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Abstract

The suitability of the synthetic faujasite zeolites X and Y as slow release ammonium and potassium fertilizers has been researched by examining their ion-exchange properties.
The resistance of the zeolites to hydrolysis in water with a neutral or only mildly acidic pH has been investigated. The change in pH and cation concentration of solutions into which zeolite was added, was monitored using pH and ion-selective electrodes. The role of atmospheric carbon dioxide in determining the final solution pH and the level of hydronium exchange was examined by conducting experiments both in air and under a nitrogen blanket. The design and construction of a computer interface, which enabled the rapid and accurate measuremnt of pH and cation concentrations in solution, and a diffusion cell, which allowed the solution to be blanketed by a flowing stream of nitrogen whilst preventing the mass transfer of water into or out of the reaction cell, is described. A discrepancy between the total number of equivalents of cation released from the zeolite and the number of equivalents of hydronium ion exchanged has been observed and is discussed.
Ion-exchange reactions between the fertilizer elements and ions likely to be present in the soil solution have been investigated. Sodium - potassium ion-exchange isotherms in zeolites X and Y were measured using different backgound pH's of between 5.5 and 2. Levels of hydronium exchange and hydrolysis of the zeolites under these conditions were measured and the results are presented and discussed. Ion-exchange isotherms for the systems ammonium - sodium, potassium - ammonium and potassium - calcium in zeolite Y were also measured and the isotherms, and the thermodynamic interpretation of the results, are presented here.
Hydronium ion-exchange and hydrolysis in zeolite Y was investigated further by the measurement of sodium - hydronium, potassium – hydronium and sodium – aluminium isotherms. The selectivity of zeolite Y for hydronium ions was found to be extremely high at lower exchange levels and these results help to explain the relatively high levels of hydronium exchange found when zeolite Y is contacted with water with a neutral or only miIdly acidic pH. It is suggested that evidence of the initial dealumination of the faujasite lattice by acid solutions is masked by exchange of cationic aluminium species into exchange sites in the zeolite. Aluminium is only released into solution when the pH falls below a certain critical value, or when the total number of hydronium and aluminium ions in the zeolite exceeds the exchange capacity in the large pore sites.
²⁷Al and ²⁹Si MASNMR studies of partially hydrolysed zeolite Y samples, carried out in conjunction with Unilever Research and the Karl Marx University in Leipzig, are used to support this hypothesis

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science Q Science > QD Chemistry
Departments:	School of Science & Technology School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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