Environ. Sci. Technol. 2003, 37, 873-879
Mineralogical Characterization of Arsenic in Uranium Mine Tailings Precipitated from Iron-Rich Hydrometallurgical Solutions B R E T T J . M O L D O V A N , †,‡ D . T . J I A N G , § A N D M . J I M H E N D R Y * ,‡ Cameco Corporation, 2121 11th Street West, Saskatoon, Saskatchewan, Canada S7M 1J3, Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5E2, and Canadian Light Source, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 0X4
Arsenic-rich uranium mine tailings from the Rabbit Lake in-pit tailings management facility (RLITMF) in northern Saskatchewan, Canada, were investigated to determine the mineralogy and long-term stability of secondary arsenic precipitates formed from iron-rich hydrometallurgical solutions. Total arsenic and iron concentrations in six ironrich samples of the mine tailings ranged from 56 to 6000 µg/g and from 12 600 to 30 200 µg/g, respectively (Fe/As molar ratios of 5.3-303). On the basis of stability field diagrams generated from pH, Eh, and temperature measurements on tailings samples (mean values of 9.79, +162 mV, and 2.8 °C, respectively), it was concluded that arsenic and iron in the tailings were stable as As5+ and Fe3+. Synchrotron-based X-ray absorption spectroscopic studies of tailings samples, fresh mill precipitates, and reference compounds showed that the arsenic in ironrich areas of the tailings existed as the stable As5+ and was adsorbed to 2-line ferrihydrite through inner-sphere bidentate linkages. Furthermore, under the conditions in the RLITMF, the 2-line ferrihydrite did not undergo any measurable conversion to more crystalline goethite or hematite, even in tailings discharged to the RLITMF 10 yr prior to sampling.
Introduction Contamination of groundwater by arsenic derived from both natural and anthropogenic sources is of global concern. The U.S. EPA (35) identified arsenic as the only carcinogen where exposure through drinking water has been demonstrated to cause human cancer. Arsenic has been identified as a contaminant in drinking water supplies in many countries (1). In Bangladesh, for example, between 35 million and 77 million of the country’s 125 million people are exposed to arsenic-contaminated drinking water (2, 3) while in the United States about 12.7 million people drink water that exceeds the drinking water standard of 10 µg/L (4). Arsenic can exist in four oxidation states (As5+, As3+, As0, and As3-). In most groundwaters, arsenic exists primarily as * Corresponding author e-mail:
[email protected]; phone: (306)966-5720; fax: (306)966-8593. † Cameco Corporation. ‡ Department of Geological Sciences, University of Saskatchewan. § Canadian Light Source, University of Saskatchewan. 10.1021/es025947a CCC: $25.00 Published on Web 02/05/2003
2003 American Chemical Society
As3+ and As5+ with As3+ being up to 60 times more toxic and generally more mobile than As5+ (5). Surface adsorption or direct precipitation of arsenic with ferric oxyhydroxide, commonly known as ferrihydrite, is a dominant control on the solubility of arsenic in natural aquatic systems (6). Ferrihydrite, a short-range ordered poorly crystalline solid, has a high adsorption affinity for the arsenate anion because of its reactivity and large specific surface area (>200 m2/g) (7). The degree of crystallinity of ferrihydrite determines its ability to effectively adsorb arsenic with the least crystalline ferrihydrite (2-line ferrihydrite) having the greatest sorption affinity for arsenic (8). The degree of arsenic adsorption on ferrihydrite (forming an arsenical ferrihydrite) is greater in solutions where arsenic and iron are coprecipitated simultaneously as compared to surface adsorption of arsenic after the formation of ferrihydrite. This is attributed to the strong adsorption of the arsenate anion to the surface of ferrihydrite, preventing FeO-Fe polymerization within the ferrihydrite lattice, thereby reducing the degree of crystallinity. In conditions where arsenic is introduced after the precipitation of ferrihydrite, much less adsorption occurs because polymerization has already progressed (9). Formation of stable arsenical ferrihydrite requires conditions where the Fe/As molar ratio in the original solution is greater than 4. An increase in the Fe/As molar ratio in the solutions from which precipitation occurs results in an increase in the stability of the arsenical ferrihydrite as evidenced by lower solubility (10). In contrast, arsenical ferrihydrite formed in conditions with low Fe/As molar ratios (4) arsenical ferrihydrite could be stable for several years (10, 12, 13). Furthermore, field studies investigating the stability of arsenate adsorption to ferrihydrite particles have focused on the aquatic environment with little emphasis on sorption processes in soils and mine wastes (14). The objectives of the current study were to (i) identify the mineralogical composition of arsenic present in mine tailings precipitated from iron-rich (Fe/As molar ratio >4) hydrometallurgical solutions and (ii) assess the long-term stability of the arsenic precipitates in these mine tailings. The objectives of this study were met using synchrotron radiationbased X-ray absorption techniques in conjunction with conventional geochemical analyses of fresh and aged mine tailings (up to 10 yr) collected from the Rabbit Lake in-pit tailings management facility (RLITMF), a long-term uranium mine tailings facility, located in northern Saskatchewan, Canada. With regard to the synchrotron studies, a structuresensitive technique known as X-ray absorption fine structure (XAFS) spectroscopy was used to investigate the local coordination environment around the arsenic atom including its oxidation state. This technique is element-specific and has been demonstrated to have the ability to determine the molecular level speciation of arsenic over the concentration range of 50 µg/g to several weight percent in mine tailings solids (15). In contrast to XRD, XAFS probes short-range order (several atomic shells from the central absorbing atom), thus making the technique useful for the analysis of short-range ordered poorly crystalline compounds such as 2-line ferrihydrite. The tailings in the RLITMF provided a wellVOL. 37, NO. 5, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Whole Rock Analyses of Iron-Rich Tailings Samples from Rabbit Lake Tailings Management Facilitya Sample number (depth in m) parameter
04 (1.5 m)
09 (3.0 m)
23 (7.3 m)
53 (21.1 m)
74 (28.5 m)
113 (46.6 m)
As (µg/g) Fe (µg/g) Fe/As (M) mg of As/kg of ferrihydrite year deposited
1 614 23 625 19.6 43 086 1999
5 040 27 091 7.2 117 205 1999
2 968 25 721 11.6 72 756 1998
55.8 12 593 303 2 790 1995
5 954 23 454 5.3 159 618 1993
5 000 30 247 8.1 104 305 1991
a
Sample depths recorded as depth below the tailings surface.
constrained system to study the long-term evolution of Fe3+/ As5+ compounds because the vertical distribution of iron and arsenic concentrations in the tailings corresponded to that in the mill feed, thus providing age-defined stratigraphic layers in the tailings management facility (16).
