Noble-metal-free photosensitizers for continuous-flow photochemical oxidation of steroid hormone micropollutants under sunlight

Abstract Given the crucial role of noble metals in promoting singlet oxygen (1O2) production, the feasibility of relying on cheaper and more abundant metals remains challenging. This work investigates the continuous-flow photocatalytic degradation of steroid hormone micropollutants, achieved with photocatalytic membranes based on noble-metal (Pd) and noble-metal-free (Zn, Mn, free-base) porphyrins. Under solar illumination corresponding to one-sun equivalent for the wavelength range 350–1150 nm, Zn- or free-base porphyrin-coated membranes exhibit a similar decrease of 17β-estradiol concentration in the permeate from 100 ng L−1 down to 15–23 ng L−1 compared to the benchmark membranes coated with Pd-porphyrin. Due to the difference in 1O2 quantum yield generation upon immobilization, the noble-metal-free porphyrins demonstrate diminished removal of estrone or 17β-estradiol (up to 62% against 85%), when illuminated with intensity equivalent to one-sixth of terrestrial sunlight. Nevertheless, the Zn-complex photobleached 3–4 times slower than free-base and Pd-porphyrins during 125 h of accelerated aging. This photostability test revealed the half-life time for immobilized Zn-porphyrin equivalent to 209 days of outdoor exposure assuming the daily average illumination of 3 h. The findings highlight that Zn-porphyrin is slightly less effective than noble-metal analogue, but may outperform Pd-porphyrin in terms of photostability and price–performance ratio.