Green Process for Uranium Separations Utilizing Molybdenum

Chapter 13

Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date (Web): October 15, 2010 | doi: 10.1021/bk-2010-1046.ch013

Green Process for Uranium Separations Utilizing Molybdenum Trioxide Mohamed Chehbouni,1 Hamed Al-Busaidi,2 and Allen W. Apblett*,2 1Department

of Chemistry, Computer, and Physical Sciences, Southeastern Oklahoma State University, Durant, OK 74701 2Department of Chemistry, Oklahoma State University, Stillwater, OK 74078 *[email protected]

Molybdenum trioxide can absorb uranium from aqueous solution with a capacity of up to 165 % by weight. The mechanism of this process is a chemical reaction that produces an insoluble uranium molybdenum oxide mineral oxide called umohoite, UMoO6·2H2O. The reaction between MoO3 and aqueous uranyl acetate was found to be first order in each reactant with a rate constant of 0.084 L/mol·min. A ‘green” process was developed whereby MoO3 adsorbed uranium from aqueous solution and then the uranium and molybdenum trioxide were separated by treatment with aqueous ammonia. The resulting ammonium uranate solid was isolated by filtration and the aqueous ammonium molybdate was converted back to MoO3 by heating. The recovery of uranium was 98.9%.

Introduction All aspects of nuclear energy production can have a potential toll on the environment. While the spent fuel produced by commercial, defense, and isotope-production reactors is an obvious hazard, fuel fabrication and reprocessing, and uranium mining, milling, and extraction activities also produce radioactive wastes that require safe disposal. In the past, insufficient attention to this problem has led to the contamination of soil and ground water with large quantities of radioactive substances, especially at U.S. Department of Energy facilities (1). For example, the concentration of uranium in contaminated soil and mine tailings was found to be 0.012% and 0.002–0.11% percent, respectively © 2010 American Chemical Society In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


(2). The Fry Canyon site in Utah provides a good example of the dangers of uranium mine tailings. The uranium concentrations measured in groundwater at this site were as high as 16,300 ppb with a median concentration of 840 ppb before remedial actions were taken (3). Uranium contamination can result from human activities other than the generation of nuclear energy (4, 5). In particular, phosphate fertilizers are an important contributor of uranium to groundwater since they often contain uranium at an average concentration of 150 ppm (6). The depleted uranium ammunition used as armor penetrators in several military conflicts has also been demonstrated as a source of drinking water contamination (7) The contamination of ground water with uranium can also arise from natural sources. Uranium will solubilize over a wide range of pH when bedrock consisting mainly of uranium-rich granitoids and granites are exposed to soft, slightly alkaline bicarbonate waters under oxidizing conditions. These conditions occur widely throughout the world. For example, in Finland exceptionally high uranium concentrations up to 12,000 ppb are found in wells drilled in bedrock (8). Concentrations of uranium up to 700 ppb have been found in private wells in Canada (9) while a survey in the United States of drinking water from 978 sites found a mean concentration of 2.55 ppb (10). However, some sites in the United States, such as the Simpsonville-Greenville area of South Carolina, have serious contamination with uranium. At that location, high amounts of uranium (30 to 9900 ppb) were found in 31 drinking water wells (11) as a consequence of the presence of veins of pegmatite in the area. While uranium in ground water is problematic for drinking water supplies, it is also a resource that could be tapped to augment the supply of uranium for energy generation. What is required is a simple efficient method to remove the uranium in the form of a useful resource while at the same time perhaps generating purified drinking water. Animal testing and studies of occupationally exposed people have shown that the major health effect of uranium is not due to radiation but is a result of chemical kidney toxicity (12). Functional and histological damage to the proximal tubulus of the kidney have been observed (13). The effects of long-term environmental uranium exposure in humans are uncertain but there is an association of uranium exposure with increased urinary glucose, alkaline phosphatase, and β-microglobulin excretion (14) as well as increased urinary albumin levels (15) As a result of such studies, the World Health Organization has proposed a guideline value of 2 ppb for uranium in drinking water while the US EPA has specified a limit of 30 ppb. Current practices for treatment of municipal water supplies practices are not effective for removing uranium. However, it has been reported that uranium removal can be accomplished by a variety of processes such as modification of pH or chemical treatment (often with alum) or a combination of the two (16). Several sorbants have been shown to be useful for removal of uranium from water. Activated carbon, iron powder, magnetite, anion exchange resin and cation exchange resin were shown to be capable of adsorbing more than 90% of the uranium and radium from drinking water. However, two common household treatment devices were found not to be totally effective for uranium removal (11). 156 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Molybdenum hydrogen bronze (also called molybdenum blue), HMo2O6, was previously investigated for application in removal of uranium from aqueous solution and possible use in a cyclic process for recovery of actinides and heavy metals (17). It was thought that the protons would provide ion exchange sites on the material in its reduced form while oxidation would remove these sites and release the toxic metals in concentrated form. However, it was discovered that oxidation of the blue reagent occurred during the adsorption process so that the reagent turned from blue to yellow. It was also found that the uptake of uranium (122% by weight) exceeded the expected capacity based on the number of protons present i.e. the proton concentration is 3.46 mEq/g while the uranium adsorption was 5.14 mEq/g. Analysis of the product from uranium uptake revealed that the bronze had reacted with uranium to produce the mineral iriginite, UMo2O9•3H2O. The reaction of HMo2O6 was found to be very selective for uranium, other actinides, and heavy metals. The oxidation of the Mo(V) centers in the bronze was found to be due to reaction with molecular oxygen as the layered structure was disassembled by reaction with uranyl ions. This result suggested that prior reduction of MoO3 to HMo2O6 was unnecessary for uranium adsorption, a hypothesis that was proven in subsequent work (18).