Site Description and Milling Process The Rabbit Lake mine site is located approximately 700 km north of Saskatoon, SK, Canada (58°15′ N and 103°40′ E). The climate is sub-Arctic with a mean annual temperature of 4 °C (ranging from -45 to 25 °C) (17). The mine site is situated within the Athabasca Basin, which contains the world’s richest uranium deposits. The Rabbit Lake mine began production in 1975, making it Canada’s longest operating uranium mine. By September 1999, in excess of 10 million t of ore were milled resulting in the production of 150 million lb of yellowcake (U3O8). The RLITMF was developed in the mined-out Rabbit Lake open-pit mine. Mine tailings have been deposited in the RLITMF since 1985, making it a useful site to study the longterm geochemical controls on arsenic solubility. The tailings are composed of residues from the ore leach process and precipitates from acid neutralization. As of September 1999, the RLITMF was approximately 425 m long and 300 m wide with a depth of 98 m at the center and contained more than 4.9 million t of tailings solids of which approximately 23 000 t was arsenic. The uranium milling process did not vary during the time that mine tailings were emplaced in the RLITMF. The ground ore was mixed with water to produce an admixture of 55% solids with 45% of the solids passing the 75-µm sieve (18). Leaching of the host rock with 93% sulfuric acid and sodium chlorate as the oxidant (Eh maintained at +660 mV) resulted in the dissolution of the primary arsenic and iron minerals. The highly oxidative conditions present in the leaching circuit resulted in the oxidation of the primary arsenic minerals to the pentavalent state (19). The insoluble barren fraction of the leaching process was separated from the uranium-bearing soluble fraction in the counter-current decantation process (CCD). The soluble fraction (pregnant aqueous) reported to the solvent extraction process where the uranium was extracted. The uranium solution was further purified in an impurity precipitation circuit and precipitated from the solution as uranium peroxide (yellowcake) using hydrogen peroxide while maintaining a pH of 4.5 with milk of magnesia. Prior to November 2000, the barren solution (raffinate) from the solvent extraction circuit was neutralized with lime (Ca(OH)2) in three solution pachucas with incrementally increasing pH values (i.e., 3.5, 6.5, and 11.0). Since November 2000, the pH of solution pachuca 3 was lowered to 8.5. The retention time in each pachuca was approximately 6 h. The precipitated solids were combined with lime-neutralized leach residue solids from the CCD circuit, thickened to a density of 35% solids, and discharged to the RLITMF.
Methods and Materials Collection of Solid Samples and Field Measurements. Core samples (n ) 42) of tailings were collected to a depth of 70.6 874
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m below the surface of the tailings near the center of the pit in September 1999 using a sonic drill rig mounted on a barge. These samples were collected using a 3.1 m long × 75 mm diameter core barrel, and in cases where the tailings material was too soft to be collected using the core sampler, they were collected using a 0.6-m-long piston sampler. Field measurements of pH, Eh, and temperature were made on all core samples using an Orion model 250 pH/ion meter, an Orion combination pH/ATC electrode for pH and temperature measurements, and a Cole Palmer ORP platinum electrode for Eh measurements. Electrodes were inserted directly in the core samples and monitored until a stable reading was displayed (typically after 15 min). All Eh readings were corrected to the standard hydrogen potential. Core samples were analyzed for As and Fe using a PerkinElmer Elan 3000 ICP-MS located in the University of Saskatchewan, Department of Geological Sciences. The ICPMS technique had reported uncertainties of (2 to (5%. Samples of neutralized raffinate were collected in the mill from solution pachucas 1 (pH 3.5) and 3 (pH 8.5) on August 22, 2001. These samples were air-dried and analyzed for As and Fe at the Rabbit Lake analytical laboratory using atomic absorption spectroscopy. X-ray Absorption Fine Structure (XAFS) Spectroscopic Analysis. Preparation of Reference Compounds and Mine Tailings Samples. Standard reference compounds were used to identify suspected arsenic minerals (including the valence state of arsenic) contained within the mine tailings and to provide controls in quantitative nonlinear least-squares curve fitting analysis of the extended X-ray absorption fine structure (EXAFS) data. Scorodite was prepared and provided to the Canadian Light Source by G. Demopoulos (McGill University, Montreal, Canada) and natural arsenical ferrihydrite (samples V2-B and FV-1) was provided by T. Pichler (University of South Florida, Tampa, FL). Reference compounds of arsenic metal (University of Saskatchewan mineral collection), arsenic trioxide (Fisher Scientific), and sodium arsenate (Sigma-Aldrich) were used to identify the valence states of arsenic minerals in the mine tailings. The reference compounds were ground to a fine powder (98% of the arsenic in the fresh mill tailings existed in the As5+ state. Furthermore, a direct overlay of the spectra for arsenic acid (As5+) and neutralized mill raffinates is presented in Figure 2. This overlay further confirmed that VOL. 37, NO. 5, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Representative As K-edge EXAFS spectra of scorodite, natural arsenical ferrihydrite (FV-1), and Rabbit Lake mill neutralized raffinate over the energy range of 11.80-12.80 keV. The inset spectra (b) illustrates the fine detail of the EXAFS region over the energy range of 11.90-12.30 keV. Note the fine structure differences (arrows) between the scorodite and the natural arsenic ferrihydrite and neutralized raffinate. The flattening and broadening of the EXAFS peaks of natural arsenical ferrihydrite and neutralized raffinate at 11.95 keV is attributed to backscattering from second neighbor Fe ions in a mono- or bidentate arsenate complex on the ferrihydrite surface. All spectra have been offset for illustration purposes. FIGURE 2. X-ray absorption near edge structure (XANES) for neutralized raffinate (pH 8.5) and arsenic acid. The solid line is the spectra for neutralized raffinate. The dotted line overlying the neutralized raffinate spectra is the XANES spectra of the reference compound arsenic acid (As5+). The agreement between the two spectra shows that greater than 98% of the arsenic in the neutralized raffinate existed in the As5+ state. Curve fitting of all mine tailings samples was completed in this manner. As5+ was the dominant form of arsenic in the fresh mine tailings (As3+ was not detected). These data showed that the valence state of the arsenic (As5+) did not change with residence time in the tailings (Table 1) and was in keeping with the stability field diagrams. EXAFS Analyses. The local coordination environment about the arsenic atom in the six mine tailings samples, neutralized raffinate, natural arsenical ferrihydrite, and scorodite was determined through the analysis of the collected EXAFS spectra. Arsenic K edge EXAFS for a natural arsenical ferrihydrite (FV-1), scorodite, and neutralized raffinate (pH 8.5) are illustrated in Figure 3. The EXAFS signals in these three systems are dominated by the nearest neighbor As-O shell, the dominant frequency in the EXAFS spectra (Figure 3), which provides the basic characteristic of the χ(k)’s decaying sine wave. The existence of the more distant As-Fe shell induces higher frequency components between 11.93 and 11.99 keV. Figure 3b highlights the subtle differences between the corresponding spectra of scorodite and the natural arsenical ferrihydrite and neutralized raffinate (pH 8.5). The spectrum for scorodite in this region shows a split peak while the neutralized raffinate and natural 2-line ferrihydrite have a peak that flattens. Weak shoulder-like features are also visible in the broad peak between 12.02 and 12.10 keV in all three cases (Figure 3b). The differences in peak shape (see arrows) between scorodite and naturally occurring arsenical ferrihydrite was attributed to the presence of a strong additional higher frequency that causes a flattening at the top of the EXAFS oscillations (11.93-11.99 keV) (9). Flattening of the EXAFS oscillations is also illustrated in the neutralized raffinate (pH 8.5) spectrum. The flattening of this EXAFS oscillation was attributed to backscattering from second neighbor Fe ions in an arsenate complex on the ferrihydrite surface (9). These findings were supported by powder X-ray diffraction (XRD) and sequential extractions 876
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FIGURE 4. Fourier-transformed spectra of a natural arsenical ferrihydrite compound (FV-1), scorodite, and Rabbit Lake mine tailings sample (04). All spectra were k3 weighted prior to Fourier transformation using a 30% Gaussian window. The As-Fe shell in the crystalline scorodite sample (approximately 2.9 Å) is welldefined with larger amplitude relative to the natural arsenical ferrihydrite and mine tailings. The spectra have not been corrected for phase shift. on the RLITMF tailings samples (27). X-ray diffraction analysis supported the presence of 2-line ferrihydrite, and sequential extraction studies showed a strong association between arsenic and the 2-line ferrihydrite. To obtain more quantitative data, we determined bond length and coordination chemistry of the corresponding As-O and As-Fe shells as a characteristic indicator to determine the appropriate structural model for arsenic in the mine tailings. Fourier-transformed EXAFS spectra from the naturally occurring arsenical ferrihydrite sample (FV-1), one mine tailings sample (04), and scorodite are presented in Figure 4. The major peak in the Fourier transform at approximately 1.29 Å (uncorrected for phase shift) was the result of the nearest neighbor As-O shell. Fitting the As-O peak was completed in both k space (Figure 5a) and R space using a single As-O shell resulted in a consistent As-O radial distance (1.68 ( 0.02 Å) for all reference compounds and
FIGURE 6. Surface structure binding illustration of arsenic adsorbed to ferrihydrite. The figure illustrates four possible sorption geometries of arsenate adsorption to the surface of ferrihydrite. The arsenate tetrahedron identified by (a) represents a bidentate complex having an As-Fe interatomic distance of 3.28 Å; (b) illustrates a shared corner monodentate complex having an As-Fe interatomic distance of 3.65 Å; (c) illustrates a bidentate bridged complex having an As-Fe interatomic distance of 3.45 Å; and (d) illustrates a tridentate complex having an As-Fe interatomic distance of 3.28-3.33 Å (after ref 9).
