Weathering and microbial activity in sulfidic mine tailings with implications in reclamation
Creators: name: Robbins, James Milton
Publisher: name: University of British Columbia
Year: 2010

description: The oxidation of sulfidic mine tailings and subsequent acid and salt accumulation results in acid drainage and poses a severe reclamation problem. The objectives of this study were to quantify the processes leading to these problems and propose feasible solutions. Characterization of 120 sulfidic surface samples, collected from the Sullivan mine tailings ponds, was carried out for dominant physiological groups of microorganisms, sulfur forms, water and acid extractable cations qualitative physical characteristics, pH, conductivity, organic matter and moisture content. Acid tolerant fungi and iron, sulfur and acid tolerant heterotrophic bacteria were enumerated by the most-probable-number technique. Iron bacteria (Thiobacillus ferrooxidans) occurred often and can be implicated as the cause of Fe⁺² oxidation in samples not buffered at near neutral pH values by bases. Sulfur bacteria (thiobacillus species were commonly found and classified as T. thioparus, T. neapolitanus, or T. thiooxidans. A pH-dependent succession of these thiobacilli occurs in the tailings. Acid tolerant heterotrophic bacteria populations were highly correlated with those of the thiobacilli suggesting a symbiotic relationship, particularly in samples with pH &gt;2.5. The numbers of acid tolerant fungi tended to increase proportionally with oxidation, suggesting increased colonization with time. Chemical analysis for major sulfur forms indicates that iron mono- sulfide oxidation is rapid resulting in the formation and persistence of Fe⁺³ (as amorphous Fe oxyhydroxides in mineralogical analysis) and elemental sulfur. Some accumulation of other oxidizable sulfur forms is indicated by the high levels of total oxidizable sulfur. A portion of this sulfur not accounted for by CS₂ extractable elemental sulfur may be present as amorphous elemental sulfur. The oxidation of elemental sulfur and other sulfur intermediates to sulfate is pronounced in the surface 0-2 cm based on the laboratory oxidation of a bulk tailings sample. Mineralogical analysis shows that gypsum is the predominant sulfate containing mineral at basic to slightly acid pH values. Under moderately to strongly acid conditions, jarosite type minerals occurred in high amounts. Mineralogical analysis also showed that the decomposition of basic minerals and chlorite occurred initially, with the dissolution of micas and K-feldspars being less rapid. The absence of detectable kaolinite or other Al silicate residues indicates that alumino-silicate dissolution may be congruent. Mineralogical results showing dissolution of minerals is supported by general increases in water extractable cations (including Fe, Al, Ca, Mg, K and Na) and decreases in these same acid extractable cations as weathering proceeds. These processes ultimately result in a highly oxidized surface in which acid production is slow. It is suggested that erosional processes be inhibited where an oxidized surface exists to prevent the exposure of reduced tailings beneath the surface. Furthermore, steps should be taken to minimize the addition of fresh tailings over oxidized surfaces. The use of correlations between the qualitative physical characteristics texture, structure and color, obtained in this study, can serve as guides to estimating the degree of tailings oxidation in the field. If more precision is required, the colorimetric determination of CS₂ extractable elemental sulfur is suggested, particularly on the more oxidized samples. Proper management of the tailings to maintain present oxidized surfaces should reduce the acid drainage considerably until reclamation is undertaken. The quantification of the changes in tailings properties with increases in weathering can be useful in evaluating reclamation strategies.