Photodegradation of steroid hormone micropollutants with palladium-porphyrin coated porous PTFE of varied morphological and optical properties

Abstract

In flow-through reactors, the photodegradation rate can be enhanced by better contact and an increase in photocatalyst loading. Both can be attained with a higher surface-to-volume ratio. While previous work focused on thin membranes (30 – 130 µm) with small pore sizes of 20 – 650 nm, this work was performed with poly(tetrafluoroethylene) (PTFE) supports. The pore sizes are in the order of 10 µm, while porosity 22.5 − 45.3% and thickness 0.2 − 3 mm are variable. It was anticipated that these porous materials would enable enhanced loading of the porphyrin photosensitiser and better light penetration for subsequent photodegradation of steroid hormone micropollutants via singlet oxygen (¹O₂) generation. The reactor surface refers to the surface within the PTFE pores, while the reactor volume is the total void space inside these pores. The surface-to-volume ratio falls between 10⁵ and 10⁶ m²/m³, higher than those of typical microreactors (10³ to 10⁴ m²/m³). The weighted average light transmittance varied from 38% with the thinnest and most porous support to 4.8% with the thickest support. Good light penetration combined with minimal absorption by PTFE enhanced the light utilisation of the porphyrins when coated in the porous supports.Changes in the support porosity of the coated supports affected insignificantly steroid hormone removal, because the collision frequency in the very large pores remained relatively constant. However, varying the support thickness, porphyrin loading (0.3 − 7.7 μmol/g), and water flux (150 − 3000 L/m².h), and the resulting HRT influenced the collision frequency and steroid hormone removal. Results were not competitive with membranes, most likely owing to the larger pore size limiting contact between micropollutants and reactive oxygen species.From photostability testing of the pristine supports, perfluoroalkyl substances (PFAS) released from the supports were found at 10 − 300 ng/L concentrations during accelerated ageing. This indicates that some PFAS are released, while quantities during water treatment operations would be extremely low. In summary, this study elucidates the capability and limitations of porous supports coated with photosensitisers to remove waterborne micropollutants.