Capture of molybdenum in pyrite-forming sediments: Role of ligand-induced reduction by polysulfides

Capture of Mo by FeS₂ is an important sink for marine Mo. X-ray spectroscopy has shown that Mo forms Mo-Fe-S cuboidal clusters on pyrite. Reduction of Mo^VI must occur to stabilize these structures. Sulfide alone is a poor reductant for Mo, producing instead a series of Mo^VI thioanions (MoO_xS_4-x^2-, x = 0-3). In solutions that contain both H₂S and S⁰-donors (i.e. polysulfides; dissolved S₈), Mo is transformed to Mo^IV or Mo^V₂ polysulfide/sulfide anions. This intramolecular reduction requires no external reducing agent. Remarkably, an oxidizing agent (S⁰ donor), rather than a reducing agent, stabilizes the reducible Mo^VI complex. Thiomolybdates and their reduction products do not precipitate spontaneously; solutions supersaturated by 10⁹ with respect to molybdenite, MoS₂, produce no precipitate in 40 days. In 10-minute exposures, pyrite can scavenge MoOS₃ ^2- and MoS₄^2- weakly at mildly alkaline pH but can scavenge an unidentified product of the S⁰-induced reduction of MoOS₃^2- very strongly. On the basis of these observations, a reaction pathway for Mo capture by pyrite is proposed. Conditions that favor Mo capture by this pathway also favor pyrite growth. Ascribing Mo capture simply to low redox potential is too simplistic and neglects the likely role of oxidizing S⁰-donors. The aqueous speciation of Mo in anoxic environments will be a function of the activity of zero-valent sulfur as well as the activity of H₂S(aq).