Vanadate Adsorption Research Articles

Facet-dependent and defect-enhanced vanadate adsorption on Mg(OH)2 surface: A combined study of DFT calculations and batch experiments

Vanadium contamination has gained increasing attention around the world and it is urgent to develop feasible approach to tackle this growing pollution (e.g., vanadate). Herein, density functional theory (DFT) calculations combined with batch experiments were used to systematically evaluate the capacity of Mg(OH)2 to adsorb vanadate, by investigating the impacts of different exposed facet and surface OH defect on the structure configurations, adsorption energy and the efficiency of vanadate adsorption. DFT calculations suggested that although the capacity of vanadate adsorption on the facets of Mg(OH)2 would be obviously influenced by the formation of various coordination complex (e.g., monodentate, bidentate and tridentate) on each facet, the capacity for these facets of Mg(OH)2 towards vanadate adsorption followed the order: (100) > (001). Moreover, such a vanadate adsorption was further enhanced by the presence of surface OH defect, owing to altered densities of states (DOS) of the adsorption site. Batch experiments verified the defect-rich Mg(OH)2 was favorable to adsorb vanadate, and both (001) and (100) facet play the great roles. This molecular-level understanding of the facet-dependent and defect-enhanced adsorption behavior of Mg(OH)2 toward vanadate provides a useful insight into designing and applying effective adsorbents for the removal of heavy metal oxyanions.

Efficient Vanadate Removal by Mg-Fe-Ti Layered Double Hydroxide

A series of novel layered double hydroxides (Mg-Fe-Ti-LDHs) containing Mg2+, Fe3+ and Ti4+ were prepared. The adsorption performance of Mg-Fe-Ti-LDHs on vanadate in aqueous solution was investigated and the effects of various factors on the adsorption process were examined, including initial vanadate concentration, adsorbent dosage, contact time, solution pH and coexisting ions. A preliminary discussion of the adsorption mechanism of vanadate was also presented. Results show that the adsorption efficiency of vanadate increased with the introduction of Ti4+ into the laminate of LDHs materials. The adsorption capacity of the materials also differed for different anion intercalated layers, and the Mg-Fe-Ti-LDHs with Cl− intercalation showed higher vanadate removal compared to the CO32− intercalated layer. Furthermore, Mg-Fe-Ti-CLDH showed higher vanadate removal compared to pre-calcination. The adsorption experimental data of vanadate on Mg-Fe-Ti-LDHs were consistent with the Langmuir adsorption isotherm model and the adsorption kinetics followed a pseudo-second order kinetic model. The pH of the solution significantly affected the vanadate removal efficiency. Meanwhile, coexisting ions PO43−, SO42− and NO3− exerted a significant influence on vanadate adsorption, the magnitude of the influence was related to the valence state of the coexisting anions. The possible adsorption mechanisms can be attributed to ion exchange and layered ligand exchange processes. The good adsorption capacity of Mg-Fe-Ti-LDHs on vanadate broadens the application area of functional materials of LDHs.

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.

Inhibition Performance Study of Aqueous Vanadate Species on Mg Alloys

This work presents the corrosion inhibition performance of vanadate on Mg alloy AZ31 and its dependence on pH and immersion time. Aqueous vanadate speciation, which has been shown to have a strong effect on corrosion inhibition, is a function of solution pH. The pH dependence of vanadate corrosion inhibition is, therefore, attributed to the pH dependence of vanadate speciation. Immersion in tetrahedrally coordinated vanadate species, present in neutral and alkaline solution, was shown to decrease corrosion rate and increase pitting potential, both of which were enhanced with longer immersion times. Alternatively, exposure to octahedrally coordinated vanadate, which predominates in acidic solution, only slightly decreased the corrosion rate. An acidic solution (octahedrally coordinated speciation) was adjusted to alkaline conditions and samples were then immersed in the adjusted alkaline solution. Inhibition on these samples was weaker than that of samples immersed in alkaline solutions that had never been acidic. Regardless of speciation, vanadate was shown to provide limited inhibition on cathodic reaction kinetics (hydrogen reduction reaction). Anodic inhibition was observed on samples treated in solutions containing tetrahedral species. Inhibition performance was also characterized by electrochemical impedance spectroscopy. Aqueous vanadate in 0.1 M NaCl increased the total impedance at all pH values (5, 7.7, 9.2), with enhanced results observed with increasing immersion time. The greatest increase in impedance for a given immersion time occurred at pH 9.2, which is consistent with the inhibition determined by corrosion rate. SEM showed that vanadate forms a film across both secondary particles and the Mg matrix, and it provided qualitative evidence that inhibition efficiency increased as the pH of the exposure solution increased. XPS results indicate that vanadate film formation is associated with the reductive adsorption of vanadate by Mg. Exposure at a pH of 5 produced vanadate predominated by V4+. Exposure at pH values of 7.7 and higher, however, produced vanadate containing predominantly V3+.

