Environ. Sci. Technol. 2003, 37, 3581-3586
Solid-Solution Reactions in As(V) Sorption by Schwertmannite KEISUKE FUKUSHI,† T S U T O M U S A T O , * ,† A N D NOBUYUKI YANASE‡ Division of Global Environmental Science and Engineering, Graduate School of Natural Science and Technology, and Institute of Nature and Environmental Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan, and Japan Atomic Energy Research Institute (JAERI), 2-4, Shirakata Shirane, Tokai-muta, Naka-gun, Ibaraki 319-1195, Japan
Sorption behavior of As(V) by synthesized schwertmannite was examined under pH 3.3 as a function of As(V) concentration in the initial solution and interpreted in term of solid-solution reactions. Results showed that schwertmannite released 0.62 mmol of SO42- for every 1 mmol of H2AsO4- and 0.24 mmol of OH- that has been sorbed. As(V) replaced SO4 up to half of the total SO4 in schwertmannite. The quantitative relationship among the three chemical compositions indicated that As(V)sorbed schwertmannite would behave as a solid solution between the As(V) free schwertmannite and schwertmannite containing the maximum level of As(V). The equilibrium constant for the anion exchange in the solid-solution reaction estimated from the reacted solution chemistry depicts the As(V) content found in precipitates formed in acid mine drainage and laboratory experiments. Although schwertmannite is metastable with respect to goethite, the transformation is significantly inhibited by sorption of As(V). The solid-solution reactions also explain the stabilization of schwertmannite by sorption of As(V).
Introduction Arsenic (As) occurs in trace amounts in the natural environment. Its natural abundance can be traced from magmatic hydrothermal fluids and its associated ore deposits and longterm accumulation in the oceans by adsorption to clay mineral deposits. However, mobilization of As due to natural processes such as weathering, sedimentation, and diagenesis results in a multitude of natural sources. Anthropogenic activities such as mining also results in mobilization of As, especially with the degradation of the ubiquitous mineral pyrite (1). As contamination of the environment results from these mobilization processes and its subsequent accumulation. Acid mine drainage (AMD) conditions are characterized by low pH and high SO4 concentration and result from the dissolution of sulfide minerals. Localized accumulation of As in areas affected by AMD conditions occurs when sulfide minerals that contains As dissolve under oxidizing conditions (2). Ochreous precipitates mainly comprising schwertmannite found in AMD conditions effectively adsorb As(V) (37), which significantly reduces As concentrations in mine * Corresponding author phone: +81-76-264-5723; fax: +81-76264-5746; e-mail:
[email protected]. † Kanazawa University. ‡ Japan Atomic Energy Research Institute. 10.1021/es026427i CCC: $25.00 Published on Web 07/11/2003
2003 American Chemical Society
drainage below background values. Schwertmannite has been noted to play an important role in reducing As concentrations in AMD conditions (3, 4). Similar iron oxides such as ferrihydrite and goethite have been proven by both macroscopic sorption examination (8, 9) and spectroscopic observation (9-12) to be effective sorbents of As(V). Despite the recognized importance of schwertmannite as a host for As in AMD conditions, the mechanism of As(V) sorption has not been systematically examined. Schwertmannite is a poorly crystalline iron oxyhydroxysulfate mineral (2-4, 13-15) that distinctively forms at low pH (ranges of 2-4) with high SO4 concentration (13-15). Its structure is similar to akagane´ite having double chains of FeO3(OH)3 octahedra sharing corners to produce square tunnels extending parallel along the c-axis (13). In akagane´ite, the structure is stabilized by Cl-, F-, or OH- occupying every second cavity. It was suggested that SO4 could play a similar role in schwertmannite. SO4 ions, however, could not occupy structural cavities without sharing the O atoms with surrounding Fe atoms because of size restrictions. This implies the severe distortion of the structure that leads to the poor crystallinity. It results to a significant amount of excess SO4 adsorbed on surface sites (2). Barham (16) has suggested that the SO4 in schwertmannite can be replaced through exchange reactions by equilibrating specimens with solutions containing other anions such as carbonate, oxalate, and chromate. Murad et al. (17) showed that As(V) was easily displaced with SO4 by suspending the schwertmannite in a solution rich in As(V). Carlson et al. (4) and Waychunas et al. (18) synthesized schwertmannite with various amount of As(V) by coprecipitation and analyzed the mineralogy and the structure. In these earlier investigations, only the solidphase characteristics of schwertmannite were dealt with. It is necessary to focus on the schwertmannite-aqueous As(V) interaction to understand the natural attenuation process that has been observed in AMD conditions. Furthermore, it is important to elucidate the As(V) sorption behavior in solution phase and estimate its quantitative parameters to predict the geochemical processes that could lead to mobilization and immobilization of arsenic in the environment.
