Nanomaterials Functionalized Reverse Osmosis Membranes: Combating Fouling and Selectivity Challenges

納米材料功能化反滲透膜:應對膜污染和選擇性挑戰

Student thesis: Doctoral Thesis

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Award date11 Jun 2021

Abstract

Desalination and potable water reuse have become indispensable components of the urban water supply in many regions around the world to address the water scarcity challenges. The use of membrane technology, in particular reverse osmosis (RO), has been reported to be promising for desalination and water reclamation in potable and non-potable applications. Despite remarkable advancement and its commercialized applications, RO membrane suffers from several limitations. For instance, when reclaiming water using RO, poor selectivity of the membrane against the contaminants of emerging concerns (CECs) which include pharmaceuticals, personal care products, endocrine disruptive compounds, disinfection byproduct (DBPs), etc. is an emerging challenge owing to their potential negative health effects. In addition, fouling of the membrane, in particular biofouling, is one the major limitations leading to several operational challenges including increased feed channel pressure, decreased permeate flux, permeate quality deterioration, frequent membrane cleaning, and its periodic replacement. Chlorine-based compounds are considered to be an efficient and cost-effective for biofouling mitigation, however, vulnerability of the polyamide layer to chlorine attack and subsequent deterioration of membrane performance is another limiting barrier in achieving sustainable membrane performance.

Herein, we begin with an overview of the removal mechanism for a wide range of CECs via different non-biological membrane-based water and wastewater treatment processes including forward osmosis (FO), RO, nanofiltration (NF), ultrafiltration (UF), and membrane distillation (MD). In addition to the important role of different process types, working principles, and their operational conditions, the contribution of membrane material and its configurations, CECs characteristics, and feed solution chemistry in enhancing CECs removal was further examined. Though RO exhibited superior selectivity against a wide range of volatile and non-volatile CECs in comparison with other membrane-based treatment processes, the permeation of low molecular weight and non-ionic CECs clearly demonstrated that pristine commercial RO membranes cannot serve as a complete barrier. In contrast, better selectivity of RO membrane was demonstrated upon the inclusion of nanomaterials. Based on the reviewed literature, the functionalization of nanomaterial over the polyamide layer was found to improve membrane selectivity in comparison to their incorporation in the support and or polyamide structure which contributed more toward better flux performance.

Next, we modified a RO membrane using graphene oxide (GO) crosslinked with a thin layer of polydopamine (PDA-GO) to obtain a bactericidal and antibiofouling surface. Optical coherence tomography (OCT) was used as a non-destructive, real-time, in situ monitoring tool to observe the fouling dynamics and to understand the material-foulant interaction. In situ three-dimensional (3D) monitoring results showed that the modified membrane was antifouling in nature and exhibited 46% less biofilm formation. In addition, the colony forming unit enumeration and results of confocal laser scanning microscopy (CLSM) revealed that the PDA-GO coated membrane exhibited a high bactericidal effect when compared with pristine, PDA, and GO-coated membrane surfaces. Investigation of bacterial cell morphology further validated that the presence of a GO layer and its functional groups combined with a thin PDA layer resulted in physical and chemical disruption of the bacterial cell wall. The high selectivity of the modified membrane against a monovalent salt validated its potential for sustainable desalination and wastewater reclamation performance.

Later, we extend our efforts to investigate the effect of GO layer positioning, attributed to different functionalization methods (i.e., mixing GO with a crosslinking agent and functionalizing GO on a pre-modified surface), on a modified membrane surface and its associated antibiofouling properties. The RO membranes were modified by GO via PDA crosslinking using two different approaches; mixing GO with PDA (co-deposition; PDA-GOco) and functionalizing GO on PDA impregnated surface (sequential deposition; PDA-GOsq). Both methods resulted in distinct membrane properties, with each exhibiting different selectivity and anti-fouling performances. OCT results revealed that the PDA-GOco membrane exhibited improved anti-scaling and anti-biofouling compared with the pristine and PDA-GOsq membranes. Contrarily, CFU-enumeration and CLSM results showed superior bactericidal effect for the PDA-GOsq coating. The investigation of the selectivity performance of the membranes against N-Nitrosodimethylamine (NDMA) which was chosen as one of the emerging CECs due to its potentially high formation from the increased utilization of disinfectants during this pandemic period, revealed that PDA-GOco membrane showed enhanced NDMA selectivity, with an 87% rejection, in comparison to both the pristine (74%) and PDA-GOsq membrane (82%). The improved roughness and hydrophilicity of the PDA-GOco membrane attributed to the formation of thin PDA layers on both sides of the GO causing its reduction and subsequent layers-stacking eventually resulted in enhanced antifouling and selectivity performance.

Finally, we investigated the mechanism of electrochemical chlorine reduction using cyclic voltammetry and sodium hypochlorite (NaOCl) as a chlorine source. To overcome the vulnerability of RO membranes to chlorine attack, a chlorine resistant RO membrane was fabricated via inclusion of carboxylated carbon nanotubes (CNTs) functionalized on the polysulfone ultrafiltration support via pressure assisted filtration. The formed 2 µm thick CNTs layer imparted a conductive property to the polyamide network (CNT-RO). The conductive membrane was used as a cathode and its resistance to chlorine was tested with and without DC voltage application using a NaOCl solution (1000 ppm: pH = 5). Attachment of chlorine onto the conductive membrane surface was examined via XPS and FTIR whereas the effect of chlorination on the wettability of the membrane surface and morphology was evaluated by contact angle measurement and SEM monitoring. In addition, the rejection of a monovalent salt and the permeate flux were used as performance indicators. Our findings revealed that compared with the pristine RO membrane, which exhibited high chlorine uptake and subsequent compromised performance (i.e., increased membrane flux and decreased salt rejection), the application of a 2V DC potential onto the conductive membrane surface successfully prevented chlorine attack and caused irreversible chlorine reduction.

As a whole, the research presented in this dissertation presents an attempt to overcome existing RO membrane challenges such as poor selectivity against CECs, membrane fouling, and low chlorine tolerance via inclusion of carbon-based nanomaterials (i.e., GO and CNTs). The application a GO layer via PDA crosslinking and subsequent induced partial reduction successfully helped in improving membrane fouling and selectivity challenges. Whereas the inclusion of CNTs and its associated conductive property in the polyamide network effectively overcome chlorine-induced degradation of polyamide network, thereby opened new direction for the revival of existing thin film composite (TFC) RO membrane to demonstrate sustainable performance.