Mitigating Uranium in Groundwater:� Prospects and Limitations (original) (raw)

Mitigation of uranium in effluents by zero valent iron: The role of iron corrosion products

The coprecipitation of U (VI) with iron corrosion products from aqueous solutions by zero valent iron was investigated. The evidence of coprecipitation was demonstrated by conducting experiments with well characterized scrap iron, pyrite and a mixture of both materials with experimental durations of up to four months. Results indicate that under anoxic conditions only less than one tenth of the immobilized U(VI) was associated with the surface of scrap iron, whereas the remaining amount is entrapped in aging corrosion products.

Treatment of Uranium in Subsurface Water - 11616

2011

Treatability studies were conducted to evaluate in-situ treatment of pond and subsurface water contaminated with uranium (U). This presentation is applicable for U in-situ treatment at multiple Department of Energy and U mine/mill sites. At one location, the water contained dissolved U and high total dissolved solids, which is typical of hard rock U mine sites. Constituents of concern included U and sulfate. The groundwater contamination at the other site was a result of an in-situ leaching process to extract U from natural porous sandstone deposits. At the conclusion of this mining process, U and selenium were present at elevated concentrations in the oxidized groundwater. Chemical treatments to remove soluble hexavalent uranium (U[VI]) included pH adjustment, flocculation, phosphate addition, and adsorption on activated red mud (a waste product from bauxite ore processing). Geochemical modeling predicted that U[VI] should be removed by phosphate precipitation and by adsorption on the iron and aluminum oxides present in the activated red mud. Geochemical modeling also indicated that the addition of lime (CaO) to precipitate dissolved sulfate as calcium sulfate (CaSO 4) would be hindered by the formation of soluble magnesium sulfate complexes that kept the sulfate in solution. Laboratory tests confirmed the model results. Under reducing conditions, bacteria can utilize an organic or inorganic substrate to reduce soluble U[VI] to U[IV], which subsequently precipitates as one of several insoluble minerals, such as uraninite or coffinite. These organisms gain energy from this process. In some instances, the reduction of U[VI] can also occur indirectly as a result of biological processes. Under reducing conditions, bacteria will reduce ferric iron (Fe[III]) to ferrous iron (Fe[II]) and sulfate to sulfide (S 2-), and these reduced species can subsequently reduce U[VI] through a direct chemical process. In either case (i.e., the direct or indirect process), the biological reduction of U[VI] results in the formation of stable and insoluble U[IV] minerals. The laboratory tests have successfully demonstrated that application of either adsorption on activated red mud, precipitation following phosphate addition, or biological treatment can lower U concentrations to acceptable levels.

Understanding the mechanism of the uranium Mitigation by zero-valent iron in effluents

The influence of co-precipitation on the uptake of U(VI) from aqueous solutions by zero valent iron in the neutral pH range was investigated. The evidence of co-precipitation was shown by conducting remobilization experiment with 0.1 M Na2CO3 and three different concentrations of HCl. The extent of this reaction was investigated by conducting separated experiments with 15 g/L of a well characterized scrap iron and 0 to 25 g/L of three different additive materials: (1) water works sludge (Fe2O3), (2) manganese nodules (MnO2) and (3) a pyrite mineral (FeS2). The response of the system on addition of these modifiers is investigated. The results show that less than one third of the immobilized U(VI) was associated with the surface of scrap iron, the remaining amount being entrapped in aging corrosion products.

Mechanism of uranium removal from the aqueous solution by elemental iron

Journal of Hazardous Materials, 2006

The effectiveness of elemental iron (Fe 0 ) to remove uranium (U) from the aqueous phase has been demonstrated. While the mitigation effect is sure, discrepancies in the removal mechanism have been reported. The objective of this study was to investigate the mechanism of U(VI) removal from aqueous phases by Fe 0 . For this purpose a systematic sequence of bulk experiments was conducted to characterize the effects of the availability and the abundance of corrosion products on U(VI) removal. Results indicated that U(VI) removal reactions did not primary occur at the surface of the metallic iron. It is determined that U(VI) co-precipitation with aging corrosion products is a plausible explanation for the irreversible fixation under experimental conditions. Results of XRD analyses did no show any U phases, whereas SEM-EDX analyses showed that U tended to associate with rusted areas on the surface of Fe 0 .

