Geochemical stability of acid-generating pyrrhotite tailings 4 to 5 years after addition of oxygen-consuming organic covers

Oxygen-consuming organic covers can serve as a reactive barrier to minimize sulfide oxidation in acid-generating tailings but may lead to metal mobilization in surface oxidized layers. We evaluated changes in the bio-geochemical stability of acid-generating, Cu-Ni pyrrhotite tailings 4 to 5 years after addition of a 50 cm-thick or a 1 m-thick biosolid cover planted with energy crops. The original tailings (pH 2.5) were oxidized in the first 10 to 40 cm, and goethite was the dominant sink for Cu and Ni, the main metal contaminants in these tailings. Both covers increased pH, nutrient availability, microbial activity and diversity in the oxidized tailings, and led to a reduction of water-soluble Cu, Ni, Fe and Al after 4 to 5 years of application. Changes in pH, humidity, organic C content, and redox conditions resulted in partial dissolution of jarosite and gypsum below the cover but goethite apparently remained stable. Under both covers, total Ni decreased in the oxidized layer, indicating remobilization, but Cu was retained. Significant accumulation of Cu as Cu sulfide at the oxidized/unoxidized tailings interface was detected only under the 1 m-thick cover, suggesting that the thinner cover may not sufficiently decrease the oxidizing conditions to mitigate acid mine drainage. Migration of nitrate and P down to the unoxidized tailings was observed under both covers and raises the concern of continued sulfide oxidation in unoxidized tailings. Although the implementation of thinner covers is economically more sustainable than thick covers, our results indicate that further research is required to establish their long-term suitability and performance to prevent acid mine drainage.