Adsorption of molybdate and tetrathiomolybdate onto pyrite and goethite: effect of pH and competitive anions.
The adsorption of two major molybdenum (Mo) species, molybdate (MoO4(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 MoO4(2-)) and goethite (or pyrite) in 0.1M 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 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 (micromol 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 > MoO4(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 approximately = MoO4(2-)-pyrite > MoS4(2-)-pyrite > MoS4(2-)-goethite. Silicate and sulfate have a negligible effect on the sorption of MoO4(2-) and MoS4(2-). The preferred adsorption by iron mineral of MoS4(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.
Xu
Nan
N
Center for Environmental Systems, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, NJ 07030, USA.
Christodoulatos
Christos
C
Braida
Washington
W
D000327:Adsorption; D000838:Anions / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000737:chemistry*; D058085:Iron Compounds / Q000737:chemistry*; D008903:Minerals; D008982:Molybdenum / Q000032:analysis*; D013440:Sulfides / Q000737:chemistry*; D013499:Surface Properties
D000838:Anions; D058085:Iron Compounds; D008903:Minerals; D013440:Sulfides; C094886:goethite; C011342:pyrite; C044659:molybdate; D008982:Molybdenum; C020809:tetrathiomolybdate; D007501:Iron