Screening of biological properties of Mo^V₂O₂S^2- and Mo^V₂O₄^-based coordination complexes: Investigation of antibacterial, antifungal, antioxidative and antitumoral activities versus growing of Spirulina platensis biomass

This paper deals with the biological potential of coordination compounds based on binuclear core [Mo^V₂O₂E₂]²⁺ (E = O or S) coordinated with commercially available ligands such as oxalates (Ox^2- ), L-cysteine (L-cys^2-), Lhistidine (L-his-), Iminodiacetate (IDA^2-), Nitrilotriacetate (HNTA^2- or NTA^3- ) or ethylenediamine tetraacetate (EDTA₄^-)]by means of various in vitro assays in a screening approach. Results suggest that the obtained complexes show weak antibacterial and antifungal properties while not being cytotoxic on cancerous and mammalian cells. In contrast, [Mo^V₂O₂E₂(L-cys₂]^2- complexes stand out as powerful antioxidant, whereas [Mo^V₂O₂E₂(EDTA]^2- associating tetraphenylphosphonium counter-cations display strong antibiotic activity. Finally, some complexes have evidenced a positive activity towards the growing of spirulina platensis together with a modification of the proportions of biological components inside the cells. These findings reveal promising bioactivity of the bridged binuclear Mo^V cores inside complexes and encourage further research for new highly active yet non-toxic molecules for biological and biomedical applications.

A. Fuior, A. Hijazi, O. Garbuz, V. Bulimaga, L. Zosim, D. Cebotari, M. Haouas, I. Toderas, A. Gulea, and S. Floquet,Screening of biological properties of Mo^V₂O₂S^2- and Mo^V₂O₄^-based coordination complexes: Investigation of antibacterial, antifungal, antioxidative and antitumoral activities versus growing of Spirulina platensis biomass, Journal of Inorganic Biochemistry, 2022, 226, 111627.

Molybdate effectively controls sulfide production in a shrimp pond model

The production of shrimp is often performed in earthen outdoor ponds in which the high input of feed and faeces on the bottom can result in deterioration of the water quality, which negatively impacts the animals and the environment. Here, we investigate the potential of sodium molybdate (Na₂MoO_4.2H₂O, sodium nitrate (NaNO₃ and sodium percarbonate (Na₂CO_3.1.5H₂O₂ to control sulphide production in a simulated shrimp pond bottom system that included the sediment, overlaying artificial seawater and organic matter input in the form of shrimp feed and shrimp faeces. Sediment depth gradient measurements of oxygen, H₂S and pH were obtained during 7 days of incubation using microelectrodes. The most significant impact in terms of H₂S, was observed for 50 mg/L sodium molybdate. At the water-sediment interface, there was up to 73% less H₂S detected for this treatment in comparison to a control treatment, while in the deeper layers of the sediment it was up to 47% less H₂S. The residual sulphate concentrations in the molybdate treated samples were 16 +/- 4% higher than the control, indicating an inhibition in sulphate reduction. Nitrate and sodium percarbonate treatments also showed a limited capacity to decrease H₂S entering in the water column, yet no clear difference in H₂S concentrations in the sediment compared to the control were observed. Molybdate treatment appears to work through the inhibition of sulphate reducing bacteria in situ for the control of H₂S production better than the chemical oxygen boosters or nitrate treatment.

F. Torun, B. Hostins, P. De Schryver, N. Boon, and J. De Vrieze,Molybdate effectively controls sulfide production in a shrimp pond model, Environmental Research, 2022, 203.

Impedimetric detection of miRNA biomarkers using paper-based electrodes modified with bulk crystals or nanosheets of molybdenum disulfide

Paper-based electrodes modified with molybdenum disulfide MoS₂ in the form of bulk crystals or exfoliated nanosheets were developed and used as a biosensing platform for the impedimetric detection of miRNAs miRNA^-155 and miRNA^-21 related to early diagnosis of lung cancer. For this purpose, MoS₂ crystals or nanosheets were used for the modification of the working electrode area of paper-based platform for the first time in this study. The proposed assay offers sensitive and selective detection of microRNAs by electrochemical impedance spectroscopy EIS technique. The entire assay, both the electrode modification and the miRNA detection being completed in 30 min and a single sample droplet 5 μL was enough to cover working electrode area which enabled analysis in low sample volumes. The limits of detection LOD for miRNA^-21 and miRNA^-155 were calculated both in buffer and fetal bovine serum media. It is found that the LOD is varying between 1 and 200 ng/mL. In comparison to nanosheets, a larger electroactive surface area was obtained with bulk MoS₂ resulting in lower LOD values on miRNA detection. The paper-based electrodes showed high specificity towards their target sequences. Moreover, they effectively discriminated single base mismatched non-target sequences. The advantages of these MoS₂ paper based electrodes include high sensitivity, and low-cost provide great potential for improved monitoring of miRNA biomarkers even in artificial serum media.

