Effects of Adsorbed Arsenate on the Rate of ... - ACS Publications

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Effects of Adsorbed Arsenate on the Rate of Transformation of 2-Line Ferrihydrite at pH 10 Soumya Das,* M. Jim Hendry, and Joseph Essilfie-Dughan Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan SK S7N 5E2 Canada

bS Supporting Information ABSTRACT: 2-Line ferrihydrite, a form of iron in uranium mine tailings, is a dominant adsorbent for elements of concern (EOC), such as arsenic. As ferrihydrite is unstable under oxic conditions and can undergo dissolution and subsequent transformation to hematite and goethite over time, the impact of transformation on the long-term stability of EOC within tailings is of importance from an environmental standpoint. Here, studies were undertaken to assess the rate of 2-line ferrihydrite transformation at varying As/Fe ratios (0.500
0.010) to simulate tailings conditions at the Deilmann Tailings Management Facility of Cameco Corporation, Canada. Kinetics were evaluated under relevant physical (∼1 °C) and chemical conditions (pH ∼10). As the As/Fe ratio increased from 0.010 to 0.018, the rate of ferrihydrite transformation decreased by 2 orders of magnitude. No transformation of ferrihydrite was observed at higher As/Fe ratios (0.050, 0.100, and 0.500). Arsenic was found to retard ferrihydrite dissolution and transformation as well as goethite formation.

’ INTRODUCTION Arsenic is a widespread contaminant of groundwaters and surface waters throughout the world.1
3 Due to its toxicity, the U.S. EPA standard for arsenic in drinking water was set in 2001 at 0.010 ppm (10 parts per billion).2 A dominant control on the mobility of arsenic is its adsorption onto metal oxide surfaces, of which one of the most reactive and naturally occurring is ferrihydrite.4 Because ferrihydrite lacks an ordered structural arrangement,4 has a very high specific surface area,5 and is commonly found in soils and sediments as natural nanoparticles (3
10 nm in size),4,6,7 an understanding of the adsorption of arsenic onto ferrihydrite has become increasingly important. Tailings resulting from mining and metallurgical operations are an important anthropogenic source of arsenic to the environment.8,9 Generally, arsenic is removed from oxic, acidic leach solutions by neutralization with lime and coprecipitation with ferric iron. In this process, the ferric iron forms ferrihydrite to which the arsenate adsorbs. As such, the arsenate
ferrihydrite system generated in mining and metallurgical operations is of great interest to geochemists and metallurgists charged with understanding the fate and transport of arsenic from the tailings to the geosphere and biosphere. Arsenate strongly adsorbs onto ferrihydrite via either monodentate or bidentate surface complexes, such as FeH2AsO4, FeOHAsO4, and FeHAsO4,10 depending upon the surface coverage.11,12 A low surface coverage produces more energetic monodentate complexes whereas a high surface coverage produces bidentate complexes.11
13 Determining the long-term stability of arsenate adsorbed onto ferrihydrite in tailings is one of the key challenges facing the r 2011 American Chemical Society

mining industry, because ferrihydrite is unstable and can convert to more crystalline and stable minerals, such as hematite and goethite. The products of ferrihydrite transformation are defined by the pH and temperature of the system.14
16 Ferrihydrite transforms to goethite under acidic or alkaline conditions (pH 2
5 or 10
14) via dissolution and reprecipitation, and favors hematite at near neutral pH (∼7) via dehydration and internal atomic arrangement at room temperature.17 However, at higher temperatures (50
100 °C) hematite formation is favored over goethite irrespective of pH.18 The rate of ferrihydrite transformation in aqueous solutions devoid of competing ions increases with increasing temperature and pH. For example, the rate of transformation increases by about 5 orders of magnitude from 25 °C (room temperature) to 100 °C at pH 10.18 The rate of transformation of ferrihydrite can be inhibited by the presence of adsorbed species. Few studies have investigated the effects of adsorbed arsenate on the rate of ferrihydrite transformation. Ferrihydrite dissolution is inhibited in the presence of arsenate species (HAsO4, H2AsO4) under oxic conditions when the system/solution is ether highly acidic (pH 1.3 at 25 °C) or highly alkaline (pH 12 at 70 °C).10,19 At 70 °C and pH 12, 50% of ferrihydrite with no adsorbed species converts to a mixture of hematite and goethite within ∼8 h; however, with the addition of 1 mol % arsenate, the rate of ferrihydrite transformation decreases by a factor of 20.19 Even after aging Received: January 12, 2011 Accepted: May 18, 2011 Revised: May 12, 2011 Published: May 27, 2011 5557

