Electrochemical Characterization of Isolated Nitrogenase Cofactors from Azotobacter vinelandii.

The nitrogenase cofactors are structurally and functionally unique in biological chemistry. Despite a substantial amount of spectroscopic characterization of protein-bound and isolated nitrogenase cofactors, electrochemical characterization of these cofactors and their related species is far from complete. Herein we present voltammetric studies of three isolated nitrogenase cofactor species: the iron-molybdenum cofactor (M-cluster), iron-vanadium cofactor (V-cluster), and a homologue to the iron-iron cofactor (L-cluster). We observe two reductive events in the redox profiles of all three cofactors. Of the three, the V-cluster is the most reducing. The reduction potentials of the isolated cofactors are significantly more negative than previously measured values within the molybdenum-iron and vanadium-iron proteins. The outcome of this study provides insight into the importance of the heterometal identity, the overall ligation of the cluster, and the impact of the protein scaffolds on the overall electronic structures of the cofactors.

Precise determination of the molybdenum isotopic composition of urine by multiple collector inductively coupled plasma mass spectrometry.

RATIONALE: Molybdenum (Mo) is predominantly expelled from the human body in urine. Consequently, urinary variability in the concentration and isotopic composition of Mo may encode valuable clinical information. To access this information, however, it is first necessary to develop and demonstrate a rapid, accurate and precise methodology capable of concentrating Mo from urine for isotope analysis. METHODS: The utility of N-benzoyl-N-phenylhydroxylamine (BPHA) to effectively separate and purify Mo from urine samples without the need for acid digestion was tested. Following this approach, applying a double-spike mass bias correction, we determined the Mo isotopic compositions of a set of urine samples by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). RESULTS: Based on replicate analyses of an in-house urine standard, this approach demonstrates an external precision on δ98/95 Mo values of better than 0.08‰ (2SD, n = 15). Application to a sample set collected from healthy individuals in Guangzhou, China, provides the first suite of δ98/95 Mo measurements from urine samples. Samples from the female participants show δ98/95 Mo (‰) values (1.31 ± 0.19‰, Ave ± 2SD, n = 14) that are consistently lower than those from the male participants (1.55 ± 0.16‰, Ave ± 2SD, n = 17). CONCLUSIONS: The employed methodology is suitable for rapid, low-blank and high-throughput Mo isotope analysis of urine samples. Although resolvable δ98/95 Mo variability is seen in this preliminary dataset, the mechanism driving this variability is unknown. High-precision Mo isotopic analysis might be added to the urinalysis tool-kit, with the potential to provide valuable clinical information in the future.

Changes in the fractionation profile of Al, Ni, and Mo during bioleaching of spent hydroprocessing catalysts with Acidithiobacillus ferrooxidans.

Spent hydroprocessing catalysts are known to contain a variety of potentially toxic metals and therefore studies on the bioavailability and mobility of these metals are critical for understanding the possible environmental risks of the spent catalysts. This study evaluates the different chemical fractions/forms of aluminium (Al), nickel (Ni), and molybdenum (Mo) in spent hydroprocessing catalyst and the changes they undergo during bioleaching with Acidithiobacillus ferrooxidans. In the spent catalyst (prior to bioleaching), Al was primarily present in its residual form, suggesting its low environmental mobility. However, Ni comprised mainly an exchangeable fraction, indicating its high environmental mobility. Molybdenum was mainly in the oxidizable form (47.1%), which indicated that highly oxidizing conditions were required to liberate it from the spent catalyst. During bioleaching the exchangeable, reducible and oxidizable fractions of all the metals were leached, whereas the residual fractions remained largely unaffected. At the end of bioleaching process, the metals remaining in the bioleached sample were predominantly in the residual fraction (98.3-99.5%). The 'risk assessment code' (RAC) and IR analysis also demonstrated that the environmental risks of the bioleached residue were significantly lower compared to the untreated spent catalyst. The results of this study suggest that bioleaching is an effective method in removing the metals from spent catalysts and the bioleached residue poses little environmental risk.

Molybdenum(VI) removal by using constructed wetlands with different filter media and plants.

