Calcium(2+) and magnesium(2+) effects on water and ammonia adsorption by soil clays
Abstract
The purpose of this study was to determine the effect of Ca2+ and Mg2+ cations on surface properties of clay-sized fraction (CSF) of the soils. Specifically, water sorption and swelling, zeta potential, and surface acidity of the samples were examined. Differences in properties between samples exchanged with Ca2+ and Mg 2+ were expected because of the differences in the size of these cations. Methods traditionally employed in studies of specimen clay minerals were utilized for the CSF of two soils with different mineralogical composition: Blount loam (fine, illitic, mesic Aeric Epiaqualfs) and Fayette silty clay loam (fine-silty, mixed, mesic, superactive Typic Hapludalfs). The amount and status of water adsorbed on the samples was examined using an Environmental Infrared Microbalance (EIRM) cell, which permitted simultaneous collection of the infrared spectra and weight of the sample in situ. The basal spacings of the samples were measured by x-ray diffraction. The zeta potential was calculated from the electrophoretic mobility of the CSF in suspension. Lastly, ammonia sorption was evaluated using two methods: the FTIR-microgravimetric method employed for the water sorption study and the ATR-FTIR method. It was found that exchangeable cations had a statistically significant effect on the water sorption of soil CSF. On average, Mg-samples adsorbed 25% more water than Ca-samples. A highly significant linear relationship was obtained between the hydration energy of the cations and the amount of adsorbed water (P > F = 0.0001). Mg-samples also exhibited enhanced H-bonding as evidenced by the blue shift of the water deformation band. The effect of exchangeable cations on H-bonding decreased with the increase in the water content of the sample. The differences in water sorption between Ca- and Mg-exchanged CSF were not reflected in the observed d-spacing. The electrophoretic mobility and the zeta potential of the Mg-exchanged CSF were significantly (P > F = 0.0063) greater than of Ca-CSF, indicating greater thickness of the diffuse double layer of the Mg-samples. Ammonia sorption was also affected by the presence of Mg on the exchange sites, though not significantly. At high (0.9) partial pressure of water, Mg-CSF adsorbed more of both NH3 and NH 4+ than Ca-CSF. While at low water contents (partial pressure of 0.02), the amount of NH4+ was not increased by Mg-saturation, the sum of ammonium and ammonia was always greater in the presence of Mg. This was explained by a positive relationship between the amount of water on the sample and NH3.
Degree
Ph.D.
Advisors
Norton, Purdue University.
Subject Area
Soil sciences
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