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Observations and modelling of the reactions of 10 metals with goethite: adsorption and diffusion processes

Authors

  • L. Fischer,

    1. aInstitute of Crop Science and Resource Conservation, Division Soil Science, University of Bonn, Nußallee 13, 53115 Bonn, Germany, and b22 Townsend Dale, Mt Claremont, WA 6010, Australia
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  • a G. W. Brümmer,

    Corresponding author
    1. aInstitute of Crop Science and Resource Conservation, Division Soil Science, University of Bonn, Nußallee 13, 53115 Bonn, Germany, and b22 Townsend Dale, Mt Claremont, WA 6010, Australia
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  • and a N. J. Barrow b

    1. aInstitute of Crop Science and Resource Conservation, Division Soil Science, University of Bonn, Nußallee 13, 53115 Bonn, Germany, and b22 Townsend Dale, Mt Claremont, WA 6010, Australia
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  • Dedication: In memory of the 100th anniversary of the birth of Prof. Dr Dr Dr h.c. Eduard Mückenhausen (17 February 1907 – 6 February 2005).

G.W. Brümmer. E-mail: bruemmer@boden.uni-bonn.de. Enquiries about the modelling: N. J. Barrow. E-mail: nbarrow@bigpond.net.au

Summary

A sample of goethite was mixed for periods which ranged from 2 hours to 8 weeks with solutions of dilute nitrate salts of Pb, Hg, Cd, Zn, Cu, Ni, Co, Mn, Cr and Al. The amount of sorption after each period was measured for an appropriate pH range for each metal. The sorption behaviour was characterized both by using characteristics of the sorption curves such as the pH at which half of the added metal was sorbed (pH50) and by fitting a model in which sorption was mainly characterized by an affinity constant and by a diffusion constant. Initial sorption, whether characterized by the pH50 or by the affinity constant, was closely correlated with the appropriate dissociation constants of the metals. The greater the affinity of the metals for hydroxide ions, the greater their affinity for the goethite surface. The metals differed in the rate at which they continued to react with the goethite. Lead had the slowest continuing reaction, cobalt the fastest. The continuing reaction was due to diffusion into the particles. It was characterized by the fitted diffusion constant and by the change with time in the pH50. For seven of the eight divalent metals, these were correlated with the ionic radius of the metals: the larger the radius, the slower the reaction. For Al and Cr, rates were slower than would be expected from the ionic radii and we suggest this shows that these ions react as the larger M(OH)2+ ions. The behaviour of Ni was consistent with oxidation of the surface species and diffusion of Ni(OH)2+ ions. The continuing reaction was similar to that observed when metal ions react with soils and suggests that their reaction with iron oxides is important in soils. The results also show that studies in which sorption is measured at only one period of reaction are incomplete and the application of equilibrium models to such results is misleading.

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