Sulfate reducing bacterial community and in situ activity in mature fine tailings analyzed by real time qPCR and microsensor

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Abstract

Sulfate reducing bacteria (SRB) play significant roles in anaerobic environments in oil sands mature fine tailings (MFTs). Hydrogen sulfide (H2S) is produced during the biological sulfate reduction process. The production of toxic H2S is one of the concerns because it may hinder the landscape remediation efficiency of oil sands tailing ponds. In present study, the in situ activity and the community structure of SRB in MFT and gypsum amended MFT in two settling columns were investigated. Combined techniques of H2S microsensor and dissimilatory sulfite reductase β-subunit (dsrB) genes-based real time quantitative polymerase chain reaction (qPCR) were applied to detect the in situ H2S and the abundance of SRB. A higher diversity of SRB and more H2S were observed in gypsum amended MFT than that in MFT, indicating a higher sulfate reduction activity in gypsum amended MFT; in addition, the activity of SRB varied as depth in both MFT and gypsum amended MFT: the deeper the more H2S produced. Long-term plans for tailings management can be assessed more wisely with the information provided in this study.

Introduction

Oil sands tailing ponds in Northern Alberta that receive and store wastes from the bitumen extraction process contain large amounts of recalcitrant and toxic organics. The current total volume of the fine tailings has exceeded 700 m3, and this volume keeps increasing as mining operations proceeded because each cubic meter of mined oil sands uses 3 m3 of water. One of the problems of oil sands tailing ponds is the very slow settlement of fine clay particles. The fine clay particles that stay in suspension in tailing ponds are called mature fine tailings (MFTs). It has been estimated that the densification of the fine clay would need over a hundred years (Eckert et al., 1996). Technologies being applied to solve the tailings densification problem include physical mechanical processes, chemical amendments, natural processes such as freeze–thaw, and in situ biological treatments (Powter et al., 2010).

Over past years, there have been big changes subject to the presence of microorganisms and their activities in tailing ponds. It has been indicated that the tailings are not just sitting in tailing ponds but are microbiologically active. Management of MFT has lately turned toward biological treatment technology. Researchers are interested to find whether microorganisms endogenous in MFT can be utilized to aid tailings densification. Foght et al. (1985) detected both aerobic and anaerobic microbes present in Syncrude Canada's Mildred Lake Settling Basin (MLSB) (Foght et al., 1985, Li, 2010). However, the activities of the existing microorganisms were very slow and had not been seriously noticed until the early 1990s when gas bubbles were observed at the surface of Syncrude's MLSB tailing ponds. Holowenko et al. (2000) reported that 60%–80% of the observed gas flux to the atmosphere across the surface area of MLSB was methane. It has been estimated that a daily flux of 12 g CH4/m2 was produced in a single pond, evidence of a very active methanogenesis in the pond. Several studies investigated the relationship between methanogenesis and MFT densification. Fedorak et al. (2003) demonstrated that the MFT densification rate can be significantly accelerated due to the produced CH4 gas by microbial activity of methane-producing archae, known as methanogens. Guo (2009) studied the influence of microbial activity on rapid MFT densification. It was found that water drainage volumes from MFT was improved due to the increase of microbial activity and the accordingly gas generation.

In some experiments, coarse sand tailings were mixed with fluid fine tailings and gypsum (CaSO4·2H2O) to yield composite tailings. Gypsum is used by oil sands companied as coagulants to accelerate particle precipitation and to enhance dewatering process of the mixture (List and Lord, 1997). As a result, the continuous accumulation of SO42  would facilitate the growth of sulfate reducing bacteria (SRB) and sulfate reduction process in MFT. During the biological sulfate reduction process, H2S could be generated. The produced toxic H2S itself is one of the concerns (Barton and Fauque, 2009); in addition, the H2S promotes corrosion of facilities (Pol et al., 1998), and the produced H2S might transport toxic organics such as NAs from MFT to cap water. All of these concerns will hinder the efficiency of landscape remediation of oil sands tailing ponds.

Up to now, most studies have focused on the sulfate inhibition of methanogenesis (Holowenko et al., 2000, Salloum et al., 2002, Ramos-Padron et al., 2011). To our best knowledge, there is little information regarding the effect of gypsum on the community characteristics and the in situ activity of SRB in MFT. And the in situ H2S data is missing in oil sands tailing ponds. Microsensors allow for the measurements of chemical variables with high spatial resolution in microbial environment (Revsbech and Jorgensen, 1986), while molecular techniques provide a way to investigate the presence and functional diversity of specific populations without isolation (Sanz and Kochling, 2007). It was the first try in this study to apply combined technologies in MFT microbial environment in order to investigate the effects of gypsum on the in situ microbial activity and functional diversity of SRB.

Section snippets

Experimental setup

Two columns (column A and column B) were constructed for the settling and the development of microbial stratified MFT. A schematic drawing of the column is shown in Fig. 1.

The columns were made from acrylic material. The cylinder column was 10 cm in diameter and 25 cm in height, had a total volume of around 2 L, and contained nine sampling ports on its wall. The sampling ports were constructed with luer locks connected with stainless steel tubing extended into the center of column. At sampling

MFT settlement

At the start point of the settling, the interface of the overlying water and sediment for the two columns were at the same level. After eight months' settling, it can be seen from Fig. 2 that the interface of the overlying water and sediment for the gypsum amended MFT column was lower than that of MFT column. Moreover, much more gas-generated void space marked with yellow circles shown in Fig. 3 can be observed in the gypsum amended MFT column than in the MFT column.

Although the mechanisms of

Conclusions

The overall agreement in the vertical distribution and microbial in situ activity of SRB supported the validity of the combined microsensor measurement and molecular biology approaches in complex MFT microbial environments. Results in this study demonstrated that the amendment of gypsum in mature fine tailings stimulated the activity of SRB; more H2S was produced in gypsum amended MFT than that in MFT. In addition, it was observed that the deeper the more active of SRB and the more H2S

Acknowledgments

The authors acknowledge the financial supports from Natural Sciences and Engineering Research Council (NSERC) of Canada, Canadian School of Energy and the Environment (CSEE), and China Scholarship Council (CSC).

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