Photo-responsive Zinc Oxide-coated alumina ceramic membrane with super-wettable and self-cleaning features fabricated by single step RF magnetron sputtering for oily water treatment

Abstract

Zinc Oxide (ZnO) coated alumina ceramic membrane with super-wettable and photo-responsive self-cleaning features was fabricated by radio frequency (RF) magnetic sputtering, and used as a filtering medium in the dead-end filtration system for oily water treatment. The chemistry of the material in conjunction with the optimum surface roughness accomplish by RF magnetic sputtering rendered a unique surface wettability, favorable for water passing oil-in-water emulsion separation. The second desirable functionality of the membrane is its self-cleaning potential under light irradiation, owing to the efficient photo-catalytic property of ZnO. Oil-surface-air and water-surface-air interfacial contact angles for the membrane surface were measured using goniometer and they are close to 0° (superhydrophilic / superoleophillic in air), whereas under the water oil-surface-water interfacial contact angle switched to 158.70° (underwater superoleophobic). The oil-in-water emulsion separation efficiency, the water permeate flux were measured at different trans-membrane pressures, and oil concentrations in oily water emulsion, and the results showed that 99.9% oil-in-water emulsion separation efficiency was maintained for a range of trans-membrane pressure, and oil concentration for a long duration of use. The highest permeate flux rate and separation efficiency (%) of the fabricated Zinc oxide-coated alumina ceramic membrane were found to be ∼325 L/m2.h and 99.9%, respectively at 2 bar using surfactant stabilized oil-in-water emulsions as feed. However, the water flux declined with reduced trans-membrane pressure, increased oil concentration and with the long use of the membrane. The advantage of ZnO-coated alumina ceramic membrane is that it is water passing, and hence the oil clogging of the membrane is quite minimal. However, with the long use the membrane is prone to the deposition of oily contaminant on the membrane surface, which resulted in low water permeate flux, and this problem was resolved, and the original permeate flux was restored by routine self-cleaning of the membrane surface by illuminating light on the photo-catalytic ZnO film on the membrane surface. In addition to the wettability studies of the membrane, the structural, chemical and morphological characterizations of the material, and the coated membrane were carried out.