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Environmental Science & Technology for 2 weeks, ferrihydrite with varying As/Fe molar ratios (0.500, 0.250, 0.125) remains unchanged at pH 8 at room temperature.20 Other solutes, such as phosphate and silicate, can also reduce the rate of ferrihydrite dissolution and phase transformation. Phosphate and silicate strongly adsorb to ferrihydrite under alkaline conditions via inner-sphere complexes, such as Fe2O2PO2, Fe2O2POOH,21 and Fe2O2Si(OH)2.22 The inhibition is attributed to surface protonation and complexation with the available ligands on the surface sites, similar to arsenate.15,23 These surface complexes that form onto Fe-bearing mineral phases, retard the rate of dissolution23,24 and inhibit the transformation to either goethite or hematite.15,19,25,26 Under alkaline conditions (pH 12), increasing the concentration of adsorbed phosphate from 0 to 1 mol % decreases the rate of transformation of ferrihydrite by an order of magnitude, although the transformation does not completely cease (at least not under the experimental conditions studied).25 Although goethite is expected to be the only stable product under pristine alkaline conditions,17 formation of surface complexes (adsorbed solutes) promotes dehydration followed by transformation to hematite rather than the dissolution and reprecipitation pathway to goethite.15,24
29 Although sulfate adsorbs onto ferrihydrite in a much weaker fashion than silicate and phosphate, it still retards the rate of ferrihydrite transformation and promotes hematite formation akin to silicate and phosphate.30 In addition to the inorganic anions discussed above, organic ligands such as oxalate,31 carboxylic acid,32 acetate,33 and simple sugars34 also inhibit ferrihydrite transformation. The objective of this study was to define ferrihydrite transformation rates over a range of adsorbed arsenate concentrations. The objective was attained using batch test methods. To reduce the number of variables, the pH and temperature were fixed. A pH of 10 was selected to represent in situ conditions in tailings generated at the world’s largest uranium mine located in northern Saskatchewan, Canada. A temperature of 75 °C was selected to expedite the transformation process, which could take years to complete at room temperature. The resulting data were used to estimate rates of transformation under environmental conditions present in the in situ tailings (∼1
2 °C, pH ∼10). The results from this study will help to determine the long-term stability of ferrihydrite in tailings from uranium mill operations in Saskatchewan and elsewhere.

’ EXPERIMENTAL SECTION Preparation of Synthetic 2-Line Ferrihydrite, Goethite, and Hematite. Synthetic 2-line ferrihydrite, goethite, and hematite

were prepared according to the methods of Cornell and Schwertmann.35 Briefly, 2-line ferrihydrite was synthesized by titrating anhydrous FeCl3 solution with dropwise addition of 1 M KOH to a pH of 7
8; goethite was synthesized by aging freshly prepared 2-line ferrihydrite in a 5 M KOH solution for 60 h at 70 °C in a water bath; and hematite was prepared by heating Fe(NO3)3 3 9H2O solution in a water bath at 98 °C for 7 d. All synthesized precipitates were washed with double distilled deionized (DDI) water until the pH of the slurry containing the mineral precipitates approached their respective pHzpc values (∼8
8.5 for 2-line ferrihydrite; ∼7.8 for goethite; ∼8.5 for hematite).36 Finally, the supernatants were removed and the precipitates were air-dried and stored at room temperature (21 °C) for further analyses. Time Series Transformations of Ferrihydrite. The rates of 2-line ferrihydrite transformation under varying As/Fe molar ratios were determined using batch-test methods. First, six

