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Britta Planer-Friedrich
Dissertation, 2004

To understand the behavior of metals and metalloids in the environment, speciation is one of the most important requirements because occurrence, sorption, mobility, potential assimilation, and toxicity depend significantly on the individual species. The release of As to groundwater in Bangladesh by reduction of As(V) to As(III) is a well-known example for the complexity and possible consequences of species conversions. Yet, inorganic As(III) and As(V) are not the only important As species. Trivalent and pentavalent organic (methylated) species act as connecting links for the transfer between hydrosphere and biosphere. The most reduced forms of organic and inorganic As species are volatile and, thus, enable transport from the hydrosphere to the atmosphere. They are also the most toxic As species known. Their accidental release has been documented as early as in the mid of the 19th century. Species-selective determination methods, however, have not been introduced until the early 1970s with the invention of the hydride generation technique. Arsenic biomethylation pathways in different organisms have been investigated, but reports about the occurrence of dissolved methylated and especially volatile As species in aquatic environments are still rare. Quantification has seldom been achieved and several compounds such as volatile sulfur- or chloroarsines have not even been qualitatively identified in nature, yet.

A major requirement for identifying volatile As compounds in the environment is a simple standardized sampling procedure that guarantees long-term stability of the sample. In the present research work a robust method for in-situ sampling of volatile metallics from aqueous environments was developed. Efficient gas-water separation is achieved with a PTFE membrane (porosity 0.1 µm) inside a PTFE collector cell that can be placed at the sediment-water interface or in the aqueous body itself. Vacuum up to -500 mbar can be applied to force the gases to a trapping system of either liquid or solid sorbents. As liquid sorbent a 1:100 diluted NaOCl solution in a PTFE bottle is used. PTFE rings in the oxidizing bottle increase the reaction surface and thus trapping efficiency that was 84% ± 6.6 for As in the laboratory. The build-up of a high pressure from the intense gas evolution during hydride generation might be responsible for the loss of some volatile As, especially at the beginning of the reaction. Quantitative evaluation of volatile metallics concentrations is achieved by referring the trapped mass in the oxidizing solution [µg] to the replaced vacuum volume [m3].

Besides this fast screening method on total volatile metallics concentrations, two species-selective sampling techniques were investigated for As. Solid phase micro extraction (SPME) fibers enabled the detection of more volatile As species in lower concentrations than solid sorbent tubes. For SPME fibers, selective sorption was observed dependent on the volatile As species. Volatile chloro-arsines (CH3)AsCl2 and (CH3)2AsCl with larger molecular weights were sorbed on polydimethylsiloxane fibers whereas polydimethylsiloxane fibers with Carboxen and divinylbenzene coating proved best for MMA, DMA, and TMA. The fibers with only Carboxen coating showed by far the largest peak areas of all fibers for DMA, TMA, and (CH3)2AsCl. The most effective sorption material in sorption tubes was also Carboxen but only TMA and traces of (CH3)2AsCl were detected. Probably the selected Carboxen type (CAR 564) is not the optimum fit yet. The volatile As species were stable on the SPME fibers for at least 3 days when wrapped in aluminum foil and stored at 4°C in the refrigerator. Quantification could not be achieved mainly because of unknown sorption affinities of the individual arsines, non-availability of calibration standards, and potential competitive sorption with other, unknown gases.

As an example for an aqueous environment predestined for the release of volatile metallics Yellowstone National Park was investigated. It is located over the largest continental hot spot world wide and contains more than 10,000 geothermal features. After the first detection of volatile As during a reconnaissance study in 2002, 5 study areas (Nymph Lake, Hazle Lake, Ragged Hills, Gibbon Geyser Basin, and Lower Geyser Basin) were sampled for their water and gas chemistry. Sulfatedominated waters with low pH and As concentrations between 20 and 2,000 µg/L were differentiated from chloride-dominated waters with near neutral pH and As concentrations between 1,050 and 11,000 µg/L. The precipitation of As minerals was thermodynamically calculated for Hazle Lake (orpiment supersaturation) and actually observed for a low temperature hot spring at Ragged Hills (precipitation of an amorphous Fe, S, and As rich material).

Volatile As was found in all samples besides volatile Al, B, Ba, Cu, Fe, K, Li, Si, Sr, Zn, and S. By species-selective sampling on SPME fibers, four volatile As species were detected: (CH3)3As, (CH3)AsCl2, (CH3)2AsCl, and (CH3)2AsSCH3. The latter three were all found for the first time in a natural environment. Modeling of the gaseous As species proved impossible because of a lack of thermodynamic constants for the species detected. The volatile inorganic As, As-S, As-F, and As-Cl species for which thermodynamic data were available showed negligible partial pressures of <10-11 Vol%. Total concentrations in the trapping solution from passive, diffusion based sampling were converted to concentrations in gas phase using Fick´s first law. Medium concentrations were 18 mg/m3 for sulfate-dominated waters and 50 mg/m3 for the chloride-dominated ones. Concentrations from active gas sampling by pumping were lower (20-100 µg/m3) but are supposed to be subject to significant dilution by withdrawing ambient air from over-pumping. The detected concentrations are significantly increased compared to the acute toxicity limit for 1 hour exposure (160 µg/m3) with respect to AsH3. However AsH3 is probably not the predominant volatile species. Even though concentrations decrease rapidly with increasing distance from a geothermal feature animals grazing and warming up at hot springs might be exposed to significant amounts of volatile metallics, particularly As.

Further research on volatile metallics in aquatic environments should focus on quantification of individual volatile species, on their chemical or microbial origin, their distribution, and their toxicity potential.

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