G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Article Volume 8, Number 11 1 November 2007 Q11001, doi: /2007gc ISSN: Dissolution kinetics of synthetic and natural meta-autunite (n) minerals, X + 3 n [(UO 2 )(PO 4 )] 2 xh 2 O, under acidic conditions D. M. Wellman Applied Geology and Geochemisty, Pacific Northwest National Laboratory, P.O. Box 999, K3-62, Richland, Washington 99352, USA (dawn.wellman@pnl.gov) K. M. Gunderson College of Earth and Energy, School of Geology and, University of Oklahoma, Norman, Oklahoma 73019, USA Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma , USA J. P. Icenhower Applied Geology and Geochemisty, Pacific Northwest National Laboratory, P.O. Box 999, K3-62, Richland, Washington 99352, USA S. W. Forrester Department of Geology, University of Nevada, Las Vegas, Las Vegas, Nevada 89154, USA [1] Mass transport within the uranium geochemical cycle is impacted by the availability of phosphorous. In oxidizing environments, in which the uranyl (UO 2+ 2 ) ionic species is typically mobile, formation of sparingly soluble uranyl phosphate minerals exerts a strong influence on uranium (n) transport. Autunite group minerals, X + 3 n [(UO 2 )(PO 4 )] 2 xh 2 O, have been identified as the long-term uranium-controlling phases in many systems of geochemical interest. Anthropogenic operations related to uranium mining operations have created acidic environments exposing uranyl phosphate minerals to low-ph groundwaters. Investigations regarding the dissolution behavior of autunite group minerals under acidic conditions have not been reported; consequently, knowledge of the longevity of uraniumcontrolling solids is incomplete. The purpose of this investigation was threefold: (1) to quantify the dissolution kinetics of natural calcium meta-autunite, Ca[(UO 2 ) 2 (PO 4 ) 2 ] 3H 2 O, and synthetic sodium meta-autunite, Na 2 [(UO 2 ) 2 (PO 4 ) 2 ] 3H 2 O, under acidic conditions; (2) to measure the effect of temperature and ph on meta-autunite mineral dissolution; and (3) to investigate the formation of secondary uranyl phosphate phases as long-term controls on uranium migration. Single-pass flowthrough (SPFT) dissolution tests were conducted over the ph range of 2 to 5 and from 5 to 70 C. Results presented here illustrate meta-autunite dissolution kinetics are strongly dependent on ph but are relatively insensitive to temperature variations. In addition, the formation of secondary uranylphosphate phases such as uranyl phosphate, (UO 2 ) 3 (PO 4 ) 2 x H 2 O, may serve as a secondary phase limiting the migration of uranium in the environment. Components: 7183 words, 8 figures, 4 tables. Keywords: uranium; phosphate; autunite; dissolution; kinetics. Copyright 2007 by the American Geophysical Union 1 of 16

2 Index Terms: 1009 : Geochemical modeling (3610, 8410); 1012 : Reactions and phase equilibria (3612, 8412); 1030 : Geochemical cycles (0330); 1042 : Mineral and crystal chemistry (3620). Received 18 May 2007; Revised 7 August 2007; Accepted 27 August 2007; Published 1 November Wellman, D. M., K. M. Gunderson, J. P. Icenhower, and S. W. Forrester (2007), Dissolution kinetics of synthetic and natural (n) meta-autunite minerals, X + 3 n [(UO 2 )(PO 4 )] 2 xh 2 O, under acidic conditions, Geochem. Geophys. Geosyst., 8, Q11001, doi: /2007gc Introduction [2] Phosphate is one of the most important components in the uranium geochemical cycle. In aqueous solutions, uranium forms strong complexes with oxygen-containing ligands, phosphate being one of the most stable, even though concentrations of phosphate in pore fluids are typically low. Because of the affinity of uranium for (n) phosphorus, autunite group minerals, X + 3 n [(UO 2 )(PO 4 )] 2 xh 2 O, have been identified as the long-term controlling phase limiting the mobility of the uranyl cation (UO 2+ 2 ) in subsurface environments. Studies of weathering of primary UO 2 deposits have highlighted the importance of autunite group minerals, which represent terminal phases in the paragenetic sequence that form during oxidative corrosion [Finch and Ewing, 1992]. In addition to natural environments, discrete autunite phases have been identified as the dominant control on uranium mobility in contaminated sediments resulting from previous operations related to either uranium mining or nuclear fuel and weapons programs [Bertsch et al., 1994; Buck et al., 1994, 1995, 1996; Elless and Lee, 1998; Morris et al., 1996; Tidwell et al., 1996]. [3] Previous experimental results have established the low solubility of many uranyl phosphate minerals [Chukhlantsev and Stepanov, 1956; Karpov, 1961; Moskvin et al., 1967; Schreyer and Baes, 1954; Vesely et al., 1965]. However, few kinetic dissolution studies of autunite and meta-autunite have been reported, and these have been mainly in solutions