Adsorption of molybdate and tetrathiomolybdate onto pyrite and goethite: Effect of pH and competitive anions

The adsorption of two major molybdenum (Mo) species, molybdate (MoO42-) and tetrathiomolybdate (MoS42-) onto two main iron minerals pyrite (FeS2) and goethite (FeOOH) is addressed to elucidate the possible mechanisms of molybdenum immobilization in anoxic sediments. Suspensions of MoS42- (or MoO42-) and goethite (or pyrite) in 0.1 M NaCl solution were equilibrated under anoxic conditions at 25 °C in the pH range from 3 to 10. The competitive effects of sulfate, phosphate, and silicate on the adsorption of MoO42- and MoS42- by pyrite and goethite are also addressed. Adsorption of MoO42- and MoS42- on pyrite and goethite is in general well described by a Langmuir model at low pH; the extent of sorption is a function of pH and the surface loading. Maximum sorption is observed in the acidic pH range (pH < 5) at low surface loading. The adsorption of molybdenum (μmol g−1) depends upon Mo species and on the type of iron mineral following the order: MoS42-–goethite>MoO42-–goethite>MoS42-–pyrite>MoO42-–pyrite. Phosphate appears to compete strongly with MoO42- and MoS42- for the sorption sites of pyrite and goethite. The strength of the phosphate competitive effect follows the sequence of MoO42-–goethite∼MoO42-–pyrite>MoS42-–pyrite>MoS42-–goethite. Silicate and sulfate have a negligible effect on the sorption of MoO42- and MoS42-. The preferred adsorption by iron mineral of MoS42-, as well as its behavior in the presence of competitive anions suggests that tetrathiomolybdate species may be an ultimate reservoir and may control Mo enrichment in the sediments.