E. Yarali, E. Eksin, H. Torul, A. Ganguly, U. Tamer, P. Papakonstantinou, and A. Erdem,Impedimetric detection of miRNA biomarkers using paper-based electrodes modified with bulk crystals or nanosheets of molybdenum disulfide, Talanta, 2022, 241, 123233.

A sensitive biosensor for glucose determination based on the unique catalytic chemiluminescence of sodium molybdate

Chemiluminescent CL reaction between hydrogen peroxide H₂O₂ and luminol was dramatically enhanced by sodium molybdate Na₂MoO₄ for 284-fold. CL mechanism investigation indicated that Na₂MoO₄ increased the production of hydroxyl radical •OH and superoxide anion •O^2- in the H₂O^2-luminol system, which could attribute to the enhanced-CL intensity and gave us new insights into the CL-enhanced property of Na₂MoO₄. The CL intensity of Na₂MoO₄^-H₂O^2-luminol system increased with the concentration of H₂O₂, based on which, a convenient and sensitive CL determination method could be developed for H₂O₂ in the concentration ranging from 0.5 to 60 μmol/L, with a detection limit of 0.25 μmol/L. Combining with glucose oxidase, the Na₂MoO₄^-H₂O^2-luminol system could also be applied for glucose detection. Glucose in human serum has been successfully detected with satisfied recoveries in the range of 96.7 % to 105.4 %.

W. Yao, X. Zhang, and Z. Lin,A sensitive biosensor for glucose determination based on the unique catalytic chemiluminescence of sodium molybdate, Spectrochim Acta A Mol Biomol Spectrosc, 2022, 265, 120401.

Tungsten and Molybdenum Oxide nanostructures: Two-dimensional layers and nanoclusters

WMo-oxides form an interesting class of materials, featuring structural complexities, stoichiometric flexibility, and versatile physical and chemical properties that render them attractive for many applications in diverse fields of nanotechnologies. In nanostructured form, novel properties and functionalities emerge as a result of quantum size and confinement effects. In this topical review, WMo-oxide nanosystems are examined with particular emphasis on two-dimensional 2-D layers and small molecular-type clusters. We focus on the epitaxial growth of 2-D layers on metal single crystal surfaces and investigate their novel geometries and structures by a surface science approach. The coupling between the oxide overlayer and the metal substrate surface is a decisive element in the formation of the oxide structures and interfacial strain and charge transfer are shown to determine the lowest energy structures. Atomic structure models as determined by DFT simulations are reported and discussed for various interface situations, with strong and weak coupling. Free-standing quasi-2-D oxide layers, so-called oxide nanosheets, are attracting a growing interest recently in the applied research community because of their easy synthesis via wet-chemical routes. Although they consist typically of several atomic layers thick - not always homogeneous - platelet systems, their quasi-2-D character induces a number of features that make them attractive for optoelectronic, sensor or biotechnological device applications. A brief account of recently published preparation procedures of WMo-oxide nanosheets and some prototypical examples of proof of concept applications are reported here. MO33 M = W,Mo clusters can be generated in the gas phase in nearly monodisperse form by a simple vacuum sublimation technique. These clusters, interesting molecular-type structures by their own account, can be deposited on a solid surface in a controlled way and be condensed into 2-D WMo-oxide layers; solid-state chemical reactions with pre-deposited surface oxide layers to form 2-D ternary oxide compounds tungstates, molybdates have also been reported. The clusters have been proposed as model systems for molecular studies of reactive centres in catalytic reactions. Studies of the catalysis of MO33 clusters in unsupported and supported forms, using the conversion of alcohols as model reactions, are discussed. Finally, we close with a brief outlook of future perspectives.

