Mill Tailings Research Articles

Comparison of two sequential extraction protocols for fractionation of natural radionuclides in soil samples

Revised BCR (Bureau Communautaire de Référence) and the Schultz modification of Tessier´s sequential extraction protocols are compared for fractionation of uranium, thorium and radium in soil samples. Six soil samples were collected near to the uranium mill tailings waste pile and some of them had enhanced levels of natural radionuclides. The samples were processed by both protocols and the radionuclide content assayed by alpha spectrometric determination. The analysis revealed that results obtained by the two protocols are not fully comparable as data are mostly protocol- and element-dependent. Nevertheless, general conclusions about potential source of the particular radionuclide, which could be drawn from the specific study carried out, are mostly similar for both protocols.

Anaerobic bioremediation of hexavalent uranium in groundwater by reductive precipitation with methanogenic granular sludge

Uranium has been responsible for extensive contamination of groundwater due to releases from mill tailings and other uranium processing waste. Past evidence has confirmed that certain bacteria can enzymatically reduce soluble hexavalent uranium (U(VI)) to insoluble tetravalent uranium (U(IV)) under anaerobic conditions in the presence of appropriate electron donors. This paper focuses on the evaluation of anaerobic granular sludge as a source of inoculum for the bioremediation of uranium in water. Batch experiments were performed with several methanogenic anaerobic granular sludge samples and different electron donors. Abiotic controls consisting of heat-killed inoculum and non-inoculated treatments confirmed the biological removal process. In this study, unadapted anaerobic granular sludge immediately reduced U(VI), suggesting an intrinsic capacity of the sludge to support this process. The high biodiversity of anaerobic granular sludge most likely accounts for the presence of specific microorganisms capable of reducing U(VI). Oxidation by O 2 was shown to resolubilize the uranium. This observation combined with X-ray diffraction evidence of uraninite confirmed that the removal during anaerobic treatment was due to reductive precipitation. The anaerobic removal activity could be sustained after several respikes of U(VI). The U(VI) removal was feasible without addition of electron donors, indicating that the decay of endogenous biomass substrates was contributing electron equivalents to the process. Addition of electron donors, such as H 2 stimulated the removal of U(VI) to varying degrees. The stimulation was greater in sludge samples with lower endogenous substrate levels. The present work reveals the potential application of anaerobic granular sludge for continuous bioremediation schemes to treat uranium-contaminated water.

Surface Complexation Modeling of U(VI) Adsorption by Aquifer Sediments from a Former Mill Tailings Site at Rifle, Colorado

A study of U(VI) adsorption by aquifer sediment samples from a former uranium mill tailings site at Rifle, Colorado, was conducted under oxic conditions as a function of pH, U(VI), Ca, and dissolved carbonate concentration. Batch adsorption experiments were performed using <2 mm size sediment fractions, a sand-sized fraction, and artificial groundwater solutions prepared to simulate the field groundwater composition. To encompass the geochemical conditions of the alluvial aquifer at the site, the experimental conditions ranged from 6.8 x 10(-8) to 10(-5) M in [U(VI)](tot), 7.2 to 8.0 in pH, 3.0 x 10(-3) to 6.0 x 10(-3) M in [Ca(2+)], and 0.05 to 2.6% in partial pressure of carbon dioxide. Surface area normalized U(VI) adsorption K(d) values for the sand and <2 mm sediment fraction were similar, suggesting a similar reactive surface coating on both fractions. A two-site two-reaction, nonelectrostatic generalized composite surface complexation model was developed and successfully simulated the U(VI) adsorption data. The model successfully predicted U(VI) adsorption observed from a multilevel sampling well installed at the site. A comparison of the model with the one developed previously for a uranium mill tailings site at Naturita, Colorado, indicated that possible calcite nonequilibrium of dissolved calcium concentration should be evaluated. The modeling results also illustrate the importance of the range of data used in deriving the best fit model parameters.

Fractionation of natural radionuclides in soils from the vicinity of a former uranium mine Žirovski vrh, Slovenia

As a result of former uranium mining and milling activities at Žirovski vrh, Slovenia, 0.6 million tons of uranium mill tailings (UMT) were deposited onto a nearby waste pile Boršt. Resulting enhanced levels of natural radionuclides in UMT could pose threat for the surrounding environment. Therefore, sequential extraction protocol was performed to assess mobility and bioavailability of 238U, 234U, 230Th and 226Ra in soils from the waste pile and its surrounding. The radionuclides associated with exchangeable, organic, carbonate, Fe/Mn oxides and residual fraction, respectively, were determined. Results showed that the highest activity concentrations for the studied radionuclides were on the bottom of the waste pile. In non-contaminated locations, about 80% of all radionuclides were in the residual fraction. Considering activity concentrations in the UMT, 238U and 234U are the most mobile. Mobility of 226Ra is suppressed by high sulphate concentrations and is similar to mobility of 230Th.

