Nanostructured membrane for arsenic removal from drinking water

Arsenic (As) removal is a major concern because inorganic As is highly toxic to all forms, is a confirmed carcinogen, and significant environmental concern. As contamination in drinking water threatens more than 150 million people all over the world. Therefore, several conventional methods such as oxidation, coagulation, adsorption, have been implemented for As removal, but due to their cost-maintenance limitations; there is a drive for advanced, low cost nanofiltration (NF) membrane-based technology. Thus, in order to address the increasing demand of fresh and drinking water, NF technology has been used for As removal to safeguard water security in this PhD research. To proceed with this, NF membrane is the pre-requisite. Mixed matrix membranes (MMMs) can be used as NF membrane for As filtration as they possess a wide range of properties, including selectivity, good permeability, antifouling behavior, mechanical strength. However, these properties of MMMs are from the tailored and designed structure, which is possible due to the fabrication process with controlled parameters and appropriate materials, such as polymer matrix with dispersed nanoparticulates. Therefore, several conventional fabrication methods such as phase inversion process, interfacial polymerization, co-casting, coating, electrospinning, etc., have been implemented for MMMs preparation. This thesis is presented in 6 chapters. Chapter 1 introduces the toxicity of As and its removal technologies. It follows with the detail description of NF technology including As removal mechanism to understand the working principle of the NF membranes in this study. In addition, the major challenge faced during membrane filtration technology known as membrane fouling is described briefly with its formation mechanism and possible steps taken to reduce the fouling by using nanoparticles. NF membrane fabrication processes also have been discussed briefly followed by the effect of intrinsic and extrinsic parameters on As removal efficiency. Finally, the objectives of this thesis are identified based on the existing literature gaps. Chapter 2 presents the research methodology employed in this study to fabricate the membranes and their characterization. Electrospinning technology used for the fabrication of membranes has been explained extensively with the effect of different parameters on the membrane morphology. The characterization section presents the working principle of various instruments used in this study to investigate the membranes such as rheometer, scanning electron microscope (SEM), Fourier Transform Infrared (FTIR), X-ray diffraction (XRD), X-ray Photoelectron Spectrometer (XPS) etc. Chapter 3 focuses on the development and characterization of functional polysulfone (PSf)-based composite NF membranes comprising two different oxides, such as graphene oxide (GO) and zinc oxide (ZnO) for As removal from water. PSf/GO and PSf/ZnO- NF membranes were fabricated by electrospinning, and subsequently examined for their physicochemical properties and evaluated for their performance for arsenite–As(III) and arsenate–As(V) rejection. The effect of GO and ZnO on the morphology, hierarchical structure, and hydrophilicity of fabricated membranes was studied using SEM, small and ultra-small angle neutron scattering (SANS, USANS), contact angle, zeta potential, and BET (Brunauer, Emmett and Teller) surface area analysis. FTIR, XRD, and XPS were used to study the elemental compositions and polymer-oxide interaction in membranes. Incorporation of GO and ZnO in PSf matrix reduced the fiber diameter but increased the porosity, hydrophilicity, and surface negative charge of the membranes. Among five membrane systems, PSf with 1% ZnO has the highest water permeability of 13, 13 and 11 L h−1 m−2 bar−1 for pure water, As(III), and As(V)-contaminated water, respectively. The composite NF membrane of PSf and ZnO exhibited enhanced (more than twice) arsenite removal (at 5 bar pressure) of 71% as compared to pristine PSf membranes, at 43%, whereas both membranes showed only a 27% removal for arsenate. In Chapter 4, the fabrication of nanofibrous mixed matrix membranes (MMMs) by electrospinning of PSf using composite particles as filler has been studied extensively to investigate As filtration performance. Dual nanofillers such as GO-ZnO and GO-ZnO-iron oxide nanoparticles were synthesized and investigated to use as filler particles for MMM preparation by TEM, XRD and XPS analysis. The effect of these composite particles on the synthesized membrane morphology, hierarchical structure, surface charge, hydrophilicity and thermal behaviour was studied using SEM, SANS, USANS, contact angle, zeta potential, differential scanning calorimetry (DSC) and BET. FTIR, XRD and XPS were used to study the elemental compositions and interaction between them in the membranes. These analyses confirmed the synthesis of hydrophilic porous nanofibrous NF MMMs with negative surface charge, which lead to better arsenite and arsenate filtration simultaneously from drinking water. The effect of extrinsic and intrinsic parameters on membrane performance for As removal was also studied extensively for efficient process optimization. An arsenite and arsenate filtration efficiency of 99% and 95%, respectively was achieved for the fabricated nanofibrous MMMs comprising GO-ZnO-iron oxides, which can be attributed to the negative surface change and nanoporosity of the membranes. Chapter 5 highlights the antifouling and antibacterial properties of electrospun nanofibrous MMMs in respect to As NF process. Three electrospun nanofibrous MMMs (P, CP and MCP; where P is pristine PSf membrane and CP and MCP are MMMs consisting GO-ZnO and GO-ZnO-iron oxides particles) have been studied. The effects of these composites on the antifouling behaviour of the membranes have been studied by characterizing the bovine serum albumin (BSA) protein adsorption on the membranes and subsequent analysis using microscopic (morphology by SEM), and BET analysis. The antibacterial properties of these membranes were also studied against Gram- positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli). The composite nanoparticles incorporated membranes showed improved antifouling properties in comparison with pristine PSf membrane. The excellent antimicrobial properties of these membranes make them appropriate candidates to contribute or overcome biofouling issues in water or wastewater treatment applications. Chapter 6 summarizes the major findings/outcomes of this research and provides the conclusions and recommendations for future research. In summary, the research presented here will contribute to the advancement of efficient As removal technology through the engineering approach by designing and fabrication of nanofibrous MMMs as well as NF process optimization. The design and composition of fabricated MMMs will potentially serve as a platform for future investigations of water filtration technology.