Carbon nitride – PVDF photocatalytic membranes for visible-light degradation of venlafaxine as emerging water micropollutant

Abstract

Metal-free carbon nitride-based membranes were prepared by embedding bulk or exfoliated graphitic carbon nitride (g-C3N4) photocatalysts in a polyvinylidene fluoride (PVDF) matrix. Supported membranes were also fabricated by immobilising these photocatalysts on a polytetrafluoroethylene (PTFE) substrate. The membranes were tested for venlafaxine (VFX) degradation at a feed concentration of 250 µg L−1 under continuous flow mode operation and using a visible-light emitting diode (LED) as energy source. The reduction of the (002) diffraction peak and the increased intensity of the infrared absorption bands confirmed that the exfoliation process decreased the number of aligned g-C3N4 layers, with a more ordered packing of tri-s-triazine units. Additionally, the small particle size (around 745 and 257 nm for bulk and exfoliated g-C3N4, respectively) and the presence of terminal amino groups in the catalysts allowed for homogeneous dispersion in the PVDF matrix. Despite their different surface roughness and cross-sectional microstructure, all the fabricated membranes exhibited similar photocatalytic filtration performance for VFX degradation, resulting in conversions above 95% after 5 h of irradiation (418 nm with an irradiance of 11.2 W m−2) and keeping the VFX concentration in the effluent negligible up to 48 h of continuous operation (mass removal rate of about 6 mg m−2 h−1). In a more complex water matrix (i.e., surface water), the photocatalytic filtration process led to 50% VFX removal (or 3 mg m−2 h−1) at the steady state. Moreover, the permeate flux remained at about 25 L m−2 h−1 throughout the continuous operation, suggesting that fouling did not occur at these conditions.