Vanadium geochemistry of oil sands fluid petroleum coke

Abstract

Vanadium has previously been linked to elevated toxicity of leachates derived from oil sands petroleum coke. However, geochemical controls on V mobility within coke deposits remain poorly constrained. Detailed examinations of pore-water and solid-phase V geochemistry were, therefore, performed on oil sands fluid petroleum coke deposits in Alberta, Canada. Sample collection focused on both active and reclaimed deposits, which contained more than 3 × 10⁷ m³ of fluid petroleum coke. Dissolved V concentrations were highest (up to 3.0 mg/Lˉ¹) immediately below the water table, but decreased rapidly with increasing depth. This trend corresponded to a transition from mildly acidic (pH 6 – 7) and oxic conditions to mildly alkaline (pH 7 – 8.5) and anoxic conditions. Scanning electron microscopy (SEM), electron microprobe analysis (EMPA) and micro-X-ray fluorescence (µXRF) mapping revealed coke particles exhibited an internal structure characterized by successive concentric layers. The outer margins of these layers were characterized by elevated V, Fe, Si, and Al concentrations, indicating the presence of inorganic phases. Micro-X-ray absorption near-edge structure (μXANES) spectroscopy revealed that V speciation was dominated by V(IV) porphyrins except at outer margins of layers, where octahedrally-coordinated V(III) was a major component. Minor to trace V(V) was also detected within fluid petroleum coke particles.