The scope of this thesis is the study (synthesis, characterization and stability evaluation) of poorly-crystalline iron arsenate (Fe/As = 1) and several crystalline metal (Fe, Al, Ga, In, La) arsenates. Scorodite is used as reference materials in both cases. The study of the crystalline materials is approached from the standpoint of determining the relationship between their structure and chemical stability and their potential as arsenic immobilizing carriers. On the other hand the study of the poorly-crystalline iron (III)---arsenate (Fe/As = 1) phases seeks to elucidate their structural development as a function of pH and the mechanism of their crystallization. / The synthesis of the crystalline metal arsenates (mansfieldite (A1AsO 4·2H2O), scorodite-mansfieldite solid solution series, gallium arsenate dihydrate, indium arsenate dihydrate and lanthanum arsenate) involves hydrothermal precipitation from equimolar metal---arsenic (V) nitrate solutions at 160°C. Several techniques, among which XRD, TGA, FEG-SEM, VP-SEM and FEG-TEM, are used to characterize the synthesized materials. A1AsO4·2H2O and InAsO4·2H 2O are found to crystallize as single crystal particles as opposed to the other arsenates that crystallize as aggregates of polycrystalline particles. In terms of stability, all synthesized metal arsenates are determined to be less stable than scorodite. / Iron (III) arsenate precipitated from equimolar Fe (III---As (V) (nitrate or sulfate) solutions by fast neutralization in the temperature range 22 to 80°C is found to consist predominantly of a poorly-crystalline FeAsO 4·(2+x)H2O phase and to lesser extent of two-line ferrihydrite. The fraction of ferrihydrite (or better arsenate-adsorbed ferrihydrite) increases with pH and temperature elevation. FEG-TEM observation combined with microanalysis revealed a nanosize-domain structure with uniform chemical composition (Fe/As) at pH 2 but widely variable chemical composition at pH 8 consistent with its postulated heterogeneous nature. The poorly-crystalline iron arsenate is found to transform to crystalline scorodite following "auto catalytic" (sigmoidal) kinetic behavior. The rate of transformation is accelerated by temperature elevation and pH reduction. The transformation is explained by a mechanism which involves heterogeneous nucleation within the amorphous precursor phase nanoparticle aggregates and self-catalyzed crystal growth sustained by short-range dissolution.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.103208 |
| Date | January 2007 |
| Creators | Le Berre, Jean-François. |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Mining, Metals and Materials Engineering.) |
| Rights | © Jean-François Le Berre, 2007 |
| Relation | alephsysno: 002666856, proquestno: AAINR38602, Theses scanned by UMI/ProQuest. |
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