Polyselenylsulfides as neglected central intermediates in Selenium cycling

Overview & Synopsis

Selenium (Se) is an essential yet toxic trace element with one of the narrowest nutritional optimums of all elements. The speciation of Se significantly influences its mobility, bioavailability, bioaccumulation, and toxicity. Se cycling in the environment has been generally understood as a sequence of bidirectional redox processes of Se species. Elemental Se (Se0) is considered a central species that is thermodynamically favoured in the redox conditions found in most environments. The identification of a novel group of dissolved species, polyselenylsulfides (with the general formula SexSy2-), is reported in this work. These species form upon the reaction of Se0 with sulfide. Using derivatisation and ultrahigh-performance liquid chromatography electrospray ionisation quadrupole time-of-flight mass spectrometry (UHPLC–ESI–qTOF–MS), polysulfides (Sx2-, x = 2–6), selenide (Se2-), polyselenides (Sex2-, x = 2–3), and previously undetected polyselenylsulfides (SeS2-, SeS22-, Se2S2-, and SeS32-) were detected. The reaction was spontaneous and proceeded rapidly at alkaline pH (completed within <24 h). A slower and incomplete reaction was observed at circumneutral pH. The fast and spontaneous reaction down to circumneutral pH suggests the ubiquitous importance of polyselenylsulfides in sulfidic environments and calls for revising the environmental fate of Se. The existence of polyselenylsulfides challenges the traditional 'selenocentric' view, which focuses on specific, bidirectional redox reactions involving Se species only. Moreover, the perception of Se0 as a terminal, insoluble species in many anoxic environments is questioned. Adoption of a "chalcogenic perspective" is proposed, wherein the Se and sulfur (S) cycles are closely interconnected, and the fate of Se is governed by the presence of sulfide.