Environ. Sci. Technol. 2006, 40, 6709-6714
Scorodite Dissolution Kinetics: Implications for Arsenic Release MARY C. HARVEY, MADELINE E. SCHREIBER,* J. DONALD RIMSTIDT, AND MARTHA M. GRIFFITH Department of Geosciences, 4044 Derring Hall, Virginia Tech, Blacksburg Virginia 24061
We have measured the rate of scorodite (FeAsO4‚2H2O) dissolution over an environmentally relevant range of pH and temperature conditions. Dissolution rates, calculated using arsenic (As) as the reaction progress variable, were slowest at pH 3 and increased with both decreasing and increasing pH. Comparison of the pH-dependence of the dissolution rates with a scorodite stability diagram suggests that our measurements of dissolution rate at pH 2 reflect congruent dissolution, and those at and above pH 3 reflect incongruent dissolution. Because As was used as the reaction progress variable, and recognizing that As may adsorb to iron hydroxides during incongruent dissolution of scorodite, the derived rates may be underestimated. The pH and temperature dependence of scorodite dissolution rates determined in these experiments have implications for the stability of scorodite at field sites and also for the potential use of scorodite to sequester As. Although scorodite dissolution is slow, it can be enhanced by up to a half order of magnitude by increases in pH and temperature.
1 Introduction Arsenic (As) is a naturally occurring toxin that damages neurological and cardiovascular systems and has also been linked to cancer (1). Natural sources of As include As-bearing sulfides, including arsenopyrite, realgar and orpiment, and iron hydroxides, which have a strong affinity for As. The rate of As release from minerals into water is controlled by mineralogy, pH, redox conditions, ionic strength, competing anions, and microbial activity (2). The weathering rates of As sulfides have been studied under a range of pH and redox conditions (3-6). However, less is known about the rate of As release from secondary minerals formed from weathering of the As sulfides, including the mineral scorodite (FeAsO4‚2H2O). Scorodite is a common weathering product of arsenopyrite (FeAsS) and is often found in mine settings (7-11). Previous work on scorodite has focused in two areas: experimental determination of its stability (8, 10, 12, 13) and precipitation of the mineral to remove As from mine waste (11, 14-16). Several studies have focused on scorodite stability in mine wastes (9, 11, 17), and suggest that arsenic solubility is controlled by the formation of scorodite at pH less than 3. As previous studies have documented, scorodite can dissolve congruently or incongruently, depending on pH. Congruent dissolution of scorodite, releasing equimolar concentrations of As and Fe, occurs at low pH by the following reaction (8): * Corresponding author phone: 540-231-3377; fax: 540-231-3386; e-mail:
[email protected]. 10.1021/es061399f CCC: $33.50 Published on Web 09/28/2006
2006 American Chemical Society
FeAsO4‚2H2O +H+ ) H2AsO4- + Fe(OH)2+ +H2O (1) At higher values of pH, scorodite dissolves incongruently, forming iron hydroxide and arsenate oxyanions (H2AsO4- or HAsO42-; pKa2 ) 6.98) (10):
FeAsO4‚2H2O + H2O ) H2AsO4- + Fe(OH)3(s) + H+ (2) FeAsO4‚2H2O + H2O ) HAsO42- + Fe(OH)3(s) + 2H+ (3) Despite the numerous publications on scorodite solubility and precipitation, little is known about scorodite dissolution kinetics. Langmuir et al. (18) estimated As release rates from scorodite dissolution based on experiments conducted on mill tailings. However, the rates were derived using solids that that contain other minerals in addition to scorodite. Because scorodite has a wide occurrence in comparison to other secondary As-bearing minerals, evaluation of the dissolution kinetics of this mineral is a critical step in quantifying As release from natural sources to water. In addition, because there is interest in using scorodite precipitation as a sink for As in metallurgical processing, quantifying the dissolution kinetics of this potential waste product is imperative. This paper reports rates of scorodite dissolution for pH between 2 and 6 and temperatures between 22 and 50 °C. The experimental results are best understood in context with the stability of iron (Fe) and As reaction products.
