Dirk Wallschläger

Speciation of arsenic in sulfidic waters

Formation constants for thioarsenite species have been determined in dilute solutions at 25°C, ΣH2S from 10-7.5 to 10-3.0 M, ΣAs from 10-5.6 to 10-4.8 M, and pH 7 and 10. The principal inorganic arsenic species in anoxic aquatic systems are arsenite, As(OH)30, and a mononuclear thioarsenite with an S/As ratio of 3:1. Thioarsenic species with S/As ratios of 1 : 1,2 : 1, and 4 : 1 are lesser components in sulfidic solutions that might be encountered in natural aquatic environments. Thioarsenites dominate arsenic speciation at sulfide concentrations > 10-4.3 M at neutral pH. Conversion from neutral As(OH)30 to anionic thioarsenite species may regulate the transport and fate of arsenic in sulfate-reducing environments by governing sorption and mineral precipitation reactions.

A critical review of the biogeochemistry and ecotoxicology of selenium in lotic and lentic environments

Anthropogenic activities resulting in elevated selenium (Se) levels in aquatic ecosystems can result in teratogenic and reproductive effects in fish and waterfowl. However, relationships between observed effects and exposure concentrations or body burdens are ambiguous. Therefore, it is critical to identify factors that affect Se ecotoxicity before defining adequate protective environmental regulations. One important political debate questions if Se ecotoxicity differs between standing (lentic) and flowing (lotic) waters and, if so, how this should be incorporated into the definition of protective criteria. In the present review, we compile and discuss the scarce literature regarding Se ecotoxicity in lotic systems, and we compare it to the substantial body of evidence for lentic systems. General differences between lentic and lotic systems with respect to ecology, hydrology, and biogeochemistry are identified and related to Se ecotoxicity. The limited knowledge regarding Se speciation in the biomagnification process is reviewed and put in context. Fundamental considerations suggest that Se ecotoxicity in lotic systems should be reduced compared to lentic systems, but we conclude that this statement is not substantiated by the existing data. Additionally, we identify critical gaps of knowledge that must be resolved in future studies before the argument can be decided conclusively.

Methylated mercury species in municipal waste landfill gas sampled in Florida, USA1

Mercury-bearing material has been placed in municipal landfills from a wide array of sources including fluorescent lights, batteries, electrical switches, thermometers, and general waste. Despite its known volatility, persistence, and toxicity in the environment, the fate of mercury in landfills has not been widely studied. The nature of landfills designed to reduce waste through generation of methane by anaerobic bacteria suggests the possibility that these systems might also serve as bioreactors for the production of methylated mercury compounds. The toxicity of such species mandates the need to determine if they are emitted in municipal landfill gas (LFG). In a previous study, we had measured levels of total gaseous mercury (TGM) in LFG in the μg/m3 range in two Florida landfills, and elevated levels of monomethyl mercury (MMM) were identified in LFG condensate, suggesting the possible existence of gaseous organic Hg compounds in LFG. In the current study, we measured TGM, Hg0, and methylated mercury compounds directly in LFG from another Florida landfill. Again, TGM was in the μg/m3 range, MMM was found in condensate, and this time we positively identified dimethyl mercury (DMM) in the LGF in the ng/m3 range. These results identify landfills as a possible anthropogenic source of DMM emissions to air, and may help explain the reports of MMM in continental rainfall.

Sorption of arsenite, arsenate, and thioarsenates to iron oxides and iron sulfides: a kinetic and spectroscopic investigation.

Sorption to iron (Fe) minerals determines the fate of the toxic metalloid arsenic (As) in many subsurface environments. Recently, thiolated As species have been shown to dominate aqueous As speciation under a range of environmentally relevant conditions, thus highlighting the need for a quantitative understanding of their sorption behavior. We conducted batch experiments to measure the time-dependent sorption of two S-substituted arsenate species, mono- and tetrathioarsenate, and compared it to the sorption of arsenite and arsenate, in suspensions containing 2-line ferrihydrite, goethite, mackinawite, or pyrite. All four As species strongly sorbed to ferrihydrite. For the other sorbents, binding of the thiolated As species was generally lower compared to arsenate and arsenite, with the exception of the near instantaneous and complete sorption of monothioarsenate to pyrite. Analysis of the X-ray absorption spectroscopy (XAS) spectra of sorbed complexes implied that monothioarsenate binds to Fe oxides as a monodentate, inner-sphere complex. In the presence of Fe sulfides, mono- and tetrathioarsenate were both unstable and partially reduced to arsenite. Adsorption of the thiolated As species to the Fe sulfide minerals also caused the substitution of surface sulfur (S) atoms by As and the formation of As-Fe bonds.