S. Surnev, and F. P. Netzer,Tungsten and Molybdenum Oxide nanostructures: Two-dimensional layers and nanoclusters, J Phys Condens Matter, 2022.

The role of hydrolysis in biological effects of molybdenum cluster with DMSO ligands

Biological applications of octahedral molybdenum cluster complexes are complicated by their hydrolytic instability, since hydrolysis leads to irreversible changes in the structure and properties of these compounds. On the other hand, if such changes are thoroughly investigated and understood, the hydrolysis process can become an important tool for regulating specific biological effects of the clusters. In this work, we demonstrate how the luminescence and biological properties cellular uptake, cytotoxicity in the dark and photodynamic effect of highly unstable cluster complex [{Mo₆I₈}DMSO₆]NO₃₄ change along with the degree of hydrolysis. Particularly, cluster solution preliminarily aged in water demonstrated lower dark and higher photoinduced cytotoxicity and higher cellular uptake in comparison with fresh solution.

E. V. Pronina, T. N. Pozmogova, Y. A. Vorotnikov, A. A. Ivanov, and M. A. Shestopalov,The role of hydrolysis in biological effects of molybdenum cluster with DMSO ligands, J Biol Inorg Chem, 2022, 27, 111-119.

Simultaneous detection of dual biomarkers using hierarchical MoS₂ nanostructuring and nano-signal amplification-based electrochemical aptasensor toward accurate diagnosis of prostate cancer

Accurate and reliable quantification of tumor biomarkers in clinical samples is of vital importance for early stage diagnosis and treatment of cancer. However, a poor specificity of prostate specific antigen PSA testing alone fostering overdetection and overtreatment, remains a great controversy in prostate cancer PCa screening. Here we report an electrochemical aptasensor using hierarchical MoS₂ nanostructuring and SiO₂ nano-signal amplification for simultaneous detection of dual PCa biomarkers, PSA and sarcosine, to enhance the diagnostic performance of PCa. In this strategy, hierarchical flower-like MoS₂ nanostructures as functional interface accelerated intermolecular accessibility and improved DNA hybridization efficiency. Moreover, the spherical SiO₂ nanoprobe that conjugated with both electroactive tags and DNA probes, allowed effective electrochemical signal amplification. By deliberately designing different hybridization modes, we individually implemented the optimization of PSA and sarcosine sensing system. Based on this, simultaneous determination of PSA and sarcosine was achieved, with limit of detection LOD down to 2.5 fg/mL and 14.4 fg/mL, respectively, as well as excellent selectivity. More importantly, using this approach, we could directly differentiate cancer patients with healthy ones for clinical serum samples. The ultrasensitive biosensor provides single-step analysis with simple operation and a small sample volume ∼12 μL, shedding new light on accurate diagnosis and early-detection of cancer in clinical applications.

R. Yan, N. Lu, S. Han, Z. Lu, Y. Xiao, Z. Zhao, and M. Zhang,Simultaneous detection of dual biomarkers using hierarchical MoS₂ nanostructuring and nano-signal amplification-based electrochemical aptasensor toward accurate diagnosis of prostate cancer, Biosens Bioelectron, 2022, 197, 113797.

Bionanohybrid composed of metalloprotein/DNA/MoS₂/peptides to control the intracellular redox states of living cells and its applicability as a cell-based biomemory device

The development of cell-based bioelectronic devices largely depends on the direct control of intracellular redox states. However, most related studies have focused on the accurate measurement of electrical signals from living cells, whereas direct intracellular state control remains largely unexplored. Here, we developed a biocompatible transmembranal bionanohybrid structure composed of a recombinant metalloprotein, DNA, molybdenum disulfide nanoparticles MoS₂, and peptides to control intracellular redox states, which can be used as a cell-based biomemory device. Using the capacitance of MoS₂ located inside the cell, the bionanohybrid controled the intracellular redox states of living cells by recording and extracting intracellular charges, which inturn was achieved by activating writing and deactivating erasing the cells. As a proof of concept, cell-based biomemory functions including writing, reading, and erasing were successfully demonstrated and confirmed via electrochemical methods and patch-clamp analyses, resulting in the development of the first in vitro cell-based biomemory device. This newly developed bionanohybrid provides a novel approach to control cellular redox states for cell-based bioelectronic applications, and can be applicable in a wide range of biological fields including bioelectronic medicine and intracellular redox status regulation.

