Environ. Sci. Technol. 2008, 42, 2439–2444
Evidence for Incorporation of H2S in Groundwater Fulvic Acids from Stable Isotope Ratios and Sulfur K-edge X-ray Absorption Near Edge Structure Spectroscopy F L O R I A N E I N S I E D L , * ,† BERNHARD MAYER,‡ AND THORSTEN SCHÄFER§ GSF-National Research Center for Environment and Health, Institute of Groundwater Ecology, Ingolstädter Landstraße 1, D-85764 Neuherberg, Germany, Department of Geoscience, University of Calgary, Calgary, Alberta, Canada T2N 1N4, and Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung (INE), P.O. Box 3640, D-76021 Karlsruhe, Germany
Received October 8, 2007. Revised manuscript received January 10, 2008. Accepted January 14, 2008.
Groundwater samples collected in a shallow oxic and reduced deep groundwater system revealed the influence of dissolved sulfide on the chemical and isotopic composition of fulvic acid associated sulfur. Stable isotope compositions of groundwater sulfate and fulvic acid sulfur and sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy data were used to determine the sources and processes affecting fulvic acid sulfur in the aquifer. A δ34S value of 2.2 ‰ for the shallow groundwater sulfate and a δ34S value of fulvic acids of 4.9 ‰ accompanied by a contribution of up to 49% of the most oxidized sulfur species (S+6) documented that fulvic acid sulfur is mainly derived from soil S compounds such as ester sulfates, with δ34S values similar to those of atmospheric sulfate deposition. In contrast, in the deep groundwater system with elevated δ34S values in groundwater sulfate of up to 20‰ due to bacterial sulfate reduction, δ34S values in fulvic acid sulfur were negative and were up to 22‰ lower compared to those of groundwater sulfate. Furthermore, reduced sulfur compounds constituted a significantly higher proportion of total fulvic acid sulfur in the deep groundwater compared to fulvic acids in shallow groundwater, supporting the hypothesis that fulvic acids act as a sink for dissolved hydrogen sulfide in the deep aquifer. Our results suggest that the combination of sulfur K edge XANES spectroscopy and stable isotope analysis on fulvic acids represents a powerful tool to elucidate the role of fulvic acids in the sulfur cycle in groundwater.
Introduction Humic substances, mainly composed of humic (HA) and fulvic acids (FA), represent a large portion of the dissolved * Corresponding author phone: +49 (0) 89 31872567; fax: +49 (0) 89 31873361; e-mail:
[email protected]. † Institute of Groundwater Ecology. ‡ University of Calgary. § Institut für Nukleare Entsorgung. 10.1021/es7025455 CCC: $40.75
Published on Web 02/27/2008
2008 American Chemical Society
organic carbon (DOC) pool in marine environments, soils, peat bogs, and groundwater systems (1–3). In marine sediments it was reported by Buscail et al. (4) and Brüchert (5) that humic substances represent around 50% of the total carbon pool and constitute a large sink for sulfur. The incorporation of dissolved sulfide into the humic fraction has been observed in marine sediments associated with anoxic waters and organic-rich muds (5, 6) and in saltmarsh sediments (7). It was also shown that 20-60% of DOC in groundwater consists of FA, whereas humic acids are of minor importance (8, 9). So far, most studies of groundwater systems concentrate on the reactions between dissolved sulfide and Fe(II) as sink for reduced sulfur species and the bacterial or chemical oxidation of sulfide containing minerals such as pyrite (e.g., ref 10). Brücher and Pratt (11) showed that sulfur isotope measurements on sulfide minerals and groundwater or porewater sulfate may provide useful information about the occurrence and the extent of bacterial sulfate reduction (BSR). Several lines of evidence suggest that reaction rates of H2S to iron sulfide minerals are faster compared to the formation of organosulfur compounds, suggesting that the latter process may become dominant after the reactive Fe-pool is completely exhausted (12). Brücher and Pratt (11), however, found that incorporation of early diagenetic sulfide into the organic fraction occurred contemporaneously with Fe-sulfide formation. In laboratory studies, Heitmann and Blodau (13) found that during the interaction of H2S with humic substances intermediates such as S2O32- are formed, and Einsiedl et al. (14) and Chen and Gupta (15) reported on the formation of polysulfides and thiols in FA during the oxidation of H2S affected by organic matter. Moreover, Brüchert and Pratt (11) interpreted decreasing δ34S values of sulfur in FA and humic acids from an organic-rich estuarine mud as indicative of the incorporation of dissolved sulfide generated by bacterial sulfate reduction. Coulson et al. (16) found, based on sulfur isotope data, that the organic S fraction in the oxic zones of peat systems was predominantly composed of sulfur derived from atmospheric deposition, whereas incorporation of sulfide was characteristic for organic S in the reduced segments of the peat lying below the water table. In contrast to marine sediments and peats, biogeochemical reactions and pathways of sulfide oxidation and the formation of sulfur intermediates during sulfide oxidation regulated by humic substances in groundwater systems are poorly understood. The objective of this study was to investigate whether groundwater FA act as a sink for H2S in a reducing deep groundwater system and whether the associated biochemical reactions are affecting the isotopic composition of groundwater fulvic acid sulfur using the novel approach of combining sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy and stable isotope measurements.