that are circum-neutral to mildly alkaline [Wellman et al., 2006]. Numerous sites involved in mining and milling, as well as nuclear energy and weapons operations, display very acidic conditions within which substantial quantities of subsurface autunite minerals have been identified in controlling the long-term fate of uranium [Bertsch et al., 1994; Buck et al., 1994, 1995, 1996; Morris et al., 1996; Tidwell et al., 1996]. The dissolution kinetics of autunite group minerals in acidic solutions has not been reported; consequently, understanding of the long-term dissolution behavior of this important uraniumcontrolling solid is incomplete. [4] A complicating factor is that, under acidic conditions, uranyl phosphate, (UO 2 ) 3 (PO 4 ) 2 4H 2 O, has also been reported to be a stable solid phase [Schreyer and Baes, 1954]. The solubility product, log K sp, of uranyl phosphate has been measured to range from 49 to 53 [Grenthe et al., 1992; Langmuir, 1997; Rai et al., 1999; Sandino and Bruno, 1992; Vesely et al., 1965], slightly less soluble than calcium (log K sp = 45) [Langmuir, 1997] or sodium (log K sp = 48) [Langmuir, 1997; Vesely et al., 1965] autunite, the two most common autunite phases. Thus release of uranium via the dissolution of autunite minerals may be followed by local saturation with respect to uranyl phosphate, resulting in secondary precipitation/dissolution reactions and re-immobilization of uranium. Therefore understanding the dissolution and re-precipitation kinetics of uranyl phosphate phases in oxidizing, acidic environments is necessary to understand the uranium geochemical cycle. [5] The purpose of this investigation was threefold: (1) to quantify the dissolution kinetics of metaautunite minerals under acidic conditions, (2) to measure the effect of temperature and ph on metaautunite mineral dissolution, and (3) to investigate the formation of secondary uranyl phosphate phases as long-term controls on uranium migration. Single-pass flow-through (SPFT) dissolution tests were conducted over the ph range of 2 to 5 and from 5 to 70 C. Results presented here illustrate meta-autunite dissolution kinetics are strongly dependent on ph but are relatively insensitive to temperature variations. Moreover, results from this investigation highlight the importance of secondary 2of16

3 Geosystems G 3 wellman et al.: meta-autunite minerals /2007GC Table 1. Source, Particle Size, and Surface Area of Autunite Minerals Autunite Composition Sample ID Source Particle Size Surface Area, m 2 /g Na 2 [(UO 2 )(PO 4 )] 2 3H 2 O Na-Autunite synthetic Ca[(UO 2 )(PO 4 )] 2 3H 2 O GHR natural Ca[(UO 2 )(PO 4 )] 2 3H 2 O GHR natural uranyl-phosphate phases limiting long-term migration of uranium. 2. Materials and Methods 2.1. Synthetic Sodium Meta-Autunite and Natural Calcium Meta-Autunite [6] Synthetic sodium meta-autunite I, Na 2 [(UO 2 ) (PO 4 )] 2 3H 2 O (herein designated Na-autunite) was prepared by direct precipitation from a mixture of uranyl nitrate with sodium phosphate, dibasic. The precipitated phase was characterized using extended X-ray absorption fine structure (EXAFS) spectroscopy, chemical digestion followed by inductively-coupled plasma-optical emission spectroscopy (ICP-OES) and inductively-coupled plasma-mass spectroscopy (ICP-MS) for elemental analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), and multipoint Brunauer- Emmett-Teller (BET) analyses [Wellman et al., 2005]. [7] Natural calcium meta-autunite I, Ca[(UO 2 ) (PO 4 )] 2 3H 2 O (herein designated GHR) was obtained from northeastern Washington State. The material was characterized using ICP-OES and ICP-MS analyses, XRD, and SEM to confirm the chemical composition, structure, and morphology of the autunite minerals as 98 99% pure autunite with calculated anhydrous structural formula consistent with Ca-autunite: Ca[(UO 2 )(PO 4 )] 2. Electron microprobe analysis suggests the autunite mineral contains 3 waters of hydration per formula unit (p.f.u.) [Morgan and London, 1996]. X-Ray diffraction patterns are consistent with the powder diffraction file (PDF) # of Ca-meta-autunite (maut I): Ca(UO 2 ) 2 (PO 4 ) 2 3H 2 O. [8] Powdered samples of Na-autunite and GHR were prepared to be within the same size fraction, from 25 to 45 mm ( 325 to +500 mesh); however, surface cracking, fractures, and basal plane cleavage of the GHR resulted in a greater surface area relative to Na-autunite (Table 1). Therefore a second size fraction of GHR autunite (75 to 150 mm, or 100 to mesh) was prepared that had a comparable surface area to Na-autunite (Table 1) and was utilized for all experiments. As has been previously noted, hydration of metaautunite to higher hydration states has not been observed [Beintema, 1938; Smith, 1984]; therefore it is believed the meta-autunite phases utilized in this