Engineered Nanoparticles with Decoupled Photocatalysis and Wettability for Membrane-Based Desalination and Separation of Oil-Saline Water Mixtures

Abstract

Membrane-based separation technologies are the cornerstone of remediating unconventional water sources, including brackish and industrial or municipal wastewater, as they are relatively energy-efficient and versatile. However, membrane fouling by dissolved and suspended substances in the feed stream remains a primary challenge that currently prevents these membranes from being used in real practices. Thus, we directly address this challenge by applying a superhydrophilic and oleophobic coating to a commercial membrane surface which can be utilized to separate and desalinate an oil and saline water mixture, in addition to photocatalytically degrading the organic substances. We fabricated the photocatalytic membrane by coating a commercial membrane with an ultraviolet (UV) light-curable adhesive. Then, we sprayed it with a mixture of photocatalytic nitrogen-doped titania (N-TiO2) and perfluoro silane-grafted silica (F-SiO2) nanoparticles. The membrane was placed under a UV light, which resulted in a chemically heterogeneous surface with intercalating high and low surface energy regions (i.e., N-TiO2 and F-SiO2, respectively) that were securely bound to the commercial membrane surface. We demonstrated that the coated membrane could be utilized for continuous separation and desalination of an oil–saline water mixture and for simultaneous photocatalytic degradation of the organic substances adsorbed on the membrane surface upon visible light irradiation.

Predicting kinetics of water-rich permeate flux through photocatalytic mesh under visible light illumination

“…A mixture of nitrogen-doped TiO 2 (N-TiO 2 ) and perfluorosilane-grafted SiO 2 (F-SiO 2 ) nanoparticles was utilized to fabricate a visible light-active photocatalytic mesh (see “ Methods ”). Please note that the synthesis of a mixture of N-TiO 2 and F-SiO 2 nanoparticles (i.e., N-TiO 2 /F-SiO 2 ) was reported in previous work 6 which demonstrated selective wettability for water over oil (i.e., hydrophilic and oleophobic wettability). Briefly, a dispersion of N-TiO 2 /F-SiO 2 in deionized (DI) water (solute concentration = 10 wt%) was sprayed onto a stainless steel (SS) 316 Twill Dutch weave mesh (SS mesh) for one minute.…”

“…In comparison, membranes possessing lower solid surface energy ( γ sv ) can repel the organic substances without needing for a water film 30 – 32 . We 6 and others 26 – 28 have reported fouling-resistant membranes that can separate oil–water mixtures with an insignificant decline in the permeate flux by combining hydrophilic (or superhydrophilic) wettability along with lower solid surface energy.…”

“…Membranes with selective wettability have been incorporated with photocatalytic materials (e.g., TiO 2 6 , 33 , N-TiO 2 34 , α-Fe 2 O 3 16 , 35 , Fe 3 O 4 36 , WO 3 37 , ZnO 38 , BiVO 4 7 , α-FeOOH 39 , MoO 3 40 , Co 3 O 4 41 , Gd 2 ZnMnO 6 /ZnO 42 ) that can degrade the organic substances deposited on the surface upon light illumination. These membranes have demonstrated that they can oxidize (or reduce) the organic substances either dissolved in a liquid (e.g., water) or adsorbed on the membrane surface when irradiated by light with an energy higher than their bandgap energy 43 , 44 .…”

“…Liu et al 47 fabricated a PVDF-Ni-ZnO composite membrane and demonstrated in situ photocatalysis-driven recovery of the water-rich permeate flux during the filtration of an aqueous solution dissolved with organic substances (e.g., humic acid, sodium alginate, bovine serum albumin). Recently, we 3 , 6 developed photocatalytic membranes by coating a commercial filter with a mixture of visible light-active iron-doped TiO 2 or nitrogen-doped TiO 2 and perfluorosilane-grafted SiO 2 . These membranes have demonstrated in situ recovery of the water-rich permeate flux upon visible light illumination during oil–water separation.…”

“…An increase in the permeate flux upon light illumination has been attributed to the photocatalytic degradation of the organic substances deposited on the membrane surface 27 , 48 , 49 . Also, such photocatalytic membranes have demonstrated that they exhibit a time-dependent evolution of the surface chemistry heterogeneity (e.g., clean and contaminated regions) upon light illumination which can be quantitatively described by the contact angle measurements 6 , 22 , 50 – 52 . To our knowledge, quantitative relationships of the evolution of surface chemistry heterogeneity on a membrane surface and that of permeate flux upon visible light illumination are lacking.…”