AbstractThe influence of surface area, oxygen, temperature, pH, light, iron and humic acid on the mobilisation of heavy metals from a mixed metal sulfide ore concentrate (OC) in aquatic environments is investigated. The main mineral constituents of OC include sphalerite, galena, pyrite and chalcopyrite. Mineralogical data indicate Cd and some Fe are substituted into the sphalerite matrix.
The relatively linear mobilisation of Zn, Fe, Cu and Cd from OC, as a function of time, in acidic, oxic and anoxic solutions suggests a surface controlled mechanism is responsible for the release of these metals from OC. The Zn, Fe and Cd mobilisation in oxic and anoxic solutions at p1-I 2 increases with increasing surface area of OC exposed to solution, which indicates the importance of surface sites in metal release from OC. Copper mobilisation in oxic solution also increases with increasing OC, but an inverse relationship between dissolved Cu and OC surface area is observed in anoxic solutions. This is attributed to exchange reactions that remove mobilised Cu from solution. In contrast to the other metals, the trend in Pb mobilisation is hyperbolic rather than linear. This suggests a diffusion controlled mechanism is responsible for Pb mobilisation from OC. Moreover, data for Pb release could be fitted to the parabolic rate law used to describe diffusion controlled dissolution. A linear plot of maximum dissolved Pb against OC surface area also illustrates the importance of surface sites in Pb mobilisation. The combined results for Pb indicate a mixed transport-surface controlled mechanism is responsible for Pb mobilisation from OC. The curvature of Pb mobilisation is also attributed to exchange reactions that remove dissolved Pb from solution. The mechanisms identified for Zn, Pb, Fe, Cu and Cd mobilisation from OC are supported by calculations of apparent activation energies.
The mobilisation of metals from OC in anoxic solutions at pH 2 can be primarily attributed to proton promoted dissolution. The release of Zn, Cu and Cd from OC is enhanced in aerated solution by oxidative dissolution. However, the mobilisation of Pb and Fe are mostly driven by proton promoted dissolution irrespective of oxygen conditions. This is attributed to the diamagnetism of PbS and FeS2. The Fe impurities make ZnS and CdS paramagnetic and thus can readily adsorb oxygen. Paramagnetic oxygen also appears to readily react with antiferrornagnetic CuFeS2.
The mobilisation of Zn, Fe and Cd is increased under illumination, which is attributed to the semiconductor nature of ZnS, FeS2 and CdS, respectively. An increase in mobilisation of Zn, Fe and Cd under light is observed with decreasing pH, due to the greater oxidising property of photoholes and increased surface protonation. The semiconductor nature of PbS and CuFeS2 is also observed at pH 2. However, at higher pH values the concentration of mobilised Pb and Cu are greater in darkness than under illumination. This is attributed to the light enhanced exchange reactions involving dissolved Pb and Cu with metal sulfides more soluble than PbS and CuS. Anoxic experiments revealed the importance of oxygen in the photo-dissolution of semiconducting sulfide minerals.
The mobilisation of metals from OC is enhanced in the presence of ferric ion. The order of reaction in Fe(III) for the mobilisation of Zn, Pb and Cu from OC are not the same as those determined by Rimstidt et al. (1994) for individual sphalerite, galena and chalcopyrite. This indicates processes such as interfacial reactions and galvanic interactions contribute to metal mobilisation in a mixed metal sulfide system. The presence of ferric ions also enhances the photo-dissolution of sulfide minerals possibly by scavenging photoelectrons and/or production of hydroxyl radicals.
The OC was conditioned with ferric nitrate at pH 4, prior to mobilisation experiments. The results indicate FeOH2 and Fe(OH)2 adsorbed onto OC greatly promote the initial mobilisation of Cu from OC. This is attributed to the direct reduction of adsorbed Fe(III) species vicinal to Cu dissolution sites. However, these adsorbed ferric species apparently contribute to a lag in the initial release of Zn and Cd from OC. Desorption of ferric species from OC enhances the mobilisation of Cu, Zn and Cd from OC during the latter stages of the experiment. Adsorbed Fe(III) inhibit Pb mobilisation from OC. However, parallel experiments with galena showed that adsorbed ferric species promotes PbS dissolution. The observed inhibition of Pb release from OC is attributed to galvanic interactions between PbS and ZnS, which decreases the anodic oxidation of PbS. Hence Fe(III) adsorbed onto Pb dissolution sites apparently block the attack of protons and oxygen.
Humic acid (HA) enhances the mobilisation of Pb and, particularly, Cu but release of Zn and Cd from OC are inhibited. The diverse effect of HA on metal mobilisation appears to be governed by the different affinities of HA for metals and the relative stability of metal-HA complexes. The mobilisation of Cu, and to a lesser extent, Zn, Pb and Cd in HA solutions are enhanced under light. This has been attributed to enhanced photo-oxidative dissolution and oxygen containing photo-intermediates that are photosensitised by HA.
|Date of Award||2003|
|Supervisor||David Parry (Supervisor)|