Invited Keynote Presentation 'Interaction of selenite with iron sulphides: a new perspective'

The geochemistry of selenium, exhibiting valence states from +VI to –II, is of key importance due to its role as a highly toxic essential micronutrient and as a significant component of high level radioactive waste (HLRW). XAS studies conducted at circum-neutral pH have shown that pyrite (FeS2 ), the most relevant redox-active mineral in Boom clay, reduces selenite to a solid-state Se(0) phase. This observation raises several questions. First, why does an Fe-free Se(0) phase form in presence of pyrite, while selenite is reduced to FeSex by troilite and mackinawite (FeS)?[1-4]. What is the exact identity of this Se(0) phase, which has been observed by several authors? Why is a dissolved, low oxidation-state selenium species encountered in association with the Se(0) phase; and what is its identity? Correlating selenium redox chemistry with sulphide mineral oxidation pathways allowed to link these observations to the different oxidation behaviour of acid-soluble and acid-insoluble metal sulphides [5]. Acid insoluble metal sulphides such as pyrite, molybdenite or tungstenite exhibit oxidative dissolution only. Upon six consequent one-electron oxidation steps, a thiosulphate anion is liberated (thiosulphate pathway). In contrast, acid soluble metal sulphides (troilite, mackinawite, sphalerite, etc.) exhibit also non-oxidative dissolution thereby liberating sulphide species (H2 oxidative dissolution in presence of FeIII cations (e.g. H2S+ S, HS- disulphide species, which may further to ,S2- ). Under ). The latter can spontaneously dimerize react (polysulphide pathway) and finally elemental sulphur. The end products of Se(IV) reduction by acid-soluble 2- , they release sulphide into polysulphide iron sulphur minerals are fairly well known, but the solid and liquid phase species present during interaction of SeO3 characterized. The solid phase reaction product could not yet be assigned as a specific phase, but clearly identified as a Se0 compound. Trigonal (grey) selenium could be excluded as a canditate.[4] The presence of an unexpected high concentration of reduced, dissolved species in presence of pyrite, led to a new pyrite-centered reduction mechanism. Based on this mechanism, a hypothesis about the identity of the unknown dissolved species was put forward. In addition, the new mechanism explains all current experimental observations, especially the presence of the currently non-identified dissolved species and the unexpected relation between Se(IV) reduction and pH.[6] [1] Breynaert, et al. (2008) ES&T. 42(10): 3595-3601. [2] Scheinost and Charlet, L. (2008) ES&T. 42(6): 1984-1989. [3] Scheinost, et al. (2008) J. Contam. Hydrol. 102(3-4): 228-245. [4] Breynaert, et al. (2010) ES&T. 44(17): 6649-6655. [5] Rohwerder and Sand (2007) in Microbial Processing of Metal Sulfides.p 35-58. [6] Kang, et al. (2011) ES&T. 45: 2704-2710Invited keynote lecturestatus: publishe