Capture of molybdenum in pyrite-forming sediments: role of ligand-induced reduction by polysulfides 1

Details Capture of molybdenum in pyrite-forming sediments: role of ligand-induced reduction by polysulfides 1 Capture of molybdenum in pyrite-forming sediments: role of ligand-induced reduction by polysulfides 1

Feb 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004gecoa..68..547v&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 68, Issue 3, pp.547-556

Computer Science

12

Scientific paper

Capture of Mo by FeS 2 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 x S 4-x 2- , X = 0-3). In solutions that contain both H 2 S and S 0 -donors (i.e. polysulfides; dissolved S 8 ), Mo is transformed to Mo IV or Mo V 2 polysulfide/sulfide anions. This intramolecular reduction requires no external reducing agent. Remarkably, an oxidizing agent (S 0 donor), rather than a reducing agent, stabilizes the reducible Mo VI complex. Thiomolybdates and their reduction products do not precipitate spontaneously; solutions supersaturated by 10 9 with respect to molybdenite, MoS 2 , produce no precipitate in 40 days. In 10-minute exposures, pyrite can scavenge MoOS 3 2- and MoS 4 2- weakly at mildly alkaline pH but can scavenge an unidentified product of the S 0 -induced reduction of MoOS 3 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 S 0 -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 H 2 S(aq).

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