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-) ontotwo 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 degrees 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 (mu mol g(-1)) depends upon Mo species and on the type of iron mineral following the order: MoS42--goethite > MoO42--goethite > MoS42--pyritc > 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 similar to MoO42--pyrite > MoS42--pyrite > MoS(4)(2-)goethite. Silicate and sulfate have a negligible effect on the sorption MoO42- and MoS42-. The preferred adsorption by iron mineral of MOS2-, 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. (c) 2005 Elsevier Ltd. All rights reserved.