Adsorption Of Molybdate Research Articles

Development of the passive film during passivation in passivating electrolytes containing sodium molybdate

The impact of sodium molybdate on passive films formed on lean duplex stainless steel (STS 329 FLD) was investigated. Elemental depth distribution was examined using glow discharge optical emission spectroscopy, while the analysis covered passive film capacitance, molybdenum's chemical state, and corrosion resistance. The addition of sodium molybdate to the passivation electrolyte resulted in a reduction of iron content in the iron-rich layer (from 48 % to 38 %), an increase in chromium content in the enhanced chromium layer (from 33 % to 42 %), and the presence of molybdenum on the film surface. Electrical impedance spectroscopy measurements indicated a significant increase in the capacitance of the passive film. Furthermore, molybdenum was identified as molybdate. Electrochemical anodic polarization curves revealed that the corrosion potential rose with the addition of sodium molybdate to the nitric acid electrolyte. This improvement in corrosion resistance was attributed to molybdate adsorption on the outermost part of the passive film, preventing chloride adsorption or penetration. Additionally, the corrosion current density decreased due to the presence of the chromium-rich layer.

Fabrication of an innovative phosphonate Schiff base adsorbent for molybdenum adsorption and its applications for molybdenum adsorption from spent hydrodesulfurization catalyst

AbstractBACKGROUNDTo investigate the recovery of molybdate from water and hydrodesulfurization catalysts, a novel synthesized phosphonate Schiff base, 4,4′‐(((2,2‐dimethylpropane‐1,3‐diyl) bis(azaneylylidene)) bis(methaneylylidene)) bis(2‐methoxyphenol) (HMI) and 4‐(((3‐((4‐(((S)‐hydroxyhydrophosphoryl)oxy)‐3‐methoxybenzylidene) amino)‐2,2‐dimethylpropyl)imino)methyl)‐2‐methoxyphenyl hydrogen phosphonate (HIMP), was utilized. This study aimed to assess the structural and property aspects of this synthesized compound using diverse characterizations, including FTIR, Thermogravimetric Analysis (TGA), mass spectrometry (GC–MS), fluorescence transfer spectra, scanning electron microscopy (SEM), and 1H‐NMR, 13C‐NMR, and 31P‐NMR.RESULTSThe study systematically examined several factors controlling molybdate recovery, including temperature, adsorbent dosage, pH, interaction time, and molybdate concentration using the batch technique. Optimal sorption effectiveness was achieved at pH 2, with an interaction time of 40 min, a 0.06 g adsorbent dose, at ambient temperature. The newly devised adsorbent exhibited an impressive adsorption capacity (Qe) of 372.54 mg of Mo per gram, with empirical data fitting the Langmuir and D‐R adsorption models. The kinetics of molybdate uptake followed second‐order kinetics. Thermodynamic aspects, including ΔG°, ΔS°, and ΔH°, were also investigated, providing appreciated perceptions into the energetic dynamics of the sorption process. Additionally, molybdate adsorption in the occurrence of other ions was explored, highlighting the selectivity of the modified phosphonate Schiff base for molybdate removal.CONCLUSIONThe study underscores the potential of the modified phosphonate Schiff base as a crucial adsorbent for molybdate elimination from both solutions and spent hydrodesulfurization catalysts. The findings offer important insights into the adsorption kinetics, thermodynamics, and selectivity of the adsorbent, enhancing our understanding of molybdate recovery processes and their broader applications. © 2024 Society of Chemical Industry (SCI).

Designing a dual barrier-self-healable functional epoxy nano-composite using 2D-carbon based nano-flakes functionalized with active corrosion inhibitors

