 238 U series and 232 Th is just a primordial parent of thorium series so one can expect that their ratio in the soil will reflect the activity ratio between the 230 Th and 232 Th in host rock.
Soil developed from a uniform lithology has a constant 230 Th/ 232 Th ratio, the same ratio as that in the rock from which the soil was formed.  Thus primordial radionuclide 230 Th and 232 Th activity in soil belonging to two different decay series can be used as an indicator for the genesis of the soil. Not much information is available for spatial distribution of 230 Th/ 232 Th ratio in soil developed on different age groups rocks. Hence soil samples from the localized area of Lambapur and Peddagattu, encompassing the youngest member of the Cuddapah supergroup rocks, overlying on the basement granite rock of lower Proterozoic/Archaean age were collected for study.
This study aims to provide application of spatial distribution of thorium and 230 Th/ 232 Th activity ratio in soil to characterize the geological setting of the environment.
| Materials and methods|| |
Study area lies in the tropical region where climate is characterized by very hot summer, during the summer (March-May) with a temperature ranging from 30°C to 46.5°C and in winter (November-January) it varies between 16°C and 29°C. The average annual rainfall in this area is about 1000 mm occurring mostly during the southwest monsoon (June-September). There are several small hillocks in this area with height ranging from 100 to 200 m. The Lambapur and Peddagattu area where the uranium minerals occur are flat topped hills with an elevation of about 300 m mean sea level. Drip irrigation is practiced in this area especially for sweet lime.
The area is close to the unconformity between the archaean/lower proterozoic Mahboobnagar granite and Srisailam formation, in a number of dissected outliers in India. The Srisailam sub-basin has a highly dissected topography on its North-Western fringes, resulting into the development of numerous isolated flat-topped hills. Lambapur outlier is one such flat-topped hill and exposes the unconformity between the Mahboobnagar granite and Srisailam formation. At Lambapur the uranium mineralization is found close to the unconformity, both in the basement granite and the overlying Srisailam pebbly arenite. Basic dykes and vein quartz within the basement granite are also mineralized close to the unconformity. The vein quartz, in addition, has associated lead and copper mineralization. The main uranium ore body is confined to the unconformity between the basement granite and the overlying Srisailam formation. The major part (more than 85%) of the main ore body occurs in granite, close to the unconformity, at shallow depth (at 1-55 m from the surface). 
Samples of surface soil from Lambapur, Peddagattu and Sheripally areas were collected as per the protocols of international atomic energy agency.  Sampling locations sites are presented in [Figure 1]. Following removal of the surface vegetation, 2-3 kg soil samples were collected from each site up to a depth of 10-15 cm. Soil samples were sealed in plastic bags for transportation to the laboratory, where they were dried at 40°C, grounded to pass through a 300 μ sieve and stored.
|Figure 1: Map showing sampling locations at Lambapur, Peddagattu and Seripally area|
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In each case between 0.5 g and 1 g of dried finely grounded soil sample was processed. Soil samples were ashed at 600°C for 6 h or more to destroy organic components. Next, samples were acid digested in a hot 1:1 HF:HNO 3 mixture for 6 h and evaporated to near dryness. The residues were treated with HClO 4 to remove any remaining organics. Residues were further digested with concentrated HNO 3 and re-dissolved in 4M HNO 3 . U and Th present in the sample solutions were co-precipitated with Fe (OH) 3 by adding ammonia solution. Ferric hydroxide precipitates were then separated from solution by decanting and centrifugation. Precipitates were rinsed with deionised water, re-centrifuged and dissolved in concentrated HNO 3 . Samples were then evaporated to near dryness and dissolved in 8M HNO 3 .
Chromatographic columns were prepared, containing approximately 1 g of Dowex 1 × 8 chromatographic resin. The resin was slurried in deionized water overnight prior to transferring into the glass columns. All aliquots were passed through the columns using gravity flow rates (approximately 1 cm 3 /min). The resin was preconditioned with 8M HNO 3 . Samples that had been digested and dissolved in 8M HNO 3 were then passed through the resin followed by 5 × 10 cm 3 wash of 8M HNO 3 . Thorium was eluted with 50 cm 3 of 8M HCl, collected into a 100 cm 3 beaker and evaporated to dryness.
