Hydrogeochemical Processes Governing Uranium Mobility: Inferences from the Anthropogenically Disturbed, Semi-arid Region of India

Khetri Copper Belt, Rajasthan is anthropogenically active and geologically belongs to the Delhi supergroup. To study the active geochemical processes controlling the elemental mobility, representative groundwater samples were analysed for heavy metals and radionuclide (U). A maximum U concentration (average 87 µgL− 1) is observed in the quartzite mine zone whereas minimum (average 13 µgL− 1) is found to be in the copper mine zone samples. Despite the presence of Jaspura and Gothra granitoid in the copper mine zone, the abundance of U is low suggesting the scavenging of U by sulphides or iron oxides. Additionally, at the confluence of two geological groups, Fe concentration is found high with a low concentration of U which further confirms scavenging of U. In the region, aquifers are shallow which supports the weathering of minerals. Thus, a high concentration of U in groundwater is attributed to mineral dissolution, faults or fractures. It is evident from the results that in the absence of iron-bearing sulphides, U concentration in groundwater would be very high compared to the current concentration. It also indicates low concentration of U in the copper mine zone, area is covered with abundant quantity of Fe sulphide rich waste. The present study recommends further research to understand the feasibility of mining waste for the removal of U contamination from groundwater.

Heavy Metal and Metalloid Contamination Assessments of Soil around an Abandoned Uranium Tailings Pond and the Contaminations’ Spatial Distribution and Variability

To investigate the heavy metal and metalloid contamination of soil around a Huanan uranium tailings pond, abandoned in 1998, we defined a study area of 41.25 km2 by a natural boundary and targeted 5 elements’ (U, Mn, As, Pb, Cr) single contamination and comprehensive pollution as the assessment contents. First, we collected 205 samples and evaluated them with the contamination factor (CF) method aiming at judging whether the single target element concentration exceeded the local background value and environmental quality standard. We obtained CF1 (the background value of a certain target element as the baseline value) and CF2 (the environmental quality standard for soils as the baseline value). Second, we evaluated the ecological risk of the key pollutant U with the risk assessment code (RAC) method, taking the 27 samples whose CF2 > 1 as examples and concluded that the environmental risk of U was relatively high and should arouse concern. Third, we selected comprehensive pollution index (CPI) to assess the compound pollution degree of five target elements. Fourth, we constructed the U contamination and CPI’s continuous distribution maps with spatial interpolation, from which we worked out the sizes and positions of slightly, moderately and strongly polluted zones. Finally, we analyzed the spatial variability of U and CPI with the aid of a geostatistical variogram. We deduced that the spatial variation of uranium was in close relationship with local topography, and probably precipitation was the driving force of U contamination diffusion, whereas CPI exhibited weak spatial dependence with random characteristics. The above work showed that 3.14 km2 soil near the pond was fairly seriously polluted, and the other 4 elements’ single contaminations were less serious, but the 5 target elements’ cumulative pollution could not be ignored; there were other potential pollution sources besides the uranium tailings pond. Some emergency measures should be taken to treat U pollution, and bioremediation is recommended, taking account into U’s high bioavailability. Further, special alerts should be implemented to identify the other pollution sources.

Remobilisation of uranium from contaminated freshwater sediments by bioturbation

Benthic macro-invertebrate bioturbation can influence the remobilisation of uranium (U) initially associated with freshwater sediments, resulting in a high release of this pollutant through the overlying water column. Given the potential negative effects on aquatic biocenosis and the global ecological risk, it appears crucial to improve our current knowledge concerning the biogeochemical behaviour of U in sediments. The present study aimed to assess the biogeochemical modifications induced by Tubifex tubifex (Annelida, Clitellata, Tubificidae) bioturbation within the sediment in order to explain such a release of U. To reach this goal, U distribution between solid and solute phases of a reconstructed benthic system (i.e. in mesocosms) inhabited or not by T. tubifex worms was assessed in a 12-day laboratory experiment. Thanks notably to fine-resolution (mm-scale) measurements (e.g. "diffusive equilibrium in thin-films" DET gel probes for porewater, bioaccumulation in worms) of U and main chemical species (iron, sulfate, nitrate and nitrite), this work (i) confirmed that the removal of bottom sediment particles to the surface through the digestive tract of worms greatly favoured oxidative loss of U in the water column, and (ii) demonstrated that both U contamination and bioturbation of T. tubifex substantially influenced major microbial-driven biogeochemical reactions in sediments (e.g. stimulation of denitrification, sulfate reduction and iron dissolutive reduction). This study provides the first demonstration of biogeochemical modifications induced by bioturbation in freshwater U-contaminated sediments.

Water pollution in Kucukcekmece Lake, Istanbul, Turkey

Water samples collected in February and August 1997 from the Kucukcekmece Lake were analysed by ICPMS-200 to determine the heavy metal concentration s (i.e., Hg, Cd, Pb, As, Cu, Cr, Co, Ni, Zn, Fe, Mn, Se, Ba, Al, and U). A comparison of these data with those of 1985, 1989, and 1995 revealed Hg and U contamination in the northern region, whereas Pb, Cu and Se contamination in the southern region of the Kucukcekmece Lake. Beside s, the influent waters were collected from the respective stream mouths, and were also analysed for heavy metal concentrations. The results showed the following contaminants: Hg, Se and U in the Ispartakule Stream, Hg, Cu and Se in the Sazli Stream, and Cu, Se and U in the Nakka Stream. On the other hand, a comparison of the results of February 1997 with those of August 1997 revealed that the amounts of As, Cu, Cr, Mn, Ba and Fe had decreased, but those of Pb, Ni, Zn and Se had increased in the lake since February 1997. Such kind of seasonal variation in concentration of heavy metals is attributed to pollution of the lake water by domestic waste, industrial effluents, and water-rock interactions.

Comparison of acid and base leach for the removal of uranium from contaminated soil and catch-box media

Several leach solutions have been developed for the removal of uranium (U) from contaminated media such as soil and military catch-box sand used for the ballistics testing of U-containing projectiles. Leach solutions of concentrated sulfuric acid and of carbonate (with and without oxidants such as peroxides) have also been used in the mining of U from high-grade ore deposits. Many of these solutions have been used at U-contaminated environmental sites with varying degrees of success. Batch leaches with six characterized U-contaminated media and seven leach solutions were conducted. After the leach solutions were reacted with the media (five soils and a catch-box sand), the solutions were analyzed for dissolved U, Ca, Si, Fe, Mn, Pb and Cr. We study the ability of acidic, basic, acidic-oxidizing and basic-oxidizing solutions to leach U from contaminated media. The results suggest the removal efficiency of soil U for each leach solution varies with the nature of the U-contamination in the media and the media composition. It is likely the forms of U in the contaminated media such as reduced, absorbed, complexed and solid species influence the solubilization of U by the leaching agents. Key words: Uranium, synchrotron X-ray fluorescence (SXRF), remediation, chromium, lead, leaching, catch-boxes