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Quantification of mineral weathering rates in sulfidic mine tailings under water-saturated conditions
Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Tailings are a fine-grained waste product produced during the metal recovery process. Tailings consist mostly of different silicates but also sulfides (e.g. pyrite), since 100 % metal recovery is not possible. Freshly processed tailings are deposited in large impoundments. If the mine tailings in the impoundments are exposed to water and oxygen, the sulfides will oxidize and release acidity and metals such as Fe, Cu, Zn, and Pb. The sulfide mineral oxidation reactions are catalyzed by sulfur and iron oxidizing bacteria (principally Acidithiobacillus ferrooxidans) that oxidize ferrous iron to ferric iron, which then oxidizes pyrite. When the leachate produced by this process discharges from the impoundment, it is called acid mine drainage, which may lead to the pollution of adjacent streams and lakes.

The intention with this thesis is to investigate and quantify mineral weathering processes and element release rates occurring in water-saturated and soil-covered sulfidic mine tailings. The study was performed in different batch and column experiments in room temperature and in the laboratory. The batch experiments were conducted for ca. three months and investigated: a) microbial and abiotic sulfide oxidation in freshly processed tailings under oxic conditions at pH 2-3 and pH 8, b) microbial oxidation of pure pyrite grains at pH 2-3 under different oxygen concentrations ranging from anoxic to oxic conditions. The column experiments, consisting of unoxidized tailings in water-saturated columns, were conducted for up to three years. In these experiments, an oxygen-saturated solution was continually pumped into the column inlet, and investigated: a) differences in oxidation rates between tailings of two different grain sizes, b) factors affecting element discharge rates, acid neutralization, and sulfide oxidation, c) the effect of ions released in a soil cover on release rates in the tailings.

Sulfide oxidation processes within the batch experiments were limited by surface kinetics. The microbial oxidation of pure pyrite at atmospheric conditions produced the most rapid rate, while the microbial oxidation of pure pyrite at anoxic conditions was slower by 1.8 orders of magnitude. Microbial and abiotic oxidation of pyrite in freshly-processed tailings resulted in pyrite oxidation rates that were intermediate between these two extremes. The results from the microbial experiments with pure pyrite indicated a positive correlation between the concentration of dissolved oxygen, ferric iron and bacterial cells (at a total cell concentration > 106 cells/mL and a dissolved oxygen concentration ≥ 13.2 µM), which implies an interdependence of these factors. The results from these batch experiments support the indirect mechanism for microbial oxidation by the ferric oxidation pathway. Pyrite oxidation rates estimated from the batch experiments may be comparable with oxidation rates in the unsaturated zone and at the groundwater table in a tailings impoundment.

Acid neutralization reactions in the column experiments resulted in the release of base cations to the column leachate. Calcite was the most important neutralizing mineral despite that it was only present in minor amounts in the tailings. It was confirmed that acidity forced the calcite dissolution. Element release rates in the column experiments were controlled by the availability of dissolved oxygen, which was a function of the water flow rate into the column. These column experiments also showed that the results are comparable with results from field studies, justifying the use of column experiments to study processes within tailings impoundments.

Place, publisher, year, edition, pages
Stockholm: Institutionen för geologi och geokemi , 2005.
Series
Meddelanden från Stockholms universitets institution för geologi och geokemi, ISSN 1101-1599 ; 321
Keyword [en]
mine tailings, pyrite, sulfides, weathering, oxygen availability, Acidithiobacillus ferrooxidans
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:su:diva-349OAI: oai:DiVA.org:su-349DiVA: diva2:192797
Public defence
2005-02-18, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 8 C, Stockholm, 10:00
Opponent
Supervisors
Available from: 2005-01-27 Created: 2005-01-27Bibliographically approved
List of papers
1. Sulfide mineral oxidation in freshly processed tailings: batch experiments
Open this publication in new window or tab >>Sulfide mineral oxidation in freshly processed tailings: batch experiments
2002 In: Journal of Geochemical Exploration, ISSN 0375-6742, Vol. 76, no 3, 139-153 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-23555 (URN)
Note
Part of urn:nbn:se:su:diva-349Available from: 2005-01-27 Created: 2005-01-27Bibliographically approved
2. Pyrite oxidation by Acidithiobacillus ferrooxidans at various concentrations of dissolved oxygen.
Open this publication in new window or tab >>Pyrite oxidation by Acidithiobacillus ferrooxidans at various concentrations of dissolved oxygen.
2006 (English)In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 225, no 1-2, 16-29 p.Article in journal (Refereed) Published
Abstract [en]

Pyrite oxidation rates were examined at various concentrations of dissolved oxygen (DO) in the presence of the sulfur and iron oxidizer Acidithiobacillus ferrooxidans. Five different batch experiments were performed at room temperature for 75 days under various DO levels (273, 129, 64.8, 13.2, and ≤ 0.006 μM), containing pyrite grains (particle size 63–250 μm) and a modified 9K nutrient medium at pH 3. The reactors were inoculated with A. ferrooxidans. In all experiments, pH decreased with time and sulfur and iron were released to the solution, indicating pyrite oxidation at all DO levels. Pyrite oxidation rates (ca. 5 × 10− 10 mol m− 2 s− 1 at 273 μM DO) from all experiments showed positive correlation with DO, Fe(III), and bacterial concentration. These rates were significantly slower than rates presented in other published studies, but this is probably due to the significantly greater Fe(III) concentration at lower pH in these previous studies. The results obtained in this study suggest that ferric iron reduction at the pyrite surface is the primarily mechanism for microbial pyrite oxidation in the presence of DO. The results from our study support the indirect mechanism of sulfide oxidation, where A. ferrooxidans oxidizes ferrous iron in the presence of DO, which then oxidizes pyrite.

Keyword
Pyrite oxidation; Rate; Oxygen; Acidithiobacillus ferrooxidans; Batch experiments
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-23556 (URN)10.1016/j.chemgeo.2005.07.020 (DOI)
Note
Part of urn:nbn:se:su:diva-349Available from: 2005-01-27 Created: 2005-01-27 Last updated: 2010-08-09Bibliographically approved
3. Comparison of Sulfide Oxidation in Unweathered Pyritic Mine Tailings
Open this publication in new window or tab >>Comparison of Sulfide Oxidation in Unweathered Pyritic Mine Tailings
2003 In: 6th ICARD, International Conference on Acid Rock Drainage: Conference Proceedings, Cairns, Queensland, Australia, July 12-18 2003, 2003, 1027-1030 p.Chapter in book (Other academic) Published
Identifiers
urn:nbn:se:su:diva-23557 (URN)
Note
Part of urn:nbn:se:su:diva-349Available from: 2005-01-27 Created: 2005-01-27Bibliographically approved
4. Element discharge from sulfidic mine tailings at limited oxygen availability
Open this publication in new window or tab >>Element discharge from sulfidic mine tailings at limited oxygen availability
Manuscript (Other academic)
Identifiers
urn:nbn:se:su:diva-23558 (URN)
Note
Part of urn:nbn:se:su:diva-349Available from: 2005-01-27 Created: 2005-01-27 Last updated: 2010-01-13Bibliographically approved
5. Evaluation of the influence of soil cover leachate on weathering in sulfide-rich tailings
Open this publication in new window or tab >>Evaluation of the influence of soil cover leachate on weathering in sulfide-rich tailings
Manuscript (Other academic)
Identifiers
urn:nbn:se:su:diva-23559 (URN)
Note
Part of urn:nbn:se:su:diva-349Available from: 2005-01-27 Created: 2005-01-27 Last updated: 2010-01-13Bibliographically approved

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