Assessing the mobility of metals and dissolved organics in pit-lake models containing PASS-treated fluid tailings
Creators: Mehravaran, Foroogh
Publisher: name: University of Alberta Library
Year: 2024

<p>Oil sands surface mining produces considerable quantities of fine fluid tailings (FFT). Several reclamation strategies are being investigated to address the environmental concerns related to long-term FFT storage. One promising sustainable tailings management approach is End Pit Lake (EPL); however, concerns remain regarding the potential release of contaminants from FFT into the capping water. Preliminary tailings treatment with innovative Permanent Aquatic Structure Storage (PASS) technology is projected to enhance the dewatering and capping water quality in the EPL. Nevertheless, little is known about the geochemical stability of PASS-treated FFT, especially for the release of potentially toxic metals and organic compounds.<br>
This dissertation thoroughly investigated the effects of PASS-treated FFT on EPL, starting with the FFT characterization, including the development of a novel method for analyzing naphthenic acids (NAs) in FFT. Subsequently, the study investigated the short- and long-term effects of PASS-treated FFT on contaminants of potential concerns (COPCs) mobility in a demonstrated EPL system. The dissertation concluded by examining COPCs desorption mechanisms from FFT to capping water.<br>
A highly efficient extraction and cleanup process was developed to assess NAs in oil sands tailings. This method used several extraction solvents, mixing conditions, extraction times, and steps. Additionally, solid phase extraction (SPE) was fine-tuned using different elution conditions and reconstitution reagents. 0.5 M NaOH outperformed ten extraction solvents for NAs recovery effectively. The orbital shaking condition was 1.27 times more effective than the static condition. Mixing for over 40 minutes increased the recovery by 32% over 5 minutes. Two extraction steps yielded 26% more recovery than a single extraction step. Methanol/formic acid (43%), and methanol/water (82%), in SPE using Hydrophilic-Lipophilic Balance (HLB) cartridges, recovered the most NAs during cleanup. Using this developed procedure, NAs were measured in oil sands tailings at 56.10–112.25 mg/kg. The largest NAs level was detected in PASS-treated FFT tailings samples. The most abundant NA species in all tailings were classical, notably with carbon numbers 16 and 18. This finding provided fresh insight for analyzing NAs in tailings sediment, which is critical for evaluating treatment efficacy and monitoring NAs at FFT reclamation sites.<br>
To investigate COPC fate in treated FFT storage, EPL models (columns of FFT/water ratio ½, 2.4 m deep, 75.4 L) were run for two years. During the first week, a high flux of contaminants from tailings deposits to capping water was quickly stabilized. Later on, the release of potentially toxic metals, ions, and dissolved organics was also decreased (PO4-3: 100%, Ca2+: 33%, Mg2+: 1%, Fe3+: 100%, Li+: 100%, Cu+: 100%, Al+3: 100%, and classical NAs: 10%). A short-term bench-scale column experiment was carried out to better understand the chemical release mechanism. It was determined that only advective mass transfer influenced the initial chemical flux, which was 0.002 m3m-2d-1, 50% lower than the first commercial-scale EPL. Lake turnover did not affect advection flow. The high viscosity of FFT and diluted oil sands process water (OSPW) reduced chemical flux. PASS treatment efficiently immobilized COPCs, paving the way for self-sustaining mine closure at the EPL system, as exemplified by Lake Miwasin and its implementation of PASS-treated FFT.<br>
Batch experiments examined the desorption of organics from treated FFT into capping water under varying dosages, contact duration, temperature, and ionic strength conditions. Higher FFT dosage and capping water organic matter increased dissolved organic carbon (DOC) desorption, whereas temperature and conductivity had minimal impact. Electrolytes controlled DOC equilibrium desorption capability, with cations (K+ and Ca2+) considerably affecting desorption from negatively charged FFT particles. Temkin isotherms displayed that desorption dynamics followed physically favorable patterns. The intraparticle diffusion kinetics model accurately predicted 0.65 mg/g of PASS-treated FFT DOC equilibrium desorption. A small percentage (5.82%) of the DOC and less than half of NAs species (39.1%) in PASS-treated FFT were potentially released. A high distribution coefficient value for DOC and NAs indicated that they were mainly stable in PASS-treated FFT deposition. <br>
The study provided an effective approach for monitoring NAs across various oil sands tailings storage systems, facilitating the assessment of tailings treatment and reclamation practices.  Research on the fate and impact of PASS-treated FFT in EPL reclamation landscapes helps explain COPCs flux from FFT to capping water. The chemical mass transfer and desorption investigation also shows that PASS treatment in this FFT management technique improves EPL system surface water quality over time.</p>