Comparison of nitrate attenuation characterization methods at the Uranium mill tailing site in Monument Valley, Arizona

Several methods for characterizing the occurrence and rate of nitrate attenuation were tested at a field site near Monument Valley, Arizona. Spatial and temporal nitrate concentration data collected from a transect of monitoring wells located along the plume centerline were analyzed to evaluate the overall rates of natural attenuation. The occurrence and rate of denitrification was evaluated through microcosm experiments, nitrogen isotopic fractionation analysis, and solute-transport modeling. First-order denitrification-rate coefficients calculated with each method were comparable. In addition, the composite natural attenuation rate coefficient was similar to the denitrification-rate coefficients, which suggests that microbially induced decay primarily controls nitrate attenuation at the site. This research highlights the benefits associated with a multiple-method approach for the characterization of natural attenuation.

Estimation of ionizing radiation impact on natural Vicia cracca populations inhabiting areas contaminated with uranium mill tailings and radium production wastes

Industrial areas in proximity to the Vodny settlement in the Komi Republic, Russia, have been contaminated by uranium mill tailings and radium production wastes. These areas, exhibiting high activity concentrations of naturally occurring radionuclides in soils, constitute a field laboratory where the effects of combined chronic exposures to α-, β- and γ-emitting radionuclides on natural plant populations can be studied. The aim of the present work was to determine dose–effect relationships and the range of doses that cause biological effects in natural Vicia cracca L. populations inhabiting the study area. The studied plant species is native to the area and is found ubiquitously. Soil and vegetation samples were taken at a reference location and six contaminated sites characterized by distinct floodplain depositional units with different enhanced levels of naturally occurring radionuclides. A large fraction of the dose at the study sites (including the reference location) was attributable to internal irradiation and 226Ra was found to be an important contributor to this component of dose. The relationship between the frequency of chromosome aberrations in seedlings' root tip cells and the absorbed dose was found to be quadratic. An exponential model provided the best result in describing the empirical dependence between the absorbed dose and both the germination capacity of seeds and the survival rate of sprouts of V. cracca. For V. cracca plants inhabiting areas contaminated with uranium mill tailings and radium production wastes, a weighted absorbed dose of 0.2 Gy (weighting factor for alpha particles = 5) during the vegetation period could be considered to be a level below which no increase in genetic variability and decrease in reproductive capacity might be observed above background.

English

The Abandoned Mine Inventory System (AMLIS) includes nearly 1100 abandoned mines in Montana. The Montana offices of the Bureau of Land Management and the U.S. Forest Service as well as the Montana Department of Environmental Quality AML Program have been working to clean up these abandoned mine lands since 1995. The agencies in general have given priority to sites with mill tailings and waste rock dumps situated in stream channels, and in Montana the clean up of impacted lands on a watershed basis through interagency cooperation has been emphasized. There is an emerging desire of the federal agencies to begin monitoring these reclaimed sites in a systematic way. The overall objective for developing the Qualitative Reclamation Assessment Handbook for Abandoned Hardrock Mine Lands is to provide a common platform to evaluate reclaimed mine sites so that federal agencies responsible for risk management and land management can easily communicate and work in partnership to accomplish their respective missions. The handbook contains assessment forms and protocol designed to evaluate pertinent attributes found at specific locations within a reclaimed mine site (repository, wetland, etc.). These attributes may include vegetation cover, status of a cap or liner, roads, evidence of AMD, and several others. The outcome envisioned by the agencies includes identifying maintenance needs, generating temporal information for trend analysis, and identifying remedial methods and technologies that have proven to be effective and those that have resulted in poor performance. The data and information collected during a reclamation assessment can then be used to evaluate the status of the reclamation work and whether or not the conditions at the site remain protective of human health and the environment.

