Britta Planer-Friedrich

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.

Discrimination of thioarsenites and thioarsenates by X-ray absorption spectroscopy.

Soluble arsenic-sulfur compounds play important roles in the biogeochemistry of arsenic in sulfidic waters but conflicting analytical evidence identifies them as either thioarsenates (= As(V)-sulfur species) or thioarsenites (= As(III)-sulfur species). Here, we present the first characterization of thioarsenates (mono-, di-, and tetrathioarsenate) by X-ray absorption spectroscopy and demonstrate that their spectra are distinctly different from those of As(III)-sulfur species, as well as from arsenite and arsenate. The absorption near edge energy decreases in the order arsenate > thioarsenates > arsenite > As(III)-sulfur species, and individual thioarsenates differ by 1 eV per sulfur atom. Fitted As(V)-S and As(V)-O bond distances in thioarsenates (2.13-2.18 A and 1.70 A, respectively) are significantly shorter than the corresponding As(III)-S and As(III)-O bond distances in As(III)-S species (2.24-2.34 A and 1.78 A, respectively). Finally, we demonstrate that thioarsenates can be identified by principal component analysis in mixtures containing As(III)-sulfur species. This capability is used to study the spontaneous reduction of tetrathioarsenate to As(III)-sulfur species (possibly trithioarsenite) upon acidification from pH 9.5 to 2.8.

Effect of sulfate, carbonate, and phosphate on the uranium(VI) sorption behavior onto bentonite

Summary Batch experiments were conducted to study the uranium U(VI) sorption onto bentonite as a function of pH (3 to 8), and initial U(VI) concentrations (5 × 10−6 and 5 × 10−5 M) in the presence and absence of sulfate, carbonate, and phosphate. Uranium sorption onto bentonite depended on the initial U(VI) concentration with a stronger sorption at lower concentrations and was high over a wide range of pH in the absence of complexing ligands. In the presence of 0.005 M sulfate, U(VI) sorption was reduced at low pH values due to the competition between SO42− and the uranyl ion for sorption sites on the bentonite surface, or the formation of uranyl-sulfate complexes. In the presence of 0.003 M carbonate, U(VI) sorption decreased sharply at a pH above 7, because of the formation of negatively charged uranyl-carbonate complexes, which are weakly adsorbed onto the bentonite. Uranium sorption onto bentonite was greatly enhanced in the presence of 0.003 M phosphate. Kinetic batch experiments carried out for 5 × 10−5 M U(VI) at pH values of 3, 5, and 8 revealed that the sorption rate was generally rapid over the first 10 min of the experiments, then slowed down appreciably after 1 to 24 h. Sulfate had little effect on the kinetics of U(VI) sorption; both in the absence and presence of sulfate, sorption equilibrium was attained after 4 h. In the presence of carbonate, attainment of sorption equilibrium required more time than in its absence. The presence of 0.003 M phosphate reduced the time required to reach sorption equilibrium across a wide range of pH compared to phosphate-free systems.

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.

Disproportionation of elemental sulfur by haloalkaliphilic bacteria from soda lakes

Microbial disproportionation of elemental sulfur to sulfide and sulfate is a poorly characterized part of the anoxic sulfur cycle. So far, only a few bacterial strains have been described that can couple this reaction to cell growth. Continuous removal of the produced sulfide, for instance by oxidation and/or precipitation with metal ions such as iron, is essential to keep the reaction exergonic. Hitherto, the process has exclusively been reported for neutrophilic anaerobic bacteria. Here, we report for the first time disproportionation of elemental sulfur by three pure cultures of haloalkaliphilic bacteria isolated from soda lakes: the Deltaproteobacteria Desulfurivibrio alkaliphilus and Desulfurivibrio sp. AMeS2, and a member of the Clostridia, Dethiobacter alkaliphilus. All cultures grew in saline media at pH 10 by sulfur disproportionation in the absence of metals as sulfide scavengers. Our data indicate that polysulfides are the dominant sulfur species under highly alkaline conditions and that they might be disproportionated. Furthermore, we report the first organism (Dt. alkaliphilus) from the class Clostridia that is able to grow by sulfur disproportionation.

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.

Thioarsenate formation upon dissolution of orpiment and arsenopyrite

Thioarsenates were previously determined as dominant species in geothermal and mineral waters with excess sulfide. Here, we used batch leaching experiments to determine their formation upon weathering or industrial leaching of the arsenic-sulfide minerals orpiment (As(2)S(3)) and arsenopyrite (FeAsS) under different pH and oxygen conditions. Under acidic conditions, as expected based on their known kinetic instability at low pH, no thioarsenates formed in either of the two mineral systems. Under neutral to alkaline conditions, orpiment dissolution yielded mono-, di- and trithioarsenate which accounted for up to 43-55% of total arsenic. Thioarsenate formation upon arsenopyrite dissolution was low at neutral (4%) but significant at alkaline pH, especially under suboxic to sulfidic conditions (20-43%, mainly as monothioarsenate). In contrast to orpiment, we postulate that recombination of arsenite and sulfide in solution is of minor importance for monothioarsenate formation during alkaline arsenopyrite dissolution. We propose instead that hydroxyl physisorption lead to formation of As-OH-S surface complexes by transposition of hydroxyl anions to arsenic or iron sites. Concurrently formed ironhydroxides could provide re-sorption sites for the freshly released monothioarsenate. However, sorption experiments with goethite showed slower sorption kinetics of monothioarsenate compared to arsenite, but comparable with arsenate. The discovery that thioarsenates are released by natural weathering and industrial leaching processes and that, once they are released, have a higher mobility than the commonly-investigated species arsenite and arsenate requires future studies to consider them when assessing arsenic release in sulfidic natural or mining-impacted environments.

