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Task 3 - Biogeochemical cycle of metals in permafrost thaw lakes: inventory of phases, species & reactions
Task 3b - Partner in charge: Oleg Pokrovsky (GET-Toulouse, partner 3)
Organometal molecules in water and sediments (inorganic/organic-Se cpds, volatile Se (S, Hg)
cpds), metals bond to reactive OM, inorganic speciation and bio-availability of metals in soils and
sediments by selective leaching - (D. Amouroux; E Tessier; M. Bueno; O. Pokrovsky; C. Cloquet)
Dissolved and Gaseous Inorganic or organic Se speciation: The chemical speciation of Se is a critical
prerequisite in assessing selenium environmental reactivity as the bioavailability and toxicity of Se are
greatly affected by its chemical forms. In natural environments, selenium can occur in four oxidation
states (-II, selenide; 0, elemental selenium; +IV, selenite; +VI, selenate) and in a variety of inorganic and
organic compounds. The organically bound Se(-II) compounds include seleno-amino acids and volatile
forms dimethylselenide (DMSe), dimethyldiselenide (DMDSe)…. Typical Se concentrations in natural
waters are generally below 1 μg(Se)/L and may be highly variable depending on geochemical background
and/or anthropogenic activities. Inorganic species, including selenite Se(IV) and selenate Se(VI), are the
major species in natural waters, and therefore selenium speciation is frequently limited to these two
species. Other dissolved Se species, operationally-defined as "organic Se" fraction, occur at lower
concentrations than Se oxyanions. The low levels of volatile selenium species can be quantified by
sensitive analytical techniques like "purge and trap" followed by GC-QICP-MS (Amouroux and Donard,
1997; Amouroux et al., 1998; Pecheyran et al., 1998). Water samples will be collected at most sites.
Determination of dissolved inorganic and organic Se species: The most suitable and sensitive technique
for selenium speciation analysis is liquid chromatographic separation coupled to inductively coupled
plasma mass spectrometry (QICP-MS) detection as it is capable of distinguishing different chemical forms
without redox pretreatments which secures species conservation. As Se concentration levels to measure in
natural waters are very low (ng(Se)/L range), a preconcentration step before sample analysis is also
necessary. On-line simultaneous preconcentration of inorganic and organic Se species was developed on
porous graphitic carbon stationary phase using heptafluorobutyric acid as injection medium, inducing an
enrichment of solutes at the top of the column which allowed large volumes (up to 1 mL) to be injected.
Combining these injection conditions and 78Se monitoring with QICP-CRC-MS, detection limits between
2 and 8 ng (Se)/L, depending on the species (SeIV, SeVI, SeMet, SeCyst), are obtained (Dauthieu et al.
2006). Because of the extremely low detection limits obtained, the method was successfully applied to
freshwater lake samples showing selenate concentration decrease with depth (102 (-1m) to 44 ng/L (-
36m)) and at the opposite an increase of selenite concentration with depth (14 (-1m) to 33 ng/L (-36m)).
In all analyzed samples the total Se concentration was higher than the sum of SeIV and SeVI, indicative of
the contribution of organic dissolved Se. Another selenium containing compound was indeed detected in
some samples (-20 to -1 m depth) which chromatographic retention time did not match with the one of
available organoselenium standards. Same analytical method can be used for the determination of
selenium species in "water extractable" fraction of soils and/or sediments (Tolu et al. 2011, Ponce de Leon
et al., 2003). Sampling and analyses will be performed by partner 4.
Determination of dissolved gaseous methylated Se (and Hg) species: For natural freshwater samples,
volatile Se species as well as dimethylmercury can be easily purged and trapped in the field, using
cryogenic preconcentration technique or adsorption on activated charcoal columns. The water vapor is
removed from the gas stream using a moisture trap (-20°C) and the volatile derivates are trapped at -
196°C onto a cryogenic trap or at ambient temperature on activated charcoal adsorbent, respectively. In
the laboratory, the field traps are thermodesorbed and analysed using a multi-element analytical method,
involving cryogenic trapping, gas chromatography and inductively coupled plasma-mass spectrometry
(CT-GC-QICP-MS). This technique allows to perform simultaneous speciation of dissolved volatile
organometal(loid) compounds of several trace elements such as Hg and Se (Amouroux et al., 1998, 2000;
Tessier et al., 2002). Water samples will be collected in surface and bottom waters among observed redox
gradient and diurnal cycle using a gas-tight Go-Flo sampler. A bulk fraction will be directly purged with
pure nitrogen gas in the field to collect gaseous species on adsorbent traps, while dissolved non-volatilespecies will be stored in PE bottle at 4°C after field filtration (0.45 μm). Gaseous and dissolved samples
will be analyzed within 15 days after the field campaign. For soil and sediment samples, the
bioavailability of Se will be evaluated through the water soluble fraction (solid/water extraction over 24h
at room temperature). Water tested for such extraction will be milli-Q, and waters from studied sites, i.e.
from lakes or "melted snow". Sampling and analyses will be performed by partner 4.
Other metal speciation: For water samples, ultrafiltration (100, 10 and 1 kDa Amicon YM Regenerated
cellulose filters installed in a portable stirred ultrafiltration cell "Amicon 8050" having 50 mL volume)
and dialysis experiments (1 and 10 kDa membranes) will be performed within one day after sampling (e.g.
Pokrovsky et al 2010). Metal speciation in solution will be assessed via computer modeling using
available codes (WHAM 6, vMINTEQ) based on results of 0.22 μm and 1 kDa fractions analysis. These
preparation, analyses and calculations will be performed by partner 3.
Bio-availability of metals: The bio-availability of metals will be estimated in different ways: 1- for
dissolved species, the bio-availability assay will be conducted using purified cultures of cyanobacteria and
heterotrophic bacteria separated from the thermokarst lake water column using sterile filtered lake water
with added different metal concentration (partner 3); 2- the phyto-availability of metals (Ni, Zn, Hg, Se) in
soils (surface active zone) surrounding the studied lakes will be determined using the stable isotope spike
equilibration technique as documented by Sterckeman et al. (2009) for Cd for various types of soils.
Metals spikes are available at CRPG (partner 2) and in the process of being calibrated. 3- metal
concentrations will be determined in vegetation (grass, small trees, berries, ...) surrounding the thaw lakes
in order to estimate the flux of metals transferring to vegetation in comparison to the available stock in
soils. Interpretations will use results from tasks 2c and 3c. Point 2 and 3 will be performed by partner 2.
Risks: The main risks here are potential technical problems in the field for sampling S-Se-Hg dissolved
gas species although partner 4 has a strong experience for such work. Another risk resides in the fact that
some species might be in a lower abundance than our detection limits. Information of these ultra low
concentrations would be also an information used for assessing metal speciation and potential toxicity.