Graphene oxide (GO) nanosheets were in-situ functionalized via polypyrrole (POP) nanoparticles using ammonium persulfate and afterward doped with sodium molybdate (GPM) and finally incorporated into the epoxy resin for achieving a nano-composite coating with dual barrier and self-healing properties. The synthesized nanoparticles were characterized by field-emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction analysis (XRD), and thermal gravimetric analysis (TGA) techniques. The release mechanism of the inhibitors from the GPM was investigated in both solution and coating phases. In addition, the modified coatings were tested electrochemically using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization as well as the pull-off test, cathodic disbonding, and salt spray. Results from FE- SEM /EDS, FT-IR, TGA, and XRD investigations supported the successful synthesis of the GPM complex and molybdate adsorption on the surface of GP. Electrochemical measurements revealed that the release of MoO4−2 from GPM resulted in the creation of a compact protective layer on the steel surface, enhanced inhibitive ability, and a low corrosion rate (1.41 A cm−2). It was shown that the charge transfer resistance of the bare steel sample in the blank solution enhanced from 2360 Ω cm2 to 8530 Ω cm2 in the presence of GPM after 1 h immersion time. The epoxy coating's ability to prevent corrosion was also improved by the GPM nanoparticles, reaching 99%.

Surface and internal modification of composite ion exchange membranes for removal of molybdate, phosphate, and nitrate from polluted groundwater

Al 2 O 3 /chitosan-multiwall carbon nanotubes (MWCNTs) were created to increase the exchange capacity of polyvinylidene fluoride (PVDF) ion-exchange membranes. The composite membranes were made by mixing Al 2 O 3 nanoparticles into the PVDF cast solution, then applying a thin coating of chitosan functionalized carbon nano tubes (Cs-MWCNTs) to the PVDF membrane surface. The structure and characteristics of the hybrid membranes were described using XRD, SEM, IR, and TG-DTA. The Al 2 O 3 -PVDF/Cs-MWCNTs membrane beat the other Al 2 O 3 -PVDF/Cs, Al 2 O 3 -PVDF, and PVDF membranes in terms of molybdate, phosphate, and nitrate adsorption. The removal efficiency, pH solution, adsorption capacity, and desorption process of molybdate, phosphate, and nitrate anions by Al 2 O 3 -PVDF and PVDF membranes were investigated. The removal effectiveness of molybdate, phosphate, and nitrate, according to the testing findings, was 94.3, 65.6, and 85.78 %, respectively. The adsorption of MoO 4 2− , PO 4 3− , and NO 3 − increased as the pH increased initially until the best adsorption was achieved, and then decreased significantly as the pH increased further. The total adsorption capabilities of MoO 4 2− , PO 4 3− , and NO 3 − for the Al 2 O 3 -PVDF/Cs-MWCNTs membrane were 65.50, 61.22, and 59.77 mg/g, respectively. Using regeneration and reuse experiments for the simultaneous adsorption of molybdate, phosphate, and nitrate during three consecutive cycles, the adsorption/desorption of Al 2 O 3 -PVDF/Cs-MWCNTs was assessed. Al 2 O 3 -PVDF/Cs-MWCNTs offer a lot of promise when it comes to eliminating MoO 4 2− , PO 4 3− , and NO 3 − from actual wastewater samples.

Adsorption of octahedral mono-molybdate and poly-molybdate onto hematite: A multi-technique approach

Molybdenum (Mo) is a key trace element and a contaminant in many environments including mine tailings and acid mine drainage systems. Under oxic conditions Mo exists in a number of forms, including mono-molybdate (Mo(VI)O42-) and various poly-molybdate species (e.g. Mo(VI)7O246-) depending on the geochemical conditions (e.g. pH). The mobility and bioavailability of Mo is often controlled by sorption to mineral surfaces, including iron (oxyhydr)oxides e.g. hematite (Fe2O3). This study uses adsorption isotherms, PHREEQC geochemical modeling, Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and X-ray Absorption Spectroscopy (XAS) to holistically characterise the molecular scale adsorption of molybdate to hematite as a function of pH (3−12) and Mo(VI) concentration (0.01 × 10−4 - 2 × 10−3 M). PHREEQC and ATR-FTIR indicated both pH and Mo concentration are important variables when forming mono- vs. poly- molybdate and suggest low pH (≤ 4) and high Mo(VI) concentration (≥ 5 × 10−4 M) contribute to the formation of a poly-molybdate surface species on the hematite surface. XAS found Mo adsorbed to hematite via an inner-sphere corner-sharing bidentate binuclear complex with an octahedral mono-molybdate structure at a Mo concentration of 0.6 × 10−4 M across the pH range, and at a Mo(VI) concentration of 5 × 10−4 M and pH over 5. This is the first direct observation of octahedrally coordinated Mo(VI) adsorption species on hematite, and this information has broad implications for the mobility and transport of Mo as a contaminant in the environment.