investigation are stable and did not hydrate during testing Buffer Solutions [9] Dissolution rates (Tables A1 and A2) were determined using a 0.01 M nitric acid (HNO 3 ) solution to investigate element release from natural and synthetic autunite minerals over the ph interval of 2 to 5 (23 C). A 1 M ammonium hydroxide (NH 4 OH) solution was used to adjust the solutions to the target ph. The compositions of the buffer solutions used are summarized in Table 2. The in situ ph values of the buffer solutions were calculated for the experimental temperatures using the thermodynamic software package EQ3/6 [Wolery, 1992] (Table 2). As evident in Table 2, the ph is constant over the temperature interval of 5 to 70 C. Geochemical modeling results indicate the Table 2. Composition of Solutions Used in Autunite Dissolution Experiments a Solution Composition 5 C 23 C 40 C 70 C M HNO M HNO MNH 4 OH M HNO MNH 4 OH M HNO MNH 4 OH a Solution ph values <25 C were calculated with EQ3NR Code V7.2b database [Wolery, 1992]. 3of16

4 Figure 1. Schematic of the single-pass flow-through apparatus. minimal concentration of ammonium hydroxide utilized for ph adjustment within these experiments is insufficient to result in exchange of sodium or calcium within the respective autunite structures. As such, it is not believed the use of ammonium hydroxide for ph adjustment affected the composition of the starting minerals Single-Pass Flow-Through (SFPT) Apparatus [10] The dissolution kinetics of autunite minerals were determined using single-pass flow-through (SPFT) apparatus over the temperature range of 5 to 70 C. The SPFT apparatus, illustrated schematically in Figure 1, consists of a syringe pump (Kloehn; model 50300) that transfers solution from an influent reservoir to Teflon reaction vessels. The reactors are perfluoroalkoxy (PFA) (Savillex) vessels that consist of top and bottom piece which, when screwed together, forms a 40 ml capacity jar. The top half contains ports for ingress and egress of solution, in addition to a third port to accommodate a nitrogen gas line. Use of syringe pumps accommodate up to six experiments to be run using the same influent solution. Transport of influent solution from the reservoir to the reaction vessels and effluent solution to collection vials was accomplished via 1.59 mm Teflon tubing. Nitrogen was continuously supplied to the reactors and the influent solution reservoirs to ensure atmospheric CO 2 did not cause deviations in the ph of the solution. The reactors were housed in a constant temperature oven (VWR Scientific Products) that was set to the experimental temperature of interest. The temperature of the solution within the reactors was measured with a digital thermocouple (Glas- Col; model TC105) accurate to within ±2 C. [11] Each reactor contained a thin layer of the powdered sample on the bottom of the reactor (Figure 1 inset). This configuration prevents fluid from being pumped directly through the sample. Advantages to this configuration are that bubbles formed in the fluid transfer lines do not become entrained in the sample, which could alter the exposed surface area. This setup also minimizes the potential for grains becoming entrained in flow currents, as may be the case for fluid flowing directly through a powdered specimen. Entrainment of particles may result in spuriously faster rates due to collisions with other grains or the 4of16

5 reactor walls generating additional surface area or high energy sites with greater reactivity. [12] Three blank samples were drawn prior to addition of the autunite sample. Effluent solution continuously flowed out of the reactors and was collected in vials next to the oven. Experiments were run until steady state conditions (constant element concentrations over time) prevailed, which was the time necessary to exchange 10 reactor volumes. Aliquots of effluent solution were periodically checked to ensure ph stability. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was used to determine the effluent concentrations of calcium (Ca), sodium (Na) and phosphorus (P); inductively coupled plasma-mass spectrometry (ICP-MS) was used to determine total uranium (U) concentration Quantification of Dissolution Rates [13] Dissolution rates obtained from the SPFT tests are based on steady state concentrations of elements released from the solid phase to the effluent solution. The rates are normalized to the element mass fraction present in the autunite composition by the following formula [McGrail et al., 1997; McGrail et al., 2000]: where r i,j r i;j ¼ C i;j C i;b qj f i S j normalized release rate based on element i at the jth sampling, g m 2 d 1 ; C i,j concentration of the element of interest, i, in the effluent at the jth sampling, g L 1 ; C i,b average background concentration of the