Quantification of Methylated Selenium, Sulfur, and Arsenic in the Environment

Biomethylation and volatilization of trace elements may contribute to their redistribution in the environment. However, quantification of volatile, methylated species in the environment is complicated by a lack of straightforward and field-deployable air sampling methods that preserve element speciation. This paper presents a robust and versatile gas trapping method for the simultaneous preconcentration of volatile selenium (Se), sulfur (S), and arsenic (As) species. Using HPLC-HR-ICP-MS and ESI-MS/MS analyses, we demonstrate that volatile Se and S species efficiently transform into specific non-volatile compounds during trapping, which enables the deduction of the original gaseous speciation. With minor adaptations, the presented HPLC-HR-ICP-MS method also allows for the quantification of 13 non-volatile methylated species and oxyanions of Se, S, and As in natural waters. Application of these methods in a peatland indicated that, at the selected sites, fluxes varied between 190–210 ng Se·m-2·d-1, 90–270 ng As·m-2·d-1, and 4–14 µg S·m-2·d-1, and contained at least 70% methylated Se and S species. In the surface water, methylated species were particularly abundant for As (>50% of total As). Our results indicate that methylation plays a significant role in the biogeochemical cycles of these element

Glass polymorphism in glycerol–water mixtures: I. A computer simulation study

We perform out-of-equilibrium molecular dynamics (MD) simulations of water–glycerol mixtures in the glass state. Specifically, we study the transformations between low-density (LDA) and high-density amorphous (HDA) forms of these mixtures induced by compression/decompression at constant temperature. Our MD simulations reproduce qualitatively the density changes observed in experiments. Specifically, the LDA–HDA transformation becomes (i) smoother and (ii) the hysteresis in a compression/ decompression cycle decreases as T and/or glycerol content increase. This is surprising given the fast compression/decompression rates (relative to experiments) accessible in MD simulations. We study mixtures with glycerol molar concentration wg = 0–13% and find that, for the present mixture models and rates, the LDA–HDA transformation is detectable up to wg E 5%. As the concentration increases, the density of the starting glass (i.e., LDA at approximately wg r 5%) rapidly increases while, instead, the density of HDA remains practically constant. Accordingly, the LDA state and hence glass polymorphism become inaccessible for glassy mixtures with approximately wg 4 5%. We present an analysis of the molecular-level changes underlying the LDA–HDA transformation. As observed in pure glassy water, during the LDA-to- HDA transformation, water molecules within the mixture approach each other, moving from the second to the first hydration shell and filling the first interstitial shell of water molecules. Interestingly, similar changes also occur around glycerol OH groups. It follows that glycerol OH groups contribute to the density increase during the LDA–HDA transformation. An analysis of the hydrogen bond (HB)-network of the mixtures shows that the LDA–HDA transformation is accompanied by minor changes in the number of HBs of water and glycerol. Instead, large changes in glycerol and water coordination numbers occur. We also perform a detailed analysis of the effects that the glycerol force field (FF) has on our results. By comparing MD simulations using two different glycerol models, we find that glycerol conformations indeed depend on the FF employed. Yet, the thermodynamic and microscopic mechanisms accompanying the LDA–HDA transformation and hence, our main results, do not. This work is accompanied by an experimental report where we study the glass polymorphism in glycerol–water mixtures prepared by isobaric cooling at 1 ba

Relationship between the efficiency of chemotrapping in nitric acid and boiling points of the studied volatile compounds.

<p>Error bars indicate the standard deviation of the measurements of triplicate samples.</p

Overview of the experimental set-ups for gas trapping experiments in the laboratory and in the field.

<p>(A) Schematic of the experimental set-up for the laboratory gas trapping experiments, with the separate in-situ production of volatile methylated As species in a gas-tight reaction vessel (left) and direct introduction of volatile methylated Se and S species (right), connected to (B), a set of glass impingers filled with concentrated nitric acid and <b>c</b>, schematic of the experimental set-up for the field gas trapping experiments, which consists of a flow-through box equipped with an air pump connected to the set of glass impingers (B). During field application, one impinger was connected to one flow-through box and the flow-through boxes were deployed in triplicate.</p

Studied volatile species, including their structure and boiling points, calculated total trapping efficiencies, and observed reactions products and structures after trapping and transformation in concentrated nitric acid.

a<p>Boiling point at 1 atm ^befficiency using a 30 mL·min⁻¹ N₂ gas flow, summed over three impingers, standard deviation from triplicate experiments.</p><p>Abbreviations: dimethyl selenide (DMSe), dimethyl diselenide (DMDSe), dimethyl sulfide (DMS), dimethyl disulfide (DMDS), monomethyl arsine (MMA), dimethyl arsine (DMA), trimethyl arsine (TMA), dimethyl selenoxide (DMSeO), methane seleninic acid (MSeA), dimethyl sulfoxide (DMSO), methane sulfonic acid (MSA), arsenate (As[V]), monomethyl arsonic acid (MMAA).</p

Chromatograms of trapping liquid- and surface water samples collected at Gola di Lago.

<p>(A) Stacked chromatogram of the gas trapping liquid sample 1 from Gola di Lago for Se (top), S (middle), and As (bottom) and (B) Stacked chromatogram of the surface water sample 1 from Gola di Lago for Se (top), S (middle), and As (bottom). Chromatograms for blanks are indicated by dashed lines. All chromatograms are five-point moving averages, and the identified compounds and their molar concentrations (on an elemental basis) are indicated at the corresponding peaks. Details of both methods are given in Table S3 in Supporting Information File S1.</p