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batches of 2-line ferrihydrite were prepared by dissolving 20 g of anhydrous FeCl3 in DDI water and titrating with 1 M NaOH to pH 7
8. The ferrihydrite precipitates were then washed using the method described above. Each of the six ferrihydrite precipitates were then resuspended (never dried) in 200 mL of DDI water in individual polyethylene bottles, resulting in a ferrihydrite concentration of ∼0.62 mol Fe/L. After the slurries were homogenized by stirring on a stir plate at room temperature, hydrated sodium arsenate (Na2HAsO4 3 7H2O) was added to each slurry under continued stirring to produce arsenate adsorbed to 2-line ferrihydrite at As/Fe molar ratios of 0.500, 0.100, 0.050, 0.018, 0.013, and 0.010. The pH of all slurries was raised to 10 ((0.05) by adding trace metal grade NaOH (0.1M) using a 10-μL pipet. Subsequently, all six polyethylene bottles were transferred to a water bath preheated to 75((2) °C. The pH was maintained at 10((0.05) during the experiment. Based on preliminary testing (data not presented), a sampling interval of 24 h was used for As/Fe molar ratios of 0.500, 0.100, 0.050, 0.018, and 0.013 and a sampling interval of 6 h was used for the As/Fe ratio of 0.010. A ∼15 mL sample (from the slurry) was pipetted from each container until the end of the experiment (14 d for As/Fe ratios of 0.500, 0.100, 0.050, and 0.018; 5 d for an As/ Fe of 0.013; and 3 d for an As/Fe ratio of 0.010). In addition, day 0 samples (before heating) from each slurry were collected to ensure no ferrihydrite transformation before the experiments began. All samples were centrifuged (at 5000 rpm for 10 min) and the precipitates were air-dried (24 h) for X-ray diffraction (XRD) and Raman spectroscopic analyses within 7 d of collection. In addition, four samples (day 0 and 14 for As/Fe ratios of 0.100 and 0.050) were collected from the polyethylene bottles and centrifuged as above. The supernatants were collected using 10-mL syringes, filtered through 0.2-μm syringe filters, and stored at room temperature for inductively coupled plasma-mass spectrometry (ICP-MS) analyses. Solid samples were processed in the manner detailed above. All ICP-MS analyses were also conducted within 7 d of sample collection. X-ray Diffraction. XRD analyses were carried out on the synthetic 2-line ferrihydrite, goethite, and hematite to confirm the purity of the iron oxy-hydroxides samples as well as on the time series samples for all As/Fe molar ratio experiments to quantify the transformation phases. All air-dried samples were gently ground using a ceramic mortar and pestle to break up larger aggregates. A small amount of methanol (∼500 μL) was dropped onto the glass slide; the dried and ground samples were then placed on the wetted slide and evenly distributed. Samples were allowed to dry for ∼5 min before XRD analyses were performed. Analyses were conducted using a Rigaku Rotoflex 200 XRD with a rotating anode (3.2 kW) and a Cu target and graphite monochromator. All XRD scans were conducted over a range in 2θ of 5 to 80 degrees at 2 degrees/minute. Raw data files were converted to separate Excel files and the relative intensities were plotted against 2θ for all scans. The phases in the transformation experiments (i.e., ferrihydrite, goethite, and hematite) were quantified using the methods applied by Das et al.18 Briefly, XRD analyses of predetermined mixtures of pure ferrihydrite and goethite (0.1
90 wt %) and pure ferrihydrite and hematite (0.1
90 wt %) were conducted. The intensities of the individual XRD scans from the transformation experiments were calibrated against the XRD scans from the predetermined mixtures using the integrated intensities. Using this method, the lower limit of detection of both goethite and hematite was 1 wt % with an accuracy of (5 wt %. 5558

