Development of Near-Infrared Mediated Photocatalytic Nanocomposites for the Treatment of Turbid Wastewaters – UpPhotoCat

Polycyclic aromatic hydrocarbons (PAHs) consisting of several fused benzene rings are found as products of the incomplete combustion of hydrocarbons (e.g. forest fire) as well as in flowback and produced water from Coal bed methane (CBM) and shale gas extraction process. These two processes are widely used in Canada and subject to a moratorium in France. For both countries, concerns have risen about PAH-contaminated wastewaters as several studies have proven their toxicity and/or carcinogenicity to humans and wildlife. This raises the question of the remediation of wastewater polluted by these molecules, especially if one takes into account the expected increase in shale gas extraction by hydraulic fracturing, which implies an equal increase in the volume of contaminated water to be remediated.

Over time, the impact of these contaminants in the aquatic ecosystem will undeniably become a major public health problem and the urgency of finding a solution to this problem is real. Unfortunately, these pollutants are refractory to bacterial metabolization, and conventional photocatalysis, based on UV radiation, is fairly inefficient because of the low penetration depth of the UV light in turbid wastewater highly loaded with organic molecules. Other technologies, more effective for the degradation of these pollutants and more profitable should be rapidly developed. Among these technologies, the activation of photocatalytic processes in the near infrared (NIR) using upconversion particles (particles that transform NIR radiation into UV radiation) will play a major role.

The proposed project is based on a collaborative work between a Canadian team (INRS-EMT - F. Vetrone) and two French teams: LRS, UPMC-SU (J. Blanchard, G. Costentin, and S. Boujday) and LCP, Paris Sud Université (C. Colbeau-Justin). It also involves two partners without requested funding: an industrial Canadian partner, Advanced CERT Inc. and an academic Singaporean partner, MSE-NTU, Terry Steele. It aims at the synthesis and application for wastewater remediation of new and innovative photocatalytic nanocomposites combining lanthanide-based upconverting nanoparticles (UCNPs) with a ZnO photocatalyst.

Upconverting nanoparticles (UCNP) are a nascent technology that employs rare earth fluorides doped by lanthanides that act as an ‘antenna’ for the NIR light, and, subsequently, convert it to UV light. Herein, the UV emission upon NIR excitation of the UCNPs will trigger the photocatalytic activity of the ZnO resulting in pollutant oxidation. This novel and innovative approach will be superior to current methodologies as it will take advantage of the high penetration depth of NIR light in turbid media to overcome the limited penetration of UV by NIR-mediated in-situ UV generation. The photocatalytic activity will be insured by a shell of ZnO, a widely used photocatalytic metal oxide, deposited on the UCNP. Our design incorporates UCNP/ZnO core shell and ‘flower-like’ composite nanoparticles. The upconversion efficiency of the UCNP chore critically depends on its nanoscale morphology, which must be tailored during the elaboration process, whereas the photocatalytic activity will depend both on the upconversion efficiency and the UV-based photocatalytic activity of the ZnO surface layer. The understanding and the optimization of the efficiency of the new composite material in photocatalytic process requires therefore deep understanding of the electron transfer and electron transport mechanisms in correlation with the elaboration process.

We expect that this approach, based on a comprehensive study of the mechanisms at each step of the elaboration, followed by testing of the photocatalytic activity on model molecules and later on real wastewater sample, will lead to a more efficient and cost-effective system for wastewater treatment and will create significant impact in the water industry.