Health, Safety & Environment
This paper presents the results obtained when studying the removal of molybdate and tungstate anions from aqueous using organically modified montmorillonite. In batch experiments, the influence of pH, reaction time, the initial concentration of Mo and W, and the influence of the amount and type of surfactant was investigated. The modification of smectite with leads to the formation of an organo-mineral complex characterized by a positively -charged surface and a high sorption capacity in the removal of Mo(VI) and W(VI). The pH effect determining the possibility of Mo and W removal from the aqueous solution. In the pH range of 1-5, the sorption of Mo and W efficiency is very high, while at pH > 5 sorption is limited or completely inhibited. DDTMA-smectite and DDDDMA-smectite were recognized as the best sorbents. With an increasing amount of surfactant used in modification process, the sorption efficiency increases. Molybdates are removed from solutions compared to tungstates to a much greater extent - maximum sorption capacities were 705 and 390 mmol/kg, respectively. Kinetic studies indicated that maximum sorption capacity is achieved faster in the case of Mo than of W, and that the main sorption mechanisms are chemisorption and ion exchange. (C) 2016 Elsevier B.V. All rights reserved.
Muir, B., Andrunik, D., Hyla, J., and Bajda, T.,The removal of molybdates and tungstates from aqueous solution by organo-smectites, Applied Clay Science, 2017, 136, 8-17.
Molybdate adsorption
High efficient removal of molybdenum from water by Fe2(SO4) 3: Effects of pH and affecting factors in the presence of co-existing background constituents
Comparatively investigated the different effects of Fe2(SO4) 3 coagulation-filtration and FeCl3 coagulation-filtration on the removal of Mo (VI). And the influence of calcium, sulfate, silicate, phosphate and humic acid (HA) were also studied.
The following conclusions can be obtained: (1) compared with the case of FeCl3, Fe2 (SO4) 3 showed a higher Mo (VI) removal efficiency at pH 4.00-5.00, but an equal removal efficiency at pH 6.00-9.00. (2) The optimum Mo (VI) removal by Fe2(SO4) 3 was achieved at pH 5.00-6.00; (3) The presence of calcium can reduce the removal of Mo (VI) over the entire pH range in the present study; (4) The effect of co-existing background anions (including HA) was dominated by three factors: Firstly the influence of co-existing background anions on the content of Fe intercepted from water (intercepted Fe). Secondly the competition of co-existing anions with Mo (VI) for adsorption sites. Thirdly the influence of co-existing background anions on the Zeta potential of the iron flocs. (C) 2015 Elsevier B.V. All rights reserved.
Zhang, X., Ma, J., Lu, X. X., Huangfu, X. L., and Zou, J.,High efficient removal of molybdenum from water by Fe2(SO4) 3: Effects of pH and affecting factors in the presence of co-existing background constituents, Journal of Hazardous Materials, 2015, 300, 823-829.
Molybdate. Simultaneous biosorption of selenium, arsenic and molybdenum with modified algal-based biochars
Ash disposal waters from coal-fired power stations present a challenging water treatment scenario as they contain high concentrations of the oxyanions Se, As and Mo which are difficult to remove through conventional techniques. In an innovative process, macroalgae can be treated with Fe and processed through slow pyrolysis into Fe-biochar which has a high affinity for oxyanions. However, the effect of production conditions on the efficacy of Fe-biochar is poorly understood. We produced Fe-biochar from two algal sources; "Gracilaria waste" (organic remnants after agar is extracted from cultivated Gracilaria) and the freshwater macroalgae Oedogonium. Pyrolysis experiments tested the effects of the concentration of Fe3+ in pre-treatment, and pyrolysis temperatures, on the efficacy of the Fe-biochar. The efficacy of Fe-biochar increased with increasing concentrations of Fe3+ in the pre-treatment solutions, and decreased with increasing pyrolysis temperatures. The optimized Fe-biochar for each biomass was produced by treatment with a 12.5% w/v Fe3+ solution, followed by slow pyrolysis at 300 degrees C. The Fe-biochar produced in this way had higher a biosorption capacity for As and Mo (62.5-80.7 and 67.4-78.5 mg g-1) respectively) than Se (14.9-38.8 mg g-1)) in single-element mock effluents, and the Fe-biochar produced from Oedogonium had a higher capacity for all elements than the Fe-biochar produced from Gracilaria waste. Regardless, the optimal Fe-biochars from both biomass sources were able to effectively treat Se, As and Mo simultaneously in an ash disposal effluent from a power station. The production of Fe-biochar from macroalgae is a promising technique for treatment of complex effluents containing oxyanions.