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Figure 1. XRD patterns for pure 2-line ferrihydrite and time series transformation phases at pH 10 and 75 °C with varying As/Fe molar ratios. (A) As/ Fe ratios of 0.500, 0.100, and 0.050 and aged for 14 d. (B) As/Fe ratio of 0.018 and aged for 0, 1, 2, 8, and 14 d. (C) As/Fe ratio of 0.013 and aged for 0, 24, 27, 30, 48, and 120 h. (D) As/Fe ratio of 0.010 and aged for 0, 6, 24, 27, 30, and 72 h. Data presented at time 0 are from the sample collected after the addition of arsenic but before heating. H defines hematite peaks.

Raman Spectroscopy. Raman spectroscopic analyses were conducted on the solid samples collected for As/Fe ratios of 0.500, 0.100, and 0.050 at day 0 to ensure that arsenate was adsorbed onto the ferrihydrite. Measurements were carried out with a Renishaw InVia Raman microscope equipped with a solid state laser diode (Renishaw) and operating at 785 nm and a 1200 lines/mm grating. Air-dried grounded samples (less than 0.5 g) were mounted onto a glass slide and the microscope was focused onto the sample using a Leica 20X N PLAN objective lens (NA = 0.40). The instrument was operated in the line focus confocal mode at a 10-s detector exposure time with 32 spectra accumulations; backscattered Raman signals were collected with a Peltier cooled CCD detector using a laser power of 0.1% (>300 mW measured at the output aperture of the laser). The instrument was calibrated using an internal Si sample, which was measured at 520 cm
1 before conducting any measurements. After collection of these spectra, all wxd files were converted to excel files where Raman shift (cm
1) was plotted vs relative counts. Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). ICP-MS analyses were performed on four aqueous and four solid samples collected on day 0 and 14 for As/Fe ratios of 0.100 and 0.050 (as these samples did not show any sign of transformation during the course of the experiments) using a Perkin-Elmer Elan 5000 ICP-MS with the following settings: RF power, 1000 W; cool gas, 10 L/min; intermediate gas, 0.8 L/min; nebulizer gas,

0.85 L/min; B lens setting, 45; P lens setting, 45; E1 lens setting, 25; S2 lens setting, 45. All Teflon vials, caps, and glassware were rinsed with Milli-Q water and then soaked in a 6 N HCl bath for 3 d. In addition, all utensils were washed with Milli-Q water and placed in an 8 N HNO3 bath for 3 d, then washed 3 times with Milli-Q water, air-dried, and stored in clean plastic containers prior to their use. Solid samples were digested using HF-HNO3. For each, approximately 100 mg of sample was placed in a Teflon jar and then ∼5 mL of double distilled HF (48
51%) and ∼5 mL of double distilled concentrated (16 N) HNO3 were added. Screw caps were subsequently placed on the jars before heating on a hot plate at 100
150 °C for 3 d. After the samples had cooled, 1
2 mL of concentrated HNO3 and 1
2 mL HF were added to dissolve any residue if present. Samples were transferred to sample bottles, and Milli-Q water added to adjust the final volume to 100 mL. All samples were filtered and the final solution was analyzed via ICP-MS for As and Fe in both solid and aqueous phases.

’ RESULTS AND DISCUSSION Characterizing the Pure Iron Oxide Phases. Comparing the XRD patterns of the synthesized minerals to standards in the Joint Committee on Powder Diffraction Standards database 5559

