Sabyasachi Rout

Risk Assessment for Natural Uranium in Subsurface Water of Punjab State, India

ABSTRACT Traces of uranium were measured by laser fluorimeter in 235 subsurface water samples collected from four districts of Punjab state in India. The concentration of U in water samples ranged between <2–644 μg/L with a mean value of 73.1 μg/L. The radiological risk was observed to be in the range of 5.55 × 10−6–1.78 × 10−3 with a mean value of 2.03 × 10−4, which is around 22% more than the maximum acceptable level (l.67 × 10−4) as per guidelines of Indias Atomic Energy Regulatory Board. The mean of chemical toxicity risk, expressed as life time average daily dose (LADD) was worked out to be 5.56 μg/kg/day with a range of 0.15–48 μg/kg/day by considering a bodyweight of 51.5 ± 8.5 kg, water ingestion rate of 4.05 L/d, and life expectancy of 63.7 yrs for an adult Indian reference man and compared with the reference dose (4.53 μg/kg/day). The average exposure level of U was comparatively high and the chemical toxicity was expected to be more. The mean of hazard quotient (LADD/ RfD) for all four districts was found to be greater than 1, indicating that groundwater may not be suitable for consumption from a chemical toxicity point of view.

Spatial distribution and accumulation of 226Ra, 228Ra, 40K and 137Cs in bottom sediments of Mumbai Harbour Bay

The present work shows the activity levels of 226Ra, 228Ra, 40K and 137Cs in bottom sediments collected from eight locations of Mumbai Harbour Bay. The study has shown that 40K and 228Ra concentration is nearly uniform throughout the studied area while 226Ra and 137Cs are more concentrated in the southern regions of the bay. The significant variation in the activity levels of radionuclides within the study site might be due to their sorption/desorption processes onto the surface of sediment materials. The low mean value of 226Ra/228Ra ratio (0.72) in the sediments indicates that 238U has relatively greater solubility and mobility than 232Th. Similarly, low activity ratio (0.18) for 137Cs/40K reflects the presence of very high content of 40K in sediment due to presence of primary minerals in sediment. Silt and clay were reported to dominate the composition of the sediment. A significant positive correlation between 226Ra and 228Ra and 137Cs and 40K suggest a similar origin of their geochemical sources and identical behavior during transport in the sediment system.

Characterization of groundwater composition in Punjab state with special emphasis on uranium content, speciation and mobility

Abstract Groundwaters (borewell and handpump) were sampled from two districts (Bathinda and Mansa) of Punjab state and analyzed for their major ionic concentrations and uranium isotope compositions in order to assess the possible origins of the waters and water–rock interactions that occurred in the deep aquifer system. The major ionic concentrations of waters were plotted on a Piper diagram and grouped into four dominant hydrochemical facies as (Na+K)-SO4+Cl type (69% – 73%), (Ca+Mg)-SO4+Cl type (6% – 21%), (Ca+Mg)–HCO3 type (4% – 6%) and (Na+K)-HCO3 type (2% – 19%). It was observed that mobility of uranium in groundwater was very much influenced by TDS (total dissolved solids). To investigate the various mechanisms for deriving the elevated uranium concentrations in groundwater, 234U/238U activity ratios (ARs) were calculated using the determined activity levels of 234U and 238U. The mean ARs was found to be near unity (i.e. secular equilibrium) in the study regions confirmed that uranium in groundwaters was mainly resulted from its host/parent rocks through weathering processes. The concentration of HCO3− in ground water showed one order of magnitude higher than the total dissolved SiO2 indicates that carbonate weathering was the dominant process due to major water–rock interaction. The uranium speciation in groundwaters was investigated by an equilibrium model calculation using MEDUSA (make equilibrium diagrams using sophisticated algorithms) under the influence of redox conditions and complexant concentration. At the observed range of pH values, the predominant redox speciation of uranium was observed as hydroxo-carbonato complexes of (UO2)2(CO3)(OH)3− and hydroxyl complexes of UO2(OH)3− which might be caused for increasing the solubility of uranium. Due to very low concentration of phosphate in groundwater, its effects on U(VI)-aqueous speciation was negligible.