element i in the influent, g L 1 ; q j flow rate at the jth sampling, L d 1 ; f i mass fraction of the element in the sample, dimensionless; S j surface area of the powdered specimen at the jth sampling, m 2. In cases where the analyte is below background count, the background concentration of the element is set at the value of the lower limit of quantification (LOQ) and the calculated dissolution rate is therefore the maximum dissolution rate. The LOQ of any element is defined as the lowest concentration calibration standard that can be determined reproducibly during an analytical run within 10% error. Flow rates were determined gravimetrically ð1þ at each sampling interval. The value of f i is calculated from the chemical composition of the solid phase. [14] Determining the standard deviation of the dissolution rates requires accounting for the uncertainty associated with each parameter in equation (1). The standard deviation of a function for uncorrelated random errors is given by vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ux 2 s f ¼ t s 2 i i¼1 where s f is the standard deviation of the function f, x i is parameter i, and s i is the standard deviation of parameter i. ð2þ [15] Substituting equation (1) in (2) and converting to relative standard deviations, ^s r = s f /x, yields sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð^s c c out i Þ 2 þð^s b c in i Þ ^s r ¼ ðc out i þ c in i Þ 2 þ ^s 2 f i þ s 2 S þ s2 q Errors for ^s c, ^s b, ^s fi, ^s S, and ^s q are 10%, 10%, 5%, 15%, and 5%, respectively. This error analysis results in typical 2s uncertainties of approximately ±35% for SPFT-measured dissolution rates (or ±0.2 log units when reported as log 10 rates). Replicates of select SPFT experiments were conducted to ensure the system yielded reproducible results. [16] It is recognized that over the course of the dissolution experiments the reactive surface area may change as a result of the dissolution process itself. Assuming autunite particles are adequately represented by rectangular prisms, size distribution of grains within the sieve fractions are normally distributed, and cracks, surface pits, and other forms of surface roughness do not strongly affect the geometric surface area of the powder, the geometric surface area can be calculated as where ð3þ S ¼ ½2ac þ 2ab þ 2bcŠN ð4þ S total surface area, m 2 g 1 ; N number of particles; a average length of the plate, m; b average width of the plate, m; c average height of the plate, m. [17] Given an expression for N, the number of particles, equation (4) can be written in terms of 5of16

6 variables that are known and quantifiable, namely total mass and density. Realizing that m ¼ abcrn ð5þ where m is mass of the sample, g, and r is mineral density of the sample, g m 3. [18] Solving equation (5) for N gives N ¼ m ð6þ abcr [19] Substituting equation (6) into equation (4) gives m S ¼ ½2ac þ 2ab þ 2bcŠ ð7þ abcr [20] On the basis of the mass loss during the dissolution rate the change in surface area can be mathematically approximated using the geometric calculation for surface area. The change in mass as a function of dissolution during single-pass flowthrough test was calculated using equation (8) [McGrail et al., 1997]: where m o " # m i ¼ m o 1 X i 1 Dt i q j C j:k Dt j þ q i C i;k ; i 1 ð8þ f k 2 j¼1 initial mass; m i mass at time i; f k mass fraction of element k; C j,k concentration of element k at time j or i; q flow rate at time j or i; Dt change in time. [21] The change in mass as a function of dissolution was measured to be 5% under all conditions within this study. Direct measurement of the mass of particles post-run was impossible since (1) the mass of meta-autunite crystals are prone to rapid hydration and dehydration reactions and (2) the formation of secondary phases confounds measuring the mass of post-reacted meta-autunite. Therefore the assigned error of 15% for surface area adequately accounts for changes in surface area occurring during the dissolution process Scanning Electron Microscopy Energy Dispersive Spectrometry (SEM-EDS) [22] Photomicrographs were obtained by means of a SEM JEOL 840 equipped with a Robinson 6.0 backscatter detector. The beam conditions were 20 KeV acceleration and a 1 na beam current. The images from the SEM were acquired using GATAN DM software version 3.2, An Oxford ISIS 300 series energy dispersive spectrometer (EDS) was used to determine chemical composition. EDS spectra were stored electronically using Oxford ISIS 300 version 3.2 software. An EDS spectrum provides information regarding the chemical composition of a particle found within a sample; while not all particles are counted for the same live-/dead-time period, a typical EDS spectrum is counted for 100 seconds with a 30% deadtime. [23] Uranyl-phosphate minerals generally have a very low electrical conductivity. The particle size of the synthetic materials was very small, <20 mm, and the autunite particles exhibited a tendency to become electrostatically charged during imaging. Therefore the samples were mounted to an aluminum plate using double-sided tape and carboncoated under a vacuum. The carbon coating provides a conductive path for the electrons and helps secure the particles. 