The efficacy and capacity of vertical-flow wetland filters on molybdenum (Mo) removal from wastewater was examined, employing reed (Phragmites australis) and cattail (Typha latifolia) as well as different adsorption granular media. Humus, cinder, modified cinder, as well as pyrite were used as filter media. A synthetic effluent with different concentrations of Mo(VI) at different hydraulic retention times was used for simulating Mo leached mine wastewater. Laboratory experiments showed that the equilibrium adsorption data were in agreement with the Langmuir isotherm model, and the maximum Mo(VI) adsorption capacities of modified cinder and pyrite were 10.01 and 6.25 mg/g, respectively. Mo(VI) removal in F5 (combination substrates of pyrite and cinder) was found to be more stable and effective than that of F1 (conventional gravel and soil filter media) during the 14-week experiment. Most of the Mo(VI) was retained in the 10-20 cm of the substrate, and adsorbed by the modified cinder and pyrite. The largest fraction of Mo(VI) retained was the water-soluble fraction on the surface of the pyrite. Cattail was more suitable for Mo(VI) absorption than reed, but the bioaccumulation accounted for a very small portion of the total removal.

A novel micellar electrokinetic chromatographic method for separation of metal-DDTC complexes.

Micellar electrokinetic chromatography (MEKC) was examined for the separation and determination of Mo(VI), Cr(VI), Ni(II), Pd(II), and Co(III) as diethyl dithiocarbamate (DDTC) chelates. The separation was achieved from fused silica capillary (52 cm × 75 µm id) with effective length 40 cm, background electrolyte (BGE) borate buffer pH 9.1 (25 mM), CTAB 30% (100 mM), and 1% butanol in methanol (70 : 30 : 5 v/v/v) with applied voltage of -10 kV using reverse polarity. The photodiode array detection was achieved at 225 nm. The linear calibration for each of the element was obtained within 0.16-10 µg/mL with a limit of detection (LOD) 0.005-0.0167 µg/mL. The separation and determination was repeatable with relative standard deviation (RSD) within 2.4-3.3% (n = 4) in terms of migration time and peak height/peak area. The method was applied for the determination of Mo(VI) from potatoes and almond, Ni(II) from hydrogenated vegetable oil, and Co(III) from pharmaceutical preparations with RSD within 3.9%. The results obtained were checked by standard addition and rechecked by atomic absorption spectrometry.

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 modeling 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 microg/l Mo) can easily be met.

Isolation, characterization, and computational studies of the novel [Mo3(mu3-Br)2(mu-Br)3Br6]2- cluster anion.

The novel trimolybdenum cluster [Mo(3)(mu(3)-Br)(2)(mu-Br)(3)Br(6)](2-) (1, {Mo(3)}(9+), 9 d-electrons) has been isolated from the reaction of [Mo(CO)(6)] with 1,2-C(2)H(4)Br(2) in refluxing PhCl. The compound has been characterized in solution by electrospray ionization mass spectrometry (ESI-MS), UV-vis spectroscopy, cyclic voltammetry, and in the solid state by X-ray analysis (counter-cations: (n-Bu)(4)N(+) (1), Et(4)N(+), Et(3)BzN(+)), electron paramagnetic resonance (EPR), magnetic susceptibility measurements, and infrared spectroscopy. The least disordered (n-Bu)(4)N(+) salt crystallizes in the monoclinic space group C2/c, a = 20.077(2) A, b = 11.8638(11) A, c = 22.521(2) A, alpha = 90 deg, beta = 109.348(4) deg, gamma = 90 deg, V = 5061.3(9) A(3), Z = 4 and contains an isosceles triangular metal arrangement, which is capped by two bromine ligands. Each edge of the triangle is bridged by bromine ions. The structure is completed by six terminal bromine ligands. According to the magnetic measurements and the EPR spectrum the trimetallic core possesses one unpaired electron. Electrochemical data show that oxidation by one electron of 1 is reversible, thus proceeding with retention of the trimetallic core, while the reduction is irreversible. The effective magnetic moment of 1 (mu(eff), 1.55 mu(B), r.t.) is lower than the spin-only value (1.73 mu(B)) for S = 1/2 systems, most likely because of high spin-orbit coupling of Mo(III) and/or magnetic coupling throughout the lattice. The ground electronic state of 1 was studied using density functional theory techniques under the broken symmetry formalism. The ground state is predicted to exhibit strong antiferromagnetic coupling between the three molybdenum atoms of the core. Moreover, our calculated data predict two broken symmetry states that differ only by 0.4 kcal/mol (121 cm(-1)). The antiferromagnetic character is delocalized over three magnetic orbitals populated by three electrons. The assignment of the infrared spectra is also provided.