Material and Methods Field Site. Four FA samples were isolated from a shallow oxic (n ) 1) and a deep reducing karst groundwater system (n ) 3) of the Franconian Alb located in southern Germany (Figure 1). The catchment area is ca. 3000 km2, and the karst aquifer consists of a shallow and deep groundwater system, the latter is ca. 200 m below surface. The shallow aquifer is hosted within Upper Jurassic carbonates. Marls form the base of the shallow aquifer and are overlain by well-bedded limestones with interspersed reef complexes that have been dolomitisized. In contrast, the deep karst groundwater system is a confined aquifer overlain by limestones and marls of the VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Geological cross section of the Franconian karst aquifer, located in southern Germany, and locations of the shallow karst spring (No. 1) and the deep wells (Nos. 2, 3, and 4) hosted in the Malm δ, E, ξ aquifer covered by Tertiary sediments. Kimmeridge or by Tertiary sediments. The deep aquifer represents an important drinking water resource for southern Germany, and several deep wells provided excellent access to groundwater samples for chemical and isotopic analyses. Average annual rainfall in the study area is approximately 750 mm/year. Water balance estimates suggest average annual evaporation between 500 and 600 mm/year and a groundwater recharge for the Franconian Alb in the range of 130-250 mm/year. The groundwater recharge rate for the deep aquifer is estimated to be only ca. 15% of that calculated for the shallow aquifer (17). Sampling. To investigate processes affecting the chemical and isotopic composition of FA in groundwater, one shallow and three deep karst groundwater systems were sampled (Figure 1). Chemical and isotopic measurements on FA and groundwater sulfate were performed for samples obtained from all four sites. Sulfur K-edge XANES spectroscopy was performed on FA isolated from the karst groundwater systems. In the deep groundwater, dissolved sulfide was detected by smell during sampling. Up to 5 L of water were sampled for the preparation of Ag2S, but sulfide concentrations in deep groundwater were insufficienttotrapAg2Sforchemicalandisotopicmeasurements. Preparation of Fulvic Acids. A water volume of around 1500 L with DOC concentrations between 0.3 and 0.9 mg/L carbon was sampled for each karst groundwater using a reverse osmosis technique for preconcentration of DOC in the field, slightly modified from Artinger et al. (18). A 500 L portion of each sample was transported in stainless steel containers to the laboratory. Repeated preconcentration by reverse osmosis in the laboratory resulted in 3 samples of 10 L of DOC-enriched water per sample, which were acidified to a pH of around 2 by addition of HCl. Hutchison et al. (19) reported that HA showed no detectable change in reduced organic S at pH values between 3.5 and 12.4 after samples were exposed to aeration for around 2 days. Nevertheless, samples from the deep, anoxic groundwater system were obtained under N2 atmosphere to maintain reducing conditions during sampling and transport to the laboratory. The isolation of humic substances (HS) followed the International Humic Substances Society (IHSS) standard procedures (20), using a XAD-8 resin column (Rohm and Haas Co., USA) for sorption at pH 2; the sorbed humic substances were eluted with 0.1 M NaOH. However, if this step is performed under normal laboratory atmosphere conditions, Hutchison et al. (19) reported an increase of oxidized forms of sulfur relative to reduced forms. Therefore, samples obtained after preconcentration were kept in a N2 atmosphere during the injection procedure, and the time span where reoxidation may have occurred was less than 1 h, thereby minimizing the risk of reoxidation. After acidification (pH 1, HCl), HA was precipitated and separated from the supernatant via 2440