Analyses of radionuclides in soil, water, and agriculture products near the Urgeiriça uranium mine in Portugal

Analyses of soils, irrigation waters, agriculture products (lettuce), green pasture, and cheese were performed in samples collected in the area of the old Urgeirica uranium mine and milling facilities, Centre-North of Portugal, in order to assess the transfer of uranium series radionuclides in the environment and to man. Soils close to milling tailings display an enhancement of radioactivity. In the drainage basin of the stream Ribeira da Pantanha, receiving drainage from the tailings piles and discharges from the acid mine water treatment plant, there was enhancement of uranium series radionuclide concentrations in water and suspended matter. Agriculture products from kitchen gardens irrigated with water from the Ribeira da Pantanha show an increase of radioactivity, mainly due to uranium isotopes. Agriculture products from other kitchen gardens in this area, irrigated with groundwater, as well pasture and cheese produced locally from sheep milk did not show enhanced radionuclide concentrations. In the Urgeirica area, some soils display radionuclide concentrations higher than soils in reference areas and, in agriculture products grown there, 226Ra was the radionuclide more concentrated by vegetables. Through ingestion of these products 226Ra may be the main contributor to the increment of radiation dose received by local population.

Effects of phosphate and Cr3+ on the sorption and transport of uranium(VI) on a silica column

Six column experiments were performed and seven breakthrough curves (BTCs) and seven displacement (desorption) curves (DPCs) of phosphate and U(VI) were obtained, which demonstrated the effects of phosphate and Cr3+ on the sorption and transport of U(VI) on a silica column at pH 3 and uranium concentration 10−5–10−6 mol/L of the influent. It was found that in the presence of phosphate sorbed preliminarily on the silica column, the amount of U(VI) sorbed on this column is significantly increased owing to the interaction of U(VI) with phosphate sorbed as compared with that in the absence of phosphate, and the breakthrough and the mean residence time of U(VI) on this column are similar to those in the absence of phosphate. While the effect of simultaneous injection of Cr3+ on the retardation of U(VI) on the silica column, the maximum concentration of BTC and the amount of U(VI) sorbed was found to be insignificant. Transport and sorption studies of U(VI) are important, since all uranium isotopes are radioactive, there is a need to understand the potential for migration away from radioactive waste storage and mill tailing sites.

Utilization of industrial waste products as pozzolanic material in cemented paste backfill of high sulphide mill tailings

In this study, the potential use of the industrial waste products including waste glass (WG), fly ash (FA), granulated blast furnace slag (GBFS) and silica fume (SF) as pozzolanic additive for the partial replacement of ordinary Portland cement (OPC) in cemented paste backfill (CPB) of sulphide-rich mill tailings was investigated. The influence of these industrial waste products on the short- and long-term mechanical performance of CPB was demonstrated. The rate of development of strength of CPB samples tended to slow down when the pozzolanic wastes were incorporated or increased in dosage in the binder phase. Severe losses (by 26%) in the strength of CPB samples produced from exclusively OPC occurred after an initial curing period of 56 days. The addition of WG (10–30 wt%) as a partial replacement of OPC was observed to aggravate further the strength losses of CPB samples. GBFS, FA and SF appeared to improve the long-term performance of CPB samples; albeit, only GBFS and SF could be incorporated into the binder phase only at certain levels i.e. up to 20 wt% GBFS and 15 wt% SF in order to maintain a threshold strength level of 0.7 MPa over 360 days. SEM studies have provided further insight into the microstucture of CPB and confirmed the formation of deleterious gypsum as the expansive phase. These findings have demonstrated that the industrial waste products including GBFS and SF can be suitably used as mineral additives to improve the long-term mechanical performance of CPB produced from sulphide-rich tailings as well as to reduce the binder costs in a CPB plant.

Challenges in the design of a prototype contactor assembly for the recovery of uranium from seawater

The total estimated quantity of uranium in oceans is around four and a half billion tonnes. Harvesting uranium from seawater is much less taxing to the environment. Thus, when uranium is harvested from seawater with near-zero environmental burdens, it will become a totally green fuel in the hands of humankind, as uranium generates primary calories without giving CO2 emissions and leaves no mill tailings at the recovery site while recovering it for use in reactors. To extract uranium from seawater, a contactor assembly that contains radiation-grafted polyacrylamid oxime (PAO) is used to trap the loosely bonded uranyl ion. Some of the design constraints for contractor assembly are the maximisation of the grafted sheet area and strength to withstand extreme marine conditions. The material should be compatible with the hot processing chemicals and be extremely cost effective, as well as lightweight. Design also has to take into account the ease of loading/unloading of grafted sheet modules, ease of replacement of grafted sheets, fixing of cage modules in seabed and load equilibrium even in high and low tide. This paper will cover the various challenges involved in designing a contactor assembly.

