Sulfate-based coagulants can suppress methanogenesis in treated oil sands fine tailings

Abstract

Bitumen extraction from mined oil sands ore generates a large volume of fluid fines tailings (FFT) that must be incorporated into either aquatic or terrestrial reclamation landforms. Mine operators are developing various tailings technologies to accelerate FFT dewatering, including the addition of chemical coagulants and flocculants. However, the impacts of these coagulants and flocculants on biogeochemical processes in treated FFT are not fully understood. We conducted anaerobic batch experiments to examine the influence of different doses (i.e., 0, 500, 1000, and 1500 ppm) of sulfate-based coagulants, including aluminum sulfate (alum) [Al₂(SO₄)₃∙nH₂O], ferric sulfate (ferric) [Fe₂(SO₄)₃∙nH₂O], and calcium sulfate (gypsum) [CaSO₄∙2H₂O], on biogenic gas production and microbial communities in treated FFT. Our results show that sulfate addition stimulated microbial sulfate reduction, which inhibited methanogenesis in coagulated FFT relative to experimental controls. Sulfate depletion preceded increased methane production in the 500 ppm gypsum experiment, while larger ferric and alum doses produced higher sulfate concentrations and larger pH decreases. 16 S rRNA sequencing revealed that Comamonadaceae, Anaerolineaceae, and Desulfocapsaceae were the major bacterial families, while Methanoregulaceae and Methanosaetaceae dominated the archaeal families in all treatments. Precipitation of iron(II) sulfides limited dissolved hydrogen sulfide concentrations in experiments where Fe availability was not limited. Our results indicate that addition of sulfate-based coagulants can stimulate microbial sulfate reduction and suppress methanogenesis. However, resumption of methane production following sulfate depletion reveals complex interactions among biogeochemical reaction pathways. Overall, this study demonstrates that biogeochemical cycling of carbon, sulfur, and iron are important considerations for the development and implementation of tailings treatment technologies.

Keywords: Biogeochemistry; Methanogenesis; Oil sands; Sulfate reduction; Treated tailings.

Fig. 1

Pore-water pH, alkalinity, and electrical conductivity for different doses (0, 500, 1000, and 1500 ppm) of coagulants (alum, ferric sulfate, and gypsum) from week 0 to week 49. Where visible, the error bars represent 1σ

Fig. 2

Dissolved concentrations of total Fe, SO₄²⁻, ΣH₂S, and ƩNH₃-N in FFT pore water for 0, 500, 1000, and 1500 ppm of alum (Al³⁺), ferric sulfate (Fe³⁺), and gypsum (Ca²⁺) from week 0 to week 49. Where visible, the error bars represent 1 σ

Fig. 3

Dissolved concentrations of CO₂ and CH₄ in FFT pore water for 0, 500, 1000, and 1500 ppm of alum (Al³⁺), ferric sulfate (Fe³⁺), and gypsum (Ca²⁺) from week 0 to week 49. Where visible, the error bars represent 1 σ

Fig. 4

Stacked column plot of Observed Species, Chao1, Shannon, and Inverse Simpson indices values for different concentrations of alum, ferric sulfate, and gypsum from week 0 to week 49. The first numbers (0, 16 and 49) represent the number of weeks of the experiment, while the numbers after the hyphen (0.0, 0.5, 1.0, and 1.5) represent coagulant doses (0, 500, 1000, and 1500 ppm)

Fig. 5

Bubble chart representing the percentage relative abundance of the 18 most abundant bacterial families. Each bubble represents % reads relative to all the major families in each sample. The first numbers (0, 16, and 49) represent the number of weeks, while the numbers after the hyphen (0.0, 0.5, 1.0, and 1.5) represent coagulant doses (0, 500, 1000, and 1500 ppm). R.A = Relative Abundance

Fig. 6

Bubble chart representing the percentage relative abundance of the 11 most abundant archaeal families. Each bubble represents % reads relative to all the major families in each sample. The first numbers (0, 16, and 49) represent the number of weeks, while the numbers after the hyphen (0.0, 0.5, 1.0, and 1.5) represent coagulant doses (0, 500, 1000, and 1500 ppm). R.A = Relative Abundance

Fig. 7

Stacked bar plot showing % relative abundance of SO₄²⁻ and S reducers as a portion of the total reads in each sample. The first numbers (0, 16, and 49) represent the number of weeks, while the numbers after the hyphen (0.0, 0.5, 1.0, and 1.5) represent coagulant doses (0, 500, 1000, and 1500 ppm)

Fig. 8

Stacked bar plot representing % relative abundance of methanogens as a portion of the total reads in each sample. The first numbers (0, 16, and 49) represent the number of weeks, while the numbers after the hyphen (0.0, 0.5, 1.0, and 1.5) represent coagulant doses (0, 500, 1000, and 1500 ppm)

Fig. 9

Plots of relationships between dissolved ΣH₂S and Fe in the alum, ferric, and gypsum batches