Antimicrobial Propensity of Graphene-based Membranes for Water Treatment Systems

Abstract

To overcome the challenge of global water shortage and scarcity, seawater desalination and wastewater reuse have been proposed over the past years. More importantly, the pressure driven processes have been preferred over the thermally driven process because of its relatively lower energy consumption. Nonetheless, membrane fouling especially, membrane biofouling remains the Achilles heels of this technology. Several control strategies have been proposed and implemented such pre-treatment of feed water, physical, chemical, and biological cleaning and membrane surface modification/use of new material for membrane fabrication. Although, cleaning practice is an inevitable aspect of the membrane-based process; however, recent studies have focused on the fabrication of novel membranes with new materials such as two dimensional (2D) materials like graphene-based material in order to control biofouling in membrane-based water treatment systems. Graphene-based membranes have a huge futuristic application as an alternative to the conventional polymeric membranes because of its controllable pore size, higher water flux, and higher salt and other material rejection. Graphene-based membranes can be fabricated by making uniform nanopores on graphene sheets or by the stacking of graphene material nanosheets, whereby, the nanochannels created by the stacking of charged and functionalized nanosheets could be used for water permeation and ion or particle rejection. Graphene-based membranes are also known for their antibiofouling/antimicrobial properties. Notwithstanding the superior performance of graphene-based membrane in terms of high water flux, high salt rejection, and low fouling tendency; contradictory evidence exists on the antimicrobial and antibiofouling properties of graphene-based membranes. Thus, this doctoral study aims to understand the intrinsic antimicrobial property of graphene-based membranes and further improve the antimicrobial activity of graphene-based membranes. To start with, the scope and outlines of the thesis were introduced in chapter 1 and chapter 2 gives the general background and the extent of work done in this research area. Due to the contradictory evidence that exists on the interaction of graphene-based surfaces with microorganisms in the literature, chapter 3 attempts to clarify to some extent these discrepancies. The role of GO layer morphology in microbial behaviors on membrane surfaces was investigated by using Pseudomonas aeruginosa PAO1 as a model bacterium. As a result, strong adhesion of P. aeruginosa PAO1 on GO membranes at an elevated GO thickness (>1.71429 mg/cm2) was found. This behavior is attributed to changes in the surface morphology, which helps harbor and trap bacterial cells. These results reveal the significance of GO layer morphology in bacterial behaviors on membrane surfaces and suggest that GO layers with considerable roughness and curvature may serve as an absorbent for bacterial cells and potentially reduce biofouling in membrane-based water and wastewater treatment systems when used in the pre-treatment unit. As reported in chapter 3, GO membrane could not inactivate bacterial growth irrespective of the thickness or morphology. Consequently, in chapter 4, the effect of degree of GO layer reduction on its antimicrobial property was investigated. The antimicrobial properties of graphene-based membranes such as single-layer graphene oxide (GO) and modified graphene oxide (rGO) on top of cellulose ester membrane were reported in this study. rGO membranes were made from GO by hydriodic acid (HI) vapor treatment. The antibacterial properties were tested after 3 h contact time with selected model bacteria. Complete bacterial cell inactivation was found only after contact with rGO membranes, while no significant bacterial inactivation was found for the control (i) GO membrane, (ii) the mixed cellulose ester (MCE) support, and the (iii) rGO membrane after additional washing that removed the remaining HI. This indicated that the antimicrobial effect was neither caused by the graphene nor the membrane support. The antimicrobial effect was found to be conclusively linked to the HI eliminating microbial growth, at concentrations from 0.005%. These findings emphasize the importance of caution in the reporting of antimicrobial properties of graphene-based surfaces. Hitherto, GO or rGO membranes do not possess enough antimicrobial properties needed to control biofouling development in membrane-based water treatment system. In Chapter 5, the antimicrobial property of GO membranes was enhanced by the incorporation of copper oxide into the matrix of the GO nanosheets. Here, the improvement of the antimicrobial properties of reduced graphene oxide by the preparation of rGO-CuO nanocomposite membranes was reported. The rGO-CuO nanocomposite was synthesized via a simple hydrothermal method, and the nanocomposite film was fabricated with the assistance of a vacuum filtration unit by filtering through a polytetrafluoroethylene filter. After characterization, the antimicrobial properties were tested against Pseudomonas aeruginosa PAO1. The fabricated nanocomposite film exhibited excellent antimicrobial activity, leading to complete bacterial inactivation. The antimicrobial properties were closely linked to the electron transfer mechanism rather than the generation of reactive oxygen species (ROS). This chapter provides insights into the antimicrobial mechanisms of copper oxide and graphene material composites. Finally, in chapter 6, the antimicrobial property of a novel rGO-CuO-Ag composite membrane was reported for developing a water treatment membrane with enhanced antibiofouling properties. The fabricated membrane showed remarkable antimicrobial performance (about 10 log reduction) of Pseudomonas aeruginosa PAO1 when compared with the pristine rGO membrane. The novel membrane exhibited a controlled and consistent small release of Ag ion which was responsible for the antimicrobial property noticeable in this chapter. Due to the controlled small release of the Ag ion, a long-lasting antimicrobial property was obtained. This chapter introduced a new surface coating or membrane material for biofilm control in biomedicine and water treatment systems. Overall, this research clarifies the possibility and direction of developing graphene-based membranes for biofouling control in membrane-based water treatment systems. Nevertheless, experiments were performed under very confined conditions and in a sense, the work is limited. Therefore, further studies such as biofilm formation over a long time, and the evaluation of biofouling phenomenon in a system operation should be conducted.