A compact solvent extraction based 99Mo/99mTc generator for hospital radiopharmacy.

A compact and portable 99Mo-99 mTc generator based on solvent-extraction, mimic to the conventional 99Mo-99 mTc alumina column generator is much-needed commodity for use in hospital radiopharmacy setup. The present study includes the development of a portable, simple and low cost 99Mo/99 mTc-generator based on MEK solvent extraction technique to obtain a very high concentration of no-carrier added (nca) 99 mTc solution, where low specific activity 99Mo source is obtained through 98Mo(n, γ)99Mo reaction in a research reactor. The unit is intended for operation under the conditions of medical radiological laboratories. Technical trials showed that the mean time of preparation of sodium [99mTc] pertechnetate radiopharmaceutical did not exceed 15 min. The quality and yield of 99 mTc-pertechnetate is upto the mark for formulation of radiopharmaceuticals.

Determination of sub-ppb cadmium in urine by solid-phase extraction and inductively coupled plasma-mass spectrometry.

In order to determine low levels of Cd in urine samples, we tried to remove Mo interference using ICP-MS with a dynamic reaction-cell technique, but failed due to the low sensitivity and the variance with the standards. We then performed solid-phase extraction (SPE) before the ICP-MS measurement. A commercially available chelating resin, NOBIAS PA-1, was used for SPE, and could effectively remove Mo from urine samples, permitting the accurate determination of Cd by ICP-MS. This SPE-ICP-MS method gave 0.012 microg Cd L(-1) as the method limit of quantification, and the mean recovery of Cd spiked with 0.0505 and 5.05 microg L(-1) was 93.1 and 97.6%, respectively.

Fractionated elution of 99Mo/99mTc generators using calibrated vials.

Fractionated elution consists in collecting the fractions of an eluate with the highest radioactive concentration. It may be useful to meet the requirements of a subset of clinical radiopharmacy procedures. This study aims to describe and evaluate straightforward procedures allowing to readily perform fractionated elution on dry and wet columns Mo/Tc generators by using calibrated vials. The main objectives of this study consisted in determining the relationship between eluate volume and elution yield using different vials calibration and assessing repeatability of the procedure. Elution vials were calibrated to obtain different eluate volumes by addition of air for wet column generator (WCG) and subtraction of saline for dry column generator (DCG) (n≥5 for each calibration). The relationship between the eluate volume and the elution yield was determined by a regression model for both DCG and WCG. Then repeatability evaluation was performed using 3-ml vial calibration. Relationships between the eluate volume (V) and the elution yield (Y) for DCG and WCG were Y=57.551 ln(V)+10.526 and Y=50.256 ln(V)+17.597, respectively. For repeatability assessment (n=30 for DCG and n=31 for WCG), the median volume and the interquartile range for DCG and WCG were 2.98 ml (2.92-3.01) and 3.28 ml (2.71-3.40), respectively, and median (interquartile range) eluate yields were 84.73% (81.30-86.33) and 81.78% (78.91-85.20), respectively. The volume was significantly higher for WCG than DCG (P=0.036) and also significantly more variable (P<0.001). The elution yield was significantly lower for WCG than DCG (P=0.025), but no difference in variability between the two generators was found (P=0.874). Easy-to-handle fractionated elution methods are compatible with both DCG and WCG. Fractionation using calibrated vials exhibits a better reproducibility with DCG than WCG generators and represents the only proposed method so far to master fractionated elution with DCG.