J. Yoon, M. Shin, D. Kim, J. Lim, H. W. Kim, T. Kang, and J. W. Choi,Bionanohybrid composed of metalloprotein/DNA/MoS₂/peptides to control the intracellular redox states of living cells and its applicability as a cell-based biomemory device, Biosens Bioelectron, 2022, 196, 113725.

Adsorption from solution

Kinetic modeling of molybdenum sorption and transport in soils

In this investigation, batch and column experiments were conducted to investigate the molybdenum (Mo) sorption and transport processes on a neutral-pH soil (Webster loam) and an acidic soil (Mahan sand) in Ca²⁺and K⁺background solutions. Batch results showed that the adsorption of Mo was strongly non-linear in both soils and amount of Mo sorbed in the acidic soil was larger than the neutral soil. The Freundlich distribution coefficients (Kf) and Langmuir sorption maxima (Smax) in Ca²⁺background solution are larger than that in K⁺solution, indicating greater Mo sorption in Ca²⁺than in K(+). Experimental breakthrough curves (BTCs) demonstrated that mobility of Mo was higher at neutral condition than that at acidic condition. A multi-reaction transport model (MRTM) formulation with two kinetic retention reactions (reversible and irreversible) well described Mo transport for Webster soil. However, MRTM model which accounts for equilibrium and kinetic sites is recommended for Mo transport in Mahan soil, reflecting different soil properties. Based on inverse modeling, the sorption forward rate coefficients (k1) obtained from Ca²⁺in both soils are larger than that from K^+, which consistent with batch experiment. Overall, MRTM model was capable of describing the Mo transport behavior under different geochemical conditions.

W. Sun, and H. M. Selim,Kinetic modeling of molybdenum sorption and transport in soils, Environmental science and pollution research international, 2020.

Selective adsorption of molybdate from water by polystyrene anion exchanger-supporting nanocomposite of hydrous ferric oxides

Molybdenum is an essential trace element for humans but can be harmful with excess assimilations or chronic exposures. In this study a polymer-functionalized nanocomposite (HFO-PsAX) was fabricated for selective adsorption of molybdate from aqueous solution. HFO-PsAX was prepared by grafting hydrous ferric oxide nanoparticles (HFOs) into the porous structure of a polystyrene anion exchanger (PsAX) by in situ synthesis method. The resultant HFO-PsAX exhibited greatly enhanced selectivity toward molybdate as compared with the matrix, PsAX, which is also a fair adsorbent for scavenging molybdate. The competitive abilities of the ubiquitous anions, i.e., chloride, carbonate, sulfate, and phosphate, on the adsorption of molybdate by HFO-PsAX followed the order: chloride < phosphate < carbonate < sulfate. The unexpectedly weak competitive ability of trivalent phosphate may be due to incompletely dissociated state and formation of molybdate-phosphate complexes. The optimal pH for the adsorption of molybdate was determined as pH approximately 4, which is associated with the dissociation constants of molybdic acid; certain adsorption capacities were also observed even under extremely alkaline condition (pH=14) for single-component molybdate solution. Temperature (10, 25, and 40 degrees C) has negligible effect on the adsorption capacities by HFO-PsAX, and Freundlich model and Dubinin-Radushkevich (D-R), Temkin model can describe the adsorption isotherms well. The adsorption potential of Temkin model is calculated as approximately 100J/mol, which is between those of physisorption and chemisorption process. Fixed-bed column adsorption experiments validated the potential of HFO-PsAX in treating Mo(VI) contaminated water for practical application, and the exhausted HFO-PsAX can be regenerated by a binary NaOH-NaCl solution (both 5% in mass) without loss in adsorption capacities.

J. Li, D. Chen, X. Liao, and B. Pan,Selective adsorption of molybdate from water by polystyrene anion exchanger-supporting nanocomposite of hydrous ferric oxides, The Science of the total environment, 2019, 691, 64-70.