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Environmental Science & Technology confirmed their composition as two-line ferrihydrite, goethite, and hematite. Two-line ferrihydrite (hereafter termed ferrihydrite) is confirmed by the two characteristic broad peaks at 2θ of 34 and 61°35 (Figure 1A), goethite by the characteristic 020, 110, and 120 peaks at 2θ of ∼18°, ∼21°, and ∼26°, respectively,35 and hematite by the 012, 104, and 110 peaks at 2θ of ∼25°, ∼39°, and ∼42°, respectively.35,37 Partitioning of Arsenic in the Aqueous and Solid Phases. ICP-MS analyses on solid and aqueous phases for As/Fe ratios of 0.100 and 0.050 indicate that >99.9% of the arsenic was present on the ferrihydrite at the onset of the experiments (day 0) and remained adsorbed onto ferrihydrite at the end of the experiment (day 14) (within the accuracy of the measurements). These results indicate that the arsenic did not desorb from the ferrihydrite during the course of the experiments. Moreover, the aqueous and solid concentrations of Fe remained unchanged, indicating that Fe was not lost from the system via an Fe(OH)4 aqueous complex during centrifugation and decanting. For the slurry at an As/Fe ratio of 0.050, for example, aqueous Fe was 1 ppm at the beginning of the experiment and remained at this concentration until the end of the experiment (14 d). In the batch experiments, arsenic was introduced as arsenate and is assumed to have remained in that form. Raman spectroscopic analyses show that the arsenic was adsorbed onto ferrihydrite at As/Fe ratios of 0.500, 0.100, 0.050 at day 0 and As(V) is the dominant solid phase arsenic species and no reduction of As(V) to As(III) was identified.38 This is confirmed by the characteristic band position at ∼836 cm
1 due to As
O bond stretching vibration 20,38 (Figure S1 in the Supporting Information). Characterization of the Iron Oxide Phases in Time-Series Experiments. Time series XRD analyses (Figure 1) show that the phase transformation of ferrihydrite is dependent on the adsorbed arsenate concentration at pH 10 and 75 °C. As the As/ Fe ratio increased from 0.010 to 0.018, the rate of ferrihydrite transformation decreased. For example, only 24 h was required to transform ∼50% ferrihydrite to hematite at an As/Fe ratio of 0.010 while 48 h was required for an As/Fe ratio of 0.013. At an As/Fe ratio of 0.018, the transformation is even slower (only ∼11% transformation is achieved after 14 d). XRD patterns for As/Fe ratios greater than 0.018 (i.e., 0.500, 0.100, and 0.050) exhibit no measurable transformation of ferrihydrite even after aging at 75 °C for 14 days (Figure 1A), while evidence of ferrihydrite transformation was first observed after 8 d at an As/Fe ratio of 0.018 (the next lowest As/Fe ratio tested). The only measurable transformation product in the presence of adsorbed arsenate was hematite (Figure 1B, C, D). The lack of goethite in the test samples is in contrast to the findings of Das et al.,18 who conducted the same experiment using pure ferrihydrite (As/Fe = 0) at pH 10 and 75 °C. They showed that although hematite dominates (∼90%), goethite formed ∼10% of the transformation product. Here, the reduction in the rate of ferrihydrite transformation when adsorbed arsenate is present can be attributed to the inhibition of nucleation and crystallization via adsorption and surface complexation of these solutes onto ferrihydrite.10
12 Furthermore, arsenate, like phosphate and silicate, adsorbs onto ferrihydrite and produces as an immobile network, and thus hinders ferrihydrite dissolution and goethite nucleation (dissolution is a precursor for goethite nucleation) and promotes hematite formation.15,28,29 Kinetics of Ferrihydrite Transformation. Values calculated for the ferrihydrite remaining in the batch-test experiments as a

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Figure 2. Plot of the reduction in 2-line ferrihydrite and formation of hematite with time at pH 10 and 75 °C for As/Fe molar ratios of 0.018, 0.013, and 0.010. Closed and open diamonds, triangles, and circles represent calculated percent of 2-line ferrihydrite transformed and percent hematite formed from XRD analyses on these samples, respectively. The data presented for an As/Fe molar ratio = 0.000 (represented by the solid and open squares for percent ferrihydrite transformed and hematite formed, respectively) were generated by Das et al. (2011) using the same methods as the current study.

