Toward sustainable remediation of oil sands fine Tailings-A review

Microbial community structural and functional differentiation in capped thickened oil sands tailings planted with native boreal species

The oil sands mining operations in Alberta have produced billions of m³ of tailings which must be reclaimed and integrated into various mine closure landforms, including terrestrial landforms. Microorganisms play a central role in nutrient cycling during the reclamation of disturbed landscapes, contributing to successful vegetation restoration and long-term sustainability. However, microbial community succession and response in reconstructed and revegetated tailings remain largely unexplored. This study aimed to monitor the structural and functional responses of microbial communities in tailings subjected to different capping and vegetation strategies over two growing seasons (GS). To achieve this, a column-based greenhouse experiment was conducted to investigate microbial communities in tailings that were capped with a layer (10 or 30 cm) of peat-mineral mix (PMM) and planted with either upland or wetland communities. DNA metabarcoding analysis of the bacterial 16S rRNA gene and fungal ITS2 region as well as shotgun metagenomics were used to asses the impact of treatments on microbial taxonomy and functions, respectively. Results showed that tailings microbial diversity and community composition changed considerably after two GS compared to baseline samples, while communities in the PMM capping layer were much more stable. Likewise, several microbial functions were significantly enriched in tailings after two GS. Interestingly, the impact of capping on bacterial communities in tailings varied depending on the plant community, leading to a higher number of differentially abundant taxa and to a decrease in Shannon diversity and evenness in the upland treatment but not in the wetland treatment. Moreover, while capping in the presence of wetland vegetation increased the energy-related metabolic functions (carbon, nitrogen, and sulfur), these functions were depleted by capping in the upland treatment. Fungi represented a small proportion of the microbial community in tailings, but the relative abundance of several taxa changed over time, while the capping treatments favored the growth of some beneficial taxa, notably the root endophyte Serendipita, in both upland and wetland columns. The results suggest that selecting the right combination of capping material and vegetation type may contribute to improve below-ground microbial processes and sustain plant growth in harsh environments such as oil sands tailings.

Direct Shear Strength Characteristics in Unsaturated Compacted Soil Surface Coverage on Pb(II)-Polluted Tailings Reservoir under Low Normal Stress

Mining activities generate a large amount of solid waste called tailings, and it often contains a certain number of heavy metals, depending on the different types of minerals. To avoid disasters caused by the diffusion of tailings and their infiltration into surface water, mines often use tailings ponds as a surface coverage structure. This study mainly focused on the shear strength of surface coverage isolation layer soil under low normal stress. The soil isolation layers with different values of dry bulk density, water content, bentonite content, and Pb(II) ion concentration were treated with direct shear strength under normal stress at 12.5, 25, 50, and 100 kPa. It was found that when the dry bulk density was increased from 1.1 g/cm3 to 1.3 g/cm3, the shear strength increased by 49.3%~117.2%, and when the water content was decreased from 24% to 16%, the shear strength increased by one to three times. With the addition of bentonite in proportions from 18% to 30%, shear strength increased by 1.9 to 116.9 kPa, and the degree of improvement was related to both dry bulk density and water content; the specimens with bentonite added were more sensitive to Pb(II) ion concentration, and when the specimen with 24% bentonite addition was compared with the specimen without the addition of bentonite, both under a Pb(II) ion concentration of 5 mg/L, the shear strength decreased by 8.1~35.6 kPa, the internal friction angle decreased by 0.5~9.7°, the cohesion force decreased by 5.8~37.7 kPa, and the shear modulus decreased by 5~12 kPa/mm. Therefore, in order to maintain good mechanical properties in the surface covering structure, it is necessary to maintain a high dry bulk density, low water content, and bentonite content of no less than 24%.