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

The adsorption of two major molybdenum (Mo) species, molybdate (MoO ₄^2-) and tetrathiomolybdate (MoS₄^2-) 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 MoS₄^2- (or MoO ₄^2-) 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 MoO₄^2- and MoS₄^2- by pyrite and goethite are also addressed. Adsorption of MoO₄^2- and MoS₄^2- 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: MoS₄^2- - goethite>MoO₄ ^2- - goethite>MoS₄^2- - pyrite>MoO ₄^2- - pyrite. Phosphate appears to compete strongly with MoO₄^2- and MoS₄^2- for the sorption sites of pyrite and goethite. The strength of the phosphate competitive effect follows the sequence of MoO₄^2- - goethite∼MoO ₄^2- - pyrite>MoS₄^2- - pyrite>MoS₄^2- - goethite. Silicate and sulfate have a negligible effect on the sorption of MoO₄^2- and MoS ₄^2-. The preferred adsorption by iron mineral of MoS ₄^2-, 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.