The vanadate adsorption on a mesoporous boehmite and its cleaner production application of chromate

Source reduction of hazardous Cr6+ containing wastes has significance in the cleaner production of chromate and environmental protection.

Adsorption of vanadate(V) on Fe(III)/Cr(III) hydroxide waste

Adsorption of vanadate(V) from aqueous solution onto industrial solid ‘waste’ Fe(III)/Cr(III) hydroxide was investigated. HCl treated Fe(III)/Cr(III) hydroxide was found to be more efficient for the removal of vanadate(V) compared to untreated adsorbent. The adsorption follows second-order kinetics. Langmuir and Freundlich isotherms have been studied. The Langmuir adsorption capacity (Q 0) of the treated and untreated adsorbents was found to be 11.43 and 4.67 mg g−1, respectively. Thermodynamic parameters showed that the adsorption process was spontaneous and endothermic in the temperature range 32–60°C. Maximum adsorption was found at system pH 4.0. The adsorption mechanism was predominantly ion exchange. Effect of other anions such as phosphate, selenite, molybdate, nitrate, chloride, and sulfate on adsorption of vanadium has been examined.

Template Synthesis and Adsorption Properties of Hierarchically Porous Zirconium Titanium Oxides

Hierarchical morphologies in metal oxides are advantageous for many applications, including controlled drug release, photocatalysis, catalysis, synthetic biomaterials, and adsorption and separation technologies. In this study, agarose gel has been used as a template to prepare zirconium titanium mixed oxide pellets with bimodal porosity. Sol-gel chemistry conducted within the agarose gel produced "coral-like" interconnected networks of oxide nanoparticles with controllable quantities of zirconium and titanium. The materials were characterized using N(2) sorption, extended X-ray absorption fine structure, X-ray diffraction, TEM, SEM, zeta potential, and thermogravimetric analysis (to measure surface hydroxyl group density). The oxides were then tested for the adsorption of vanadyl and vanadate to determine which Zr mole fraction exhibited the highest capacity and fastest kinetics. The material containing 25 mol % Zr exhibited the highest surface area (322 +/- 8 m(2)/g) of the compositions investigated and also displayed a superior adsorption rate and capacity. Vanadate adsorption occurred with faster kinetics than did vanadyl adsorption. A comparative study demonstrated that the macro/meso pore structure had improved transport properties over a monomodal mesopore structure of similar Zr/Ti composition. The faster vanadate adsorption kinetics is attributed to enhanced surface accessibility in a hierarchical material.

Effect of salinity on vanadate biosorption by Halomonas sp. GT-83: Preliminary investigation on biosorption by micro-PIXE technique

Thirty-eight soil samples were collected from crude oil contaminated land in south of Iran. Initial screening of a total of 100 bacterial isolates, resulted in the selection of one isolate with maximum adsorption capacity of 52.7 mg vanadate/g dry weight. It was tentatively identified as Halomonas sp. according to morphological and biochemical properties and named strain GT-83. Removal of vanadate by biosorption with Halomonas sp. GT-83 was very sensitive to solution pH. Vanadate adsorption decreased with increasing pH, with maximum adsorption capacities achieved in at pH 3.0 in the absence and in the presence of increasing concentrations of salt. Vanadate-salt biosorption studies were also performed at this pH value. Equilibrium uptakes of vanadate increased with increasing vanadate concentration up to 600 mg/l. Maximum metal removal (91.8%) took place at pH 3.0 with initial vanadate concentration of 100 mg/l, which got reduced (84.8%) in the presence of 50 g/l salt. The equilibrium sorption data were analyzed by using Freundlich isotherm. The specific uptake of vanadate increased at low cell concentration and decreased when cell concentration exceeded 0.75 g/l. The paper also demonstrates the potential value of micro-PIXE in biosorption studies.