Experimental All reagents were commercial products of ACS Reagent grade purity or higher and were used without further purification. Water was purified by reverse osmosis followed by deionization. Bulk pyrolyses at various temperatures were performed in an air atmosphere in a digitally-controlled muffle furnace using approximately 1 g samples, a ramp of 10°C/min and a hold time of 4 hr. X-ray powder diffraction (XRD) patterns were recorded on a Bruker AXS D-8 Advance X-ray powder diffractometer using copper Kα radiation. Crystalline phases were identified using a search/match program and the PDF-2 database of the International Centre for Diffraction Data (19). Scanning Electron Microscopy (SEM) photographs were recorded using a JEOL Scanning Electron Microscope. Uranium concentrations of aqueous solutions were determined by XRF spectroscopy on a Thermo Electron QuanX XRF spectrometer. A calibration curve for the instrument was generated using a NIST-traceable standard at 5 different concentrations that bracketed the region of interest.

Reaction of MoO3 with Uranyl Acetate MoO3 (1.00 g, 6.95 mmol) was added to 100 ml of a 0.100 M uranyl acetate solution (10.0 mmol) and the mixture was heated at reflux for 7 days. Upon cooling, a yellow solid was isolated by filtration through a fine sintered glass filter. After drying in vacuum at room temperature, the yield of yellow solid was found to be 3.23 g. Thermal gravimetric analysis indicated a water content of 9.24%. XRD analysis after firing at 600°C indicated the formation of UMoO6. Overall, the yield of this product was 2.97 g or 99.7% based on MoO3. Infrared spectrum 157 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

(DRIFTS, solid diluted in KBr, cm-1): 3582 w, 3513 vs, br, 3195 w, 2928 w, 2150 w, 1630 s, 1611 s, 1402 s, 918 vs, 889 vs, 859 vs, 821 vs, 724 m, 642 m, 541 m.