Impact of ageing on the fate of molybdate-zerovalent iron nanohybrid and its subsequent effect on cyanobacteria (Microcystis aeruginosa) growth in aqueous media.

Nanoscale zerovalent iron (nZVI) has been proposed to remediate heavy metal ions in the subsurface. However, the fate of metal-nZVI hybrid has not been fully investigated. In this study, we investigated (1) the long-term removal performance of nZVI for molybdate (Mo(VI)); (2) the relationship between the ageing of Mo-nZVI hybrid in specific solution chemistries and the remobilization of Mo(VI) from the hybrid; and (3) the effects of Mo-nZVI hybrid on cyanobacteria (Microcystis aeruginosa). Results showed that although common ions have limited influence on the removal ratio of Mo(VI) by nZVI, they do impact the structure evolution and transformation of the Mo-nZVI nanohybrid formed thereafter. Ageing time was crucial for the chemical stabilization of Mo-nZVI hybrid, but common groundwater ions retarded the stabilizing process, which may lead to a significant remobilization of Mo(VI) from the hybrid after exposure to water bodies. While low levels of Mo(VI) ions could stimulate the growth of M. aeruginosa, aged Mo-nZVI hybrid inhibited the growth of M. aeruginosa, except when ageing occurred in the presence of HPO42-/CO32- (which also retarded hybrid stabilization). This study shows that nZVI can immobilize Mo(VI) ions in groundwater, and the derived metal-nZVI hybrid can effectively suppress the potential growth of M. aeruginosa in river water.

The Use of a Molybdenum Polyoxometalated Compound to Increase the Amount of Extractives from Wood Wastes.

The treatment of wood wastes of Castanea sativa L., Quercus frainetto, Larix decidua, and Paulownia tomentosa S. in autoclave in the presence of micrometric crystals of H3PMo12O40 showed an impressive increase of the amount of extractives. The extractives were mainly constituted of insoluble compounds that were analyzed by using gas chromatography-mass spectrometry (GC-MS) after acetylation. The GC-MS analysis of the chloroform soluble fraction of the extractives obtained from sativa showed the presence of methyl hexadecanoate and octadecanoic acid, that of the extractives of frainetto showed the presence of octadecanal and some long chain hydrocarbons. decidua extracts showed the presence of large amounts of sesamin, while the extractives of P tomentosa revealed the presence of 4-hydroxy-3,5-dimethoxybenzaldehyde, 4-hydroxy-3,5-dimethoxycimmanaldehyde, and relevant amounts of long chain hydrocarbons. The insoluble fraction showed the presence of relevant amounts of several carbohydrates and, in the case of C. sativa, of inositol.

Competitive adsorption and desorption of arsenate, vanadate, and molybdate onto the low-cost adsorbent materials alum water treatment sludge and bauxite.

When low-cost adsorbents are being used to remove contaminant ions (e.g. arsenate, vanadate, and molybdate) from wastewater, competitive adsorption/desorption are central processes determining their removal efficiency. Competitive adsorption of As, V, and Mo was investigated using equimolar oxyanion concentrations in single, binary, and tertiary combinations in adsorption isotherm and pH envelope studies while desorption of previously adsorbed oxyanions was examined in solutions containing single and binary oxyanion combinations. The low-cost adsorbent materials used were alum water treatment sludge (amorphous hydroxy-Al) and bauxite ore (crystalline Al oxides). Adsorption isotherm and pH envelope studies showed that Mo had only a small effect in decreasing adsorption of As and V but V and As had substantial and similar effects in reducing adsorption of the other. As had a greater effect than V in reducing adsorption of Mo and it was concluded that the affinity of oxyanions for the surfaces of water treatment sludge and bauxite followed the order As > V >> Mo. In 0.3 M NaCl electrolyte, desorption of previously adsorbed oxyanions amounted to 0.3-3.4% for V and As, and 11-20% for Mo. As had approximately four times greater effect than Mo in increasing desorption of V while V had about three times the effect of Mo in increasing desorption of As. Thus, the order of oxyanions in inducing desorption of the other oxyanions (i.e. As on V and As) was the same as that for adsorption selectivity: As > V >> Mo. Water treatment sludge was a more effective adsorbent than bauxite because it had a greater adsorption capacity for all three anions and, in addition, they were held more strongly so desorption in the background electrolyte was proportionately less. It was concluded that at similar molar concentrations, arsenate would tend to reduce adsorption of vanadate as well as displace vanadate already held on adsorbent surfaces while both anions will compete effectively with molybdate. The limiting factor for simultaneous removal of As, V, and Mo from multielement solutions by adsorption will therefore be the removal of Mo.

