Green Mines Green Energy: Impact of 6 Years of Organic Cover on Metal Mobility in Acidic Tailings.

The Green Mines Green Energy research program was initiated in 2006 by Natural Resources Canada to demonstrate the potential to rehabilitate several metal mine sites in Northern Ontario for biofuel production. This presentation summarizes the impact of 6 years of a papermill biosolids cover on the stability of metal contaminants in acidic tailings at one of the sites. Sampling was conducted in 2013 on field plots from oxidized, Cu/Ni tailings (pH 2.5), which have been cultivated for the last 6 years with canola, corn or sunflower crops established in a thick (1 m) cover of papermill biosolids. The 15 cm layer of tailings below the cover was collected for microbiological, chemical and mineralogical characterization, along with a control of unamended tailings for comparison. Regardless of the treatment, goethite is the main sink for Cu (312 mg kg^-1) and Ni (416 mg kg^-1), the two main contaminants of concern in these oxidized tailings. Compared to the control tailings, the nutrient contents (nitrate and K) increased in the amended tailings, along with microbial (acid-producing and sulfate-reducing bacteria) and cellulase activity. An increase of up to 1.5 pH unit was also measured in the tailings underneath the organic cover, and was associated with a significant reduction of the most mobile (water-extractable) pool of Cu, Ni, Fe and Al in the 0-15 cm layer of amended tailings. The increased pH would favor metal sorption to goethite and precipitation of fresh Fe- and Al-oxyhydroxides as potential sinks for metal contaminants. Overall, the approach of using biosolids as a cover improved the “soil health” in the underlying oxidized tailings, but the thickness and degradability of the cover are important factors to consider because of the availability of biosolids and the cost to transport them may limit their use as covers on tailings. Current studies evaluate the option of using a reduced amount of biosolids mixed with a more recalcitrant source of organic C such as biochar.