Environ. Sci. Technol. 2009, 43, 7252–7257
Zero-Valent Sulfur and Metal Speciation in Sediment Porewaters of Freshwater Lakes ´ T E S S I E R * ,‡ FEIYUE WANG† AND ANDRE Department of Chemistry, and Department of Environment and Geography, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada, and INRS-ETE, Universite´ du Que´bec, Que´bec, Que´bec G1K 9A9, Canada
Received December 9, 2008. Revised manuscript received February 3, 2009. Accepted February 10, 2009.
We measured the aqueous solubility of rhombic sulfur and used this information to incorporate, in speciation codes, the thermodynamic constants reported in the literature for the formation of polysulfide complexes. Using the values of pH and total concentrations of dissolved zerovalent sulfur, sulfide, humic substances, trace metals (Ag, Cd, Cu, Hg, methylmercury, Pb, Zn) and major ions measured in anoxic porewaters of nine oligotrophic Canadian lakes as input to these speciation codes, we show that the porewaters of these lakes are undersaturated or close to saturation with respect to rhombic sulfur and that sulfide and polysulfide ligands play a dominant role in controlling metal speciation in freshwater anoxic environments when they are present at micromolar levels. The study also highlights the need for further research on the formation constants of metal complexes with sulfide and polysulfides.
Introduction Knowledge of metal speciation is the key to understanding cycling, mobility and biological effects of metals in the aquatic environment. Laboratory measurements have shown that reduced sulfur ligands such as sulfide and polysulfides form strong complexes with class-B metals such as Ag(I), Cd(II), Cu(I), Hg(II), Pb(II), and Zn(II) (1-7). Inorganic polysulfides 2and their protonated forms) are unbranched chain (Sn+1 polymers, comprising n atoms of zerovalent sulfur (n g 1) and one of sulfide, which can be readily formed in natural waters by oxidation of sulfide, reaction of sulfide and sulfur, or by disproportionation of thiosulfate (8, 9). Polysulfide speciation in natural waters can be calculated from the measurement of total dissolved zerovalent sulfur (ΣS(0)), sulfide (ΣS(-II)), and pH, using the appropriate thermodynamic constants (3). Despite the presumed geochemical and environmental importance of polysulfides in anoxic environments, there are only limited data available on their abundance (9-14) and on their role in controlling metal speciation (15, 16). Methods for the measurement of low ΣS(0), even in small volumes of sediment porewater typically sampled, have been available for several years (9, 17); the paucity of data on zerovalent sulfur probably arises from difficulties in mastering the sampling and measurement techniques to avoid artificial oxidation. * Corresponding author phone +1 418 654 2632; fax: +1 418 654 2600; e-mail:
[email protected]. † University of Manitoba. ‡ Universite´ du Que´bec. 7252
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 19, 2009
In this study, we first measure the solubility of rhombic sulfur and use this information to express the reactions of polysulfide formation with respect to dissolved zerovalent sulfur and to incorporate their equilibrium constants into computer speciation codes. We then use our detailed analyses of anoxic porewaters from lake sediments to calculate their saturation state with respect to rhombic sulfur, and to estimate the relative importance of reduced S and dissolved organic ligands in controlling the speciation of various Class-B metals in these environments.
