publications > open file report > ofr 01-007 > analytical methods
Geochemistry of Sulfur in the Florida Everglades:
Sediment cores were collected by piston coring using 9 cm inner diameter Plexiglas core tubes (Orem et al., 1997). The cores were sectioned into segments ranging from 2-10 cm thick. The core segments were placed in zip-lock plastic bags, returned to the lab, and were then frozen until analysis. There was brief exposure to air when the samples were transferred between containers.
Aliquots of selected freshly thawed core segments were assayed gravimetrically for disulfides (DS), acid-volatile-sulfides (AVS), sulfate and organic-sulfur (OS) after separation of these sulfur fractions by an HCl-CrCl2-Eschka sequential extraction scheme (described in detail by Bates et al., 1993), similar to methods were used by Tuttle et al. (1986) and Canfield et al. (1986). A brief description is given here: acid-volatile sulfides were extracted from the samples using hot 6N HCl under a nitrogen atmosphere in a sealed reaction vessel and then reprecipitated as silver sulfide in a separate test-tube filled with 5% silver nitrate. Disulfides within the HCl-insoluble residue were then reduced by chromous chloride in hot 6N HCl under nitrogen and reprecipitated as silver sulfide. The sediment residue was then filtered. Sulfate in the filtrate (including sulfate derived from both pore-water and solid-phases in the sample) was precipitated as barium sulfate. Organic-sulfur in the residual sediment was oxidized to sulfate by fusion with Eschka's mixture (magnesium oxide and calcium carbonate) and then precipitated as barium sulfate. The percentage by weight of each sulfur fraction in the sediment was determined gravimetrically from the silver sulfide or barium sulfate recovered.
All water samples were collected in clean, dry 500 milliliter Nalgene bottles. Surface water was collected from about midway between the water surface and the sediment surface. Most water sample bottles were topped off and did not contain any air space. Samples were kept on ice during transit to the laboratory, where they were continuously refrigerated. Usually no more than two weeks elapsed between collection and the beginning of analysis.
Water was filtered through 0.4 micrometer Nuclepore filter pads before analysis in order to remove particulates. The volume of the filtrate was measured to the nearest milliliter. The samples were then transferred to volumetric flasks, and the contents were acidified to pH 4 with concentrated HCl. Samples were then heated on a hot plate, ant barium chloride (10%) was added after they began to boil. After the sample volume had been reduced to about 100 ml, the samples were filtered through 0.4 micrometer Nuclepore filter pads collect the precipitated barium sulfate. Recovery was determined after drying the filter pads in a desiccator. The sulfate concentrations of the samples were calculated from the mass of sulfate recovered and the measured volume of the water sample.
Sulfur in elemental sulfur fertilizer was oxidized to sulfate by fusion with Eschka's mixture for two hours at 800 oC. The dry fusion mixture was then slowly cooled and then suspended in boiling distilled-deionized water for 30 minutes. The suspension was filtered to remove solid residue, and the recovered solution was then treated as described above for water samples.
The recovered barium sulfate was converted to SO2 by combustion on a vacuum line and was then isolated by vacuum line methods. The 34S/32S of SO2 was determined using a Finnigan MAT 251 stable isotope mass spectrometer, and the results are reported in delta notation (34S) as parts per thousand deviation from Canyon Diablo Triolite (CDT) reference standard (Thode et al., 1961). Smaller samples (less than 1 milligram of sulfur) were concentrated using liquid nitrogen for mass spectrometric analysis.
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:03 PM (KP)