Factors affecting the measurement of mercury emissions from soils with flux chambers

Air-surface exchange of mercury (Hg) above an arid geothermal area was measured with three parallel flux chamber experiments. The different experimental designs were intercompared with each other, with regard to the magnitude of the measured Hg fluxes and their response to environmental changes. Qualitatively, the measured Hg fluxes agreed well throughout the diurnal cycle, and in their response to environmental events and experimental manipulations, but quantitatively, there were significant discrepancies between the individual flux results. On average, the three designs yielded Hg fluxes agreeing within a factor of 2, but even more pronounced differences were observed during midday high emission periods and during apparent nighttime deposition events. The chamber flushing rate appears to have a very significant impact on the measured fluxes and on the response behavior to environmental change. This study demonstrates that both experimental differences and small-scale regional variability introduce large uncertainty in the estimation of natural Hg air-surface exchange by different flux chamber techniques. Also, the impact of environmental parameters on Hg air-surface exchange was studied. Rain events led to a strong increase in the Hg emissions, even when the covered soil remained dry, suggesting that the apparent chamber footprint is larger than the actually covered area. Exclusion of sunlight led to decreases in Hg emissions. Statistical analysis revealed the strongest correlations between the measured Hg fluxes and radiation and wind speed. Weaker correlations were observed with air and soil temperature and wind direction (probably due to local Hg sources). Fluxes were also inversely correlated with relative humidity.

Arsenic speciation in sulfidic waters: reconciling contradictory spectroscopic and chromatographic evidence.

In recent years, analytical methods have been developed that have demonstrated that soluble arsenic-sulfur species constitute a major fraction of dissolved arsenic in sulfidic waters. However, an intense debate is going on about the exact chemical nature of these compounds, since X-ray absorption spectroscopy (XAS) data generated at higher (mmol/L) concentrations suggest the presence of (oxy)thioarsenites in such waters, while ion chromatographic (IC) and mass spectroscopic data at lower (μmol/L to nmol/L) concentrations indicate the presence of (oxy)thioarsenates. In this contribution, we connect and explain these two apparently different types of results. We show by XAS that thioarsenites are the primary reaction products of arsenite and sulfide in geochemical model experiments in the complete absence of oxygen. However, thioarsenites are extremely unstable toward oxidation, and convert rapidly into thioarsenates when exposed to atmospheric oxygen, e.g., while waiting for analysis on the chromatographic autosampler. This problem can only be eliminated when the entire chromatographic process is conducted inside a glovebox. We also show that thioarsenites are unstable toward sample dilution, which is commonly employed prior to chromatographic analysis when ultrasensitive detectors like ICP-MS are used. This instability has two main reasons: if pH changes during dilution, then equilibria between individual arsenic-sulfur species rearrange rapidly due to their different stability regions within the pH range, and if pH is kept constant during dilution, then this changes the ratio between OH(-) and SH(-) in solution, which in turn shifts the underlying speciation equilibria. This problem is avoided by analyzing samples undiluted. Our studies show that thioarsenites appear as thioarsenates in IC analyses if oxygen is not excluded completely, and as arsenite if samples are diluted in alkaline anoxic medium. This also points out that thioarsenites are necessary intermediates in the formation of thioarsenates.

Mechanism and significance of mercury volatilization from contaminated floodplains of the German river Elbe

Abstract Several aspects of atmospheric emissions of mercury from contaminated wetlands were studied in the floodplains along the river Elbe (Northern Germany). The volatilization process manifests itself in a well behaved height profile of atmospheric mercury concentrations in the boundary layer around the soil/air interface (air and soil air). This profile was described by a detailed 9-point vertical gradient, which follows the mathematical description c (Hg)=27 h −0.5 above the ground. This power function relationship is discussed theoretically and validated by comparison to other atmospheric mercury height profiles described in the literature. Understanding of the phase transfer mechanism is improved by the finding that rain events increase the mercury flux to the atmosphere up to threefold due to increased soil moisture. A mechanism is proposed, which indicates the coupling between aquatic and atmospheric remobilization of mercury from soils, and thereby emphasizes that wetlands play an important role for mercury turnover in biogeochemical cycles due to their characteristic properties. Finally, some estimates of the long-term behavior of the emissions and their importance for the regional Hg budget are derived.