Vanadium removal by metal (hydr)oxide adsorbents

Vanadium is listed on the United States Environment Protection Agency (USEPA) candidate contaminant list # 2 (CCL2), and regulatory guidelines for vanadium exist in some US states. The USEPA requires treatability studies before making regulatory decisions on CCL2 contaminants. Previous studies have examined vanadium adsorption onto some metal hydroxides but not onto commercially available adsorbents. This paper briefly summarizes known vanadium occurrence in North American groundwater and assesses vanadium removal by three commercially available metal oxide adsorbents with different mineralogies. GTO (Dow) is TiO2 based and E-33 (Seven Trents) and GFH (US Filter) are iron based. Preliminary vanadate adsorption kinetics onto GFH, E-33 and GTO has been studied and the homogenous surface diffusion model (HSDM) is used to describe the adsorption kinetics data. The effects of pH, vanadium concentration, and volume/mass ratio are assessed. Vanadium adsorption decreases with increasing pH, with maximum adsorption capacities achieved in at pH 3–4. Results indicate that all adsorbents remove vanadium; GFH has the highest adsorption capacity, followed by GTO and E-33. Data are best fit with the Langmuir model rather than Freundlich isotherms. Both the sorption maxima (Xm) and binding energy constant ( b) follow the trend GFH>GTO>E-33. Naturally occurring vanadium is also removed from Arizona ground water in rapid small-scale column tests (RSSCTs). Metal oxide adsorption technologies currently used for arsenic removal may also remove vanadium but not always with the same effectiveness.

Development and Characterization of Corrosion Resistant Coatings Using the Natural Biopolymer Chitosan

In the present study, chitosan was used as the basis for corrosion resistant coating. Chitosan suffers from two inherent weaknesses as a coating material namely high hydrophilicity and poor adhesive strength with Al 2024 T3 alloy. In the present study, chitosan structure was modified using epoxy functional silane and vanadate. Performance of coatings was tested using EIS, adhesion testing and salt spray testing. The best salt spray performance was observed in case of chitosan-(3-Glycidoxypropyl)-trimethoxysilane-vanadate coatings, which lasted 450 hours in salt spray chamber. UV/Visible measurements showed release of vanadate by chitosan increases with increasing solution pH. This property of chitosan provides self-healing characteristics to these coatings. When the solution pH is readjusted to a lower value, chitosan can re-adsorb released vanadate. FTIR spectra of chitosan gel showed adsorption of vanadate at protonated amine sites. Chitosan showed a maximum in the vanadate adsorption capacity in the pH 3 to 5 range.

Treatment of a vanadium-containing effluent by adsorption/coprecipitation with iron oxyhydroxide

Competitive adsorption experiments showed that vanadate is slightly more strongly adsorbed than phosphate on iron oxyhydroxide. Spent corrosion inhibitor containing vanadate (10 −4 M) and phosphate (10 −2 M) was treated by selectively precipitating the phosphate as apatite by addition of lime. The vanadate was then coprecipitated by addition of waste pickle liquor (FeCl 2(aq)) plus further lime, with aeration to convert Fe(II) to Fe(III). Residual vanadate was <2 × 10 −6 M and phosphate <1.6 × 10 −4 M. Adsorption of vanadate on apatite was negligible.

Desorption of phosphate from three Finnish mineral soil samples during adsorption of vanadate, molybdate and tungstate

Adsorption of V(V) and Mo(VI) from 10-4 M and 10-5 M solutions and W(VI) from a 10-4 M solution (in 0.02 M KCI) by three Finnish mineral soils was studied in two series of experiments. In the first experiment, the adsorption of V, Mo and W by soil and the desorption of P were measured at the soils’ natural pH after an equilibration time of 3, 5, 22, 29, 46 and 70 h. Adsorption of molybdate occurred mainly within the three first hours, whereas adsorption of vanadate and tungstate were slower processes. During the first few hours, the presence of molybdate seemed to increase the desorption of phosphate most effectively, but after a longer equilibration period, the differences between additions of V, Mo, and W became smaller. In the second experiment, the adsorption process was followed as a function of the acidity of the suspension (pH 2.3-7.5; for W pH 2.8-7.5). Adsorption of V(V), Mo(VI) or W(VI) resulted in a statistically significant increase in the amounts of P desorbed from all three soils over the pH range studied. The aqueous chemistry of V(V), Mo(VI) and W(VI) is briefly discussed.

Adsorption of vanadate on γ-alumina: Comparison with other isopolyanions

Impregnation of γ-Al 2O 3 with vanadate ions in a solution at fixed pH was performed with a fluidized bed technique. In conjunction with X-ray photoelectron spectroscopic measurements, it was shown that vanadium is well dispersed on alumina and the upper limit for “monolayer” coverage was deduced. Samples prepared by other techniques were compared with this first series. The limiting values for adsorption at equilibrium to obain monolayer coverage of different isopolyanions were compared. It was deduced that the number of fixation sites on γ-Al 2O 3 is about 0.33 ± 0.04 nm −2 whatever the chemical nature of the anionic species [V, Mo, W, Cr and Co as Co(CN) 6 3−].