FIGURE 5. Curve fitting to mine tailings sample 09. Panel a illustrates the data and fit for the As-O shell (the solid line represents the experimental data and the dotted line the fit). Panel b illustrates the data and fit for the As-Fe shell with the same representation for the experimental data and fit. mine tailings samples (Table 2). The As-O tetrahedron remained relatively rigid in all samples analyzed, evidenced by the constant interatomic distances and was independent of the compound to which it was bound. These results show that the radial distance for the As-O shell cannot be used to identify the mineralogy of arsenic-bound species in this environment. The second major peak in the Fourier transform was at 2.5-3.3 Å (uncorrected for phase shift) and was attributed
to As-Fe bonding with some multiple scattering contributions on the lower R side of this range. Fitting the As-Fe peak in both k space (Figure 5b) and R space using a single As-Fe shell resulted in a range in values for coordination number (CN) and interatomic distance (R (Å)) between scorodite and the natural occurring arsenical ferrihydrite compounds as well as the six samples of mine tailings (Table 2). Fitting of a single Fe shell in the natural arsenical ferrihydrite compounds and tailings samples yielded reasonable fits with a coordination number of 2 and radial distances over the range of 3.24-3.29 Å (Table 2). An interatomic distance of 3.28 Å is indicative of arsenic adsorption to 2-line ferrihydrite through As-Fe bidentate binuclear bridging (Figure 6), where the arsenate anion was connected to edge-sharing ferric octahedra (9, 28, 29). A bidentate arsenate complex is favored in cases where the arsenate anion is coprecipitated with ferric iron, as in the Rabbit Lake milling circuit (9). The presence of monodentate and tridentate arsenate complexes in synthetic arsenical ferrihydrite systems have been identified (9). It is unlikely that the arsenate complex in the tailings was monodentate or tridentate (Figure 6). In the case of monodentate linkages, As-Fe interatomic distances were longer relative to both bidentate and tridentate complexes and were in the range of 3.60-3.65 Å (9). Furthermore, monodentate arsenate complexes occurred in samples where the arsenate was adsorbed after the precipitation of ferrihydrite (9). In the case of tridentate arsenate complexes, As-Fe interatomic
TABLE 2. Arsenic K-edge EXAFS Fitting Results of Rabbit Lake Mine Tailings and Selected Reference Compoundsa As-O shell
As-Fe shell
test sample
Fe/As
N
R (Å)
σ2 (Å2)
∆Eo (eV)
N
R (Å)
σ2 (Å2)
∆Eo (eV)
scorodite neutralized raffinate (pH 3.2) neutralized raffinate (pH 8.5) FV-1 V2-B 04 09 23 53 74 113
1.0 7.4 7.4 8.9 11.8 19.6 7.2 11.6 303 5.3 8.1
4.0 4.0 4.5 4.0 4.6 5.7 4.0 4.0 4.0 4.4 4.4
1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68
0.0022 0.0030 0.0022 0.0017 0.0022 0.0054 0.0017 0.0014 0.0022 0.0024 0.0030
2.3 2.1 2.5 2.2 2.5 2.5 2.4 2.7 2.5 2.5 2.5
4.0 2.1 2.2 2.1 2.0 2.3 2.0 2.1 2.0 2.2 2.0
3.35 3.28 3.29 3.29 3.24 3.26 3.25 3.26 3.27 3.24 3.25
0.0062 0.0093 0.0121 0.0106 0.0092 0.0106 0.0092 0.0094 0.0087 0.0106 0.0107
2.3 2.1 2.5 2.2 2.5 2.5 2.4 2.7 2.5 2.5 2.5