Molybdate Recovery by Adsorption onto Silica Matrix and Iron Oxide Based Composites.

Aggressive industrial development over the last century involved different heavy metals being used, including high quantities of molybdenum, which need to be treated before discharge in industrial waters. Molybdenum’s market price and industrial applicability make its recovery a big challenge. In the present study the possibility to recover molybdenum ions from aqueous solutions by adsorption on a composite material based on silica matrix and iron oxides—SiO2FexOy—was evaluated. Tests were performed in order to determine the influence of adsorbent material dose, initial solution pH, contact time and temperature over adsorption capacity of synthesized adsorbent material. For better understanding of the adsorption process, the obtained experimental data were modelled using Langmuir, Freundlich and Sips adsorption isotherms. Based on the obtained data, it can proved that the Sips isotherm was describing with better orderliness the studied process, obtaining a maximum adsorption capacity of 10.95 mg for each gram of material. By modelling the studied adsorption process, it was proven that the pseudo-second order model is accurately describing the adsorption process. By fitting experimental data with Weber-Morris model, it was proven that adsorption is a complex process, occurring in two different steps, one controlled by diffusion and the second one controlled by mass transfer. Further, studies were performed in order to determine the optimum pH value needed to obtain maximum adsorption capacity, but also to determine which are the adsorbed species. From pH and desorption studies, it was proven that molybdate adsorption is a physical process. In order to establish the adsorption mechanism, the thermodynamic parameters (ΔG0, ΔH0 and ΔS0) were determined.

Simultaneous Removal of Molybdate and Chromate Ions from Industrial Wastewater using Biosorbents Derived from Stems of Murraya koenigii: Thermodynamics, Isothermal and Kinetic Investigations

The removal of molybdate and chromate from industrial wastewater is one of the major tasks of water remediation methods. The disposal of ill-treated effluents containing these toxic heavy metal oxyanions into aqueous environment, effects aquatic life, ecosystems and endogenous the human life. The adsorptive methods available so far are to removal either molybdate or chromate ions and not their simultaneous removal. In the present investigation, a bioadsorbent derived from Murraya koenigii plant has the potential to remove both molybdate and chromate ions simultaneously at pH 2.5. The adsorbent was characterized using XRD and FTIR besides the assessment of conventional physico-chemical parameters. Various extraction conditions were investigated and optimized using simulated solutions of individual as well as mixtures of molybdate and chromate ions. The optimum conditions for simultaneous removal are: pH: 2.5; dosage of adsorbent: 2.5 g/L; contact time: 120 min; rpm: 300; temp.: 30 ± 1 ºC. The extraction was marginally effected by common co-ions. The adsorbents can be regenerated and reused for three cycles. Thermodynamic parameters revealed that the adsorption of molybdate and chromate onto the surface of the adsorbent is endothermic and spontaneous. Further, the magnitude of ΔH values and IR data confirmed that the nature of adsorption is ‘ion exchange and/or a sort of surface complex formation’. Kinetics of adsorption was analyzed by various models and of them, pseudo-second-order model explains well. Of the various isotherm models analyzed, Langmuir model fits well and thereby indicating the homogeneity surface of the adsorbent and unform distribution of active sites. The developed method was applied to treat real wastewater samples collected from industrial and mining effluents and found to be highly effective. The novelty of the present investigation is that a simple and effective bioadsorbent is developed for the simultaneous removal of highly toxic molybdate and chromate ions from the industrial wastewaters.

Soil chemical properties and nutrition of conilon coffee fertilized with molybdenum and nitrogen

Molybdenum (Mo) availability is strongly affected by soil pH, which determines the dynamics of electrical charges and the adsorption of molybdate. This study evaluated the effects of nitrogen (N) and Mo application on the chemical properties of a Latossolo Amarelo (Oxisol) and in Coffea canephora nutrition and productivity throughout two productive cycles under field conditions. The experiment was conducted from June 2018 to May 2020. The experimental design used was in randomized blocks, in a 2 × 5 factorial [...]

