The process used for extraction of bitumen in Canada’s oil sands generates large volumes of water as a by-product, referred to as oil sands process-affected water (OSPW), which is stored on site in tailings ponds for water recycling. Although the oil sands mining companies are obligated to return this water to the environment eventually, they are currently operating on a zero-discharge policy, due in part to the water’s toxicity, attributed primarily to bitumen-derived organics. Fractions of these organics are highly persistent, and remain potently toxic even after decades of aging, thus requiring the treatment of OSPW to enable safe discharge. The industry has subsequently begun exploring remediation solutions, with a focus on passive technologies, as these do not require energy and would demand fewer resources.
Initial investigations in the field identified naphthenic acids (NAs), a class of organics naturally present in petroleum, as comprising the organic contamination in OSPW, though innovative analytical techniques have revealed far more complexity. In fact, OSPW contains thousands of different species of organic compounds, a subset of which are conventional NAs. While classic NAs are known toxicants, recent studies have shown other classes contribute significantly to the toxicity of OSPW, namely base-neutral organics, and may be implicated in its hormone-disrupting effects. Since the organic species found in OSPW can vary widely based on location and age of the tailings pond, some researchers have proposed that it may be possible to predict the toxicity of a specific OSPW by identifying all of the species of organic contaminants found in that water.
It has consequently become evident that water treatment technologies should be evaluated based on their ability to remove the specific classes of organic contaminants responsible for the water toxicity. In terms of technologies studied for OSPW treatment, solar photocatalysis (PC) has demonstrated to be particularly effective at eliminating classical NAs, and may be the only advanced oxidation process suitable for passive treatment systems, given the vast sunlight-exposed surface area of tailings ponds. However, studies have yet to assess the capacity of PC to treat base-neutral organics.
The objective of this work was therefore to evaluate the performance of photocatalysts employed as a passive treatment in the form of buoyant particles at degrading the organic contaminants in OSPW, while monitoring the degradation of all classes of organic compounds, paying special attention to the toxic and persistent species.
Methodology
Our current work began with the goal of improving the efficiency of the solar photocatalytic process toward practical water treatment applications. Most studies focus on the use of nanoparticle photocatalysts, as these are commercially available, though they are impractical for passive deployment, as they require vigorous mixing, are difficult to recover, and may be accidentally released to the environment. Since sunlight at the water surface drives solar PC, we hypothesized that keeping all photocatalysts near the water surface where they are most active by attaching them to a buoyant base would enable us to eliminate mixing requirements, material to a stable buoyant inorganic material using a porous adhesive that did not hinder the performance of the catalyst.
We also aimed to gain an unprecedented perspective of the PC treatment of OSPW by following thousands of species simultaneously throughout the process, allowing us to capture a more holistic picture of the chemical changes occurring during treatment. As not all OSPW organics are equally toxic, tracking the elimination of key classes in a complex mixture, rather than simply measuring reduction of bulk organics, may represent a new standard for evaluating treatment solutions for petroleum-impacted waters.
Outcomes
We investigated the performance of the solar photocatalyst material in OSPW by varying operating parameters and monitoring their behaviour. In addition, we performed tests using the traditional free TiO2 nanoparticle material for comparison, and observed that our engineered material performed equally well in environmentally relevant conditions. Most notably, the buoyant photocatalysts maintained their performance over ten treatment cycles, outlining their robustness and potential for large-scale application.
The organic contaminants were analyzed at several points throughout the treatment process with seven different analytical techniques, three of which enabled the differentiation of all classes of compounds being degraded. As expected, all methods showed a decrease of contaminant concentrations with treatment time, which was consistent with the destruction of the organics. However, prior to complete degradation, we observed the transformations of each of the organic families and noted with passive biodegradation may be another promising strategy to increase overall treatment efficiency.
Conclusions
With recent insights into the relationship between contaminant chemical structure and OSPW toxicity, it is clear that specific classes of organics, comprising only a minority of the total organic content, constitute the majority of the toxicity associated with OSPW. We implemented a thorough multi-step analysis of the organic mixtures that enabled the tracking of individual contaminant classes in the context of PC treatment of the organic content as a whole. Revelation that the toxic base-neutral organics were eliminated in the earliest stages of the PC reaction, and that the more complex NAs were preferentially degraded, may indicate that only relatively low solar doses would be required to detoxify OSPW, thereby improving the treatment efficiency as compared to the solar exposures required for complete organics degradation. This study also demonstrates application of a buoyant photocatalyst formulation as a passive treatment concept for oil sands remediation challenges. Further studies to develop this paradigm of a passive system are ongoing in our lab.
Leshuk, T., Peru K.M., de Oliveira Livera, D., Tripp, A., Bardo, P., Headley, J.V., Gu, F. (2018). Petroleomic analysis of the treatment of naphthenic organics in oil sands process-affected water with buoyant photocatalysts. Water Research, 141, 297-306.
Frank Gu, Department of Chemical Engineering
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