Treatment of moybdenum(VI)-containing groundwater using chitosan nanoparticle: adsorption mechanism and performances

A novel nano-adsorbent prepared by hydrothermal carbonization of chitosan was used for removing moybdenum (Mo(VI)) from groundwater. Optimal adsorption parameters were determined via varying pH, time, concentrations, and temperatures. The results indicated that the removal rate of Mo(VI) by chitosan carbonization nanoparticles (CCN) was depended on pH values and the maximum adsorption efficiency could be achieved when the pH is in the range of 1.5-3.3. Kinetic studies showed that Mo(VI) could be removed rapidly, and the experimental results fitted pseudo-second-order kinetic model well. Mo(VI) adsorbed by CCN was described well by Langmuir model and the theoretical maximum adsorption capacity reached 192.308 mg g^-1 at 303.15 K. Thermodynamic parameters indicated that the adsorption process was endothermic, entropy increasing, and spontaneous. Electrostatic interaction and hydrogen bonding were the major adsorption mechanisms. Regeneration and real groundwater treatment experiments revealed that Mo(VI) could be removed efficiently by CCN, and CCN could be recycled for a long term. Altogether, CCN might be a green, efficient, and recyclable adsorbent for Mo(VI) removal from groundwater.

J. J. Lian, M. Yang, S. S. Wang, B. Chen, F. J. Zhou, and Z. L. Liu,Treatment of moybdenum(VI)-containing groundwater using chitosan nanoparticle: adsorption mechanism and performances, Desalination and Water Treatment, 2019, 167, 258-268.

Molybdate adsorption

Selective adsorption of molybdate from water by polystyrene anion exchanger-supporting nanocomposite of hydrous ferric oxides

Molybdenum   is an essential trace element for humans but can be harmful with excess assimilations or chronic exposures. In this study a polymer-functionalized nanocomposite (HFO-PsAX) was fabricated for selective adsorption of molybdate from aqueous solution. HFO-PsAX was prepared by grafting hydrous ferric oxide nanoparticles (HFOs) into the porous structure of a polystyrene anion exchanger (PsAX) by in situ synthesis method. The resultant HFO-PsAX exhibited greatly enhanced selectivity toward molybdate as compared with the matrix, PsAX, which is also a fair adsorbent for scavenging molybdate. The competitive abilities of the ubiquitous anions, i.e., chloride, carbonate, sulfate, and phosphate, on the adsorption of molybdate by HFO-PsAX followed the order: chloride < phosphate < carbonate < sulfate. The unexpectedly weak competitive ability of trivalent phosphate may be due to incompletely dissociated state and formation of molybdate-phosphate complexes. The optimal pH for the adsorption of molybdate was determined as pH approximately 4, which is associated with the dissociation constants of molybdic acid; certain adsorption capacities were also observed even under extremely alkaline condition (pH=14) for single-component molybdate solution. Temperature (10, 25, and 40 degrees C) has negligible effect on the adsorption capacities by HFO-PsAX, and Freundlich model and Dubinin-Radushkevich (D-R), Temkin model can describe the adsorption isotherms well. The adsorption potential of Temkin model is calculated as approximately 100J/mol, which is between those of physisorption and chemisorption process. Fixed-bed column adsorption experiments validated the potential of HFO-PsAX in treating Mo(VI) contaminated water for practical application, and the exhausted HFO-PsAX can be regenerated by a binary NaOH-NaCl solution (both 5% in mass) without loss in adsorption capacities.

J. Li, D. Chen, X. Liao, and B. Pan,Selective adsorption of molybdate from water by polystyrene anion exchanger-supporting nanocomposite of hydrous ferric oxides, The Science of the total environment, 2019, 691, 64-70.

Adsorption of molybdenum by melanin

BACKGROUND: Melanin is detectable in various sense organs including the skin in animals. It has been reported that melanin adsorbs toxic elements such as mercury, cadmium, and lead. In this study, we investigated the adsorption of molybdenum, which is widely recognized as a toxic element, by melanin.

METHODS: Molybdenum level of the mouse skin was measured by inductively coupled plasma mass spectrometry. The pigmentation level of murine skin was digitalized as the L* value by using a reflectance spectrophotometer. An in vitro adsorption assay was performed to confirm the interaction between molybdenum and melanin.

RESULTS: Our analysis of hairless mice with different levels of skin pigmentation showed that the level of molybdenum increased with an increase in the level of skin pigmentation (L* value). Moreover, our analysis by Spearman's correlation coefficient test showed a strong correlation (r = - 0.9441, p < 0.0001) between L* value and molybdenum level. Our cell-free experiment using the Langmuir isotherm provided evidence for the adsorption of molybdenum by melanin. The maximum adsorption capacity of 1 mg of synthetic melanin for molybdenum was 131 mug in theory.