Observation of surface precipitation of ferric molybdate on ferrihydrite: Implication for the mobility and fate of molybdate in natural and hydrometallurgical environments

Adsorption of molybdate (Mo(VI)) on the surfaces of ferrihydrite is one of the most critical processes that control its mobility and fate in the environment. However, the sorption mechanism and the effect of pH on the speciation of Mo(VI) on ferrihydrite surfaces are not well understood. In this study, X-ray diffraction (XRD), Raman, Fourier transform infrared (FTIR), and Mo K-edge and L3-edge X-ray absorption spectroscopy (XAS) have been utilized to characterize the Mo(VI) species sorbed on ferrihydrite under various pH conditions. XRD, Raman, and FTIR results show that at acidic pH, surface precipitation of poorly crystalline ferric molybdate (PCFM) occurs under apparently undersaturated conditions (theoretical log IAP < log Ksp) and is enhanced by the aging process, whereas Mo(VI) is mainly present as surface adsorbed species at circum-neutral pH. The Mo K-edge and L3-edge X-ray absorption near edge structure (XANES) analyses show that a mixture of tetrahedrally and octahedrally coordinated Mo(VI) simultaneously exists at pH 3–7 and the octahedral Mo(VI) species decreases with increasing pH. The Mo-Fe interatomic distances (3.52–3.56 Å) derived from EXAFS fittings suggest the corner-sharing complexation of both MoO4 and MoO6 with FeO6 octahedra. As the pH decreases from 7 to 3, the coordination number of the Mo-Fe shell (CNMo-Fe) increases from 0.6(3) to 1.9(3), possibly due to the gradual transformation of surface adsorbed Mo(VI) to PCFM. These findings on the observation of Mo(VI) complexation, surface precipitation, and their marked pH dependence during the Mo(VI) adsorption on ferrihydrite have important implications for both understanding the mobility and fate of Mo(VI) in natural and hydrometallurgical industry impacted environments and developing optimal applications for the remediation of Mo contamination in aqueous environments.

Kinetic characteristics of mobile Mo associated with Mn, Fe and S redox geochemistry in estuarine sediments

Estuarine sediments are crucial repositories and incubators of molybdenum (Mo) during its transport from rivers to the ocean. Here, Mo mobility and related processes in estuarine sediments were explored using high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques. Better correlations were observed between dissolved Mn and Mo than between dissolved Fe and Mo, implying that Mn geochemistry plays a key role in dissolved Mo mobility via molybdate adsorption onto abundant Mn oxides and its substantial release upon intense Mn reduction. As a result, oxic intertidal sediments functioned as Mo sinks, and anoxic subtidal sediments functioned as Mo sources. The opposite vertical distributions between DGT-Labile S and DGT-Labile Mo indicated that the availability of labile Mo can be blocked by aqueous sulfide. However, the corresponding high concentrations of DGT-Labile S and dissolved Mo at subtidal sites demonstrated that the abundant dissolved Mo remobilized via Mn reduction was not effectively solidified by sulfide. Simulation with the DIFS model further verified that redox conditions and induced physicochemical processes are crucial factors controlling Mo mobility, with relatively low dissolved Mo concentrations but an adequate and steady resupply capacity of the bioavailable molybdate in intertidal sediments.

Hollow Molybdate Microspheres as Catalytic Hosts for Enhancing the Electrochemical Performance of Sulfur Cathode under High Sulfur Loading and Lean Electrolyte

AbstractLithium–sulfur battery possesses a high energy density; however, its application is severely blocked by several bottlenecks, including the serious shuttling behavior and sluggish redox kinetics of sulfur cathode, especially under the condition of high sulfur loading and lean electrolyte. Herein, hollow molybdate (CoMoO4, NiMoO4, and MnMoO4) microspheres are introduced as catalytic hosts to address these issues. The molybdates present a high intrinsic electrocatalytic activity for the conversion of soluble lithium polysulfides, and the unique hollow spherical structure could provide abundant sites and spatial confinement for electrocatalysis and inhibiting shuttling, respectively. Meanwhile, it is demonstrated that the unique adsorption of molybdates toward polysulfides exhibits a “volcano‐type” feature with the catalytic performance following the Sabatier principle. The NiMoO4 hollow microspheres with moderate adsorption show the highest electrocatalytic activity, which is favorable for enhancing the electrochemical performance of sulfur cathode. Especially, the S/NiMoO4 composite could achieve a high areal capacity of 7.41 mAh cm−2 (906.2 mAh g−1) under high sulfur loading (8.18 mg cm−2) and low electrolyte/sulfur ratio (E/S, 4 µL mg−1). This work offers a new perspective on searching accurate rules for selecting and designing effective host materials in the lithium–sulfur battery.

