Adsorption of molybdate by synthetic hematite under alkaline conditions: Effects of aging
Hematite is a common primary/secondary mineral in mine drainage and mine waste settings that can adsorb dissolved metals and metalloids. This study explored the ability of synthetic hematite to retain one such contaminant, molybdate, on its surfaces under highly alkaline (pH ca10) conditions.
X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM), and specific surface area (BET) analyses show that synthetic hematite particles are stable and able to adsorb molybdate.
Raman spectra show that the hematite efficiently adsorbs molybdate and retains it on its surfaces via strong inner-sphere surface complexation.
Inductively coupled plasma-mass spectrometry (ICP-MS) data indicate that hematite aged (7 and 9 days) at high and room temperatures (75 and 25°C) retains adsorbed molybdate and that molybdate sorption increases with aging.
SEM images show that aged hematite particles with adsorbed molybdate are similar in size and shape to pure hematite and exhibit no significant reduction in surface area.
These findings are valuable for understanding the fate of molybdenum in mine wastes and mill tailings environments where the ferrihydrite to which it is adsorbed can transform to hematite. (C) 2012 Elsevier Ltd. All rights reserved
Das, S. and Hendry, M. J., Adsorption of molybdate by synthetic hematite under alkaline conditions: Effects of aging, Applied Geochemistry, 2013, 28, 194-201.
The heterogeneous isotopic anion exchange between calcium molybdate and sodium molybdate solutions have been studied by using 99Mo as tracer.
Atun, G., Bodur, N., Ayyildiz, H., Ayar, N., and Bilgin, B., Kinetics of isotopic exchange between calcium molybdate and molybdate ions in aqueous solution, Radiochimica Acta, 2007, 95, 177-182.
Molybdate, [MoO4]2-, was adsorbed reversibly by pyrite forming labile bidentate, mononuclear surface complexes. Tetrathiomolybdate, [MoS4]2-, formed Mo-Fe-S cubane-type clusters. Because of the high affinity of [MoS4]2- for FeS2 and its resistance to desorption thiomolybdate species may be the reactive Mo constituents in reduced sediments and may control Mo enrichment in anoxic marine environments.
Bostick, B.C., Fendorf, S., and Helz, G. R., Differential adsorption of molybdate and tetrathiomolybdate on pyrite (FeS2), Environmental Science & Technology, 2003, 37, 285-291.
The adsorption of molybdate (MoO42-) and tetrathiomolybdate (MoS42-) by pyrite (FeS2) and goethite (FeOOH) has been studied in relation to molybdenum immobilization in anoxic sediments and the competitive effects of sulfate, phosphate, and silicate on the adsorption of MoO42- and MoS42- by pyrite and goethite. Suspensions of MoS42- (or MoO42-) and goethite (or pyrite) in 0.1 M NaCl solution were equilibrated under anoxic conditions at 25°C, pH 3―10. Adsorption of MoO42- and MoS42- on pyrite and goethite was Langmuir-type at low pH. Maximum sorption is observed in the acidic pH range (pH < 5) at low surface loading. Adsorption decreased: MoS42-/goethite > MoO42-/goethite > MoS42-/pyrite > MoO42-/pyrite. Phosphate competes with MoO42- and MoS42- for the sorption sites of pyrite and goethite Phosphate competition decreases: MoO42-/goethite = MoO42-/pyrite > MoS42-/pyrite > MoS42-/goethite. Silicate and sulfate have a negligible effect on the sorption of MoO42- and MoS42-. That MoS42- is the most strongly adsorbed species by goethite and least susceptible to competition by phosphate suggests that tetrathiomolybdate species may be an ultimate reservoir and may control molybdenum enrichment in the sediments.
Xu, N., Christodoulatos, C., and Braida, W., Adsorption of molybdate and tetrathiomolybdate onto pyrite and goethite: Effect of pH and competitive anions, Chemosphere, 2006, 62, 1726-1735.
Molybdenum(molybdate) adsorption to iron oxohydroxides - isotope fractionation
Note: Molybdenum was applied as an aqueous solution of sodium molybdate, Na2MoO4.2H2O. The species adsorbed is the molybdate ion or a protonated species.
The isotopic fractionation of molybdenum during adsorption to iron oxyhydroxides under variable Mo/Fe-mineral ratios and pH is reported.
Molybdenum isotopes have great potential as a paleoredox indicator, but this potential is currently restricted by an incomplete understanding of isotope fractionations occurring during key biogeochemical processes. Iron oxyhydroxides can readily adsorb molybdate, highlighting the potential importance of this removal pathway for the global molybdenum cycle. Furthermore, adsorption of molybdate to iron oxyhydroxides is associated with preferential uptake of the lighter molybdenum isotopes.
