Molybdate sorption

Wu et al. (2001) found that molybdate sorption was affected insignifically in the presence of equimolar amounts of selenate however, selenate sorption was significantly reduced in the presence of molybdate at pH < 7.0, where a 30% decrease in sorption was noticed (Figure 5.7). 

Sulfate was poor at preventing arsenate and molybdate sorption onto metal oxides and soils (Wu et al., 2001 Violante et al., 2005b), but reduced the sorption of selenate significantly (Wu et al., 2001). The alleviating effect of sulfate application on arsenic toxicity has also been reported (review by Kitagishi and Yamane, 1981). 

Transport of Mo(VI) through laboratory columns packed with alluvium was retarded by sorption on iron oxide coatings. The amount of Mo(VI) sorbed decreased as pH and the ratio of PO4 to Mo(VI) increased. Molybdate sorption could be described by the Freundlich isotherm. 

Stollenwerk, K. G. (1991). Simulation of Molybdate Sorption with the Diffuse Layer Surface-Complexation Model. U.S. Geological Survey water-resources investigations report 91-4034 47-52. 

Xie, R. J., and MacKenzie, A. F. (1991). Molybdate sorption-desorption in soils treated with phosphate. Geoderma 48 321-33. 

Guibal, E., Milot, E. C., and Roussy, J. (2000). Influence of hydrolysis mechanisms on molybdate sorption isotherms using chitosan. Separation Science and Technology 35, 1020-1038. 

The effect of other inorganic anions (sulfate, molybdate, silicate), low molecular mass organic ligands (LMMOLs, such as oxalate, malate, citrate, tartrate and succinate), and fulvic or humic acid on the sorption of arsenate and arsenite onto variable charge minerals and soils has been studied (Roy et al. 1986 Grafe et al. 2001 Liu et al. 2001 Violante et al. 2005a,b). 

Dambies, L., Guibal, E., Rose, A., Arsenic (V) sorption on molybdate-impregnated chitosan beads, Colloids and Surfaces A, 170 (2000), 19-31. 

During the electrolytic preparation of composite cathodes from solutions of Ni or Co salts with molybdate or tungstate, the current efficiency for deposition of the two metals is far from 1(X)%, so cathodic Hj evolution, with codeposition (sorption) of the H intermediate, is unavoidable. Hence it is virtually certain that these composite cathode materials are formed as hydride materials. It was suggested in Ref. (75) that this may be one of the reasons for their excellent electrocatalytic behavior in the HER, in contrast to that of bulk, thermally prepared alloys of the same metals, Ni and Mo. In this respect, hydrided metals may behave like Pt cathodes where the HER proceeds with good electrocatalysis on a full monolayer of UPD H and, under appreciable applied current densities, on a Pt surface region containing apparently some significant quantity of three-dimensionally sorbed H (136). 

A Ni cathode that has become deactivated by impurity deposition or supposed H sorption under conditions of cathodic polarization can be activated (177) by electrodeposition of Mo species from added molybdate in solution, as found in a similar way with Co. The in situ activation is ascribed to formation of a spongy Mo-base deposit ( MoO) on the Ni during the first day of continuous water electrolysis. 

Several inorganic ion exchangers like the zirconium salts of phosphates, silicate phosphates, molybdate phosphates, and tungstate phosphates showed selective sorption properties for potassium dissolved in sea water and brines. The potassium capacity of zirconium phosphate was found to be 25 mg K+/g. The selectivity for potassium increased with higher drying temperatures of the exchangers. The potassium ion sorption rate exceeded that of other cations40). 

Preconditioning of the sodium tungstate solution. The pH is adjusted to 1.2-1.A by electrodialysis (see Section 5.2.4.7.). Electrodialysis is preferred to the addition of acids, because additional anions affect the sorption capacity of the resin and furthermore result in stable tungstate solutions. Subsequently, molybdate is converted into tiiiomolybdate by addition of sodium sulfide solution. 

