Transient release of Ni, Mn and Fe from mixed metal sulphides under oxidising and reducing conditions
Résumé
The potential release of metals from anoxic sediments exposed to oxygen was investigated by using a synthetic preparation of metal sulphides dominated by solid phase FeS. The technique of DGT (diffusive gradients in thin-films) was used to measure sulphide and Fe, Mn and Ni in the anoxic metal-sulphide slurry, which had a pH of 6.4. Speciation calculations based on these data showed there was moderate supersaturation with respect to amorphous FeS in the solution phase. Measurements made using DGT with a range of diffusion layer thicknesses showed that when Fe, Mn and Ni are removed from solution there is fairly rapid (minutes) release from the solid phase, that is reasonably well sustained. This presumed desorptive release will be responsible for elevated concentrations of some metals in solution when sediments are resuspended. Oxidation of the slurry by bubbling with air rapidly (hours) removed Fe, Mn and Ni from the pore water solution. While Fe concentrations in solution remained low after the removal, Mn and Ni were transiently released. These results were consistent with initial rapid oxidation of Fe(II) to oxyhydroxides, which remove Mn(II) and Ni by adsorption. The slower oxidation of FeS then releases Mn and Ni, but these too are eventually removed by adsorption to iron oxyhydroxides. These data suggest that oxidation of metal sulphides will contribute to the release of metals from sediment disturbed by dredging or remedial aeration, but it is likely to be short lived, with complete removal within a day.