Abstract
Advancement of membrane technology in recent years has increased due to its low energy consumption, no use of chemicals, high removal capacity and easy accessibility of membrane material. Among various membrane types, thin film composite (TFC) nanofiltration (NF) membrane has been studied intensively due to its advantages of achieving promising water flux and rejection towards various pollutants at relatively low operating pressure. The technique is widely used in wastewater treatment, water purification, water softening, food processing, and bio-separation. Widely used NF membranes are thin film composite (TFC) polyamides. These membranes offer high flux and good selectivity, but they are inherently susceptible to fouling. To develop a high performance TFC membrane, various nanomaterials, such as graphene oxide and metal organic frameworks (MOFs), have been investigated due to their unique functionalities (e.g., hydrophilicity, antimicrobial functionality, porosity, mechanical property,and high surface area).
The main focus of this work was to develop composite filler materials [(zeolitic imidazole framework-8@ functionalized graphene oxide (ZIF-8@fGO), and functionalized zeolitic imidazole framework-8 @ sulfone graphene oxide (fZIF-8@SGO)] to enhance the properties of the TFC membrane such as enhanced hydrophilicity, high rejection of pollutants, improved antifouling properties and prolong the lifespan of the membrane. All the nanomaterials were synthesized and characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), Raman spectroscopy, thermogravimetric analysis (TGA), scanning electron microscope (SEM), transmission electron microscope (TEM) and x-Ray photoelectron spectroscopy (XPS). On one hand membranes were also analyzed using SEM, attenuated total relectance-Fourier transform infrared (ATR-FTIR), and atomic force microscope (AFM).
In the first study, five different ratios of ZIF-8@fGO (0.1,0.5,0.8,1.0,1.3):1 nanocomposite materials were synthesized and incorporated on the polyamide layer of the prepared TFC membranes. Eight membranes [ bare thin film membrane (M1), TFC incorporated with fGO (M2), TFC incorporated with ZIF-8 (M3), TFC
vii
membranes incorporated with five different concentrations of ZIF-8@fGO (M4-M8)] were fabricated on top of the in-house prepared PES support layer (M0) using two monomers of m-Phenylenediamine (MPDA) and trimesoyl chloride (TMC). The water contact angle of membranes incorporated with nanomaterials (M2-M8) exhibited high hydrophilicity and increased water flux of 64 L.m-2.h-1 relative to pristine bare thin film membrane (M1) with 32 L.m-2.h-1. Furthermore, these TFC membranes presented high antifouling properties with almost 91% flux recovery ratio compared to 60, 80, and 82% obtained for M1, M2, and M3, respectively. All the TFC NF membranes incorporated with the ZIF-8@fGO exhibited ≥90% rejection toward As3+, Ba2+, Li+, and Sr2+, with M7 showing the highest rejection of 98% for As.
Secondly, ZIF-8 and GO were both functionalized with 3-amino-1,2,4-triazole and sulfanilic acid to achieve fZIF-8@SGO. The same concentration as part one was also investigated for membrane application. fZIF-8@SGO composites at different fZIF-8 loading onto SGO were successfully synthesized via the in-situ growth method. 0.5wt% of fZIF-8@SGO were embedded onto the thin polyamide layer fabricated on top of the PES substrate. The fabricated membranes (bare thin film membrane (M1), TFC membrane incorporated with SGO (T1), TFC membrane incorporated with fZIF-8 (T2), TFC membranes incorporated with five diffrernt concentrations of fZIF-8@SGO (T3-T7)] were evaluated by monitoring water flux, textile wastewater rejection, and fouling resistance. The pure water permeability of membranes was determined under different pressure filtration in dead-end mode at (700, 800, 900, 1000, and 1100) kPa. The membrane fouling resistance was characterized under constant flux operation using bovine serum albumin as a model foulant. The results confirmed that modified TFC membranes exhibited improved fouling resistance with ∼90% water flux recovery with high fZIF-8@SGO loading and the recovery was maintained for at least six cycles. The nanocomposite membranes rejected above 99% of textile wastewater through a size exclusion mechanism. M7 showed better performance relative to other composite membranes.
Incorporating both ZIF-8@fGO and ZIF-8@SGO composite materials into thin film composite membranes managed to overcome low flux while retaining high rejection which is a major obstacle facing this technology. Furthermore, these composites aided in the fouling resistance of TFC membranes due to their excellent properties resulting in prolonging the lifespan of the TFC membranes.