Biogeochemical Implications of Sulfate-Based Coagulants in Treated Oil Sands Fluid Fine Tailings
Date
2024-06-19
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0002-8127-9984
Type
Thesis
Degree Level
Doctoral
Abstract
Oil sands mining operations generate a large volume of fluid fine tailings (FFT), requiring incorporation into aquatic and terrestrial reclamation landforms. These tailings are a mixture of inorganic solids, oil sands processed-affected water (OSPW), and residual bitumen. The initial solid contents, which range between 25 to 35 % (w/w), are dominated by quartz and clay minerals (i.e., kaolinite, illite, and illite-smectite (I-S) mixed layer) with minor carbonates, trace sulfides, and oxides. Associated OSPW contains elevated concentrations of Na+, Cl-, HCO3-, and NH4+, plus trace elements, naphthenic acids (NAs), and residual hydrocarbons. Elevated Na+ concentrations relative to other cations (e.g., K+, Ca2+, and Mg2+) hinder aggregation of silt- and clay-size particles (≤ 44 µm), thereby slowing FFT settlement and dewatering, which precludes prompt integration into reclamation landscapes. Consequently, FFT inventories within tailings ponds have grown steadily over time, approaching 1.3 billion m3 by 2020. Regulators have issued directives intended to curtail these growing inventories and promote progressive mine reclamation. These directives, coupled with slow settlement behaviour, prompted oil sands operators to develop various technologies that accelerate dewatering with the addition of chemical coagulants. However, the influence of these coagulants on FFT biogeochemistry is not fully understood.
The main goal of this thesis was to examine the biogeochemical implications of chemical coagulant treatments on oil sands FFT and to understand the effects of cation exchange reactions associated with gypsum addition on the isotopic signature of FFT pore-water. This research uses the integration of analytical methods and anaerobic batch experiments to examine how different doses (i.e., 0, 500, 1000, and 1,500 ppm) of coagulants (i.e., alum [Al2(SO4)3.nH2O], ferric sulfate [Fe2(SO4)3.nH2O], and gypsum [CaSO4.2H2O]) affect FFT geochemistry, mineralogy, and microbiology over time. The research also uses Na-Ca exchange batch experiments with different masses (0.025, 0.05, 0.1, 0.2, 0.4, 0.8, 1.6, 2.4, and 3.2 g) of reference clay minerals (kaolinite, illite, and I-S) and FFT to determine the extent of Ca uptake during cation exchange reactions associated with gypsum amendments using the magnitude of isotopic fractionation.
The anaerobic batch experiments showed that both treated and untreated FFT samples host diverse microbial communities with a range of metabolic capabilities. Sulfate and Fe(III) reduction and methanogenesis are the key biogeochemical redox processes in FFT deposits. The interactions among these processes lead to the complex biogeochemistry of FFT. Sulfate reduction suppresses CH4 production, while the H2S produced reacts with microbially-derived Fe(II) to form Fe(II) sulfide minerals, which promote the sequestration of metals, which include As, Ni, V, and Zn. Coagulant addition enhances mass-transfer reactions (e.g., carbonate minerals dissolution-precipitation, ion exchange, adsorption-desorption, and sulfide minerals precipitation), which determines the release or attenuation of major and trace elements. The sulfate-based coagulants also decreased solution pH (alum: 7.16 to 4.50, ferric sulfate: 7.72 to 6.28, and gypsum: 8.03 to 7.41) and alkalinity (alum: 840 to 20 mg L-1, ferric sulfate: 920 to 280 mg L-1, and gypsum: 920 to 420 mg L -1) and increased electrical conductivity (EC) (alum: 3.78 to 9.06 mS cm-1, ferric sulfate: 2.31 to 10.71 mS cm-1, and gypsum: 5.98 to 10.30 mS cm-1) due to elevated Na+, Cl-, and SO42-. The Ca - Na exchange experiments revealed that cation exchange dominates Ca uptake in acidic environments, and the magnitude of isotopic fractionation depends on the extent of Ca uptake. Illite and I-S exhibited the most Ca uptake and isotopic fractionation compared to kaolinite. The clay minerals present in FFT control the Ca uptake and the magnitude of isotopic fractionation. Heavier isotopes (e.g., 44Ca) were preferentially attached to the clay surfaces in most samples, which could be controlled by variation in Ca2+ coordination number (CN).
Description
Keywords
Oil sands, Tailings, Process-affected water, Biogeochemistry, Methanogenesis, Sulfate reduction, Coagulants, Clay minerals, Stable isotopes, Ion exchange, Coordination number
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Geological Sciences
Program
Geology