A simple equation to describe sorption of anions by goethite

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    A simple equation to describe sorption of anions by goethite would be useful as a means of characterizing batches of goethite and in studies of plant uptake of anions from the sorbed form. A suitable relationship between solution concentration (c) of phosphate or citrate and their sorption (S) by goethite at a constant pH or at different pH values is c = a exp(bS)S/(S(Max) - S), where b is a parameter, S(Max) is the maximum sorption, and a is a parameter at constant pH. In the middle range of sorption (from about 30% to about 70% of maximum sorption) this equation approximated to a Tempkin equation, but the full equation is more useful as it applied over the whole sorption range. The values of a varied with pH. This variation could be explained by changes in the electric potential of the adsorbing surfaces and in the degree of dissociation of the anions. The parameter a could therefore be replaced by a function of pH. The effects were consistent with formation of bidentate phosphate complexes and tridentate citrate complexes with the goethite surface.

    Original languageEnglish
    Pages (from-to)151-155
    Number of pages5
    JournalEuropean Journal of Soil Science
    Issue number1
    Publication statusPublished - 1 Mar 1999

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