function of time for As/Fe ratios of 0.010, 0.013, and 0.018 (Figure 2) support the above observations. In keeping with the literature on ferrihydrite transformation,18,31,39 the trends in these data were approximated using the first-order reaction equation ½At ¼ ½A0 e
kt

ð1Þ

where [A]t is percent ferrihydrite remaining at time t, [A]0 is the percent ferrihydrite before phase transformation (100%), k is the rate constant (hr
1), and t is time. First-order ferrihydrite transformation reactions using eq 1 (Figure 2) and the equation parameters for each As/Fe ratio (Table 1) were determined and compared to data for an As/Fe ratio of 0.000.18 In all cases, the agreement between the measured and simulated kinetic data, based on R2 values, is good (Table S1). The rates of transformation of ferrihydrite can be described by first-order reactions with the rate of ferrihydrite transformation decreasing as the As/Fe ratio increases. The rate of transformation (k) decreases linearly as the As/Fe ratio increases. The effect of a reduction in the transformation rate is clearly evident at small concentrations of adsorbed arsenate (i.e., an As/ Fe ratio of 0.010 vs 0.000; Table 1). Overall, these data show a 3 orders of magnitude reduction in the rate of transformation from 1.18  10
1 to 3.00  10
4 as As/Fe ratios are increased from 0.000 to 0.018 (Table 1). In keeping with the rates of transformation of the ferrihydrite, the formation of hematite during ferrihydrite transformation can also be described using first-order kinetics (Figure 2) with comparable R2 values (Table S1). As was also the case for the ferrihydrite transformation, the rate of hematite formation decreases with increasing As/Fe ratio (Table S1). The inhibition of ferrihydrite transformation by arsenate is supported by the literature. For example, Jia et al.20 did not observe 2-line ferrihydrite transformation in XRD analyses conducted after aging ferrihydrite for 2 weeks at As/Fe molar ratios of 0.500, 0.250, and 0.125 at 21 °C and pH 8. Although 5560

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Table 1. First-Order Reaction Rate Constants for the Transformation of Ferrihydrite at 75 and 1 °C and pH 10 at As/Fe Molar Ratios Ranging from 0.000 to 0.018a time for ferrihydrite to transform (in years)

a


1


1

As/Fe

k75 (hr )

k1 (hr )

0.000

1.18  10
1

0.010


2

0.013 0.018

10%

50%

99.9%

8.04  10
6

2

10

130

3.83  10

2.61  10
6

5

30

400

2.03  10
2 3.00  10
4

1.38  10
6 2.04  10
8

40 600

260 3900

3400 52000

Estimation of times to transform 10, 50, and 99.9% of the ferrihydrite at 1 °C are also included. The rate constant for As/Fe of 0.000 is from Das et al.18

their experimental conditions were different from those in the present study, their results suggest that ferrihydrite remains stable for a long period of time irrespective of pH and temperature when As (V) is present in the system. In addition, Paige et al.10 showed that ferrihydrite transformation is greatly reduced with the addition of arsenate under acidic (pH 1.3) conditions at 25 °C with varying As/total solid ratios (i.e., 3, 5, 15, and 50 mol %). This retardation phenomenon was also reported by Paige et al.,19 where ferrihydrite transformation was inhibited with increasing mol % of As/(As þ Fe) ratios from 0 to 7 under alkaline (pH 12) conditions at 60 °C for 500 h; no transformation was recorded when As/(As þ Fe) ratios were >1. As the temperature of most natural environments is well below the temperature used in this experiment, the reaction rates at these lower temperatures must be approximated from the data collected at 75 °C. As a first step, the first-order kinetic rate constant for the transformation of ferrihydrite as a function of temperature at pH 10 was determined using a rearranged expression of the Arrhenius equation:40   kx Ea 1 1 ln ¼
ð2Þ k75 R T75 Tx