Kinetic Studies of Uranium Uptake Using MoO3 Excess of MoO3 (5 mmol, 0.72 g) was reacted at room temperature with 100 ml of an aqueous solution of 0.250 mmol of uranium acetate that was buffered with sodium acetate / acetic acid (pH of 4.7). The mixture was stirred magnetically and aliquots of 5 ml of the reaction mixture were withdrawn at regular intervals and the concentration of uranium and the pH were determined. This experiment was repeated in the presence of 10, 15, and 20 mmol of MoO3. The rate constant of the reaction for each experiment was determined and the overall order of the reaction was established. Recovery of Uranium and Molybdenum Trioxide Uranium was recovered from the uranium molybdate phase by treatment with a strong base. In this procedure, 1.00 g of the uranium/molybdenum trioxide product was stirred overnight with 100 ml of a 15% solution of ammonium hydroxide. The reaction mixture was separated by filtration through a 20 µm nylon membrane filter. The solid product was washed copiously with distilled water and then dried in a vacuum desiccator to yield 0.701 g of a yellow solid that was subsequently identified as ammonium uranate, (NH4)2U3(OH)2O9·2H2O, by XRD analysis. The filtrate was evaporated and the solid obtained was determined to be ammonium molybdate by XRD analysis.

Results and Discussion A prolonged reaction of molybdenum trioxide with an aqueous solution of uranyl acetate was performed in order to determine the maximum uptake of uranium and identify the product of the reaction. After one week, the MoO3 had absorbed 165% by weight of uranium, and amount that equates to 6.94 millimoles of uranium per gram of MoO3 and surpasses the 122% by weight observed for HMo2O6 (17). The uranium-containing products in both cases have the characteristic yellow color of hexavalent uranium and the infrared spectra of products are consistent with a structures consisting of hydrated uranyl ions and molybdenum oxide octahedra. These results imply that the differences in uranium uptake are due to varying ratios of uranium to molybdenum in the product rather than to differences in uranium oxidation states. X-ray powder diffraction analysis (Figure 1) of the solid product from uranium uptake by MoO3 revealed that it consisted mainly of the mineral umohoite [(UO2)MoO4(H2O)](H2O) in contrast to the iriginite, [(UO2)Mo2O7(H2O)2](H2O), that was produced by the reaction of HMo2O6 with uranyl acetate. In the latter case it appears that the presence of a proton and, initially, a Mo(V) center stops the adsorption of uranium at one equivalent per two moles of molybdenum oxide. The XRD pattern of the product from the reaction of uranyl acetate with 158 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


MoO3 contains several unidentified peaks, the strongest of which is centered at approximately 2θ = 15°. It is believed that these correspond to a more hydrated form of UMoO6 than umohoite. Supporting evidence for this hypothesis was provided by thermal gravimetric analysis, which clearly demonstrated a water content of 9.24% or approximately 2.43 molar equivalents of water per UMoO6 formula unit. Also, when the product from the reaction between uranyl acetate with MoO3 was not dried before XRD analysis, the peaks for the proposed hydrated phase were significantly more intense than they were in the dried sample. Finally, heating of the uranyl acetate/MoO3 product to 600°C produced phase-pure UMoO6 (Figure 1), eliminating the possibility of the presence of a crystalline phase with a different ratio of uranium to molybdenum other than one to one. The reaction of MoO3 with uranyl ions to produce umohoite appears to preserve part of the structure of MoO3. The latter compound is composed of MoO6 octahedra that are interconnected through corner linking to form infinite chains. These chains are further linked by edge-sharing to form double-layer sheets. These stack together via Van der Waals forces to give the final layered structure (Figure 2) (20). Reaction of uranyl ion with MoO3 leads to separation of the edge-shared chains of MoO6 octahedra. These chains are intercalated with chains of UO7 decahedra to yield umohoite with a new layered structure shown in Figure 3 (21). The structural changes suggests that the mechanism of the reaction might involve pulling out of chains of molybdenum oxide octahedral as the uranyl ions react with MoO3. Such a reaction would be expected to give a fibrous product as it does in the reaction of molybdenum hydrogen bronze with uranyl nitrate. However, the SEM image of the MoO3/uranyl acetate product provided in Figure 4 shows square plates and rectangles. Notably, the new particles are much smaller than those of the MoO3 reagent used. This result suggests a second mechanistic possibility in which the molybdenum trioxide particles are completely dissolved in a dissolution/precipitation process that generates new particles with different morphologies. Unfortunately, the experiments that were performed cannot confirm this mechanism. Future experiments with large single crystals of molybdenum trioxide may shed further light on the mechanism. Previously, complete morphological rearrangement was also seen for the reaction of HMo2O6 with uranyl acetate (17). An important parameter for application of MoO3 in removal of uranium from water is the rate of reaction under ambient conditions. Therefore, the kinetics of the uranium uptake reaction were determined under condition where the MoO3 was present in excess. Under these conditions, plotting the natural logarithm of uranium ion concentration versus time yields a straight line (Figure 5), demonstrating that the reaction is first order in the uranyl ion. The slope of the plots yielded the rate constants listed in Table 1.

