Chemistry and mineralogy of siliceous ferrihydrites and implications to soil fertility
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Date
1988
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Abstract
The objectives of this investigation were to study the mineralogy, surface composition and reactivity of Si-containing ferrihydrites, and to evaluate these materials as Fe amendments for calcareous soils. Ferrihydrite with Si/Fe molar ratios from 0 to 1 were synthesized by the reaction of Fe2(SO4)3 with NaOH to a pH of 8.2 in the presence of Na2SiO3. X-ray diffraction, transmission electron microscopy, infrared spectroscopy, and x-ray photoelectron spectroscopy results provided evidence to support the probable incorporation of Si into the ferrihydrite structure for samples with Si/Fe molar ratios [greater than or equal to] 0.10. The ZPC of oxides decreased with increasing Si content, indicating that Si was specifically adsorbed on the oxide surface and/or was incorporated as an integral part of the oxide. The ferrihydrite samples were incubated for 36 h at 91 °C and pH 12.5, only those samples with Si/Fe molar ratios [less than or equal to] 0.05 were transformed to goethite and hematite. The thermal study indicated that the temperature of hematite formation was related to the Si/Fe molar ratio of the ferrihydrite samples. IR vibrations of hematite were dependent on particle morphology, particle size, the presence of Si, and the conditions of hematite synthesis. The phosphate adsorption maxima for samples with Si/Fe molar ratios [less than or equal to] 0.05 increased with the increase in Si/Fe molar ratio; whereas, it decreased for samples with Si/Fe molar ratios $ge$0.10. For samples with Si/Fe molar ratios [greater than or equal to] 0.10, the phosphate anions displaced silicate; whereas, for samples with Si/Fe molar ratios [less than or equal to] 0.05, HPO4[-2] exchanged preferentially with surface-OH groups to form binuclear-bridging complexes with the oxide surface. The greenhouse study indicated that ferrihydrite with a Si/Fe molar ratio of 0.75 and Fe2(SO4)3 were equally effective in reducing Fe chlorosis in calcareous soils; however, FeSO4 was a much less effective source of Fe.
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Typescript (photocopy).
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Major soil science