VANADIUM AND NICKEL GEOCHEMISTRY OF FLUID PETROLEUM COKE DEPOSITS AT AN OIL SANDS MINE, ALBERTA, CANADA

Petroleum coke is a byproduct of oil sands bitumen upgrading in Athabasca Oil Sands Region (AOSR) of northern Alberta, Canada. The total petroleum coke inventory in this region currently exceeds 100 million tonnes and more than 1 billion tonnes will ultimately be stored in mine closure landscapes. A detailed core and groundwater sampling program was conducted in 2014 to examine the geochemical characteristics of fluid petroleum coke deposits at a mine located approximately 40 km northwest of Fort McMurray, Alberta, Canada. The bulk coke composition was dominated by C (84.2 ± 2.3 wt. %) and S (6.99 ± 0.26 wt. %), whereas Si (9210 ± 3000 mg kg⁻¹), Al (5980 ± 1200 mg kg⁻¹), Fe (4760 ± 1200 mg kg⁻¹), and Ti (1380 ± 430 mg kg⁻¹) were present in lesser amounts. Vanadium (1280 ± 120 mg kg⁻¹) and Ni (230 ± 80 mg kg⁻¹) – which have previously been linked to elevated toxicity of coke leachates – were present at the highest concentrations among potentially-hazardous minor and trace elements. Bulk X-ray absorption near edge structure (XANES) spectra indicated that V and Ni largely occurred as V(IV) and Ni(II) in porphyrin complexes. However, micro-scale XANES measurements revealed distinct zonation in the distribution and speciation of both V and Ni. These micro-scale measurements coupled with finite difference modeling suggest that V(III) was present in distorted octahedral sites of phyllosilicates, while Ni(II) is likely hosted in distorted porphyrin complexes and oxides. Elevated pore-water V and Ni concentrations were observed within a narrow (1–3 m) depth interval immediately below the water table. Vanadium concentrations up to 3 mg L⁻¹ occurred at pH greater than 7 in a mixing zone of meteoric water and oil sands process affected water (OSPW) identified from δ²H and δ¹⁸O data. Elevated Ni concentrations up to 50 μg L⁻¹ were observed within this mixing zone only at locations where pore-water pH was less than 6.5. Vanadium concentrations were limited under these slightly lower pH conditions, while dissolved V and Ni concentrations were generally low at greater depths under mildly alkaline (i.e., pH > 8.0) and anoxic conditions characteristic of OSPW. This research provides new insight into the source and mobility of V and Ni in oil sands mine landscapes and, more generally, the environment.