State-of-the-art regarding selenium geochemistry under Boom Clay conditions

Selenium (Se) is a group VII element whose general chemistry resembles that of sulfur, i.e. there are four major valence states (VI, IV, 0, -II). Its environmental speciation can be quite complex with a variety of both inorganic and organic species occurring within its biogeochemical cycle. In this presentation, the state-of-the-art regarding Se geochemistry (speciation and mobility) under Boom Clay conditions is clarified and remaining uncertainties are discussed. The major underlying message of the presentation can be summarised as follows: when conducting lab-scale tests to elucidate the Se geochemistry, the observed processes and kinetics and the speciation after a certain equilibration time depend heavily on the initial Se speciation. The question is therefore raised whether PA has to account for these different slow processes (kinetic approach), or should adhere to the thermodynamics alone. The inorganic SeO42- species is only very weakly adsorbed on available surfaces (illite, pyrite), especially in the circumneutral pH range. Its mobility is therefore almost unretarded and only limited by possible reduction to less soluble forms. However, there are strong indications that except for selected phases typical for the near-field, such as green rust, this reduction can only be achieved through a microbial pathway. Therefore, this species might persist for an unspecified amount of time in a repository system. Selenium(IV) is a mildly-oxidized form of selenium and subject to a lot more geochemical mechanisms upon release into the environment. Its adsorption as a function of pH, Se concentration and electrolyte background concentration on conditioned illite was experimentally measured and can be modelled with the 2SPNE SC/CE approach [1] using inner-sphere surface complexes. Moreover, it is much more easily subjected to redox reactions than Se(VI): inorganic Fe(II)-bearing surfaces such as pyrite (FeS2) and organic molecules such as ascorbic acid and –probably- natural organic matter can cause reduction (mostly to selenium(0), as is demonstrated by XAS). The reduction of Se(IV) is known to exhibit somewhat slow kinetics, but is very rapid on repository lifetime scale. In more complex environments such as Boom Clay, the variety of mechanisms to which Se(IV) is exposed, can lead to a competition for Se between different processes and geochemical phases. Selenium(0) is an insoluble form which is stable in mildly reducing conditions and is formed in most lab-scale systems upon reduction of Se(IV) by geochemical phases typical for an underground repository. It exists in several allotropic forms but only for the stable, trigonal (grey) Se(0) some thermodynamic data are available. Solubilities for amorphous Se(0) in synthetic groundwater are operationally determined and are situated around 2-3×10-8 mol•l-1. There are also strong indications that this species interacts with dissolved organic matter, which might increase its solubility under repository conditions. Finally, selenium(-II) is stable in strongly reducing conditions and its solubility is limited by very insoluble metal (e.g. iron) precipitates. Se(-II) formation is thermodynamically favoured under most undergorund repository conditions but up to now, very little evidence exists that it is actually formed as a reaction product following the reduction of more oxidised Se formsstatus: publishe