Summary Band 30

Fischer, Lorenz: Investigations of the ad-/desorption and diffusion processes of heavy metals and aluminium with goethites of different crystallinity - Experimental results and process modelling. Bonner Bodenkundl. Abh. 30 (2000).

 

Adsorption and desorption experiments were carried out with ten metals (Al, Cr, Mn, Co, Ni, Cu, Zn, Cd, Hg and Pb) and seven synthetic goethites of different crystallinity (specific surface area 35.1 to 158.9 m2 g-1) for a metal concentration of 1 µM. With these investigations metal and goethite properties which influence the adsorption of the metals on the outer goethite surfaces and the subsequent diffusion into the interior of the goethite particles should be determined. To characterize these processes binding constants for the adsorption of the metals on the outer goethite surfaces and diffusion coefficients for the diffusion of the metals into the goethite particles were calculated with the extended Four-Layer-Model of BOWDEN (1977) and BARROW (1983, 1987).

The adsorption of heavy metals on the outer surfaces of goethite is strongly influenced by their hydrolytic properties. Metals with a high affinity for hydroxyl ions in solution also have a high affinity for the hydroxyl groups of the goethite surfaces. Therefore, the calculated binding constants of the different metals were closely related to their hydrolysis constants in solution.
A synthetic goethite with Si in the pores between its domains plus adsorbed Si on outer surfaces (Si-goethite) showed only a marginal change of the binding constants of Pb and Mn in relation to the pure goethites, whereas the binding constants of Ni, Cu and Cd were relative low for this goethite. Probably Si occupies sites on the outer goethite surfaces to which Ni, Cu and Cd have a particularly high affinity.

The diffusion into the interior of the goethite particles decreases with increasing ionic radius of the metals. This result shows, that it is easier for smaller ions to penetrate the micropores or the smaller point and line defects of goethite. In contrast to the other divalent ions, Co and Mn seemed to be oxidized on the outer goethite surfaces and diffuse in the trivalent form into the interior of goethite.

With decreasing goethite crystallinity, diffusion of Mn, Cu, Cd and Pb into the goethite particles increases markedly. It can be concluded that the total amount of Pb and high proportions of Mn, Cu and Cd diffuse into the micropores between the domains of the goethites; the proportion of these micropores increases with decreasing goethite crystallinity.

For Ni and Cr, which diffuse mainly into point and line defects of the goethite crystals, no effect of the goethite crystallinity on their diffusion was observed. Probably the well crystalline goethites also contain many of these defects.

The incorporation of Si in the pores between the domains markedly decreases the diffusion of Mn, Cd and Pb into the goethite particles. In contrast to these results Cu shows a relatively high diffusion coefficient for Si-goethite. Probably Cu diffuses mainly into the smaller defects of this goethite. For Ni and Cr, which also mainly diffuse into smaller defects of the goethite structure, no effect of the Si on the diffusion into the interior of the goethites was observed.

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