Effect of Sesquioxide Coatings on Surface Charge of Standard Mineral and Soil Samples1
- W. H. Hendershot and
- L. M. Lavkulich2
In nature, Fe and Al hydrous oxides often form the interface between crystalline soil minerals and the soil solution. To understand better the effect of these coatings on surface charge properties, standard minerals and cleaned C horizon samples were coated with either Fe or Al hydrous oxide and the point(s) of zero salt effect (PZSE) were measured by potentiometric titration. Five standard mineral samples (including illite, muscovite, kaolinite, microcline, and quartz) and three C horizon samples of mixed mineralogy were artificially coated with Fe and Al hydrous oxide material by precipitation of FeCl3 or AlCl3 solutions with the mineral and soil samples. The samples were then washed free of Cl- and aged. Uncoated samples and samples of pure sesquioxide material were prepared using similar procedures. Depending on the nature of the underlying surface, the addition of sesquioxide coatings to the samples changed the surface charge properties in different ways. In every case the buffering capacity of the samples was increased by the addition of the sesquioxide coatings. The uncoated samples that had relatively high buffering capacities had their surface charge properties changed more by the Al- than the Fe-coating material. For example, the illite, muscovite, Memekay C horizon, and Marblehill C horizon all responded to the Al-coating with a distinct shift of the PZSE to higher pH. The less reactive samples (kaolinite, microcline, quartz, and the Cox Bay C horizon) responded little to the Al coatings but showed a strong response to the Fe coatings. The results indicate that in a system containing both pH-dependent and permanent exchange sites there is no effective means of separating the two components. Below the point of zero net charge (PZNC) of the coating material, the measured cation exchange capacity (CEC) will be decreased to a point below the permanent CEC by the coatings, whereas at pH values above this point the measured CEC will be increased. The PZSE of the pH-dependent charge material cannot be measured separately by potentiometric titration; this method measures the PZSE of the samples as a whole. At any pH the CEC measured will be the result of a complex interaction between the two types of exchange sites.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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