a Coordination number (N), interatomic distance (R), Debye-Waller parameter (σ2), and threshold energy difference (∆E ) were determined from o FEFF7 fitting of R-space and k-space data with theoretical phase and amplitude functions (see text). Fitting uncertainties of the interatomic distances (R) for the As-O and As-Fe shells were (0.02 Å. The value Fe/As is expressed as a molar ratio. The coordination numbers (N) for the As-Fe shell were fixed during the fitting process.
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distances were determined to be in the range of 3.28-3.33 Å. In this case, the As-Fe coordination number would be in the range of 4-6 (9). This was not observed in the mine tailings or natural arsenical ferrihydrite. In addition, a tridentate arsenate complex would result in considerable strain to the arsenate tetrahedron and is therefore not thermodynamically favored. The As-Fe interatomic distances for the natural arsenical ferrihydrite compounds and mine tailings samples (including raffinate precipitates at pH 3.2) differed from the As-Fe interatomic distances of scorodite by 0.07-0.12 Å. Fitting uncertainties of the interatomic distances for the As-O and As-Fe shells were (0.02 Å. Attempts to fit scorodite-like distances to the natural arsenical ferrihyrdrite compounds and mine tailings samples by constraining the distances in the fitting process yielded very poor fits thereby excluding scorodite as a reasonable model for the arsenic system in the tailings. Scorodite is unstable at pH levels greater than 2, and arsenical ferrihydrite is reported to be the stable solid phase (30). It was concluded that scorodite solubility was incongruent above pH 2 (30). Scorodite precipitated below pH 2 has a net negative surface charge; however, the surface potential becomes positive at pH levels greater than 2. At pH 2, the surface of the scorodite becomes coated with what was assumed to be ferrihydrite (31). Ferric arsenate solids formed at pH 5 and at temperatures below 125 °C are amorphous and consist primarily of arsenate sorption on poorly crystalline (2-line) ferrihydrite (8). Long-Term Stability of Precipitated Arsenical Ferrihydrite. Arsenic contained within uranium mine tailings is a primary contaminant in terms of its long-term potential to adversely impact downstream receptors. Thus, an understanding of the long-term stability of arsenical ferrihydrite in mine tailings is crucial to quantify the contaminant transport potential of arsenic from the mine tailings to the regional groundwater. EXAFS analysis of the tailings samples showed that arsenic was adsorbed to 2-line ferrihydrite through inner-sphere bidentate linkages and was stable in the RLITMF for up to 10 yr with no sign of crystallization of the 2-line ferrihydrite to goethite or hematite. The lack of crystallization in the tailings samples, and thus the lack of release of arsenic to the tailings pore water, may be attributed to the presence of arsenic adsorbed to the surface of the ferrihydrite complex. Coprecipitation of arsenic with ferrihydrite was shown to inhibit the transformation of ferrihydrite to goethite or hematite (32). The arsenate solid loading on the ferrihydrite in the mine tailings ranged from 2 790 to 159 618 mg of As/kg of ferrihydrite (Table 1). Arsenate loading on ferrihydrite throughout this range was found to inhibit the