Impact of lime treatment on tailings dewatering and cap water quality under an oil sands end pit lake scenario

摘要

Oil sands tailings have a limited ability to dewater and can also release toxic water, so finding a cost-effective tailings reclamation strategy has been challenging. End pit lakes (EPLs) are being examined as one potential approach to reduce fluid fine tailings (FFT) inventories for the oil sands industry. This study investigated the effect of adding lime (650 to 4000 ppm) on FFT dewatering and improving cap water quality under the simulated EPL scenario. The results illustrated that a high lime dose of 3500 ppm achieved the highest FFT water recovery, decreased the cap water alkalinity at the end of the experiment (after 90 d), and increased the possibility of cation exchange (at day 0). In contrast, the degradation of petroleum hydrocarbons was slightly enhanced at low lime dose of 650 ppm in comparison to high doses. In addition, the 650 ppm dose resulted in minimal change in the microbial cell counts at day 90, compared with high lime doses that resulted in a large reduction in the cell counts. Comparisons of toxicity of the FFT pore water after lime addition exhibited higher toxic effects for lime dosages >1600 ppm. Nevertheless, at all lime doses, low cap water toxicity (i.e., <1.0 Toxicity Unit; TU) at day 90 was attained (e.g. for the 3500 ppm lime dosage, toxicity was reduced from 0.83 TU (day 14) to 0.58 TU (day 90)). The low water toxicity for the cap water (day 90) can be ascribed to the reduction of cap water pH over time due to the dissolution of atmospheric carbon dioxide into cap water. In this study, the significant implications for the use of lime treatment to improve water quality in EPLs were highlighted. We provided an initial understanding of how lime treatment could benefit the long-term success of FFT remediation within EPLs as self-sustaining aquatic ecosystems in the final reclaimed landscape.