(56a) Photocatalytic ZnO Foams for Organic Micropollutant Degradation

The presence of micropollutants - pharmaceuticals, pesticides, phthalates and hormones - in water is a major global health and environmental challenge. Conventional wastewater processes are not capable of effectively removing these compounds, often found at very low concentrations, µg L^-1 or even ng L^-1, leading to their discharge in surface and ground water. Advanced oxidation processes (AOPs) while effective at removing micropollutants, have not yet been deployed at industrial scale due to cost and safety concerns associated with the potential leaching of nanoparticle slurries on one hand, and the low activity of immobilised photocatalyst configurations on the other.

Here we report on novel photocatalytic foams, whose activity is given by the material itself rather than grafted nanomaterials, as previously done [1]. ZnO photocatalytic foams were prepared using a two-step procedure of liquid templating and sintering: Air was first incorporated into a dispersion of ZnO particles, surfactant and polymer. Upon crosslinking reaction and drying, a highly porous self-supporting foam, composed of pure oxide particles sintered together into an interconnected monolith, was produced. Morphological and structural characterisation was performed using FE-SEM, Raman, XRD and microcomputed tomography. Degradation experiments were performed using carbamazepine (10 µmol L^-1) as a model pollutant in both recirculating and single-pass reactors under UV irradiation with flow rates ranging from 100 to 500 mL min^-1 and 0.7 to 5.0 mL min^-1, respectively, under oxygen saturation and adsorption-desorption equilibrium time of 40 min.

Under recirculation conditions at 500 mL min^-1, 76% degradation of carbamazepine was obtained after 2 h compared to 4 h for a batch reactor, with a quantum yield of 0.60 and electrical energy per order below 1.0 kWh m^-3. At lower flow rates, the effect of mass transfer resistances was significant, with a reduced degradation performance. The foams showed high chemical and mechanical stability, with minimal Zn leaching and reuse for 5 cycles and 10 hours of operation in total. Increasing the length of the foams from 2.1 to 6.3 cm led to a further increase in degradation of carbamazepine to 95% under the same conditions.

Under single -pass conditions, full degradation of carbamazepine was obtained in under 30 minutes for the lowest flowrate due to a combination of both photolysis and photocatalysis, with an enhanced quantum yield of 0.9.

These findings show that photocatalytic foams have comparable photocatalytic activity to and higher quantum yield and energy efficiency of nanoparticle slurries without the risks associated with nanoparticle leaching into the environment, which necessitates costly downstream removal. These results open the way to the safe industrial use of photocatalysis for the removal of micropollutants from water.

References:

T. Tasso Guaraldo, J. Wenk, and D. Mattia, "Photocatalytic ZnO Foams for Micropollutant Degradation," Advanced Sustainable Systems, p. 2000208, 2021, doi: https://doi.org/10.1002/adsu.202000208.