Materials and Methods Preparation of Schwertmannite. Schwertmannite was prepared similar to the method described by Bigham et al. (14). A solution of 0.02 M Na2SO4 was warmed to 60 °C, and Fe(NO3)3‚9H2O was added to yield 0.02 M in Fe solution. The produced suspension was held at 60 °C for 12 min, then cooled, and dialyzed for 2 weeks with deionized water using cellulose membranes. The water used in the dialysis was changed every day. The resulting suspension was filtered through a 0.2-µm cellulose membrane and later freeze-dried. The Fe content of the synthesized schwertmannite specimen measured by inductively coupled plasma-mass spectrometer (ICP-MS; Hewlett-Packard HP4500) analysis of triplicate sample dissolved with 6 M HCl solution was 7.46 ( 0.10 mmol/g. The SO4 content measured by ion chromatography (TOSOH ion chromatograph system 8010 series) of triplicate samples extracted with 0.01 M NaOH solution was 1.41 ( 0.03 mmol/g. Specific surface area by the multi-point BET method using a Beckman Coulter SA3100 instrument with N2 as a adsorbate was 154 m2/g. Sorption Experiments. Sorption of As(V) by the schwertmannite suspension was examined as a function of initial As(V) concentrations at constant ionic strength (I ) 0.01 M, NaNO3). The pH of the solution was adjusted to 3.95 ( 0.04 VOL. 37, NO. 16, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Equilibrium Constants Used for Speciation-Saturation Analyses and Sorption Modelinga reaction -
H+
H3AsO4(aq) ) + H2AsO4 HAsO42- ) H+ + H2AsO43+ AsO4 + 2H ) H2AsO4FeOH2+ + H+ ) Fe3+ + H2O Fe(OH)2+ + 2H+ ) Fe3+ + 2H2O Fe(OH)3(aq) + 3H+ ) Fe3+ + 3H2O Fe(OH)4-+ 4H+ ) Fe3+ + 4H2O FeSO4+) Fe3+ + SO42FeHSO42+ ) H+ + Fe3+ + SO42Fe(SO4)2- ) Fe3+ + 2SO42Fe2(OH)24+ + 2H+ ) 2Fe3+ + 2H2O Fe3(OH)45+ + 4H+ ) 3Fe3+ + 4H2O HSO4- ) H+ + SO42OH- + H+ ) H2O NaSO4- ) Na+ + SO42FeAsO4+ 2H+ ) Fe3+ + H2AsO4FeOOH + 3H+ ) Fe3+ + 3H2O a
Temperature 25 °C.
b
Ref 19. c Ref 21.
d
log K
source
-2.24 6.86 18.35 2.19 5.67 12.56 21.60 -4.04 -2.48 -5.38 2.95 6.3 -1.98 14.00 -0.70 -3.35 -1.4
b b b c c c c c c c c c c c c b d
Ref 14.
by addition of 0.1 or 0.01 M HNO3. At this pH, As(V) occurs in the form of H2AsO4- (98-99%) (19). Duplicate 40-mL solutions having different As(V) concentrations (0 M (As(V)free system), 10-7-10-2.5 M) with support electrolyte were prepared in 50-mL polycarbonate centrifuge tubes. A series of suspensions in centrifuge tubes contain 40 mg of schwertmannite, while another series without schwertmannite serves as the blank solution. Total solids in the suspension (1 g/L) correspond to 7.46 mmol/L total Fe and 1.41 mmol/L total SO4. The suspensions with the As(V)-containing solution and blank solutions were placed in a reciprocal shaker at 25 °C for 24 h. The pH of the suspensions and blank solutions was measured with a pH meter (Cyber-Scan 2000 with calibrated pH electrode) and filtered through 0.2-µm cellulose membranes afterward. Reacted solid samples left on the filter paper were washed with deionized water to remove the excess salts and air-dried. These are served for characterization by X-ray diffractmetory (XRD). The amount of sorbed As(V) (solid-phase As(V) concentration) was calculated from the difference of As concentrations between blank solution and reacted solution of same initial As(V) concentration. The speciation-saturation analyses of the reacted and blank solutions and subsequent sorption modeling were performed using the computer code REACT (20). Table 1 shows a summary of equilibrium constants at 25 °C. Activity coefficients were calculated from the extended Debye-Hu ¨ ckel equation (22).
Results The efficiency of As(V) sorption by schwertmannite from the aqueous solution is greater than 98% when the initial As(V) concentration is 2 and with As concentrations
