CAPTURE OF MOLYBDENUM IN PYRITE-FORMING SEDIMENTS: ROLE OF LIGAND-INDUCED REDUCTION BY POLYSULFIDES

Abstract

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 MoVI must occur to stabilize these structures. Sulfide alone is a poor reductant for Mo, producing instead a series of MoVI thioanions (MoOxS4-x2-, x = 0-3). In solutions that contain both H2S and S0-donors (i.e. polysulfides; dissolved S8), Mo is transformed to MoIV or MoV2 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 MoVI 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 MoOS32- and MoS42- weakly at mildly alkaline pH but can scavenge an unidentified product of the S0-induced reduction of MoOS32- 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).