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Towards cleaner fertilisers; Solution ion-exchange methods for reducing cadmium levels in reactive phosphate rock

The aim of this study was to characterise reactive phosphate rock (RPR); a form of rock phosphate which can be used directly as fertiliser, and investigate the potential to reduce cadmium levels in RPR by application of calcium ion solutions derived from waste mussel shells. Analytical techniques including ICP-MS, XRD, XRF, SEM-EDX, and FTIR were used in the characterisation of reactive phosphate rock, as provided by Terracare. These techniques allowed for changes to both the macro and trace element composition of reactive phosphate rock to be measured and monitored as it was subjected to a calcium ion solutions and variety of chemical and physical reaction conditions, with the aim of selectively extracting cadmium ions, by ion exchange with calcium, while leaving the general structure of RPR unchanged. The solubility of phosphate ions in RPR was tested at varying pH, the intention being to determine the pH range of solutions that could be used for cadmium extraction while minimising phosphate losses. Calcium ion solutions were applied to reactive phosphate rock under a range of different reaction conditions. Changes to calcium ion concentration, contact time, pH, temperature, microwave irradiation, use of different counter ions and addition of potential cadmium stabilising ligands were tested for their effects on reactive phosphate rock. Promising results were observed, with elevated levels of cadmium found in many extraction solutions. Calcium chloride solution of 1 mol L⁻¹ concentration was found to be the most effective at extracting cadmium from RPR. Mussel shells were characterised by analytical methods including ICP-MS, XRD, XRF, UV-Vis, and FTIR. The effects of heating to temperatures between 300°C and 900°C to the physical and chemical properties of mussel shells were studied. Mussel shells were used as a basis for calcium ion solutions by dissolving crushed mussel shell in mineral acids. These shell solutions were compared with solutions of pure calcium salts for their effectiveness in extracting cadmium ions. Overall the results of this study indicate that it is possible for significant amounts of cadmium to exchange from RPR into calcium ion solutions with comparatively small phosphate losses. It appears that mussel shells could provide a suitable source of calcium ions for this purpose. Overall indications are that cadmium extraction from RPR by ion exchange with calcium ion solutions is a concept worthy of further research. If the preliminary results prove to be accurate then further developments have the potential to contribute to improved environmental outcomes for both the fertiliser and shellfish industries in New Zealand.
Type of thesis
The University of Waikato
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