Polyvinylidene fluoride incorporated with titania-zirconia dual layer hollow fiber photocatalytic membrane for oily wastewater treatment

Abstract

The need for a more effective oily wastewater treatment is necessary to minimize oil and grease content in the wastewater and produce maximum amount of treated water that is suitable to be discharged into open water course. Even though degradation of oily wastewater was found to be promising through photocatalysis process, excellent performance can be achieved through the combination of photocatalysis with membrane separation process. In this study, zirconium dioxide (ZrO2) was combined with titanium dioxide (TiO2) to improve the specific surface area of TiO2. The TiO2-ZrO2 hybrid photocatalysts were then embedded in the outer layer of the polyvinylidene fluoride (PVDF) dual layer hollow fiber (DLHF) membrane to produce a photocatalytic membrane for oily wastewater treatment. In the first stage of the study, the coupling of ZrO2 content from 1 to 20% into TiO2 was designed to enhance the oily wastewater adsorption capacity and the photodegradation performance. The 1% TiO2-ZrO2 hybrid photocatalysts synthesized in this study revealed a higher (second highest) specific surface area of 136.7 m2/g in comparison to single TiO2 (39.9 m2/g). This characteristic is desired as it can boost the photocatalytic activity. The hybrid photocatalysts also displayed reduced optical band gap energy which is desirable as it allows better absorption of photons to excite the electrons into the valence band. The second stage of the study involved fabrication of PVDF DLHF membrane embedded with 1 wt.% of TiO2-ZrO2 hybrid photocatalysts in the outer layer of the membrane (DL-ZT1). The fabricated membrane was optimized in terms of air gap from 5 cm to 50 cm. The membrane spun at lower air gap of 5 cm showed the formation of long finger-like structure around 65.0 ± 3.3 µm in length on the outer layer of the membrane due to the immediate phase inversion on the outer side of the fiber. Cross-sectional image of the membrane showed that the membrane is free from delamination which indicated mutual diffusion of polymer during co-extrusion. The membrane displayed lowest contact angle of 71.70° ± 2.58°. The low contact angle was attributed to the low air gap of 5 cm that promoted the growing of microvoids on the outer layer. Under crossflow filtration condition, the membrane also demonstrated highest water and oily wastewater permeation flux as well as oil rejection percentage of 85.4% without UV light irradiation. In the third stage of the study, the membrane was optimized in terms of photocatalysts loading from 0 to 1 wt.% in the outer layer dope composition. The photocatalytic activity of the membrane was investigated using the submerged membrane photoreactor (sMPR) at oily wastewater concentration of 1000 and 10,000 ppm. At 1000 ppm concentration, DL-ZT1 was found to have initial oily wastewater permeation flux of 97.71 L/m2.h without UV light irradiation and the flux increased to 321.62 L/m2.h under UV light irradiation. As a result, DL-ZT1 recorded total organic carbon (TOC) degradation of 91.8%. Despite showing reduced TOC degradation at higher oily wastewater concentration of 10,000 ppm, DL-ZT1 recorded oil rejection percentage of 96%. DL-ZT1 exhibited a great potential of photocatalytic membrane for oily wastewater treatment. In comparison to single layer hollow fiber membranes, the DLHF membranes has better performance due to the embedded nanomaterials localized on the outer layer and which made possible reduction in membrane fouling.