Geochemical properties of 40-year old forested pyrrhotite tailings and impact of organic acids on metal cycling

Abstract

Under humid climate, surface layers of old Cu–Ni pyrrhotite tailings impoundments are typically oxidized and characterized by depletion of sulfides and secondary precipitation of gypsum, jarosite and goethite; metal contaminants are dominantly associated with goethite. In eastern Canada, historical reclamation of such tailings consisted of a period of amendment with lime, fertilizers, and seeding with grass/legume mixtures, followed by forestation. The current study combines characterization of a 40-year old site planted with jack pine and laboratory experiments to evaluate the mineral evolution and the role of organic acids on mineral weathering of oxidized pyrrhotite tailings. In contrast to a nearby control site with no vegetation, gypsum, jarosite and goethite were absent or present in negligible amount in the root-colonized 0–15 cm surface layer of the forested tailings. The results suggested dissolution of the initial secondary minerals with changes in geochemical properties (e.g., increase in pH and organic C content) and pedogenetic processes in the forested tailings. In contrast to the notable loss in total Fe and S from the 0–15 cm layer, Cu and most of the Ni were retained. A 2-month column leaching experiment was conducted to evaluate the impact of repeated additions of low amounts of oxalate, a common organic acid in the rhizosphere, on metal mobilization in oxidized pyrrhotite tailings at pH 2.5, 4 or 6. Gypsum was readily leached regardless of pH or oxalate treatments, whereas jarosite dissolution increased with increasing pH and was further enhanced by the input of oxalate. Indirect evidence of accelerated goethite dissolution by oxalate was provided by the close correlation between dissolved Fe and Cu in oxalate treatment at pH 4 and 6. The oxalate exhibited an increased efficiency in mobilizing Cu as the pH increased. This study emphasizes the contribution of organic acids in metal cycling and their potential impact on the long-term change in Cu and Ni speciation in the root-colonized tailings which may affect metal uptake and migration in the environment.