Reductive precipitation of uranium (VI) by zero-valent iron

Environmental science & …, 1998

This study was undertaken to determine the effectiveness of zero-valent iron (Fe 0 ) and several adsorbent materials in removing uranium (U) from contaminated groundwater and to investigate the rates and mechanisms that are involved in the reactions. Fe 0 filings were used as reductants, and the adsorbents included peat materials, iron oxides, and a carbon-based sorbent (Cercona Bone-Char). Results indicate that Fe 0 filings are much more effective than the adsorbents in removing uranyl (UO 2 2+ ) from the aqueous solution. Nearly 100% of U was removed through reactions with Fe 0 at an initial concentration up to 76 mM (or 18 000 mg of U/L). Results from the batch adsorption and desorption and from spectroscopic studies indicate that reductive precipitation of U on Fe 0 is the major reaction pathway. Only a small percentage (<4%) of UO 2 2+ appeared to be adsorbed on the corrosion products of Fe 0 and could be desorbed by leaching with a carbonate solution.

Investigating the Mechanism of Uranium Removal by Zerovalent Iron

Environmental Chemistry, 2005

Environmental Context.Groundwater is the water that fills the spaces between sand, soil, and rock below the water table. It discharges into ecologically sensitive wetlands and is used as drinking water or in agriculture and industry. Inappropriate waste disposal and poor land management can contaminate groundwater and may minimize its use for decades. The common method for pumping contaminated groundwater to the surface for treatment is costly and labour intensive. Zerovalent iron is a new, more cost-effective method of groundwater remediation. . Zerovalent iron (ZVI) has been proposed as a reactive material in permeable in situ walls for groundwater contaminated by metal pollutants. For such pollutants that interact with corrosion products, the determination of the actual mechanism of their removal is very important to predict their stability in the long term. From a study of the effects of pyrite (FeS2) and manganese nodules (MnO2) on the uranium removal potential of a selected ZV...

Removal of uranium(VI) from the aqueous phase by iron(II) minerals in presence of bicarbonate

Applied Geochemistry, 2009

23 Uranium(VI) mobility in groundwater is strongly affected by sorption of mobile U(VI) 24 species (e.g. uranyl, UO 2 2+ ) to mineral surfaces, precipitation of U(VI) compounds, such 25 as schoepite (UO 2 ) 4 O(OH) 6 * 6H 2 O), and by reduction to U(IV), forming sparingly 26 soluble phases (uraninite; UO 2 ). Especially the latter pathway would be very efficient for 27 long-term immobilization of uranium. In nature, ferrous iron is an important reducing 28 agent for U(VI) because it frequently occurs either dissolved in natural waters, sorbed to 29 matrix minerals, or structurally bound in many minerals. Redox reactions between U(VI) 30 and Fe(II) depend not only on the availability of Fe(II) in the environment, but also on 31 the chemical conditions in the aqueous solution. Under natural groundwater condition 32 U(VI) forms complexes with many anionic ligands, which strongly affect its speciation. 33 Especially carbonate is known to form stable complexes with uranium, rising the 34 question if U(VI), when complexed by carbonate, can be reduced to UO 2 . The goal of this 35 study was to find out if Fe(II) when structurally bound in a mineral (as magnetite, Fe 3 O 4 ) 36 or sorbed to a mineral surface (as corundum, Al 2 O 3 ) can reduce U(VI) to U(IV) in 37 presence of bicarbonate. Batch experiments were conducted under anaerobic conditions 38 to observe uranium removal from the aqueous phase by the two minerals in dependence 39 of bicarbonate addition (1 mM), uranium concentration (0.01-30 μM) and pH value (6-40 10). Immediately after the experiments, the mineral surfaces were analyzed by X-ray 41 photoelectron spectroscopy (XPS) to obtain information on the redox state of uranium 42 bound to the solid surfaces. XPS results gave evidence that U(VI) can be reduced both by 43 magnetite and by corundum amended with Fe(II). In presence of bicarbonate the amount 44 of reduced uranium on the mineral surfaces increased compared to carbonate-free 45 3 solutions. This can be explained by the formation of Fe(II) carbonates on the mineral 46