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

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