3. Results and Discussion [24] Dissolution rates for GHR and Na-autunite were measured over the ph(23 C) range of 2 to 5 and temperature range from 5 to 70 C. All ph values discussed in the text of this manuscript will be denoted as ph(23 C) which corresponds to the ph value measured at 23 C. The corrected in situ solution ph values were calculated with the EQ3NR code (version 7.2b [Wolery, 1992]). Experiments were monitored until steady state conditions were achieved, ensuring effluent ph and element concentrations were invariant with respect to time, and the system was maintained at constant chemical affinity. Steady state effluent concentrations are listed in Tables A1 and A2 in Appendix A Sodium (Na-Autunite) and Calcium Meta-Autunite (GHR) Dissolution [25] Figure 2a illustrates the release rate of uranium from Na-autunite across the ph(23 C) range 2 to 5 and the temperature range of 5 to 70 C. Under the temperature range investigated, the rate of autunite dissolution decreases as a function of increasing ph. Within experimental error, there is no measurable difference between the uranium dissolution rates quantified at 5 and 23 C under the ph(23 C) range of 2 to 5. Additionally, the dissolution rates quantified at ph(23 C) = 3 are equal, within 6of16

7 Figure 2. (a) The log 10 uranium release rate as a function of temperature-corrected ph for Na-autunite in 0.01 M HNO 3 solution. (b) The log 10 phosphorus release rate as a function of temperature-corrected ph for Na-autunite in 0.01 M HNO 3 solution. experimental error, under the temperature range of 5 to 70 C. The ph dependence of autunite dissolution is not linear as a function of increasing ph at 70 C. The rate of autunite dissolution calculated on the basis of the release of uranium at ph(23 C) = 4 at 70 C is equal to that quantified at ph(23 C) = 5 at 70 C. Comparable behavior is observed for the dissolution rate of Na-autunite at 5 and 23 being equal at ph(23 C) = 4 and 5. This suggests a secondary phase may be influencing the apparent dissolution rate. Performing linear regression analysis on the data from 5 to 40 C indicates the phdependent dissolution of autunite is constant and can be quantified by the power law coefficient, h = 1.28 ± The constant value of the slope over the temperature interval indicates the power law coefficient, h, is independent of temperature. [26] Figure 2b depicts the release rate of phosphorus across the ph(23 C) range 2 to 5 under the temperature range of 5 to 70 C. The apparent release of phosphorus generally decreases as a function of ph; however, deviations in the release rate of phosphorus quantified at ph(23 C) afford a non-linear behavior. The release behavior of phosphorus displays a rapid decrease in rate, nearly two orders of magnitude, with increasing ph(23 C) from 2 to 3. Over the ph range of 3 to 4 the 7of16

8 Figure 3. (a) The log 10 uranium release rate as a function of temperature-corrected ph for GHR in 0.01 M HNO 3 solution. (b) The log 10 phosphorus release rate as a function of temperature-corrected ph for GHR in 0.01 M HNO 3 solution. release rate of phosphorus is significantly slower, decreasing by 5-fold, and a minimum in phosphorus release is reached at ph(23 C) = 4 to 5, mol m 2 s 1. [27] The apparent release behavior of uranium and phosphorus from calcium autunite was comparable to that observed for Na-autunite. Figure 3a illustrates the release of uranium from GHR specimens under the ph(23 C) range of 2 to 5 from 5 to 70 C. As observed during the dissolution of Naautunite, the rate of release at 5 and 23 was within experimental error and at 70 C the dissolution rate at ph(23 C) = 3 was within error of those calculated at all other temperatures investigated. The rate of autunite dissolution calculated at ph(23 C) = 4 at 70 C is equal to the rate of autunite dissolution at 40 C, ph(23 C) = 4, and to the dissolution rate of autunite under the temperature range of 23 to 70 C at ph(23 C) = 5. Comparable dissolution rates at varying ph and temperature conditions suggests that a possible secondary phase may be forming which influences the apparent dissolution rate. The general release behavior of uranium from GHR decreased as a function of increasing ph. Performing linear regression analysis on the data from 5 to 70 C indicates the power law coefficient for GHR is h = 1.16 ± 0.15, which is within error of that quantified for the release of uranium from Na- 8of16