159 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 1. XRD patterns of product from reaction of uranyl acetate with MoO3 as isolated and after heating to 600°C.



Figure 2. Layered structure of MoO3 viewed down the a axis (left) and the c axis (right).



Figure 3. Structure of umohoite viewed down the axis (left) and the c axis (right). Lighter shaded octahedra are MoO6 while darker shaded decahedra are UO7 (21). Spheres are water.

Table 1. Rate Constants for Varying MoO3 Concentrations Concentration (M)

Kobs (min-1)

0.05

0.0042

0.10

0.0080

0.15

0.0120

0.20

0.0165



Figure 4. SEM pictures of molybdenum trioxide (top) and the product from its reaction with uranium acetate (bottom).



Figure 5. Pseudo-first order kinetics for the reaction of 0.20 mmol of MoO3 with 100 ml of 0.025 M UO22+.

The order of the reaction in molybdenum trioxide was determined by plotting the logarithm of kobs versus the logarithm of the concentration of MoO3 (Figure 6). The slope of the resulting graph was 1.0, indicating that the reaction is first order in MoO3. Therefore, the reaction is first order in both MoO3 and in UO22+ overall follows a second-order rate law (Equation 1). The rate constant was derived from a plot of kobs versus the concentration of MoO3, Figure 7, that gave a slope of 0.084 L/mol·min. Note that the “concentration” of MoO3 is being expressed in terms of molarity despite the fact that it is an undissolved solid. Since the reaction transforms one solid into another, it is difficult to give a rate constant in terms of surface area.



Figure 6. Order of reaction in MoO3.

Figure 7. kobs versus [MoO3].



Figure 8. Green cycle for uranium removal from water. The uranyl molybdate product (umohoite) obtained on the reaction of molybdenum bronze with uranyl nitrate was treated with a 15% solution of ammonium hydroxide. The reaction was stirred for 12 hours and the reaction mixture was separated by filtration. The X-ray powder diffraction pattern of the residue corresponded to ammonium uranate {(NH4)2U3(OH)2O9.2H2O}, which has applications in the nuclear power industry. The ammonium uranate can be further converted to UO3 upon heating to 600°C. The recovery of uranium was 98.9 %. Evaporation of the filtrate produced ammonium molybdate, {(NH4)2( Mo2O7)}, that was identified by XRD. Molybdenum trioxide (MoO3) could be recovered on heating the ammonium molybdate product to 242°C as determined by thermal gravimetric analysis. Hence a complete cycle for uranium concentration can be developed in which the only reagents consumed are ammonium hydroxide. Potentially the ammonia could be recovered and reused to yield a process with no waste products (Figure 8).

Conclusions In conclusion, it has been demonstrated that molybdenum trioxide has an extremely high capacity for absorption of uranium by a reaction that generates the insoluble mineral umohoite. MoO3 has considerable promise for application in environmental remediation, water treatment, and uranium recovery from ores.


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Green Process for Uranium Separations Utilizing Molybdenum

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