Fate of dissolved organics in oil sands process water during long-term storage: Mechanistic insights into toxicity removal and microbial processes.

Abstract

Refining oil sand process water (OSPW), a byproduct of bitumen extraction from oil sands in Canada, presents significant environmental challenges due to its complex makeup. This complexity is mainly due to the presence of naphthenic acids (NAs), which play a substantial role in contributing to the toxicity of OSPW. Although various treatment technologies have been explored, the long-term behaviour of OSPW dissolved organics under different storage conditions has not been studied extensively. This study is the first to delve deeply into the natural attenuation of OSPW under diverse controlled conditions, focusing on the effects of temperature, dissolved oxygen, and ozone pretreatment on water quality, NAs degradation, toxicity, and bioavailability. Our results revealed the critical role of temperature in OSPW characteristics, with long-term storage at 4 °C demonstrating minimal degradation of dissolved organics, providing the first empirical support for current OSPW storage practices. In contrast, at 20 °C, ozoned OSPW exhibited maximum reduction in the following parameters: total NAs, 72.6 %; chemical oxygen demand, 25.3 %; acute toxicity towards A. fischeri by 60.7 %; and bioavailability of organics by 35.2 %. This suggests that ozone pretreatment facilitates the biodegradation process by breaking down NAs into more readily metabolized compounds, which are further degraded by microbial activity over time. Furthermore, the study identified evolving microbial communities during OSPW storage, highlighting the presence of Bacillus and Fontimonas genera, which may play a role in organics degradation but require further investigation into their specific functions. These findings provide critical insights into the long-term dynamics of organics in OSPW and provide a foundation for optimizing management strategies.