Fractionations between the solid and dissolved phase (δ98Mo) increase at higher pH, and also vary with mineralogy, increasing (δ98Mo/parts per thousand) in the order magnetite (0.83 ± 0.60) < ferrihydrite (1.11 ± 0.15) < goethite (1.40 ± 0.48) < hematite (2.19 ± 0.54).
Small differences in isotopic fractionation are also seen at varying Mo/Fe-mineral ratios for individual minerals.
The observed isotopic behaviour is consistent with both fractionation during adsorption to the mineral surface (a function of vibrational energy) and adsorption of different molybdate species/structures from solution.
The different fractionation factors determined for different iron oxyhydroxides suggests that these minerals exert a major control on observed natural molybdenum isotope compositions during sediment deposition beneath suboxic through to anoxic (but non-sulfidic)bottom waters.
Molybdenum isotopes can provide important information on the spatial extent of different paleoredox conditions, providing they are used in combination with other techniques for evaluating the local redox environment and the mineralogy of the depositing sediments.
Goldberg, T., Archer, C., Vance, D., and Poulton, S. W., Mo isotope fractionation during adsorption to Fe (oxyhydr)oxides, Geochimica et Cosmochimica Acta, 2009, 73, 6502-6516.
The removal of sulfate and molybdate anions (among other anions) from mining liquid effluents is attracting much interest because of environmental legislation and the need for water recycling and reuse. Adsorption of sulfate and molybdate ions on chitin-based materials was investigated. From mining effluents, 71% sulphate and 85% Mo from a Cu-Mo flotation mill effluent were removed. The regeneration of the adsorbent material was possible through the anions desorption in alkaline medium.
Moret, A. and Rubio, J., Sulphate and molybdate ions uptake by chitin-based shrimp shells, Minerals Engineering, 2003, 16, 715-722.
Bead sorbent Perloza MT 50 was used for selective removal of metal W, Mo, V, Ge, and Sb oxoanions. All experiments were carried out by dynamic column sorption. Sorption of tungstate and molybdate anions was successful. The sorption capacity decreased with increasing concentration of accompanying anions (chlorides, sulphates) and with increasing pH (3.5-5.5). Sorption of vanadate anion was possible but the sorption capacity was very low. Sorption of Ge(IV) and Sb(III) oxoanion was negligible.
Mistova, E., Parschova, H., and Matejka, Z., Selective sorption of metal oxoanions from dilute solution by bead cellulose sorbent, Separation Science and Technology, 2007, 42, 1231-1243.
Chemically modified seaweed
This paper provides some data on interaction of molybdate with functional groups of an adsorbent and compares molybdate adsorption with tungstate adsorption.
Seaweed is a heterogeneous mixture of polysaccharides which may sorb metal ions. Oxoanions of tungsten, molybdenum, vanadium, germanium and antimony were sorbed by seaweeds, Ascophyllum nodosum, modified by crosslinking with (1) hexamethylenediamine (NS-1), which partially removed carboxylate giving a sorbent with OH, NH2 and residual CO2- and (2) with epichlorhydrin (DS-1), giving a matrix more accessible to polyoxoanions. Breakthrough concentrations were determined in dynamic column sorption mode. Tungstate, molybdate, and vanadate were most adsorbed. Data for molybdate and tungstate sorption at pH 3.5 and 5.5 in the presence of chloride and sulfate are in the table.
Molybdate and tungstate – effect of variables on sorption capacity
pH atinitial molybdate and tungstate concentrations (1 mg/L) and chloride and sulfate (both 100 mg/L)
For tungstate in acidic solution (pH 3.5) uptake is much greater for DS-1 than NS-1 attributed to
- dominance in acid solution of isopolyoxoanions W12O4110- and H2W12O406- which are said to have reactive W-O sites capable of forming polyol complexes with saccharide OH- groups of the seaweed unlike WO42-, dominant in neutral and alkaline solutions.
- The functional groups of NS-1 being less accessible to the large tungstate polyoxoanions than DS-1.
For molybdate in acidic solution the dominant species are protonated molybdates and heptamolybdate, Mo7O246- . The uptake of molybdate by NS-1 is greater than the uptake of tungstate presumably because the binding sites of NS-1 are more accessible to the smaller molybdate polyoxoanions than to tungstate.
Otherwise, the uptake of molybdate is generally less than the uptake of tungstate, attributed to lower stability of the molybdate complexes and their greater sensitivity to increase of pH.
A five-fold increase of chloride and sulfate caused, at the most a 10% decrease in molybdate and 13 % for tungstate. These anions hardly compete with molybdate and tungstate for the binding sites.
Mistova, E., Parschova, H., Jelinek, L., Matejka, Z., Plichta, Z., and Benes, M., Selective sorption of metal oxoanions from dilute solution by chemicaly modified brown seaweed Ascophyllum nodosum, Separation Science and Technology, 2008, 43, 3168-3182.