Laboratory column experiments were used to identify potential rate-controlling mechanisms that could affect transport of molybdate in a natural-gradient tracer test conducted at Cape Cod, Mass. Column-breakthrough curves for molybdate were simulated by using a one-dimensional solute-transport model modified to include four different rate mechanisms equilibrium sorption, rate-controlled sorption, and two side-pore diffusion models. The equilibrium sorption model failed to simulate the experimental data, which indicated the presence of a ratecontrolling mechanism. The rate-controlled sorption model simulated results from one column reasonably well, but could not be applied to five other columns that had different input concentrations of molybdate without changing the reaction-rate constant. One side-pore diffusion model was based on an average side-pore concentration of molybdate (mixed side-pore diffusion) the other on a concentration profile for the overall side-pore depth (profile side-pore diffusion). 

The sorption of TcOj by roots of hydroponically grown soybean seedlings (Glycine max.) was shown to be linear from 10 M pertechnetate solutions for at least 6 h and to exhibit characteristics of carrier-mediated transport commonly associated with the sorption of nutrient ions in higher plants. Analyses of TcO uptake in the presence of individual nutrient anions revealed the sorption to be competitively inhibited by sulphate, phosphate, selenate. and molybdate indicating the use of common transport mechanisms [54],... 

There are some similarities in the chemical behaviors of the molybdate anion (M0O42 ) and the sulfate anion (S04 ). Both are binegative and tetrahedral in structure. They can compete for sorption sites, and in biological systems they are taken up, transported, and excreted along many of the same routes (Haight and Boston, 1973). 

The sorption characteristics of anions, particularly plant-nutrient anions (such as phosphate, sulfate, and molybdate) and toxic elements (such as As and Se), have been extensively studied. Several excellent reviews summarizing anion sorption have been published (Parfitt, 1978 Kingston, 1981 Mott, 1981 Barrow, 1985). Emphasis in both laboratory and field studies has been on the affinity of anions for metal (e.g., Fe, Mn, or Al) oxide phases. 

The sorption of dissolved Mo onto soils has been well studied because of the probable role of sorption in controlling the availability of Mo to plants. Research has focused on defining the characteristics of Mo sorption on soils and soil components and on the competition of anionic plant nutrients, such as phosphate, sulfate, and molybdate, for surface sites. Various empirical and surface-complexation models have been used to interpret such data. 

Suarez et al. (36) use a combination of FTIR spectroscopy, electrophoretic mobility and pH titration data to deduce the specific nature of anionic surface species sorbed to aluminum and silicon oxide minerals. Phosphate, carbonate, borate, selenate, selenite and molybdate data are reviewed and new data on arsenate and arsenite sorption are presented. In all cases the surface species formed are inner-sphere complexes, both monodentate and bidentate. Two step kinetics is typical with monodentate species forming during the initial, rapid sorption step. Subsequent slow sorption is presumed due to the formation of a bidentate surface complex, or in some cases to diffusion controlled sorption to internal sites on poorly crystalline solids. 

Identification of the specific species of the adsorbed oxyanion as well as mode of bonding to the oxide surface is often possible using a combination of Fourier Transform Infrared (FTIR) spectroscopy, electrophoretic mobility (EM) and sorption-proton balance data. This information is required for selection of realistic surface species when using surface complexation models and prediction of oxyanion transport. Earlier, limited IR research on surface speciation was conducted under dry conditions, thus results may not correspond to those for natural systems where surface species may be hydrated. In this study we review adsorbed phosphate, carbonate, borate, selenate, selenite, and molybdate species on aluminum and iron oxides using FTIR spectroscopy in both Attenuated Total Reflectance (ATR) and Diffuse Reflectance Infrared Fourier Transform (DRIFT) modes. We present new FTIR, EM, and titration information on adsorbed arsenate and arsenite. Using these techniques we... 

Molybdenum. Sorption of Mo onto am-Fe(OH)3 was examined by Goldberg et al. (27) using FTIR. The Mo04 aqueous species is dominant in most natural systems (pK j for molybdic acid = 4.2). The ATR-FTIR spectrum for Mo04 reported by them showed peaks at 933, 885, and 835 cm in reasonable agreement with the earlier published vibrational spectra (Table I). The Mo, am-Fe(OH)3 ATR difference spectrum showed peaks at 928 and 880 cm similar to aqueous Mo04 thus adsorbed Mo was not detected. 

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