Enhanced photocatalytic efficiency via improved contact in a solar-driven membrane reactor for steroid hormone removal

The application of solar-driven photocatalytic membranes (PM) for steroid hormone elimination from surface water is encouraging. However, both the limited availability and utilization of oxidizing species (singlet oxygen) inside the PM and the limited diffusion of reactants may constrain the destruction of organic pollutants. To facilitate the contact between the liquid phase and PM surface, poly(vinylidene-fluoride) membranes with small pore size are used to confine and evenly distribute organic photocatalyst (porphyrin) molecules. The porphyrin-coated membranes with nanopores (20 nm, NM20) provide a greater surface area and four times faster degradation of 17β-estradiol than PMs based on microfiltration membranes (pore size in range 100 – 650 nm). Stacking multiple NM20 membranes on top of each other extends contact time and results in a concentration of 17β-estradiol in the permeate of 2 ng L–1, but the gain in 17β-estradiol removal is limited by light propagation. Alternatively, adjusting the flux (below 150 L m–2 h−1) of the membrane with pore size of 200 nm is effective in reducing the concentration of 17β-estradiol in the permeate from 100 ng L–1 to 1 ng L–1. These results ensure the compliance with the European Commission guidance value for 17β-estradiol. The degradation of more chemically stable testosterone (compared to 17β-estradiol and estrone) using NM20 membranes is improved relative to PM with larger pores (200 nm). This study confirms that improved contact, achieved by carefully choosing the membrane support, accelerates the photocatalytic degradation of micropollutants.