Environ. Sci. Technol. 1997, 31, 2136-2140
Release of Sorbed Sulfate from Iron Oxyhydroxides Precipitated from Acid Mine Drainage Associated with Coal Mining SETH ROSE* AND A. MOHAMAD GHAZI Department of Geology, Georgia State University, Atlanta, Georgia 30303
Batch experiments were used to investigate the release of sulfate sorbed on X-ray amorphous iron oxyhydroxide precipitates that formed in acid mine drainage (AMD) (pH ) 2.8-3.2) from the Stearns Coal Belt in southeastern Kentucky. The sediments were characterized by high sulfate concentrations (∼600-1000 mmol/kg) and iron/sulfate ratios that ranged from 6.6 to 8.6 as determined by dissolution in 10 M hydrochloric acid. The results of a ligand exchange experiment (using separate preparations of 0-0.25 N sodium nitrate, chloride, phosphate, bicarbonate, and oxalate) indicated that ∼60-70% of the total sulfate will be retained in the presence of monovalent ligands. This may be indicative of a “bidentate” bridging mechanism bonding iron and sulfate. Most of this sulfate would likely be stable upon the secondary iron oxyhydroxides associated with acid mine drainage. Sulfate desorption increased directly with pH. At neutral pH, approximately 33-50% of the total sulfate present in these precipitates was released to solution. The bicarbonate ion released ∼60% of the total sulfate from one of the AMD precipitates. These results imply that acid neutralization methods (e.g., application of crushed limestone) used to stabilize metals in AMD can have the unwanted effect of raising sulfate concentrations within impacted watersheds.
Introduction The biogeochemical oxidation of pyrite [FeS(s) + 15/4O2(aq) + 7/2H2O(l) f Fe(OH)3(s) + 2H+ + SO42-, modified from ref 1] present in coal spoil results in the production of secondary iron oxyhydroxide phases (2-4) and elevated hydrogen, sulfate, and metal ion loading (5). The rate-determining step in this process involves the aqueous oxidation of ferrous iron, which can be catalyzed by microbiological activity (6), notably by the sulfur-oxidizing genus Thiobacillus ferroxidans (1). Factors such as downstream distance from a mining operation (7), colloid loads (8), pH perturbations (9, 10), and dilution (11, 12) ultimately control the site-specific degree of the pollution problem. Although there has been a great deal of research interest paid to metals associated with acid mine drainage (AMD), less attention has been given to the fate of sulfate. However, sulfate contamination often persists and may be the best indicator of the impact of mining on a drainage system (13). Sulfate present in AMD can sorb to iron oxyhydroxides and precipitate to form a variety of minerals (both stable and metastable) such as gypsum, jarosite, natrojarosite, schw* Corresponding author phone: 404-651-2272; fax: 404-651-1376; e-mail:
[email protected].
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ertmannite, and amorphous sulfate-bearing iron oxyhydroxide phases (2, 4, 14-17). Furthermore, aqueous sulfate concentration and iron/sulfate ratios are factors that influence the types of iron oxyhydroxide phases that form in AMD (2). Recent studies have shown that iron oxyhydroxide sulfates are formed in the AMD environment and have analyzed various structural considerations pertaining to sulfate in minerals such as schwertmannite (14-16, 18). However, due to the metastable nature of the iron oxyhydroxides (2) and the related iron oxyhydroxide sulfates (18), it has not been definitively determined whether these phases constitute a permanent “sink” for sulfate. The problem is complicated by the wide range of stability of sulfate associated with iron oxyhydroxide surfaces. This is principally attributable to the partitioning of sulfate upon sites of variable bond energy ranging from strong bidentate oxygen bridges to surfaces where weak electrostatic attractions predominate (19-22). The objective of the present study was to experimentally analyze the fate of sulfate adsorbed on sedimentary “coats” or “ochres” that precipitated in AMD from the Stearns Coal District in southeastern Kentucky. The results indicate that most of the sulfate associated with these contaminated sediments is not easily desorbed; however, a substantial proportion of the total sulfate load is labile, particularly at pH values that are greater than those associated with acid mine waters. Therefore, these results have implications for reclamation and remediation projects, particularly those involving the pH control of metals.
Study Area and Site Descriptions The study area is located in southeastern Kentucky (McCreary and Whitley Counties) in the Stearns Coal Belt, which is part of the Appalachian Coal Basin (see coordinates on Table 1). Drainage from this district flows into the south fork of the Cumberland River. The Stearns coal beds (Nos. 1, 1 1/2, and 2) include disseminated pyrite and occur as 35-160 cm seams in the Lower Pennsylvanian Beatty Shale Member and Marsh Creek Sandstone Tongue of the Lee Formation (23-25). This is a sulfurous coal characterized by a total sulfur content ranging between 1000 and 40 mg/L, respectively), and heavy metal concentrations in the high ppb/low ppm range (Table 2). The precipitates associated with these sampling locations are yellow and reddish brown and clearly stand out from nonimpacted soils as ochres. Pleasant Run Creek streamflow is less contaminated than the coal spoil and slope toe sites (Table 2); however, it is definitely impacted by AMD (pH ) 3.2, sulfate ) 176 mg/L, and Mn > 5 mg/L).
S0013-936X(96)00970-4 CCC: $14.00
1997 American Chemical Society
TABLE 1. Description of Samples and Locations sample location
sample description
coal spoil, State Road 1363; Barthell Quadrangle, McCreary Co., Kentucky, 36°42′/84°35′
toe of a hillslope, State Road 1363; Barthell Quadrangle, McCreary Co., Kentucky, 36°42′/84°35′
Pleasant Run Creek, State Road 92; Hollyhill Quadrangle, Whitley Co., Kentucky, 36°42′/84°35′
Bucks Branch tributary to Jellico Creek, Hollyhill Quadrangle, Whitley Co., Kentucky, 36°39′/84°16′
designation: coal crusts precipitates forming upon coal spoil and plant detritus; pHprecipitate ) 2.70 designation: coal crust drainage; water samples from acid mine drainage (AMD) associated with spoil designation: slope toe soil yellow brown precipitates forming upon soils receiving acid mine drainage from a coal spoil; pHprecipitate ) 2.86 designation: slope toe drainage; water samples from associated AMD designation: boulder crusts orange brown precipitates upon limestone boulders located