9 Figure 4. The log 10 dissolution rate for (a) uranium and (b) phosphorus from Na-autunite and GHR as a function of temperature. autunite, h = 1.28 ± 0.06, suggesting the mechanism of uranium release from both Na-autunite and GHR is the same. [28] Phosphorus release rates from GHR are shown in Figure 3b. As observed with the release of phosphorus from Na-autunite, GHR exhibits a deviation in the release of phosphorus with increasing ph. Under the temperature range of 5 to 70 C the rate of phosphorus release at ph(23 C) = 4 is equal to the rate of release at ph(23 C) = 5, mol m 2 s 1. This again suggests secondary mineral solubility may be influencing the apparent phosphorus concentrations. [29] Unlike ph the effect of temperature on the dissolution of autunite is minimal. The effect of temperature was evaluated with both Na-autunite and GHR over the ph(23 C) = 2 to 5 as a function of temperature ranging from 5 C to70 C. Figure 4 displays the temperature dependence based on the rate of uranium (Figure 4a) and phosphorus (Figure 4b) release from Na-autunite and GHR. The results indicate that the dissolution rate, based on uranium and/or phosphorus, is invariant as a function of temperature. However, as previously noted, the rates decrease by nearly 4 orders of magnitude as ph increases from ph(23 C) = 2 to 5. The ph-dependence of autunite dissolution under acidic conditions (Na-autunite, h = 1.28 ± 0.06, GHR, h = 1.16 ± 0.15) is comparable, but slightly greater, to that quantified under alkaline conditions (Na-autunite, h = 0.91 ± 0.08, GHR, h = 0.88 ± 0.03) [Wellman et al., 2006]. Comparison of the dissolution rate under acidic conditions, presented here, and alkaline conditions, previously presented in [Wellman et al., 2006] illustrates the dissolution of autunite is in accordance with the general trend with regard to the rate dependence of 9of16

10 dissolution on ph: the ph decreases in the acidic range and increases under alkaline conditions. The minimum rate of dissolution is measured at ph(23 C) 5 6. This is the ph pzc for autunite, or the conditions under which the net total particle charge is zero. Thus it may be generalized that the dissolution rate of autunite is related to the surface charge imparted to the surface by the sorption of H + and/or OH, in the absence of other complexing ligands. The slow reaction rates under conditions approaching the ph pzc are contributed to by the decrease in the rate of sorption of H + and/or OH to the surface and the concomitant decrease in the rate-limiting hydrolysis of uranium within the autunite sheet Effect of Secondary Phase Solubility on the Apparent Dissolution Rate of Meta- Autunite [30] Single pass flow through testing precludes conditions conducive to back precipitation of the starting solid phase. However, during dissolution, as the concentration of elements in solution increases, the saturation state will approach the equilibrium constant with respect to some solid phase or phases exhibiting higher solubility than the starting phase. The saturation index (SI) compares the ion activity product (Q) to the equilibrium constant (K) and can be expressed mathematically by Q SI ¼ log 10 K If Q < K then SI < 0 and the solution is under saturated, if Q > K then SI > 0 and the solution is super saturated, but if Q = K then SI = 0 and the solution is in equilibrium (or near-saturated) with respect to a potential solid phase. [31] Geochemical models EQ3NR [Wolery, 1992] and MINTEQA2 [Allison et al., 1991] were applied to steady state concentrations to evaluate the aqueous speciation and saturation state of the effluent solutions with respect to key minerals, solids, and aqueous phases. It is important to note that because of the complex chemistry of uranium, there is significant debate within the literature regarding the stoichiometry and the thermodynamic values assigned to aqueous uranium species and secondary mineral phases. Although the thermodynamic databases from numerous literature sources were used to update the computer codes [Alwan and Williams, 1980; Chen et al., 1999; Finch, 1997; Grenthe et al., 1992; Kalmykov and Choppin, ð9þ 2000; Langmuir, 1978; Nguyen et al., 1992; O Hare et al., 1976, 1988; Sergeyeva et al., 1972; Vochten, 1990], the solubility calculations are based on current knowledge but may have significant associated uncertainty. [32] Figure 5 presents the predicted solubility of (UO 2 ) 3 (PO 4 ) 2 x H 2 O. Results suggest under all ph conditions, the aqueous uranium concentrations quantified during the dissolution of Na-autunite are saturated with respect to (UO 2 ) 3 (PO 4 ) 2 x H 2 O under the temperature range of 5 to 70 C. The results for aqueous uranium concentrations obtained during the dissolution of GHR (Figure 