Oxidative Transformation of Trithioarsenate Along Alkaline Geothermal Drainages—Abiotic versus Microbially Mediated Processes

Trithioarsenate is the predominant arsenic species at the source of alkaline, sulfidic geothermal springs in Yellowstone National Park. Kinetic studies along seven drainage channels showed that upon discharge the major initial reaction is rapid transformation to arsenite. When aerating a trithioarsenate solution in the laboratory, 10 to 20% of trithioarsenate dissociates abiotically before reaching a steady state with arsenite and thiosulfate. In the geothermal springs, trithioarsenate is completely converted to arsenite and rate constants of 0.2 to 1.9 min−1 are 40 to 500 times higher than in the laboratory, indicating microbial catalysis. Abiotic transformation of trithioarsenate to arsenate requires the presence of a strong oxidizing agent in the laboratory and no evidence was found for direct transformation of thioarsenates to arsenate in the geothermal drainage channels. The simultaneous increase of arsenite and arsenate observed upon trithioarsenate dissociation in some hot springs confirms that the main reaction is thioarsenate transformation to arsenite before microbially catalyzed oxidation to arsenate. In contrast to previous investigations in acidic hot springs, microbially catalyzed arsenate production in near-neutral to alkaline hot springs is not inhibited by the presence of sulfide. Phylogenetic analysis showed that arsenate production coincides with the temperature-dependent occurrence of organisms closely related to Thermocrinis ruber, a sulfur-oxidizing bacterium.

Stabilization of thioarsenates in iron-rich waters

In recent years, thioarsenates have been shown to be important arsenic species in sulfidic, low-iron waters. Here, we show for the first time that thioarsenates also occur in iron-rich ground waters, and that all methods previously used to preserve arsenic speciation (acidification, flash-freezing, or EDTA addition) fail to preserve thioarsenates in such matrices. Laboratory studies were conducted to identify the best approach for stabilizing thioarsenates by combination and modification of the previously-applied methods. Since acidification was shown to induce conversions between thioarsenates and precipitation of arsenic-sulfide minerals, we first conducted a detailed study of thioarsenate preservation by flash-freezing. In pure water, thioarsenates were stable for 21d when the samples were flash-frozen and cryo-stored with a minimal and anoxic headspace. Increasing headspace volume and oxygen presence in the headspace were detrimental to thioarsenate stability during cryo-storage. Addition of NaOH (0.1M) or EtOH (1% V/V) counteracted these effects and stabilized thioarsenates during cryo-storage. Addition of Fe(II) to thioarsenate solutions caused immediate changes in arsenic speciation and a loss of total arsenic from solution during cryo-storage. Both effects were largely eliminated by addition of a neutral EDTA-solution, and thioarsenates were significantly stabilized during cryo-storage by this procedure. Neutralization of EDTA was required to prevent alteration of thioarsenate speciation through pH change. With the modified method (anoxic cryo-preservation by flash-freezing with minimal headspace after addition of neutralized EDTA-solution), the fractions of mono- and dithioarsenate, the two thioarsenates observed in the iron-rich ground waters, remained stable over a cryo-storage period of 11d. Further modifications are needed for the higher SH-substituted thioarsenates (tri- and tetrathioarsenate), which were not encountered in the studied iron-rich ground waters.

Determination of inorganic selenium species in rain and sea waters by anion exchange chromatography-hydride generation-inductively-coupled plasma-dynamic reaction cell-mass spectrometry (AEC-HG-ICP-DRC-MS)

The inorganic selenium (Se) species selenite, selenate and selenocyanate in waters are determined by anion exchange chromatography-hydride generation-inductively-coupled plasma-dynamic reaction cell-mass spectrometry (AEC-HG-ICP-DRC-MS) with instrumental detection limits of 0.15, 0.27 and 0.19 ng Se L−1, respectively. Sea water has to be diluted ten-fold prior to analysis to overcome chromatographic interferences, so practical method detection limits for this matrix are around 2–3 ng Se L−1. The species are separated by gradient elution with NaOH and converted to SeH2 by prereduction with iodide in HCl at 100 °C, followed by subsequent reaction with KBH4, before the SeH2 is introduced into the plasma after aerosol and water vapor removal. Quantification using the main Se isotope 80Se was possible by employing, for the first time, a mixture of two reaction gases: methane for eliminating the 40Ar2+ dimer, and ammonia for eliminating a significant interference from HBr+ caused by bromine present in the employed reagents. Oxidation of Se+ to SeO+ using O2 as the reaction gas was also attempted, but yielded incomplete (10–25%) conversion. In addition, the HBr+ interference was not eliminated by this approach, because the interfering molecular ion was also oxidized to HBrO+. The optimized method was successfully applied to the determination of Se speciation in uncontaminated sea water and in rain water. In rain water, an unidentified Se species was detected, which we believe to be a monomethylated Se species.