Purification and properties of sulfite oxidase from human liver.

Sulfite oxidase has been purified to near homogeneity from human liver. Properties of the molecule have been investigated and compared to those of the rat liver enzyme which has been isolated in a pure form. Both proteins exist as dimeric molecules with one molybdenum and one cytochrome b5-type heme per sub-unit. The human enzyme has a slightly larger subunit molecular weight (61,100 vs. 57,200 daltons) and is a more negatively charged molecule. Decreased reactivity of the human enzyme with various electron acceptors suggests the presence of nonfunctional molybdenum centers in a portion of the molecules isolated. Human liver sulfite oxidase cross-reacts strongly with antibody prepared against the rat liver enzyme. Human enzyme activity is precipitated by antibody at concentrations about twofold greater than required for comparable complexation of rat sulfite oxidase.

A novel molybdenum disulfide nanosheet self-assembled flower-like monolithic sorbent for solid-phase extraction with high efficiency and long service life.

A novel material consisting of molybdenum disulfide (MoS2) nanosheet that self-assemble into flower-like microspheres which aggregate to form a monolithic matrix with a micro or nano-scaled mesopore structure was successfully synthesized and used as an efficient sorbent for solid-phase extraction (SPE) due to its large specific adsorption area and good stability. The extraction properties of the as-prepared sorbent were evaluated by high-performance liquid chromatography with variable wavelength detection (HPLC-VWD) by analyzing four flavonoids (apigenin, quercetin, luteolin, and kaempferol). Under optimal conditions, the LODs and LOQs were found to be in the ranges of 0.1-0.25 and 0.4-0.5µgL-1, respectively, and wide linear ranges were obtained with correlation coefficients (R) ranging from 0.9991 to 0.9996. Compared with commercial C18 and Alumina-N sorbents, the as-prepared sorbent showed high extraction efficiency at different concentrations of flavonoids. After 100 uses, the extraction ability of the self-assembled MoS2 nanosheet monolithic sorbent had no evident decline, denoting a long service life. Finally, the SPE-HPLC-VWD method using the as-prepared sorbent was applied to flavonoid analysis in beverage samples with satisfactory results.

Mechanochemical processing of molybdenum and vanadium sulfides for metal recovery from spent catalysts wastes.

This work describes the mechanochemical transformations of molybdenum and vanadium sulfides into corresponding molybdate and vanadate, to serve as a new environment-friendly approach for processing hazardous spent hydrodesulphurization (HDS) catalysts solid waste to achieve an easy recovery of not only molybdenum and vanadium but also nickel and cobalt. Co-grinding the molybdenum and vanadium sulfides with oxidants and sodium carbonate stimulates solid-state reactions without any heating aid to form metal molybdates and vanadates. The reactions proceed with an increase in grinding time and were enhanced by using more sodium carbonate and stronger oxidant. The necessary conditions for the successful transformation can be explained on the basis of thermodynamic analyses, namely a negative change in Gibbs free energy.

Removal of molybdate from water by adsorption onto ZnCl2 activated coir pith carbon.

Removal and recovery of molybdate from aqueous solution was investigated using ZnCl2 activated carbon developed from coir pith. Studies were conducted to delineate the effects of contact time, adsorbent dose, molybdate concentration, pH and temperature. Two theoretical adsorption isotherms, namely, Langmuir and Freundlich were used to describe the experimental results. The Langmuir adsorption capacity (Q0) was found to be 18.9 mg molybdate/g of the adsorbent. Adsorption followed second order kinetics. Studies were performed at different pH values to find out the pH at which maximum adsorption occurred. The pH effect and desorption studies showed that ion exchange and chemisorption mechanism were involved in the adsorption process. Thermodynamic parameters such as DeltaG0, DeltaH0 and DeltaS0 for the adsorption were evaluated. Effect of foreign ions on adsorption of molybdate has been examined. The results showed that ZnCl2 activated coir pith carbon was effective for the removal and recovery of molybdate from water.