Materials and Methods Reagents and Solutions. All solutions of redox sensitive solutes were prepared with deoxygenated ultrapure Milli-Q water (>18 MΩ cm), purged with N2 for at least 30 min and transferred to a glovebox (Anaerobic System model 1025, Forma Scientific Inc.) filled with ultrapure N2 (99.996%). Sulfide stock solutions were freshly prepared from anhydrous Na2S (Aldrich) in 0.01 M NaOH and standardized by iodometry. Elemental sulfur solutions were freshly prepared by dissolving rhombic sulfur (S(R)8(s)) powder (Alfa Products; 99.99% pure, with X-ray diffraction pattern characteristic of S(R)8(s)) into 10 mL of tetrahydrofuran (THF) and diluting to 100 mL with double-distilled ethanol. Colloidal sulfur (Scol) was prepared by dissolving S(R)8(s) in an alkaline sulfide solution inside an N2-flushed glovebox, and then acidifying the solution to a pH of about 3 with HNO3. The suspension of the white milk-like Scol was then purged with N2 to remove remaining sulfide, and pH was adjusted to near neutral with 1 M NaOH. Solubility of Sulfur in Water. In the glovebox, S(R)8(s) or Scol was added in excess to wide-mouth glass jars (125 mL; VWR Trace Clean; equipped with airtight Teflon-lined caps) each containing a small Plexiglas dialysis sampler (“peeper”; 9.5 × 3.5 cm; two columns of two cells); the reaction vessels were tightly closed and agitated continuously in an incubator maintained at 25 ( 0.2 °C. Before use, a clean peeper was kept under N2 for g1 wk, the cells were filled with Milli-Q water, and each column of cells was covered either with a Gelman HT-200 polysulfone (0.2 µm pore size) or with an Amicon PM 10 (10 000 Dalton molecular cutoff; 0.001 µm pore size) membrane; a thin Plexiglas sheet, with holes fitting the cell apertures, was fixed with nylon screws to isolate the cells. The assembled samplers were kept under N2 for at least another week prior to being used; it is critical to remove O2 from the Plexiglas to avoid its slow release (18). At predetermined times, two 1.5 mL samples (one from each column) for dissolved zerovalent sulfur measurement were removed from a peeper with a N2-purged syringe and injected through Teflon septa into N2-purged vials containing 2 mL of double distilled ethanol, 40 µL of THF, 0.4 mL of 1 M NaNO3 (Merck, Suprapur), and 10 µL of 1 M HNO3 (Anachemia, Environmental grade). pH, measured immediately after sample collection using a combined microelectrode (Microelectrode Inc., model MI-710), varied between 6.4 and 7.5. Under the conditions of the experiments, concentrations of solutes in the peeper cells were >99% of those in the reaction vessel after 3 h (2). Dissolved zerovalent sulfur was analyzed by square wave cathodic stripping voltammetry (SWCSV; ref 17), using a computer-controlled BAS-100B (Bioanalytical Systems Inc.) polarographic analyzer. Field Sampling and Analyses. Overlying water and porewater samples were obtained at various seasons from nine headwater, oligotrophic lakes, using 1 cm interval in situ Plexiglas dialysis samplers (two columns of horizontal cells; cell volume of 4 mL; ref 19). Location of these lakes in 10.1021/es8034973 CCC: $40.75
2009 American Chemical Society
Published on Web 03/16/2009
TABLE 1. Ranges of pH and T, of Sulfide (∑S(-II)) and Zero-Valent Sulfur (∑S(0)) Concentrations, of the Ratio ∑S(0): ∑S(-II) and of the Saturation Factor (SF) in Various Anoxic Environments anoxic environment
pH
Tb (°C)
ΣS(-II) (µM)
ΣS(0) (µM)
ΣS(0): ΣS(-II)
SFa
reference
L. Be´dard L. Despe´riers L. Holland L. N11 L. N56 L. Syndicat L. Tantare´ L. Vose L. Clair L. Croche hot brines groundwater brook salt marsh salt marsh salt marshes L. Pavin groundwater
6.67-6.80 5.79-6.39 7.19-7.58 6.35-7.12 6.20-6.44 7.16-7.64 6.10-6.99 6.48-6.81 6.13-6.65 5.70-6.03 6.40-6.45 7.43-7.62 6.9 5.05-8.02 6.68-7.17 6.80-7.10
NA NA NA NA NA NA NA NA NA NA 68-80 14 14.5 NA NA NA ∼5 27-44
0.45-3.57 0.23-15.3 12.0-38.5 0.06-0.18 0.57-14.8 0.13-3.78 0.021-6.13 0.18-3.94 0.01-4.41 0.28-14.24 480-590 780-800 1640-2020
0.16-2.13 0.58-9.23 0.70-7.6