transformation of ferrihydrite to the more crystalline goethite or hematite. The transformation of ferrihydrite was significantly retarded at or above arsenate solid loading of 29 455 mg of As/kg of ferrihydrite (33). Our results show that arsenate loading as low as 2790 mg of As/kg of ferrihydrite in the mine tailings was sufficient to retard the transformation of ferrihydrite for up to 5-yr storage in the RLITMF. Conditions present in the RLITMF (pH 9.79, Eh ) +162 mV, and temperature ) 2.8 °C) as well as XANES results of the collected mine tailings indicate that arsenic exists in an oxic environment predominantly as As5+. There is the potential for stabilization of arsenate partitioned to poorly crystalline iron (hydr)oxides in oxic environments (33). Generated stability field diagrams based on conditions present in the RLITMF further support the presence of As5+ as the stable arsenic species with amorphous ferric hydroxide being the stable iron phase in these mine tailings. Scorodite or crystalline ferric oxides (R-FeOOH) were not stable under the conditions found in the RLITMF. Transformation of 2-line ferrihydrite to goethite or hematite is only possible under 878
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geochemical conditions that lie within the stability field of the more crystalline products (33). Our findings are, however, in contrast to thermodynamic calculations that predict that arsenical ferrihydrite will decompose to the more crystalline goethite (R-FeOOH) with the subsequent release of arsenic to the tailings pore fluids (11, 34). Test work on arsenic free ferrihydrite at 24 °C in aqueous suspensions over the pH range of 2-12 showed that after 3 yr most of the ferrihydrite had transformed into goethite or hematite (30).
Acknowledgments The authors acknowledge the Cameco Corporation, the Cameco-NSERC Research Chair, and the Canadian Light Source for financial support. The authors also thank G. Demopoulos and T. Pichler for providing the well-characterized reference compounds. The authors are grateful to the staff of the Pacific Northwest Consortium-Collaborative Access Team (PNC-CAT; in particular Steve Heald and Dale Brewe) and the Advanced Photon Source (APS), Argonne National Laboratory, Chicago, for their technical support. The Canadian participation at the PNC-CAT/APS was supported by NSERC of Canada. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract W-31-109-Eng-38.
Literature Cited (1) Smith, A. H.; Lingas, E. O.; Rahman, M. Bull. WHO 2000, 78 (9), 1093. (2) Khan, A. W. J. Prev. Social Med. 1997, 16 (1), 65. (3) Dhar, R. K. Groundwater Arsenic Contamination and Sufferings of People in Bangladesh May Be the Biggest Calamity in the World. Presented at the International Conference on Arsenic Pollution of Groundwater in Bangladesh: Causes, Effects and Remedies, Dhaka, Bangladesh, 1998. (4) Russel, N. Arsenic and Old Waste: U.S. Congress Debates Bush Environmental Policy; International Committee of the Fourth International (ICFI): 2001. (5) Korte, N. C.; Fernando, Q. Crit. Rev. Environ. Controls 1991, 21, 1. (6) Pierce, M. L.; Moore, C. B. Water Res. 1982, 16, 1247. (7) Fuller, C. C.; Davis, J. A.; Waychunas, G. A. Geochim. Cosmochim. Acta 1993, 57, 2271. (8) Swash, P. M.; Monhemius, A. J. Synthesis, Characterisation