Identifying New Methods for Oil Sands Waste Treatment

One of the greatest challenges facing the extraction of bitumen from Alberta’s oil sands is dealing with the massive quantities of waste products generated in the extraction process. The waste generated, known as tailings sludge, is composed of silica and clay particles, water, ions, residual bitumen and trace organic species. Currently, typical waste treatment involves addition of a chemical additive followed by disposal into large tailing ponds. The chemical additive causes the fine silica and clay particles to aggregate. These aggregations then settle during deposition into the tailing ponds, and a dewatering step must occur before the solid waste can be used as a foundation for soil deposits and reforestation. This dewatering step is critical in the waste treatment process, and remains a challenge largely due to the enormous volume of existing tailings waste. New chemical agents or physical methods of dewatering are needed for optimal dewatering of tailing ponds. But first, we need to take a step back and ask: What are the molecular characteristics of the surfaces of these particles in contact with water? More specifically, can we find spectroscopic methods that can probe then dewatering process? If so, then we assess which additives are the most promising for dewatering and determine their mechanism of action.

In the Gibbs lab we use nonlinear optics (NLO) to study the structure and order of water at the silica surface. Using NLO methods we can determine the effects of different ionic species on water structure at the silica/aqueous interface, which could have direct implications in the dewatering of silica in tailings sludge. For more information, please visit Nonlinear Optics.

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Point-of-Care Diagnostics

 

One of the main research areas of the group is the development of point-of-care diagnostic tools focusing on nucleic acid amplification that provide robust results and versatile applications.

Specifically, we have developed isothermal self replication DNA strategies using destabilizing probes for the identification of specific targets.  Such approach is being developed for the identification of mRNA sequences as well as single nucleotide polymorphisms. In a similar manner, our group has also developed DNA templated chemical ligation strategies to achieve enzyme-free self replication.

Nonlinear Optics

We use second-order nonlinear optical (NLO) methods to probe the water structure at the silica/aqueous interface. NLO methods differ from conventional linear optics (e.g. reflection, refraction, one-photon fluorescence), in that they rely nonlinearly on the intensity of the input light. For second-order NLO methods, signal is only generated where there is a break in inversion symmetry. As a result, signal can only arise in certain ordered systems including certain types of crystals as well as surfaces and interfaces. We use this unique feature of second order NLO methods to our advantage: it allows us to selectively probe the silica/aqueous interface, with signal arising only from molecules right at the interface, and molecules from the bulk solution or amorphous silica generating no response.

Sum Frequency Generation

One specific NLO method we use is broadband vibrational sum frequency generation (SFG) spectroscopy. This techniques combines two separate light sources (a narrowband visible and a broadband IR laser), which interact with the sample to generate light at a frequency that is the sum of the frequency of the two incidence sources. In our silica/water experiments we tune the IR light source such that it is vibrationally resonant with the OH stretch of water, thereby selectively probing the response from water molecules at the the interface.
SFG_Water

Since only ordered systems can generate SFG signals, we can add different additives to the system and observe changes in the SFG spectrum of water to gain insight on disruption of the water organization at the silica/ aqueous interface. For example, simple addition of NaCl can disrupt ordering of interfacial water, which is manifested as a decrease in intensity in the SFG spectrum. In this case, not only is water being displaced at the interface by cations, but the cations also screen the negative charge of the silica, which in turns decreases the net order of water beyond the first cation layer at the surface. In our lab, we are currently examining the effects of different additives at varying concentrations and pH on SFG spectral shapes and intensities. These changes in water structure at the silica surface could have direct implications for dewatering of silica particles in tailing ponds. For more information on the application of SFG spectroscopy in developing methods for oil sands waste treatment, please see Identifying New Methods for Oil Sands Waste Treatment.

Second Harmonic Generation

In addition to SFG, our lab also utilizes another NLO method called second harmonic generation (SHG). SHG is a specific form of SFG, where two photons of identical frequency interact at the samplet to generate light at double the frequency. For this reason, SHG is often referred to as “frequency doubling.”

NLO_EnergyDiagrams.jpg

SHG is a simpler measurement compared to SFG, as it requires only one incident light source.  In addition, SHG intensity from the sample can be related to the degree of water order at the silica/water interface. However, for practical reasons visible or NIR lasers are used for SHG and therefore SHG is not able to selectively probe the vibrational modes of water. As a result, contributions from silica may interfere with the water response, confusing interpretation of the results. Nevertheless, interferences from silica are thought to be small, and SHG can be used to gain insight on overall changes in water ordering.

Electrokinetic analysis

We use electrokinetic analysis to complement NLO methods and further our understanding of the electrical double layer (EDL) of silica/water. In the EDL, a negatively charged surface attracts hydrated counterions. These ions accumulate near the surface at the outer Helmholtz plane (OHP), and screen the negative surface charge.

EDL
The electircal double layer (EDL) for negatively charged silica/water interface in the presence of monovalent cations.

Water molecules between the surface and the OHP make up the Stern layer. While there is still a net negative charge beyond the OHP, albeit reduced by the presence of counterions, this generates a potential which can align the dipoles of water in what is referred to as the diffuse layer. This potential decreases exponentially with distance away from the surface, as ions in the diffuse layer screen more and more of the negatively charged surface. Using streaming potential measurements, we can measure the zeta potential (~ the potential at the outer Helmholtz plane) of the silica/water interface in the presence of different ionic species. Measuring the zeta potential should offer insight on relative cation adsorption. We are currently investigating trends in measured zeta potentials and their relation to trends observed in out SFG spectroscopy measurements.