| The mobility of Molybdenum (Mo) in soils and sediments under well aerated or anoxic conditions depends on several factors, including soil mineralogy and the presence of competitive oxyanions. Batch experiments addressing the adsorption of two major Mo species, molybdate (MoO42-) and tetrathiomolybdate (MoS42-), on two iron minerals, pyrite (FeS2) and goethite (FeOOH), were performed to elucidate the possible mechanisms of molybdenum immobilization in anoxic sediments. Suspensions of MoS42- (or MoO4 2-) and goethite (or pyrite) in UM 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 MoO 42- and MoS42- by pyrite and goethite were also addressed. The Charge Distribution Multi Site Complexation (CD-MUSIC) model was used to predict competitive adsorption of MoO4 2- and other anions (i.e., phosphate, sulfate, silicate and tungstate) using model parameters obtained from fitting single ion adsorption envelopes on goethite. Finally, the competitive interaction for the adsorption of tungstate, molybdate, and phosphate were investigated by estimating the values of the competitive coefficients obtained using the Sheindorf-Rebuhn-Sheintuch (SRS) equation.; The adsorption of MoO42- and MoS4 2- on pyrite and goethite is well described by the Langmuir model at low pH, the extent of sorption is a function of pH and the surface loading. Maximum sorption is observed under acidic conditions (pH<5) at low surface loading. The adsorption of molybdenum (mumol g-1) depends on predominant Mo (VI) species and on the type of iron mineral and follows the order: MoS42----goethite > MoO4 2- -goethite > MoS42----pyrite > MoO 42----pyrite. Phosphate appears to compete strongly with MoO42- and MoS42- for the sorption sites at the pyrite and goethite surfaces. The strength of the phosphate competitive effect follows the sequence: MoO4 2----goethite ∼ MoO42---pyrite > MoS 42----pyrite > MoS42----goethite. Silicate and sulfate have a negligible effect on the sorption of MoO 42- and MoS42-. The preferred adsorption by iron minerals of MoS42-, as well as its behavior in the presence of competitive anions, suggests that tetrathiomolybdate species may be an ultimate depository and may control Mo (VI) enrichment in the sediments.; CD-MUSIC modeling results fairly well describe the experimental adsorption envelopes of molybdate over the pH range 3.5 to 10, furthermore, model predictions of the molybdate adsorption isotherm are satisfactory. Modeling results suggest that the diprotonated monodentate complexes, FeOW(OH)5-0.5 and FeOMo(OH)5-0.5, were the dominant complexes of adsorbed W (VI) and Mo (VI), respectively, to goethite 110 faces at low pH.; The interactions of MoO42- + WO4 2-, MoO42- + PO43-, and WO42- + PO43- solutions with goethite were studied to better understand the competitive sorption of these anions in natural environments. Experimental results indicate that tungstate has strong adsorption affinity for goethite. Molybdate adsorption onto goethite is reversible whereas tungstate and phosphate adsorption are irreversible. The SRS equation shows that tungstate and phosphate have a comparable competitive effect, and both may significantly compete with Mo (VI) adsorption. The experimental results also suggest that goethite could be used effectively as a natural adsorbent to remove phosphate over large pH ranges, whereas tungstate may be effectively removed at pH 5 from tungstate contaminated streams. |
