Vanadium uptake

At the present time, considerable insight into the biochemistry of vanadium in tuni-cates is associated with the mechanism of vanadium uptake. We shall concentrate on this topic, first presenting the biological context in which to view this remarkable feat of metal ion accumulation. 

The redox reactivity of divanadium salen615 and other complexes555 have been investigated. Disproportionation of [ V(salen) 2(//-0)] was observed under electrochemical conditions.6 The reduced, [Vin(salen)]+ complex was found to be the essential species in the catalysis of the electroreduction of 02 by four electrons in CH2C12.616 While [ V(salcn) 2(/x-0)] was proposed to be the active species in the redox reaction, more recently [Vin(salen)]+ was identified as a reservoir from which the active species forms.616 Schiff base complexes encapsulated in zeolite Y are catalytically active in the oxidation of thioanisole with H202.14 Acid-promoted disproportionation of a Vlv phenolate under anaerobic conditions was proposed as a model reaction for the vanadium uptake in tunicates.139... 

As a result of the interaction of these many factors, it is difficult to forecast the extent to which any heavy metal may be concentrated in a given organism. This section will therefore treat three widely differing examples the copper-algae interaction, metal ions and bivalves, and vanadium uptake by tunicates. 

For risk assessment in the case of vanadium uptake urine is the matrix of choice. The collection of urine is non-invasive and is practical under routine conditions. Moreover this parameter is more sensitive for diagnostic purposes than the vanadium concentration in blood. A tentative biological threshold limit value of 50 mg/kg creatinine has been proposed for urinary vanadium (Lauwerys, 1983). 

Vanadium uptakes were calculated from the concentration changes of the supernatant solutions before and after ion-exchange. Vanadium was first all oxidized to V " " and then determined potentiometrically by employing a 0.01 mol dm (NH4)2Fe(S04)2 solution as titrant. In the preparation of VNaTiP, sodium released to the solution was determined by atomic absorption method on a Varian Techtron model 1100 spectrophotometer. 

A.ctive driers promote oxygen uptake, peroxide formation, and peroxide decomposition. At an elevated temperature several other metals display this catalytic activity but are ineffective at ambient temperature. Active driers include cobalt, manganese, iron, cerium, vanadium, and lead. 

Figure I. Oxygen uptake by supported clusters and vanadium oxide. Samples were pre-reduced and re-oxidized at the temperatures indicated on the abscissa. Silica-supported polyoxometalates PVl ( ), PV3 (A), PVI4 ( ). Bulk V2O5 (+, after [10])...

Apparent activation energies for the catalytic reactions were as expected about 80 kJ/mol for the formation of formaldehyde and 60 kJ/mol for the formation of acetaldehyde from the respective alcohols (Figure 3). The turnover rates of the samples were calculated either on the basis of the number of vanadiums (all of which could be assumed to be accessible) or by assuming that oxygen uptake counted the catalytic sites ... 

Catalytic activities of the silica-supported vanadium oxides in either 3% methanol or in 5% O2 and 3% ethanol. Oxygen uptake was measured at 625 K. O2 and 3%... 

Figure 3. Arrhenius plots for the formation of formaldehyde or acetaldehyde from methanol or ethanol, normalized by the number of vanadiums (open symbols) and by the amount of oxygen uptake measured at 625 K (filled symbols). Lines on the right panel are calculated from the data reported by Oyama and Somorjai [11].

It was discovered nearly 20 years ago that V(V) as vanadate and V(IV) as vanadyl can mimic some of the effects of insulin (stimulate glucose uptake and oxidation and glycogen synthesis) (512, 513). Vanadate is an effective insulin mimetic in the diabetic rat (514), but has proved to be too toxic for human use. Vanadyl, as VOS04, is also unsuitable because high doses are needed on account of its poor oral absorption. Vanadium complexes with organic ligands have proved to be less toxic and can have improved aqueous solubility and lipophil-icity. 

Monolayer coverage of vanadium oxide on tin oxide support was determined by a simple method of low temperature oxygen chemisorption and was supported by solid-state NMR and ESR techniques. These results clearly indicate the completion of a monolayer formation at about 3.2 wt.% V2O5 on tin oxide support (30 m g" surface area). The oxygen uptake capacity of the catalysts directly correlates with their catalytic activity for the partial oxidation of methanol confirming that the sites responsible for oxygen chemisorption and oxidation activity are one and the same. The monolayer catalysts are the best partial oxidation catalysts. 

The effect of chemisorption temperature on the ammonia uptake capacity of 6.5 wt% V20c/Ti02 is shown in Fig. 1. Ammonia chemisorption capacities increase with temperature upto 150°C and then decrease with further Increase up to 400°C. It is worth noting that there is considerable NH uptake even at 400°C. These results are in accordance with the reported literature. A number of studies have been reported on the acidic character of supported transition-metal oxides (22,34-38). Ammonia on V20g can be either adsorbed in the form of NH species on Bronsted acid sites or coordlnatively bonded to vanadium ions on Lewis acid sites (39,40). The latter species were observed up to 250°C,... 

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