and Solubility Testing of Solids in the Ca-Fe-AsO4 System. Presented at the Sudbury ‘95, Conference on Mining and the Environment, Sudbury, ON, Canada, 1995. (9) Waychunas, G. A.; Rea, B. A.; Fuller, C. C.; Davis, J. A. Geochim. Cosmochim. Acta 1993, 57, 2251. (10) Krause, E.; Ettel, V. A. Ferric Arsenate Compounds: Are They Environmentally Safe? Solubilities of Basic Ferric Arsenates. Impurity Control and Disposal; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, PQ, Canada, 1985. (11) Robins, R. G. The Stability and Solubility of Ferric Arsenate: An Update. EPD Congress ‘90; The Minerals, Metals, and Materials Society: Warrendale, PA, 1990. (12) Dove, P. M.; Rimstidt, J. D. Am. Mineral. 1987, 72, 845. (13) Nordstrom, D. K.; Parks, G. A. Am. Mineral. 1987, 72, 849. (14) Bowell, R. J.; Parshley, J. V. Environmental Geochemistry of Arsenic and Relevance to the Mining Industry; U.S. EPA: Cincinnati, OH, 2001; p 3. (15) Brown, G. E.; Foster, A. L.; Ostergren, J. D. Proc. Natl. Acad. Sci., U.S.A. 1998, 96, 3388. (16) Donahue, R.; Hendry, M. J.; Landine, P. J. Appl. Geochem. 2000, 15, 1097. (17) Cameco. Environmental Impact Statement: Collins Bay, A.-Zone, D.-Zone, and Eagle Point Development; Cameco Corporation: Saskatoon, SK, Canada, 1992. (18) Bharadwaj, B. P.; Jarvi, J. W.; Lam, E. K. The Rabbit Lake MillTwenty Years of Milling; Cameco Corporation Ltd.: Saskatoon, SK, Canada, 1995. (19) Demopoulos, G. P.; Kondos, P. Controlled Oxidative Leaching Technique for Arseniferous Uranium and Other Ores; Department of Mining and Metallurgical Engineering, McGill University: Montreal, PQ, Canada, 1987. (20) Jiang, D. T.; Crozier, E. D. Can. J. Phys. 1998, 76, 621.
(21) Ressler, T. J. Phys. IV 1997, 7 (C2), 269. (22) Rehr, J. J.; Albers, R. C. Rev. Mod. Phys. 2000, 72, 842. (23) Sayers, D. E.; Bunker, B. A. Data Analysis in X-ray Absorption, Principles, Applications, Techniques of EXAFS, SEXAFS, and XANES; John Wiley & Sons: New York, 1988; p 211. (24) Ravel, B. J. Synchrotron Radiat. 2001, 8, 314. (25) Kitahama, K.; Kiriyama, R.; Baba, Y. Acta Crystallogr. 1975, B31, 322. (26) Jiang, D. T.; Mineralogical Association of Canada. Synchrotron Radiat.: Earth, Environ. Mater. Sci. Appl. 2002, 30, 65. (27) Pichler, T.; Hendry, M. J.; Hall, G. E. M. Environ. Geol. 2001, 40 (4-5), 495. (28) Fendorf, S.; Eick, M. J.; Grossl, P.; Sparks, D. L. Environ. Sci. Technol. 1997, 31 (2), 315. (29) Manceau, A. Geochim. Cosmochim. Acta 1995, 59, 3647. (30) Robins, R. G. Am. Mineral. 1987, 72, 842.
(31) Robins, R. G.; Wong, P. L. M.; Nishimura, T.; Khoe, G. H. Basic Ferric Arsenates-Non-existent; Randol Gold Forum: Golden, CO, 1991, p 197. (32) Jambor, J. L.; Dutrizac, J. E. Chem. Rev. 1998, 98, 2549. (33) Ford, R. G. Environ. Sci. Technol. 2002, 36, 2459. (34) Robins, R. G. The Stabilities of Arsenic(V) and Arsenic(III) Compounds in Aqueous Metal Extraction Systems. Hydrometallurgy: Research, Development and Plant Practice; The Metallurgical Society of AIME: Warrendale, PA, 1983. (35) U.S. EPA. Arsenic in Drinking Water: Arsenic Research Plan; Office of Water: 1998.
Received for review July 8, 2002. Revised manuscript received November 24, 2002. Accepted December 16, 2002. ES025947A
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