Role of organic matter and sesquioxides on variable charge of three soils from Galicia, Spain

Abstract

The charge properties of three soils developed on granulite and serpentinite were investigated. These soils contain large amounts of organic matter, free iron oxide, and clay, and are therefore a good medium for measuring the interactions between those components. Cation‐ and anion‐exchange capacities (CEC and AEC) were measured in percolation experiments with and without ammonium acetate buffer pretreatment, using various electrolyte concentrations. The measurements were repeated after removal of sesquioxides and/or organic matter. In addition, potentiometric titrations were carried out to determine the ‘point of zero salt effect’ of each sample. Untreated samples showed a mutual blocking of positive charge on sesquioxides and negative charge on organic matter, leading to decreased CEC and negligible AEC values. Because the AEC is fully blocked, the natural soils do not have a ‘point of zero net charge’ (PZNC), and the apparent ‘points of zero salt effect’ (PZSE) obtained in potentiometric batch titrations are artifacts. Positive charge appeared only after the removal of organic matter. Percolation with acetate buffer strongly increased the measured CEC in samples containing organic complexes. After removal of organic matter, a slight buffer effect remained. This is attributed to the release of sesquioxides from organic complexes and clay surfaces by preferent binding to the acetate anion.