Polyselenylsulfides as neglected central intermediates in Selenium cycling

Selenium (Se) is an essential yet toxic trace element; it has one of the narrowest nutritional optimums of all elements. The speciation of Se significantly influences its mobility, toxicity, bioavailability, and bioaccumulation. Se environmental cycling 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. This study reports the identification of a novel group of dissolved species called polyselenylsulfides (with the general formula Se_xS_y^2-). 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, polysulfides (S_x^2-, x = 2-6), selenide (Se^2-), polyselenides (Se_x^2-, x = 2-3), and previously undetected polyselenylsulfides (SeS^2-, SeS₂^2-, Se₂S^2-, and SeS₃^2-) were identified. 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 that only involve Se species. Moreover, the perception of Se⁰ as a terminal, insoluble species in many anoxic environments is questioned. The adoption of a 'chalcogenic' perspective is proposed, wherein the selenium and sulfur cycles are closely interconnected and the fate of Se is governed by the presence of sulfide.