Johansson, C. L., Paul, N. A., de Nys, R., and Roberts, D. A.,Simultaneous biosorption of selenium, arsenic and molybdenum with modified algal-based biochars, Journal of environmental management, 2016, 165, 117-23.
Molybdate adsorption Goethite
The mobility of Mo in soils and sediments depends on several factors including soil mineralogy and the presence of other oxyanions that compete with Mo for the adsorbent's retention sites. Batch experiments addressing Mo adsorption onto goethite were conducted with phosphate, sulfate, silicate, and tungstate as competing anions in order to produce competitive two anions adsorption envelopes, as well as competitive two anions adsorption isotherms. Tungstate and phosphate appear to be the strongest competitors of Mo for the adsorption sites of goethite, whereas little competitive effects were observed in the case of silicate and sulfate. Mo adsorption isotherm from a phosphate solution was similar to the one from a tungstate solution. The charge distribution multi-site complexation (CD-MUSIC) model was used to predict competitive adsorption between MoO42- and other anions (i.e., phosphate, sulfate, silicate and tungstate) using model parameters obtained from the fitting of single ion adsorption envelopes. CD-MUSIC results strongly agree with the experimental adsorption envelopes of molybdate over the pH range from 3.5 to 10. Furthermore, CD-MUSIC prediction of the molybdate adsorption isotherm show a satisfactory fit of the experimental results. Modeling results suggest that the diprotonated monodentate complexes, FeOW(OH)(5)(-0.5) and FeOMo(OH)(5)(-0.5), were respectively the dominant complexes of adsorbed W and Mo on goethite 110 faces at low pH. The model suggests that Mo and W are retained mainly by the formation of monodentate complexes on the goethite surface. Our results indicate that surface complexation modeling may have applications in predicting competitive adsorption in more complex systems containing multiple competing ions.
Xu, N.,Christodoulatos, C.,and Braida, W.,Modeling the competitive effect of phosphate, sulfate, silicate, and tungstate anions on the adsorption of molybdate onto goethite, Chemosphere, 2006, 64, 1325.
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)
| NS-1 | DS-1 | |||
|---|---|---|---|---|
| molybdate | tungstate | molybdate | tungstate | |
| pH 3.5 | 673 | 55.9 | 577 | 1058 |
| pH 5.5 | 275 | 464 | 123 | 775 |
For tungstate in acidic solution (pH 3.5) uptake is much greater for DS-1 than NS-1 attributed to
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.
| NS-1 | DS-1 | |||
|---|---|---|---|---|
| molybdate | tungstate | molybdate | tungstate | |
| pH 3.5 | 673 | 55.9 | 577 | 1058 |
| pH 5.5 | 275 | 464 | 123 | 775 |
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.
Competitive adsorption of molybdate, phosphate and sulfate on alumina
Anion adsorption on the aluminum oxide, gibbsite, was investigated as a function of solution pH (3-11) and equilibrium solution molybdate (3.13, 31.3, or 313 μmol/L), phosphate (96.9 μmol/L), or sulfate (156 μmol/L) concentration.
Adsorption of all three anions decreased with increasing pH.
Electrophoretic mobility measurements indicated a downward shift in point of zero charge, indicative of an inner-sphere adsorption mechanism for all three anions.
The constant capacitance model, having an inner-sphere adsorption mechanism, was able to describe molybdate and phosphate adsorption; whereas the triple-layer model with an outer-sphere adsorption mechanism was used to describe sulfate adsorption.
Competitive adsorption experiments showed a reduction of molybdate adsorption at a Mo/P ratio of 1:30 and 1:300 but no reduction at a Mo/S ratio of 1:52 and 1:520. These concentrations are realistic of natural systems where molybdate is found in much lower concentrations than phosphate or sulfate.
Using surface complexation constants from single-ion systems, the triple-layer model predicted that even elevated sulfate concentrations did not affect molybdate adsorption.
The constant capacitance model was able to predict the competitive effect of phosphate on molybdate adsorption semiquantitatively
[The constant capacitance model: a surface complexation model. Goldberg, S. 1992. Use of surface complexation models in soil chemical systems. Adv. Agron. 47:233–329.]
Goldberg, S., Competitive Adsorption of Molybdenum in the Presence of Phosphorus or Sulfur on Gibbsite, Soil Science, 2010, 175, 105-110.