5b) are comparable to those of Na-autunite (Figure 5a). Thus it is suggested the apparent dissolution of Naand GHR-autunite is influenced by solubility limits and secondary formation of (UO 2 ) 3 (PO 4 ) 2 x H 2 O. However, the molar ratio of uranium to phosphorus in (UO 2 ) 3 (PO 4 ) 2 x H 2 O is 1.5. Figure 6 displays the ratio of uranium to phosphorus release rate for Na-autunite and GHR as a function of temperaturecorrected ph. The results indicate the aqueous concentration of uranium to phosphorus is stoichiometric at ph(23 C) = 2, with an anomalously high point for Na-autunite at 70 C, U:P = The ratio of uranium to phosphorus for Na-autunite and GHR at ph(23 C) = 3 was disperse ranging from 0.30 to At ph(23 C) = 4 and 5, the ratio of uranium to phosphorus ranged from 0.02 to Therefore, although geochemical modeling results suggest (UO 2 ) 3 (PO 4 ) 2 x H 2 O as a possible phase controlling the aqueous concentration of uranium and phosphorus from the dissolution of autunite, they do not completely explain the deviation in the uranium to phosphorus ratio with increasing ph. [33] To support the predicted formation of secondary phases and further elucidate deviations in the aqueous uranium to phosphorus ratio, SEM was conducted on GHR-autunite that had been subjected to SPFT tests conducted at ph(23 C) = 2, 70 C, at a flow rate of 100 ml/d. Figure 7 clearly displays the formation of a secondary phase. Micrographs obtained at 3000x magnification show the needle-like crystal morphology characteristic of (UO 2 ) 3 (PO 4 ) 2 x H 2 O (Figure 7b). EDS analysis indicates the composition of the secondary phase is consistent with (UO 2 ) 3 (PO 4 ) 2 x H 2 O. SEM-EDS analyses in conjunction with geochemical modeling results suggest (UO 2 ) 3 (PO 4 ) 2 x H 2 O may be an important phase controlling the release of uranium from autunite under acidic conditions. Additional phases which could contribute to the 10 of 16

11 Figure 5. Geochemical thermodynamic modeling results depicting the predicted solubility of (UO 2 ) 3 (PO 4 ) 2 x H 2 O and steady state uranium concentrations from (a) Na-autunite and (b) GHR. removal of phosphorus from effluent solution were not evident in SEM analyses. However, the precipitation of secondary phases generally proceeds through the formation of an initial amorphous gel which cannot be identified via SEM. Further investigation is needed to understand the removal of phosphorus from solution Cation Release Rates [34] Release of interlayer cations (i.e., Na + or Ca 2+ ) from minerals is generally subject to two separate reactions: matrix dissolution and alkali-hydrogen exchange. On the basis of the saturation state of the system, one or both of these mechanisms may contribute to release of interlayer cations from the structure. For example, when the system is near saturation, the activities of dissolved species near and/or in contact with the solid phase increase resulting in a decrease in the matrix dissolution rate. Concurrently, the chemical potential difference between autunite and solution will be the driving force for cation diffusion. The dominant reaction for release of sodium depends therefore on which reaction is fastest and a distinction can be made between them, as discussed below. [35] The total release of interlayer cations to solution can be obscured by the formation of secondary phases that sequesters Na or Ca. Geochemical modeling did not indicate the formation of any phases that incorporated sodium or calcium, under any experimental conditions. Therefore there should be no competing mechanism influencing the apparent release of sodium or calcium from the respective autunite mineral. 11 of 16

12 Figure 6. and GHR. Ratio of the uranium to phosphorus release rate as a function of temperature-corrected ph for Na-autunite [36] Figure 8 shows the release rate of sodium and calcium from Na-autunite and GHR, respectively, in 0.01 M HNO 3. The results indicate, within experimental error, the release of both Na + and Ca 2+ are equivalent and independent of ph and temperature. However, this is misleading because dissolution of the meta-autunite matrix will also contribute to the concentration of dissolved cations in solution. Therefore two distinct mechanisms, ion exchange and matrix dissolution, account for the release of Na + or Ca 2+ into solution. This phenomenon is more fully discussed in a companion paper on sodium borosilicate glass dissolution (Icenhower et al., submitted manuscript). 4. Conclusions [37] The dissolution kinetics of both natural Ca meta-autunite, Ca[(UO 2 )(PO 4 )] 2 3H 2 O, and synthetic Na meta-autunite (Na-autunite), Na[(UO 2 )(PO 4 )] 3H 2 O were measured using SPFT experiments to evaluate the effects of ph(23 C) from 2 to 5 under the temperature range of 5 to 70 C. Performing linear regression analysis on the data from 5 to 70 C indicates the power Figure 7. SEM photomicrograph of GHR-autunite that had been subjected to SPFT tests conducted at ph = 2, 70 C, at a flow rate of 100 ml/d illustrating the formation of uranyl-phosphate, (UO 2 ) 3 (PO 4 ) 2 x H 2 O, during dissolution. 12 of 16