Selective recovery of molybdenum from spent HDS catalyst using oxidative soda ash leach/carbon adsorption method.

The petroleum refining industry makes extensive use of hydroprocessing catalysts. These catalysts contain environmentally critical and economically valuable metals such as Mo, V, Ni and Co. In the present study, a simple hydrometallurgical processing of spent hydrodesulphurization (HDS) catalyst for the recovery of molybdenum using sodium carbonate and hydrogen peroxide mixture was investigated. Recovery of molybdenum was largely dependent on the concentrations of Na2CO3 and H2O2 in the reaction medium, which in turn controls the pH of leach liquor and the presence of Al and Ni as impurities. Under the optimum leaching conditions (40 g L(-1) Na2CO3, 6 vol.% H2O2, room temperature, 1h) about 85% recovery of Mo was achieved. The leach liquor was processed by the carbon adsorption method, which selectively adsorbs Mo at pH around 0.75. Desorption of Mo was selective at 15 vol.% NH4OH. With a single stage contact, it was found possible to achieve >99%, adsorption and desorption efficiency. Using this method, recovery of molybdenum as MoO3 product of 99.4% purity was achieved.

Separation of [(99m)Tc]pertechnetate and molybdate using polyethylene glycol coated C18 and C30 resins.

Hydrophobic adsorbents such as C18 and C30 were coated with PEG and subsequently used for the separation of Mo/Tc. The most effective resin for adsorbing PEG was the C18-U resin, which demonstrated a coating capacity of 97.6±2.8mg PEG per g of resin. The ability to adsorb pertechnetate was proportional to the amount of PEG coated on the hydrophobic resin. The [(99m)Tc]pertechnetate recovery during the separation of cyclotron produced (99m)Tc from (100)Mo was 91.8±0.3% (n=2). The resultant product met relevant USP monograph specifications.

Preconcentration and determination of vanadium and molybdenum in milk, vegetables and foodstuffs by ultrasonic-thermostatic-assisted cloud point extraction coupled to flame atomic absorption spectrometry.

A new ultrasonic-thermostatic-assisted cloud point extraction procedure (UTA-CPE) was developed for preconcentration at the trace levels of vanadium (V) and molybdenum (Mo) in milk, vegetables and foodstuffs prior to determination via flame atomic absorption spectrometry (FAAS). The method is based on the ion-association of stable anionic oxalate complexes of V(V) and Mo(VI) with [9-(diethylamino)benzo[a]phenoxazin-5-ylidene]azanium; sulfate (Nile blue A) at pH 4.5, and then extraction of the formed ion-association complexes into micellar phase of polyoxyethylene(7.5)nonylphenyl ether (PONPE 7.5). The UTA-CPE is greatly simplified and accelerated compared to traditional cloud point extraction (CPE). The analytical parameters optimized are solution pH, the concentrations of complexing reagents (oxalate and Nile blue A), the PONPE 7.5 concentration, electrolyte concentration, sample volume, temperature and ultrasonic power. Under the optimum conditions, the calibration curves for Mo(VI) and V(V) are obtained in the concentration range of 3-340µgL(-1) and 5-250µgL(-1) with high sensitivity enhancement factors (EFs) of 145 and 115, respectively. The limits of detection (LODs) for Mo(VI) and V(V) are 0.86 and 1.55µgL(-1), respectively. The proposed method demonstrated good performances such as relative standard deviations (as RSD %) (≤3.5%) and spiked recoveries (95.7-102.3%). The accuracy of the method was assessed by analysis of two standard reference materials (SRMs) and recoveries of spiked solutions. The method was successfully applied into the determination of trace amounts of Mo(VI) and V(V) in milk, vegetables and foodstuffs with satisfactory results.