The dry residue was dissolved in 9 cm 3 of an electrolyte solution for electrodeposition.  The pH of the solution was adjusted to approximately pH 1.8 using 10% H 2 SO 4 . The solution was transferred to an electrodeposition cell containing a platinum electrode. The thorium was electrodeposited on a stainless steel disc for 2 h at 0.95 A. At 1 min prior to the end of this stage, 1 cm 3 of concentrated ammonia solution was added to the cell before switching off the current. The cell was disassembled and the disc was then quickly rinsed and allowed to dry prior to measurement.
The electrodeposited thorium was measured using high-resolution alpha spectrometry. Single channel alpha spectrometer Model 7401 (manufactured by Eurisys Measure) with passivated implanted planar silicon detector (450 mm 2 active area) was used. The counting time for samples varied from 25,000 s to 50,000 s with a corresponding minimum detection limit of the spectroscopy system to be 0.6-0.4 mBq respectively in the region of interest for thorium isotopes.
Spectrophotometric determination was based on measuring the absorption of the complex compound of thorium with Arsenazo III in the visible part of the spectrum at λmax of 660 nm. Purified and separated thorium content of the soil samples were prepared in flasks of 10 cm 3 by adding of the specific sample volume and 1.0 cm 3 of 0.02% solution of Arsenazo III. After thorough mixing, the absorbance at 660 nm against a reagent blank was measured.
| Result and discussion|| |
The accuracy and reliability of the spectrophotometric determination of thorium in soil is verified by cross method analysis and spike recovery study. Randomly selected samples were analysed by both inductively coupled plasma atomic emission spectroscopy and ultraviolet-visible spectroscopy for thorium estimation. The results are in good agreement with each other and the correlation coefficient of 0.96 was observed [Figure 2].
|Figure 2: Comparison of thorium concentration by two techniques ultraviolet-visible spectroscopy and inductively coupled plasma atomic emission spectroscopy|
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Spike recovery study for spectrophotometric determination of thorium has been carried out. The spike recovery study was carried out by spiking thorium standard in soil samples. The results of the mean and the standard deviation (SD) for spike recovery were plotted [Figure 3]. The dashed and dotted lines represent two and three SD of the mean. The recovery of spiked thorium was found to be consistent in the range of 90-97% with an average of 94% ± 2%.
Thorium concentration in the surface soil samples from Lambapur, Peddagattu and Seripally area are given in [Table 1]. Thorium concentration in the study area is varying from 15 ± 3 mg/kg to 132 ± 15 mg/kg with a mean of 68 ± 9 mg/kg. A wide variation in concentration of thorium in localized soil has been observed which characterize the diverse geology of the proposed mining study area.
The average thorium concentration and their variation observed in the present study area are compared with those reported for the normal background regions of other countries [Table 2]. Soil samples showed enhanced level of 232 Th as compared to reported world-wide. Sartandel et al.  have reported 232 Th in a range from 7.9 to 76.9 mg/kg for soil samples in the Lambapur, Mallapuram and Peddagattu study area at proposed mining site.
|Table 1: Thorium concentration and isotopic activity ratio in soil samples |
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|Table 2: Comparison of 232Th concentration in soil samples of Lambapur, India with the reported for other countries |
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A typical alpha spectrum of thorium isotopes and their daughters obtained from an electroplated thorium of soil samples is presented in [Figure 4]. Total eight alpha peaks were observed. [Figure 4] shows three peaks of thorium isotopes viz. 232 Th (4.012 MeV), 230 Th (4.687 MeV) and 228 Th (5.423 MeV) and the remaining five peaks are very short lived daughter products of 232 Th decay chain viz. 224 Ra (5.685 MeV), 212 Bi (6.051 MeV), 220 Rn (6.288 MeV), 216 Po (6.779 MeV) and 212 Po (8.784 MeV) build up during counting.
