Immobilization of Bi2WO6 on Polymer Membranes for Photocatalytic Removal of Micropollutants from Water – A Stable and Visible Light Active Alternative

Abstract In this work, bismuth tungstate Bi2WO6 is immobilized on polymer membranes to photocatalytically remove micropollutants from water as an alternative to titanium dioxide TiO2. A synthesis method for Bi2WO6 preparation and its immobilization on a polymer membrane is developed. Bi2WO6 is characterized using X‐ray diffraction and UV–vis reflectance spectroscopy, while the membrane undergoes analysis through scanning electron microscopy, X‐ray photoelectron spectroscopy, and degradation experiments. The density of states calculations for TiO2 and Bi2WO6, along with PVDF reactions with potential reactive species, are investigated by density functional theory. The generation of hydroxyl radicals OH• is investigated via the reaction of coumarin to umbelliferone via fluorescence probe detection and electron paramagnetic resonance. Increasing reactant concentration enhances Bi2WO6 crystallinity. Under UV light at pH 7 and 11, the Bi2WO6 membrane completely degrades propranolol in 3 and 1 h, respectively, remaining stable and reusable for over 10 cycles (30 h). Active under visible light with a bandgap of 2.91 eV, the Bi2WO6 membrane demonstrates superior stability compared to a TiO2 membrane during a 7‐day exposure to UV light as Bi2WO6 does not generate OH• radicals. The Bi2WO6 membrane is an alternative for water pollutant degradation due to its visible light activity and long‐term stability.

Table S1.Fluorescence intensity after 2 min of irradiation and the responding concentration of umbelliferone using calibration of Figure S10.6.1% of OH • radicals react to umbelliferone when utilizing a 0.1 mM coumarin solution [1] .

EPR measurement
Only the irradiated DMPO solution with TiO 2 membrane shows paramagnetic species.The other solutions are not paramagnetic and thus do not indicate the formation of OH • radicals.
Figure S12 shows the EPR spectrum of the DMPO irradiated with the TiO 2 PVDF membrane together with its DMPO-OH adduct simulation.The isotropic EPR parameters are g 0 =2.0057, a 0 ( 14 N)=1.5 mT, and a 0 ( 1 H)=1.5 mT.These values are in good agreement with the literature for the DMPO-OH adduct [2] .In addition, further hyper fine structure signals are observed.
The assignment of these signals to a DMPO radical adduct could not be performed.The adduct DMPO-OOH can be excluded because the half-life of DMPO-OOH is approx.80 seconds [3] , and the irradiation experiment was not carried out directly in the EPR resonator.
Even if not all signals could be assigned, the results of the EPR measurements prove that OH • radicals are generated during irradiation of the TiO 2 membrane and not with the Bi 2 WO 6 membrane.

Figure S1 .
Figure S1.Elemental ratio of Bi evaluated via XPS for Bi 2 WO 6 applied on the PES membrane and synthesized at different concentrations of reactant salts (here stated as concentration of Na 2 WO 4 in g L -1 ).The reactant salts Na 2 WO 4 *H 2 O and Bi(NO 3 ) 3 *5H 2 O were used at a ratio of 1:2.9, respectively.

Figure S6 .
Figure S6.Degradation of propranolol (Pro) over time at 3 different pH values with the Bi 2 WO 6 PVDF membrane (photocatalysis) under irradiation with UVA-lamp.Bi 2 WO 6 was synthesized for 24 h and with the concentration of the reactant salt Na 2 WO 4 *H2O adjusted to 14.3 g L -1 and immobilized with method 3 on the PVDF membrane.

Figure S8 .
Figure S8.Calculated density of states of TiO 2 and Bi 2 WO 6 , and the corresponding Fermi levels (dashed lines) at 8.2 eV and 10.1 eV, respectively.

Figure S9 .
Figure S9.Projected density of states of TiO 2 (a) and Bi 2 WO 6 (b), showing the elemental contributions to the total density of states.

Figure
Figure S11.X band EPR spectra of aqueous reaction solution of DMPO irradiated (UVA) 15 min in advance for 60 min.Two reference samples were generated.Aqueous solutions of DMPO were irradiated -only water (H 2 O) and with a PVDF membrane (Membrane).Membranes with immobilized photocatalysts (Membrane + Bi 2 WO 6 and Membrane + TiO 2 ) in aqueous DMPO solutions were irradiated.

Figure
Figure S12.X band EPR spectrum of the aqueous reaction solution of the TiO 2 PVDF membrane with DMPO, and the simulation of an DMPO-OH adduct (simulated DMPO-OH).Additional signals of an unknown species are marked with asterisks.