Polyselenylsulfides as neglected central intermediates in Selenium cycling.

Area of Science:

• Environmental Chemistry
• Geochemistry
• Biogeochemistry

Background:

• Selenium (Se) is an essential trace element with a narrow optimal nutritional range, and its environmental behavior is dictated by speciation.
• Environmental cycling of selenium is traditionally viewed as redox processes involving specific Se species, with elemental Se (Se⁰) as a central, favored species.
• The environmental fate and mobility of selenium are significantly influenced by its chemical form and interactions with other elements.

Purpose of the Study:

• To identify and characterize novel dissolved selenium species formed from the reaction of elemental selenium with sulfide.
• To investigate the formation conditions and kinetics of these newly discovered selenium-sulfur species.
• To reassess the environmental cycling of selenium in sulfidic environments and challenge existing models.

Main Methods:

• Utilized derivatization coupled with ultrahigh-performance liquid chromatography electrospray ionization quadrupole time-of-flight mass spectrometry (UHPLC-ESI-QTOF-MS).
• Analyzed reaction products of elemental selenium with sulfide under varying pH conditions.
• Identified and quantified polysulfides, selenide, polyselenides, and novel polyselenylsulfides.

Main Results:

• Identified a novel group of dissolved polyselenylsulfide species (general formula Se_xS_y^2-), including SeS^2-, SeS₂^2-, Se₂S^2-, and SeS₃^2-.
• Demonstrated that polyselenylsulfides form spontaneously and rapidly (<24 h) from the reaction of elemental selenium with sulfide at alkaline pH, and slower at circumneutral pH.
• Detected polysulfides (S_x^2-), selenide (Se^2-), and polyselenides (Se_x^2-) alongside the novel polyselenylsulfides.

Conclusions:

• The spontaneous formation of polyselenylsulfides in sulfidic environments necessitates a revision of selenium's environmental fate and cycling.
• Challenges the traditional 'selenocentric' view by highlighting the interconnectedness of selenium and sulfur cycles through polyselenylsulfide formation.
• Proposes a 'chalcogenic' perspective, emphasizing that selenium's environmental behavior is strongly governed by the presence of sulfide, questioning elemental selenium's role as a terminal species.