Goethite selenite adsorption

Zhang, P.C. Sparks, D.L. (1990) Kinetics and mechanism of sulfate adsorption and desorption on goethite using pressure jump relaxation. Soil Sci. Soc. Am. J. 54 1266-1273 Zhang, P.C. Sparks, D.L. (1990) Kinetics of selenate and selenite adsorption/desorption at the goethite/water interface. Environ. Sci. Technol. 24 1848-1856... 

Zhang, R-C. and Sparks, D.L. (1990) Kinetics of selenate and selenite adsorption/ desorption at the goethite/water interface. Environ. Sci. Technol., 24, 1848-1856. 

Kinetics of Selenium Adsorption. Zhang and Sparks 4G) examined selenate and selenite adsorption and desorption on goethite using pressure jump relaxation techniques. Selenate produced a single relaxation, that was interpreted as outer-sphere complexation with surface protonation based on fitting to the triple layer model. The forward rate constant was 10 L mol s Selenite adsorption was proposed to occur via two steps, an initial outer-sphere complex and subsequent replacement of a water molecule by formation of inner-sphere complexes of both HSeOj and SeOj, based on optimized fits using the triple layer model. The model optimized fit for the pK, of the surface species was approximately 8.7. Forward rate constants for the first step were on the order of 10 L -mor -s for HSeOj and 10 L -mor -s for SeOj. Forward rate constants for the formation of the inner-sphere complexes were 100 and 13 s respectively for HSeOj and SeOj. Agreement between the equilibrium constant obtained from batch and kinetic studies was taken as confirmation of the proposed reactions. 

Direct in situ X ray (from synchroton radiation) adsorption measurements (EXAFS) (Hayes et al., 1987, Brown et al., 1989) permit the determination of adsorbed species to neighboring ions and to central ions on oxide surfaces in the presence of water. Such investigations showed, for example, that selenite is inner-spherically and selenate is outer-spherically bound to the central Fe(lll) ions of a goethite surface. It was also shown by this technique that Pb(II) is inner-spherically bound to 5-AI2O3 (Chisholm-Brause et al., 1989). 

The four layer model (Bowden et ah, 1980 Bousse and Meindle, 1986) also locates different adsorbing ions in different planes. It has been used to model adsorption of phosphate, citrate and selenite (Bowden et ah, 1980) and borate (Bloesch et al., 1987) on goethite and competitive adsorption of Ca and Cd on ferrihydrite (Cowan et al., 1991). 

The selenite and sdenate ions illustrate the effect of oxidation state on adsorption behaviour. The selenite ion (SeO ) adsorbs strongly and specifically on goethite and shifts the iep of both goethite and ferrihydrite to lower pH values (Su and Suarez,... 

Phosphate must be applied as fertilizer to the soil. Ideally it is added in quantities sufficient to guarantee optimal yields, but not in excess in order to avoid P transportation into other compartments of the ecosystem. The amount added should be based on an accurate estimation of the plant-available fraction of P already present in a soil.This is an old and difficult task and a large number of extraction methods have been used since intensive land use was practised. Recently methods have been worked out in which a strip of filter paper impregnated with an Fe oxide (2-line ferri-hydrite) is dipped into a soil suspension and the amount of P adsorbed by the paper is taken as being plant-available (Sissingh,1988 Van der Zee et ah, 1987 Sharpley, 1993 Sharpley et ah,1994 Kuo and Jellum, 1994 Myers et ah 1997). Anion and cation resins extracted more P from four heavily fertilized soils than from goethite (Delgado Torrent, 2000). Other oxyanions adsorbed by soil Fe oxides are silicate, arsenate, chromate, selenite ( ) and sulphate. Adsorption of sulphate led to a release of OH ions and was substantially lowered once the Fe oxides were selectively removed (Fig.16.17). 

Glasauer, S.M. Doner, H.E. Gehring, A.U. (1995) Adsorption of selenite to goethite in a flow-through reaction chamber. Eur. J. Soil Sci. 46 47-52... 

Specific adsorption of selenite on goethite increases the pH of the suspension and the negative charge on the oxide surface. Isotherms at constant pH are represented by,... 

Adsorption of anions at mineral surfaces is important in soils because of the limit this process imposes on the availability of plant nutrients such as P, S, and Mo which occur naturally as anions and are added in anionic form in fertilizers. Anion adsorption is also relevant in geochemistry, ore processing, and other fields where minerals with high surface areas are brought into contact with aqueous solutions of anions. Selenite and goethite were chosen for this study because in Western Australia a selenium deficiency in pastures has been shown to be related to the incidence of white muscle disease in sheep (3), and according to workers quoted by Rosenfeld and Beath (9) selenium in soils of higher... 

Adsorption Isotherms. The dependence of the amount of selenite adsorbed on pH and solution concentration of selenite is illustrated by the curves in Figures 1 and 2. These show that the amount of selenite taken up by goethite reaches a maximum value, r0(PH), at constant pH which cannot be exceeded by increasing the solution concentration and that this maximum value varies with pH. In the pH region studied ion size is unlikely to be the only factor limiting adsorption because even at low pH, where the maximum is greatest, the area of surface available to the ion is always greater than the area it would be expected to occupy ( 20 A.2/ion). 

The adsorption of selenate (SeO ) by goethite (a-FeOOH) is thought to result primarily in outer-sphere surface complexes, whereas the adsorption of selenite (Se02 ) is thought to result primarily in inner-sphere surface complexes. Develop equations for the time constants rx and r2 in the adsorption kinetics of these two species and compare the resulting equations for t2. (Hint Consider Eqs. 4.40-4.45.)... 

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