13 Figure 8. The log 10 cation exchange rate for Na-autunite (sodium release rate) and GHR (calcium release rate) in 0.01 M HNO 3 solution as a function of temperature-corrected ph. Table A1. Single-Pass Flow-Through Experimental Conditions and Dissolution Rates of Synthetic Sodium Autunite a in 0.01 M HNO 3 Sample ID Surface Area, m 2 T, C ph, (23 C) Flow Rate, ml/d U, ug/l U Rate, mol/m 2 /s P, mg/l P Rate, mol/m 2 /s Na, ug/l Na Rate, mol/m 2 /s U:P SYN E E-10 (8.94E-11) SYN E E-12 (7.89E-13) SYN E E-13 (2.01E-14) SYN E E-13 (2.63E-14) SYN E E-10 (2.94E-11) SYN E E-12 (7.66E-13) SYN E E-13 (2.32E-14) SYN E E-14 (4.61E-15) SYN E E-10 (1.83E-11) SYN E E-11 (3.53E-12) SYN E E-12 (2.42E-13) SYN E E-13 (2.35E-14) SYN E E-10 (1.83E-10) SYN E E-12 (1.55E-12) SYN E E-13 (8.36E-14) SYN E E-13 (5.71E-14) E-10 (6.98E-11) E-12 (8.15E-13) E-13 (4.11E-14) E-13 (8.24E-14) E-10 (3.24E-11) E-12 (5.48E-13) E-13 (1.33E-13) E-12 (2.20E-12) E-10 (1.24E-10) E-11 (3.14E-12) E-12 (6.79E-13) E-12 (6.10E-13) E-11 (1.84E-11) E-12 (1.33E-12) E-12 (2.13E-13) E-12 (5.47E-13) E-10 (1.11E-11) E-10 (1.59E-10) E-12 (1.96E-12) E-10 (6.13E-11) E-10 (6.88E-11) E-10 (1.60E-11) E-12 (8.70E-13) E-10 (1.32E-10) E-10 (4.81E-11) E-10 (2.53E-11) E-11 (1.95E-11) E-10 (3.27E-11) E-11 (9.96E-12) E-10 (4.23E-11) E-10 (3.62E-11) E-10 (6.54E-11) a Rate uncertainties calculated as described in the text are listed in parentheses below dissolution rate. 13 of 16

14 Table A2. Single-Pass Flow-Through Experimental Conditions and Dissolution Rates of Natural GHR Autunite in a 0.01 M HNO 3 Sample ID Surface Area, m 2 T, C ph (23 C) Flow Rate, ml/d U, ug/l U Rate, mol/m 2 /s P, mg/l P Rate, mol/m 2 /s Ca, ug/l Ca Rate, mol/m 2 /s U:P GHR E E-10 (2.63E-11) GHR E E-12 (7.43E-13) GHR E E-13 (4.96E-14) GHR E E-14 (1.87E-14) GHR E E-10 (2.52E-11) GHR E E-12 (5.72E-13) GHR E E-13 (2.96E-14) GHR E E-14 (5.94E-15) GHR E E-10 (1.12E-10) GHR E E-11 (2.92E-12) GHR E E-12 (2.46E-13) GHR E E-13 (3.55E-14) GHR E E-10 (1.37E-10) GHR E E-12 (1.39E-12) GHR E E-12 (2.61E-13) GHR E E-13 (4.62E-14) E-10 (2.21E-11) E-12 (6.82E-13) E-13 (5.79E-14) E-130 (1.30E-13) E-10 (2.63E-11) E-12 (4.04E-13) E-13 (1.10E-10) E-13 (1.34E-13) E-10 (9.71E-11) E-11 (2.59E-12) E-12 (7.08E-13) E-12 (3.44E-13) E-10 (7.91E-11) E-12 (1.21E-12) E-12 (8.63E-13) E-12 (2.64E-13) E-10 (2.07E-11) E-10 (8.89E-11) E-12 (9.02E-13) E-11 (1.56E-11) E-10 (2.98E-11) E-10 (9.38E-11) E-12 (5.46E-13) E-10 (3.29E-11) E-10 (5.76E-11) E-11 (1.71E-11) E-11 (1.91E-11) E-11 (9.27E-12) E-12 (1.75E-12) E-10 (3.62E-11) E-10 (8.08E-11) E-11 (1.51E-11) a Rate uncertainties calculated as described in the text are listed in parentheses below dissolution rate. law coefficient for GHR is h = 1.16 ± 0.15, which is within error of that quantified for the release of uranium from Na-autunite, h = 1.28 ± 0.06, suggesting the mechanism of uranium release from both Na-autunite and GHR is the same and the valence state of the interlayer cation does not partake in the mechanism of matrix dissolution for autunite. The constant value of the slope over the temperature interval indicates the power law coefficient, h, is independent of temperature. [38] Comparison of the dissolution rate under acidic conditions and alkaline conditions illustrates the dissolution of autunite is in accordance with the general trend with regard to the rate dependence of dissolution on ph. It may be generalized that the dissolution rate is related to the surface charge imparted to the surface by H + and/or OH ;the rate increases both with increasing positive surface charge with decreasing ph values of the solution and with increasing negative surface charge with increasing solution ph values. The minimum rate of dissolution is measured at ph(23 C) 5 6, the ph pzc for autunite. [39] Additionally, the aqueous concentration of uranium and phosphorus released during the dissolution of autunite were non-stoichiometric over the range of experimental conditions investigated. Evaluation of the thermodynamic stability of uranium-phosphate phases was performed using the thermodynamic data packages EQ36 and MIN- TEQA2, with an updated database. The predicted saturation indices suggest the apparent release behavior for uranium and phosphorus is influenced by the solubility limits for (UO 2 ) 3 (PO 4 ) 2 x H 2 O. SEM-EDS analyses supports the formation of uranyl phosphate, (UO 2 ) 3 (PO 4 ) 2 x H 2 O, which may serve as a secondary phase limiting the 14 of 16

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