Ion adsorption mechanisms in oxidic soils; implications for point of zero charge determinations

In order to clarify the adsorption mechanisms for various ions (e.g., P, K, Ca, Mg), ion adsorption studies were carried out on a range of soils of widely different mineralogical compositions. The soils were classified as: Hydrandept, Gibbsihumox, Eutrustox and Haplustoll and belonged, respectively, to the Honokaa, Halii, Wahiawa and Waialua soil series in Hawaii.Soils containing mostly hydrous Fe and Al oxides, thus colloids of the variable-charge type, retain ions through various types of adsorption mechanisms. As a result, certain interactions between cation adsorption and anion adsorption occur which affect the ion exchange processes and thereby the determination of the point of zero charge (PZC).In this study adsorbed phosphate increased the adsorption of cations and lowered the PZC and adsorbed Ca interfered with PZC determinations unless these were carried out with CaCl₂ as supporting electrolyte. It is likely that this latter interference will have influenced the results of many earlier studies because of the very strong adsorption of Ca by oxidic colloids. In this study only part of the adsorbed Ca could be recovered from the two most highly weathered soils, by repeated extractions with NH₄-acetate at pH 7.The results indicate that many soils of the tropics can be characterized effectively by their PZC's. One should be aware, however, of the effects of strongly adsorbed ions commonly present in these soils and thus use those supporting electrolytes for the PZC determinations which counteract these effects.