Rhizosphere Microbial Community Composition Affects Cadmium and Zinc Uptake by the Metal-Hyperaccumulating Plant Arabidopsis halleri

ABSTRACT The remediation of metal-contaminated soils by phytoextraction depends on plant growth and plant metal accessibility. Soil microorganisms can affect the accumulation of metals by plants either by directly or indirectly stimulating plant growth and activity or by (im)mobilizing and/or complexing metals. Understanding the intricate interplay of metal-accumulating plants with their rhizosphere microbiome is an important step toward the application and optimization of phytoremediation. We compared the effects of a “native” and a strongly disturbed (gamma-irradiated) soil microbial communities on cadmium and zinc accumulation by the plant Arabidopsis halleri in soil microcosm experiments. A. halleri accumulated 100% more cadmium and 15% more zinc when grown on the untreated than on the gamma-irradiated soil. Gamma irradiation affected neither plant growth nor the 1 M HCl-extractable metal content of the soil. However, it strongly altered the soil microbial community composition and overall cell numbers. Pyrosequencing of 16S rRNA gene amplicons of DNA extracted from rhizosphere samples of A. halleri identified microbial taxa (Lysobacter, Streptomyces, Agromyces, Nitrospira, “Candidatus Chloracidobacterium”) of higher relative sequence abundance in the rhizospheres of A. halleri plants grown on untreated than on gamma-irradiated soil, leading to hypotheses on their potential effect on plant metal uptake. However, further experimental evidence is required, and wherefore we discuss different mechanisms of interaction of A. halleri with its rhizosphere microbiome that might have directly or indirectly affected plant metal accumulation. Deciphering the complex interactions between A. halleri and individual microbial taxa will help to further develop soil metal phytoextraction as an efficient and sustainable remediation strategy.

Bacterial Communities in Bangladesh Aquifers Differing in Aqueous Arsenic Concentration

At Titas, Bangladesh, two aquifers of different arsenic concentrations and redox conditions were investigated to link variations in geochemistry to in situ bacterial diversity characterized by T-RFLP (terminal restriction fragment length polymorphism) and clone library analysis. While the shallow aquifer was characterized by reduced gray sediments with a higher share of easily mobilized sedimentary arsenic (2.6% was easily mobilized from 18 mg/kg of total arsenic available in sediments) and higher aqueous arsenic concentrations of 120 ± 6 μg/L (45% arsenite), the deeper aquifer consisted of brown oxidized sediments with lower aqueous arsenic concentrations, predominantly as arsenate (60 ± 6 μg/L; 3% arsenite) and a higher share of tightly bound arsenic (only 0.6% of 53 mg/kg total sorbed arsenic was easily mobilized). The bacterial communities of both aquifers were dominated by putative aerobic or denitrifying populations of Pseudomonas, Elizabethkingia and Pantoea. The shallow aquifer was more diverse in bacterial populations of aerobic, facultative and anaerobic bacteria, an observation which may be correlated to more variable geochemical conditions resulting in arsenic mobilization and re-sorption. The deeper aquifer showed higher abundance of aerobic bacterial populations including the presence of iron-oxidizing Sideroxydans possibly of importance for the sorption of arsenic on oxidized iron hydroxides. From the arsenic-affected shallow aquifer, As(III) oxidizing isolates of Comamonas and Microbacterium were obtained, which may provide information on suitable conditions for arsenic immobilization useful for future bioremediation efforts. Supplemental materials are available for this article. Go to the publisher’s online edition of Geomicrobiology Journal to view the free supplemental file.

Thioarsenate transformation by filamentous microbial mats thriving in an alkaline, sulfidic hot spring

Thioarsenates dominate arsenic speciation in sulfidic geothermal waters, yet little is known about their fate in the environment. At Conch Spring, an alkaline hot spring in Yellowstone National Park, trithioarsenate transforms to arsenate under increasingly oxidizing conditions along the drainage channel, accompanied by an initial increase, then decrease of monothioarsenate and arsenite. On-site incubation tests were conducted using sterile-filtered water with and without addition of filamentous microbial mats from the drainage channel to distinguish the role of abiotic and biotic processes for arsenic species transformation. Abiotically, trithioarsenate was desulfidized to arsenate coupled to sulfide oxidation. Monothioarsenate, however, was inert. Biotic incubations proved that the intermediate accumulation of arsenite in the drainage channel is microbially catalyzed. In the presence of sulfide, microbially enhanced sulfide oxidation coupled to reduction of arsenate to arsenite could simply enhance abiotic desulfidation of trithioarsenate and potentially also monothioarsenate. However, we were also able to show, in sulfide-free medium, direct microbial transformation of monothioarsenate to arsenate. Some arsenite formed intermediately, which was subsequently also microbially oxidized to arsenate. This study is the first evidence for microbially mediated thioarsenate species transformation by (hyper)thermophilic prokaryotes.