Photocatalytic removal of steroid hormone micropollutants under simulated sunlight using TiO$_2$-coated electrospun nanofibers on poly(vinylidene fluoride) membrane

Solar-driven membrane photocatalysis holds promise for micropollutant removal. TiO$_2$ is an environmentally benign and abundant semiconductor, but its limited light-harvesting capacity in the visible light region poses a challenge to its application as an effective photocatalyst. Herein, the degradation of steroid hormone (SH) micropollutants was investigated using TiO$_2$ nanoparticles immobilized on a poly(vinylidene fluoride) microfiltration membrane (TiO$_2$-PVDF) under simulated solar light (350–1150 nm) at environmentally relevant micropollutant concentrations (100 ng/L estradiol, E2). Key rate-limiting factors influencing SH removal were identified under various operational conditions. The results demonstrated that photon availability was the primary limiting factor for E2 degradation at light intensities below 40 mW/cm$^2$. As the light intensity increased, the catalyst surface area (or catalyst loading) accessible to the light became the main constraint. The TiO$_2$ surface area was thus increased by spinning PVDF nanofibers of varying thickness onto the membrane substrate to enhance the surface area available for TiO$_2$ loading. ... mehrA thicker nanofiber layer provided more surface area for TiO$_2$ attachment (from 0.11 ± 0.01 to 0.33 ± 0.02 mg/cm$^2$ as the thickness increased from 70 to 149 µm). However, this improvement came at the expense of light transmission, which decreased substantially from 55 to 19 %. Increasing the nanofiber mat thickness from 70 to 149 µm did not significantly enhance the mass removed (from 18 ± 3 to 22 ± 2 ng/cm$^2$), attributed to the reduced light transmission counterbalancing the benefits of increased TiO$_2$ loading. Amplifying the light irradiance achieved limited enhancement in E2 removal, due to the reduced collision frequency between reactants within the enlarged pores of the nanofiber mat. These findings highlight the importance of striking innovative photocatalytic membrane design that achieves a balance between maximizing catalyst loading and preserving sufficient light penetration, while avoiding excessive pore enlargement. Such an approach is essential for improving the efficiency of micropollutant treatment in membrane photocatalysis.