|Figure 4: α spectra of thorium isotope and their daughters obtained from a soil sample|
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230 Th/ 232 Th activity ratios for each of the soil samples are shown in [Table 1]. The reported uncertainty for individual analysis in the [Table 1] is the propagated uncertainties associated with the counting statistics of the sample. 230 Th/ 232 Th activity ratios in the study area soil samples range from 0.21 ± 0.07 for the sample from site S-11 to 0.68 ± 0.05 in the sample from site S-5. 230 Th/ 232 Th in the soils will be same as those in parent rock while the absolute concentration of these nuclides varies between the soil and rock samples. Thorium is being mobilised during weathering process, but no differential mobilisation of thorium isotopes occurs. Soils and the rocks from which the soil formed have 230 Th/ 232 Th activity ratio within analytical uncertainty.  Scott  also observed that extensive redistribution of thorium between a rock and developed soil occurred during the weathering process with no significant fractionation of the thorium isotopes. Sarin et al.  also made similar observations.
Frequency and cumulative distribution for 230 Th/ 232 Th in soil is shown in [Figure 5]. A significant variation in 230 Th/ 232 Th activity ratio has been observed in the study area. 37.5% of the samples are observed with activity ratio in the range of 0.40-0.60 with an average of 0.52 fairly close to reported ratio of 0.57 for soil developed on granite rock.  The result indicates that the host rock in the study area is mainly granite as earlier reported elsewhere.  About 25% of the samples are reported with activity ratio 0.60-0.70 with an average of 0.65 which is fairly close to reported ratio of 0.66 ± 0.02 for soil developed on sandstone type of rock.  The result helps further to characterize geological setting of the area showing the presence of the also host Srisailam arenite, a sedimentary type of rock.  Nearly 37.5% samples showing activity ratio in the range of 0.20-0.40 indicates the presence of other type of rocks. Olley  has reported 230 Th/ 232 Th activity ratio to be 0.571 ± 0.010, 1.25 ± 0.02, 0.76 ± 0.03; 0.66 ± 0.02 and 0.95 ± 0.06 for the soil developed on granite, basalt, mix sediments, sandstone, rhyolite type of rock respectively. Dramis et al.  has reported 230 Th/ 232 Th ratio to be varying from 0.146 ± 0.007 to 0.356 ± 0.018 for micritic limestone. Since soils developed for a single uniform rock type have a uniform ratio and soils developed on different rock type can have distinctive 230 Th/ 232 Th ratio. The result indicates the development of soil due to weathering of different types of rocks such as granite, arenite and altered dyke in the study area. It is therefore lithology in the study area is the dominant factor controlling the heterogeneity of 230 Th/ 232 Th ratio in soil.
|Figure 5: Frequency and cumulative distribution of 230Th/232Th activity ratio in soil|
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228 Th/ 232 Th activity ratios in the soil samples were found to be in the range of 0.87 ± 0.05-1.07 ± 0.15 with a mean of 0.96 ± 0.02. Being 228 Th a daughter product of 232 Th decay chain, these nuclides will be in secular equilibrium in all type of host rocks. Thus soil developed even on different lithology, generally will not show deviation from state of secular equilibrium for 232 Th decay chain nuclides. The activity ratio indicates that 232 Th is in equilibrium with its daughter 228 Th. The results are similar to reported elsewhere. Jia et al. have reported 228 Th/ 232 Th for environmental soil samples in the range of 0.971 ± 0.065-1.10 ± 0.08 with mean 1.04 ± 0.05.
| Conclusions|| |
Soil monitoring data can be very effective in characterizing the underlying rocks, because the soils are largely derived from the rock by physical denudation processes. A wide variation of thorium concentration and activity ratio of 230 Th/ 232 Th in soil samples of localized study area were observed. This significant variation indicates the presence of different age of underlying host rocks. Thus isotopic thorium activity in soil may be utilized as a chronological indicator for underlying rocks characterization.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2]