ARTICLE pubs.acs.org/est
Examining Mechanisms of Groundwater Hg(II) Treatment by Reactive Materials: An EXAFS Study Blair D. Gibson,* Carol J. Ptacek, Matthew B.J. Lindsay, and David W. Blowes Department of Earth and Environmental Sciences, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada
bS Supporting Information ABSTRACT: Laboratory batch experiments were conducted to examine mechanisms of Hg(II) removal by reactive materials proposed for groundwater treatment. These materials included granular iron filings (GIF), 1:1 (w/w) mixtures of metallurgical granular Fe powder + elemental S (MGI+S) and elemental Cu + elemental S (Cu+S), granular activated carbon (GAC), attapulgite clay (ATP), ATP treated with 2-amino-5-thiol-1,3,4thiadiazole (ATP-a), and ATP treated with 2,5-dimercapto1,3,4-thiadiazole (ATP-d). Following treatment of simulated groundwater containing 4 mg L 1 Hg for 8 or 16 days, the solution pH values ranged from 6.8 to 8.8 and Eh values ranged from +400 to 400 mV. Large decreases in aqueous Hg concentrations were observed for ATP-d (>99%), GIF (95%), MGI+S (94%), and Cu+S (90%). Treatment of Hg was less effective using ATP (29%), ATP-a (69%), and GAC (78%). Extended X-ray absorption fine structure (EXAFS) spectra of Hg on GIF, MGI+S, and GAC indicated the presence of an Hg—O bond at 2.04 2.07 Å, suggesting that Hg was bound to GIF corrosion products or to oxygen complexes associated with water sorbed to activated carbon. In contrast, bond lengths ranging from 2.35 to 2.48 Å were observed for Hg in Cu+S, ATP-a, and ATP-d treatments, suggesting the formation of Hg—S bonds.
’ INTRODUCTION Mercury contamination of soils, sediments, and groundwater is a pervasive environmental problem throughout the world. This potentially fatal neurotoxin can have adverse impacts on ecosystem function and human health. Anthropogenic Hg contamination may be derived from mining and mineral processing,1 biomass combustion,2 ammunitions manufacturing,3 chlor-alkali production,4 and other industrial activities. Mercury released from these sources often is dominated by inorganic species. Soils and sediments can be effective sinks for inorganic Hg species due to the high potential for complexation with humic matter and iron oxyhydroxide mineral surfaces.5 Under anoxic conditions, one of the most important attenuation mechanisms is reaction with sulfur-containing compounds because Hg is highly chalcophilic. Dissolved Hg can be attenuated by reactions with thiol functional groups on natural organic matter,6,7 or by reactions with Fe sulfide minerals such as mackinawite [FeS]7 9 to form sparingly soluble cinnabar [α-HgS] or metacinnabar [β-HgS] minerals which are stable under anoxic conditions. However, sediment dredging operations or natural resuspension events can lead to the oxidation of Fe and Hg sulfide minerals and result in release of Hg back into the aquatic environment.10,11 At sites where Hg can become mobile, steps must be taken to lower Hg concentrations and decrease Hg mobility. The addition of reactive media may provide an effective environmental management r 2011 American Chemical Society
and remediation option to reduce the potential for Hg release at these sites.12 Technologies that utilize reactive materials to promote in situ treatment have the potential to limit Hg mobility, methylation and bioaccumulation. Various materials including activated carbon,13 clay minerals,14 zeolites,15 and Fe(III) (oxy)hydroxides16,17 have been utilized for treatment of dissolved Hg. Mercury forms complexes with both inorganic and organic ligands at surfaces of these materials; therefore, treatment capacities are dependent on specific surface areas and competition for complexation sites. The efficacy of these materials for Hg complexation can be enhanced through pretreatment with synthetic thiol-functionalized compounds, which promotes the formation of relatively strong Hg—S bonds.18 20 Materials that promote in situ H2S production and the precipitation of Hg-sulfide phases also are utilized for Hg treatment.21,22 These materials all have potential to promote groundwater treatment; however, understanding mechanisms of Hg treatment is central to the development and implementation of effective management and remediation strategies. Received: June 30, 2011 Accepted: November 8, 2011 Revised: November 7, 2011 Published: November 08, 2011 10415
dx.doi.org/10.1021/es202253h | Environ. Sci. Technol. 2011, 45, 10415–10421
Environmental Science & Technology In this study, laboratory batch experiments were conducted to evaluate the mechanisms of Hg(II) removal from anoxic groundwater following treatment by a variety of sorptive and reactive materials. Solid phase reaction products were examined by X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. These techniques previously have been utilized to determine Hg speciation in mine wastes23 and contaminated soils,24 and to characterize the coordination geometry during treatment of Hg.18,20,21 Results of these experiments provide insight on the sorptive or reactive mechanisms of Hg(II) treatment in a variety of materials. This information is critical to the evaluation of in situ technologies for treatment of Hg in soils, sediments, and groundwater.
’ MATERIALS AND METHODS Reactive Material Preparation and Characterization. Mechanisms of Hg(II) removal were evaluated for seven different treatment materials that promote treatment through sorption or precipitation reactions. Removal by sorption mechanisms was evaluated with the application of attapulgite (palygorskite) clay (ATP) and granular activated carbon (GAC). The source of attapulgite clay was Basco Salt Mud (Zemex Industrial Minerals Inc., Attapulgus, GA), and activated carbon was purchased from a chemical supplier (Sigma-Aldrich Ltd., Canada). Attapulgite clay is a hydrated magnesium aluminum silicate that forms needle-like chain structures and compared to bentonite and kaolinite clays, attapulgite typically has higher surface area and sorptivity.25 Portions of the attapulgite clay were further pretreated with two different thiadiazole compounds (Sigma-Aldrich Ltd.): 2-amino-5-thiol-1,3,4-thiadiazole (ATP-a), and 2,5-dimercapto1,3,4-thiadiazole (ATP-d). Heterocyclic thione compounds readily form complexes with aqueous metals,26 and experiments with 2,5-dimercapto-1,3,4-thiadiazole have shown a high affinity to adsorb aqueous Hg ions.27 The clay-thiadiazole mixtures were prepared by dissolving 10 g of the thiadiazole compound in a 100 mL ethanol suspension containing 100 g of powdered attapulgite clay. These suspensions were allowed to equilibrate for approximately 10 h, and the treated clays were then rinsed with 200 mL of deionized water and allowed to air-dry. A color change in the clay from gray to yellow was observed after reaction with 2,5-dimercapto-1,3,4-thiadiazole (presumably due to the presence of thiol groups), though no color change was observed after reaction with 2-amino-5-thiol-1,3,4-thiadiazole. Removal via secondary precipitation reactions was examined using different metal and metal sulfur mixtures. Granular iron filings (GIF) were applied to a simulated groundwater containing only aqueous Hg(II) to investigate the removal mechanisms under SO4-limited conditions. The GIF were sieved to between 500 and 1000 μm and (oxy)hydroxide coatings were chemically removed prior to use (see Supporting Information (SI)). Additional treatment materials included heterogeneous mixtures of powdered copper metal and powdered metallurgical granular iron (H2Omet 56, Quebec Metal Powders Ltd., Canada) with granular elemental sulfur (Georgia Gulf Sulfur Corp., Valdosta, GA). The use of elemental copper to treat aqueous Hg through amalgamation has been described.28 For the current experiments, additional amendment with elemental sulfur was examined to assess alternate mechanisms of aqueous Hg removal. A heterogeneous copper sulfur reaction mixture (Cu+S) was prepared by dry mixing 0.5 g of elemental copper powder (
