Remediation of crude oil contaminated oily wastewater using nanostructured ZnO-decorated ceramic membrane: Membrane fouling and their mitigation using photo-catalytic self-cleaning process

Abstract

In membrane-based oil water separation, the fouling of oil on the membrane pores due to the adsorption of oily feed components (crude oil) is a major disadvantage. In order to address the problem of membrane fouling, in this work, the filtration membrane itself was coated with a photoactive semiconducting material to self-clean the fouled membrane by photo-catalytic degradation. In addition to the self-cleaning, the other major functionality of the coating is to render a favorable surface wettability, conducive for water passing oil water separation. The basic membrane is porous alumina (Al₂O₃) substrate, on which a layer of crystalline Zinc Oxide (ZnO) thin film was coated by single step RF magnetron sputtering. The fabricated membrane is super-hydrophilic in the air and super-oleophobic underwater, and this contrasting wettability is crucial for the water passing oil water separation. The advantage of preferentially water passing membrane is that the oil clogging on the membrane surface is less than that of the oil passing membrane. However, after prolonged use with crude oil-contaminated feed, the accumulation of organic pollutants on the membrane surface hinders the smooth permeation of water through the membrane. The oil water filtration system involves cross flow through the ZnO-deposited Al₂O₃ membrane with the application of trans-membrane pressure. The membrane achieved an excellent separation efficiency (99.66 %) of oil and water from the oil emulsified water, and high permeates flux (908.4 L/m².h.bar) for 200 ppm crude oil contaminated oily feed (surfactant stabilized crude oil-in-water emulsion). After a prolonged use of 540 min, the permeate flux declined due to the accumulation of organic pollutants present in the oily water. The original flux was restored by initiating photocatalytic degradation of organic pollutants by the exposure of UV radiation on the used membrane surface. The ZnO-deposited Al₂O₃ membrane shows an excellent light induced photocatalytic self-cleaning and antifouling with flux recovery ratio of around 88 %. In addition to this application, this manuscript also presents the morphological, elemental, structural, and topological characterizations of coated membrane.