2 Methods 2.1. Scorodite Synthesis. Scorodite was synthesized using a method modified from Demopoulos et al. (14). First, 1 M ferric chloride solution was mixed with 1 M sodium arsenate solution in a 1:1 ratio. To this solution, concentrated HCl was added to decrease the pH to near zero. This solution was then heated to 95 °C while being stirred. Next, 1 M NaOH was added to the solution over a period of 30 min to increase pH to 1.5. The solution was aged with stirring at 95 °C for 8 h until a yellow-white precipitate formed. After the solid was recovered by filtration, it was purified through repeated washing, centrifuging, and decanting. The X-ray diffraction (Scintag XDS2000 powder diffractometer) pattern of the solid showed sharp peaks, indicating well-crystallized scorodite. The specific surface area of the synthesized scorodite was determined by N2-BET to be 9.5 m2/g. 2.2. Temperature and pH Dependence Experiments. The effects of temperature and pH on scorodite dissolution rates were determined in a stirred pH-stat batch reactor (Brinkmann Metrohm 719 Stat Titrino). All experiments were conducted in 0.01 M NaNO3 (400 mL). At the start of each experiment, the 0.01 M NaNO3 solution was titrated to the desired pH using 0.01, 0.1, or 1 M NaOH or HNO3. Once the pH stabilized, 0.1 g of scorodite was added to the solution. The pH was continuously monitored and adjusted to maintain constant pH during each experiment. Samples were collected at 1, 15, 30, 60, 120, 180, and 240 min. After collection, the samples were filtered (0.22 µm membrane) and preserved with HNO3. Room temperature (22 ( 2 °C) experiments were conducted at the full range of pH values (2, 3, 4, 5, and 6). Additional experiments were conducted at 35 °C and 50 °C for pH 4 and 6. For the 35 °C and 50 °C experiments, the pH-stat batch reactor was immersed in a water bath to maintain constant temperature. Experimental details are presented in Table 1. VOL. 40, NO. 21, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Experimental Conditions and Calculated Rates for Scorodite Dissolution Experiments pH
trial
T (°C)
acid/base (mL) added before scorodite
acid/base (mL) added after scorodite c
0.01 M NaNO3 (mL)
scorodite (g)
log rate (mol m-2 s-1)
R 2 values for polynomial fit
2 2 2 3 3 3 4 4 4 5 5 5 6 6 4 4 6 6
14 15 16 8 9 10 11 12 13 4 5 6 17 18 19 20 21 22
22 ( 2 22 ( 2 22 ( 2 22 ( 2 22 ( 2 22 ( 2 22 ( 2 22 ( 2 22 ( 2 22 ( 2 22 ( 2 22 ( 2 22 ( 2 22 ( 2 35 ( 0.5 50 ( 0.5 35 ( 0.5 50 ( 0.5
4.55 a 4.40 a 4.34 a 4.35 b 3.95 b 4.32 b 2.78 b 2.52 b 2.32 b 3.33 c 2.47 c 2.60 c 1.42 c 3.25 c 2.20 b 2.42 b 1.06 c 1.24 c
none none none none none none 2.36 2.18 1.92 1.90 2.14 2.26 1.12 2.26 2.78 3.40 3.86 4.46
400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
-9.81 -9.83 -9.94 -10.02 -10.29 -9.94 -10.04 -10.04 -9.98 -9.75 -9.89 -9.79 -9.62 -9.71 -9.83 -9.53 -9.47 -9.35
0.95 0.97 0.96 0.90 0.88 0.87 0.96 0.97 0.96 0.95 0.99 0.98 0.99 0.99 0.95 0.96 0.94 0.98
a
1.0 M HNO3.
b
0.1 M HNO3.
c
0.01 M NaOH
2.3. Analytical Methods. Arsenic concentrations in solution were determined by graphite furnace atomic absorption spectroscopy (GFAAS) or inductively coupled plasma atomic emission spectroscopy (ICP-AES), depending on the range of concentrations in samples. Lower concentrations (200 µg L-1) were analyzed on ICP-AES (SpectraFlame Modula Tabletop) which is capable of analyzing a wider range of concentrations but it has a detection limit of 25 µg L-1. Iron was analyzed using ICP-AES; the Fe detection limit is approximately 5 µg L-1. Spikes, reference standards, sample duplicates, and laboratory replicates were included in the analytical procedures to ensure that quality control objectives were met.
3. Results and Discussion 3.1. Controls on Scorodite Dissolution Kinetics. In the temperature experiments, the concentration of dissolved As increased with time (Figure 1), with the highest As concentrations released at the highest temperature (50 °C). The corresponding Fe concentrations were significantly lower; Fe concentrations were less than 1 µM at pH 4 and were below detection (