Thermodynamic parameters of U (VI) sorption onto soils in aquatic systems

The thermodynamic parameters viz. the standard free energy (∆Gº), Standard enthalpy change (∆Hº) and standard entropy change (∆Sº) were determined using the obtained values of distribution coefficient (kd) of U (VI) in two different types of soils (agricultural and undisturbed) by conducting a batch equilibrium experiment with aqueous media (groundwater and deionised water) at two different temperatures 25°C and 50°C. The obtained distribution coefficients (kd) values of U for undisturbed soil in groundwater showed about 75% higher than in agricultural soil at 25°C while in deionised water, these values were highly insignificant for both soils indicating that groundwater was observed to be more favorable for high surface sorption. At 50°C, the increased kd values in both soils revealed that solubility of U decreased with increasing temperature. Batch adsorption results indicated that U sorption onto soils was promoted at higher temperature and an endothermic and spontaneous interfacial process. The high positive values of ∆Sº for agricultural soil suggested a decrease in sorption capacity of U in that soil due to increased randomness at solid-solution interface. The low sorption onto agricultural soil may be due to presence of high amount of coarse particles in the form of sand (56%). Geochemical modeling predicted that mixed hydroxo-carbonato complexes of uranium were the most stable and abundant complexes in equilibrium solution during experimental.

Understanding the solid phase chemical fractionation of uranium in soil and effect of ageing.

The aim of the present work is to understand the solid phase chemical fractionation of Uranium (U) in soil and the mechanism involved. This study integrated batch experiments of U(VI) adsorption to soil, study of U in different soil fractions, ageing impact on fractionation of U and spectroscopic investigation of adsorbed U(VI) using X-ray Photoelectron Spectroscopy (XPS). For the study three soils, pedogenically different (S1: Igneous, S2: Sedimentary and S3: Metamorphic) were amended with U(VI) and chemical fractionation of U was studied by sequential extraction after an interval of one month and 12 months. It was found that there occurs a significant rearrangement of U in different fractions with ageing and no correlation was observed between the U content in different fractions and the adsorbents of respective fractions such as soil organic matter (SOM), Fe/Mn oxides (hydroxides) carbonates, soil cation exchange capacity (CEC). XPS study revealed that surface enrichment of U mainly governed by the carbonate minerals and SOM, whereas bulk concentration was controlled by the oxides (hydroxides) of Si and Al. Occlusion of U-Fe-oxides (hydroxides) on silica was identified as an important mechanism for bulk enrichment (Increase in residual fraction) and depletion of U concentration in reducible fraction.

Equilibrium isotherm and kinetics modeling of U(VI) adsorption by natural soil systems

This research examined the adsorption capability of natural soils of different origin for U(VI) at soil–water interface and the mechanism involved. Adsorption kinetics study indicates that adsorption followed the pseudo-second order kinetics irrespective of soil types and was not solely controlled by the diffusion step rather surface complexation and ion-exchange plays major role. Study of adsorption isotherms indicates adsorption involves chemisorption and soil with high Fe and Mn content better fit to Langmuir model. The study provides a better insight for site selection for radioactive waste (uranium) disposal facilities as well as soil suitable for back fill or permeable reactive barrier to inhibit migration of U(VI).

Spectroscopic investigation of uranium sorption on soil surface using X-ray photoelectron spectroscopy

A study was carried out to understand the sorption of uranium (U) onto soil surface and identify the species of U on soil surface using X-Ray Photoelectron Spectroscopy (XPS). For the study soil was amended with uranyl nitrate and surface speciation study was carried out by investigating the energy region for U in spectrum. Analysis of spectrum revealed that U is present in U(VI) state. Deconvolution of XPS spectrum of U(VI) sorbed on soil surface revealed that U(VI) species such as, UO22+ and (UO2)x(OH)y(2x−y)+ form complex with silanol, aluminol and goethite sites. The possible surface complexation is: ≡Al(OH)2UO22+, ≡SiO2UO2, ≡SiO2(UO2)3(OH)5 and ≡Fe(OH)2UO2.

A review of the studies on environmental transport and speciation analysis of radionuclides in India

Department of Atomic Energy, India, has a systematic environmental monitoring program to ensure that the impact of radionuclides on human and environment is well within the limits, stipulated by the regulatory body. Concentrations of radionuclides are estimated in air, water, soil, sediment and biota in the environment around the power plant site on a regular basis. The environmental migration pattern and impact of radionuclides in the environment depends upon not only on the total concentration but the physico-chemical nature of radionuclide also. This paper presents a brief review of the studies of environmental transportation of radionuclides and influence of speciation with special reference to Indian conditions. A fairly good amount of results are available on the influence of speciation of radionuclides on the wet and dry deposition and preliminary results are available on the speciation analysis in dietary items, in soil and in ground water. Initial studies are concentrated on tritium, 137Cs, Uranium, 90Sr and their chemical analogues.