Vanadium Geochemistry of Oil Sands Fluid Petroleum...

Vanadium Geochemistry of Oil Sands Fluid Petroleum Coke
https://www.osti.gov/servlets/purl/1355035

ABSTRACT
11 Vanadium has previously been linked to elevated toxicity of leachates derived from oil sands
12 petroleum coke. However, geochemical controls on V mobility within coke deposits remain
13 poorly constrained. Detailed examinations of pore-water and solid-phase V geochemistry were,
14 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
m
3
15
of fluid petroleum coke. Dissolved V concentrations were highest (up to 3.0 mg L-1 16 ) immediately
17 below the water table, but decreased rapidly with increasing depth. This trend corresponded to a
18 transition from mildly acidic (pH 6 – 7) and oxic conditions to mildly alkaline (pH 7 – 8.5) and
19 anoxic conditions. Scanning electron microscopy (SEM), electron microprobe analysis (EMPA)
20 and micro-X-ray fluorescence (µXRF) mapping revealed coke particles exhibited an internal
21 structure characterized by successive concentric layers. The outer margins of these layers were
22 characterized by elevated V, Fe, Si, and Al concentrations, indicating the presence of inorganic
23 phases. Micro-X-ray absorption near-edge structure (µXANES) spectroscopy revealed that V
24 speciation was dominated by V(IV) porphyrins except at outer margins of layers, where
25 octahedrally-coordinated V(III) was a major component. Minor to trace V(V) was also detected
26 within fluid petroleum coke particles.
27 INTRODUCTION
28 Large quantities of petroleum coke are generated during upgrading of oil sands bitumen
29 to synthetic crude oil. This carbonaceous material exhibits elevated S and trace element
contents,1
30 making it largely unsuitable for combustion or industrial applications. Consequently,
greater than 108
31 t of petroleum coke is currently stockpiled in the Athabasca Oil Sands Region
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(AOSR) of northern Alberta, Canada.2
32 Mine closure landscapes could contain more than ten
times this amount upon conclusion of mining.3
33 Although petroleum coke offers utility as a
34 construction material for mine closure landscapes, previous studies have reported toxic effects on
invertebrates4-6 and plants.7
Baker et al.8
35 observed increased V uptake by a macrophytic green
36 alga and benthic invertebrates in coke-amended sediment from a reclamation wetland.
Puttaswamy et al.5
37 found that coke leachates were toxic to an aquatic invertebrate, and identified
38 dissolved V as a principal source of this toxicity. Aeolian transport of fine particles from subaerial stockpiles has led to more widespread distribution in the AOSR.9
39 Additionally, large
40 stockpiles of oil sands petroleum coke have also been reported in urban industrial areas in major
U.S. cities.10 41 Consequently, a detailed understanding of V sources and mobility is critical for
42 assessing and mitigating long-term environmental risk associated with this oil sands bitumen
43 upgrading byproduct.
44 Vanadium occurs naturally at elevated concentration in oil sands bitumen and other
heavy oil deposits.11,12 45 Petroleum coke, generated during thermal conversion of the non46 distillable bitumen fraction into volatile compounds, becomes enriched in trace elements present
47 in heavy oils. Fluid petroleum coke from the AOSR typically exhibits total V concentrations of
1000 to 2000 mg kg-1
.
1,12-16 48 The V speciation of petroleum coke is dominated by V(IV) as the
vanadyl ion (VO2+ 49 ) in porphyrin-like atomic coordination environments within asphaltenic
micelles.17,18 50 These stable metalloporphyrin complexes are generally resistant to both weathering
and thermal decomposition.12 51 Substantial V(IV) porphyrin breakdown during natural weathering
has, however, been observed in Early Toarcian Age (183 Ma) black shales.19 52 Additionally,
53 thermal decomposition of these stable complexes has been reported at temperatures exceeding
400°C, 12 54 which is consistent with fluid coking temperatures of 480 to 565°C used during oil