Coupling Geothermal Heating with BTEX Bioremediation in the Subsurface

Abstract

There has been a worldwide interest in renewable energy technologies as a means of reducing reliance on fossil fuels, mitigating the effects of climate change, and reducing greenhouse gas emissions. One such technology is geothermal heating, where the constant subsurface temperature is used to cool or heat building interiors via heat pumps. In Canada, the use of geothermal heat pumps (GHPs) has become a popular option for heating and cooling buildings. It is anticipated that, in the near term, most large buildings will incorporate GHPs as part of their climate control strategy. However, little is known about the environmental impacts of geothermal heating on the subsurface environment. The present thesis examined the effect of geothermal heating on groundwater flow and remediation efforts, whereby the heat generated by geothermal systems may aid in addressing urban pollution. "Geothermal remediation" could leverage the subsurface heating resulting from geothermal systems to accelerate biodegradation of certain petroleum-based pollutants at brownfield sites, while providing building(s) with sustainable heating and cooling. This idea coincides with the rising momentum towards sustainable and green remediation in Europe and the United States. To ensure that Geothermal remediation is achievable, the effect of heat on bioremediation needs to be examined. This research investigated the heat effects on the bioremediation potential of pure culture and consortia and their potential for (Benzene, Toluene, Ethylbenzene and Xylene(s)) BTEX degradation as a pollutant. In the present thesis, soil microorganisms with the potential to degrade BTEX were isolated using an enrichment method from soil samples collected at different depths from geothermal boreholes. The microbes were screened and optimized for BTEX degradation at three different temperatures (15, 28 and 40 °C). The bacterial strains Microbacterium esteraromaticum and Bacillus infantis exhibited the highest degradation compared to other isolated strains and the reference strain, Pseudomonas putida. All four BTEX compounds were metabolized 2 times faster at 28 °C and 40 °C. Metabolomics data showed that BTEX was metabolized entirely to acetaldehyde and carbon dioxide by these selected strains. The catechol 1,2-dioxygenases, catechol 2,3-dioxygenases, and toluene monooxygenase enzyme activity confirmed the tol and tod degradation pathways. Furthermore, the present work offers new insights into the responses of soil microbial communities to electron acceptors under anoxic conditions, indicating that intrinsic microorganisms can be successfully stimulated for in-situ bioremediation (ISB) with electron acceptors as a supplement. The investigation revealed a maximum BTEX biodegradation of 57% by B. infantis under sulfate reduction and overall, 98% by M. esteraromaticum in combined nitrate and sulfate reduction. To understand the soil matrix influence and to mimic geothermal heating effects, small-scale soil batch experiments and continuous soil column experiments with cyclic temperature were performed. The results revealed that cyclic temperature of 5 °C to 40 °C (shallow low enthalpy geothermal temperature range) enhanced the BTEX biodegradation by 2-fold in silty loam soil (> 80%) in comparison to constant aquifer temperature (12 °C) (40%). Finally, a metagenomics study was performed on soil samples from different depths at three temperatures (15, 28, and 40 °C) to provide insight into how geothermal heat could impact the soil microbiome and its effect on bioremediation activities. Potential known strains for BTEX biodegradation such as Pseudomonas, Arthrobacter, Bacillus, as well as some novel strains such as Microbacterium, Janthinobacterium, Methylotenera, were found to be dominant at 28 °C and 40 °C. Since microbial abundance and diversity decreased drastically at 15 °C; these findings showed the potential of geothermal heating as a sustainable heat source for ISB of pollutants.