UBC Theses and Dissertations
Surface charge properties of selected soils Hendershot, William Hamilton
The aim of this thesis was to examine the use of surface charge characteristics as a measure of pedogenic development in selected soils of the humid, temperate environment of southwestern British Columbia. A secondary objective, using a wider range of soil types, was to gain further knowledge of the factors which influence the values of ZPC (the zero point of charge) and σi (its displacement from the zero point of titration, ZPT), so that these measurements might be better understood. Measurement techniques, using potentiometric titration, have a significant effect on the values of ZPC and σi obtained in the laboratory. The fast-adsorption procedure measures the fast reaction taking place at the particle surface, while the slow-adsorption procedure measures the additional slow reaction resulting from the incorporation of H⁺ and OH⁻ ions into the structure of oxide coatings. Therefore the values obtained using the slow-adsorption procedure result in higher ZPC values and lower σi values than the fast-adsorption procedure. NaCl-saturation of the exchange complex prior to titration results in the exchange of strongly bonded ions such as Al³⁺ and P0₄³⁻ and thus alters the properties of the surface. In the case of untreated samples with ZPC values below pH 5.0 and σi values less than -0.2 meq/l00g there was a shift to lower ZPC and σi values; if the values for the untreated samples were above this, then the shift was in the opposite direction. The fast-adsorption method using untreated samples is therefore considered to be the most accurate means of measuring the surface charge properties as they exist in the field. Indirect evidence suggested that the presence of organic matter caused a decrease in the ZPC and σi values. This evidence is in the form of simple correlation coefficients between organic C and σi. The results of partial correlation analysis removing the effect of organic matter indicate strong relationships between ZPC and soil age and also between σi and soil age. Evidence of this relationship is also supported by direct measurement. The removal of a portion of the organic matter using a pretreatment of Na0Cl resulted in a significant increase in both the ZPC and values. Potentiometric titration is a useful measure of surface charge characteristics and of soil genesis. In soils with large amounts of clay minerals, as the sesquioxide content increased the ZPC became better defined and moved to a higher pH and σi approached zero. In sandy soils, the values of ZPC and σi are only significantly related to soil age when the effect of soil organic matter content has been eliminated. On the other hand, the ΔpH value, defined as the difference between ZPC and pH(KCl), compensates for differences in organic matter and provides a good indicator of the extent of soil development.
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