Solar-light active tungsten oxide coated ceramic membrane with unique surface wettability fabricated by RF magnetron sputtering: Remediation of oily wastewater and photo-catalytic self-cleaning

Abstract

A thin film monoclinic WO3 was coated on porous alumina ceramic membrane by RF magnetron sputtering (RFMS), and used as a self-cleaning filtering medium for the remediation of oily water (oil-in-water emulsion). An optimum concentration of oxygen vacancy was introduced in WO3 lattice by optimizing the sputtering parameters. The excess of electron concentration arises from the introduction of oxygen vacancy enabled the induction of highly polar hydroxyl radicals on the membrane surface and this enhanced the water affinity of the surface. The resulting WO3 coated porous alumina ceramic membrane exhibited a unique combination of surface wettability: super hydrophilic in air with static contact angle 0°, super oleophilic in air with contact angle 0°, and super oleophobic under the water with contact angle close to 159.5°. These favorable wetting conditions was utilized to develop an efficient membrane for water passing oil water separation system which eventually exhibited substantially increased oil water separation efficiency, permeate flux, and critical pressure to withstand the higher applied trans-membrane pressure. In addition to the efficient oil water separation, the novelty of the membrane is its self-cleaning capability, where the organic pollutants that clogged the membrane pore during its operation was photo-catalytically removed under the natural sunlight. The self-cleaning feature of the membrane was possible also due to the presence of high concentration of electrons, comparable to that of noble metal introduced by oxygen vacancy in WO3 lattice. The collective oscillation of electron constellation resonates with the irradiated light source and resulted in surface plasmonic resonance (SPR), and consequent anomalous light absorption for efficient photo-catalytic self-cleaning of the membrane under natural sunlight. It was found that after a prolonged use, the WO3 coated porous alumina ceramic membrane showed a sharp decline of permeate flux due to clogged pores, and this was rectified and initial flux wad restored by simply exposing the used membrane to sun light.