CONCLUSION: Our in vivo and in vitro results showed a new aspect of melanin as an adsorbent of molybdenum.

W. Chen, K. Hashimoto, Y. Omata, N. Ohgami, A. Tazaki, Y. Deng, L. Kondo-Ida, A. Intoh, and M. Kato,Adsorption of molybdenum by melanin, Environ Health Prev Med, 2019, 24, 36.

Aqueous speciation nitric acid

Application of chitosan in removal of molybdate ions from contaminated water and groundwater

Water pollution by heavy metals represents a serious problem around the world. Among various treatment techniques for water remediation, adsorption is an effective and versatile method due to the low cost, effectiveness and simplicity. Chitosan is a cationic polysaccharide with an excellent adsorption capacity of heavy metal ions. Chitosan has a high molybdate adsorption capacity (265+/-1mgg(-1)) at 20 degrees C and pH 2.7. Participation of hydroxyl groups in the adsorption of molybdate anions was confirmed by FT-IR analysis. SEM images showed that morphological surface changes happen after Mo(VI) adsorption. Continuous adsorption data were best fitted by Modified Dose- Response model. Scale-up of continuous processes was achieved applying bed depth service time (BDST) model. Application of chitosan in molybdate removal from real groundwater samples suggest that this polysaccharide is a good option to be used for household purposes.

Bertoni, F. A., Gonzalez, J. C., Garcia, S. I., Sala, L. F., and Bellu, S. E.,Application of chitosan in removal of molybdate ions from contaminated water and groundwater, Carbohydrate polymers, 2018, 180, 55-62.

Sorption of molybdenum(VI) ions on natural mineral sorbents

The sorption of Mo(VI) ions on natural sorbents (zeolite tuffs and clinker) is investigated. It is estimated that zeolite tuffs sorb polymer forms of Mo(VI) ions in acidic media, while the monomer form of molybdate ion is sorbed in basic media. It is concluded that the rate of sorption of molybdenum(VI) ions on zeolite tuffs is regulated by an intradiffusional mechanism. Values of the activation energy indicate that the adsorption of molybdenum(VI) ions on zeolite tuffs is governed by a physical mechanism. The excretion of crystalline phase (probably calcium molybdate) on surfaces of zeolite tuff is estimated via scanning electron microscopy

.
Sanzhanova, S. S., and Zonkhoeva, E. L.,Sorption of molybdenum(VI) ions on natural mineral sorbents, Russian Journal of Physical Chemistry A, 2017, 91, 2219-2224.
[tuff = type of rock made of volcanic ash ejected from a vent during a volcanic eruption. clinker = a hard mass of fused stony matter formed in a furnace.]

ADSORPTION

The removal of molybdates and tungstates from aqueous solution by organo-smectites

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 MoO₄^2- 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 (MoO₄^2-) and tetrathiomolybdate (MoS₄^2-) 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 MoO₄^2- and MoS₄^2- by pyrite and goethite. Suspensions of MoS₄^2- (or MoO₄^2-) and goethite (or pyrite) in 0.1 M NaCl solution were equilibrated under anoxic conditions at 25°C, pH 3―10. Adsorption of MoO₄^2- and MoS₄^2- 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: MoS₄^2-/goethite > MoO₄^2-/goethite > MoS₄^2-/pyrite > MoO₄^2-/pyrite. Phosphate competes with MoO₄^2- and MoS₄^2- for the sorption sites of pyrite and goethite Phosphate competition decreases: MoO₄^2-/goethite = 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-. That MoS₄^2- 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, Na₂MoO_4.2H₂O. 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 W₁₂O₄₁^10- and H₂W₁₂O₄₀^6- which are said to have reactive W-O sites capable of forming polyol complexes with saccharide OH^- groups of the seaweed unlike WO₄^2-, 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, Mo₇O₂₄^6- . 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.

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 MoO₄^2- ions on Fe-treated tri-calcium phosphate

The Fe-treated calcium phosphate (Fe-TPO₄) has been prepared with FeCl₃ aqueous solution. The sorption of (MoO₄^2-)-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 MoO₄^2- ions on Fe- TPO₄ is an endothermic process.

Serrano Gomez, J., Bonifacio Martinez, J., and Lopez Reyes, M. C., Adsorption of MoO₄^2-- 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.]