Differential adsorption of molybdate and tetrathiomolybdate on pyrite (FeS2)

Molybdenum is a nutrient important for a variety of biological functions, most notably nitrogen fixation. Molybdenum availability is limited through sorption reactions, particularly in environments rich in sulfide minerals. This study examines the sorption of two major molybdenum species, molybdate (MoO4(2-)) and tetrathiomolybdate (MoS4(2-)), on synthetic pyrite (FeS2) as a function of solution composition. Both MoO4(2-) and MoS4(2-) partitioned strongly on FeS2 under a range of conditions and ionic strengths. Molybdate and tetrathiomolybdate adsorption obeyed a Langmuir isotherm with a calculated site density between 2 and 3 sites/nm2 under acidic and circumneutral conditions, which decreased to less than 1 site/ nm2 at pH 9. Although both MoO4(2-) and MoS4(2-) adsorbed most strongly under moderately acidic conditions, MoO4(2-) readily desorbed while MoS4(2-) remained adsorbed even at high pH. The reversibility of MoO4(2-) adsorption suggests the formation of labile surface complexes while MoS4(2-) likely forms strong inner-sphere complexes. X-ray absorption spectroscopy was used to determine the structure of the surface complexes. Molybdate formed bidentate, mononuclear complexes on FeS2. The Mo-S and Mo-Fe distances for tetrathiomolybdate on pyrite are consistent with the formation of Mo-Fe-S cubane-type clusters. The high affinity of MoS4(2-) for FeS2, as well as its resistance to desorption, supports the hypothesis that thiomolybdate species are the reactive Mo constituents in reduced sediments and may control Mo enrichment in anoxic marine environments.