Environmental stability of the processing waste from a sulfide mining district of Namibia, Africa - A model rhizosphere solution approach

Jaroslav Novotný

Master's thesis

Abstract:

Berg Aukas located in northern Namibia is a historic mining area, where Zn, Pb and V ores were mined and roasted on site until 1978, when the mine was closed without any remediation. Kinetic batch leaching of the flotation wastes (tailings) in 0.5 mM solutions of citric, oxalic and acetic acids was performed to simulate the release of trace metals in the rhizosphere-like environment. There was an assumption that low-molecular-weight organic acids (LMWOAs), originating from roots of cover vegetation on the abandoned mine tailings dams (>30 years), may induce trace element solubilisation and thus cause metal leaching to subsoil layers or groundwater. The experiment demonstrated that only citric acid may significantly contribute to mineral waste alteration accompanied by contaminant mobilization; the maximum dissolution rates corresponded to 2% (Cd), 0.8% (Pb) and 0.3% (Zn) of their total amounts. While in contrast, the role of oxalic and acetic acids in the complex process of contaminant leaching was mainly negligible and/or comparable with water. The precipitation of newly formed Fe (hydr)oxides followed by the re-adsorption of Pb, Cd and eventually Zn from the model solutions seems to be the principal mechanism which leads to the metals stabilization, as predicted by speciation modelling. Conclusion is that the relatively low concentrations of metals were released from the flotation waste in the presence of model LMWOA solutions in Berg Aukas tailings dams. Considering the neutral and slightly alkaline (and lowly organic) conditions in tailings, leaching of Zn, Pb and Cd, being the main contaminants in the flotation waste, to subsoil layers or groundwater promoted by LMWOAs poses a relatively low environmental risk for the area compared to wind and water-borne dispersion of contaminated tailings with potential negative consequences for adjacent ecosystems and residential communities.

Abstract:

Berg Aukas located in northern Namibia is a historic mining area, where Zn, Pb and V ores were mined and roasted on site until 1978, when the mine was closed without any remediation. Kinetic batch leaching of the flotation wastes (tailings) in 0.5 mM solutions of citric, oxalic and acetic acids was performed to simulate the release of trace metals in the rhizosphere-like environment. There was an assumption that low-molecular-weight organic acids (LMWOAs), originating from roots of cover vegetation on the abandoned mine tailings dams (>30 years), may induce trace element solubilisation and thus cause metal leaching to subsoil layers or groundwater. The experiment demonstrated that only citric acid may significantly contribute to mineral waste alteration accompanied by contaminant mobilization; the maximum dissolution rates corresponded to 2% (Cd), 0.8% (Pb) and 0.3% (Zn) of their total amounts. While in contrast, the role of oxalic and acetic acids in the complex process of contaminant leaching was mainly negligible and/or comparable with water. The precipitation of newly formed Fe (hydr)oxides followed by the re-adsorption of Pb, Cd and eventually Zn from the model solutions seems to be the principal mechanism which leads to the metals stabilization, as predicted by speciation modelling. Conclusion is that the relatively low concentrations of metals were released from the flotation waste in the presence of model LMWOA solutions in Berg Aukas tailings dams. Considering the neutral and slightly alkaline (and lowly organic) conditions in tailings, leaching of Zn, Pb and Cd, being the main contaminants in the flotation waste, to subsoil layers or groundwater promoted by LMWOAs poses a relatively low environmental risk for the area compared to wind and water-borne dispersion of contaminated tailings with potential negative consequences for adjacent ecosystems and residential communities.

Keywords

Low-Molecular-Weight Organic Acids, Solution, Leaching, Flotation Waste, Metal, Rhizosphere

Keywords

Low-Molecular-Weight Organic Acids, Solution, Leaching, Flotation Waste, Metal, Rhizosphere

Language used: English

Date on which the thesis was submitted / produced: 17. 3. 2015

Thesis defence

• Supervisor: doc. RNDr. Aleš Vaněk, Ph.D.

Czech University of Life Sciences Prague

Faculty of Agrobiology, Food and Natural Resources

Master programme / field:
Natural Resources and Environment / Natural Resources and Environment