Fate of phosphate added to mature fine tailings (MFT) of oil sands tailing ponds

Phosphate nutrients can be added to oil sand tailing ponds (OSTPs) to bioremediate naphtha. However, the disappearance of phosphate from the fluid after being added to mature fine tailings (MFT) has been reported but not clearly elucidated. Its fate in three MFT samples (with quartz, kaolinite and illite as dominant minerals) which have a complex pore water chemistry and residual bitumen, was studied. This is the first study to our knowledge to examine the fate of phosphate added to MFT in OSTPs. In kinetic adsorption studies, phosphate removal from the fluid increased with time until equilibrium was achieved without significant precipitation. With Pond 7 MFT, there was an initial spike of ∼70% removal which decreased to an equilibrium of 40%, indicating an initial weak, reversible adsorption. In adsorption isotherm studies, losses were similar with MFT of Pond 2/3 (at 5 and 15 m) achieving a maximum phosphate adsorption of ∼0.90 mg/g and Pond 7 (12.5 m) MFT was ∼0.44 mg/g. When bitumen was removed from Pond 7 MFT, the specific surface area increased four times and the maximum adsorbed increased 4.5 times to 2.00 mg/g, indicating that bitumen limited adsorption. When normalized for surface area, bitumen-free Pond 7 MFT adsorbed only 1.4 times more phosphate compared to when bitumen was present. Neither Langmuir nor Freundlich models described well its adsorption onto MFT, though Langmuir was better. Finally, if microorganisms use phosphate in solution, then phosphate sorbed onto MFT solids may desorb based on the adorption-desorption equilibrium and hence can be available for microbial activity. Findings may be more broadly applied to other types of tailing ponds and contaminated environments.