Removal of Molybdate Anions from Water by Adsorption on Zeolite-Supported Magnetite
Industrial wastewater may contain high molybdenum concentrations, making treatment before discharge necessary. In this paper, the removal of molybdate anions from water is presented, using clinoptilolite zeolite coated with magnetite nanoparticles. In batch experiments the influence of pH, ionic strength, possible interfering (oxy)anions, temperature and contact time is investigated. Besides determination of kinetic parameters and adsorption isotherms, thermodynamic modelling is performed to get better insight into the adsorption mechanism; molybdenum is assumed to be adsorbed as a FeOMoO2(OH) 2H2O inner-sphere complex.
At the optimum pH of 3, the adsorption capacity is around 18 mg molybdenum per gram adsorbent.
The ionic strength of the solution has no influence on the adsorption capacity.
Other anions, added to the molybdenum solution in at least a tenfold excess, only have a minor influence on the adsorption of molybdenum, with the exception of phosphate.
Adsorption increases when temperature is increased.
It is demonstrated that the adsorbent can be used to remove molybdenum from industrial wastewater streams, and that the limitations set by the World Health Organization (residual concentration of 70 mu g/l Mo) can easily be met.
Verbinnen, Bram, Block, Chantal, Hannes, Dries, Lievens, Patrick, Vaclavikova, Miroslava, Stefusova, Katarina, Gallios, Georgios, and Vandecasteele, Carlo, Removal of Molybdate Anions from Water by Adsorption on Zeolite-Supported Magnetite, Water Environment Research, 2012, 84, 753-760.
Adsorption of MoO42- ions on Fe-treated tri-calcium phosphate
The Fe-treated calcium phosphate (Fe-TPO4) has been prepared with FeCl3 aqueous solution. The sorption of (MoO42-)-Mo-99 ions from aqueous solution using Fe- TPO4 as adsorbent is investigated by batch experiments. Experiments have been performed as a function of shaking time, pH, solute concentration and temperature and the experimental rate data are tested with the pseudo-second-order rate model. The adsorption data as a function of molybdate concentration obey the Freundlich isotherm. Thermodynamic parameters ΔH0, ΔS0, ΔG0have been calculated with data of temperature studies. The positive value of ΔH0 indicates that the adsorption of MoO42- ions on Fe- TPO4 is an endothermic process.
Serrano Gomez, J., Bonifacio Martinez, J., and Lopez Reyes, M. C., Adsorption of MoO42-- ions on Fe-treated tri-calcium phosphate, Indian Journal of Chemical Technology, 2012, 19, 167-172.
Adsorption from solution: Sequestration inside ferritin.
When the iron core of equine spleen ferritin is reduced, anions in solution cross the protein shell and enter the ferritin interior as part of a charge balancing reaction.
Anion sequestration inside ferritin during iron core reduction was monitored using ion selective electrodes, inductively coupled plasma emission, and energy-dispersive X-ray spectroscopy.
The requirement for anion translocation to the ferritin interior occurs because upon iron core reduction, two OH- ions per iron are released or neutralized inside ferritin leaving a net positive charge.
Halides and oxoanions were tested as anionic substrates for this reaction.
A general trend for the halides showed that the smaller halides accumulated inside ferritin in greater abundance than larger halides, presumably because the protein channels restrict the transfer of the larger anionic species.
In contrast, oxoanion accumulation inside ferritin did not show selectivity based on size or charge. Vanadate and molybdate accumulated to the highest concentrations and nitrate, phosphate and tungstate showed poor accumulation inside ferritin.
Fe(II) remains stably sequestered inside ferritin, as shown by electron microscopy and by column chromatography.
Upon oxidation of the iron core, the anions are expelled from ferritin, and OH- ions coordinate to the Fe(III) to form the original Fe(O)OH mineral.
Anion transport across the ferritin protein shell represents an important mechanism by which ferritin maintains proper charge balance inside the protein cavity. (c) 2011 Elsevier Inc. All rights reserved
Hilton, Robert J., Zhang, Bo, Martineau, L. Naomi, Watt, Gerald D., and Watt, Richard K., Anion deposition into ferritin, Journal of Inorganic Biochemistry, 2012, 108, 8-14.
[Ferritin is a ubiquitous iron-storage protein found inside cells. A globular layer of protein encapsulates iron(III) hydroxyphosphate. In humans, ferritin acts as a buffer against iron deficiency and iron overload. Free iron, i.e. Fe2+ ions, is toxic to cells, catalysing the formation of hydroxyl radicals from reactive oxygen species. Apoferritin (i.e., the iron-free protein) binds to iron(II) which is then oxidised and stored in the iron(III) state.]