Figure 3. Plots of As/Fe molar ratios versus the predicted time for transformation of 10, 50, and 99.9% of the 2-line ferrihydrite at pH 10 and 1 °C as represented by the circles, squares, and triangles, respectively. The lines represent hyperbola functions, similar to the Michaelis
Menten model, for 10, 50, and 99.9% transformation (from left to right).

where kx is the rate constant at the desired temperature, k75 is the rate constant at 75 °C, Ea is activation energy of ferrihydrite transformation at pH 10, R is the molar gas constant, Tx is the absolute temperature at the desired temperature, and T75 is the absolute temperature for 75 °C. The application of this equation assumes that the activation energy obtained from the slope of the Arrhenius plot of four different temperatures (25, 50, 75, and 100 °C) at pH 10 (Figure S2; Table S2) determined for the transformation of ferrihydrite under pristine conditions (As/Fe = 0.000)18 is the same as that for arsenate adsorbed onto ferrihydrite. The speed of the reactions in As-doped systems significantly decreased at even 75 °C compared to pristine conditions. Thus, the rate of transformation at temperatures such as 25 or 50 °C would decrease such that complete transformation could take years. As such, interpretation or calculation of activation energies from such systems would not be possible in a short time frame. The activation energy of an As-doped system is not considerably higher than in pristine conditions,4 and as such we used the activation energy of ferrihydrite transformation under pristine conditions as calculated by Das et al.18 in our calculations; this is conservative and would reflect a worst case scenario. As an example of the application of this equation, rate constants for the transformation of ferrihydrite at As/Fe ratios of 0.000, 0.010, 0.013, and 0.018 at 1 °C were determined (Table 1). Based on these rate constants, the times required to transform 10, 50, and 99.9% of the ferrihydrite to hematite were

calculated using eq 1. The decrease in the reaction rate of ferrihydrite transformation at 1 °C was similar to that observed at 75 °C, with a 2 orders of magnitude decrease in the rate from 1.38  10
6 to 2.04  10
8 when the As/Fe ratio increased from 0.013 to 0.018. Environmental Significance. The data generated in this study are important to understanding the long-term hydrometallurgical and geochemical controls on arsenate concentrations in high pH (pH 10), neutralized mill tailings. Specifically, the increased inhibition of ferrihydrite transformation with increased concentrations of adsorbed arsenate should increase the stability of the ferrihydrite and thus ensure that the arsenate remains adsorbed. As an example application of the kinetic data presented above, consider the tailings generated from uranium mill tailings at Cameco Corporation’s Key Lake uranium mill in northern Saskatchewan, Canada, the largest U2O8 producing mill in the world. Since 1996, mill tailings from this mill have been deposited in the Deilmann TMF. In this TMF, knowledge of the long-term geochemical controls on arsenic (of which there is an estimated 17 000 tonnes) is important because its dominant sink is adsorption onto 2-line ferrihydrite generated in the mill41 and regulators require an understanding of the geochemical controls over a 10 000 year time period.42 The Deilmann tailings are alkaline and oxic (mean pH 9.8; mean Eh þ200 mV),38 with an ambient temperature of 1
2 °C. In addition, the mean As/Fe 5561

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molar ratios of solids samples from the two tailings bodies in the DTMF (reflecting the two dominant ore bodies milled) are 0.019 and 0.100.43 The As/Fe ratios in the two tailings bodies in the DTMF are both greater than the ratio of 0.018, at which no transformation was observed after 14 d of aging at pH 10 and 75 °C. These data suggest that the in situ rate of transformation will be very slow. To estimate the time to convert ferrihydrite at 1 °C at the in situ tailings As/Fe molar ratios, the As/Fe molar ratio was plotted versus the time needed to convert 10, 50, and 99.9% ferrihydrite (Figure 3). These three sets of three points (for As/Fe ratios of 0.010, 0.013, and 0.018 only, as no ferrihydrite transformation was observed at ratios of 0.050, 0.100, and 0.500) fit a hyperbola function (eq 3) similar to the Michaelis
Menten model in enzyme kinetics.44 A nonlinear regression analysis was conducted with Sigmaplot R ¼