Inhibition of microbial souring with molybdate and its application under reservoir conditions

Souring induced by sulfate reducing microorganisms (SRM) represents a severe problem in the petroleum industry. In addition to conventional biocides and nitrate, alternative SRM inhibitors such as molybdate have been proposed recently for controlling microbial souring. We used oilfield-derived microbial consortia, rock and fluids to test molybdate as a specific SRM inhibitor for a microbial enhanced oil recovery (MEOR) application where souring might occur as a side effect. SRM cells were quantified and dissolved molybdate, sulfate and gaseous hydrogen sulfide were measured under different dynamic conditions in sandpacks with and without residual oil. In batch experiments, 0.5 mM molybdate inhibited SRM growth whereas hydrogen sulfide and mineral precipitations were observed in bottles amended with 100 mM nitrate. However, significant molybdate adsorption onto reservoir rock occurred and maximum Langmuir saturation was estimated to be ≥ 34 μmol g−1. Residual oil allowed a further propagation of molybdate into sandpacks, but a pH < 6 and sulfide concentrations >11 μMH2S aq limited the efficiency of molybdate due to rapid adsorption. Under favorable souring conditions, we also observed the localized formation of macroscopic iron sulfide precipitations. These resulted in a four-fold permeability decrease after the injection of SRM substrates for 40 days and a calculated mean sulfate reduction rate of 52 μM h−1. However, delayed sulfate reduction in molybdate-preflushed sandpacks suggests an inhibitory effect even if molybdate is partially adsorbed. Sulfate reduction was not detected when molybdate was continuously injected with MEOR nutrients into sandpacks demonstrating its inhibitory efficiency in case it is applied in early phases of field operations with a potential risk of souring.

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≈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°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 ≈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.

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 0.3M 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.

Removal of molybdenum(VI) from aqueous solutions using nano zero-valent iron supported on biochar enhanced by cetyl-trimethyl ammonium bromide: adsorption kinetic, isotherm and mechanism studies.

A new carbonized pomelo peel biosorbent (MCPP) modified with nanoscale zero-valent iron (NZVI) and cetyl-trimethyl ammonium bromide was prepared and employed for the adsorption of molybdate (Mo(VI)) from aqueous solution. We investigated the effects of various conditions on Mo(VI) adsorption and evaluated the results based on adsorption kinetics models and isotherm equations. The kinetic data fitted to the pseudo-second-order model. The Langmuir model best described the adsorption of Mo(VI) on MCPP. The values of changes in Gibbs free energy, standard enthalpy, and standard entropy revealed that the adsorption process was feasible, spontaneous and endothermal. X-ray diffraction, Fourier transform infrared and X-ray photoelectron spectroscopy measurements suggested that Mo(VI) adsorption occurred via both the reduction and surface adsorption. Thus, biochar, prepared from fruit residue, can be applied to remove Mo(VI) from aqueous solutions. More importantly, our results provide a sustainable approach for Mo(VI) removal from wastewater by means of functional modification.

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°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 MoVI 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.

B1 Effect of Concentration and pH on the Adsorption of Molybdate on Nano-balll Allophane

B1 Effect of Concentration and pH on the Adsorption of Molybdate on Nano-balll Allophane

Continuously increasing δ98Mo values in Neoarchean black shales and iron formations from the Hamersley Basin

