Removal of oil in oilfield produced water using photocatalytic graphitic carbon nitride nanofibers deposited on ceramic membrane

Abstract

Oil and gas exploration and production generates billions of barrels of oilfield produced water (OPW) annually, thus making OPW the largest abundant by-product in oil and gas industry. Therefore, an efficient and effective separation of OPW has become a major challenge as it is a cornerstone to water management process that needs to meet regulatory standard for discharge and disposal to the environment. Membrane separation technology has previously delivered reliable performance of separation for treatment of OPW. However, severe fouling issues on the membrane has called for alarming an urgent technological advancement on membrane filtration. To address this concern, this study successfully synthesized a novel and potential membrane material to perform efficient and sustainable separation performances. Asymmetric ceramic hollow fiber membranes from Al2O3 precursor were fabricated using spinning technique based-phase inversion followed with high temperature sintering process. Al2O3 hollow fiber membranes were further coated with graphitic carbon nitride (GCN) incorporated polyacrylonitrile (PAN) nanofibers via direct electrospinning technique. GCN in bulk (bGCN) and nanosheets (nsGCN) configurations were synthesised from facile thermal decomposition of urea precursor. Meanwhile, bGCN was converted to nsGCN via liquid exfoliation method using isopropanol (IPA) to improve the photocatalytic properties of GCN. Decent morphological structure, well dispersed GCN, high specific surface area of nanofibers and large opening of nanofibers mesh that permitted oil droplet to permeate have been identified as the factors contributing to the excellent photodegradation of GCN nanofibers even at low loading of GCN at 1:10 to polymer ratio. Narrowed band gap energy of nsGCN as compared to bGCN, demonstrated enhancement on percentage degradation of NF-nsGCN on OPW under UV light irradiation at 96.6%. The results also revealed that synergetic effects of concentrate and degradation of oil molecules were the major important factors to obtain the high photodegradation efficiency of OPW. Due to outstanding features showcased by NF-nsGCN, in this study, self-supported photocatalytic nanofiber was coated on Al2O3 hollow fiber membrane surface using newly designed electrospinning technique to form hybrid photocatalytic nanofiber-coated membrane. Interestingly, NF-nsGCN/Al2O3 membrane established the highest pure water flux (PWF), OPW permeate flux, and oil rejection percentage at 816 Lm-2h-1, 640 Lm-2h-1, and 99%, respectively in 180 min-filtration. These findings concluded that sparse mesh-structure, high water affinity, and smooth morphology of nanofiber coatings, were the plausible parameters that significantly improve membrane performances. On top of that, NF-nsGCN/Al2O3 membrane also sustained the highest permeate flux (577 Lm-2h-1) and oil rejection (97%) in three cycle of filtrations, which confirmed the excellent cleaning performance of membrane in prolonged membrane operation. Excellent photodegradation ability of NF-nsGCN nanofiber permitted the nanofibers coating to in situ degrade the adsorbed oil contaminants under UV irradiation. Hence, it sustained high permeate flux and oil rejection of the membrane in repeating filtration system. In conclusion, this study recommends the potential application of the NF-nsGCN-coated Al2O3 hollow fiber membrane as the highly potential novel membrane for the treatment of industrial OPW.