Seasonal variation in suspended particulate matter vis-a-vis meteorological parameters at Kolar Gold Fields, India

Particulate matter concentrations and meteorological parameters were monitored to assess the impact of other industries likely to be set up at Kolar Gold Fields, India. Particulate matter less than 10 µm was the highest in winter due to the influence of gold mill tailings dumps while total suspended particulate matter was generally higher in spring probably due to the influence of uncovered area near the monitoring station. In summer, intermittent rain lowered the particulate concentrations. Correlations of particulate matter with meteorological parameters were insignificant but significant with multiple regressions. The study showed the air quality is likely to be worse without adequate pollution control measures.

Bacterially enhanced dissolution of meta-autunite

The release of U from the mineral meta-autunite {l_brace}Ca[(UO{sub 2})(PO{sub 2})](H{sub 2}O){sub 6}{r_brace} was evaluated using spectroscopy, aqueous geochemistry, and electron microscopy in a minimal media with the dissimilatory metal-reducing bacterium Shewanella putrefaciens 200R. The onset of anaerobic conditions resulted in the rapid release of U and phosphate to solution followed by the reprecipitation of meta-autinite. Spectroscopy measurements (XANES) indicated that the U was not released via reduction during the bacterial incubations, but instead dissolution was promoted by uptake and immobilization of P by the bacterial cells. Our results suggest that U(VI) in 'refractory' P mineral phases may be mobilized from U mill tailings and/or U disposal sites and that the nutrient status (P) of the geologic setting may be a predictor for the lability of U in these environments.

Geochemistry and Mineralogy of Mill Tailings Impoundments from the Panasqueira Mine (Portugal): Implications for the Surrounding Environment

Selected geochemical results are presented from a study of the Panasqueira mine (Sn–W mineralization), which is one of the Portuguese test sites for the e-EcoRisk project ( http://www.e-ecorisk.info/ ). These data permit a better understanding of the dynamics inherent to leaching, transport, and accumulation of some elements (mainly As, but also Cu, Pb, Zn, and Cd) in different sampling media and their environmental relevance. Stream sediment and water samples were found to be contaminated by the stored waste material in the tailing ponds; this effect could be observed for a considerable distance downstream.

Analytical risk-based model of gaseous and liquid-phase radon transport in landfills with radium sources

An analytical model of gaseous and liquid-phase radon transport through soils is derived for environmental modeling of landfills containing uranium mill tailings or Ra-226 sources. Processes include radon diffusion in both the gas and liquid phases, advection of soluble radon in percolating water, radioactive decay, equilibrium partitioning between gas and liquid phases, and emanation from different source terms. A probabilistic framework for the radon-transport model is introduced that provides uncertainty and sensitivity analyses for risk-based assessments. Uncertainty analyses are used to compare simulated performance metrics (e.g., radon surface flux) against regulatory standards. Sensitivity analyses are used to identify key parameters and processes that impact the variability of the simulated results. The models and analyses are illustrated with a probabilistic performance assessment of the Mixed Waste Landfill at Sandia National Laboratories in Albuquerque, NM.

Combined effects of spatially variable flow and mineralogy on the attenuation of acid mine drainage in groundwater

Quantifications of the spreading of acid mine drainage (AMD) in groundwater are needed for risk assessments of mining sites. However, due to subsurface heterogeneity, available field data may prove insufficient for deterministic process descriptions, even at well-characterized sites. Here, the probabilistic LaSAR-PHREEQC model is used to consider multicomponent reactions and transport in heterogeneous (flow and geochemistry) groundwater surrounding a mine waste site, with specific focus on the spreading of Zn. Model results, using field data from a mill tailings impoundment in northern Sweden (including major component geochemistry), indicate that precipitation of smithsonite (ZnCO3) may drastically delay the downstream arrival of Zn, but may also cause a peak concentration once the retained Zn is released. The amount of smithsonite formed is, however, minute and its spatial variation large, such that detection of smithsonite in soil samples may be difficult. Results further show that even a low degree of flow heterogeneity can effectively smooth otherwise distinctive temporal concentration changes attributed to the considered chemical reactions, and thereby mask the attenuation processes. By contrast, the existence of preferential flow paths can cause temporally separated concentration peaks in response to a single chemical reaction chain, even in a geochemically homogeneous domain, making the interpretation of the concentration curves non-trivial. The stochastic modelling results for Zn considering flow and/or mineralogical heterogeneity indicate a less efficient Zn attenuation than predicted by standard, deterministic reactive-transport models. In addition, in all considered probabilistic Zn and SO42- scenarios, the spatial variability in downstream pollutant concentration was high, implying that a relatively large number of point samples are needed to determine field-scale mean concentrations.