We present Mo-, C- and O-isotope data from black shales, carbonate- and oxide facies iron formations from the Hamersley Group, Western Australia, that range in age from 2.6 to 2.5billion years. The data show a continuous increase from near crustal δ98Mo values of around 0.50‰ for the oldest Marra Mamba and Wittenoom formations towards higher values of up to 1.51‰ for the youngest sample of the Brockman Iron Formation. Thereby, the trend in increasing δ98Mo values is portrayed by both carbonate facies iron formations and black shales. Considering the positive correlation between Mo concentration and total organic carbon, we argue that this uniformity is best explained by molybdate adsorption onto organic matter in carbonate iron formations and scavenging of thiomolybdate onto sulfurized organic matter in black shales. A temporal increase in the seawater δ98Mo over the period 2.6–2.5Ga is observed assuming an overall low Mo isotope fractionation during both Mo removal processes. Oxide facies iron formations show lowest Mo concentrations, lowest total organic carbon and slightly lower δ98Mo compared to nearly contemporaneous black shales. This may indicate that in iron formation settings with very low organic matter burial rates, the preferential adsorption of light Mo isotopes onto Fe-(oxyhydr)oxides becomes more relevant.A similar Mo-isotope pattern was previously found in contemporaneous black shales and carbonates of the Griqualand West Basin, South Africa. The consistent and concomitant increase in δ98Mo after 2.54billion years ago suggests a more homogenous distribution of seawater molybdate with uniform isotopic composition in various depositional settings within the Hamersley Basin and the Griqualand West Basin. The modeling of the oceanic Mo inventory in relation to the Mo in- and outflux suggests that the long-term build-up of an isotopically heavy seawater Mo reservoir requires a sedimentary sink for isotopically light Mo. The search for this sink (i.e. adsorption onto Mn-oxides in well oxygenated surface oceans and/or subaerial environments or incomplete thiomolybdate formation in weakly sulfidic settings) remains debated, but its relevance becomes more important closer to the Great Oxidation Event and is probably related to already weakly oxidizing conditions even prior to the 2.5Ga “whiff of oxygen”.

Molybdate adsorption by birnessite

Molybdate (MoO42−) adsorption by manganese (Mn) oxide was investigated using a synthetic birnessite. Experiments were carried out in a batch experiment as a function of time (1min to 28 d), pH (2–10) and the competitive anions sulphate (SO42−), phosphate (PO43−), selenate (SeO42−) and selenite (SeO32−). Furthermore, MoO42− adsorption was described as a function of equilibrium concentration at pH4–7 and the data were evaluated with the Freundlich and Langmuir equations. The amount of adsorbed MoO42− was strongly dependent on time and reached roughly an equilibrium after three days. An increase in the pH by 1.15 units within 28days indicates ligand exchange of surface hydroxyls with MoO42−. Molybdate adsorption showed high sensitivity to pH and reached a maximum at pH3, near the pKa1 and pKa2 for molybdic acid. The Freundlich equation adequately reconstructed the adsorption data. Molybdate adsorption also conformed to the Langmuir equation for the investigated pH values. The competition sequence of anions for MoO42− adsorption by Mn oxide was SeO32−>SeO42−>PO43−>SO42−, assuming a strong adsorption mechanism of MoO42− onto the surface of birnessite. These results indicate that Mn oxides have a strong effect on the adsorption of MoO42− especially due to their higher specific surface area compared to aluminium or iron oxides. This is important regarding the availability of MoO42− in soils under agricultural conditions (pH4–7) as well as for the treatment of soils and groundwater affected by elevated intake from industries.

Adsorption of molybdate on molybdate-imprinted chitosan/triethanolamine gel beads

Mo (VI)-imprinted chitosan (CTS)/triethanolamine (TEA) gel beads (Mo (VI)-ICTGBs) (ICTGBs=imprinted chitosan triethanolamine gel beads) were prepared by using ion-imprinted technology, in which TEA and molybdate solution were used in coagulation bath. The spectrum of FT-IR implies that bonding are formed between TEA and the primary hydroxyl of CTS, and ion gel reaction happen between CTS and molybdate; XRD patterns also prove the change among CTS, TEA and molybdate. SEM images and N2 adsorption show that the surface area increases obviously after eluting Mo (VI) ions. The adsorption isotherm of Mo (VI)-ICTGBs imply that the adsorption process is according with Freundlich model. Adsorption kinetics suggests that the pseudo-second order adsorption mechanism is predominant for this adsorbent system of Mo (VI)-ICTGBs. The Mo (VI)-ICTGBs show high adsorption capacity and good selectivity for Mo (VI) anions in the coexistence system at pH=6.0. The Mo (VI)-ICTGBs have a good application prospect, because it is with a simple and rapid technique and good durance.