Multifunctional Zwitterionic-WS₂ Membranes for Enhanced Dye Removal and Biofouling Resistance in Forward Osmosis

It is becoming increasingly crucial to develop innovative materials that exhibit qualities absent in conventional water treatment processes, which are inefficient at removing emergent contaminants. This article details the development of a bioinspired membrane enhanced with tungsten disulfide (WS₂) nanosheets for operation in forward osmosis (FO) mode, thereby integrating the benefits of polymeric and semiconducting materials in the removal of model pollutants and avoiding the formation of biofouling. This membrane was synthesized by modifying the polysulfone (PSF) polymer backbone with the zwitterionic group diethylamine N-oxide (DEAO) to increase the hydrophilic properties of the final material. Subsequently, WS₂ nanosheets were introduced into the polymeric solution to integrate their photosensitive characteristics into the resulting membrane. The synthesized membranes were meticulously analyzed by utilizing scanning electron microscopy (SEM), surface ζ-potential measurements, and water contact angle (CA) assessments. Furthermore, the efficacy of the fabricated membranes was evaluated based on their water permeance and ability to reject model dyes, including rhodamine B (RhB) and sulforhodamine (SR101). The results indicate that the photodegradation of RhB is approximately 50% after 5 h of light exposure, while the adsorption of SR101 exceeds 80% after the same 5 h without light. The dye rejection performances of these membranes are nearly 30 and 75% higher than those of the control membranes, respectively. Furthermore, the PSF-DEAO@WS₂ membrane can degrade over 60% and reject nearly 99% of RhB in forward osmosis tests under light conditions, with a significantly outstanding water flux. In addition, the membranes with embedded WS₂ nanosheets were able to inhibit the proliferation of Escherichia coli on their surface. The WS₂ nanosheets are probable to generate reactive oxygen species (ROS), which expedite the degradation process in the presence of light and prevent the formation of biofouling. Density functional theory (DFT) computations of electronic-band edges as a function of the number of WS₂ monolayers show that water oxidation/reduction energies are within the band gap for one monolayer. In the interim, the dye is removed from the water by mostly strong van der Waals interaction of the SR101 with the membrane's surface, as shown by DFT computations of adsorption energies. The overall approach of this work may pave the way for the fabrication of membranes with multiple functionalities.