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centrifugation. After three repetitions of this procedure, HA was freeze-dried. The fulvic acid (FA) containing supernatant was purified by sorption on smaller XAD-8 columns, followed by alkaline elution. Finally, purified FA was passed through a cation-exchange resin (BioRad AG 50W-X8) to remove excess salt, and the resulting solution was freeze-dried. Water Chemistry and Isotope Analysis on Groundwater Sulfate and Fulvic Acids. The standard field measurement program included temperature (°C), electrical conductivity (µS/cm), and pH values. Water samples obtained from the shallow and deep groundwaters for chemical measurements of major cations (Na+, K+, Ca2+, and Mg2+) and anions (Cl-, SO42-, and NO3-) were 0.45 µm field filtered, collected in 0.5 L plastic bottles, and stored at 5 °C prior to analysis. Standard ion chromatography (Dionex DX 100) was used for determining the concentrations of major cations and anions with an analytical error of less than (3%. Samples for determination of DOC contents were collected in 10 mL glass bottles. DOC contents were determined using a TOC analyzer (Shimadzu TOC-VCPH). The detection limit was 0.1 mg/L DOC with an analytical uncertainty of (0.1 mg/L. Sulfide concentrations were determined by accumulating sulfide as described by Cline (21). The detection limit for H2S was 0.01 mg/L. To prevent reoxidation of sulfide during sampling for isotope analyses, sulfide was trapped as ZnS by adding up to 50 mL of 20% Zn-Acetate. Water bottles were partially filled with Zn-Acetate solution before sampling, and 5 L of groundwater was introduced into the Zn-Acetate solution, where dissolved sulfide would rapidly precipitate as ZnS. In the laboratory, water samples were filtered through a 0.45 µm pore size Millipore syringe filter to remove ZnS. The filtered water samples were acidified to pH < 4 to remove HCO3-. Dissolved sulfate for isotope analysis was precipitated as BaSO4 with 5 mL of 0.2 M reagent grade BaCl2 solution. The precipitate was recovered by centrifugation, carefully washed, and dried prior to isotope analyses. Isotope analysis was performed by isotope ratio mass spectrometry (IRMS, Thermo Electron MAT 253) after complete conversion of BaSO4 to SO2 via high temperature combustion (1000 °C) with WO2 and V2O5 in an elemental analyzer (EA, EuroVector). For the measurements of δ18O of sulfate, CO was produced through pyrolysis of BaSO4 at 1450 °C, and isotope analysis was conducted by IRMS. Sulfur isotope ratios of fulvic acids were determined by thermal decomposition of the organic sample in an elemental analyzer, followed by CF-IRMS using the technique described by Yun et al. (22). Sulfur and oxygen isotope ratios are reported in parts per thousand (‰) using the conventional delta notation (eq 1), δsample(‰) ) [(Rsample - Rstandard) ⁄ Rstandard] × 1000
(1)
TABLE 1. Concentrations of Major Dissolved Species and Isotope Compositions of Groundwater Sulfate and Fulvic Acids Sulfur EC Na+ K+ Ca2+ Mg2+ Cl- SO42- NO3- DOC H2S δ34S [‰] δ18O [‰] δ34S [‰] karst groundwater pH T [°C] [µs/cm] [mg/L] [mg/L] [mg/L] [mg/L] [mg/L] [mg/L] [mg/L] [mg/L] [mg/L] sulfate sulfate fulvic acids shallow aquifer No. 1 deep aquifer No. 2 No. 3 No. 4
7.1 10
649
5.1
0.6
86.8
31.7
6.4
20.8
19.2
0.9
n.d.
2.2
2.3
4.9
7.1 19.9 7.3 16.5 7.1 16.1
528 625 428
3.3 4.3 34.4
1.5 1.3 3.2
71.6 81.6 38.7
18.7 28.4 12.8
1.4 1.2 1.5
24.5 20.5 13.5
