Sorption to Binary and Ternary Component Systems

The sorption of ions in laboratory systems formed by a few (mainly two or three) soil components has received attention, albeit not very extensive, from several research gronps (note that here binary and ternary refer to the number of soil components in the literature, the term ternary often refers to two components plus one adsorbing ion). We will summarize here some important results. 

FIGURE 14.4 Cu sorption to the solid phase in binary systems hematite-fulvic acid at three different ionic strengths. Experimental data ( ) 0.1 M (O) 0.03 M (A) 0.01 M, and corresponding model calculations (lines) based on the additivity assumption. (Reprinted from Geochimica et CosmochimicaActa, 65, Christl, I. and Kretzschmar, R., Interaction of copper and fulvic acid at the hematite-water interface, 3435-3442. Copyright 2001, with permission from Elsevier.) 

In a similar study (Orsetti, Quiroga, and Andrade 2006), the binding of Pb(II) to the humic acid-goethite binary system and the individual components was 

In an another stndy related to the additivity rule (Alessi and Fein 2010), the sorption of Cd(II) in mixtures of kaolinite. Bacillus subtilis bacterial cells, and iron oxy-hydroxide (HFO) was performed in the absence and presence of an organic ligand, acetate. The resnlts indicate that for systems containing B. subtilis, HFO, and kaolinite, the component additivity approach is a reasonable predictor of metal distribution, with the accuracy limited by the accuracy of the stability constants of the important surface complexes. However, in systems including acetate, the additivity rule predictions significantly nnderestimate the extent of adsorption above pH 5, likely due to the formation of ternary Cd-acetate surface complexes on each surface. 

From the studies discussed above, albeit further research may be necessary to reach firm conclusions, an emerging picture is that when considering mixtures of mineral components, the additivity rule could predict reasonably well the extension of ion binding, but in the presence of organic matter, this is no longer valid. 

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