Abstract
Methods have been developed to study the dissolution mechanisms of a mixed Zn/Pb sulfide concentrate, from the McArthur River Mine, in seawater and subsequent trace metal cycling in the vicinity of the Bing Bong loadout facility, Gulf of Carpentaria, Australia.Submersible pump and in-line filtration protocols are presented for samplecollection in a remote pristine coastal environment. A rapid dithiocarbamate /diisobutyl ketone solvent extraction procedure, with Hg exchange back-extraction,was developed for the simultaneous quantitative determination of Cd, Co, Cu, Fe,Ni, Pb and Zn in seawater. The procedure was compatible with both GFAAS and ICP-MS analysis, achieved low ppt detection limits, and was adaptable for the determination of highly contaminated samples or marine porewaters. A similar 8- hydroxyquinoline extraction technique and a direct GFAAS method are alsopresented for the determination of Mn. These methods were essential to studying trace metal dynamics in dissolution and mobilisation trials in the laboratory and field.
The dissolution of Zn/Pb mixed sulfide concentrate in seawater proceeded by atwo phase process: the rapid dissolution of an existing oxidised surface layer followed by slow surface controlled dissolution from the exposed metal sulfide surface. The relative rate of dissolution during the initial and secondary phases was Pb>Zn>Cd>>Cu and Pb>>Zn>(Cd,Cu), respectively, with the relative rate of Cd, Cu and Zn dissolution being several orders of magnitude faster in the initial phase.For Pb, the initial and secondary phases were indistinguishable with Pb rapidly attaining equilibrium in solution (closed system). This was controlled by adsorption / desorption of an activated species at the concentrate surface, possibly PbCO3. PbS oxidised most readily in air or solution. Iron impurities appeared to enhancethe overall metal sulfide dissolution mechanism, but Fe dissolution was not evident due to adsorption onto the concentrate and/or precipitation of Fe oxy-hydroxides. Oxygen was essential for at least Cd, Cu and Zn dissolution.
The dissolution of both Cd and Zn from the exposed metal sulfide surface proceeded linearly with time, showing minimal re-adsorption onto the concentrate surface. Natural organic ligands and salinity changes appeared to have a relatively negligible effect on Cd and Zn mobilisation. CuS dissolution was totally dependent upon organic complexation, with metal sulfide lattice exchange reactions dominating in the absence of an organic ligand promoted dissolution mechanism.Pb dissolution was also enhanced by natural organic ligands, which increased Pb dissolution beyond the solubility of PbCO3. Added fulvic acid enhanced Cu and Pbdissolution, but inhibited Zn and Cd dissolution. Photo-sensitised dissolution mechanisms were absent in seawater, except in the presence of added fulvic acidwhich increased the rate of ZnS and CdS dissolution via the apparent generation of energetic singlet oxygen.
In the presence of sediment, the rate of metal dissolution from concentrate into seawater only exceeded the rate of sediment uptake for the initial rapid dissolutionphase. Sediment adsorption was initially rapid onto external (surficial) sites and then became slower and diffusion limited for internal (sub-surface) sites. Therelative rate of sediment adsorption of mobilised metals was Pb>Zn>Cd. The presence of anoxic sediment was an important scavenging mechanism for Pb andparticularly Cd. Organic complexation appeared to be an important controlling mechanism for dissolved Ni, Cu and Cd cycling in the marine environment.
The metabolism of organic matter (seagrass) and the formation of a highly reactive iron sulfide pool (AVS) dominated sediment chemistry in the Bing Bong swing basin. The AVS pool efficiently scavenged metals entering the sediment through metal sulfide lattice exchange reactions. Porewaters of this anoxic sediment, contaminated with concentrate, had low concentrations of trace metals and temporary suspension of this sediment resulted in a minimal and transient mobilisation of metals. Laboratory and field studies showed that only heavy metals associated with a rapidly oxidising FeS pool would be released during temporary sediment suspension and that the mobilised metals would be rapidly re-adsorbed or co-precipitated with Fe oxy-hydroxides. It was concluded that the physicochemical characteristics of the Bing Bong swing basin were conducive topreventing heavy metal contamination outside of the immediate vicinity of the swing basin in the event of accidental concentrate spillage or through continual resuspensionof concentrate contaminated sediment.
| Date of Award | 1999 |
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| Original language | English |
| Supervisor | David Parry (Supervisor) & J. H. Swinehart (Supervisor) |