Microstructural and geochemical evolution of paste tailings in surface disposal conditions

Surface paste tailings disposal has recently been proposed in the mining industry for the management of mill tailings. This new disposal method can represent an advantageous alternative to traditional tailings impoundments, as it favours water recycling and the control of free water during deposition, reduces the need for costly water-retaining dykes, and facilitates site rehabilitation. In this paper, the microstructural and geochemical properties of surface paste tailings were assessed. Column leaching tests were conducted on sulphide-rich paste tailings deposited under different configurations (cemented and uncemented). The leached samples were characterized and the results were compared to those obtained on the reference (unleached) samples. The paste microstructure was characterized by mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) coupled with energy dispersive spectroscopy (EDS) microanalysis. A part of the study focussed on the impact of cracks, which were found to influence the paste reactivity observed for some disposal configurations. The paper illustrates how to use paste technology and a small amount of ordinary Portland cement (0.66 wt.% in configuration L2) to stabilize sulphide-rich acid generating tailings (AP = 953 kg CaCO 3/t).

Measurement of Radon Diffusion Coefficients for Japanese Surface Soils

The radon diffusion coefficient for soil, D, is a very important parameter used to estimate radon dose for uranium-bearing waste. Many Ds were measured in the uranium mill tailing remediation action project in the US, and a formula for the estimation of diffusion coefficient, Rogers's formula, was proposed. However, it is uncertain whether Rogers's formula is applicable to Japanese soils because most of them have come from volcanic ash and contain much water. This paper describes the development of a measurement apparatus for D using a lump response transient method and a step response transient method, and presents measured D values for Japanese surface soils. Measured alpha ray count curves are good in agreement with those of theory. This shows that radon transportation in soil can be described using Fick's law. Furthermore, the measured effective D values are good in agreement with those of Rogers's formula. This means that Rogers's formula can be applied to Japanese soils.

Radon Diffusion Coefficients for Soils: Previous Studies and their Application to Uranium-Bearing Wastes

Radon diffusion in soils has been studied over the years by many researchers. The application of such studies to the evaluation of radiation exposure caused by radon from uranium-bearing wastes disposed in a shallow land site is very important. The present paper surveyed closely relevant studies and elucidated the inherent nature of radon diffusion in terms of the definition of radon diffusion coefficients. Then, basic features of measurement methods for determining radon diffusion coefficients in soils were explained. Furthermore, theoretical aspects of radon diffusion in soils were discussed in terms of microscopic radon diffusion in soils and large-scale radon diffusion through cover soil defects for uranium mill tailings. Finally, in order to apply the radon diffusion studies to uranium-bearing waste disposal in shallow land sites, new challenges were presented: elucidation of radon diffusion in uranium-bearing wastes and cover-soil cracks, and demonstration of the validity of applying only radon diffusion in the evaluation of radiation exposure caused by radon, which would come through Japanese cover soils for uranium-bearing waste disposal.

Use of gold mill tailings in making bricks: a feasibility study

Mill tailings dumps at Kolar Gold Fields, Karnataka, are creating environmental problems. One of the solutions to these problems is to use the mill tailings for some useful purpose. This study examined the possibility of making bricks from the mill tailings with some additives in laboratory experiments. Samples of the mill tailings and the additives were analysed for particle size distribution, Atterberg limits and specific gravity. The plasticity index of the mill tailings being zero, they could not be used directly for making bricks. Therefore some additives that had plasticity or binding properties were mixed with the mill tailings. Ordinary Portland cement, black cotton soils and red soils were selected as additives. Each of the additives was mixed separately with the mill tailings in different proportions by weight and a large number of bricks were prepared using metallic moulds. The bricks were termed as cement-tailings bricks or soil-tailings bricks, depending on the additives used. The cement-tailings bricks were cured for different periods and their corresponding compressive strengths were determined. The bricks with 20% of cement and 14 days of curing were found to be suitable. The soil-tailings bricks were sun-dried and then fired in a furnace at different temperatures. The quality of bricks was assessed in terms of linear shrinkage, water absorption and compressive strength. The cost analysis revealed that cement-tailings bricks would be uneconomical whereas the soil-tailings bricks would be very economical.