Immobilisation of arsenic in synthetic mineral phases
This thesis presents work relating to the synthesis, structure and stability of various arsenate phases having potential to immobilise high concentration arsenic wastes. Such wastes arise from mining and hydrometallurgy operations and the high arsenic concentrations, arising in tailings dams for example, represent an environmental concern. Large quantities of highly contaminated waste sludge are also generated by the co-precipitations of arsenic on ferric hydroxide floccs from low arsenic concentration waste streams. The disposal of this waste is becoming a prominent issue when cleaning minesite run off and drinking water. It is important therefore to consider the environmental impact of discarded arsenic residues and this project addresses the potential for its immobilisation in low stability minerals capable of return to mine sites. Two groups of phases with the potential for immobilisation of arsenic and other toxic metals are studied in this thesis. The first group are zinc arsenate zeolite analogues. These open framework structures also present the possibility of immobilization of other waste materials by ion exchange. The second group are a series of cadmium arsenate phases, which offer the potential for cadmium and arsenic immobilisation. This thesis examines the crystal structure, and synthesis of both groups of phases and examines their solubility and stability thus assessing their potential as waste immobilisation tools. The results of these studies has shown that although it is possible to make phases which are entirely composed of waste metals these phases are not stable enough for direct use in waste immobilisation processes. The crystal structures of several previously unpublished phases have been determined and a solubility product has been calculated for one of the phases.