Rmax T KþT

ð3Þ

where R is the As/Fe molar ratio, T is the predicted time for transformation of ferrihydrite, K is a constant which increases with an increasing percentage of ferrihydrite transformed, and Rmax is the maximum As/Fe ratio up to which ferrihydrite would not transform to other crystalline phases. R2 values as an indication of goodness of fit were 0.9937, 0.9935, and 0.9935 for As/Fe ratios of 0.010, 0.013, and 0.018, respectively. K and Rmax values obtained from the fits (Table S3) show that ferrihydrite would not transform to any other crystalline phases (goethite or hematite) and would remain indefinitely as a poorly crystalline material at As/Fe ratios g0.0184. As the two tailings bodies in the DTMF have mean As/Fe ratios of 0.019 and 0.100, this plots predicts no transformation of ferrihydrite under the physical (∼1 °C) and chemical (∼pH 10) conditions that have been maintained in the tailing facilities for the past 20 years. Thus, our data predict that ferrihydrite within these tailings (including some samples with As/Fe ratios greater than 0.250) should remain stable for several tens of thousands of years at the low ambient temperatures (∼1 °C). Arsenate concentration appears to play an important role in this predicted stability. Das et al.18 calculated the conversion time for the transformation of ferrihydrite (in pristine conditions; As/ Fe = 0.000) to goethite or hematite under in situ pH (pH ∼10) and temperature conditions (1
2 °C) for mine tailings in northern Saskatchewan, Canada, at ∼3 years for 10% conversion and ∼90 years for 100% conversion. Their findings suggest that the ferrihydrite in these tailings (without arsenate) may not be stable over the long-term, and confirm the notion here of the importance of arsenic adsorption on the long-term stability of ferrihydrite in this tailings environment. Although both phosphate and silicate have been shown to occupy surface sites of iron oxides surface45 and thus present possible competition with the arsenate, detailed geochemical analyses at the DTMF show both phosphate and silicate at very low concentrations in the porewater and neutralized raffinate;43 thus, it is unlikely they would compete or affect arsenate adsorption onto ferrihydrite surfaces. The results of the estimates of transformation rates are supported by spectroscopic observations of in situ tailings samples, which show no measurable conversion of ferrihydrite over the past ∼20 years.41 In addition, field studies from the SainYrieix gold mining district of France indicate As-associated ferrihydrite remains stable over centuries, with no Fe(III) oxides,

such as hematite or goethite, being reported.46 Finally, the potential for the development of dissimilatory iron reduction in the tailings is a valid concern with regard to the long-term stability of the ferrihydrite and adsorbed arsenate, and we are currently investigating its impact in this system. The findings of the present study indicate that the rate of ferrihydrite transformation depends not only on the pH and temperature of the system but also on the presence of adsorbed arsenate. In the in situ conditions of mine tailings from northern Saskatchewan, Canada, ferrihydrite is expected to remain stable for more than 10 000 years and represent a long-term sink for arsenate.

’ ASSOCIATED CONTENT

bS

Supporting Information. Table S1: First-order reaction rate constants for the formation of hematite at 75 °C and pH 10 at As/Fe molar ratios ranging from 0.000 to 0.018. Table S2: First-order reaction rate constants for the transformation of ferrihydrite at pH 10 and temperatures of 25, 50, 75, and 100 °C for an As/Fe molar ratio of 0.000. Table S3: K and Rmax values calculated from the As/Fe molar ratios versus predicted time in Figure 3 for the transformation of ferrihydrite at pH and 1 °C. Figure S1. Raman spectra of arsenate adsorbed onto ferrihydrite with As/Fe ratios of 0.500, 0.100, and 0.050. Figure S2. An Arrhenius plot depicting the transformation of 2-line ferrihydrite for an As/Fe molar ratio of 0.000 at pH 10 and temperatures of 25, 50, 75, and 100 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; phone: 306-966-5686; fax: 306966-8593.

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